Hi All
Some times, I read about pot influence on sound quality. The adduced reason is the usually low current ( e.g. 100k input pot of a pre) present at the contact interface between the involved materials with subsequent rectifying effect and associated distortion.
Someone suggest to put the pot at the output of the preamp. and to arrange things in such a way to have a continuous current across the contact.
Others advise to build an apposite pot circuit where the pot is powered, for example, by a battery.
I never saw such a circuit ad I am asking if someone in these fora knows about such a circuit or has some ideas on how
to design it.
Thank for any info
Federico
Some times, I read about pot influence on sound quality. The adduced reason is the usually low current ( e.g. 100k input pot of a pre) present at the contact interface between the involved materials with subsequent rectifying effect and associated distortion.
Someone suggest to put the pot at the output of the preamp. and to arrange things in such a way to have a continuous current across the contact.
Others advise to build an apposite pot circuit where the pot is powered, for example, by a battery.
I never saw such a circuit ad I am asking if someone in these fora knows about such a circuit or has some ideas on how
to design it.
Thank for any info
Federico
First of all, this is very vague and also a long time ago.
I remember I have read an interview with an amplifier designer in, I think, the UK magazine Hifi News and Record Review in the early eighties. I think it was about Audiolab but I'm absolutely not sure. The designer explained how important it was to have all parts being biased. For that reason the amplifier was designed with a single power supply, because then all capacitors have more or less inherent biasing. In many dual power supply amplifiers with differential inputs the coupling capacitors just have almost no DC across them. The same was said to be true for connectors and potentiometers. A DC bias current is supposed to keep a current flowing in the contact, so no small signal rectification can occur due to metal to metal oxide barriers, since there are no zero crossings. Relays have a minimum current specification for the same reason. For very small currents the on-resistance is not guaranteed.
A scratching volume control was considered a small penalty for improved sound. The direction of the DC current through the wiper was also important. I think it had something to do with metal from the wiper depositing material on the carbon track, or the other way around.
I googled around a bit, but I could not find anything on the web. Maybe someone still has all these old HNRR copies.
Steven
I remember I have read an interview with an amplifier designer in, I think, the UK magazine Hifi News and Record Review in the early eighties. I think it was about Audiolab but I'm absolutely not sure. The designer explained how important it was to have all parts being biased. For that reason the amplifier was designed with a single power supply, because then all capacitors have more or less inherent biasing. In many dual power supply amplifiers with differential inputs the coupling capacitors just have almost no DC across them. The same was said to be true for connectors and potentiometers. A DC bias current is supposed to keep a current flowing in the contact, so no small signal rectification can occur due to metal to metal oxide barriers, since there are no zero crossings. Relays have a minimum current specification for the same reason. For very small currents the on-resistance is not guaranteed.
A scratching volume control was considered a small penalty for improved sound. The direction of the DC current through the wiper was also important. I think it had something to do with metal from the wiper depositing material on the carbon track, or the other way around.
I googled around a bit, but I could not find anything on the web. Maybe someone still has all these old HNRR copies.
Steven
A relatively recent series of articles in Electronics World magazine by Cyril Bateman has shown that capacitor distortion is increased when a dc voltage is superimposed on the signal. In the series he showed the distortion measuements for several types of passive components, including pots. He was demonstrating the capabilities of his distortion meter design (available as a kit!), measuring 0.0000 something-rediculously-low. Distortion in pots increases with wiper current. The amplifier bloke would not have been able to perform such measurements with the equipment available at the time, so they were opinions, not facts.
Feedback volume control
I built and am rebuilding a very simple and good sounding preamp using AD825s and a Noble pot in the feed back loop.
Since the input signal is not attenuated the input signal to noise ratio is maximized. The volume control is controlling the gain from a constant input source.
I have my current circuit using a 50K pot with 22K input resistors.
This give a max gain of a little over two. (this is just to keep from blowing speakers in testing)
The pream runs off 9 volt batteries, has a mono output and draws 18ma.
It also cost about $40.00 to build. (Dact sells a similar design for about $200. with out a volume contol or power source)
In spite of all this it sounds great in my test rig.
I built and am rebuilding a very simple and good sounding preamp using AD825s and a Noble pot in the feed back loop.
Since the input signal is not attenuated the input signal to noise ratio is maximized. The volume control is controlling the gain from a constant input source.
I have my current circuit using a 50K pot with 22K input resistors.
This give a max gain of a little over two. (this is just to keep from blowing speakers in testing)
The pream runs off 9 volt batteries, has a mono output and draws 18ma.
It also cost about $40.00 to build. (Dact sells a similar design for about $200. with out a volume contol or power source)
In spite of all this it sounds great in my test rig.
Thank you guys
for the kind responses.
I will continue to search on the web
I probably read the same interview.
Also remember that the same argument arose about
SE trafo versus Push-Pull. Somebody said that PP trafo were
better due to the absence of constant flux that will
bring to easy saturation than SE. Other said that SE was
better since avoid strong nonlinearities at the zero
crossing.
Bye
federico
for the kind responses.
I will continue to search on the web
I probably read the same interview.
Also remember that the same argument arose about
SE trafo versus Push-Pull. Somebody said that PP trafo were
better due to the absence of constant flux that will
bring to easy saturation than SE. Other said that SE was
better since avoid strong nonlinearities at the zero
crossing.
Bye
federico
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