BA500v2 Mic Pre Amp BUILD THREAD 500/51X

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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by Joe Malone »

adalogue wrote:
Mon Apr 25, 2022 3:14 pm
One other thing I noticed, with DI jumpers in, if input pad switch is down, l get 440 ohms between xfmr green and 0v and still a short between xfmr blue and 0v. These tests were done without the pots plugged in.

Some of the resistors got hot when I was trying to solder the switches in, I should have pulled the board to finish them. I lifted one end of the ones that looked like they got hot and they tested OK (Rpad and two 470R).
The soldering pads below the green/black text have soldering going over the ground plane which is bad as any hole in the solder mask will give a dead short to 0v. Also a couple of the 10 pin black DI header soldering do the same by the looks.

Is the black DI header also fitted the correct way around?

Do you have the white plastic bushes between the JLM14 and the pcb?
Joe :-)
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adalogue
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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by adalogue »

Joe Malone wrote:
Mon Apr 25, 2022 5:05 pm
The soldering pads below the green/black text have soldering going over the ground plane which is bad as any hole in the solder mask will give a dead short to 0v. Also a couple of the 10 pin black DI header soldering do the same by the looks.

Is the black DI header also fitted the correct way around?

Do you have the white plastic bushes between the JLM14 and the pcb?
I desoldered the green pads, short remains. Also inspected the 10 pin headers and look ok.
Yes, DI header is correct orientation
Yes, white plastic bushings are installed.

I can get the short to move between green and red wires with phase switch. I desoldered phase switch pins but issue remains. I’ll keep trying to trace out the rest of the phase switch connections, I suspect the issue is somewhere around there. Even with a magnifying glass I’m having a hard time following the traces.

If you could shoot me this section of the schematic I would be ever so grateful for further ohm’ ing.
Adam
Seattle, WA

adalogue
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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by adalogue »

Progress! Traced the issue back to J1 header. Desoldered J1 ribbon connection and GND short went away. Ohm’d the ribbon cable and found two shorts in the ribbon cable. Re-terminated the ribbon cable and GND short issue was fixed! Now getting nice balanced input impedance with good variation from Z pot.

However, I can’t decrease the VU meter trim pot enough. Even +4 dBU out pins the needle. I also have asymmetrical voltages on Vout+ and Vout-. Output impedance is good (200 ohms @ 1 kHz total, 100 each leg), and the transformer appears to be connected properly (1+ = in, 1- = GND, 2+ = Vout+, 2- = 3+, and 3- = Vout-). Scope image below. Ch1 = 1+, Ch3 = Vout+, Ch4 = Vout-. I doubled check all my resistor values around the VU buffer. Thanks Joe!
FF0015A6-A1CD-4DE1-A171-0692414F3559.jpeg
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Last edited by adalogue on Tue May 03, 2022 4:04 pm, edited 1 time in total.
Adam
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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by Joe Malone »

adalogue wrote:
Mon May 02, 2022 10:20 am
:roll: Progress! Traced the issue back to J1 header. Desoldered J1 ribbon connection and GND short went away. Ohm’d the ribbon cable and found two shorts in the ribbon cable. Re-terminated the ribbon cable and GND short issue was fixed! Now getting nice balanced input impedance with good variation from Z pot.

However, I can’t decrease the VU meter trim pot enough. Even +4 dBU out pins the needle. I also have asymmetrical voltages on Vout+ and Vout-. Output impedance is good (200 ohms @ 1 kHz total, 100 each leg), and the transformer appears to be connected properly (1+ = in, 1- = GND, 2+ = Vout+, 2- = 3+, and 3- = Vout-). Scope image below. Ch1 = 1+, Ch3 = Vout+, Ch4 = Vout-. I doubled check all my resistor values around the VU buffer. Thanks Joe!
FF0015A6-A1CD-4DE1-A171-0692414F3559.jpeg
Crazy about the cable as we have made a jig to fit our ribbon cable tester to take the solder in end but looks like a rejected cable made it into a kit sorry.

Transformer output with no reference resistors to the ground means only the internal capacitance to the primary give a ground reference so you will always have what seems like different output levels on a floating transformer out until it has resistance on +out and -out to ground then everything will measure with the same output. Basically, this is 100% fine is to add to 10k to 0v your measurements will be even.

Does the 25 turn VU trimpot have 104 on top of it? Can you send me a photo of the top of the BA500 pcb with no DOA opamp in so I can check it.
Joe :-)
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adalogue
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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by adalogue »

Joe Malone wrote:
Tue May 03, 2022 10:11 am
Transformer output with no reference resistors to the ground means only the internal capacitance to the primary give a ground reference so you will always have what seems like different output levels on a floating transformer out until it has resistance on +out and -out to ground then everything will measure with the same output. Basically, this is 100% fine is to add to 10k to 0v your measurements will be even.

Does the 25 turn VU trimpot have 104 on top of it? Can you send me a photo of the top of the BA500 pcb with no DOA opamp in so I can check it.
Good call! I neglected to consider the high Zin of my scope. I added two 10k resistors a crossed each channel and indeed it looks better! Note in this photo I’d have Ch4 inverted so I can use the Math function to add the signals for total RMS.

Yes, trim pot has 104 on top. Photos follow!
70779F17-4946-4533-9268-B650B1955873.jpeg
445F6717-FC7E-4453-8166-2DDDEA861B94.jpeg
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Adam
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adalogue
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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by adalogue »

Joe:

I believe my gain resistors are correct, I've verified 60.5 dB max gain @ 1kHz (Zin @ detent). Is it going to mess anything up if I change a resistor value in the VU meter buffer stage to reduce the signal level feeding the VU meter? Since the unit clips at about +28 dBU, I'd like to decrease the minimum VU meter sensitivity range by about 24 dB. If you can propose a resistor substitution (ID and value) I'd really appreciate it.
BA500V2.png
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Adam
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toneflake
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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by toneflake »

Dear Joe
I'm building kit. want to see pics but web don't show them. I change browser but don't work.
I need info like BA500 previous version...PCB are almost finished.

BA500 vr2 PCB looks need 9 pcs of 10ohm, but in kit parts included 10pcs.

please advise me.

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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by Joe Malone »

toneflake wrote:
Mon May 16, 2022 7:25 pm
Dear Joe
I'm building kit. want to see pics but web don't show them. I change browser but don't work.
I need info like BA500 previous version...PCB are almost finished.

BA500 vr2 PCB looks need 9 pcs of 10ohm, but in kit parts included 10pcs.

please advise me.
Hi Did you solve the browser photo issue? As long as you are logged into the forum the photos should show and clicking on them they will enlarge them. Just tested them on all latest browsers and it all works here. You will often have some spares like extra 10R etc. Let me know if you are still having problems.
Joe :-)
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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by Joe Malone »

adalogue wrote:
Mon May 16, 2022 3:40 am
Joe:

I believe my gain resistors are correct, I've verified 60.5 dB max gain @ 1kHz (Zin @ detent). Is it going to mess anything up if I change a resistor value in the VU meter buffer stage to reduce the signal level feeding the VU meter? Since the unit clips at about +28 dBU, I'd like to decrease the minimum VU meter sensitivity range by about 24 dB. If you can propose a resistor substitution (ID and value) I'd really appreciate it.
BA500V2.png
The normal trimpot range is roughly centered for 0VU = +4dBu so not sure why yours is not adjusting correctly.
Joe :-)
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adalogue
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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by adalogue »

Joe Malone wrote:
Mon May 30, 2022 7:55 pm
The normal trimpot range is roughly centered for 0VU = +4dBu so not sure why yours is not adjusting correctly.
Joe:

Possible to get typical voltage values at VU buffer IC pins for +4 dBU output?

Adam
Adam
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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by Joe Malone »

adalogue wrote:
Tue May 31, 2022 6:30 am
Joe Malone wrote:
Mon May 30, 2022 7:55 pm
The normal trimpot range is roughly centered for 0VU = +4dBu so not sure why yours is not adjusting correctly.
Joe:

Possible to get typical voltage values at VU buffer IC pins for +4 dBU output?

Adam
Pin 5 is the input to the Vu buffer coming directly from the JLM99v output via 33R in series with 470uF cap.
Due to the output transformer 1:2 ratio, it is 6dB down from the output.
So if you have 1.23vac (+4dBu) on the output you should roughly have 0.6vac (-2dBu) at pin 5 of TL072.
If trimpot is set to be 100k (Gain of 1.1) then pin 7 will be roughly 0.66vac and so will pin 1 but out of phase to pin 7.
If trimpot is set to be 10k (Gain of 2) then pin 7 will be roughly 1.23vac and so will pin 1 but out of phase to pin 7.
The 2 opamp outputs feed 2 BAT85 diodes for rectification then a series 1k resistor to the meter.
Joe :-)
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Alex1990
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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by Alex1990 »

Hello Joe

Can I just check the pot wiring for the BA500 when using the JLM14. I can’t quite make out from the pics with 100% certainty what’s going on and don’t want to go wrong!

Do we have a seperate black wire going between the middle legs of each pot, and are we joining up each of the three pairs of legs on the alpha pot?

And the middle & bottom legs on each pot connected with a thin wire in the picture? Then the black wire from GB board to the middle leg of gain pot… let me know if I’ve missed something!

Also just a small noob question regarding the orientation of the BD681 transistor on the FETDI board? Think it’s text facing inwards but should check!

Thanks a lot :)

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Re: BA500v2 Mic Pre Amp BUILD THREAD 500/51X

Post by Joe Malone »

Alex1990 wrote:
Mon Jun 06, 2022 3:37 am
Hello Joe

Can I just check the pot wiring for the BA500 when using the JLM14. I can’t quite make out from the pics with 100% certainty what’s going on and don’t want to go wrong!

Do we have a seperate black wire going between the middle legs of each pot, and are we joining up each of the three pairs of legs on the alpha pot?

And the middle & bottom legs on each pot connected with a thin wire in the picture? Then the black wire from GB board to the middle leg of gain pot… let me know if I’ve missed something!

Also just a small noob question regarding the orientation of the BD681 transistor on the FETDI board? Think it’s text facing inwards but should check!

Thanks a lot :)
Yes all 3 pairs join together. One Black wire going to the bottom 4 x Z pot terminals and bottom 2 x gain pot terminals. Red wire going to top 2 x Z pot terminals. Yellow wire to top Gain pot terminal.
Pot wiring for JLM14 (1).jpg

The BD681 metal side is shown by the thicker white line on the overlay. Metal side is down away from the relay.
BA500v2 angle with DI fitted (1).jpeg
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Joe :-)
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