[BUILD] CAPI FC526~500 Series~FET Limiter Kit~Official Support Thread

[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.

 

[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.

 

[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.

 

[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

 

[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?

 

[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.

 

[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.

 

[Yannis]

I was OK with the built and the use of both the VP312 and the VP28 which are really nices units,  but I'm more in trouble with the FC526XFMR.

I had two problems first. Frist I fried the DTO5 with a bad testing move. I replaced damaged resistors and transitors and it seems OK now. I have put 5089 to match the Hfe of provided 5088, my 5088 have a lower Hfe
The other issue was leds went crazy. I figured out there was a shortcut in the ribbon cable so I soldered it directly on the board and it seems OK too

But it's always not OK the beginning of the audio path seems to be OK :

TP1 : 5.7 mVAC (expected : 5.8)
TP2 : 123 mVAC (expected : 117.7)
TP3 : 983 mVAC (expected : 939) (maybe hot)

TPP 9 : 9.99 V DC so OK
MTR point #11 : 2.5 V DC so OK

But I have nearly nothing in TP4 and next.

The output is a little hot : Module's outtput : 8.09 dBu (expected : 7.75) maybe because of the 5089 transistors

And the release pot leads to a big strange volume reduction when it goes towards CCW

What does I need to check between TP3 and TP4 ? A similar question had been requested in the post #444 of this thread but I can't find anwers.

Help would be much appreciated thanks !

 

[Dr.Weezil]

I'm going to jump in with my most pressing concern and likely roll into other possible issues with my builds.

I've got two (2) FC526-XFMRs completed and started on the calibration process this morning.  I did not get very far and have given up for the day.

My first concerns have to do with getting the correct signals into the units and monitoring the output correctly.  I only have Pro Tools and a Fluke 117 meter.  I have adjusted the Advanced Meter Type Settings so that 0dBu = -18dBFS.  For Q Bias of VVR, I thus set the output of my signal generator to -48dB (producing a graphical meter reading of -30dB) and matched the incoming signal out of the bypassed FC526 #1 through my mixer/mBox Mini to produce a matching -48dB.  With input and output pots set to max, attack and release at the fastest, 20:1 selected, the output was around 7.1-7.2dBu (or -10.8 to -10.9dB).  Turning RV2 CW until it did not change did very little, the steady level was about -7.2dBu.  I opted to set the bias voltage per the reading from the plastic bag and dialed in RV2 by measuring the DC voltage at TP7.

The second FC526 gave an output right in the correct range per the Calibration guide 7.8dBu or -10.2dB.  I double checked against the bag value at TP7 and tweaked to match.

I then moved onto the meter calibration.  Going back and forth between RV8 and RV1 does not appear to be getting the meter to calibrate per the guide's instructions.  If I set RV1 to the specified 2.487V at TP10, all of the LEDs go out, as is to be expected.  The instructions for then adjusting RV8 have the opposite effect on both of my units; with the GR toggle on, turning RV8 CW increases the measured voltage at TP10, while turning it CCW decreases the voltage.  Decreasing the voltage also makes the LEDs come back on.  This is opposite of what the directions say needs to happen.  Further, there is no appreciable difference in DV voltage at TP10 when GR is turned on or off.  I'm left going back and forth, adjusting RV1 to 2.487 and RV8 to 2.252.

Basically, I'm not convinced I'm performing the calibration procedures correctly and would appreciate any help either Jeff or anyone else in the thread could offer.

Thanks.

EDIT:

I identified a resistor error, 470R/4k7 swap, in the DTO5 of unit #1.  The input transistors were running hot and sure enough, there it was.  Will be fixing that issue and hopefully will permit easier biasing of that unit.  #2 is fine for biasing.  Meter is still a question mark for both.

EDIT 2:

I do not believe gain reduction is working on either unit.  I realized that the meter calibration was failing because there was no input signal being attenuated by the unit.  Input and output controls end up at 1 and 10, respectively, but do not yield a 7dB reduction of input.  Essentially input and output are the same with GR switched to on.  Ugh.

EDIT 3: TP09 value for each unit: -9.99VDC, -10.04VDC.

 

[geardude]

on a scale of one to boned would I be if I fired up this module upside on the jig?

Didn’t sound right and quickly unplugged and re orientated..notifying quickly the error of my ways....fired it up no smoke & the pre calibration test points on the meter board check out however when going into Qbias and flipping to active but it’s overloading instead of hanging around -5dbfs.

Going to start going through the board checking for any resistors that may be out of place but wanted to know if any particular area I should check/would’ve been blown since I flipped it around like a dumb dumb 

 

[Yannis]

Hello !
What should I test between TP3 and TP4 ? which one is the HP filter capacitor ?

 

[ChristianFrentzen]

Hi everyone,
I'm super thrilled that I've managed to put together 2x Capi VP28, 2x LC25 and 2x FC526 xfmr. The VP28 and LC25s already made for quite some big and "thick" sound.
I did find a solder bridge on one of my FC526 - which at first made two R10 resistors to smoke a little, but they still work. I turned the unit off immediately and OpAmps are fine, too. Now both units work and I've already tested some audio being compressed. I am waiting to get a new DMM delivered which can measure as low as 0.1mV - so I can see what's happening at all TPs and calibrate both compressors. I am using a website (szynalski) to generate some test tones running through the modules. When it's all calibrated I will run some tracks through it and make an extensive youtube video, because I really think there is a great demand for it.

I am really happy that I stumbled across Jeff Steiger's amazing gear, so thank you for making this really awesome gear!

Oh btw - I don't own a extender so I can run the module outside my lunchbox. Some TPs can be reached without an extender, and soldering 2-3 leads to those testpoints which can't be reached might do the job for the calibration. I'm in Germany and I'm not sure where to get a suitable extender. If you have a recommendation, I'd be very happy!

Have a great one!
Christian

 

[Yannis]

Lucky you ! better if this one works the first time ! ;D

I asked myself this question and I would go for the XT500 Lunchbox connector extender from soundskulptor in our geographic area.

Not ordered so if someone knows another good one in Europe fell free to tell us !

 

[ChristianFrentzen]

Ok... So I'm pretty much trying to calibrate my two FC526 xfmr units.

As far as basic currents go there seems to be everything pretty ok. When in bypass everything is running through, input level and output level are the same. At TP9 I do have 9.99V on one unit and 10.06 at the second unit. It should only be between -9.98 and -10.02. Why could this be?

Also when calibrating the gain reduction meter I have my input and output knobs set at very different positions than what the guide says. It's more like Input 10:00 and Output 12:00. "Or if not, adjust the knobs as needed".

For making some measurements a little easier - regarding my audio levels - I'm assuming the following:
knowing that at 0dBFS my interface sends an output voltage of +13dBU I will know from there that my dBs can be translated. So if I am expecting a signal of lets say +7dBu that means my output fader should be at -5dB, right?

 

[ChristianFrentzen]

Alright. I got everything solved. It takes a little time to get familiar with the compressor, its TPs and voltages. My new DMM was needed to really see what's happening here.

Unit #1
TP1: 5,9 mV AC
TP2: 120,7 mV AC
TP3: 964 mV AC (slightly higher than what the guide says)

Unit #2
TP1: 6mV AC
TP2: 120,9 mV AC
TP3: 965 mV AC (same as with Unit1)

Also I get the following on both units:
TP9: -10,06V DC (it's a little higher than I expected - but
Meter Resistor String „Top“: 2,501V DC (excellent!)


So I calibrated both units just now and put them in a listening test. I love the way they sound!!!
I might let them run for a while and then run another calibration just to make sure they are properly calibrated. I don't know if this should be done every once in a while...

Hey Jeff, you making amazing gear. Really.

 

[ChristianFrentzen]

I've encountered something when comparing both FC526 xfmr units that I buildt.

I sent test tones through both compressors. When in bypass, input and output are the same on both units - what is expected. When I activate both units, they deliver exactly the same output level. But that changes when I turn on GR - one unit "grabs" the signal a little earlier. I can see this visually while turning up both inputs at the same time and I can hear it. 2-3 LEDs are going on fairly quickly on one unit while the other one's LEDs are coming on slower. So when both units are starting to reduce gain I am seeing that there is a 1,5-2 dB difference between both output levels. I've tried the calibration a few times. I tried it the way it was described in the guide, and I can easily get a 7dB GR and then adjust the trimmers to achieve the described target voltages at TP10 (2.245V DC with GR and 2.487V without). It's not too bad and I can easily compensate the slight 1,5-2dB differences in output with my compressors. But I am intending to use them for stereo audio and I guess if everything is done correctly then levels should be similar. Is there anybody who knows what this could be?

 

[ChristianFrentzen]

Seems like one compressor is less "sensitive" when it comes to GR. I sent some rhythmic material through both units and I can see that one comp is working very smoothly - I can easily turn the input from almost nothing to full on and LEDs are bouncing and increasing as I turn the input up.

On the other unit (same signal) there is a lot of range where no LEDs are moving - and then suddenly it's super sensitive. Any help would be appreciated!