View Full Version : DC offset question.
farflungstar
03-05-2018, 14:52
So to cut a long story short my ARC ref 75 has just been returned after 3 months at the service department. Diagnosis of fault DC on the input of one channel - busted FETs.
I've hooked it up but not switched on yet. I measured for DC at the outputs of the Truth pre. With the multimeter set at 200mv I get 0.02 on both channels. Please can someone tell me if this is ok? It seems very very little but I'm so paranoid and dont want another 3 month absence and 900€ bill.
Firebottle
03-05-2018, 15:45
I think a little more clarity is need on the reading.
If the multimeter is set on the 200mV range then isn't the 0.02 a reading of 20uV?
Adrian can you repeat the measurement on the 2V range and report back?
Good reply Andrew but I'm a bit unsure of the reported reading.
337alant
03-05-2018, 17:19
The offset seems very low TBH
Could the damage have occurred by careless hot phono plugging between pre and power where the signal makes a connection but no ground so you get the classic oscillation screech ?
Alan
Firebottle
03-05-2018, 17:58
Could be Alan, I have to admit to damaging things myself doing that :doh:
What a dork I am. Sorry cancel that, was.
Another possibility is a knackered output cap on the preamp (or input cap on the amp...) allowing dc into the amp. Wouldn't it need a fair level of dc to destroy the fets? Would 20mV be enough?
Edit: 20mV dc coming out of a pre sounds like abysmal design if it's 'normal' for the pre... I'd be angry if I got that out of my power amps, I'd be straight inside and checking/adjusting the bias.
Sent from my GT-I9195 using Tapatalk
sq225917
03-05-2018, 19:56
Yup. Should be 20uv at that range.
If no show on 2v he's good.
20mv on a preamp is attrocious, it should sub 1mv
farflungstar
03-05-2018, 20:10
I have to admit I'm confused by the multimeter. I've included a pic of the settings. I did actually adjust the DC offset internally to zero, and perhaps the 0.02 reading is dodgy connection of the probes on the RCA outputs. I could use alligator clips when adjusting internally. (The maker guided me through the process and both channels read 0.00). I don't want any DC offset - nada. But I find it hard to interpret the meter.
Could I connect a coupling cap across live/earth at the outputs to get rid of any? If so what kind of value?
Here's the meter...
https://uploads.tapatalk-cdn.com/20180503/ee864240afccf0ace2185bf0c20e89b8.jpg
Firebottle
03-05-2018, 20:42
Put it on 2000m and see what it reads.
What did you use to adjust internally to zero?
If it is showing 0.02 on the 200m range that is more than likely the expected error of the multimeter and is nothing to worry about.
Though to put your mind fully to rest (we are all behind you in this) report what the 2000m range reads.
No! The cap goes in series with the output.
Edit: not aimed at Firebottle :)
Sent from my GT-I9195 using Tapatalk
farflungstar
03-05-2018, 20:45
Put it on 2000m and see what it reads.
What did you use to adjust internally to zero?
If it is showing 0.02 on the 200m range that is more than likely the expected error of the multimeter and is nothing to worry about.
Though to put your mind fully to rest (we are all behind you in this) report what the 2000m range reads.Hang on....
farflungstar
03-05-2018, 20:49
Hang on....Ok - on 2000 both channels read the same - they fluctuate (maybe the probes) between 000 and -000.......
Oh, there are small pots to adjust DC internally.
Sounds like the pre is behaving itself. Panic over :)
Sent from my GT-I9195 using Tapatalk
Firebottle
04-05-2018, 06:28
I concur, good to go Adrian :thumbsup:
It might indeed! It would help to know what pre it is, otherwise we all go round in circles. Also do the Jfets occur after the input tube, or is it a driver? If the former I'd be inclined to look at the 6H30P tubes, in my experience they can misbehave if stressed.
Sent from my GT-I9195 using Tapatalk
farflungstar
04-05-2018, 12:26
I would be more cautious. How did the amp get damaged in the first place? It's a very expensive repair to go through again. Why won't connecting and using the same equipment as before give the same results as before - ie a blown power amp?
It would worry me.
Maybe the fault wasn't caused by a DC offset from the pre-amp but by ultrasonic oscillation.
I'm just advising caution, that's all.This is my worry. I really can't say for absolutely sure what caused it (the techie said it was DC at the input) - but what other equipment did it? The problem came up (or was very apparent) when I bought that ill fated CDP and connected it direct to the amp using its inbuilt volume. That was returned but I cannot be 100% sure it was the cause.
I checked the DC at the outputs of the truth pre after the techie told me about the DC - one channel (at the 200m setting) was 00.2 the other 01.7 - but had I checked it at the 2000 setting I don't know what that would have been. For this reason I contacted the maker (who said it was negligible and not likely to cause any issue) and adjusted the input buffers and output buffers to 0 on both channels using the 200m setting on the meter. In theory there is no DC in the circuit except perhaps the anomoly of bad probe contacts on the outputs when measuring.
The truth is never switched off and I never connect or disconnect stuff when things are switched on. The only components are the cart, Vida, hashimoto, truth, ref 75, Tannoys.
Is there a possibility that the small input tubes in the 75 could have an effect? It was soak tested using them at the centre without a problem but I have a brand new set of power and signal tubes.
It's silly but I still haven't switched the amp on - I just dare not.
The truth has caused no problems with the Hiraga amp but of course it's a different animal to the ARC.
Primalsea
04-05-2018, 12:42
Can you advise your setup, is it the same as in your signature? It looks like your Truth phono amp is connected via the poweramp via a transformer based volume control/preamp? If this is the case any DC on the output of the Truth will effectively be blocked by the volume control as transformers do not pass DC. However, out of hearing band oscillation could be passed on to the poweramp.
Many multimeters dont tend to be good at measuring at the frequency extremes but you could try measuring the Truth’s outputs with the lowest AC volts setting with and without the inputs grounded.
farflungstar
04-05-2018, 12:45
It's simple - Vida phono stage into hashimoto sut into Truth ldr pre into amp - no transformers. Though now I wish I still had the old slagle autoformers....
farflungstar
04-05-2018, 12:50
This is my worry. I really can't say for absolutely sure what caused it (the techie said it was DC at the input) - but what other equipment did it? The problem came up (or was very apparent) when I bought that ill fated CDP and connected it direct to the amp using its inbuilt volume. That was returned but I cannot be 100% sure it was the cause.
I checked the DC at the outputs of the truth pre after the techie told me about the DC - one channel (at the 200m setting) was 00.2 the other 01.7 - but had I checked it at the 2000 setting I don't know what that would have been. For this reason I contacted the maker (who said it was negligible and not likely to cause any issue) and adjusted the input buffers and output buffers to 0 on both channels using the 200m setting on the meter. In theory there is no DC in the circuit except perhaps the anomoly of bad probe contacts on the outputs when measuring.
The truth is never switched off and I never connect or disconnect stuff when things are switched on. The only components are the cart, Vida, hashimoto, truth, ref 75, Tannoys.
Is there a possibility that the small input tubes in the 75 could have an effect? It was soak tested using them at the centre without a problem but I have a brand new set of power and signal tubes.
It's silly but I still haven't switched the amp on - I just dare not.
The truth has caused no problems with the Hiraga amp but of course it's a different animal to the ARC.One more doubt - I bought the 75 second hand, and the CDP shortly afterwards - there is the possibility that the ref 75 was faulty when I bought it - this is the problem - so manyy variables I feel like dumping the lot.
I just remeasured the pre outputs having rigged up an RCA plug to the meter probes. Right 000 left -000 - set at 2000m. At 200m I get right 00.4 left -00.2.
I have replaced ALL tubes with new.
Should I try it?
If you know of anyone with an oscilloscope then it might be worth looking for ultrasonics which might be there as Andrew has suggested? It seems a long shot but safety is always sensible. If not and you are worried then perhaps get a cheap charity shop type amp and use that as a burner to see if any issues arise?
StanleyB
04-05-2018, 17:39
If you know of anyone with an oscilloscope then it might be worth looking for ultrasonics which might be there as Andrew has suggested? It seems a long shot but safety is always sensible. If not and you are worried then perhaps get a cheap charity shop type amp and use that as a burner to see if any issues arise?
If you have a digital multimeter, put it on the lowest value of the AC range And measure the AC voltage. Some multimeters can display the value of a frequency in terms on an AC voltage.
farflungstar
04-05-2018, 18:02
If you know of anyone with an oscilloscope then it might be worth looking for ultrasonics which might be there as Andrew has suggested? It seems a long shot but safety is always sensible. If not and you are worried then perhaps get a cheap charity shop type amp and use that as a burner to see if any issues arise?I wish I did. I don't understand oscillation, what could cause it?
I'm going to open up the truth again tomorrow as fine adjust so I get absolute zero DC book matter is negligible.
It's simple - Vida phono stage into hashimoto sut into Truth ldr pre into amp - no transformers. Though now I wish I still had the old slagle autoformers....
Any DC on the output of the Vida will be blocked by the Hashimoto SUT. The Truth line-level attenuator would pass DC as it is a simple resistor L-pad; as would the Slagle autoformer.
You have already measured the output of the chain to be free of DC (to within the accuracy of your multimeter), so as others have suggested check for any VHF oscillation.
With the inputs to your valve amp shorted, what is the DC reading across the output to your speakers?
farflungstar
04-05-2018, 18:37
Any DC on the output of the Vida will be blocked by the Hashimoto SUT. The Truth line-level attenuator would pass DC as it is a simple resistor L-pad; as would the Slagle autoformer.
You have already measured the output of the chain to be free of DC (to within the accuracy of your multimeter), so as others have suggested check for any VHF oscillation.
With the inputs to your valve amp shorted, what is the DC reading across the output to your speakers?The Truth is not like another pre - having seen inside it I truly do not understand how it works, I do know that it has a very wide bandwidth and is buffered both at input and output. This I could also see in the box, and is where adjustment pots for DC are located. There is nothing but light/or not between the two boards. I could see no circuitry for this though it has an extraordinary power supply. I won't put pictures without permission - and I don't want to go off topic - oscillation... Shorting the input at amps? Holy cow.
There are 3 possible culprits for the damage, the Truth, The amp (maybe it was broken when I got it, or the CDP I bought and returned. All three are equally possible.
Now oscillation.... Cry, slash wrists
If I found someone with an oscilloscope what should be do/look for?
Think you won't be happy until you get another pre. And I don't blame you
The Truth is not like another pre - having seen inside it I truly do not understand how it works, I do know that it has a very wide bandwidth and is buffered both at input and output. This I could also see in the box, and is where adjustment pots for DC are located. There is nothing but light/or not between the two boards. I could see no circuitry for this though it has an extraordinary power supply. I won't put pictures without permission - and I don't want to go off topic - oscillation... Shorting the input at amps? Holy cow.
There are 3 possible culprits for the damage, the Truth, The amp (maybe it was broken when I got it, or the CDP I bought and returned. All three are equally possible.
Now oscillation.... Cry, slash wrists
If I found someone with an oscilloscope what should be do/look for?
If the Truth has buffer stages at both the input and the output and if they are DC coupled throughout (and the presence of DC zero adjustment pots suggests that it is), then mis-adjustment of these pots could easily cause DC to be present on the output. But your measurements have already shown that this is not the problem.
Shorting the inputs of your power amplifier will not cause it any harm at all; it is no different to using a 'passive' pre-amp with the volume set to zero. It is unlikely your valve power amp is producing DC on the output, since the output is transformer coupled to the speakers, but a small amount of insulation breakdown between primary and secondary windings could occur. Unlikely, but worth checking.
I wouldn't get too exercised Adrian, it's most likely that summat went/was wonky in the amp which has now been fixed, and nothing more. The amp has a clean bill of health, I presume from a reputable service agent?
Re. Oscillation as I understand it it's a PITA that may pop up for all sorts of surprising reasons. Unity gain op amp circuits are certainly one possiblity as the OP amps are often unstable without gain (poor spec or faked ICs), another is unusual capacitive coupling effects at the input stage, and the interface between pre output and power amp input is a likely source of such potential mismatches. They are easy enough to find with an oscilloscope and a probe set up with a small sensing coil moved around the two boxes.
One thing that should kill oscillation is bandwidth limiting, but you need to know where it is occurring and the approximate frequency. As an example you could then apply an appropriate inductor to dampen the excitation.
Sent from my GT-I9195 using Tapatalk
StanleyB
05-05-2018, 06:23
Two things that grabbed my attention are: the Truth is never switched off, and the engineer said it was DC at the input.
If an amp or preamp is never switched off, one or more components will eventually fail. Very often it is due to the soldering on one or more components starting to break down. That in turn will lead to things like thermal run away, and components failure due to excessive voltages from any circuit part malfunction. Zener diodes and transistors are typical examples of that.
There is no way any engineer can say that the output stage of an amp has failed because of DC at the input, unless that engineer has access to the equipment that connects to the amp and has tested them together to show that the DC at the input is producing such a large swing that the output fails. But even then, a steady state DC voltage of even 100mV or more would be well within the capability of the output stage to handle that offset. It would be the speakers that would more likely go faulty before the amp since speakers cannot handle a large DC offset for too long.
Any qualified audio engineer who wishes to challenge me on that, please step forward.
I would agree. Also if it's never off then it's connected and disconnected while on which isn't good practice
farflungstar
05-05-2018, 06:55
Two things that grabbed my attention are: the Truth is never switched off, and the engineer said it was DC at the input.
If an amp or preamp is never switched off, one or more components will eventually fail. Very often it is due to the soldering on one or more components starting to break down. That in turn will lead to things like thermal run away, and components failure due to excessive voltages from any circuit part malfunction. Zener diodes and transistors are typical examples of that.
There is no way any engineer can say that the output stage of an amp has failed because of DC at the input, unless that engineer has access to the equipment that connects to the amp and has tested them together to show that the DC at the input is producing such a large swing that the output fails. But even then, a steady state DC voltage of even 100mV or more would be well within the capability of the output stage to handle that offset. It would be the speakers that would more likely go faulty before the amp since speakers cannot handle a large DC offset for too long.
Any qualified audio engineer who wishes to challenge me on that, please step forward.To clarify. The Truth should have zero DC at input and output - when initially tested there was a DC reading on the offending channel output AND a reading on both channels at the input buffers. This is now resolved. No DC.
Yes it's kept on all the time, as per the makers advice - but nothing is ever disconnected unless it is switched off. (The truth has no on/off switch).
There are very very specific rules for using the Truth which have to be followed. It cannot be operated without a source connection as this can cause a huge 10v DC surge through the output. I have obeyed this rule.
There are less than half a dozen components and nothing like resistors or diodes. I'm waiting to hear from the maker about the oscillation possibility.
The official ARC service dept said that DC on the input was the most likely cause, and that due to the amps balanced nature I wouldn't have heard anything to indicate it (a hum).
I use CARDAS unbalanced to balanced convertors/connectors to attach the truth to the 75 - could they cause a problem?
Also, the hashimoto sut is the X version which trades a few bottom Hz for a far more extended top end - I've heard SUTs can ring at high frequencies, could this be the oscillation?
I still haven't switched the amp on. I've swapped out all tubes for new ones. I just stare at it. Even if I buy a cheap passive to keep me going I may be depriving myself of a stunning, and blameless preamp.
Why don't you connect up the Slagle to the ARC and some speakers and connect an mp3 player/phone to the Slagle and play some music. That takes everything else out of the equation. You could also open up the cdp and check if there are caps on the output (there usually are) and then you could try that as a source.
Why don't you connect up the Slagle to the ARC and some speakers and connect an mp3 player/phone to the Slagle and play some music. That takes everything else out of the equation. You could also open up the cdp and check if there are caps on the output (there usually are) and then you could try that as a source.
he sold the slagle...keep up :ner::D
I am horrified to read that your preamp will output 10V dc if it isn't connected to a source (I am wondering if that mightn't be load) . What if the source is connected but turned off, what if there is a blocking capacitor, what if there isn't in the power amp (as is the case here)?
This is unstable behaviour and has no place in a purportedly high end component. If it was mine I would now remove the Truth and replace it with a buffer, pretty much any buffer, still using your passive attenuator, and enjoy the amp.
Was it you who sold me a homebrew BB OPA627 'pre' some years ago? If so I'd be happy to post it to you if you'll return it/buy it back when you're done, no time constraints? It is neutral, runs off a 9V battery, and doesn't misbehave. If it wasn't you then it ain't a beauty but functions fine! PM me if interested?
Sent from my GT-I9195 using Tapatalk
he sold the slagle...keep up :ner::D
Oops, I was just looking at his list of kit in his signature.
Ok then direct connect your phone to the amp and play music, even simpler. That way you can test the amp.
As Jazid says put a buffer in, or stick some nice quality DC blocking caps in line. HiFiCollective have a very nice range, or I'm sure many here could advise.
[emoji106]
Even better, and don't forget to turn the volume right down on the phone first!
Sent from my GT-I9195 using Tapatalk
Primalsea
05-05-2018, 10:29
I am horrified to read that your preamp will output 10V dc if it isn't connected to a source (I am wondering if that mightn't be load) . What if the source is connected but turned off, what if there is a blocking capacitor, what if there isn't in the power amp (as is the case here)?
This is unstable behaviour and has no place in a purportedly high end component. If it was mine I would now remove the Truth and replace it with a buffer, pretty much any buffer, still using your passive attenuator, and enjoy the amp.
Was it you who sold me a homebrew BB OPA627 'pre' some years ago? If so I'd be happy to post it to you if you'll return it/buy it back when you're done, no time constraints? It is neutral, runs off a 9V battery, and doesn't misbehave. If it wasn't you then it ain't a beauty but functions fine! PM me if interested?
Sent from my GT-I9195 using Tapatalk
Yes, I’m a bit worried by this too. If the source has relay switching or muting at the outputs then a momentary activation could cause the Truth to output a large DC voltage, based on what has been said.
Adrian, I really feel for you. You have invested a lot to have a good system and it has gone pairshaped. However, often the most seemingly dire issues can be the simplest to resolve, although they don’t seem so at the time.
Im sure we can help, but I think we should start right at the beginning and help you work through each piece of equipment one by one methodically.
If you’re interested can you list your system setup source to power amp and we will go step by step.
farflungstar
05-05-2018, 10:53
SP10/Reed/Allaerts/Vida/hashimoto hm-x/Truth/ARC Ref 75/Tannoy Kensington - all interconnects VDH Integration. Speaker cables Townsend Isolda.
It's typical that I'm in this position towards the end of the academic year when money is tight! I might just buy a very cheap passive to use for the summer and in September I can buy one of the Slagle Emias (very expensive). Just silly if in fact the truth isn't/wasnt the problem - but that doubt persists. Unhappy bunny .
How would i fit a DC blocking cap at the outputs?
Primalsea
05-05-2018, 11:31
Okay. For now we can ignore the TT, cart, SUT.
Vida- looking at the circuit I can see some relays. I rekon that two of these are for disconnecting the output of the Vida during switch on, off and possibly if there is a fault or an overload. Sometimes the relay will switch the central pin of the output phono socket to ground, but this depends on the design and sometimes the relay may just disconnect the pin of the phono socket. Effectively this will either ground the input of the Truth, or just disconnect it. Either way the input of the Truth will be disconnected at some point. Even if the relay grounds the input there will be a very short period of time when the relay is between connecting the output of the Vida and switching it to ground (0V).
Here I suggest that this could be an incompatibility with the Vida and the Truth. However, we should still check the Vida with a multimeter (DMM from now on) and a oscilloscope if possible, this will be the next stage.
Before we move on can anyone add anything else, or see any technical issue with what I have said?
farflungstar
05-05-2018, 12:00
Okay. For now we can ignore the TT, cart, SUT.
Vida- looking at the circuit I can see some relays. I rekon that two of these are for disconnecting the output of the Vida during switch on, off and possibly if there is a fault or an overload. Sometimes the relay will switch the central pin of the output phono socket to ground, but this depends on the design and sometimes the relay may just disconnect the pin of the phono socket. Effectively this will either ground the input of the Truth, or just disconnect it. Either way the input of the Truth will be disconnected at some point. Even if the relay grounds the input there will be a very short period of time when the relay is between connecting the output of the Vida and switching it to ground (0V).
Here I suggest that this could be an incompatibility with the Vida and the Truth. However, we should still check the Vida with a multimeter (DMM from now on) and a oscilloscope if possible, this will be the next stage.
Before we move on can anyone add anything else, or see any technical issue with what I have said?I think the relays are part of the power supply to the Vida, they click Street switch on. I will clarify with the maker if a source that is switched off has an effect - though I can't believe it would, that is crazy. The amp is never on unless everything else is powered up.
I haven't measured the truth with the Vida switched on, connected yes. I will check it later and see if that has any effect on readings.
SP10/Reed/Allaerts/Vida/hashimoto hm-x/Truth/ARC Ref 75/Tannoy Kensington - all interconnects VDH Integration. Speaker cables Townsend Isolda.
It's typical that I'm in this position towards the end of the academic year when money is tight! I might just buy a very cheap passive to use for the summer and in September I can buy one of the Slagle Emias (very expensive). Just silly if in fact the truth isn't/wasnt the problem - but that doubt persists. Unhappy bunny .
How would i fit a DC blocking cap at the outputs?
Okay. For now we can ignore the TT, cart, SUT.
Vida- looking at the circuit I can see some relays. I rekon that two of these are for disconnecting the output of the Vida during switch on, off and possibly if there is a fault or an overload. Sometimes the relay will switch the central pin of the output phono socket to ground, but this depends on the design and sometimes the relay may just disconnect the pin of the phono socket. Effectively this will either ground the input of the Truth, or just disconnect it. Either way the input of the Truth will be disconnected at some point. Even if the relay grounds the input there will be a very short period of time when the relay is between connecting the output of the Vida and switching it to ground (0V).
Here I suggest that this could be an incompatibility with the Vida and the Truth. However, we should still check the Vida with a multimeter (DMM from now on) and a oscilloscope if possible, this will be the next stage.
Before we move on can anyone add anything else, or see any technical issue with what I have said?
You've got a lovely kit list, its not a disaster, just a compatibility problem. Once again I suggest that you get the Truth out of the system for now. It can come back once you have made sure everything else is happy if you can bring yourself to do so. An element that is not unconditionally stable could/will make a mockery of any help given irrespective of whether it has amazing sonic attributes when behaving... The rest of the kit is available on the open market so shouldn't suffer such issues.
So saying - for safety and to tick it off the list - do as Primalsea suggests and check the output from the VIDA for DC using your multimeter on the 200mV setting and report back. Using your SUT will block the dc but using a SUT as a dc blocker is wrong in many ways. If you can get access to an oscilloscope it would help strike off potential issues but if not no stress.
I'd also try the phone straight into the power amp, it's overwhelmingly likely to work straight away, costs you just a cheap cable (I have old phones that provided them in the box), and will give you some music while you work :)
lordmortlock
05-05-2018, 12:50
This is my worry. I really can't say for absolutely sure what caused it (the techie said it was DC at the input) - but what other equipment did it? The problem came up (or was very apparent) when I bought that ill fated CDP and connected it direct to the amp using its inbuilt volume. That was returned but I cannot be 100% sure it was the cause.
As I read through this I can't help but feel sorry for the seller of the Resolution audio Opus 21 CD player, as it sounds like his unit may not have been the cause of the problem after all.
He got a bit of a kicking on here iirc, and in light of this thread does that not seem rather unfair?
Although an unfortunate experience for both parties, it is "all water under the bridge", and so not really helpful to bring it up now.
farflungstar
05-05-2018, 13:03
As I read through this I can't help but feel sorry for the seller of the Resolution audio Opus 21 CD player, as it sounds like his unit may not have been the cause of the problem after all.
He got a bit of a kicking on here iirc, and in light of this thread does that not seem rather unfair?Jake - I can't say what caused the damage, the damage became apparent only when the Opus was directly connected to the amp. Before that point all had seemed well. It was only after that that I thought it best to get the amp checked. The Opus may have been coincidence, the truth may be blameless, the Vida likewise, it may even have been a previous fault - I don't know, but I did try the Opus with both the Hiraga amp and the system at school and it was definitely faulty.
lordmortlock
05-05-2018, 13:23
Fair enough Adey. Given your last sentence perhaps the two issues (Arc and Opus) were unrelated.
Checking the Vida is just prudence.
Quite possibly the Opus could have had a drying capacitor when sold but any dc was stopped by the sellers kit (which is the whole point of input PLUS output caps), clearly not so in Adrian's kit, and anyway might have become rapidly worse in its new home. Or it could be the Truth going unstable. Or a dodgy valve running away in the power amp and the techie just slightly deflecting the cause away from ARC?
We may never know, just trying to make sure Adrian's kit works without issue in future.
Sent from my GT-I9195 using Tapatalk
farflungstar
05-05-2018, 14:12
Checking the Vida is just prudence.
Quite possibly the Opus could have had a drying capacitor when sold but any dc was stopped by the sellers kit (which is the whole point of input PLUS output caps), clearly not so in Adrian's kit, and anyway might have become rapidly worse in its new home. Or it could be the Truth going unstable. Or a dodgy valve running away in the power amp and the techie just slightly deflecting the cause away from ARC?
We may never know, just trying to make sure Adrian's kit works without issue in future.
Sent from my GT-I9195 using TapatalkI'm driving myself to distraction over this because of the many variables. I've put in all new tubes into the ARC. I'm going to measure the Vida for DC, remeasure the truth for DC. But even if all is 0 because I dont know the cause of the fault I just don't know if I should power up. They only other option is to buy something to replace the truth and wait till it arrives, then it's a question of buy cheap but get the best in a few months or try to get something excellent at a decent price now. It's so frustrating having already v waited 3 months for the amp to be sorted.
Primalsea
05-05-2018, 14:29
I think the relays are part of the power supply to the Vida, they click Street switch on. I will clarify with the maker if a source that is switched off has an effect - though I can't believe it would, that is crazy. The amp is never on unless everything else is powered up.
I haven't measured the truth with the Vida switched on, connected yes. I will check it later and see if that has any effect on readings.
Adrian,
From looking at a photo of the Vida circuitboard there seems to be 4 relays. I suspect that two of these are to isolate the output until the powersupply has stabilised. These are to stop load thumps at power on. The relays will engage shortly after the power button has been pressed. This is quite common in source components.
Next would be to modify a set of interconnects so you can ground the inputs of the Vida. You can do this by stuffing some alu foil inbetween the pin and the outer ring on the phonoplugs at one end of the interconnects. Do this to both left and right and so that the foil contacts both the outer ring and the pin of the RCA phono plug. When you plug the other end of the interconnects into the input of the Vida this will ground the inputs. You do this so that you measure the circuit in isolation and not any noise that is being picked up and amplified by the Vida.
If you can do this and take DC and AC voltage measurements at the outputs of both channels of the Vida, using the smallest range on the DMM, and post a photo of the DMM for each of the 4 measurements (DC left/right, AC left/right) we can see what is happening with the Vida and hopefully discount it.
I strongly recomend that you work through each bit of kit separately in a methodical manner rather than just performing shotgun tests.
Primalsea
05-05-2018, 14:32
I'm driving myself to distraction over this because of the many variables. I've put in all new tubes into the ARC. I'm going to measure the Vida for DC, remeasure the truth for DC. But even if all is 0 because I dont know the cause of the fault I just don't know if I should power up. They only other option is to buy something to replace the truth and wait till it arrives, then it's a question of buy cheap but get the best in a few months or try to get something excellent at a decent price now. It's so frustrating having already v waited 3 months for the amp to be sorted.
The point is to remove the variables and perform tests in a methodical way that will gradually give us a good picture of what is going on. A complex problem is usually a series of simple problems all combined together. Forget the Truth for the moment, we can get to measuring that later.
What about the phone idea? Costs nothing (if you have a smart phone), just a simple cable. At least you will know the power amp is working fine and have music.
Alternative solutions include cheap class d amps which can be had on amazon for £25, next day delivery, that you can plug the (checked for dc) Vida into and off you go, music. If the amp complains then it's the Vida, if not connect the Vida to the Truth and then the cheapo amp. If the amp now complains, it's the Truth. If all's ok it's between the ARC and the Truth, so you could either buy a cap coupled buffer with volume control, Pass B1 &c is probably available reasonably cheaply and pretty transparent, or even a pair of line level 1:1 transformers to isolate the dc between Truth and Arc. This will get you going. But using two dc coupled components in series is a recipe for disaster, most especially if one kicks out 10V when it's unhappy.
Sent from my GT-I9195 using Tapatalk
farflungstar
05-05-2018, 15:06
The point is to remove the variables and perform tests in a methodical way that will gradually give us a good picture of what is going on. A complex problem is usually a series of simple problems all combined together. Forget the Truth for the moment, we can get to measuring that later.Hmmm, I don't like the idea of shorting the inputs on a 5K phonostage - but I just tried something.
With the truth switched off (it cannot be on without a source and the vida is the source) I measured the Vida at switch on, only for DC, set at 200m. Both channels behaved the same at start up, fluctuating between -00.0/00.0/00.1/00.2/00.0 - then between 00.0/00.1 for the few minutes I monitored. This may be the noise you mentioned?
Set at 2000m the readings were 000.
Then i reconnected it to the truth and monitored the truth at start up, set at 200m.
01.8, gradually falling to 00.4 on one channel, - 00.4 the other.
Set at 2000m, 000.
Now, I dont understand what 01.8 means at the 200m setting - I'm stupid and it's Chinese to me.
What I do know is that although I never turn off the truth so it shouldn't get that initial spike, here in Spain, in summer and winter the electricity often goes down 2 or 3 times in a day. It can drop for only a second and has happened with the system on.
Now, when I first measured the truth the other week the offending channel was reading 01.7 at 200m 'steady state' meaning not at switch on, so this could have doubled when electricity failed and the unit switched back on before dropping back to 01.7.
Could this the problem.....
Jazid - the amp should be fine after a 900€ service and soak test. It just needs to be switched on and the new tubes biased. I get the phone thing, but my phone headphone Jack doesn't work.
I've posted a wanted ad for a transformer based volume controller. I'm also waiting for Pieter from Tribute to get back to me about pricing for two mono units. My preference is the Slagle Emia, but at 3k I just can't afford that until September. Im kicking myself for selling my old slagle, and for backing out of the LM VO2 from Jake last year. We live and learn. But looking at those numbers from the Truth, and knowing the vagaries of the supply here in Spain I think I have to ditch the Truth.
Primalsea
05-05-2018, 16:42
I’m sorry Adrian, but I cannot help you. I feel for what you are dealing with, I really do. As I have explained you need to have a methodical approach to solving this, especially if you have limited knowledge of the technical issues. Performing shotgun tests the results of which you don’t know how to interpret is not helping.
Those figures (assuming 0-2mV) aren't scary and are ordinary enough for kit at start up. I'd say you have found that the Vida isnt the obvious problem. The truth is currently behaving properly so no issue at present there. We are left with ultrasonics which are a possible cause but difficult to spot without an oscilloscope, a glitch caused by the unstable behaviour of the Truth, or a dodgy cd player which is no longer available.
Oscillation is problematic as it is likely to be generated at the interface between two components so swapping kit may hide rather than eliminate it as a cause. If the cd player didn't behave into your amp, or your Hiraga, or another amp, then it is a possible cause, but why it was misbehaving we'll never know for sure.
I agree with you that your best course for future safety is a capacitor coupled interface, tvc, buffer, trafo, pre, or whatever. The Truth maybe should be sold or mothballed for whenever you live in a zone without power cuts, have a cap/trafo coupled power amp.
Sent from my GT-I9195 using Tapatalk
farflungstar
05-05-2018, 16:52
Why is the output of a phonostage being fed into a step-up transformer? Surely the output of the step-up transformer should be fed into the phonostage? :scratch:
My suspicion, though this is just speculation, is the The Truth is a bit of a wacky "minimalist/purist" design using video op-amps running at unity gain and in order to use the minimum number of components there is no DC blocking cap or series resistor at the output. A series resistor of about 100 or 200 ohms is considered good practice because it isolates the op-amp from capacitive loads which can send the op-amp into oscillation. An examination of The Truth in isolation might show no problems but when the interconnecting cable and the input to the ARC amp are added there may be enough capacitance to send the op-amp into oscillation.
Anyway, that's just guesswork, but it is a possibility.Jajaja - no of course the hashimoto feeds the Vida, the Vida the truth. Sorry about that.
I'd assumed that was what was happening, Hashimoto into Vida into Truth... Either way the dc doesn't look like coming from that end of things.
Sent from my GT-I9195 using Tapatalk
Oops, posts crossed :)
Why is the output of a phonostage being fed into a step-up transformer? Surely the output of the step-up transformer should be fed into the phonostage? :scratch:
My thoughts exactly, the SUT would probably run into saturation, and whilst it would block DC, there would be a risk of magnetising the core. But I assumed Adrian had described his system accurately.
It's difficult to find much technical information about the Truth, but their claim that the input impedance is 'near infinite' does suggest the use of op-amps.
farflungstar
05-05-2018, 21:57
My thoughts exactly, the SUT would probably run into saturation, and whilst it would block DC, there would be a risk of magnetising the core. But I assumed Adrian had described his system accurately.
It's difficult to find much technical information about the Truth, but their claim that the input impedance is 'near infinite' does suggest the use of op-amps.There are no opamps in the box. And yup I wrote it arse about tit. Sorry.
farflungstar
06-05-2018, 15:24
One more question, please humour my stupidity.
I have nulled the DC at output to 00.0 but this changes depending on volume position (voltage applied to the light source LED), to -01.7 at full output, the same happens when the balance control is used full turn.
I am assuming it is measuring a negative voltage, or that ground is actually now carrying voltage. Is this correct? If so might this be bad for the connected amp?
Firebottle
06-05-2018, 16:04
1.7 mV, positive or negative is very low and should be no problem for any amp.
The ground doesn't carry a voltage.
farflungstar
06-05-2018, 16:10
1.7 mV, positive or negative is very low and should be no problem for any amp.
The ground doesn't carry a voltage.Thank you, I just don't get why the volume/voltage pushes the meter from null into a negative reading when increased (it powers the led).
But I agree 1.7mv is tiny.
Thank you, I just don't get why the volume/voltage pushes the meter from null into a negative reading when increased (it powers the led).
But I agree 1.7mv is tiny.
Without seeing the circuit diagram for the Truth, it is difficult to say for sure, but I suspect the LED and the buffer amps might share the same power
supply, which is not, IMO, a good idea.
farflungstar
07-05-2018, 10:21
I have emailed Audio Research about compatability whey the Truth and the jfets input stages of the ref 75. The Truth is measuring perfectly but ARC's topology may be an issue - let's see what they say.
Meanwhile I've bought and am waiting for a Luxman AT3000 TVC to replace the Truth, so I can finally switch on the newly returned 75 without worry.
Very nice! And good move IMHO :)
Sent from my GT-I9195 using Tapatalk
From what I can gather, The Truth uses video buffer amps before and after an LDR volume control. The video buffers are op-amps with a specialised purpose. Because they're designed to work at video rather than audio frequencies I would expect they need extra care to make sure they don't go into ultrasonic oscillation.
My thoughts exactly, though my knowledge of electronic circuitry is insufficient to make specific suggestions.
Powered by vBulletin® Version 4.2.3 Copyright © 2026 vBulletin Solutions, Inc. All rights reserved.