D-AOC PCBs - the building and help Thread

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Gold

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Potato Cakes said:
With the 500R trimmer installed, I am a bit lost with trying to match the two channels and have that shown on the metering. I can get the same reduction in levels with adjusting both of the trimmers, but then the metering is off, and I am using a nice set of Sifam VU meters. When I adjust the meters to match, then the zero is off. The mechanical zeros are set correctly.  I'm chasing my tail trying to find a way to set these properly. Does anyone have any suggestions on a procedure to set this? Is there any use to adding a trimmer to the stereo link connection?

I’m not sure where the 500R trimmer is. To get a handle on the level and threshold you need two external meters. One meter at the output and one to measure internally.

I started by measuring the level at the input. If both channel match, measure after the input pot. If those match move through the circuit. I made sure that levels were matched when measuring at the input to the Compression pot. 

Then check the compression steps. If the gain reduction when measured at the output matches well enough for you  then adjust the output for unity gain. 

I prefer to know where the threshold is. I think having both the Input control and the compressor control is confusing and redundant. I’d view the Input control more as a compression range extender than an Input control.

When doing this I’d remove all the diodes to make sure they aren’t skewing the results

Once that is done then work on calibrating the meter to match the external measurements.

I didn’t have much luck with stereo link. Judging from other posts I don’t think many have. I’m not using it. I certainly wouldn’t use it while testing individual channels. You don’t want any interaction between channels for testing.

PS if you have pots it might remove some variables by starting with pots.
 

Potato Cakes

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I had seen early in this thread that a 500R trimmer can be installed at the Vactrol + leg to match the two precisely. I agree (now that I've been doing tests) that Input and Threshold are probably redundant, but that's how my panel layout is so I'm going to figure out how to make that work.

The audio levels seem to be match, I guess I'm just wondering how to correct the voltage difference between the meters when there is gain reduction. I realize it's a VU meter and all it's inherent flaws and that it should be used as a reference when using with program material. But the purpose was to be able to use this as mastering compressor, so I would like the meters to match more closely if possible.

I'll mess around more with this after I sort out the buzzing hum.

Thanks!

Paul
 

Gold

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Have you checked the gain reduction with an external meter? You can’t expect the gain reduction meter to be better than the actual gain reduction happening. The meter and gain reduction won’t be well matched over a wide range. I picked a tight match over a small range.  Mine is very tightly matched up to about 4dB gain reduction. After that it goes to hell. I could have done a looser match over a wider range
 

Potato Cakes

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I have been checking with my DAW. The tracking on the VU meters closely matches drop I see in the software, but what I have been messing around with is when they are stereo linked, I'm using only one threshold control which is where the actual gain reduction differs quite a bit from the meters, but still the same level. I vaguely understand why, but not entirely. I'll look at this in more depth later after I figure out the buzz.
Thanks!

Paul
 

Gold

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My experience with the  stereo link matches yours. Judging by other reports  stereo link doesn’t work very well. I never run compressors linked for small amounts of gain reduction
 

Potato Cakes

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The buzz is a little more daunting to find than I had hoped. Tubes have been swapped. Output control switch connections swapped to no avail. The toroidal transformer was moved around as well. I thought the position of the output transformer would be an issue, but connecting channel 1 to the output transformer of channel 2 yields complete silence. Also, with no transformer on channel 2  still gives the same noise I am trying to find. I know that it is happening somewhere after the switch itself as the noise level does not change directly with adjusting the output control. When turned almost all the way up is when the buzz disappears but then comes back at the last two switch positions.

In doing the Kubi mods, I had changed values of Cx02, 04, and 05. I put a 1uF back in C205 just to see if anything would transpire from it but as I thought it didn't change anything.

I should note that the buzz is heard not at a level that one can immediately hear, but when monitoring at higher levels it is plainly audible. So it is usable as is, but channels should match each other in noise performance. I'll poke around some more on the thread and see if anyone else had this issue. I'd love to have this done and in the rack today.

Thanks!

Paul
 

Potato Cakes

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I think I found the issue I read regarding hum/buzz. Reading back through this thread and I saw some people did have a breakdown in the thermal insulation between the LM350 and the heat sink, so I checked mine and there was 1 ohm resistance between ground and the center pin. I removed the screw holding the two together and pulled the thermal pad and separated from heat sink and I was still getting this near continuity. Same with the LM350 removed. Same when I removed the heat sink. When I removed the board completely this went away. I reinstalled it still without the regulator or heat sink and could not replicate the issue. Very bizarre. At any rate, I'm going to have order LM350T and get a bigger heat sink while I am at it. The 1 ohm  between regulator and ground explains why when I disconnected  the transformer from the main PCB, I was getting a 60Hz hum (no buzz) from at the output. This was same for channel 1 but much lower. The only thing I don't understand is why I was not getting any noise on channel 1 and the fact that the noise went away at a particular switch position on the output.

I'll report back in about a week or so when I am able to resume working on this. Hopefully that will be it and then start figuring out how to set it up and interpret the metering versus what is actually happening to the audio.

Thanks!

Paul
 

delaymix

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This might make sense... but if so, the question is, why is there noise in channel 2 and in channel 1 there isn't? Logic tells me that if the noise were generated by LM350 there should be noise in both channels.

My unit works quite well, I built it a long time ago. In general, the compressor is quite quiet, except that the right channel has the highest background noise level than the left. As of today I have not yet discovered what it is that generates it.
I made the mods of Kubi. At the time he gave me many headaches because two of the vactrols came out defective and until I discovered it was crazy. It almost ends my patience.
Nor have I obtain a stereo alignment as satisfactory as I would like, as the measurement of the vumeters does not seem entirely accurate or matched especially when working in stereo link mode.
Perhaps it would be a good idea to try to make a manual of procedures to match both the compression and meters in stereo mode. What do you think?
 

Gold

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My unit has all four channels pretty tightly matched. Level and  frequency response are within 0.1dB of each other. Noise and distortion are within about a dB from channel to channel.  All channels of Gain Reduction are within a quarter dB of each other up to about 4dB gain reduction. After that they start drifting. With 10dB of gain reduction there is about 1.5dB from channel to channel. It gets way worse fast after that.

So far no one has reported success with stereo linking, including myself.
 

living sounds

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My unit stopped compressing on one channel. I switched sidechain tubes, made no difference.

The voltage at R221 won't change despite lowering the threshold, but I can see distortion going up while the amplitude stays the same. Can the vactrols have died (I've got two on each channel via the Kubi advanced mod)?

 

gnarmageddon

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Gold said:
My unit has all four channels pretty tightly matched. Level and  frequency response are within 0.1dB of each other. Noise and distortion are within about a dB from channel to channel. 

I want to book a session at Salt so I can hear a D-AOC at work. Been on the fence about building one. Now if I could just get a project mixed that's vinyl-worthy.
 

Gold

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If you just want to hear it you are welcome to stop by.  My contact information is at saltmastering.com
 

Potato Cakes

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Parts came in, time to report back.

Replacing the LM350 for the heater regulator fixed the hum problem. I mounted it off board and doubled the thermal insulation sheets as well. Now the unit is dead quiet, measuring the same if not better than some of the solid state designs I've built. I still need some replacement festoon bulbs for the meters as I managed to burn my out during initial testing. Then we can rack this up and call it done.

After spending some time with it listening to it and messing around with it, here are my thoughts for this circuit:

First, a quick review of how mine is setup:

Kubi mods - LL1521 1:2.8 input, EA 2503 Hi Nickel output 1:3, Stepped controls, matched/noise tested NOS tubes from Christian Whitmore, Dual VTL5C2/2 Vactrols (thanks again, Paul)

As others have noted, it's not intrusive on the audio it is affecting, especially in the extreme low end, which is something I have had problems with other compressor circuits. It also requires to really listen to what is happening as the meters only seem to indicate that something is happening and not a real accurate gauge of the gain reduction taking place, but this has been the case for as long as VU meters have been in existence, at least in audio. Using a 500R trimmer position leg of the vactrol (with the dual setup it is the leg closest to Vx02) I am able to match the gain reduction to each channel when linking them using tone and when listening to program material after this adjustment I can not detect any shift in the stereo image. The metering however is not as closely matched as discussed in this thread. It seems that replacing R121 with a 10k trimming would allow the metering to be adjusted for gain reduction being displayed, unless I am reading the schematic wrong, which is always a possibility. For my purposes, I think I'm just going leave it as I know the actual audio is matched.

Also with stereo linking, I found it useful to turn one channels threshold all the way or almost all the way off and then just use one threshold control. This seems to give a more detailed range over the gain reduction.

The dual vactrols seem to make the differences in threshold/slope mods less apparent. The no mod threshold is much lower and almost negates the need for any sort of additional mods. I was debating between the single or dual vactrol and I ended on the dual. I sometimes think having a board to switch between the two would be handy, but for this circuit having a bunch of options is not needed for myself when it's primary role will be for mastering. I do hear some changes with the threshold mods as posted at the beginning of this thread, but not nearly as apparent as I was expecting, so more tests will be in order down the road to see if they make any difference for my purposes.

I chose a 1:3 transformer but I think a 1:4 as prescribed might be more suited as I find myself wanting a little more gain at the top of the range. R125 might be adjusted for more output as this appears to be a feedback resistor. Someone please correct me if this is not true.

All of my listening tests were with program material and using the DAOC for individual tracks may produce difference opinions. I'll post a pic when the meter lamps are replaced.

After three years of collecting dust, I'm mostly glad it's just done.

Thanks!

Paul
 

weiss

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living sounds said:
My unit stopped compressing on one channel. I switched sidechain tubes, made no difference.

The voltage at R221 won't change despite lowering the threshold, but I can see distortion going up while the amplitude stays the same. Can the vactrols have died (I've got two on each channel via the Kubi advanced mod)?

are you using ne2 lamps?
 

Potato Cakes

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Finally, finally, finally done. After years of the completed board sitting around and collecting dust, it's now finally done with working VU lamps and in the rack. Thanks for everyone's help on this. I've already been able to put it to use and it's going to be a great compliment to the rest of the DIY gear in my rack, which is pretty much all of the gear in the rack.

Thanks!

Paul
 

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