Whistle Rock Audio ML12 preamp BUILD/HELP thread!

[dissonantdissident]

Another pic.

 

[mikefatom]

Thanks for posting the pics.

I can't see anything wrong right away.
Can you verify that the gain switch resistors are stuffed correctly?
You can also try swapping the gain switch from a working channel to see if the problem is with the switch.

Let me know what you find.

Thanks,

Mike

 

[dissonantdissident]

Well, I checked for cold solder joints, and reflowed solder over the couple that weren't super shiny, with change in functionality. So I don't think it's the soldering job. I'm attaching a pic of the first card's underside to this, and second one to the next post. Will try swapping the gainswitches, but I checked to make sure that none of the wires touched, etc. They're stuffed right with clean, shiny solder joints, too. I know the circuit isn't very complicated, but I've never done repair diagnostics. I've got a DMM and everything, though. Card one pic below.

 

[dissonantdissident]

Card 2 pic.

 

[mikefatom]

It's hard to tell with the photos.

Try swapping the gain switches and if that doesn't narrow down the problem, we can do a Skype call and I can walk you through some troubleshooting steps.

Keep in touch.

Mike

 

[dissonantdissident]

Oh man, I thought I was pretty much all done, but after racking it, I've noticed loud pops, whenever a switch is engaged. Also, it seems as though the rack case is bizarrely microphonic.  Sounds good, outside of these issues, though! My guess is that I didn't get the grounding scheme right. The groung on the preamp cards are going to the PSU, obviously. I took the ground from the XLRs and tied that to the star Ground point on the PSU, as well. From there to socket. Do I need to take the XLRs to chassis, then chassis to star ground?

Anyway, the ones I did with the CAPI iron and Red25s are sound especially amazing.

 

[mikefatom]

Hey dissonantdissident,

The pops can from the phantom power being on.  Some mics don't draw the same current through pin 2 and 3 of the XLR connection.  This can introduce a DC voltage across the transformer input primaries which then makes a pop when the PAD is switched in and out.

You got the grounding correct.  All pin 1s go to the star ground point.

What do you mean by microphonic though?  What are the symptoms?

 

[dissonantdissident]

Like if I tap the case, it seems to be coming through  the preamps. I'll double check the pops, but it seemed to be whenever I engaged any button, regardless of the phantom power. I don't even think it mattered if there was a mic plugged in, at all. When I get home tonight, I'll try and figure out a way to more accurately describe things.

 

[dissonantdissident]

So, I've been busy with home repair stuff, the last couple months. Anyway, getting back to this project. By microphonic, I mean that you can hear just touching the switches, even if they're not engaged. Even the phase switch. You've got to turn the gain way up and crank the DAW, but it's definitely there. Not really sure what could cause that. I'm going to be moving the PSU out of the case to a separate enclosure, and I'm hoping that it will drop the noise level, which is currently higher than my CAPI vp cards. The channels with the CAPI iron and the RED25 opamps sound really stellar, though. Once I get the PSU moved, I'm hoping the Cinemag/BLU18 channels get to shine, but they've got too much mains hum, right now. Tried some shielding foil around the toroid, but it doesn't really seem to do much of anything.maybe need to get it on there tighter, though.

 

[roger.unc]

Hi Mike - I have the ML PSU bench assembled and I'm getting +54V on the fourth (phantom) rail. The PSU is being powered by a VPT36-1390 2x18V which is actually putting out almost 21V.  I have JP1 set to doubler (rail 4 was showing +78v on tripler) and JP2 set to 1-2.  Rails 1 & 2 are set for -/+18V and rail 3 for +12V.  The adjustment pot does not affect rail 4 output in any way. Any ideas before I start replacing resistors on rail 4 to get the output down to +48V?  Thanks, Roger.

 

[MountCyanide]

How about focusing on that trimmer (adjustment pot) ?
Sometimes I've had  to turn them several times before I see anything move (they have 25 turns).
Or if you'd done that one too many times the wiper can break and you're stuck at a set resistance.
In that case, of course, replace.

 

[roger.unc]

How about focusing on that trimmer (adjustment pot) ?
Sometimes I've had  to turn them several times before I see anything move (they have 25 turns).
Or if you'd done that one too many times the wiper can break and you're stuck at a set resistance.
In that case, of course, replace.

Yep, thanks - the trimmer is on my radar (even though I listen very carefully for the first faint click when adjusting those!).  My initial assumption was that 6 volts over the nominal value of 48v may exceed what can/should be trimmed and may show other issues with the build.  We'll see!

 

[roger.unc]

Yep, thanks - the trimmer is on my radar (even though I listen very carefully for the first faint click when adjusting those!).  My initial assumption was that 6 volts over the nominal value of 48v may exceed what can/should be trimmed and may show other issues with the build.  We'll see!

Replaced RV4 (that was fun!). Rail 4 is still steady at 53.9V.  The voltage out of the bottom pin of RV4 changes when the screw is actuated (e.g., from 4v  to 5v or so) but the voltages on U4 (L to R facing the front of the chip) stay steady at:

Vadj = 44.4v
Vout = 45.2v
Vin = 45.2v

So, maybe a bad voltage regulator? Or, it's fine and I'm going to have to do some math...

 

[roger.unc]

Yep, thanks - the trimmer is on my radar (even though I listen very carefully for the first faint click when adjusting those!).  My initial assumption was that 6 volts over the nominal value of 48v may exceed what can/should be trimmed and may show other issues with the build.  We'll see!

Replaced RV4 (that was fun!). Rail 4 is still steady at 53.9V.  The voltage out of the bottom pin of RV4 changes when the screw is actuated (e.g., from 4v  to 5v or so) but the voltages on U4 (L to R facing the front of the chip) stay steady at:

Vadj = 44.4v
Vout = 45.2v
Vin = 45.2v

So, maybe a bad voltage regulator? Or, it's fine and I'm going to have to do some math...

Indeed, a bad VR.  Per Mike's suggestion, I substituted a TL783 for the LM317 on U4. I'm now trimmed to +48v. Thanks, Mike!

Mouser P/N: 595-TL783CKCSE3

 

[mikefatom]

Hi Roger,

Glad you got it fixed!

Enjoy,

Mike

 

[mikefatom]

So, I've been busy with home repair stuff, the last couple months. Anyway, getting back to this project. By microphonic, I mean that you can hear just touching the switches, even if they're not engaged. Even the phase switch. You've got to turn the gain way up and crank the DAW, but it's definitely there. Not really sure what could cause that. I'm going to be moving the PSU out of the case to a separate enclosure, and I'm hoping that it will drop the noise level, which is currently higher than my CAPI vp cards. The channels with the CAPI iron and the RED25 opamps sound really stellar, though. Once I get the PSU moved, I'm hoping the Cinemag/BLU18 channels get to shine, but they've got too much mains hum, right now. Tried some shielding foil around the toroid, but it doesn't really seem to do much of anything.maybe need to get it on there tighter, though.

For the best SNR and hum reduction, an external PSU is the way to go.  Although I have never had "microphonics" in the preamps I have built, I can image that it could be there given enough gain, dodgy caps and/or loose parts etc...

Have you been able to get to the root cause?

Cheers,

Mike

 

[justinheronmusic]

Are there any Jensen transformers that are compatible with the ML12?

 

[dribro]

Yes, ages to finish these. 8 pres and DI with gar 2520, ea2622 and original 2623 OT. Superb. Metal work always kills me though...

 

[dribro]

Another one

 

[dribro]

Two more..