critterkllr

I put the pot in place, but I’m just having a really difficult time following the calibration instructions. 

If I’m using pro tools to send a sine, do I set it to -30db in the signal generator? If so, do I set it to Pk-Pk or RMS?

Or do I measure voltage using a DMM and adjust it to whatever hits the mark? Do I measure Pk-Pk or RMS?

Do I measure from the test point or +input to ground? Or between the + and - of the output?

I have pro tools, a DMM, and an oscope.
« Last Edit: May 08, 2020, 12:55:43 PM by critterkllr »


jsteiger

I put the pot in place, but I’m just having a really difficult time following the calibration instructions.  I’m the third person this has been passed along to that doesn’t get it.

If I’m using pro tools to send a sine, do I set it to -30db in the signal generator? If so, do I set it to Pk-Pk or RMS?

Or do I measure voltage using a DMM and adjust it to whatever hits the mark? Do I measure Pk-Pk or RMS?

Do I measure from the test point or +input to ground? Or between the + and - of the output?

I have pro tools, a DMM, and an oscope. How do I do this? Would Hairball Audios instructions work for this?
Someone posted DAW tips a while back on this thread.
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critterkllr

Someone posted DAW tips a while back on this thread.

I can’t find any actual tips. Just lots of issues calibrating.

Okay, so I’m goin to be as detailed as I can be. I am using a psu with a db25 connector, and a DIYRE extender to lay the module out.

I’m sending a 1k sine wave from pro tools. I’m using an oscope to measure the +/- +4 input pins on the extender and am getting .024 VAC RMS.

Once verified, I take the extender and plug in the compressor.

When I look in Pro Tools, that monitoring signal is too low to make any sense of it. So I boosted the bypassed signal up to -3dbvu using the VU style meter. When I power on the unit, it jumps to around +.25dbvu.

Am I in the ballpark? Your instructions say I should have a massive jump from -30dBu way up to +7.75dBu. So am I way off?

critterkllr

Okay, I made some progress. The person that built it soldered a lead into TP9. It was bent down and touching something.

Now, when I switch from bypass I go from -49 to -11 in my DAW. If my math is correct, that should mean it’s calibrated to -19 dbfs and that should be the same as -30 to +7.75! Woohoo!

Got the fet into it’s conductive range and it’s down by 1db.

Meter is calibrated and she is good to go! Hard to believe I'm the third person to look at this and it took so long. Thanks, Jeff.
« Last Edit: May 08, 2020, 05:12:50 PM by critterkllr »

jsteiger

You can easily measure ACV at the output of your DAW signal generator) to verify the -30dBu level. Simply hook your DMM's red probe to XLR pin 2 and black probe to XLR pin 3. To test the response of your meter, 1kHz @ 0dBu = .775V (RMS). If your meter is not measuring .775V AC check it at 400Hz. Not all DMMs can accurately measure 1kHz.

For reference:
0dBu = .775V AC (RMS)
-30dBu = 24.5mV AC (RMS)

Here's a helpful calculator
http://www.sengpielaudio.com/calculator-db-volt.htm
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GrantB

FC526 XFMR here, it "works" but has low signal levels throughout.  TP1 = 4.94mV with input -30dBu/24.49mV balanced.  Trace all the way back to the inputs, ~9.8 mV on both + and - input all the way back to the edge connector.  Same result with rack powered off.  Disconnect card and pins 8 and 10 of the extender return to 12.24 mV.

Power pins and PRs show correct voltages.  Have tried different slots.

Bad relay or transformer winding?

jsteiger

FC526 XFMR here, it "works" but has low signal levels throughout.  TP1 = 4.94mV with input -30dBu/24.49mV balanced.  Trace all the way back to the inputs, ~9.8 mV on both + and - input all the way back to the edge connector.  Same result with rack powered off.  Disconnect card and pins 8 and 10 of the extender return to 12.24 mV.

Power pins and PRs show correct voltages.  Have tried different slots.

Bad relay or transformer winding?
Possible misplaced resistors around the input transformer or 2 transistor discrete follower. Or maybe something is wrong with the relay circuit and both +/- legs are not being switched over properly?
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GrantB

Possible misplaced resistors around the input transformer or 2 transistor discrete follower. Or maybe something is wrong with the relay circuit and both +/- legs are not being switched over properly?

To isolate the problem, I removed the input relay, jumped the signal across the relay I/O pins, and removed the input transformer.  Same resulting AC voltages on the inputs as before.  Now only R71, R72, R73, C39, and C38 are in circuit , and they all measured  in spec with no DC conductance to ground. 

At this point, I could only conclude that my interface (Sound Devices MixPre 10T) is incapable of driving  this resistor network without drooping ~2mV rms, despite the professional-sounding spec of +18dBu balanced outs.

So after putting it all back together, I buffered my test signal through a ML2 and voila, TP1 = 5.94 mV, unit is outputting +7.75 dBu.

Now I am left with two questions:
1. What is the intended input impedance of this compressor?
2. Can my input resistors be increased to be more compatible with my interface while maintaining compatibility with transformer balanced outputs, and if so with what values?

jsteiger

The input Z of the FC526 XFMR is about 4k5. Any respectable pro audio device should be able to drive that like it's not even there.

I ASSume your interface would sh!t the bed when looking into an LA2A or 1176 or anything with a 600 ohm input impedance?

I would not change anything on the comp. Just slightly increase the level from your interface.
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GrantB

The input Z of the FC526 XFMR is about 4k5. Any respectable pro audio device should be able to drive that like it's not even there.

I ASSume your interface would sh!t the bed when looking into an LA2A or 1176 or anything with a 600 ohm input impedance?

I would not change anything on the comp. Just slightly increase the level from your interface.

Thanks Jeff.  Yeah I just started playing with REW and so far this interface has been a bust for that use.  I've been using a synthesizer through a DI box and my multimeter to calibrate 500 modules so far with no problems.


 

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