[Magnusp71]

Hi all

Just completed my first SS unit. The build was very good and the guides worked very well. After having completed initial setup, I am however experiencing some issues conducting the compressor setup. I have gone through the “usual suspects list” and found no bad solders or wrongfully places components. Here is what I am experiencing:

Setup test:

• Power voltages check = OK.
• Input signal at 1 VAC in bypass mode = OK.
• Bypass to IN = VAC immediately drops to 0 VAC. When I turn up the gain, the VAC increases steadily. but when I reach approx gain 40, the VAC jumps from 1,5VAC/1kHz to 7,5VAC/2,5kHz. When further increasing the gain the VAC more rapidly increases until reaching 21VAC/2,5kHz at gain 60. From here the output increases to 21,4 VAC/2,5kHz at gain 70. No further output can be achieved when turning the gain knob fully CW from here, but I can hear a sinus tone from the output transformer. 
• Adjust gain pot to 15 VAC = OK.
• Turn PEAK pot CW and check that it makes the output voltage go down to 1VAC or less = Output voltage decreases steadily reaching 5VAC/2,5kHz at peak 10 and 2,5VAC/2,5kHz at peak 15. From here, the frequency starts to lower, reaching 1,5VAC/1kH at peak 20 and 1VAC/1kHz at peak 30. Going further, 0,3VAC/1kHz at peak 60 and 0,1VAC/0kHz at peak 100.

Further test:

• Send a good constant sine signal on input, like 1VAC measured between pins 2&3 of the input XLR. PEAK at 0 = OK.
• Connect the voltmeter black probe to 0V and trace the signal on.
  
  • R2 right pin: Should be similar to the input voltage = 0,5 VAC (50% of input)
  • R5 low pin: Should be similar to the input voltage when GAIN is on 100 = 0,5 VAC (50% of input)
  • TP1: should go above 10V when GAIN is at 100 = 13,2 VAC.

Hopefully, some of you might be able to help me move forward in my troubleshooting, as I am kind of stuck here
Thank you

[JPK]

the VAC jumps from 1,5VAC/1kHz to 7,5VAC/2,5kHz
What gives you the frequency display? Is it the DMM? Is it reliable?
Did you try listening to the output signal? Do you hear a frequency change? or a saturation?

[Magnusp71]

Hi Jean-Pierre

Yes - the kHz reading is from the DMM. Not sure if its 100% reliable but I can hear a frequency change at the output. 

I have attached a picture of the input xlr reading at 1 VAC, a video showing my DMM a behaviour when turning the knobs, and audio file of the output and a picture of the output waveform.

Hope that there is an easy fix to this issue

Br, Magnus

LINK to video and audio file: Download

[JPK]

The circuit self oscillates. Check the value (marking 470) and solder joints of capacitor C4.
Same thing for R6 and R7.

[Magnusp71]

Hi Jean-Pierre

I appreciate the assistance. I checked values and did a re-solder for C4, R6 and R7. Nothing seemed out of order and the unit still behaves as before (VAC jump from 1,5 V to 11 V and kHz increase from 1 to 2,5 while turning gain knob). 

Any other suggestions? 

Video here - LINK

All the best, 
Magnus

[JPK]

Remove U5 and U6 and check if the behavior is the same (this will deactivate the side chain).
Next, remove U2, U3, U4, one at a time and check.

[Magnusp71]

Hi JPK

I did as instructed but it turned out that this wasn't the issue. The real issue was with my audio interface which the output I was measuring on was connected to. Once disconnected, things completely changed.

The unit is now calibrated and I have proceeded to step 7 - the sound check. 

Generally, the unit seems to be working properly and sounds gooood, but I've noticed a few things: 

• When adding a lot of gain reduction, the transients from "pop-sounds" actually becomes a lot louder than in the original waveform even though the LA502 is used as a limiter - see image below. Maybe I was expecting the waveform to look more compressed in the limiter setup, but maybe I was wrong? 
• When setting the unit up with my audio interface - Apogee Duet 2 - I very easily get sort of a "ringing sinus tone that becomes very loud when turning up the gain. The sinus tone keeps ringing in the unit even after DAW output is turned off. Is this a issue with the unit or something in the way my DAW and interface is setup?

Thank you for your assistance. Upon finishing this troubleshooting, I am ready to move on to the next DIY project.

Br, Magnus

[JPK]

When adding a lot of gain reduction, the transients from "pop-sounds" actually becomes a lot louder than in the original waveform
This is because the optical compressor is slow. The fast transients can't be caught by the compressor.

When setting the unit up with my audio interface - Apogee Duet 2 - I very easily get sort of a "ringing sinus tone that becomes very loud when turning up the gain.
Probably some feedback through the audio interface/DAW combination.