Hello,
on the image below is a volume control concept for YAMAHA CP2000 PA which will be used for measuring purposes. The idea behind it is to deliver exact power to specified load (2,4,8 and 16 Ohms), while fed with constant signal level (1,22 V unbalanced) and 1kHz sine.
Currently, the device is designed with two relay controlled resistor voltage divider stages based on LME4562(49720) connected as voltage followers. First stage will be used to set up the correct load (0,-3,-6,-9 dB). Second divider will set up the exact power level to the load (from 1W to full range of Yamaha CP2000). Last stage is made from multiturn trimmer (represented as two separated resistors) and LME4562 connected as an inverting amplifier. I/O circuits are from THAT corporation IC's.
I made some tests on a breadboard and I've got a problem with great amount of excessive noise on the lower voltage levels (Especially from the second stage). I tried to change the ladders total resistance (from 1K ohms to about 50 K ohms) but without significant results. My guess is that this problem is caused by some kind of oscillations.
Do you have any ideas how to prevent this noise without changing the voltage followers? Or it this concept an absolute dead end ? I also have an idea for different approach using THAT2181 VCA circuits instead of LME4562, but my measuring possibilities are limited.
Thank you for your time and sorry for my poor English.
View attachment 663811
on the image below is a volume control concept for YAMAHA CP2000 PA which will be used for measuring purposes. The idea behind it is to deliver exact power to specified load (2,4,8 and 16 Ohms), while fed with constant signal level (1,22 V unbalanced) and 1kHz sine.
Currently, the device is designed with two relay controlled resistor voltage divider stages based on LME4562(49720) connected as voltage followers. First stage will be used to set up the correct load (0,-3,-6,-9 dB). Second divider will set up the exact power level to the load (from 1W to full range of Yamaha CP2000). Last stage is made from multiturn trimmer (represented as two separated resistors) and LME4562 connected as an inverting amplifier. I/O circuits are from THAT corporation IC's.
I made some tests on a breadboard and I've got a problem with great amount of excessive noise on the lower voltage levels (Especially from the second stage). I tried to change the ladders total resistance (from 1K ohms to about 50 K ohms) but without significant results. My guess is that this problem is caused by some kind of oscillations.
Do you have any ideas how to prevent this noise without changing the voltage followers? Or it this concept an absolute dead end ? I also have an idea for different approach using THAT2181 VCA circuits instead of LME4562, but my measuring possibilities are limited.
Thank you for your time and sorry for my poor English.
View attachment 663811
well, that's embarrassing. It worked in a preview and I can't edit the original post. 
.
View attachment bp_volume_control.pdf
.View attachment bp_volume_control.pdf
"Noise"? Hiss? Hum? Radio?
What happens if you jumper to the bottom of the attenuator (your "0" point)? i.e. is your "ground" noisy?
For an otherwise clean audio system working voltage mode, lowest hiss will be with *small* value resistors. However your max is only a few Kohm so that should not be excessive hiss.
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