Balanced attenuator for Aleph J

[Bassling]

I am about to build the Aleph J, and will drive it balanced straight from a dac with digital volume control, so I really want to put a selectable attenuator into the path. I was going to use a 4p5t rotary switch attenuator, but am not clear how to connect it between the xlr inputs and the aleph j boards. If this has been asked before then I am sorry, but I couldn't find it through the search. If you could also suggest appropriate values for the 5t then I would be eternally grateful.
Thank you.

 

[rayma]

not clear how to connect it between the xlr inputs and the aleph j boards.

You only need a 2P5T switch for stereo. Balanced Step Attenuator & Aikido Hybrid Amplifier

 

[Bassling]

Wow - thank you for getting back so quickly. What a neat design. Have you heard it? Does this approach change the sound (apart from making it quieter, of course)?

 

[rayma]

Does this approach change the sound (apart from making it quieter, of course)?

Use good resistors, like Vishay-Dale 1% metal films. I haven't used this one,
but have made my own. You can scale all the resistance values by a common
factor to suit your system, for example, divide all values by two, etc.

 

[Bassling]

The input to the aleph j is a 22k/220k divider. Can I reconfigure the attenuator to take this into account, or would you just build it as it is described? Many thanks - I will leave you alone after this!

 

[rayma]

The input to the aleph j is a 22k/220k divider. Can I reconfigure the attenuator
to take this into account, or would you just build it as it is described?

This amp wants to be driven by a low source impedance (<2k), so the gain is not affected.
You might consider scaling all the resistors way down, maybe by a factor of five or ten,
if the DAC can drive that. Also, is there a DAC output coupling capacitor to consider?
If so, then a lower attenuator resistance could reduce the bass range.

 

[plasnu]

Here is another solution I chose. I connect balanced output of Wolfson DAC "chip" directly to 10K:10K, 600 transformer, then Aleph J.

It probably depends on the DAC, but in my case, bypassing several opamps of the balanced output of the DAC has a big impact on sound quality. The output level of the DAC chip is much lower than that of DAC balanced output, and probably no attenuator is necessary. With transformer, you can remove the input capacitors from AJ, which would be another benefit. If you do this, check the DAC chip output bias, my one is 5V.