Headphone Outputs: Build-Out vs. No Build-Out

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mediatechnology
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Re: Headphone Outputs: Build-Out vs. No Build-Out

Post by mediatechnology »

Well I've heard the combination of wiper and endstop resistance produce leakage - usually fixed by some DeOxit or Blue Shower etc. on ones that were OK before.
Panpots and their often high wiper current can produce leakage.
And yes, it often sounds thin.

But adding resistance in series with a non-inverting input to fix it?
I don't see how that works.

Another possibility is that R8 could also be used to prevent latch-up on power-up if the pot was at minimum or full throttle though I doubt that's a 4580 problem.

Perhaps we need an older Guru (RIP 2011 Gurus) to tell us why.
Or, call Rane and ask.
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Re: Headphone Outputs: Build-Out vs. No Build-Out

Post by ricardo »

mediatechnology wrote:Well I've heard the combination of wiper and endstop resistance produce leakage - usually fixed by some DeOxit or Blue Shower etc. on ones that were OK before.
Panpots and their often high wiper current can produce leakage.
And yes, it often sounds thin.
I think your original guess of capacitance on the pots is correct. We're talking about very low levels here. Sometimes down to the noise level but BBC commissioning engineers will complain of audible hum -10dB below the white noise level.
Another possibility is that R8 could also be used to prevent latch-up on power-up if the pot was at minimum or full throttle though I doubt that's a 4580 problem.
Its not latch-up either. At Calrec, it was most common on 5532/4 circuits which don't latch. Any non-inverting stage coming straight off a pot. (Evil TL07xs latch)
Perhaps we need an older Guru (RIP 2011 Gurus) to tell us why.
Isn't John old enough? :lol:
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JR.
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Re: Headphone Outputs: Build-Out vs. No Build-Out

Post by JR. »

I'm old enough but I can't think of a good explanation... (maybe I can't remember it) :lol:

One possibility is a dirty pot ground... (corrupt grounds).. When the pot is off ground the noise is coming in through a divider with source... When turned down it is full pot ground feeding in..

Say in console where faders are grounded away from the opamp...

But that is not the right fix for it,,, the fader ground should be clean and tied to the gain stage ground.

I am still not familiar with this in properly designed circuits... In a headphone circuit if the current from the cans are talking into pot ground it might oscillate (but again not an issue with judicious ground layout).

Sorry...

JR

EDIT- giving this some more thought this "noise on pot ground" makes a little more sense...

Visualize the actual network when a typical pot wiper feeds a + input. When anywhere between full up and full down, the wiper has a source impedance of as high as a few K effectively in series. Any noise from the ground end is padded down by the inverse of the pot setting. While there is no, added input capacitance in the subject schematic, there can a be several pF in the package and layout. When full up it is also driven from a low impedance but any added noise will be swamped by source noise, when it is full down, any RC formed by the wiper R and and circuit C will get pushed to a very HF and there is no source noise to conceal the ground noise now feeding in at wide bandwidth.

This is a bit if a hypothetical, and makes assumptions about the circuit, but adding a R in series with the viper, insures that the RC never drop below that fixed R value. I repeat this is more of a bandaid, and the real issue is that the ground is not very clean... This more commonly shows up in pot kill testing, when a path is actually noisier when turned full down, than up a little.

Another reason to treat everything differentially.. i,e, in the original schematic C17 gnd would connect directly to bottom of pot, And wiper leg would get 2k/20k divider. This would only balance for full up and full down, but full down is where you need it to ignore ground noise

or not... mostly just guessing here.... /edit]
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mediatechnology
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Re: Headphone Outputs: Build-Out vs. No Build-Out

Post by mediatechnology »

Well I had thought about R8 (the wiper series resistor) as forming an LPF with the op amp's input capacitance.

Let's pluck 10 pF out of thin air and use the "1.00K" value of R8. The -3 dB point is 15 MHz and change.

Maybe the 4580 just doesn't like 0 Ohm impedance sources.
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Re: Headphone Outputs: Build-Out vs. No Build-Out

Post by JR. »

Yes... But we don't know the circuit that all the anecdotal reports of hearing the phenomenon is coming from. Many fader wipers are connected using shielded cable wiring that can have more than 10pF.

Sometimes designers that don't understand why some fix worked will keep using it, even when it isn't necessary.

I am just throwing out a guess...

JR
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mediatechnology
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Re: Headphone Outputs: Build-Out vs. No Build-Out

Post by mediatechnology »

Well, I've e-mailed the Engineering Director of Rane to ask. :)
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Re: Headphone Outputs: Build-Out vs. No Build-Out

Post by JR. »

Good... I'l be interested to hear an explanation for those choices.

JR
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mediatechnology
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Re: Headphone Outputs: Build-Out vs. No Build-Out

Post by mediatechnology »

Dennis wrote me back and the engineer for that product will be responding next week when he returns to the office.
I really appreciate him responding.
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Re: Headphone Outputs: Build-Out vs. No Build-Out

Post by JR. »

Good, Dennis was supportive when I was trying to get the AES standards committee to come up with a standard definition for EQ bandwidth. Rane was one of the audio companies hip to the issue.

I wish AES was as responsive. :lol:

JR
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Re: Headphone Outputs: Build-Out vs. No Build-Out

Post by JR. »

He was one of the analog application guys working at national semi back in the day.

in the 'late 80s he wrote an article in one of the sound magazines and titled it "Audio Mythology" as a homage to my old RE/P column of the same name. He thought I was some old gray hair who was probably retired or dead... I introduced myself to him at a NSCA show with the old joke "reports of my demise are premature."

Now I am an old gray hair but I wan't back then.. :lol: I suspect Dennis is too, now.

JR
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