DC blocking capacitors audibility.

[Sebba]

In Texas Instruments' snaa031a paper I've come across the following text

........In listening tests at TI's sound room evaluating different circuit components used in the LM4702 demo
amplifier, there was one part whose negative effect on audible signal quality was undeniable. A DC
blocking capacitor on the input of the LM4702 degraded sound quality. In multiple listening tests, with
different participants and at various locations around the country, the negative effects of even the best film
and foil polystyrene DC blocking input capacitors in the audio signal path was confirmed.
It is therefore recommended that DC blocking capacitors not be used in the signal path for mid to high-end audio
equipment.........

Unfortunately, no measurements provided. I wonder what might be the reason for those diferences ?
In my experience, if the DC blocking cap is large enough (RC<1Hz) to ensure low impedance in the audio band, (very low voltage drop) the capacitors should be "translucent"

 

[Roland68]

In Texas Instruments' snaa031a paper I've come across the following text

........In listening tests at TI's sound room evaluating different circuit components used in the LM4702 demo
amplifier, there was one part whose negative effect on audible signal quality was undeniable. A DC
blocking capacitor on the input of the LM4702 degraded sound quality. In multiple listening tests, with
different participants and at various locations around the country, the negative effects of even the best film
and foil polystyrene DC blocking input capacitors in the audio signal path was confirmed.
It is therefore recommended that DC blocking capacitors not be used in the signal path for mid to high-end audio
equipment.........

Unfortunately, no measurements provided. I wonder what might be the reason for those diferences ?
In my experience, if the DC blocking cap is large enough (RC<1Hz) to ensure low impedance in the audio band, (very low voltage drop) the capacitors should be "translucent"

At TI and other manufacturers, you will find many references to tonal effects in various areas, especially in older documents.
Among other things, also on the sound effect when supplying OPAmps in the small signal range or power supplies of power amplifiers.
Due to the very controversial discussions in the trade press and the discussion groups/forums that were emerging at the time, this was stopped a long time ago. Which does not mean that these tests are no longer taking place. But since even such large companies cannot measure and prove tonal differences.....

 

[PaulD]

Do they describe the tests, and were they double blind and level matched? I suspect not. People hear all sorts of things when they know what to listen for... (i.e. it's biased) Search out the Wurcer & Groner article in Linear Audio and also the Cyril Bateman articles (they are a bit old). There is an AES paper (number 8350, titled something like "Capacitor “Sound” in Microphone Preamplifier DC Blocking ...").

My bet is it's an old article and it was not a properly controlled test. To my knowledge (probably flaky now) there have been almost no reported audible effects of DC blocking capacitors when done properly.

 

[Sebba]

At TI and other manufacturers, you will find many references to tonal effects in various areas, especially in older documents.
Among other things, also on the sound effect when supplying OPAmps in the small signal range or power supplies of power amplifiers.
Due to the very controversial discussions in the trade press and the discussion groups/forums that were emerging at the time, this was stopped a long time ago. Which does not mean that these tests are no longer taking place. But since even such large companies cannot measure and prove tonal differences.....

This paper dates 2006/2013 But in my opinion it is very unprofessional to use "sound quality" term in engineering doccuments, eg. opams datasheets.

 

[Roland68]

Do they describe the tests, and were they double blind and level matched? I suspect not. People hear all sorts of things when they know what to listen for... (i.e. it's biased) Search out the Wurcer & Groner article in Linear Audio and also the Cyril Bateman articles (they are a bit old). There is an AES paper (number 8350, titled something like "Capacitor “Sound” in Microphone Preamplifier DC Blocking ...").

My bet is it's an old article and it was not a properly controlled test. To my knowledge (probably flaky now) there have been almost no reported audible effects of DC blocking capacitors when done properly.

These tests were not only blind, the subjects also did not know what it was about. If you're thinking of any dealer or manufacturer comparison demonstrations, forget it, it's light years away.
You don't seem to be clear about which companies are involved here and what the goal was. They are industrial companies, not HiFi manufacturers. It was never about sharing the results with audio enthusiasts/hobbists because these companies simply don't care.
Such tests take place on a level and with requirements that most users (or rather all) cannot even begin to imagine here. Even a simple test costs at least $30,000 to prepare, administer, and score, excluding materials and equipment. And those are all the numbers from the past, today it should be much more expensive and often a deal breaker.

 

[Sokel]

That's what they say:

It's TI we're talking about,not the random snake oil dealer.

 

[Roland68]

This paper dates 2006/2013 Yes, also in my opinion it is very unprofessional to use "sound quality" term in engineering doccuments, eg. opams datasheets.

Do you realize that most of the important and used ICs come from these manufacturers? Without these manufacturers, most of the devices and measurements in this forum would not even exist, and above all not the performance that today's devices achieve.

I don't want to question the development performance of the developers at the audio device manufacturers, but most of them are based on the work and circuit proposals of TI and the other manufacturers in this industry area.

So if these manufacturers with their tens of millions of hours of measuring experience don't have the experience to talk about sound-relevant influences, who does?
This is documentation that is not intended for you at all, but for developers and manufacturers. It is extremely kind of TI to make these documents available to everyone, not every manufacturer does that.

I personally find your post very pretentious, but that's just my personal opinion and your post only reflects your opinion.
However, such statements have meant that TI and other manufacturers no longer share certain findings that can be described as scientific, taking into account the acquisition. The bottom line is that this is a loss for every end user (audio).


Addition:
I don't know of any statements from these manufacturers about the tonal quality of the component or OPAmps, but only about the tonal effect of the circuit. So pure information and development tips for the developers.
Very reprehensible.

 

[Sebba]

I personally find your post very pretentious, but that's just my personal opinion and your post only reflects your opinion.

No, I simply would like to know what is engineering/scientific explaination for TI findings of "DC blocing caps sound degradation"
If a DC blocking capacitor afects "sound quality" then it MUST change the electical signal (in amplitude, time, freq. domain)
And as i wrote, if the DC blocking cap is large enough (RC<1Hz) to ensure low impedance in the audio band, (very low voltage drop) the capacitor distortions should be below the audibility level. Unlike in for example in crossover filters, where the voltage drop on capacitors is signigicant and it can add some distortions.

 

[Roland68]

No, I simply would like to know what is engineering/scientific explaination for TI findings of "DC blocing caps sound degradation"
If a DC blocking capacitor afects "sound quality" then it MUST change the electical signal (in amplitude, time, freq. domain)
And as i wrote, if the DC blocking cap is large enough (RC<1Hz) to ensure low impedance in the audio band, (very low voltage drop) the capacitor distortions should be below the audibility level. Unlike in for example in crossover filters, where the voltage drop on capacitors is signigicant and it can add some distortions.

And that's exactly what they don't seem to understand. It is an industrial IC circuit manufacturer. It is neither a university nor a research institute. Your task is to develop IC circuits, test them and create circuit proposals (very simplified) and make them available to the developers. It is definitely not their job to research "phenomena" that are not directly triggered by their products. This is a matter of research and such research can quickly cost 6, 7 or even 8 digit amounts.

Why should an industrial manufacturer make these expenses available without real benefit?
And when one of these manufacturers invests such large amounts in research in this area, do you think they will publish it as long as it gives them an advantage over the competition?

There are some TI white papers that have pointed out the sonic effects of circuits. And here again the hint that it was never about the sound of their components.
The more publicly everything with "sonic effects" was shifted to the snake oil area, the less was published on these topics. I think everyone can imagine why that is.

And that is not clear to many either. The contents of such white papers are checked umpteen times before they are published, even more than scientific publications and specialist books.

And one final note.
We owe most of the developments in the amplifier and small signal area to a few handfuls of people, much less than 100. They also have a large part of the knowledge that affects the sound.
Do you really think that given the current situation, even one of you would comment on the subject? And thereby expose yourself to hostilities and discussions that could lead to nothing but a waste of time?
As long as there is no way to verifiably measure and categorize sound, no one with the right knowledge will publish anything about it.

 

[LTig]

Douglas Self has measured the influence of coupling electrolytic caps with his AP (see chapter 2 in his book Small Signal Audio Design) and found, that distortion strongly depends on the voltage across the cap. To use elcaps you have to choose them with high enough capacity so that even at 20 Hz the voltage is low enough (less than 80mV) to not add audible distortion.

I assume the test done by TI did not compare let's say a 100 uF elcap with a 100 uF film cap as the latter would be crazy large and expensive. If they used 1uF caps then for sure the elcap distorts at lower frequencies much more than the film cap.

 

[Philbo King]

The TI doc would be meaningful if it listed test results for the types of capacitors, their nominal values, the DC bias level and the impedances of the circuit versus frequency. Without that data, it all boils down to voodoo hand waving.

 

[Ricardus]

Another thread about this topic started by a user with under 10 posts.

Beware.

 

[JeffS7444]

In Texas Instruments' snaa031a paper I've come across the following text

........In listening tests at TI's sound room evaluating different circuit components used in the LM4702 demo
amplifier, there was one part whose negative effect on audible signal quality was undeniable. A DC
blocking capacitor on the input of the LM4702 degraded sound quality. In multiple listening tests, with
different participants and at various locations around the country, the negative effects of even the best film
and foil polystyrene DC blocking input capacitors in the audio signal path was confirmed.
It is therefore recommended that DC blocking capacitors not be used in the signal path for mid to high-end audio
equipment.........

Unfortunately, no measurements provided. I wonder what might be the reason for those diferences ?
In my experience, if the DC blocking cap is large enough (RC<1Hz) to ensure low impedance in the audio band, (very low voltage drop) the capacitors should be "translucent"

I can imagine there might be problems if they were using, say, a 1 uF 50V electrolytic capacitor in an application which exposed it to just a few millivolts of DC.

 

[wwenze]

Another thread about this topic started by a user with under 10 posts.

Beware.

Tho at this point the user seems to have a much better message-to-like ratio and especially better word-to-like ratio than the opposition.

 

[Sebba]

Douglas Self has measured the influence of coupling electrolytic caps with his AP (see chapter 2 in his book Small Signal Audio Design) and found, that distortion strongly depends on the voltage across the cap. To use elcaps you have to choose them with high enough capacity so that even at 20 Hz the voltage is low enough (less than 80mV) to not add audible distortion.

I agree. Some years ago I measured with a sperctrum analizer and also listened to music with a high gain, low noise battery powered headphone amplifirer connected parallel to different types of capacitors in a RC divider. When the RC was high enough eg. >1sec. no distortion were seeen or heard, only a clear music/signal attenuated on capacitor's ESR.

 

[solderdude]

It's TI we're talking about,not the random snake oil dealer.

Yet... the application note has 22uF DC blocking input caps (which can be bypassed) and 200uF caps in the feedback path which can not be bypassed.
Those will certainly be electrolytics (bipolar I hope) and are essentially in the signal path.
The latter is to ensure unity gain for DC to get the lowest possible DC output offset.

 

[Sokel]

Yet... the application note has 22uF DC blocking input caps (which can be bypassed) and 200uF caps in the feedback path which can not be bypassed.

I think they have to talk to each other in TI then

 

[solderdude]

I can see too small value electrolytic output caps in speaker amps (say 1000uF) could be problematic when they are not in the feedback path though.

 

[tmtomh]

Dumb question time: what's the audio/hi-fi application for DC blocking caps in the signal path? I've heard of - and used - DC blocking caps in the AC power chain (e.g. DC blocker between a component's power cord and the AC mains plug) to prevent mechanical toroidal transformer hum. But in the signal path?

 

[egellings]

In Texas Instruments' snaa031a paper I've come across the following text

........In listening tests at TI's sound room evaluating different circuit components used in the LM4702 demo
amplifier, there was one part whose negative effect on audible signal quality was undeniable. A DC
blocking capacitor on the input of the LM4702 degraded sound quality. In multiple listening tests, with
different participants and at various locations around the country, the negative effects of even the best film
and foil polystyrene DC blocking input capacitors in the audio signal path was confirmed.
It is therefore recommended that DC blocking capacitors not be used in the signal path for mid to high-end audio
equipment.........

Unfortunately, no measurements provided. I wonder what might be the reason for those diferences ?
In my experience, if the DC blocking cap is large enough (RC<1Hz) to ensure low impedance in the audio band, (very low voltage drop) the capacitors should be "translucent"

Was this testing done doing curated blinded listening tests at matched volume levels? If not, the claims are suspect.