Sound Skulptor forums
Build help and troubleshooting => MU524 Vari-Mu Tube Compressor => Topic started by: Elektrovolt on August 18, 2024, 06:31:52 PM
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Hi all!
I've finished a pair of MU524s last week and did the calibration routine four times on each of them.
the difference in output level and gain reduction is quite a lot, I need to set the input pot on of of them 1 dot to the left to get the same levels on both.
In step 6 of the setup manual (audio path test) the 1v signal gives 3,177 VAC on the left one and 3,450 VAC on the right unit.
GR is different too and output levels differ around 1,2 dB.
I do not mind that two units have little differences in settings, but around one dot seems a bit much.
All voltages are within specs, except for the output level when the out pot is CW which gives over 20VAC on both units which seems a lot.
What could cause the difference in signal levels and the high output?
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The input pot difference could be caused by the tolerance on the pot resistance value (20%). You can check the resistance value of the pots between the 2 side pins, pot at max.
Regarding the 20VAC output it might be OK. Simply check that the input transformer is mounted in the correct orientation, label facing towards the back of the module.
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Thanks JP,
I was already suspecting the input potentiometer tolerance, they measure a bit different from each other. The transformers are mounted correctly and all audio path resistors are correct also.
The pots measure:
Full values are 9,7 kohms and 10,8kohms
Wiper set exactly in the middle: 0,95 kohms and 1,08kohms.
So there is a difference between both.
I do have some good quality log pots 25k from a Sound Devices mixer. Would that work too, (with an 18k resistor between pins 1 and 3) or should they really be 10k?
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It will work with the 25k pots, with a smaller range in the max attenuation area. The 18k resistor won't change the result much so I wouldn't care.
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Ah. yes, of course. I will try and order some Alpha or other brand pots and match them as well as possible because I prefer the controls to be identical.
I am really happy with the sound btw!