relative impedance of balance pot to volume pot

[jazzzman]

I am trying to figure out best values for balance and volume pots on an SCA-35 clone I am planning to build. Dynaco had 750k for balance and 250k for volume, which I do not understand. I know that Rod Elliott suggests using a balance pot of 2 to 2.5 times the impedance of the volume pot. ESP - A Better Volume Control I would think that because the balance pot feeds the volume pot one would want the output impedance of the balance pot to be about one tenth to one twentieth the input impedance of the volume control. Does anyone understand the considerations that would lead to a much larger impedance on the balance?

As an alternative, I was considering putting an ST-35 clone in the SCA case, but I understand that the input triode on the ST-35 (as opposed to the pentode used in the SCA) raises miller effect considerations when fed from a volume control. Does anyone know the calculations for minimizing that effect?

Thanks for your help.

 

[Palustris]

Since you are building a clone I see no reason to go with a volume and balance control when two separate 100k ohm volume controls, one for each channel, will achieve the same results. I found the stereo volume controls don't track each other exactly, so I went with two single 100k ohm controls. I use Mouser part# 588-5000E as a dial plate and Mouser Part #588-5150E for the knob. Calibrate both channels (e.g., set the knob set screw) so that when there is, say, 2VAC output across an 8 ohm resistor, both controls are showing the same point on the dial. It is easy, then, to achieve balance when adjusting volume just set the two dials to the same point.

 

[rayma]

I would think that because the balance pot feeds the volume pot one would want the output impedance
of the balance pot to be about one tenth to one twentieth the input impedance of the volume control.
Does anyone understand the considerations that would lead to a much larger impedance on the balance?

A high value of the balance control relative to the volume control is necessary to avoid loading
the source. A high value also gives more attenuation with a given amount of rotation, with a linear pot.

 

[Eli Duttman]

I too strongly favor the dual mono control approach. Sourcing suitable MN taper balance controls is anything but easy. Palustris has provided us with an elegant solution to the tracking issue.

If you want superior pots., as opposed to costly stepped controls, PEC KA series hot melted carbon parts, from DigiKey are the way to go. PEC controls sound GOOD and they are milspec tough.

 

[jazzzman]

Thanks for the suggestions. Two volume controls may be the way I end up going. It seems simpler and sure fire. However, I like the idea of a balance control. I was thinking of putting together a stepped balance control similar to this: DACT audio balance controls

But I am concerned about the impedances. Wouldn't I want the output impedance of the balance to be 1/10 to 1/20 of the input impedance of the volume pot?

In turn, shouldn't the input impedance of the balance be about 10 to 20 times the output impedance of my source?

If so, why have the balance impedance be 2 to 2.5 times the impedance of the volume?

Thanks for any insight into this.
Jazzzman

 

[Eli Duttman]

If so, why have the balance impedance be 2 to 2.5 times the impedance of the volume?

When you rotate a MN taper balance control to the user's left, resistance is added, in series, to the right channel's control. The signal path is balance control 1st and then on to the volume control. Remember, you are dealing with a logarithmic situation and lots of resistance is needed to obtain a comparatively small effect.

In the center position, a MN taper balance control is just wire. Even though each section of the control has 3 connections, you use only 2.

Perhaps a reason why L/R mono controls are not popular is the fact to move the signal to the left, you lower the right channel's control. That's somewhat counter intuitive. A MN taper balance control does the same thing, in "a more user friendly manner".