Aquaboy Deluxe Compander Biasing

[Christof]

Hi all, greetings from Australia!

I've just built up an Aquaboy deluxe delay (MN3101/MN3008)- it was a smooth build and the delay works and sounds pretty awesome, but even after calibration on a scope there is some residual distortion on transients that's a bit annoying. It actually improves significantly if I run the circuit down at 12V and re-bias the BBDs, even though BBD headroom is reduced. Pretty sure I've tracked it down to the compander biasing, but I could do with some guidance!

I followed the advice in the build instructions and socketed R13 and R14, but they seem to control only the "output 1" (pin 7) bias on the compander, and this can be adjusted to ~7.5V ok, but Output 2 (pin 10) is stuck down at 4.8V, and does not change when R13/14 values are adjusted.
Also, when increasing the input level (200Hz, sine) the first section to clip is at pin 10 of the compander, and it is assymetrical and sounds pretty nasty. If there is a way to reduce the gain in this section I think that would help as well- can anyone help me progress from here?


Many Thanks,

[Scruffie]

Get the datasheet and a copy of the compander cookbook and enjoy.

[madbean]

If you are running the circuit at 12v, try changing the biasing resistors so the output is closer to 6v, rather than 7.5v.

[Christof]

Thanks madbean, I can rebias the compressor side at Vcc=12V using R13/14, but would like to work out how to adjust the expandor side bias as well.
I am trying to get distortion levels at Vcc=15V as good or better than at 12V, which should be intuitive but for the expandor clipping I'm seeing with increasing supply voltage.
After looking through the NE570 datasheet, looks like R37 can be adjusted to change the expandor bias so I'll try that. To reduce the levels at the expander and stop the clipping I may just look at adding a small resistor at R27 or R55. I have noticed that repeats run away with the feedback knob at ~2 o'clock, even with the T1 trimpot fully CCW, so I can afford to throw away some signal.

[Christof]

Sorted

Expandor side is well biased now with a 10kOhm resistor at R37. Reduced the BBD2 output gain by reducing R28 and R29 to 33kOhm to stop the expandor from clipping so early.

Much more happy with the level of distortion now!

[madbean]

Thanks for the update. I have tried messing with that expandor resistor before but never did end up on 10k. Glad you got it worked out.