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My amp has audible intermodulation distortion. Any solution?

post #1 of 13
Thread Starter 

Hello, I was reading this page http://people.xiph.org/~xiphmont/demo/neil-young.html and took the test mentioned there to check if my equipment had intermodulation distortion. I'm currently using an Audio-GD FUN as DAC/AMP. According to that page, I might listen something if my system has a non-linearity and that's what happened: with all those files, and turning up the volume a lot, I could hear noises and whistles.

 

Is it normal to have this kind of results? Is there a way to mitigate/eliminate that kind of distortion or should I simply start looking for a new DAC/AMP?

 

Thank you in advance.

post #2 of 13
Quote:
Originally Posted by aroldan View Post
 

Hello, I was reading this page http://people.xiph.org/~xiphmont/demo/neil-young.html and took the test mentioned there to check if my equipment had intermodulation distortion. I'm currently using an Audio-GD FUN as DAC/AMP. According to that page, I might listen something if my system has a non-linearity and that's what happened: with all those files, and turning up the volume a lot, I could hear noises and whistles.

 

Is it normal to have this kind of results? Is there a way to mitigate/eliminate that kind of distortion or should I simply start looking for a new DAC/AMP?

 

Thank you in advance.

 

Don't use it in 24/96 mode and the audible intermodulation products will vanish. I have the same issue (whistling) with both of my ostensibly 24/96 capable DAC/Amps. Used in 16/44.1 mode none of the intermodulation stuff is audible. That said I do not normally hear distortion when playing music at 24/96 so Monty's example must be pretty extreme.

 

 

But, no it is not normal you should never hear supersonic components folded back into the audible range with a properly designed DAC as the aliases should be filtered out. 

 

My DAC has been measured to have IMD at 0.019% to 0.06% in the volume ranges I use it in but the test used was 60hz and 8Khz i.e normal music signals.

 

Unless you suspect you will be regularly listening to music with a very loud supersonic content I'd be inclined not to worry about it. Monty's sample has a 30K tone at -7db and a 33K tone at close to 0db - a very extreme example and even then I have to crank up the volume a lot to hear it.

 

  It is vaguely possible that it is your source that is the problem not the DAC, however I tried this test with two different computers digital outputs and had the same result which makes me think in my case it is the DAC - ho hum...


Edited by nick_charles - 12/5/13 at 11:37am
post #3 of 13
Thread Starter 

Thank you for the answer. I tried on a different computer and had the same result. While it's true that I have to turn the volume really up to hear the noises, I thought ACSS circuity provided high linearity and low distortion by using non-feedback design, virtually eliminating IMD. I emailed AudioGD to see if my device lacks of that circuity (which I'm supposed to have).

post #4 of 13
Quote:
Originally Posted by aroldan View Post
 

Thank you for the answer. I tried on a different computer and had the same result. While it's true that I have to turn the volume really up to hear the noises, I thought ACSS circuity provided high linearity and low distortion by using non-feedback design, virtually eliminating IMD. I emailed AudioGD to see if my device lacks of that circuity (which I'm supposed to have).

 

There have been question marks about the technical quality of some Audio-GD products in the past,  http://www.head-fi.org/t/435669/review-of-audio-gd-dac-19mk3#post_5866411  in one case a Galaxy S Smartphone had better measured performance  http://dl.project-voodoo.org/RMAA/reports/nfb-12-galaxys-voodoosound-load-hd650.htm 

 

Audio-GD AFAIK do not give out measured performance only component specs which makes it hard to make a sensible judgment on the objective quality - but actually their products measure well enough

post #5 of 13
Quote:
Originally Posted by nick_charles View Post
 

 

Don't use it in 24/96 mode and the audible intermodulation products will vanish. I have the same issue (whistling) with both of my ostensibly 24/96 capable DAC/Amps. Used in 16/44.1 mode none of the intermodulation stuff is audible. That said I do not normally hear distortion when playing music at 24/96 so Monty's example must be pretty extreme.

 

 

But, no it is not normal you should never hear supersonic components folded back into the audible range with a properly designed DAC as the aliases should be filtered out. 

 

My DAC has been measured to have IMD at 0.019% to 0.06% in the volume ranges I use it in but the test used was 60hz and 8Khz i.e normal music signals.

 

Unless you suspect you will be regularly listening to music with a very loud supersonic content I'd be inclined not to worry about it. Monty's sample has a 30K tone at -7db and a 33K tone at close to 0db - a very extreme example and even then I have to crank up the volume a lot to hear it.

 

  It is vaguely possible that it is your source that is the problem not the DAC, however I tried this test with two different computers digital outputs and had the same result which makes me think in my case it is the DAC - ho hum...

 

IMD is not necessarily a result of aliasing (fold back), they just sometimes come up when analog components are pushed to their limits.

 

What I think those guys are saying is that by constraining the BW of the signal one can make it easier for the analog components as they don't have to be uber-linear in the non-audible range, to avoid their IMD products affecting the audible range.

 

It's a trade off though as constraining the BW requires a filter which if not properly designed or unrealistically constrained may also audibly distort the signal. I think about 96 kHz sampling seems like a nice BW to work with. While probably not a big deal, 44.1 kHz is a bit more restrictive.

 

DACs and Amps have limitations and perform to a certain spec and conditions.

post #6 of 13
Quote:

Originally Posted by nick_charles View Post

 

That said I do not normally hear distortion when playing music at 24/96 so Monty's example must be pretty extreme.

 

It is indeed an extreme example. The idea behind those samples was 'if your DAC/system can handle this, then you can be pretty sure your system is clean enough to do any listening tests you want without intermodulation being a concern.'

 

It's not a foolproof test, but it should be pretty good.

post #7 of 13
Quote:
Originally Posted by xiphmont View Post
 

 

It is indeed an extreme example. The idea behind those samples was 'if your DAC/system can handle this, then you can be pretty sure your system is clean enough to do any listening tests you want without intermodulation being a concern.'

 

It's not a foolproof test, but it should be pretty good.

 

Hello Monty, welcome to the dark side. In another thread I wondered if the intermodulation could be at the headphone end rather than in the DAC? 

 

Have you tested any systems using single driver headphones where the test did not produce audible grunty ?

 

Try and stay away from the dedicated source forum, it will drive you nuts !

post #8 of 13
Quote:
Originally Posted by nick_charles View Post
 

 

Hello Monty, welcome to the dark side. In another thread I wondered if the intermodulation could be at the headphone end rather than in the DAC? 

 

Have you tested any systems using single driver headphones where the test did not produce audible grunty ?

 

Try and stay away from the dedicated source forum, it will drive you nuts !

 

Oh, I've been on the dark side for a long time.  People see the things I write and for some reason think I'm some kind of high-end audio hater.  I love [especially] vintage equipment and headphones, I just try to avoid delusions.

 

If we're being pedantic, headphones will all introduce IM, it's unavoidable in a physical transducer, but it's also usually inaudible.  The samples are useful partly because they test the whole system; they're limited because they don't identify where the problem is, only when there is a problem.

 

In any case, yes, I have combos that do not produce any audible IM over most of their range.  Practically everything does if you push it hard enough.  Output stages seem more likely to be a problem than transducers, or at least, when they go bad they go really bad.  Transducer issues are usually subtle (and even when a transducer is misbehaving, it will still usually sound quite good).  Amp issues will rip your head off.

post #9 of 13
Thread Starter 

Thank you! It now clarifies the results of the tests.

post #10 of 13
Quote:
Originally Posted by xiphmont View Post
 

 

Oh, I've been on the dark side for a long time.  People see the things I write and for some reason think I'm some kind of high-end audio hater.  I love [especially] vintage equipment and headphones, I just try to avoid delusions.

 

If we're being pedantic, headphones will all introduce IM, it's unavoidable in a physical transducer, but it's also usually inaudible.  The samples are useful partly because they test the whole system; they're limited because they don't identify where the problem is, only when there is a problem.

 

In any case, yes, I have combos that do not produce any audible IM over most of their range.  Practically everything does if you push it hard enough.  Output stages seem more likely to be a problem than transducers, or at least, when they go bad they go really bad.  Transducer issues are usually subtle (and even when a transducer is misbehaving, it will still usually sound quite good).  Amp issues will rip your head off.

 

I'm so giving this a try. THANK YOU!

post #11 of 13
Quote:
Originally Posted by xiphmont View Post
Output stages seem more likely to be a problem than transducers, or at least, when they go bad they go really bad.  Transducer issues are usually subtle (and even when a transducer is misbehaving, it will still usually sound quite good).  Amp issues will rip your head off.

 

Transducers (at least the most commonly used dynamic ones) usually have distortion that decreases with frequency, while amp distortion typically increases at high frequency because there is less negative feedback to correct the distortion. Therefore, the former produce IM mainly by loud bass modulating the mid and high frequency range (this normally does not happen to a significant extent in a well designed modern amp that is not clipping), while high frequency distortion can indeed sometimes be higher in the amplifier - especially a badly designed one - than in the headphones. Also, an amplifier is more likely to produce high frequency clipping or even slew limiting, while a transducer is more prone to "clipping" type of issues at low frequencies.

post #12 of 13

Just limit the range of reproduced frequencies with an extremely narrow band pass filter - IMD problem solved! 1 kHz seems like a good choice. :D 

post #13 of 13
Quote:
Originally Posted by stv014 View Post
 

 

Transducers (at least the most commonly used dynamic ones) usually have distortion that decreases with frequency, while amp distortion typically increases at high frequency because there is less negative feedback to correct the distortion. Therefore, the former produce IM mainly by loud bass modulating the mid and high frequency range (this normally does not happen to a significant extent in a well designed modern amp that is not clipping), while high frequency distortion can indeed sometimes be higher in the amplifier - especially a badly designed one - than in the headphones. Also, an amplifier is more likely to produce high frequency clipping or even slew limiting, while a transducer is more prone to "clipping" type of issues at low frequencies.

 

All true.  The transducer behavior is also a function of excursion decreasing with frequency.

 

That said, its still necessary to characterize individual systems by their actual behavior to know how they behave for sure.  Tweeter diaphragms may have to move by only a microscopic amount while a woofer is throwing two inches, but that doesn't mean we haven't measured tweets to be at fault for introducing audible IM in the past. 

 

Physical systems have so many messy tradeoffs, it seems like we have to explore them all! :-)

 

[In the mail today:  a pair of Koss KO747Q!  Case in point about tradeoffs so messy, makes you wonder why anyone bothered :-]

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