Balance control in preamp

[vanvliet]

Hi to everyone! I have built an aikido preamp using ecc88 tubes and an alps 50k pot at the input. I want to implement a LR channel balance control and I have one of J. Broskie's Trim 1 single-channel, stepped attenuator. I will only attenuate one channel, so basically I will use a 2 resistor voltage divider in one channel and the stepped attenuator on the other channel to implement the "balance control".

Is it better to put the stepped attenuator and voltage divider before the input or at the output of the preamp? And based on this decision what should be the impedance of the balance circuit?

For example, with 2 resistors 8.66K and 11K I can create a -5dB voltage divider with 20K impedance and populate the stepped attenuator board with resistors accordingly.
If I put this after the alps pot the input impedance of the preamp will be raised to 70K, with minimal consequences I assume... Or not?

But what happens if I put the balance control at the output of the preamp? what is a good trade-off of putting enough load to the preamp output without ruining the ratio of preamp output impedance vs the power amp input impedance?

Thanks for the help!!

 

[vanvliet]

Just to give a bit more detail, my pre amp drives a power amp with 100K input impedance.
I assume if I add the balance between the pre amp and the power amp I have to calculate the pre amp's output coupling capacitors corner frequency using the impedance of the balance circuit. Am I right?

 

[rayma]

But what happens if I put the balance control at the output of the preamp? what is a good
trade-off of putting enough load to the preamp output without ruining the ratio of preamp
output impedance vs the power amp input impedance?

You want low preamp output impedance, and also high amplifier input impedance.
A ratio of 1:5 is adequate (a loss of 1.6dB), though 1:10 and up is better.

The preamp (with or without pad) also must drive cables, and not lose hf due to
a high output impedance.

It's probably best to have the pad at the preamp input, but what are your sources,
can they drive the pad? If they are tube, maybe not. Why do you need to attenuate
only one channel?

 

[vanvliet]

Thank you!
I understand. So adding the balance pad between the pre amp and power amp (at the preamp output) might be detrimental to HF. Basically, it will raise the output impedance of the pre amp.

Now if we consider putting the balance pad to the input of the pre amp, this is the impedance that other sources will see. If the impedance of the balance circuit is 20K and afterwards a 50K log pot, input impedance would be only 20K? Then is it fine to have the same 50K pot? Or should it better be 200K (1:10)?

I think modern sources like a cd player, phono stage etc could drive an input impedance of 20K?
From tubecad:

A Trim-1 can be used as a balance control, by using -1dB steps and using only one in the left channel and using a -5dB two-resistor fixed attenuator on the right channel. Then turn the Trim-1 to its sixth position (-5dB), so the channels match.

 

[rayma]

Then is it fine to have the same 50K pot?
I think modern sources like a cd player, phono stage etc could drive
an input impedance of 20K?

The volume pot can stay the same. You'll need to take into account
its value whan calculating the amount of attenuation you want,
since it will load the pad.

Most sources will be able to drive 20k, but maybe not some tube sources.

 

[vanvliet]

Great. They are all solid state, so 20K input impedance would be fine.

So better configuration is sources -> balance pad -> pot -> preamp -> power amp

Thanks again. Now everything seems more clear

 

[rayma]

Great. They are all solid state, so 20K input impedance would be fine.

The accepted standard is for a 10k minimum load, so 20k is usually safe.

 

[Zung]

Both volume and balance controls should be at the input. You can implement them as shown in the original text:

Insert an appropriate value resistor in serie with the pot of the other channel to get 2 or 3dB of attenuation and you're there.

I personally prefer fine resolution, so a pot can be used instead of the stepped attenuator.

 

[vanvliet]

Thanks, very helpful. This is the plan then.