VU BUFFER & PEAK LED KIT BUILD THREAD

[Joe Malone]

thank you for your quick reply, I appreciate it.
In this post, you mentioned that the 3K3 buffer resistor may need to be lowered, dated October 26, so I just started to experiment with a value and see what happens.
It was doing the same, ie no VU meter movement after power down and power up again with no resistor value changes, so lowering it to 1K, as far as I can see, has no effect on what it is doing when powered down and back up again.

I am using a TN-73-HS-9213H VU meter.
The measured internal DC resistance is 50 OHMS on a digital meter. It has no diodes or resistors inside.
Using +4dBm, I only get a max deflection between 3 and 2 on the VU meter.
Any advise would be appreciated, thank you.

Wow that resistance is very low for a meter. Can you cal +4dBu = +4dBm = (1.23vac RMS on multimeter) to correct 0VU?
and then if you put in +7dBu does the meter show +3 ?

If not let me know as the output 3k3 ressitor on the buffer will have to be made smaller in value.

To answer your question as follows:

What power rail is the buffer running on? Volts & Amps? 12VDC @ 1.5A
Is it SMPS? Is the power supply running anything else? It uses a 7812 regulator, well filtered and running nothing else
Input reference level to the buffer? Balanced or unbalanced? If unbalanced have you tied -in to 0v? unbalanced and yes, I tried -in to 0V
What are the meter specs for the meters you have tried? see above
Is it wired in with other audio gear etc in the same case? no, one driver per VU meter
If so how are the power supplies wired? Photos are best for this.

Cannot see answers to my questions below. +4dBm should be adjusted to read 0VU. so 2 or 3 is more than needed.

Can you cal +4dBu = +4dBm = (1.23vac RMS on multimeter at input) to correct 0VU?
and then if you put in +7dBu does the meter show +3 ?

What is feeding the input of the buffer?

I have a pair of TN-73-HS-9213H VU meters here and my notes written on them say
"Meter works on buffers without mod for +4dBm = 0VU but are 1000uA 55ohm so needle movement is too slow for VU meter.
Changing series 3k3 to 2k2 resistor makes VU scale accurate and needle moves faster but still too slow for real VU"
TN-73-HS-9213H on right with backlight on in photos attached
Tests for -5 +3VU & 0VU with 2k2 fitted for R7R

[erhard58]

sorry, my bad, when I stated 2 or 3, I meant -2 or -3


Can you cal +4dBu = +4dBm = (1.23vac RMS on multimeter at input) to correct 0VU?
and then if you put in +7dBu does the meter show +3 ? I will need to check that again with my tone generator tomorrow


What is feeding the input of the buffer? the audio signal out from the the EQ. So the preamp circuit goes to the EQ. The output of the EQ goes to the buffer as well as the main preamp output for external connection.

The VU meters are used in a hybrid tube, direct playback tape head preamp, which has a user adjustable EQ for playback calibration.
The preamp by itself works flawlessly, just having issues with the buffer at the moment.

[Joe Malone]

sorry, my bad, when I stated 2 or 3, I meant -2 or -3


Can you cal +4dBu = +4dBm = (1.23vac RMS on multimeter at input) to correct 0VU?
and then if you put in +7dBu does the meter show +3 ? I will need to check that again with my tone generator tomorrow


What is feeding the input of the buffer? the audio signal out from the the EQ. So the preamp circuit goes to the EQ. The output of the EQ goes to the buffer as well as the main preamp output for external connection.

The VU meters are used in a hybrid tube, direct playback tape head preamp, which has a user adjustable EQ for playback calibration.
The preamp by itself works flawlessly, just having issues with the buffer at the moment.

Does the EQ have a balanced output you can wire across?

[erhard58]

the EQ output is unbalanced, I have just send you the EQ schematic via pm.
Note, the DEFEAT function/switch on the EQ is not used

[Joe Malone]

the EQ output is unbalanced, I have just send you the EQ schematic via pm.
Note, the DEFEAT function/switch on the EQ is not used

I cannot get to the message as it says it is on hold. Not sure what that means. But best to email it to me joe at jlmaudio.com

[erhard58]

OK, emailing now, thanks Joe, really appreciate it!

[Joe Malone]

OK, emailing now, thanks Joe, really appreciate it!

Ok looking at the circuit the main thing is to make sure you go to the vu buffer from the centre of the switch before the output CR HPF.

For these meters I would change R11 & R12 to 2k2 so scale is correct and with 1.23vac measured with multimeter between +in and -in adjust for 0VU.

If you have instability due to lots of gain change R1 from 47k to a smaller value like 10k which will give you 5 x the gain so gain pot will be less and stability greatly improved.

Make sure the 0v from the buffer is connected to your opamps power supplies 0v.

Also since your circuit is opamp-based the buffer can run directly across the +12v to +18v rails the opamps are running on.

[erhard58]

OK, so connect the buffer before the 1uF C107 and C207, just confirming
OK, will change those two resistors on the buffer to 2K2 and see how that goes.
Thank you Joe, will let you know how it goes.

[Joe Malone]

OK, so connect the buffer before the 1uF C107 and C207, just confirming

Yes correct

OK, will change those two resistors on the buffer to 2K2 and see how that goes.
Thank you Joe, will let you know how it goes.

Cool let me know how it goes. Make sure you have 1.23vac at the input

[erhard58]

OK,

changing the buffer input to that position did it!...phew!!!!!
Changing those resistors to 2K2 made it better, but I still cannot get 0dB VU, I can with 1K, so will leave it at that.
Thank you again for your time and effort in getting this sorted, I really very much appreciate it!
I purchased a pair of your SEW 315 VU meters, will try those and see if I can stick with the original parts values, I am assuming I will.

[djmiggymigz]

Hello JLM forum!! I have a question about this buffer circuit. Currently working on a 312 preamp design from scratch (relatively new to this side of actually designing and building things out vs. diy kits that just need assembling). I'd love to add a VU meter to the build. For those familiar, based on the OG 312 schematic, the output transformer has 2 spare leads (vio/gry) that were used to send signal to the VU meter in their consoles etc.

Talking with some folk on GDIY I was warned that due to the windings and wiring of the output transformer, sending signal through the 2 spare leads actually sends a signal that it around 6dB quieter than the actual output signal. Is this buffer circuit able/designed to compensate for that loss? I think so from looking at the schematic but wanted to confirm!

[Joe Malone]

Hello JLM forum!! I have a question about this buffer circuit. Currently working on a 312 preamp design from scratch (relatively new to this side of actually designing and building things out vs. diy kits that just need assembling). I'd love to add a VU meter to the build. For those familiar, based on the OG 312 schematic, the output transformer has 2 spare leads (vio/gry) that were used to send signal to the VU meter in their consoles etc.

Talking with some folk on GDIY I was warned that due to the windings and wiring of the output transformer, sending signal through the 2 spare leads actually sends a signal that it around 6dB quieter than the actual output signal. Is this buffer circuit able/designed to compensate for that loss? I think so from looking at the schematic but wanted to confirm!

Yes the buffer can make up the 6dB as well as isolate the distortion the meter would add back into the audio output by the irregular load on the transformer as the diodes switch.

[djmiggymigz]

Okay sweet thank you!! One more (beginner) question. Are C4 and C6 PSU bypass caps? Gonna be running power straight from the 500 series card and GDIY advised I'll just need bypass caps!

[Joe Malone]

Okay sweet thank you!! One more (beginner) question. Are C4 and C6 PSU bypass caps? Gonna be running power straight from the 500 series card and GDIY advised I'll just need bypass caps!

Yes you will not need anything. Just wire to +16v and 0v and feed spare transformer winding to +/-in and adjust trimpot for output 0VU = +4dBu

[djmiggymigz]

Wowww thank you!! That's awesome. Almost ordered my PCB's having given up on the VU. Very happy to have found JLM!