Very basic question about volume controls

garybdmd

Member
2016-01-03 5:13 am
Hi all, this is a very basic question about volume controls.

I am building a preamp and the output impedance is 1k. It could be linked to an amplifier with 10k input impedance, 100k input impedance or 1meg input impedance depending on which amp I use.

I wanted to make a volume control between the preamp and amp.

I was guessing that the volume pot should be about 10x's greater than the input impedance of the amplifier section. I was thinking that would make a good voltage divider but wouldn't draw current down the volume pot. Is this correct thinking? Or should I make it just 10x's greater than the output impedance of the preamp which would be 10k pot? On one hand using a large pot would insure the load is high on the preamp even when the volume pot is at lower volumes, but on the other hand, the signal has to go through a lot of resistor to get to the wiper and I though maybe that would be too noisy bc I've read in general high resistance = high noise.

I know this is a basic question, thanks for looking and your knowledge.

Does anyone know of the convention to use in this situation?
 

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The volume control, ideally, should be lower resistance than the input impedance of the amplifier. So if your amplifier has a nominal 50k impedance then a 22k or less Pot would be fine. If you have a 10k input impedance and a 50k pot, the wiper, from a possible 25k centre point, will be feeding a 10k resistance from a possible 25k source so a heavy load will effect the calibration of the dial. If that makes any sense.
 

rayma

Member
2011-04-29 8:37 pm
Yes i could on each amp easily. Sounds like what im hearing is use a pot
equal to or less than the input impedance of the amplifier.

It's better to have as low a value as possible, yet without loading the source too much.
Determine the lowest loading your source can have for good performance,
and choose a control somewhat larger than that. If the control is only 2 or 3 times
the source impedance, you'll lose a slight amount of gain, due to the voltage divider action.

If your source has an output coupling capacitor, a control too low in value could cause
a higher frequency bass roll off. The lower the value of the control, the less noise and
HF roll off due to the cable, and from the input capacitance of the amplifier. That said,
if the control is inside the amplifier, it can be higher in value than if outside.
 
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garybdmd

Member
2016-01-03 5:13 am
One follow up question. In guitars sometimes there is a small "treble bleed" capacitor that bypasses the volume control. And it totally grounds out when volume is at zero, but allows a little high frequency to bleed through when the volume is turned down at the lowers numbers like 2-3. But at high volumes its totall bypassed. Sorry im on phone cannot draw now. But it goes from top of pot to wiper i beleive. Is this ever used in situations like this or general amplification. Just wondered
 

rayma

Member
2011-04-29 8:37 pm
In guitars sometimes there is a small "treble bleed" capacitor that bypasses the volume control.
And it totally grounds out when volume is at zero, but allows a little high frequency to bleed
through when the volume is turned down at the lowers numbers like 2-3. But at high volumes its totall bypassed.

That's an old trick. If done carefully, it can improve the sound by balancing out the load capacitance
on the control, at least through a certain range. Otherwise, it just boosts the highs, with the amount
and the frequency depending on the setting of the control. I first saw this used in hi-fi gear in the
original Theta preamp, the one with slide pots.
 
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I recommend a minimum Source:Receiver impedance ratio at least 1:5 and as high as 1:20 for good voltage transfer.
That puts your Source to vol pot connection as a 1k Source feeding into a 5k to 20k vol pot.
Considering now the vol pot to Receiver and assuming you chose a 20k vol pot (for this example), the output impedance of a 20k vol pot varies between zero ohms and 5k ohms.
Using this latter value and a 1:20 ratio as for the first part, you end up with 20k*5 = 100k for the Receiver.

1K source >> 20k vol pot >> 100k Receiver gives worst case impedance ratios of 1:20
It all works for good transfer of voltages.

Next you need to consider high frequency roll off.
Each Source has an output impedance. Each connection cable (and any RF attenuation built into the next stage) has some capacitance. Together they form an RC filter that rolls off the high frequencies.
Take a worst case 1nF cable capacitance for a very long interconnect.
The 1k Source and 1nF of capacitance gives a roll off predicted by the equation
F-3dB = 1 / {2 Pi R C} and for the example comes to 1/2/3.14/1000/0.000000001=159kHz
Your Source can easily drive a long interconnect.
Now the 20k vol pot. It's maximum output impedance is actually (vol pot+Rsource)/4 = 21k/4 = 5k25
If we reduce the cable length by half and take a low capacitance version we could end up with 200pF of capacitance. Add on another 47pF inside the Receiver for a total 247pF.
The HF roll off is now about 1/2/3.14/5250/0.000000000247 = 123kHz (You can see why we use n=10^-9 and p=10^-12)
The shorter lower capacitance cable rolls off the treble a bit lower than the source and long cable.
This tells you that you must use shorter cables AFTER the source that has the higher output impedance.
That's why one finds the vol pot inside the Receiver, that equates to the shortest cable and equates to the least capacitance.

A 2m interconnect that has 50pF/m only has 100pF of capacitance. A 20k vol pot can easily drive that 2m cable without rolling off the treble.
A 100k vol pot would roll off the treble.

A buffered vol pot can be designed to drive any length of cable. That allows you to locate your vol pot in the best location for usability, i.e. next to your main listening seat and forget about loading problems.
 
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