VU meter frequency response

[edseven787]

I have just completed building the stereo VU meter kit- I have a question. I am using it with very high quality meters- they are pretty flat in frequency response. When I use the kit- there is quite a drop-off at high frequency- starting at 3k- going down almost 3db at 16k and over 4 at 20k!!!! Is this normal???
Thanks Much,
ED S.

[Joe Malone]

I have just completed building the stereo VU meter kit- I have a question. I am using it with very high quality meters- they are pretty flat in frequency response. When I use the kit- there is quite a drop-off at high frequency- starting at 3k- going down almost 3db at 16k and over 4 at 20k!!!! Is this normal???
Thanks Much,
ED S.

HI Ed
Copy the reply email I had sent you as you do not seem to have seen it.

Not sure which VU kit you are using ? But there is no bandwidth limit added on the circuit but if you are using extreme gain from the trimpot you maybe running out of bandwidth from the TL072.

To help I need to know the value the gain pot is set to now and which vu2 or vu + peak led etc to check the drop off here.
And a couple of photos of the wiring and your meter info. What feeds the input and is it balanced or unbalanced?

2 Things to watch out for if your meter has a built-in diode bridge make sure the one on the buffer is disabled.
Also do not run an external series resistor with your meter as it is already on the buffer pcb.

[edseven787]

Thanks so much for the reply!!! A few things- the kit is the stereo VU meter kit- the meters are real VU SIFRAM- input is balanced from a professional tape machine. The meters do have internal rectifiers so (as per the instruction) I have omitted the diodes on the board. I will measure the pot and get back to you with that. Appreciate your help!!

[Joe Malone]

Thanks so much for the reply!!! A few things- the kit is the stereo VU meter kit- the meters are real VU SIFRAM- input is balanced from a professional tape machine. The meters do have internal rectifiers so (as per the instruction) I have omitted the diodes on the board. I will measure the pot and get back to you with that. Appreciate your help!!

OK so a VU2 kit. Do both meters roll off the same at 20khz?

What power supply is the buffer running on 12v plug pack etc?

[edseven787]

Hello Again!
This is very baffling! Yes L+R do the same. The pots seem to be about half way- to give 0 DB. I am using (for testing) two 9 volt batteries in series to give 18 volts as the power (should I try a +/- supply?). I do have 2 other sets of meters- they all act the same way!!! Should I try a different IC- maybe a LM358?? Kind of crazy- thanks much for your help-
ED

[Joe Malone]

Hello Again!
This is very baffling! Yes L+R do the same. The pots seem to be about half way- to give 0 DB. I am using (for testing) two 9 volt batteries in series to give 18 volts as the power (should I try a +/- supply?). I do have 2 other sets of meters- they all act the same way!!! Should I try a different IC- maybe a LM358?? Kind of crazy- thanks much for your help-
ED

A LM358 should be worse as it has less bandwidth and offset issues. A OPA2132 may work but until I get a moment to get one of our AL39 setup on a buffer I will not know for sure

[edseven787]

You were right all along!!!! It was too much gain. I will see how I can use it with my tape machine outputs. Sorry for the trouble!!! Many Thanks-
ED S.

[Joe Malone]

You were right all along!!!! It was too much gain. I will see how I can use it with my tape machine outputs. Sorry for the trouble!!! Many Thanks-
ED S.

HI Ed
Ok cool. But if you need that much gain then a high bandwidth fet input dual opamp like OPA2132 should work. Just have to watch out it doesn't oscillate as no bandwidth limiting caps are fitted to the PCB as TL072 doesn't need them.