balanced preamp 2025

[MASantos]

How about an onboard psu for the attenuator? 
shouldn’t add too much real estate. 

[kevin gilmore]

only need one power supply for both channels and its not a lot of power.  also runs whatever microcontroller you decide to use.

[arcanaking]

Hi, Dr. Gilmore. It is thankful to have a new circuit for the preamp. I have some stupid questions to ask if the circuit change from leduino controls to simple toggle switches and attenuator.

1.According to the preamp2025g.pdf, does it mean when the signal comes from the XLR input socket, the signals will go to the R61 (+ signal route) and R60 (- signal route)? After the process of the input section, the Q47, Q48 (+ signal route) and Q43, Q44 (- signal route) will connect to the attenuator input pin. Then it will comes to the R81 (+ signal route) and R80 (- singal route) to enter the output section. At the end, the(+ signal route) will comes from the R82, R87 and the (- singal route ) will comes from the R90, R95 to the XLR output socket? If I want to replace the digital parts with the alps rk27 or even khozmo attenuator, is it correct to replace the components and connect to the above pins?

2. According to the output socket, if having no microcontroller parts, is it better to use the headphone protector board for the connection to control the switch?

3. In comparison to the susy dynalo and the cfa3, it seems this design requires more transistors. Do all the transistors are recommended to complete the match pairing process? Or some of the transistors play a crucial roles that requires match pairing?

4. Unlike the susy dynalo and the cfa3, it is not JFET input, can the circuit still accept 50k attenuator?

5. What is the J1 and J5 do for the gain? How should I connect to these two terminal block?

[kevin gilmore]

relays are dual coil latching.  so 2 might be tricky.

3) lots and lots of cheap transistors.  best to go with the "B" versions, matching is always better.

4) 50k attenuator NOT NEEDED. thats the point. there is no commercial product with an output voltage that will  saturate the input.

5) very much like the ayre kx-r.  individual resistor goes between the 2 pins. for increased gain should you want or need it. schematc shows everything at 0db gain.