Impedance Matching/Mismatch

[Simon_LDT]

I'm currently trying to understand Impedance matching and I'm a bit stuck on a few things. I've been trying to connect my headphone amp to the outputs on my DAC, but when I do I get distortion which sounds like something overloading. In the meantime, I've just placed my pre-amp in between (connecting the tape out to the headphone amp) but I'd like to remove it from the chain if possible.

Headphone amp is a Schiit Asgard and I was told by one of the engineers that it has an input impedance of 100k ohm. My DAC is an Arcam D33 and the manual states it has an output impedance of 47 ohm. From my understanding, it seems this should be okay as the amp has a higher impedance. My Arcam pre is a C31 and the manual says it has <50 ohm output impedance. Any reason as to why it works fine with the pre in the chain but not without it?

My other query is regarding passives. I'd like to try one but I'm not sure if my amp would match well. My amps have a quoted input impedance of 22k ohm, is that high enough for a passive?

[struth]

more likely just to much gain from dac for head inputs

[Simon_LDT]

Would some kind of attenuation between DAC and Headphone amp fix this? If so, would that degrade the sound?

[struth]

you would be best seeing what output voltage is from dac and see iff its acceptable to the headamp.might need a bit of attenuation and im not a fan of external attenuators

[RothwellAudio]

more likely just to much gain from dac for head inputs
Well, too high a signal from the DAC would be more accurate as DACs don't really have gain, but essentially yes. Still, you would expect a headphone amp to be able to cope with the output from a DAC without overloading.


Would some kind of attenuation between DAC and Headphone amp fix this? If so, would that degrade the sound?
Yes, attenuators would fix this. No, there won't be any degradation of the sound - it's overloading as it is!

http://www.rothwellaudioproducts.co.uk/html/attenuators.html

[RothwellAudio]

I'm currently trying to understand Impedance matching and I'm a bit stuck on a few things.
The source impedance (also known as the output impedance) should be a lot lower than the load impedance (aka the input impedance), preferably by a factor of 10 or more. The 100k input impedance of your headphone amp should be fine with any source up to 10k output impedance. In practice you'll find that almost everything these days has a much lower output impedance than 10k.


Any reason as to why it works fine with the pre in the chain but not without it?
Not that I'm aware of.


My other query is regarding passives. I'd like to try one but I'm not sure if my amp would match well. My amps have a quoted input impedance of 22k ohm, is that high enough for a passive?
Higher than 22k would be nice but you'll ok. A pot impedance of 10k would be about right but I wouldn't go higher than 10k.

[Arkless Electronics]

There's no such thing as impedance matching in the context of hi fi. The matching of SUT's to cartridges is about as close as we get... It is a concept used mainly in RF circuitry. Even the term impedance is usually not strictly correct unless one wishes to be really pedantic as resistance is usually more apt a term. As Andrew says, all you really need is for the driving impedance to be much higher than the input impedance.

For the op I would check if the DAC has a selectable output level and can be turned down. Unlikely in the case of the Arcam but many pro DACs can have a higher output level selected.

[RothwellAudio]

Headphone amp is a Schiit Asgard...

Is it a mkI or a mkII? According to this page http://schiit.com/products/asgard-2 the mkII has "significantly improved performance". Maybe the mkI was a load of schiit :lol:

[Simon_LDT]

Thanks for the input Andrew, Jez.

It's the mark 1 I have of the Asgard (some said it was better than the mark 2 and vice versa - If I remember correctly I got it from a fellow member here).

I assume the pre-amp is somehow lowering/making the output signal from the DAC acceptable somehow which could be why it's fine when going through it? The DAC has a 2.2v output, no idea on what the Asgard can accept voltage-wise, maybe I should ask.

Would a passive pre solve this at all (I would assume not with there being no active circuitry but I'm not really sure)?

[Arkless Electronics]

It would appear that you have a pretty weird issue going on here. Any headphone amp is going to be designed to take the standard 2V output of domestic digital sources these days. A quick look at the specs for your headphone amp suggest a gain switch on the rear panel which you could try using.
A passive pre or in line attenuator (electrically identical) should solve the problem if it's overload related but as I said there should be no issue here.

[RothwellAudio]

Would a passive pre solve this at all (I would assume not with there being no active circuitry but I'm not really sure)?
The tape output of your pre-amp may put out a lower signal level than what comes in. At least, that wouldn't be impossible.
If the passive replaced your active pre-amp and it simply routed the signal from the DAC out to the headphone amp, no it wouldn't be a solution. A pair of attenuators world be a solution.
BTW, I notice the Asgard 2 has a high/low gain switch on the back. Does yours? If so, have you got it set to low gain?

[Arkless Electronics]

It's possible but unlikely that the tape outs would be at a lower level than the input. A Passive pre WOULD be a solution to the problem as would attenuators as they do exactly the same job in exactly the same way. Neither should be required though. I mentioned the low/high gain switch in my last post....

[RothwellAudio]

I mentioned the low/high gain switch in my last post....
Yes, sorry, I didn't see your last post - we must have been typing at the same time.

[Simon_LDT]

The Asgard mk1 doesn't have the low/gain switch. Good to hear a passive could solve this, that way I can kill 2 birds and see if it'll work in place of my active in the system at the same time. If not, will try some attenuators.

A big thanks for your help guys, I will report back when I have an update.

[RothwellAudio]

A Passive pre WOULD be a solution to the problem as would attenuators as they do exactly the same job in exactly the same way.

Good to hear a passive could solve this, that way I can kill 2 birds and see if it'll work in place of my active in the system at the same time.
I think you may have misunderstood what Jez and I were saying. I think Jez's comment above refers to using a passive pre-amp in addition to your active pre-amp to reduce the signal level going to the headphone amp. He's not talking about swapping the active pre-amp for a passive one and feeding the headphone amp from the passive pre-amp's tape outputs.

Just get a pair of attenuators and the problem is solved.

[Arkless Electronics]

I was referring to replacing the active pre amp with a passive and using the main outputs, not the tape out, to drive the headphone amp.

[Spectral Morn]

Simon try a different make of interconnect between the DAC and the headphone amp.

[RothwellAudio]

I was referring to replacing the active pre amp with a passive and using the main outputs, not the tape out, to drive the headphone amp.
Ah yes, that would make sense.

[Simon_LDT]

I was referring to replacing the active pre amp with a passive and using the main outputs, not the tape out, to drive the headphone amp.

That's what I'm thinking to try. I don't think many passives have tape outs, from those I've seen/read about.


Simon try a different make of interconnect between the DAC and the headphone amp.

I don't have many different interconnects around but I did try 4 different ones and no luck (Van Damme, Klotz MC500 and Mark Grant's Canare & G1000HD).

[Arkless Electronics]

There is still something not right here whatever "cure" is used.... No modern amp should be overloaded by a CD player (or other digital source with 2V output) as this is the de facto "standard" input these days! As with many such problems it would no doubt be obvious within a few minutes what the issue is if I was there with my test gear but diagnosis via internet is another matter!

[struth]

Does it get very loud very quickly?

[Spectral Morn]

That's what I'm thinking to try. I don't think many passives have tape outs, from those I've seen/read about.



I don't have many different interconnects around but I did try 4 different ones and no luck (Van Damme, Klotz MC500 and Mark Grant's Canare & G1000HD).

I recall years ago coming across a similar issue that was solved via a cable swap, the fact you have tried several with no change shows its not that. It was a very long shot though.

I have however had problems with kit with very high output overloading inputs which manifests in a similar way to how you described. Sometimes companies are not totally honest with their specs. First time I tried a Chord DAC 64 many years ago it overloaded the input on the amp I was using at the time. That company subsequently changed the amp design to accommodate higher output sources.

Some modern highly compressed and loud CDs can tip things too, what is just on the edge of being ok with normal CDs can be overloaded by this type of CD.

[RothwellAudio]

There is still something not right here whatever "cure" is used.... No modern amp should be overloaded by a CD player (or other digital source with 2V output) as this is the de facto "standard" input these days!
I agree. I wonder how long the mkI version was available and why it's been superseded by the mkII. Even so, how would it even get to market if it can't handle 2V? And where in the chain is its volume control? If it was the first thing the signal encountered it would be a simple matter to turn it down to prevent overload.
On the assumption that it's only quite small/light, maybe it wouldn't be a bad idea to send it to Jez for checking out and/or repair.

[Arkless Electronics]

I agree. I wonder how long the mkI version was available and why it's been superseded by the mkII. Even so, how would it even get to market if it can't handle 2V? And where in the chain is its volume control? If it was the first thing the signal encountered it would be a simple matter to turn it down to prevent overload.
On the assumption that it's only quite small/light, maybe it wouldn't be a bad idea to send it to Jez for checking out and/or repair.

That's the crux of it, where's the vol control? Indeed as I suggested it should not have got to market if it can't handle the standard 2V output of the sources 90% of customers will be using with it!

I wasn't for a moment suggesting that the op send it to me for any tests, more just a throwaway comment that as in 90% of such cases it would be dead easy and take minutes if one was present with suitable test gear to see it for ones self. If the op really wants me to check it then I can do that but would need the source as well as the headphone amp and if I found a problem I would have to modify it to suit I guess. It may be not an overload problem but a case of the headphone amp oscillating when it sees a certain source impedance or combination of that and output level. Another possibility is that the source doesn't like a capacitive load it sees in the headphone amp and oscillates.... Without an oscilloscope the op would never know.

[Simon_LDT]

Just to update, I'm currently using a Tisbury passive (mk2) and going through that there is no distortion either. I've got it set at 0db on the dip switches and plugged into ouput 2 which is set to looped. In my limited knowledge, I take it that it's basically acting like a tape out as it's sending the input (DAC) straight to the output, taking out the volume pot?

I can only assume there is some kind of incompatibilty between my DAC and headphone amp (could be the oscillating maybe like you mention Jez). Very strange indeed.

[RothwellAudio]

Yes, it's strange that it works fine with the Tisbury and the other peramp, but not when the DAC goes straight to the headphone amp - the signal level should be the same in all cases.

[Simon_LDT]

Well, well, well... Just had an interesting development. Having lived with the Tisbury a few days, I like the sound so am leaving it in place for a few weeks before switching back to the active to compare.

Anyhow, I've been connecting up all sources and while doing so, I left a set of IC's in the 2nd output of the DAC still going into the active pre. Now, when I went to listen, there was now distortion coming through the speakers! After more fiddling with cables to see why this was suddenly happening I found that by removing the 2nd set of DAC outputs the sound returned to normal. I haven't yet re-tried the headphone amp because I am sure when I did before I still had both outputs active but I will report back when I do.

If it turns out it's the DAC at fault, why would having IC's plugged in both outputs cause this distortion? Surely it's designed to output to both otherwise why have them? (btw, the active pre was not in use when this distortion occured).

[Simon_LDT]

Yep, just tried the headphone amp straight from the DAC, without having the 2nd set of IC's connected and no distortion at all. Looks like it's the DAC at fault.

Bit annoyed about this, looks like I'll have to send it back to Arcam.

Anyone know what the problem might be?

[Simon_LDT]

Just found this in the manual:

''The D33 DAC provides a pair of unbalanced audio
outputs on standard RCA phono connectors and a set
of balanced audio outputs on male XLR connectors. All
audio outputs are available simultaneously, allowing
multiple devices to receive the D33’s output signal.
However, to maintain the cleanest possible signal path
the pair of unbalanced outputs is not individually
buffered. This means that if two amplifiers are
connected, both must be powered on to avoid degrading
the signal.''

Could that be the issue? I shall have to try both amps on and see whether the problem goes away.

[struth]

could wel be.. yeah try both on.

[Simon_LDT]

Yep, just tried with both amps switched on and the problem goes away completely. Looks like it's not a fault with the outputs but just the way they're implemented. I don't know the technical reasons why or what is going on but I assume because the outputs are not individually buffered that is what is causing the poor distorted sound when both connected amps are not on.

Obviously I don't really want to leave my amps on when not in use so I guess my options are to either go through the active tape out or passive loop out on the pre amps or maybe try a buffer between DAC and amp/s.

[RothwellAudio]

Yep, just tried with both amps switched on and the problem goes away completely. Looks like it's not a fault with the outputs but just the way they're implemented. I don't know the technical reasons why or what is going on...
What are the amps that the DAC is connected to? (Sorry if you've already mentioned this.)
I don't understand why powering-up the amps cures the problem unless an un-powered amp has a much lower input impedance than a powered amp. That would be unusual but not impossible and would provide an explanation - the DAC trying to drive a low impedance (or even short circuit) would produce severe distortion.

[Simon_LDT]

What are the amps that the DAC is connected to? (Sorry if you've already mentioned this.)
I don't understand why powering-up the amps cures the problem unless an un-powered amp has a much lower input impedance than a powered amp. That would be unusual but not impossible and would provide an explanation - the DAC trying to drive a low impedance (or even short circuit) would produce severe distortion.

Arcam P1 Monos (22k input impedance)
Schiit Asgard (100k input impedance)

[RothwellAudio]

Arcam have a very comprehensive service manual for the P1 on their website and I took a look. It appears (though I haven't studied all 26 pages of the manual) that the phono inputs are shorted unless the amp's logic control circuits un-mute them, ie it has to be powered-up or else the input impedance is a short circuit. That would explain why the DAC was distorting.

[Simon_LDT]

Arcam have a very comprehensive service manual for the P1 on their website and I took a look. It appears (though I haven't studied all 26 pages of the manual) that the phono inputs are shorted unless the amp's logic control circuits un-mute them, ie it has to be powered-up or else the input impedance is a short circuit. That would explain why the DAC was distorting.

Thanks for taking the time to look at this, really appreciate it. Does that also happen with the XLR input too? Reason I ask is because when I had the active Arcam C31 connected I was using balanced and the same problem occured, now with the passive Tisbury I'm using phono input.

[RothwellAudio]

To be honest I'm pretty confused about what you have and what is connected to what. The P1 power amps seem to have a mute circuit at their RCA inputs but not their XLR inputs. I don't about the active pre-amp.

[Simon_LDT]

To be honest I'm pretty confused about what you have and what is connected to what. The P1 power amps seem to have a mute circuit at their RCA inputs but not their XLR inputs. I don't about the active pre-amp.

Apologies, I will try explain better. For the last 2 years I've used the Arcam C31 pre and Arcam P1 amps, always connected via XLR. I then added the Arcam D33 DAC, connected via it's phono outputs into the pre-amp. When I then added the Asgard h/phone amplifier and using the 2nd set of outputs on the DAC for connecting it, that is when the distortion occured so I resorted to using the tape out on the pre-amp instead.

Fast forward about a year and I'm thinking of adding a passive (never tried one before) which is the main reason I started the thread and thought I'd try get to the root of the problem with the h/phone amp and DAC also.

I then acquired the tisbury passive so now that has replaced the C31 pre in the chain. The only difference in connection now is that the tisbury > P1 amps are now via phono rather than XLR. All other connections are the same, and using the same cables.

[Spectral Morn]

Yep, just tried with both amps switched on and the problem goes away completely. Looks like it's not a fault with the outputs but just the way they're implemented. I don't know the technical reasons why or what is going on but I assume because the outputs are not individually buffered that is what is causing the poor distorted sound when both connected amps are not on.

Obviously I don't really want to leave my amps on when not in use so I guess my options are to either go through the active tape out or passive loop out on the pre amps or maybe try a buffer between DAC and amp/s.

Just dawns on me that I had that problem with a Chapter Audio pre amp. If I connected my Anthony Gallo cross over bass amp using the RCA outs and had the main amp connected to the XLRs there was a loud hum :doh: Only just remembered that reading your post above.

[struth]

Dare say the 2 amps coul be on when using it. shouldnt cause any problems if the other amp isnt class A.

[RothwellAudio]

For the last 2 years I've used the Arcam C31 pre and Arcam P1 amps, always connected via XLR. I then added the Arcam D33 DAC, connected via it's phono outputs into the pre-amp. When I then added the Asgard h/phone amplifier and using the 2nd set of outputs on the DAC for connecting it, that is when the distortion occured so I resorted to using the tape out on the pre-amp instead.

Er...I'm still confused :scratch:
So the DAC was never connected directly to the power amps anyway? It always went via the C31? Did the distortion only occur with the C31 switched off? In that case, presumably the C31 has a similar input muting arrangement as the P1, The muting on the P1 shouldn't make a scrap of difference to things that are connected to the C31. Are the C31 and P1s configured so that one switch powers them all?

Anyway, the solution is simple enough - have the C31/P1 powered up when using headphones.

[RothwellAudio]

Just dawns on me that I had that problem with a Chapter Audio pre amp. If I connected my Anthony Gallo cross over bass amp using the RCA outs and had the main amp connected to the XLRs there was a loud hum :doh: Only just remembered that reading your post above.

Doesn't sound like the same problem at all - more like a straight forward earth loop in that case.

[Simon_LDT]

Er...I'm still confused :scratch:
So the DAC was never connected directly to the power amps anyway? It always went via the C31? Did the distortion only occur with the C31 switched off? In that case, presumably the C31 has a similar input muting arrangement as the P1, The muting on the P1 shouldn't make a scrap of difference to things that are connected to the C31. Are the C31 and P1s configured so that one switch powers them all?

Anyway, the solution is simple enough - have the C31/P1 powered up when using headphones.

I now see I've missed a vital bit of info... :doh:

When I had the DAC connected to the C31 and then into the P1's, I never tried them through speakers when I introduced the headphone amp into the mix, connected to the DAC outputs also. I had only tried the H/phone amp direct from the DAC without the P1's turned on, noticed the distorted sound and in an effort to fix it (thinking it was some kind of incompatibility issue), went through the tape out on the C31 which I found worked.

It's only now I've got the Tisbury that I initially just hooked up my P1's to see what a passive could do without the active C31 in the mix (all was fine). I then tried the phones and that worked too. It's only when I then decided to keep the passive in my set-up as I wanted to live with it for a few weeks that I plugged everything in, leaving the H/phone amp plugged in to the DAC output, went to play music through the P1's and I now had distortion there too. That is when I started unplugging cables to find the fault and then discovered that by unplugging the H/phone amp, sound returned to normal. When I tried with everything plugged in the other day and all amps turned on and sound through both phones and speakers was good.

For now I've reverted back to using the loop out on the Tisbury for the phones as I don't like leaving my amp on when I'm not listening via speakers (can't mute them). I will see in time if there is any difference between going direct for phones or going via the passive.

[RothwellAudio]

When I had the DAC connected to the C31 and then into the P1's, I never tried them through speakers when I introduced the headphone amp into the mix, connected to the DAC outputs also. I had only tried the H/phone amp direct from the DAC without the P1's turned on, noticed the distorted sound and in an effort to fix it (thinking it was some kind of incompatibility issue), went through the tape out on the C31 which I found worked.

Nope, still not getting it. The P1s weren't turned on but the C31 was still between the DAC and P1s? Was the C31 turned on? If that's the case, the DAC wasn't connected directly to the P1s.

[Simon_LDT]

Nope, still not getting it. The P1s weren't turned on but the C31 was still between the DAC and P1s? Was the C31 turned on? If that's the case, the DAC wasn't connected directly to the P1s.

The C31 was still between the DAC and P1's, connected in output 1 on the DAC. The H/phone amp connected to output 2 at the same time and turned on = Distortion. The only time I don't get distortion is when only 1 set of phono leads are connected to the DAC output, soon as I plug in the 2nd set I get distortion on both phones and speakers, unless both amps are turned on.

The way I got around it before (avoiding using both phono outputs from the DAC) was to instead plug the H/phone amp to the C31 tape out, meaning the DAC was only connected directly to the C31, nothing in the 2nd set of outputs. I'm now using the same set-up but with the tisbury in place of the C31. If I want to avoid any distortion while having the H/phone amp connected directly to the DAC, I have to have all amps turned on. If I don't fire up the P1's at the same time I get distortion and vice versa.

[RothwellAudio]

The C31 was still between the DAC and P1's...
Yes, but was the C31 powered up or not?

...to avoid any distortion I have to have all amps turned on if I want the H/phone amp connected directly to the DAC.
All amps?
I think there's been a lot of confusion here because I don't know what you mean by "amp". Are you refering to the preamp or the monoblock power amps?
It should be possible to have the C31 powered up to prevent he DAC/headphone amp from distorting without having to power up the P1s as well.

[Simon_LDT]

Yes, but was the C31 powered up or not?

All amps?
I think there's been a lot of confusion here because I don't know what you mean by "amp". Are you refering to the preamp or the monoblock power amps?
It should be possible to have the C31 powered up to prevent he DAC/headphone amp from distorting without having to power up the P1s as well.

The only time the C31 is powered on is if I'm using the P1's too. I never tried turning on just the C31 while the H/phone amp is on as I have no reason to do so.
By all amps I do mean the pre and power. Are you saying that technically I just need to power on the C31 while using the H/phone amp and not the monos also?

[RothwellAudio]

Are you saying that technically I just need to power on the C31 while using the H/phone amp and not the monos also?
I'll stick my neck out and say "yes" to that one.
Could end up with egg on my face for this :confused: :lol:

[Simon_LDT]

I'll stick my neck out and say "yes" to that one.
Could end up with egg on my face for this :confused: :lol:

Haha. :lol: Getting confused a bit myself, doesn't help that I don't know much technically.

Maybe this might help:

DAC (phono output 1) > C31 > P1's
DAC (phono output 1) > C31 > H/phone amp (via tape out)

That is what I've been doing the past year or so and that way I can have either the P1's on OR H/phone amp on (I never had both on at the same time) with no issues, the C31 being on in both instances.

DAC (phono output 1) > C31 > P1's
DAC (phono output 2) > H/phone amp

That caused distortion whenever I had just the DAC and H/phone amp on, everything else off. I never tried turning on the C31 at the same time, nor using the C31 > P1's while the H/phone amp was turned off (I assume would cause distortion) or H/phone amp on (which I assume would be fine), I always unplugged it and reverted back before continuing, so I never had all amps on at once.

DAC (phono output 1) > Tisbury > P1's
DAC (phono output 2) > H/phone amp

That caused distortion when I tried the phones without the P1's on and vice versa. Soon as I tried with all amps on, both were fine

[struth]

I would expect also that c31 on would be enough to remove the short

[anthonyTD]

Trying to get my head around this, and just re-read through some of the earlier posts, I am a bit confused with what your refering to here Jez,
Dont you mean lower, as far as the driving Impedance, or am I not reading you correctly ?:scratch:
A...
all you really need is for the driving impedance to be much higher than the input impedance.

[RothwellAudio]

DAC (phono output 1) > C31 > P1's
DAC (phono output 2) > H/phone amp
That caused distortion whenever I had just the DAC and H/phone amp on, everything else off. I never tried turning on the C31 at the same time
Powering up the C31 should be enough to cure the distortion.


I would expect also that c31 on would be enough to remove the short
Yes, no need to switch the P1s on as well.

The OP just has to try it and report back to us.

[RothwellAudio]

Trying to get my head around this, and just re-read through some of the earlier posts, I am a bit confused with what your refering to here Jez,
Dont you mean lower, as far as the driving Impedance, or am I not reading you correctly ?:scratch:
A...

It was probably just a slip of the tongue/brain/keyboard and what he meant was the exact opposite of what came out.

[anthonyTD]

I was hoping that was the case!:)
It was probably just a slip of the tongue/brain/keyboard and what he meant was the exact opposite of what came out.

[Arkless Electronics]

It was probably just a slip of the tongue/brain/keyboard and what he meant was the exact opposite of what came out.


Trying to get my head around this, and just re-read through some of the earlier posts, I am a bit confused with what your refering to here Jez,
Dont you mean lower, as far as the driving Impedance, or am I not reading you correctly ?:scratch:
A...

Ooops.. Lower of course! It was indeed a "slip of the tongue/brain/keyboard"

[Simon_LDT]

Powering up the C31 should be enough to cure the distortion.


Yes, no need to switch the P1s on as well.

The OP just has to try it and report back to us.

Yep, that also works (C31 on without power amps on too). Still don't like the idea of having the pre on for no real reason when using headphones so will be using the Tisbury and going through the loop output on that, at least for the time being.

Just want to thank all those that replied and helped me to understand the situation, especially Andrew. :)

[RothwellAudio]

Just want to thank all those that replied and helped me to understand the situation, especially Andrew. :)
Glad to be of assistance.