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PAS Stepped Attenuator query

ctjetta

ctjetta

It's not a perfect world
I'm considering replacing the volume control in my PAS 3 with a stepped attenuator. If I understand correctly, the volume pot has an affect on the phono stage of a PAS preamp. So to keep things correct a 250k ohm replacement would be required.
Stepped attenuators come in three varieties- ladder, series and shunt. I know they all do the same thing, only differently. Is there a reason to use one version versus another in the PAS application given how it will interact with the phono stage?
I would think there would, be but I begin to confuse myself when I think too much about it. Or anything, for that matter.
 
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if you are concerned about the volume pot effect on the phono stage, you shouldn't be.
the volume pot is after the phono stage, ie in the line stage. so it "may" affect the
loading on the output.

and if you are still concerned, then add a cathode follower to lower the output impedance
so any impedance effects is minimized. research Koren mods.

while there are lots of theories and swags about the differences, with little exception,
all volume controlled preamp/amps "suffer" or "benefit" from this. if you truly want
a non-interference then look into controlling line stage gain as a theoretical improvement
substitute for simple resistive volume control.

IOW not worth the investment and you may or may not hear the difference.

three options
1. blueprint the PAS, upgrade old CC resistors to metal films. tune the power supply to
restore the original voltage operating points. rewire the back panel to switch selector with
better wires (and shielded for the phono section), add bypass caps everywhere. upgrade
caps to modern materials, change the light to LED (blue preferably), buy new boards
and repopulate using the old as a template, upgrade back panel RCAs, remove AC to
the back sockets, rewire the amfm, tape inputs, for phono by moving the filter network to the
PCB, replace and use better sockets. DO NOT touch the switch selector - there are
multiple ground schemes there. (I had about 6 PAS and did all the above in stages)
2. you could add opamp buffers to the line stage - this involves using older low current
devices (the pas power transformer doesn't allow use of high draw opamps) and you could
squeeze by if you change the incandescent to LEDs, but there's enough room for a
small transformer.
3. there are kits on ebay that add remote controlled volume pots to any device.

Bob
 
ctjetta said:
I'm considering replacing the volume control in my PAS 3 with a stepped attenuator. If I understand correctly, the volume pot has an affect on the phono stage of a PAS preamp. So to keep things correct a 250k ohm replacement would be required.
Stepped attenuators come in three varieties- ladder, series and shunt. I know they all do the same thing, only differently. Is there a reason to use one version versus another in the PAS application given how it will interact with the phono stage?
I would think there would, be but I begin to confuse myself when I think too much about it. Or anything, for that matter.
Click to expand...
All good questions, too bad nobody answered them yet. I am curious too.
 
You want a ladder or series type so it presents the same 250k load to the phono stage as the original pot would. Shunt types will have variable loading depending on volume position. In theory a ladder should be quieter since you've only got 2 resistors in the mix at any time, series has a variable number based on position, and the more resistors you series up, the more the noise adds. Also if you're really concerned about tracking accuracy, stacking resistors stacks tolerances as well, so there is a possibility of greater channel imbalance. In reality I seriously doubt it would ever be enough to notice the imbalance though.
 
The three types:
Series Stepped: All resistors are in a line and the switch moves between them. Noise is additive.

Shunt: Switch selects a divider resistor with a common shunt. Problem is highly variable load from one end to the other.

Ladder: Switch selects pairs. More accurate but more expensive.​

Here's what I previously wrote about noise.

---

I did some research into Johnson noise to refresh my faded memory.

Johnson noise in resistors works in such a way that a series of n resistors each of value Rs has sqrt(n) more noise than a single resistor of value (n x Rs). Correct me if I've misunderstood this.

The magnitude of noise voltage for a resistor varies as sqrt(R x K) where K is some constants, temperature, etc. So we can say the noise varies as sqrt(R) since all the other factors are constant for a single vs. string. Each resistor is independent of the others in terms of noise, and its noise is a function of its resistance multipled by some other factors. But it will vary as a f(R).

So that means to compare Rt as a single resistor vs Rt = R1 + R2 + ... Rn resistors in a string, the maximum effect (all resistors in sequence) is going to be:
[sqrt(Rt)] vs [sqrt(R1) + sqrt(R2) + ... sqrt(Rn)]

In a series string the values are all equal, say Ri. So this reduces to:
[sqrt(Rt)] vs [n x sqrt(Ri)]

So if the string is, say 250 kΩ built from (25) 10 kΩ resistors then Rt = 250,000 and Ri = 10,000. So the noise would be:
sqrt(250,000) vs [25 x sqrt(10,000)]
500 vs (25 x 100)
500 vs 2,500​

So, in this case, at the maximum value, I think the single resistor has 1/5 of the noise of the string. Noise in a string of n resistors appears to vary as sqrt(n).

The question is how noisy is a 1% metal film resistor. Not very. A carbon composite is very noisy.
 
I thought there was a reason not to use the shunt type, but I just wasn't sure.
This for my "stock" PAS- upgraded boards, same circuits as original. Will definitly

I am also going to build a "PAS clone", will have an Akido line stage with a Z-PH10 phono stage from Erhard Audio. Will a shunt version be OK with this kind of preamp? I ask if only to save a few bucks on a shunt style, if possible
 
Retrovert said:
The question is how noisy is a 1% metal film resistor. Not very. A carbon composite is very noisy.
Click to expand...
So, maybe a series stepped attenuator better metal film resistors, may not equal a ladder type, but may be more than acceptable in actual use?
Is this what I'm to understand?
 
Accuracy is very likely not critical for this application, so ladder is significant overkill.

Most use the series-stepped to good effects.

Remember, the original potentiometer isn't as good as the stepped replacement.
 
right, the accuracy and imbalance is not a significant concern in a volume control if we're talking series vs ladder. If you want the absolute best you can get, go for the ladder. If you want something that is perfectly fine, better than the original, and not going to cost an arm and a leg, a series is perfectly fine.


No idea on the non-stock phono board. I'd ask the maker, but on a hunch I'd presume it won't like the variable loading either.
 
AFAIK, the noise from a string of 10k resistors to give 250k will be about the same as a single 250k resistor. That assumes little to no excess noise, just the thermal noise. That's a reasonable assumption for metal films, foils and wirewounds. Carbon and thick film, not as good.

Now, as a divider, say you pick off a point in the middle. The impedance is the parallel combination of the upper and lower halves, assuming it's driven from a low impedance. For the 250k, 125k up and 125k down gives a parallel combination of 62.5k, so you get the thermal noise of 62.5k. (plus excess noise, which for metals and wire is near zero)
 
ConradH said:
AFAIK, the noise from a string of 10k resistors to give 250k will be about the same as a single 250k resistor. That assumes little to no excess noise, just the thermal noise. That's a reasonable assumption for metal films, foils and wirewounds. Carbon and thick film, not as good.
Click to expand...

Very true. Wirewounds have essentially no noise. Beautiful resistors, aside from the drawback of being considerably more expensive.

But, to respond to the noise figure you cited, I believe the Johnson-Nyquist noise from the series string is as I above described it: varies as the sum of the square roots of the values. In consequence n resistors of size r it is not the same as a single resistor of size n x r. This makes sense from handwavy principles because the physical path is, of necessity, longer with more inter-particle junctions and more jostling of electrons. Think of the grains in a a carbon-composite resistor as the extreme case.

The fact is that metal film has very low noise does not mean it is immune from the summation effect. It does, instead, mean that the overall amount of Johnson-Nyquist noise is low enough to be negligible and insignificant for this application.
 
Retrovert said:
Very true. Wirewounds have essentially no noise. Beautiful resistors, aside from the drawback of being considerably more expensive.

But, to respond to the noise figure you cited, I believe the Johnson-Nyquist noise from the series string is as I above described it: varies as the sum of the square roots of the values. In consequence n resistors of size r it is not the same as a single resistor of size n x r. This makes sense from handwavy principles because the physical path is, of necessity, longer with more inter-particle junctions and more jostling of electrons. Think of the grains in a a carbon-composite resistor as the extreme case.

The fact is that metal film has very low noise does not mean it is immune from the summation effect. It does, instead, mean that the overall amount of Johnson-Nyquist noise is low enough to be negligible and insignificant for this application.
Click to expand...

The noise formula for thermal (Johnson-Nyquist) noise contains only variables for temperature, Boltzmann's constant and the resistance. How you construct the resistor has no effect. Excess noise (that I believe is what you're talking about) is a whole 'nuther thing and is often expressed as ENR (excess noise ratio), which is the normalized measure of noise above and beyond the inescapable baseline thermal noise. Size and 37 other things probably matter for ENR. If we can't choose the value, this is the only thing we have control over, choosing the lowest noise parts for the application.

Some time ago I built the Geller Labs "JCan", which is an amplifier that allows one to measure resistor noise. Not a true lab instrument, but surprisingly good. In general, metal films were indistinguishable from Johnson-Nyquist theory- whatever excess noise was there was so small as to be undetectable. Even most carbon comps were better than you'd expect, but some carbon pots- wow! I was already converted to stepped attenuators, but that sealed the deal. Excess noise also increases with current through the resistor, so results are application dependent. The worst noise problems I've had were plate resistors in a preamp and accidentally-used thick film resistors in a low noise amplifier. Plate resistors pass a lot of current, so IMO need to be chosen carefully. Thick films I've measured recently were very good, so I might have just been unlucky with a batch, but I still avoid 'em.
 
CC pots suck, the drifting off value of the tracks, and the fact that they do not drift together causes a lot of headaches.
 
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