The volume pot - The hidden villain of preamp
 

Those who live far from major cultural centers, we have no access to live music and must settle for listening to a recording.
If the recording engineer is an OPAMP lover, or he was taken too seriously Niquist-Shannon theorem, we can do nothing, the die is cast.:rolleyes:
When designing a Preamp only remains for us, as design goals, the basic and standard

a) Low distortion, preferably predominantly second harmonic (the lesser evil)
b) Good dynamic response.
c) Good bandwidth.
d) Small phase shift.
e) Low output impedance.
f) Low noise.

Like many others, I always assumed that valves are noisy and little can be done about it.
Wrong ! noise may get worse ! much worse !

§- Thermal Noise

Thermal noise can be defined as the noise generated by thermal agitation of the charge carriers.
For an ideal resistor R at absolute temperature T over a bandwith Δf, the RMS noise voltage is given by

Vn = √(4kTRΔf)

Where k is Boltzmann’s constant.

The following analysis is only conceptual, and calculations are courtesy of the software.

As a happy owner of a 100K Alps Black Beauty, designing a line-preamp, found real difficulties and some surprises.

1.- A traditional approach

Input => Pot => CC + CF => Output

i) Phase shift

As a reasonable design goal, is expected +/- 5º phase shift from 20Hz to 20KHz.
The worst case regarding the pot is at half the resistance, ie 50K + 50K when the output impedance reaches a maximum, supposing an ideal source of Z=0 ohm.
The only valve that I know for this requisite and a reasonable low gain is the ECC82/12AU7.
In the first simulation can be seen that hardly achieve +/- 5º phase shift.

ii) Noise

Meanwhile, the thermal noise produced by the pot over a range from 10Hz to 100KHz is 6.4µV RMS (second simulation).
For a tipical gain of 22dB, at the output the noise is about 80µV RMS (third simulation), only due to the pot !

2.- Nice try

Input => CC + CF => Pot => Output

With this scheme, the noise of the pot is not amplified, but it is a disaster in terms of the output impedance.

3.- Another approach

Input => CC => Pot => CF => Output

i) Phase shift

In the fourth simulation we can see that the goal of +/- 5º is easily achieved over a range from 10Hz to 100KHz.
Not bad, right?:D

ii) Noise

In the fifth simulation, we see that now the output noise is about 7µV RMS !
Remember that the volume pot is not the only source of noise, neither the only resistor, and mathematical models of valves don't take noise into account.
However not a bad result...:cool:

There is a flaw in your pot first noise simulation - you forgot to include the driving source impedance. If this is low compared to the pot value then the worst case noise resistance due to the pot is one quarter of the pot value I.e 25k and the noise voltage is halved. In any case, 100k is too large for an input pot and only makes frequency response and phase shift distortion due to the Miller effect worse.

Cheers

Ian

I'm still using 100K stepped attenuators to maintain compatibility with older tube gear, but in general ruffrecords makes a good point.. A 25K or 50K pot or attenuator would be a much better choice wrt input tube miller capacitance provided that all sources contemplated perform adequately into the proposed load impedance that pot or attenuator represents. (What a mouthful.. :p )

Another obvious option is to choose tubes with low Cag and low mu, still a lower resistance pot would probably be a bigger win.

Hi Ian

Sorry, that you say is already contemplated in 1.- i) and in the simulation.
Pot noise is about 6.4µV RMS, then with a gain of 22dB we obtain about 80µV RMS.

I agree with you that 100K is a little high, but if the previous stage is a phono pre, can prevent another CF, also is the only one I have. :D

Hi Kevin

I have simulated this option with the 6AQ5 in triode connection, but I fear for microphonics. :)

Understood but I am unclear why you use a bandwidth of 100KHz. In a 20KHz bandwidth the noise is 2.2 times less which gives less than 40uV at the output. This is about -90dBu and well below the level of the noise contributed by the tubes themselves.

The other thing that is unclear is the assumed source impedance when the pot is after the first stage. If that is low enough that after 22dB of gain to only give 7uV at the output then there was no need for a 100K pot in the first place.

Cheers

Ian

Thanks guys, I love this post: suscribed!!!!!

Sorry Ian, is a bad habit of mine, for me the standard audio range is from 10Hz to 100KHz.
This is due to the fact that there are MC cartridges arriving at 70KHz and very good tweeters too.
Hear it or not, the noise is there, the listening experience is more complex than we would like.
Your compadre Tim De Paravicini, says in a more elegant.
Also remind you that not always the noise density is constant as in the case of the pot.


We are talking about a preamp, and with high transconductance tubes, I would not be so sure. :confused:

The source impedance was assumed low from the beginning for simplicity, as you request in post#2
When the volume pot is after the first stage, it is assumed that the voltage levels are high, you should put capacitors, with less than 100K, should resign the low end or use huge capacitors.
It is a matter of compromise, or taste. :)

I can't see ultra violet light but it is still there. It is pointless including noise outside the audible range.



The only tube you mentioned is the ECC82 so there is no indication we are talking about hi gm tubes.

In which case my point about 100K being the wrong value to choose still stands.

It is a matter of good or bad engineering.

Cheers

ian

I can not either see the background radiation of the universe, but there are people who it bothers.
Please, read Tim De Paravicini, about infra and ultra sounds, also are part of the musical experience.;)

Sorry Ian, I forgot to mention that this is a thought experiment that attempts to show the best place for the volume pot.
In the example above, I used the ECC82 because interelectrodic capacitances and low gain.
I also forgot to mention I'm trying to design a Hi-End Preamp, valves I use, for now I keep in secret.:D

OK, if the preceding stage impedance is rather high, put a 10K pot if it makes you happy.
BTW. What part of "As a happy owner of a 100K Alps Black Beauty" and "also is the only one I have" you don't understand? :D

I agree with you ! :p