All righty,
After going through the mess I did fixing all my mistakes and finding bad parts, I feel the need to compile and share all the tips I had to figure out to help others here who are struggling with this build to get it off of the bench and into the studio. It's a great sounding compressor and definitely worth the effort.
First, it's not an insanely difficult build since there is minimal off board wiring, but it is not something for a beginner as there are heaps of parts with varying values and keeping track of them and their orientation can become overwhelming. Plus, chasing down traces and comparing them to the schematic will make your eyes cross as the connections are highly condensed on the individual daughter boards and is very easy to loose track of what is going where.
Also, don't forget that with these boards, the trimmers are to be turned opposite of the calibration instructions per Gustav.
With all of that aside, here we go, in no particular order:
- 1/4W resistors are fine to use in place of the 0.6W as listed on the BOM
- Unless you have a verified source of NOS transistors, use only modern equivalents. I had a bad batch of BC182B's that didn't know were faulty. Here are the ones I used - BC184C=MPSA18(backwards orientation), you can also use BC550C, BC182B=BC550C, BC212B=BC560C, BC212L=PN4355 - B and C legs have to be crossed, the PN4355 should be angled downward (towards the main board when the daughter card is connected) to ensure the crossed legs are not touching - just double check this, BC184L=2N3417
- The LC Network (CL1,2) are not on the BOM. I used this part:
https://www.mouser.com/ProductDetai...=/ha2pyFaduiDbeIvXMFHbA4Jqa4pQl60iVMLBFv8s0M=- Stereo matching the expander section:
www.musictoolz.pl/pliki/instrukcja-obslugi-344.pdf
- Use this link to calculate between RMS and dBu:
dB dBu dBFS dBV to volts audio conversion digital - calculator volt to dBu and dBV dB mW SPL dB decibels 0 dBFS - convert dB volt normal decibels relatioship relation explanation analog audio absolute level true rms convertor converter decibel to dbfs converter calculation online attenuation loss gain ratio reference audio engineering sound recording dBFS dBVU 0 dB audio logarithm level converter peak to peak p-p impedance voltage pro consumer audio digital analog recording level - sengpielaudio Sengpiel Berlin - It's helpful to calculate all of the dBu values to VAC(RMS) and write them down so that following along with the calibration process is easier.
- For the compressor input threshold, there is no mention of it in the calibration process. I ended up on having 8k5 ohms resistance between the wiper and the bottom leg as viewed when installed. I don't know if that is correct but I started there and everything seems to work out.
- When you get to Step 11 of the calibration, make sure that FET Bias and Inverter Set are turned all the to where you have most level you can get from those trimmers. If both trimmers are about halfway when you start, and you turn the FET one way, the output level will change. Same with the Inverter Set. My notes say full CCW for FET Bias and full CW for Inverter Set. I think that is correct, but I am not willing to double check it as my build is all buttoned up and finished after what seems to be more than a dozen calibration attempts. You're output signal should be closer to 0.69VAC when both trimmers are all the way in their correct starting positions. I think that makes sense.
- My Target Value A was 1.07VAC
- Start with the Compression Ratio full CW
- I changed the FET Law trimmer (R78) to 50k as I found that I needed more resistance to do the procedure. However, I also found that I needed to start with the trimmer set so where the wiper and the bottom leg that connects to the Control VPositve needs to be around 1k, because any lower the meter bounces around and looks like something else is wrong when you start the expander calibration. After this discovery, I thought that maybe 20k would be fine, but I had already installed the 50k trimmers so I just left them. If you want to be safe, you can use 50k. This may have something to do with using the modern equivalent transistors.
- The LED for the expansion indicator did not come on for me until about -3dB of reduction and not as the calibration instructions say. Again this may have something to do with using the modern transistors. I tried to adjust the values of R86 and R85, but nothing changed as far as the LED indication was concerned, so I left it as is.
- If you can find sockets to connect the daughter cards to the main PCB that will greatly aid you in the event you need to troubleshoot. If you can't find the correct size sockets (as in my case) then I would install the Input and Output cards first and check to make sure you are getting audio to pass with the front panel PCB connected. You should be able have a functioning Input and Output controls when the System is set to In. Then one at a time, install the Compressor card, Limiter card, and then the Expander card in that order, checking signal and the relevant calibration steps each time you install a card. Once you see that everything is working, then you can start the full calibration.
- Put a marker of some sort on a printed copy of the Initial Setup Procedure as you go through each calibration step. There are sections that are repeating a series of steps and if you have been working on this for a long period of time or if you are calibrating for the 10th time, it becomes easy to lose your place and/or skip over a step that will adversely affect the final outcome and you will have to start over. Again.
- Don't put the wrong part in the wrong place. After desoldering many connection points, my growing impatience caused me to break some to of the solder pads and then I had to trace where the connection was supposed to be going and solder jumper in place. So just stuff the boards correctly the first time. And then solder them correctly. Just like we all try to do when we start building any project. Seems easy enough...
- The large majority of these solder pads are very tiny and some of them are very close together. Make sure you have quality, thin gauge solder and fine solder tip. The iron temp should be set to at least 700F and the solder should effortlessly melt and flow into place. If your soldering skills do not land in the scale of really good to exceptional, then you will probably be in for a rough ride sorting out cold solder joints and bridges. Also, I would definitely clean the solder pads before starting this guy just to be certain.
These are all things I landed on after a massive number of attempts ending in failure and frustration. These are by no means the absolute true methods of doing this project, but if you follow the Colourbook and use these tips in conjunction with it, you will have a working unit.
On a side note, I was able get what seemed to be proper metering responses using a VU, 1mA, and a cheap DC type. My build wound up using a 1mA.
Here a link the process I had to go through when I started debugging my initial build. It may have some more information that someone may find helpful.
https://groupdiy.com/index.php?topic=67481.0
Thanks!
Paul
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