Ad VU meter api 312

[ruffrecords]

[My lasting memory is that it had a funny smell] -- Might have been the fish.....

/

Now you mention it, the smell was vaguely fishy. It was very distinctive. I would definitely know it if I smelled it again.

Cheers

Ian

 

[djmiggymigz]

Hey yall Im back!! Got my 312 circuit dialed, ordered and built! Only issue i'm now having is with the VU meter! So I ended up using this buffer circuit courtesy of @zzzzz and I took the chance with some cheapo meters from Aliexpress.

https://groupdiy.com/threads/wtb-vu-buffer-driver-pcb.82932/page-2#post-1107221

So the buffer circuit is powering the lamp of my meter perfectly however, I cant seem to get any signal reading! When I power on the unit the meter jumps as they usually do and with everything absolutely cranked I'm getting just a very very slight amount of reaction from the meter. Reading back thru this thread, I saw some say that with cheap VU's they were needing upwards of +10dBu of signal (4 Volts). Could this be where my issue is as well? I've tried adjusting the VU gain trimmer (feedback of the TL072) all which ways and still nothing. The schematic calls for a 5K trimmer there. Should I maybe up that value to be able to push the opamp even more?? Perhaps I should pop a different opamp in the that can provide more gain for the meter?

Or maybe I'm having an entirely different issue. Any thoughts would be appreciated! Im this close to finishing this pre!! Only this VU issue is standing in my way! Other than that, the rest of the pre amp is working perfectly and sounds great!

 

[zzzzz]

Hey yall Im back!! Got my 312 circuit dialed, ordered and built! Only issue i'm now having is with the VU meter! So I ended up using this buffer circuit courtesy of @zzzzz and I took the chance with some cheapo meters from Aliexpress.

https://groupdiy.com/threads/wtb-vu-buffer-driver-pcb.82932/page-2#post-1107221

So the buffer circuit is powering the lamp of my meter perfectly however, I cant seem to get any signal reading! When I power on the unit the meter jumps as they usually do and with everything absolutely cranked I'm getting just a very very slight amount of reaction from the meter. Reading back thru this thread, I saw some say that with cheap VU's they were needing upwards of +10dBu of signal (4 Volts). Could this be where my issue is as well? I've tried adjusting the VU gain trimmer (feedback of the TL072) all which ways and still nothing. The schematic calls for a 5K trimmer there. Should I maybe up that value to be able to push the opamp even more?? Perhaps I should pop a different opamp in the that can provide more gain for the meter?

Or maybe I'm having an entirely different issue. Any thoughts would be appreciated! Im this close to finishing this pre!! Only this VU issue is standing in my way! Other than that, the rest of the pre amp is working perfectly and sounds great!

Interesting, do you have any other meters to test it out with? Polarity is correct? Can you hit the meter input (disconnect from your pre) with a signal generator to see how much it takes to get it moving?

I’ve used the little VU buffer PCB on many projects now with various makes and models of meters and haven’t had any issues with it.

Edit: saw your other thread. Since 500 series is regulated 16v, no need to use the rectifier or LM317 regulator scheme I have included on the meter buffer board. Can just jump all of that and take the voltage straight from the V+ of your 500 module.

Likewise, what signal location are you using for the input of the meter? Have you tried just connecting it directly to the same output transformer secondary used for your main out? I’d probably take it from the same signal going to the OT primary (after the coupling cap on the 2520 output)

 

[djmiggymigz]

I do have a few other random meters I can test with, good idea! I’ll try the signal generator as well and see what that does.

I’m pretty sure polarity is correct but I did try it both way and it didn’t seem to make a difference for the VU. However, swapping the polarity to the VU seemed to muck with the preamp controls I.e when I swapped the tx leads to the buffer it also reversed the direction of my input gain and output attenuation controls??

 

[djmiggymigz]

One question I was trying to figure out: for the VU trim resistance, is less resistance more gain or opposite?

 

[djmiggymigz]

Edit: saw your other thread. Since 500 series is regulated 16v, no need to use the rectifier or LM317 regulator scheme I have included on the meter buffer board. Can just jump all of that and take the voltage straight from the V+ of your 500 module.

Oh okay cool thanks for clarifying that! I’ll take all that out today then and see what happens!

Likewise, what signal location are you using for the input of the meter?

im taking it directly from the violet/grey leads of the output transformer. I confirmed with a scope that signal was making it at least to the input of the buffer board. 1kHz was showing there at the input clean but a bit quieter compared to the other tx leads (which was expected). Then I seemed to be losing the signal within the buffer board but didn’t figure out exactly where that was happening. Back to the shop today! Thanks for the help!

 

[zzzzz]

sorry when I said “polarity” I meant signal and or power rather than the meter itself, my b.

I would probe the meter board on the output pins of the opamp to make sure it’s passing signal

 

[djmiggymigz]

Okay update:

I've rebuilt a brand new buffer pcb just to double check everything and also account for the things to leave out/jump.

- Im not using any of the rectifier circuit so I have deleted V+ Adj., IC2, C7, C8, and R16. I have D5 jumped with a spare lead.

- Tested and it seems my VU's have internal rectifiers (the meter react to multimeter probes in Ohms mode. I also get 0 VU going straight from my signal generator @ 1kHz, 0.4V) So to account for that I have deleted D8 and R14 and jumped D7.

- Everything plugged back in and still nothing. I'm powering the board from the 500 series card and running signal directly into it from my signal generator.

- Confirmed with scope that 1kHz is hitting the inputs, Pins 2 & 3 on TL072, and is leaving Pin 1. Also confirmed that the trimmer is indeed trimming/boosting the signal leaving the op amp on pin 1. Scope confirmed that I'm getting signal all the way to the +- pins on the VU meter itself but still no reaction???

I'm immensely confused as to why it isn't working! Could it be a polarity issue? Like the +- signal hitting the meter is just cancelling out? But why would that be the case with both the signal generator as well as the tx leads? Pretty sure it's not the VU meter itself since it responds to both the multimeter leads in Ohms mode as well as signal coming directly from my generator. Any ideas??

I'll include a pic. Maybe something is supposed to be jumped that isnt? Maybe my test didnt prove anything and my meter doesnt have internal rectifiers meaning I need to install D8, D7, and R14?

EDIT!!!: I got it!!! Repopulated D7, D8, and R14 and it works!!! Turns out my VU's do not have internal rectifiers I guess! WOOOO Thanks for the help yall! Gonna plug into the studio now and calibrated the VU to match our SSL console VU's