Yes this what we all thought remains the question though if there is going to be some amount of DC leaking from the P3A to the wiper making a scratching pot .
Kind regards
Sakis
the leakage from a big electrolytic that has been freshly reformed will be less than a microamp. Sometimes much less.
A smaller pair of back to back electrolytics that were reformed a few years ago are likely to leak less than a microamp as well.
Film capacitor leakage will be down around or below nanoamps.
A smaller pair of back to back electrolytics that were reformed a few years ago are likely to leak less than a microamp as well.
Film capacitor leakage will be down around or below nanoamps.
DC leakage is highly unlikely in the P3A. You have several of these amplifiers in your workshop as far as I know, so why not measure across the 22K input resistor with a DC millivoltmeter (no audio source connected) and prove the point?
I have "tuned in"
The current flow gives rise to a voltage.
You are worried about a voltage on the wiper and the current flow that ensues, creating a deterioration in the wiper contact.
My opinion via that previous post of mine is that leakage currents of less than a microamp (and for film caps, less then a nanoamp) are not going to be a problem.
The back feed of input offset is blocked by the input cap just before the power amplifier.
The forward feed of an output offset is passed through the pot track with the wiper tapping off a little but that little has to also pass through the cap. The wiper current can be ignored.
The current flow gives rise to a voltage.
You are worried about a voltage on the wiper and the current flow that ensues, creating a deterioration in the wiper contact.
My opinion via that previous post of mine is that leakage currents of less than a microamp (and for film caps, less then a nanoamp) are not going to be a problem.
The back feed of input offset is blocked by the input cap just before the power amplifier.
The forward feed of an output offset is passed through the pot track with the wiper tapping off a little but that little has to also pass through the cap. The wiper current can be ignored.
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i am not worried about any leak that comes from the pre I expect that if any it will be just OK when this is terminated to ground by the resistance of the potentiometer .
My concern is if there is a tiny amount of DC leaking from the other side ( the amplifier if used without capacitor) to the wiper of the pot
My concern is if there is a tiny amount of DC leaking from the other side ( the amplifier if used without capacitor) to the wiper of the pot
I would not recommend a DC coupled amplifier that could be connected to a wide variety of unknown pre-amps/Sources.
But a Power Amplifier has an input offset. That input offset is enough to put a DC across the wiper contact.
This is why ywe put in a DC blocking capacitor BETWEEN the vol pot and the power amplifier.
There are many who shown the vol pot directly on the Power Amp input. In my view this is wrong.
But a Power Amplifier has an input offset. That input offset is enough to put a DC across the wiper contact.
This is why ywe put in a DC blocking capacitor BETWEEN the vol pot and the power amplifier.
There are many who shown the vol pot directly on the Power Amp input. In my view this is wrong.
as said before the specific implementation is an integrated amp so NO the chance that an unknown preamp is connected to the amp is zero Other than this i completely agree with you .
This is what i wanted to know :But a Power Amplifier has an input offset. That input offset is enough to put a DC across the wiper contact.
Kind regards
Sakis
This is what i wanted to know :But a Power Amplifier has an input offset. That input offset is enough to put a DC across the wiper contact.
Kind regards
Sakis
An integrated amplifier still has sources as it's various inputs.
Those should be AC coupled to prevent mishaps.
This discussion seems to be about how and where to provide that protection.
The DC blocking capacitor goes immediately before the power amplifier input, not at the input to the integrated amplifier.
Once that capacitor is fitted, the DC on the vol pot wiper, either integrated inside the amplifier chassis or as a separate pre-amp, is negligible.
That was my point in posting some guidance on the leakage currents.
The protected input protects both the power amplifier and the previous vol pot.
Those should be AC coupled to prevent mishaps.
This discussion seems to be about how and where to provide that protection.
The DC blocking capacitor goes immediately before the power amplifier input, not at the input to the integrated amplifier.
Once that capacitor is fitted, the DC on the vol pot wiper, either integrated inside the amplifier chassis or as a separate pre-amp, is negligible.
That was my point in posting some guidance on the leakage currents.
The protected input protects both the power amplifier and the previous vol pot.
One can omit the input capacitor to make the amplifier DC coupled at it's input.
Most power amplifiers would ALSO require the DC blocking capacitor in the NFB loop to also be removed to reduce the output offset back to a similar level available from an AC coupled amplifier. Mixed AC & DC coupling generally does not work.
A DC coupled amplifier requires various protection measures to prevent damage to equipment.
I do not recommend DC coupling unless one can identify the risks and design the systems to alleviate the risks.
It is not simply a case of plug it in, "it'll be alright".
Most power amplifiers would ALSO require the DC blocking capacitor in the NFB loop to also be removed to reduce the output offset back to a similar level available from an AC coupled amplifier. Mixed AC & DC coupling generally does not work.
A DC coupled amplifier requires various protection measures to prevent damage to equipment.
I do not recommend DC coupling unless one can identify the risks and design the systems to alleviate the risks.
It is not simply a case of plug it in, "it'll be alright".
Been setting up a P3A and procedure was to push the bias to about 300ma ( heat sink and so on could handle it ) to warm up everything and then re set the bias to about 75ma while observing stability and general thermal behavior of the specific implementation of the new integrated board version 5242.4
In between and since the P3A was hooked up i thought about listening to it at this high bias ...
Sounded bad ....any users operating the P3A at this high bias to share their opinion ?
Kind regards
Sakis
In between and since the P3A was hooked up i thought about listening to it at this high bias ...
Sounded bad ....any users operating the P3A at this high bias to share their opinion ?
Kind regards
Sakis
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Joined 2009
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what do you mean by 'sounded bad' ?
higher current - well D. Self and others have pointed out that there is an optimal bias current (voltage drop across Re) for Class AB output stages. Deviate from this and the distortion increaases. At low signals though, if you keep in Class A it should sound fine shouldn't it ? could increased transconductance from higher current increase risk of parasitic oscillations
higher current - well D. Self and others have pointed out that there is an optimal bias current (voltage drop across Re) for Class AB output stages. Deviate from this and the distortion increaases. At low signals though, if you keep in Class A it should sound fine shouldn't it ? could increased transconductance from higher current increase risk of parasitic oscillations
Didn't bother to measure anything since in my set up bias is always about 75ma though the impression i got by listening to it was that something was holding the amp back , an idea of compression , lack of "air" in the low frequencies,while also high wasn't that crisp
I thing the wise thing to do will be to measure and look very closely what changes in aspect of distortion when bias is that high ...
Machine is no longer in my hand but will do that in my next setup and let you know .
Kind regards
Sakis
I thing the wise thing to do will be to measure and look very closely what changes in aspect of distortion when bias is that high ...
Machine is no longer in my hand but will do that in my next setup and let you know .
Kind regards
Sakis
