The volume pot - The hidden villain of preamp

[Bigun]

10Hz to 100kHz might make sense when considering the bandwidth of a feedback amplifier but I think it's silly thinking this is relevant for hearing no matter what Tim has to say about it. Why not use a more 'conventional' range for now and then extend the findings later if the results are interesting enough to warrant it ?

....The quietest pre-amp is no-preamp.

[ilimzn]

Actually, it does make sense to keep the high end roll-off higher than one would normally expect, given the definition of 'audio band'. It has been shown that frequencies as high as an octave over 20kHz do have some (admitedly relatively small) impact, and in fact this is even built into high-res digital formats (one notable example is SACD), where there is a low order 50kHz filter that can be applied. I myself have been involved in a study about echo and phase shift perception where it was quite conclusively demonstrated that the human auditory system does indeed perceive phase shifts and time shifts that suggest audibility over 20kHz. But even leaving that aside, one would be well advised to look at the whole amplification chain and not only at one element - it forms a set of cascading filters. Simply put, you do not want the phase shift and roll-off to add up, so it pays to keep your passband quite significantly wider (within reason!) than 20kHz, especially when it's done easily enough at no real extra cost. In fact, one good rule of thumb for various components you expect to have connected in a chain, except perhaps the power amp is to keep amplitude response to within +-0.1dB 20Hz to 20kHz. This actually implies extending the lower and upper cut-off by nearly a decade (assuming a low order character to the slopes). Power amps (tubes especially) are a bit more problematic in that regard WRT output transformer, but even then, expecting an 'unknown in advance' or changeable amplification chain, you want the least possible elements to dominate the response, and normally these would be your speakers, certainly if there has to be another one, then let it be only one, and this would then fall to the power amp, where it's most difficult to extend the passband if tubes are involved.

[popilin]

Originally Posted by Bigun

10Hz to 100kHz might make sense when considering the bandwidth of a feedback amplifier but I think it's silly thinking this is relevant for hearing no matter what Tim has to say about it. Why not use a more 'conventional' range for now and then extend the findings later if the results are interesting enough to warrant it ?

OK, let's put aside the opinion of Tim.
Can you show me that the noise, seen as thermal agitation of electrons within the valve, say at 100KHz, has no influence on the behavior of the electron cloud in the audible spectrum ?
I dare not...
The next estimates will be in the range of 20Hz to 20KHz, happy ?

Originally Posted by Bigun

....The quietest pre-amp is no-preamp.

And it is even quieter when you hear vinyl.
Vinyl without a preamp is the quietest thing in the world of audio.

[popilin]

Hi ilimzn

Thanks for your feedback, I can only agree with you.

With regard to valve amplifiers, when designing a transformer, I try to achieve at least 50KHz, which is quite difficult for the capacitances involved.
The last trick is to use NOMEX, due to its low dielectric constant.

For the same reason I'm working on a hybrid, valve voltage amplifier, current amplifier with BJTs.

[ilimzn]

popilin, I'll have to remember that trick

BTW interesting treatise, but it has been done before, mostly in the world of sand and in one case that is very dear to me, the 'pot' is in fact a custom made ALPS 'stepped' attenuator with 180 steps - they did not bother to make it actually have tactile steps, it acts as a pot - and it's resistance end to end is only 3k.
However, in the case of tubes, most standard tubes will overwhelm any pot generated noise by their own (via several mechanisms). As you know it's possible to combat this with a high gm tube, although it's not that simple. Tubes have several noise producing mechanisms and some are quite dependent on manufacturing quality, one important one being grid leakage current, which is mostly dependent on vacuum quality but not exclusively. This current is by definition 'noisy' and I am mentioning it because it can be significant in high gm tubes, especially as there is a trade-off between tube current and noise, but increased tube current can reduce noise but brings bias closer to zero, which increases grid leakage current, increasing noise. For this reason it's always advisable to use the lowest resistance pot that is feasible given the design goals and standard conformance.

One more area where choosing the right pot value is important is exactly the use of high-gm tubes, as these often have relatively higher input and reverse transfer capacitances. More than noise, the bandwidth of the circuit can vary a lot depending on pot position, which is one reason 'passive preamps' may sometimes have unexpectedly sub-par performance (usually this is more the case with solid state as capacitances are usually also nonlinear). From that standpoint your approach also has merit, and I would say more so than for noise considerations.

[popilin]

Hi ilimzn

My post is not intended to be a treatise, just another trick to the community.

In the world of sand, is simple, place a low value pot between two opamps, and the problem disappears.

Your post is almost a description of the problems encountered during the design, especially the variation of bandwidth with the position of the pot, however improved with approach 3.- and a suitable output stage, the Allen Wright SLCF there does wonders...

Regarding the noise, I can only deal with thermal noise, I'm just a TV repairman.

For triodes, RF Engineers use the approximation

r(eq) ≈ 2.5 / gm

In the worst case, the ECC82 has a transconductance

gm = 2.2 mA/V

Thereby

r(eq) ≈ 1136 ohm

Assuming that noise density is constant, and T = 313ºK, the equation of post#1 for a bandwidth of 20KHz, gives

Vn ≈ 62µV RMS

Of course, the use of high transconductance valves, greatly improves the noise reduction.

[ruffrecords]

Originally Posted by popilin

I also forgot to mention I'm trying to design a Hi-End Preamp,

Ah, I had not realised that. You are obviously well informed, would you be so good as to define 'Hi-End' for me please?

Cheers

Ian

[DF96]

2.5/gm gives a useful estimate of shot noise for mid-frequency RF purposes. It fails at higher (e.g. VHF) and lower (e.g. audio) frequencies, because valve noise is then dominated by grid noise and flicker noise respectively. Flicker noise can't easily be calculated as it varies so much from sample to sample; it has to be measured.

[popilin]

Originally Posted by DF96

2.5/gm gives a useful estimate of shot noise for mid-frequency RF purposes. It fails at higher (e.g. VHF) and lower (e.g. audio) frequencies, because valve noise is then dominated by grid noise and flicker noise respectively. Flicker noise can't easily be calculated as it varies so much from sample to sample; it has to be measured.

Thanks DF !, you're always the reference.

[popilin]

Originally Posted by ruffrecords

Ah, I had not realised that. You are obviously well informed, would you be so good as to define 'Hi-End' for me please?

Cheers

Ian

Hi Ian

I respect you a lot and last thing I want is to fight with you, don't screw up, please.