View Full Version : Croft specs - too much gain?
I’ve recently acquired a Croft 25R pre/phono and 7R power amp to use with my Harbeth M30.1. My sources are a Technics SL-1200G with Audio Technica VM540ML (5mV) and a Pro-Ject DAC (2V output). With no music playing I’m getting very noticeable hiss through the speakers, when I am playing music I get very little range of use on the volume control. Not had any hiss with my Croft integrated phono, and seem to have a bit more range on the volume control on that amp too.
I’ve been hunting around and found some published specs for these amps on the Audio Affair website. See below for the product name and it’s specs:
Croft RIAA: 2mV for 0.5V output
Croft RIAA RS: 2mV for 0.5V output
Croft 25 line: 0.15V for 0.5V output
Croft 25 phono: 1mV for 0.5V
Croft 7 power: 0.5mV for 45W @ 8 Ohms
Please correct me if I’m wrong here, but if the Croft 25 pre will output 0.5V to the Croft 7, the 7 will give its full output of 45W @ 8 Ohms. So, this output only requires 1mV from a cartridge, or 0.15V from a digital source. Any more than that and you’ll start cramping your volume control, right?
Most MM carts are around 5mV and most digital sources are around 2.0V, which in my limited understating would seem to be vastly too much output for a preamp with these specs. I assume this is the cause of my limited volume control range.
The other issue is the hiss/noise. With the given specs it seems the 25 and 7 are perfectly matched, with the 0.5V preamp’s output and the 0.5V power amp’s sensitivity - so why the hiss when no music is played?
I tried my Technics into the 25R phono input, then output the Croft’s line output (which bypasses the volume control and mute switch) into my NAD integrated and had no hiss at all. Sounded fine.
I must add that the 25R/7R do sound absolutely wonderful in my system, notwithstanding the above issues.
Can anyone offer any technical advice on this please?
Audio Al
01-05-2020, 12:24
Send Glenn a email and ask for his advice
glenn@croftacoustics.co.uk
Send Glenn a email and ask for his advice
glenn@croftacoustics.co.uk
Thanks Allen, I’ll do that. Sounding fab here otherwise - really impressed.
The problems of level matching are cropping up numerous times on the forum, and they are easy to resolve if a clear understanding is obtained.
Of course it would be much better if the industry standardised levels more, as is the case in broadcasting where numerous pieces of equipment are swapped into service all the time, necessitating interchangeability, and hence compatibility to facilitate this.
All amplifiers have a range of level operation which gives an optimum performance, the lowest level limited by quiescent noise in the amp, and the highest limited by design, this involving gain structure, devices, and power rail voltages.
Optimum matching between pre and power amps is achieved when their respective ranges are matched; that is, when the lower level limits are operating together, and the higher ones also, but a bit of headroom is always a bonus on a preamp.
When these ranges are matched, the best S/N ratio is achieved, and the volume pot will give the best range of volume over its rotation; none of the 'crammed up at one end' and easily too loud*, or turned up full, but too quiet, because the power amplifier is not being fed enough voltage to drive it to produce the desired volume.
*Many pots are also poorly matched at the lower end of their range, resulting in possible unequal levels from each channel at that end of the pot range.
The CD player industry likes to have O/Ps of much higher voltage than most other line level sources - at a guess they consider that this level is best high so that it does not in any way limit performance; it is always good to have level and/or gain in hand to 'play with'.
Many line level sources have about a 150mV O/P level, tuners for example, and so a CD player fed into a preamplifier will, compared with this, overload the I/P by about 21 to 22dB. This is problematic when switching from one source to another for obvious reasons, and so attenuating the higher CD O/P is sensible.
In line attenuators are convenient and negate the need for fitting internal components either in the CD player O/P or in the preamp I/P ccts.
The vinyl problem is that cartridges are very individual in their characteristics, this resulting from the designer's attempts using innovation, to reach higher performances, and as a result characteristics, including O/P levels vary, as do the needs or not, for capacitative loading, hence the adjustment available in many preamps.
What is the position of the volume control for comfortable listening? Does it have to be set too low, or too high?
What is the position of the volume control for comfortable listening? Does it have to be set too low, or too high?
I’m barely cracking it open playing vinyl, Barry. Digital I just lower the DAC output by 30% before it gets to the pre.
Feeding 4mV (from your cartridge) into a preamp having an input sensitivity of 1mV for full output, ought not cause any problems, as the overload headroom should be something like 20x the nominal sensitivity (or 26dB). The fact that you barely have the volume control from minimum, suggests there is something wrong with the disc input of the preamp.
As Al has suggested, contact Glen Croft for advice.
Feeding 4mV (from your cartridge) into a preamp having an input sensitivity of 1mV for full output, ought not cause any problems, as the overload headroom should be something like 20x the nominal sensitivity (or 26dB). The fact that you barely have the volume control from minimum, suggests there is something wrong with the disc input of the preamp.
As Al has suggested, contact Glen Croft for advice.
From what other Croft preamp owners have said, they all have limited range on their volume controls. I’ve emailed Glenn now so hopefully it’ll be sorted soon.
hifi_dave
01-05-2020, 16:13
Cen be easily corrected. Don't worry.
A common problem - have you ever used any Naim ?
Cen be easily corrected. Don't worry.
A common problem - have you ever used any Naim ?
Yes it sounds like a change to a ECC82 in the 7R might do the trick. A previous owner of nearly the same setup, except he had a 7 rather than a 7R, was recommended by Glenn to use an ECC82 which solved all issues. I’m curious as to why the 25R/7R has such a volume control issue in the first place. But an electronics expert, I am not!
And no, never owned any Naim. What are their volume controls like?
Feeding 4mV (from your cartridge) into a preamp having an input sensitivity of 1mV for full output, ought not cause any problems, as the overload headroom should be something like 20x the nominal sensitivity (or 26dB). The fact that you barely have the volume control from minimum, suggests there is something wrong with the disc input of the preamp.
As Al has suggested, contact Glen Croft for advice.
That is a 12dB increase.
Just got word from Glenn and he says an ECC82 will be fine, but he recommends a NOS 12BH7A.
hifi_dave
01-05-2020, 16:25
And no, never owned any Naim. What are their volume controls like?
From 6 o/c to 9 o/c, full volume.
From 6 o/c to 9 o/c, full volume.
That’s pretty dumb. Is that so that newbies think it must be really powerful in the demo? :lol:
I did have demo of a Naim Nova once but the dealer was so snooty, acted as if I didn’t deserve Naim. I asked him if Naim make a phono stage that would go with the Nova and he said no in a very condescending way and offered no alternatives. Mind you, he was very impressed with his BMW which he mentioned twice. Really weird!
Audio Al
01-05-2020, 16:44
I have a new unused one of these
https://www.ebay.co.uk/itm/12BH7-12BH7A-LANGREX-PREMIUM-LONG-PLATE-VALVE-TUBE/273595792187?hash=item3fb3947b3b:g:ObAAAOSwR2Fdemc w
I have a new unused one of these
https://www.ebay.co.uk/itm/12BH7-12BH7A-LANGREX-PREMIUM-LONG-PLATE-VALVE-TUBE/273595792187?hash=item3fb3947b3b:g:ObAAAOSwR2Fdemc w
Yes please Allen! :D
Firebottle
01-05-2020, 17:31
The Croft phono stages are high gain, too high in my opinion.
Audio Al
01-05-2020, 17:34
Yes please Allen! :D
OK will be posted ASAP :)
That’s pretty dumb. Is that so that newbies think it must be really powerful in the demo? :lol:
I did have demo of a Naim Nova once but the dealer was so snooty, acted as if I didn’t deserve Naim. I asked him if Naim make a phono stage that would go with the Nova and he said no in a very condescending way and offered no alternatives. Mind you, he was very impressed with his BMW which he mentioned twice. Really weird!
You've stumbled on the traits of a neurotic trying to keep up delusions of self worth, importance, and exclusivity, much more of that did happen than now.
You've stumbled on the traits of a neurotic trying to keep up delusions of self worth, importance, and exclusivity, much more of that did happen than now.
Haha I went home and plugged in my 15Wpc Leben and was much happier :)
The Croft phono stages are high gain, too high in my opinion.
That was the impression I got, and just a change in a couple of resistors in the cct would fix it.
hifi_dave
02-05-2020, 09:06
Or, if it's a problem to you, swap a valve but really, it doesn't affect the majority of owners.
Or, if it's a problem to you, swap a valve but really, it doesn't affect the majority of owners.
I’m surprised to hear that Dave, as it’s a fairly common issue to read about on forums. As I showed in the specs on my first post, the preamp gives the full 0.5V the power amp needs for it’s rated output at only 1mV input. Now, a standard 5mV MM cart won’t overload it, but this is the cause of the volume control cramping, which in my system is too loud at 3 o’ clock. It’s worse with a digital source where the preamp gives the 0.5V for a tiny 0.15V input, yet most sources are about 2V. I make that about 13 times too much. Why such high sensitivity? It just leads to numerous threads like this trying to sort it, or people moving the equipment on quickly which harms the value retention, it’s just puzzling and frustrating.
The hiss is apparent even with my Harbeth M30.1, if I had Tannoys or other reasonably sensitive speakers it would be a very serious problem. I just genuinely don’t understand why it’s been designed this way, I mean what’s the purpose in making a super high-gain amp that causes hiss and volume cramping even with low sensitivity speakers? What’s the benefit of doing things that way rather than designing for real world source scenarios? Not trying to attack Croft - long may they continue, they sound great and I’m keeping mine. Just needs a design tweak to make them more useable.
Audio Al
02-05-2020, 11:18
I can only think it must be the 7R as the 25R was totaly silent in my use with various amps
I can only think it must be the 7R as the 25R was totaly silent in my use with various amps
My previous 25R was nice and quiet with a Radford STA25/3, and also yesterday I used my new 25R line-outs into a NAD integrated and it was silent too. Didn’t get much range on the volume control with the Radford, although it was a bit better in that way than the 25R/7R combo.
I’ll get it sorted one way or another. If the new valve doesn’t sort it - and I’m assured it will - I’ll ask Glenn nicely to change the resistors to whatever.
hifi_dave
02-05-2020, 12:43
I’m surprised to hear that Dave, as it’s a fairly common issue to read about on forums. As I showed in the specs on my first post, the preamp gives the full 0.5V the power amp needs for it’s rated output at only 1mV input. Now, a standard 5mV MM cart won’t overload it, but this is the cause of the volume control cramping, which in my system is too loud at 3 o’ clock. It’s worse with a digital source where the preamp gives the 0.5V for a tiny 0.15V input, yet most sources are about 2V. I make that about 13 times too much. Why such high sensitivity? It just leads to numerous threads like this trying to sort it, or people moving the equipment on quickly which harms the value retention, it’s just puzzling and frustrating.
The hiss is apparent even with my Harbeth M30.1, if I had Tannoys or other reasonably sensitive speakers it would be a very serious problem. I just genuinely don’t understand why it’s been designed this way, I mean what’s the purpose in making a super high-gain amp that causes hiss and volume cramping even with low sensitivity speakers? What’s the benefit of doing things that way rather than designing for real world source scenarios? Not trying to attack Croft - long may they continue, they sound great and I’m keeping mine. Just needs a design tweak to make them more useable.
Forums are forums but in the real world, where I sell Croft every week and have done for way over 30 years, I rarely have anyone point this out as a problem. On the few occasions it has been an issue, the gain has been reduced without any fuss.
As to "hiss", Croft are about as quiet as they get and using a line input are deathly quiet.
Forums are forums but in the real world, where I sell Croft every week and have done for way over 30 years, I rarely have anyone point this out as a problem. On the few occasions it has been an issue, the gain has been reduced without any fuss.
As to "hiss", Croft are about as quiet as they get and using a line input are deathly quiet.
On my Croft Integrated Phono, I agree it’s nice and quiet, but the hiss I’m getting now with 25R/7R/M30.1 is the loudest I’ve ever heard by some margin - more than my 50+ year old Radford - and several others have reported the same. Maybe my particular amp is faulty? But then Glenn even knows what to do to stop it so it seems to be a known issue or deliberate characteristic. Perhaps your many customers don’t like to make a fuss? I generally don’t, but hiss at this level is abnormal.
So perhaps some are ‘deathly quiet’ and other units are ‘loudest in memory’? I can easily hear it on low level parts of the music (am currently playing a DAC with its digital volume reduced by 50% and am still only at 1:30 on the 25R volume control!) and when no music is playing the Croft hiss is the loudest sound in the room. It’s not normal at all. As I say, maybe my unit is faulty but I think it’s obvious from the specs that this is the result one would expect.
Anyway, I have a 12BH7A in the post so fingers crossed.
Here’s a photo showing the volume control position for normal listening level in my living room with my DAC as the source at full output (2V). As you can see it’s barely cracked open:
https://i.imgur.com/Gel5kbU.jpg
The output from a CD player will easity overload the 150mV sensitivity of the line input. You need a 14dB pad between the two items.
But as you are also experiencing the same problem on 'disc', the entire gain of the preamp needs to be reduced. I would contact Glenn for advice.
The output from a CD player will easity overload the 150mV sensitivity of the line input. You need a 14dB pad between the two items.
But as you are also experiencing the same problem on 'disc', the entire gain of the preamp needs to be reduced. I would contact Glenn for advice.
Makes you wonder why it’s designed like that in the first place! Does anyone have a line source that outputs 0.15v these days?
Also, at 5mV my cart is 5x higher output than is needed for the power amp to give it’s rated output. Sadly, I don’t happen to have an old 1.05mV EMT cart here though...
In your view does this explain the hiss I’m getting and are the hiss and volume control issue separate problems? I didn’t have this problem with the Radford power amp although I still did have a pretty cramped volume control on the 25R. It seems the 7R is more the culprit but the 25R is too to an extent.
southall-1998_mk2
02-05-2020, 15:04
I'm no electronics expert. But I sincerely find Croft gear a bit overpriced. Let alone secondhand prices. Sound wise is 'meh' to my ears. Each to their own.
S.
I'm no electronics expert. But I sincerely find Croft gear a bit overpriced. Let alone secondhand prices. Sound wise is 'meh' to my ears. Each to their own.
S.
Well that’s not really relevant to the thread, but I do really like the Croft sound and I’ve not seen any valve preamps at a similar price and consider them a bargain. I played an Analogue Productions 45rpm Ben Webster LP yesterday and it was incredible, absolutely sublime, so each to their own indeed. If you find Croft overpriced, I’d be very interested to know what you consider to be good value for money.
southall-1998_mk2
02-05-2020, 15:21
Well that’s not really relevant to the thread, but I do really like the Croft sound and I’ve not seen any valve preamps at a similar price and consider them a bargain. I played an Analogue Productions 45rpm Ben Webster LP yesterday and it was incredible, absolutely sublime, so each to their own indeed. If you find Croft overpriced, I’d be very interested to know what you consider to be good value for money.
Sorry about that, Greg. Just slipped out! Not wishing to troll your thread. The most important thing is that you are happy. Sod what others say, including me!
S.
Sorry about that, Greg. Just slipped out! Not wishing to troll your thread. The most important thing is that you are happy. Sod what others say, including me!
S.
No worries, was interested in your ideas of what stuff is good value though.
Frazeur1
02-05-2020, 16:17
The hiss factor, regardless of the actual volume pot positions seems wrong to me. I only have the standard Micro 25/Series 7, and with my Duevel Venus at 88/89 dB sensitivity(and I realize that number isn’t the only part of the equation), my pots are in a similar position, maybe a touch higher, at an average of 70 or so dB loudness in a 14 x 11.5 room. I have so very little to no hiss from speakers at near full bore with no music playing.
I did swap the ECC83 valve in the Series 7 for an ECC82, and this did lower the gain a slight smidge, maybe in my case of going from say one o’clock to two o’clock on the pot settings. I did that swap months ago as it was talked about on another forum and curiosity got the best of me, knowing really that the gain resistors really need changed to get things “fixed”, which I will do in time.
Try Glenn’s advice and go from there. Needless to say, I really like the Croft gear, but nothing is ever totally perfect, but he will get you sorted, don’t get discouraged!
hifi_dave
02-05-2020, 16:24
There is something faulty here and Croft will sort it out. The Crofts are amongst the quietest amps I have ever stocked - virtually no hum or hiss at all. Was this set bought s/hand ?
As I said, I sell Croft each week and have done since the early 80s and one of the reasons I took it on in the first place, was that the noise from the likes of Audio research, Conrad Johnson, Counterpoint etc, etc were getting me down. The Croft units were silent and even when thumped didn't play up, unlike the others.
for what it's worth, I don't think I've ever had a Croft amp hiss (unless there was a faulty valve somewhere). they are remarkably quiet, even compared to SS designs (which can hum). I've had limited usable volume knob travel in some combinations, but even with modern DACs I can usually comfortably get to the 3 o'clock notch. & in my experience that was never correlated to any form of hiss. so my vote goes to the people saying there must be a fault in the signal path.
Audio Al
02-05-2020, 18:24
I'm no electronics expert. But I sincerely find Croft gear a bit overpriced. Let alone secondhand prices. Sound wise is 'meh' to my ears. Each to their own.
S.
It's what I call thread crapping :bog:
southall-1998_mk2
02-05-2020, 18:25
It's what I call thread crapping :bog:
I couldn't care less.
S.
Audio Al
02-05-2020, 18:54
I couldn't care less.
S.
You did not need to explain as it was obvious
for what it's worth, I don't think I've ever had a Croft amp hiss (unless there was a faulty valve somewhere). they are remarkably quiet, even compared to SS designs (which can hum). I've had limited usable volume knob travel in some combinations, but even with modern DACs I can usually comfortably get to the 3 o'clock notch. & in my experience that was never correlated to any form of hiss. so my vote goes to the people saying there must be a fault in the signal path.
Could it be a dodgy valve? I used a Croft pre-amp for several years and it had no hiss or hum. No amplifier should exhibit hiss or hum whether it is valve or solid state. if it does it is either faulty or rubbish.
Could it be a dodgy valve? I used a Croft pre-amp for several years and it had no hiss or hum. No amplifier should exhibit hiss or hum whether it is valve or solid state. if it does it is either faulty or rubbish.
Late last night I tried the Croft preamp outputs into my NAD integrated and there wasn’t a trace of hiss. So it seems it’s the power amp. Someone told me the pre has 10.5dB of voltage gain and the power amp another 31.0dB which is a huge amount and must be the route of the issue.
hifi_dave
03-05-2020, 09:11
Did you buy the units new or s/hand ?
The power-amp is normally, virtually silent.
Did you buy the units new or s/hand ?
The power-amp is normally, virtually silent.
It was pre-owned and about 18 months old. When something’s not brand new you never know if there might be a fault, but I’ve heard from someone who bought a new 25R/7 4 years ago and had the same issue from the start via the Harbeth M30.1 like I have here. Glenn sent him an ECC82 to sort it which did the trick.
After the sale, the seller of my 7R told me he had hiss problems with the 25R/7R too, but when used individually they didn’t cause hiss. Eg DAC feeding the 7R caused no hiss, but DAC volume had to be ramped down to -50dB for normal listening because of the high gain of the 7R; the 25R also gave no hiss problems when used with a different power amp. He told me he used attenuators and a 12BH7A to solve it, and offered them to me.
A typical gain of a power amp may well be between 20 to 30dB, and if its I/P sensitivity is about 0.5V which is typical, that would produce an O/P voltage of approximately 5 to 15 V, 10V across 8 ohms giving an O/P power of about 12W, and 15V about 28W.
That all seems reasonable to me.
I've been pondering the possibility of a feedback loop which had become open cct, and has resulted in a jump in gain.
A typical gain of a power amp may well be between 20 to 30dB, and if its I/P sensitivity is about 0.5V which is typical, that would produce an O/P voltage of approximately 5 to 15 V, 10V across 8 ohms giving an O/P power of about 12W, and 15V about 28W.
That all seems reasonable to me.
I've been pondering the possibility of a feedback loop which had become open cct, and has resulted in a jump in gain.
Why would a power amp have an input sensitivity of 0.5V? Is it because most of the circuits were basically designed before 2.0V digital sources appeared so no one wants to change them? I’ve noticed old Quad amps are 0.5V too, and seen modded ones where it’s been changed to 1.5V presumably for use in a modern setting. My old Nagra power amp had a switch to change it from 1V to 2V which was very handy. I don’t understand the wide variation and the purpose of it when we’re all using the same sources - 2V for digital and a few mV for vinyl. It’s like volume controls too, they’re all 9 o’ clock at best for normal listening when there’s a whole pot to use, just why?
hifi_dave
03-05-2020, 09:42
I think the comments by the seller are a clue. The amp is faulty, probably a noisy valve and Croft will sort it out.
Normally, they are totally silent with ear up to the speaker and yes, I too use Harbeths from P3ESR to M40.2.
Why would a power amp have an input sensitivity of 0.5V? Is it because most of the circuits were basically designed before 2.0V digital sources appeared so no one wants to change them? I’ve noticed old Quad amps are 0.5V too, and seen modded ones where it’s been changed to 1.5V presumably for use in a modern setting. My old Nagra power amp had a switch to change it from 1V to 2V which was very handy. I don’t understand the wide variation and the purpose of it when we’re all using the same sources - 2V for digital and a few mV for vinyl. It’s like volume controls too, they’re all 9 o’ clock at best for normal listening when there’s a whole pot to use, just why?
The (arbitrary) 0.5V was arrived at through the years of the early 'evolution' of amplification, and became a default standard, as so many other things in society have done, and I presume that this level was regarded as sufficiently high to be relatively immune to external interference. It is very easy to with hindsight, criticise earlier things when a greater understanding has been achieved.
In broadcasting a voltage standard of 0.775V was a standard derived from a power level of 1mW in a 600 ohm resistor, this derived form maximum power transfer theory in telecommunications, and became the standard. But power transfer efficiency is not a real consideration with signals in which low currents are involved, but maximum S/N ratio is very much so. Arguably since there are metres of cable in broadcast studios carrying signals at this level, it may be advantageous to use a higher level to improve S/N ratio and crosstalk.
From another thread a link was posted to PFM which was;
"I plan to use a Croft Basic preamp with a Quad 707 power amp. I have read that the gain of the Croft can be reduced by changing a resistor per channel. I hope this combination will provide power from a 150W power amp and the magical midrange from a valve pre amp. "
It’s all things to think about. If the new valve doesn’t work out as planned I’ll ask Glenn about resistor changes. Would be a quick job I guess.
Have similar issue with my Rotel poweramp.
Hi gain poweramp into a sensitive pre, with efficient speakers...you can hear the noise floor.
Preamp has tubes on the input stage, and tried mitigating it with Different tubes, but may go down the resistor change route myself.
It’s all things to think about. If the new valve doesn’t work out as planned I’ll ask Glenn about resistor changes. Would be a quick job I guess.I purchased a Micro 25 basic pre amp new several years ago, and at the time was using a leak st20 power amp, Glenn lowered the output of the Pre amp to match the input sensitivity of the leak.
When I replaced the basic with a exdemo standard micro 25 I contacted Glenn and he was happy to provide the information to change a feedback resistor (I believe) per channel. He also offered to complete this if required at a reasonable cost.
Sent from my SM-G950F using Tapatalk
Forums are forums but in the real world, where I sell Croft every week and have done for way over 30 years, I rarely have anyone point this out as a problem. On the few occasions it has been an issue, the gain has been reduced without any fuss.
To be fair Dave the issue raised is well articulated and clearly of genuine 'real world' concern. I say this as a loyal and long term Croft owner (purchased from yourself !) I just thought your comment to be a little towards the dismissive side. But yes, I'm sure the issue can be sorted by GC if the valve change doesn't address it.
hifi_dave
04-05-2020, 09:03
Sorry to appear dismissive but as someone who has probably sold more Croft than anyone since the early 80s, this has never been an issue. What happens though, is that it gets mentioned on a forum somewhere and then becomes 'fact' and gets spread around, so we end up with these great products being run into the ground.
Occasionally, when a customer uses a Croft pre with another amp, such as a Quad, I order the pre with lower gain and then there is no problem.
As to the 'hiss' in this case, it is most likely a noisy valve, which Croft will replace FOC. In this case, the amp is s/hand and we have no way of knowing what has happened to it.I have never, in the 12 years or so that this latest Croft range been around, had a fault of any description and there isn't much you can say that of.
In a similar vein, one of the posters here had a Rega amplifier and he phoned to complain about a 'hum' on the left channel. In discussions it turned out that he hadn't actually experienced the 'hum' himself but had read about it on forums. The major 'hum' he could hear was when the amp was switched to phono, turned up to full volume and with ear to speaker, a 'hum' could just about be detected. Is that a real problem ?
Firebottle
04-05-2020, 11:34
In discussions it turned out that he hadn't actually experienced the 'hum' himself but had read about it on forums. The major 'hum' he could hear was when the amp was switched to phono, turned up to full volume and with ear to speaker, a 'hum' could just about be detected. Is that a real problem ?
It does make you wonder about the gullibility of some folks..
My 12BH7A should be here in a couple of days. Which one do I need to swap it for? There are two ECC83S and an unknown Tesla one that’s a bit smaller than the other two:
I have my spare Croft Integrated here so could try swapping over an ECC83 to see if it was a faulty valve.
https://i.imgur.com/HCa9Cgy.jpg
Emailed Glenn and he told me which one to swap. Did the swap over...and the hiss is still there, so doesn’t seem to be a faulty valve.
Have to say I’m very much enjoying the music with the 25R/7R and can see me keeping them long term, which is why I’m trying to sort out this niggle.
Firebottle
04-05-2020, 15:53
It's the valve that's on its own, the closer one in the above picture.
What have you changed the ECC83 for?
Do you get hiss with the volume pots on min (sorry you may have already said)?
If you fit shorting plugs to the 7R inputs instead of the connection to the 25R do you still get hiss?
It's the valve that's on its own, the closer one in the above picture.
What have you changed the ECC83 for?
Do you get hiss with the volume pots on min (sorry you may have already said)?
If you fit shorting plugs to the 7R inputs instead of the connection to the 25R do you still get hiss?
Yep, I confirmed which is the audio valve with Glenn. Thanks though. I just switched the 7R ECC83 for the same valve from my Croft Integrated. I’ve got two NOS 12AT7WB here but I don’t think they are suitable.
The hiss is independent of the volume position of the preamp. Once I turn the preamp volume up near the top of its range there’s a slight increase in hiss but that’s to be expected.
Don’t have any shorting plugs here, used to have some Russ Andrews Shorties but no idea where they are now.
Firebottle
04-05-2020, 17:14
The 12AT7 is a lower gain valve and won't cause any damage if you try it. I suggest you do.
The 12AT7 is a lower gain valve and won't cause any damage if you try it. I suggest you do.
You’re sure it’ll work ok?
They are GE JAN 12AT7WB.
Firebottle
04-05-2020, 17:43
Yes absolutely. The 12AT7 is equivalent to ECC81.
Yes absolutely. The 12AT7 is equivalent to ECC81.
How long should I wait for the amp to cool down before removing a valve, is it dangerous for a long time after switch-off?
Right, I’ve changed the 7R audio valve from the ECC83 to a ECC81. Don’t think there’s any difference. Still hissing like before.
Firebottle
04-05-2020, 18:42
OK. Power the 7R without anything connected to the inputs.
You may get a sort of hum from the input not being connected to a low impedance source, but listen and see if there is hiss.
OK. Power the 7R without anything connected to the inputs.
You may get a sort of hum from the input not being connected to a low impedance source, but listen and see if there is hiss.
Right, I’ll try that now...
Should I put the ECC83 back in first? How long should I wait to do that if so?
OK. Power the 7R without anything connected to the inputs.
You may get a sort of hum from the input not being connected to a low impedance source, but listen and see if there is hiss.
I got the hum like you said, but hiss was much less than before and was a bit hard to hear through the hum. There was minimal hiss.
It’s worth noting that the guy who sold me this amp told me it didn’t hiss at all when directly connected to his DAC with the DAC as volume control. The 25R he had didn’t have any hiss with another power amp he tried either. But when used together the 25R/7R had hiss, just like I’m finding. So individually the Crofts were fine, together they hiss.
Firebottle
05-05-2020, 06:57
There is another option with Croft gear, one that Glenn offers.
The output/line stage of the 25R (and other models) can be converted to a zero gain buffer (cathode follower). This removes the gain given by this stage normally and will allow further travel on the volume pots.
I have modified a Croft pre into this configuration previously.
There is another option with Croft gear, one that Glenn offers.
The output/line stage of the 25R (and other models) can be converted to a zero gain buffer (cathode follower). This removes the gain given by this stage normally and will allow further travel on the volume pots.
I have modified a Croft pre into this configuration previously.
But will that stop the hiss? Is it the preamp gain that’s likely causing it?
Firebottle
05-05-2020, 09:50
I believe it will and yes the gain is too high.
I believe it will and yes the gain is too high.
Oh great, sounds like we’ve identified the cause and solution of the problem. If I were to have the preamp mod you mention, would I also be able to comfortably use a Quad 3O3 with the preamp too? I note that both the 7R and 3O3 have 0.5V sensitivity but I don’t know if any other factors are relevant.
I’ve had the 12BH7A valve fitted to the power amp input for a few days now and it’s a lot better in terms of hiss and there’s a bit more range on the volume control too. Still not as quiet as my NAD integrated but quiet enough to not really matter anymore.
Audio Al
12-05-2020, 16:22
:D:)
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