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Old 13th March 2013, 01:38 AM   #1
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Default Volume potentiometer: the best way to do it...

Hi,
Being in the process of finishing my integrated 300B parallel SET (and having been poking my nose inside some schematics of open reel tape recorders from the 60's) I'm facing the question: What's the best place to put the volume control?
The schematics I'm following puts it on the signal path just before the grid of the first stage.
The prototype I built shunts the signal to ground on the same position through a resistor. The reason that made me opt for this topology was that the signal didn't go through the pot, thus eliminating one factor of sound degradation.
However the older schematics I've been studying use a different approach: the pot gradually shunts the signal to ground just before the grids of the power stage, after the coupling capacitor. I suppose it would also be a valid and quiet solution because it also avoids the signal path (besides the designers of the 'golden era' must have had their reasons, right?).
What's your opinion about these approaches?

Thank you for your contributions!

Renato
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Old 13th March 2013, 02:03 AM   #2
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what level of sound (signal) degradation do you expect through a reasonable quality pot?

And if its too much, have you considered a stepped attenuator?

By putting attenuation at the very front, you are managing the lowest power signal. Hence you will have least current/voltage across the item and hence the least interaction and so least degradation.

IMO.
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Old 13th March 2013, 02:08 AM   #3
Loren42 is offline Loren42  United States
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The biggest issue for me is the noise. Not the thermal noise, but the noise from the contact and wiper, which degrades over time and use.

That kind of noise is still an issue for your cited configuration.

The best solution is a step attenuator and one that only has two resistors in the circuit at a time.

Thermal noise should be a lower concern if this is a power amp and the input signal level is relatively high compared to something like a phono section or some other circuit loaded with gain.
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Old 13th March 2013, 05:48 AM   #4
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The "Kiwi" is correct. Place the control(s) at the amp's I/Ps.

Differences of opinion can be found about what is the "best" control. Some folks swear by Penny and Giles, which are (sic) conductive plastic. Other people prefer stepped attenuators, either series or ladder. IMO, the best value in controls are the milspec, hot molded carbon, parts by PEC of Canada. They sound damned good and answer Loren's wiper objection. PEC pots. use a spring loaded block of carbon, not a piece of metal that wears the track out, as the wiper. Check the PEC data sheet out. DigiKey is the North American distributor for PEC. I have no idea who to buy from in the EU.
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Old 13th March 2013, 06:40 AM   #5
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Hi!

IMHO the best place for the volume pot is at the output of a linestage with low output impedance. This enables the use of low resistance pots (600 Ohm Pots) or even autoformer volume controls.

IME pots do impact the overall sound.

Best regards

Thomas
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Old 13th March 2013, 06:59 AM   #6
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The optimum place depends on the signal level.
As an example; think about RIAA or microphone amplifier with very low input level.
If the volume control was placed at the input, the signal to noise will be much degraded when volume would be adjusted (decreased),
because the noise generated by the amplifier remains at the same level.
Only the signal level is decreased.

In case of line level (0.3...2 V) the volume control can be placed at the input of the amplifier,
because the overall signal to noise level is mainly determined by the stages preceeding the volume pot.
Now adjusting volume (signal) will also adjust the noise from preceeding stages and S/N remains good.
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Old 13th March 2013, 07:22 AM   #7
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Hi!

The reason why I prefer the volume control at the output of the linestage is because I prefer low impedance volume controls. The linestage acts as a buffer which presents a high impedance (easy) load to the sources and has low output impedance to drive a 600 Ohm volume control pot or a autoformer volume control.

Best regards

Thomas
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Old 13th March 2013, 08:16 AM   #8
JMFahey is offline JMFahey  Argentina
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Quote:
The prototype I built shunts the signal to ground on the same position through a resistor. The reason that made me opt for this topology was that the signal didn't go through the pot, thus eliminating one factor of sound degradation.
Ah!!, but the signal "goes through the pot" anyway and whatever effect the pot has on signal, it's still there.
What exactly is what worries you about pots?
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Old 13th March 2013, 08:58 AM   #9
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Before.
The older schematics are from which year ? Probably from a time when there were no sources that could output 2v, but instead much less.
Now it could be possible that with 2v, the input stage is distorting/clipping.
Also use a stepped attenuator instead of an ordinary pot or you'll throw away a lot of the 300b magic
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Last edited by danny_66; 13th March 2013 at 09:00 AM.
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Old 13th March 2013, 11:32 AM   #10
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Eli Duttman View Post
The "Kiwi" is correct. Place the control(s) at the amp's I/Ps.

Differences of opinion can be found about what is the "best" control. Some folks swear by Penny and Giles, which are (sic) conductive plastic. Other people prefer stepped attenuators, either series or ladder. IMO, the best value in controls are the milspec, hot molded carbon, parts by PEC of Canada. They sound damned good and answer Loren's wiper objection. PEC pots. use a spring loaded block of carbon, not a piece of metal that wears the track out, as the wiper. Check the PEC data sheet out. DigiKey is the North American distributor for PEC. I have no idea who to buy from in the EU.
I use the Mil-Spec PEC pots a lot for non-audio applications in my business.

I have found some problems with these pots.

First, I have received batches where some are of the wrong taper and resistance. This is not so much an issue with the manufacture (I think, but the supplier).

Second, I have one PEC log pot in my power amp that has become scratchy. These pots are rarely used and they are sealed pots, so I can't really say why it is scratchy.

I have torn a few of these down and will say that the wiper and spring assemblies are pretty robust looking.
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