Potato Cakes
Well-known member
All righty then,
I went ahead and built out the extra board I had laying around, put it in and it closely matched the other one right out the gate, like I knew it should since all the parts were the same. This wound up being easier than trying to replace parts on the other one. The boards are obviously fine, I just either have a bad part or I missed finding a misplaced one and I felt a fresh start, at least for my time and sanity would be best. Maybe some day I'll feel the need to replacing all of the diodes and transistors on the board I messed up to get it working. But not today.
So now that I have both working close to each other, I have a bit more precise way to match the distortion for both channels. For measuring I used the Blue Cat free analyzer that shows distortion. After you do the Fine Tuning I step and install the properly calculated values for R7 and R8, put trimmers on R56 and R22 for both channels as per the instructions starting with the stock BOM values. Make sure match the values between the corresponding resistor numbers. Set the release for both channels to full CW and with the auto release engaged (if installed) as I found that the amount of distortion and which harmonics are emphasized changes with the position of the Release. In this position, I found it easier to match both with having a descending level of odd harmonics, which is what I preferred in this circuit. I put the Attack full CCW for both just to be sure, but this doesn't actually seem to change distortion levels. Both input and output controls are set to 12 o'clock. Because I said so.
Now we fire up the tone generator and route signal through the Blue Cat analyzer while also keeping an eye on the output level. For me, the distortion for both were already pretty close, with one channel about 2dB lower in volume. This is where your skills of finesse and patience will be needed. As per the build manual, R22 will change the distortion and the peak to peak output, and R56 will change output while having a different effect on distortion. Pretty sweet, right? The idea is to get the distortion waveforms to match and to have the same output, and this will require you to make slow adjustments and making a mental note of what is affecting what. Patience and finesse. So as in the build manual, you start by determining which one is least distorted or the you like the best. You might know this from simply looking at the waveform. Or you may have to turn off your generator and play some program through it (don't change the settings on the panel!) and listen to which one sounds "better". Then you go back and match the waveform and level of the "bad" channel to the good one. What you might run into, as I did, is that the channel having R22 and R56 being adjusted to match the other may not be able to get enough level so both outputs are even. For me this meant 0.5dB, which is fine when listening back but not when you are OCD about matching meters and numbers. If this is the case, you would lower R56 just enough on the higher output channel so that both have equal output and then double check your waveform.
Playing back program material in THD mode retained the stereo image much better than before without any noticeable shift to instruments that are meant to be centered. And if you want to change the harmonic content of the distortion, you can change this after the fact by adjusting the Release control and engaging the Auto Release if that is installed, so you are not stuck with the attributes of the distortion after you close the lid.
This circuit reminds me to a fair degree of sameness to a Chandler Zener Limiter that I rented years ago. Not particularly suited for mastering/program material (which is what I rented the Zener to do but was unable to smoothly do), but definitely for some aggressive drum bus and guitar/bass tracks. I haven't messed with vocal tracks through this yet.
It should be noted that I went overboard matching D1/D3 and D2/D4 and kept them within 0.001VDC of each other for both 1k and 10k measurements and not too far from that between the pairs themselves.
Last couple of things for me to work out: I still get pops with switching the relays as before. It spikes the meter, but the actual sound is more like a clock switching noise and not a tremendous explosion as the display would suggest. Still needs to be sorted. The additional 100k from the middle pin Bypass 2 header to ground didn't seem to do anything for me in this regard. I also have the radio station thing to sort out. One channel has slightly more interference and after swapping positions inside the case, it was clear it has to do with what is mounted on the board and not the switches or cabling. One thought is that when I did my distortion calibration I was at too high of a level that is raising the noise floor. I get about +7dB from unity with the THD engaged and both input and output knobs set to 12 o'clock. I am using stepped controls with 11 positions. I could readjust R56 to be at unity and lower the S/N ratio in theory, but that wouldn't solve the mystery of the radio interference. However, in its current state the static interference is pretty faint and can only be heard when there is no signal being generated and the volume of the speakers are turned up. In use, I don't even hear it when it's processing audio and even if I don't figure this out it will still be for processing tracks. So for now, I've put the lid on it and I'll get back to tinkering with it after I finish the last build I have on my bench.
If anyone has any suggestions for trying to sort out the faint radio static I would appreciate it. I know it's somewhere between the bypass connections, as it is perfectly clean when the circuit is disengaged.
Really great project and I'm glad I finally got my mine up and running.
Thanks!
Paul
I went ahead and built out the extra board I had laying around, put it in and it closely matched the other one right out the gate, like I knew it should since all the parts were the same. This wound up being easier than trying to replace parts on the other one. The boards are obviously fine, I just either have a bad part or I missed finding a misplaced one and I felt a fresh start, at least for my time and sanity would be best. Maybe some day I'll feel the need to replacing all of the diodes and transistors on the board I messed up to get it working. But not today.
So now that I have both working close to each other, I have a bit more precise way to match the distortion for both channels. For measuring I used the Blue Cat free analyzer that shows distortion. After you do the Fine Tuning I step and install the properly calculated values for R7 and R8, put trimmers on R56 and R22 for both channels as per the instructions starting with the stock BOM values. Make sure match the values between the corresponding resistor numbers. Set the release for both channels to full CW and with the auto release engaged (if installed) as I found that the amount of distortion and which harmonics are emphasized changes with the position of the Release. In this position, I found it easier to match both with having a descending level of odd harmonics, which is what I preferred in this circuit. I put the Attack full CCW for both just to be sure, but this doesn't actually seem to change distortion levels. Both input and output controls are set to 12 o'clock. Because I said so.
Now we fire up the tone generator and route signal through the Blue Cat analyzer while also keeping an eye on the output level. For me, the distortion for both were already pretty close, with one channel about 2dB lower in volume. This is where your skills of finesse and patience will be needed. As per the build manual, R22 will change the distortion and the peak to peak output, and R56 will change output while having a different effect on distortion. Pretty sweet, right? The idea is to get the distortion waveforms to match and to have the same output, and this will require you to make slow adjustments and making a mental note of what is affecting what. Patience and finesse. So as in the build manual, you start by determining which one is least distorted or the you like the best. You might know this from simply looking at the waveform. Or you may have to turn off your generator and play some program through it (don't change the settings on the panel!) and listen to which one sounds "better". Then you go back and match the waveform and level of the "bad" channel to the good one. What you might run into, as I did, is that the channel having R22 and R56 being adjusted to match the other may not be able to get enough level so both outputs are even. For me this meant 0.5dB, which is fine when listening back but not when you are OCD about matching meters and numbers. If this is the case, you would lower R56 just enough on the higher output channel so that both have equal output and then double check your waveform.
Playing back program material in THD mode retained the stereo image much better than before without any noticeable shift to instruments that are meant to be centered. And if you want to change the harmonic content of the distortion, you can change this after the fact by adjusting the Release control and engaging the Auto Release if that is installed, so you are not stuck with the attributes of the distortion after you close the lid.
This circuit reminds me to a fair degree of sameness to a Chandler Zener Limiter that I rented years ago. Not particularly suited for mastering/program material (which is what I rented the Zener to do but was unable to smoothly do), but definitely for some aggressive drum bus and guitar/bass tracks. I haven't messed with vocal tracks through this yet.
It should be noted that I went overboard matching D1/D3 and D2/D4 and kept them within 0.001VDC of each other for both 1k and 10k measurements and not too far from that between the pairs themselves.
Last couple of things for me to work out: I still get pops with switching the relays as before. It spikes the meter, but the actual sound is more like a clock switching noise and not a tremendous explosion as the display would suggest. Still needs to be sorted. The additional 100k from the middle pin Bypass 2 header to ground didn't seem to do anything for me in this regard. I also have the radio station thing to sort out. One channel has slightly more interference and after swapping positions inside the case, it was clear it has to do with what is mounted on the board and not the switches or cabling. One thought is that when I did my distortion calibration I was at too high of a level that is raising the noise floor. I get about +7dB from unity with the THD engaged and both input and output knobs set to 12 o'clock. I am using stepped controls with 11 positions. I could readjust R56 to be at unity and lower the S/N ratio in theory, but that wouldn't solve the mystery of the radio interference. However, in its current state the static interference is pretty faint and can only be heard when there is no signal being generated and the volume of the speakers are turned up. In use, I don't even hear it when it's processing audio and even if I don't figure this out it will still be for processing tracks. So for now, I've put the lid on it and I'll get back to tinkering with it after I finish the last build I have on my bench.
If anyone has any suggestions for trying to sort out the faint radio static I would appreciate it. I know it's somewhere between the bypass connections, as it is perfectly clean when the circuit is disengaged.
Really great project and I'm glad I finally got my mine up and running.
Thanks!
Paul
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