VU meter retrofit basics?

[Mbira]

Hi folks,
I'm building an 8 channel tube preamp that is roughly based on the G9.  I would like to put in a switchable analog meter where I can see the level after the gain and then switch it to see the level at the output.  I saw this schematic at JLM audio:
http://www.jlmaudio.com/VU/VU%20buffer%20&%20Peak%20LED%20131201.pdf

I see it has a balanced input, so I can see that working if I tie it directly to the balanced XLR output, but how do I connect a meter to the gain stage where it is not a balanced signal?

I'm attaching a pic of where I think I would want to measure the gain stage.  Does this seem like an appropriate place?  I'm assuming that my buffer stage will need to be calibrated differently there in order to show something meaningful on the meter. 

So, my main question here is - is it appropriate to put a buffered output there after the plate for the meter, and if so can anyone point me to a schematic that would work in something like that? 

 

[JohnRoberts]

I am not the resident tube guy, but a balanced input is just a fancy (better) differential, so you should be able to just connect the + to hot, and - input to 0V (or signal gnd).

JR 

 

[boji]

I can attest the JLM buffer works unbalanced. It's how I'm using it. Since I know less than JR about tube circuits, what is the max V out after dc blocking cap? TL072 has Vmax In of +36v so I imagine you'll be ok.

Edit: Unless someone warns that tube circuits need something more robust, I'd try it out and see if it meets your needs as the buffer is a low part, relatively simple thing to cook up. Maybe hit the buffer output with a voltmeter before hooking up the VU to be safe.

 

[moamps]

Does this seem like an appropriate place?..

IMO, no. The right place is after R15 47k resistor. Also you should use the buffer with high impedance input.

 

[Mbira]

IMO, no. The right place is after R15 47k resistor. Also you should use the buffer with high impedance input.

Thanks.  Can you tell me why after R15?  I'd love to learn!

 

[moamps]

Thanks.  Can you tell me why after R15?  I'd love to learn!

The output from the gain stage is at that point. The signal at the anode of the second tube doesn't change for all settings of the gain in the attenuation mode. Please read the circuit description part on gyraf site
http://www.gyraf.dk/gy_pd/g9/g9pd.htm

The buffer should have high impedance input (for example 220k or higher) in order to not change significantly the gain of the gain stage  with it's impedance during the measurement. (a simple version of Δx ⋅Δ p≥ h  )
JLM's buffer has only 47k input impedance.

I strongly suggest designing the buffer for each channel and placing it close to the gain stage, not using one buffer for all channels. This way you will get much better isolalation of each channel's gain stage from others and from noise inducing by measuring switch and cables. If your VU meter already has rectifier inside (as Sifam do), your buffer can be simple, just non-inverting opamp set for single power supply in this case.

 

[Mbira]

The output from the gain stage is at that point. The signal at the anode of the second tube doesn't change for all settings of the gain in the attenuation mode. Please read the circuit description part on gyraf site
http://www.gyraf.dk/gy_pd/g9/g9pd.htm

hmm...In that case, I may be even better off putting it right after the Low Cut and before the output attenuator.

The buffer should have high impedance input (for example 220k or higher) in order to not change significantly the gain of the gain stage  with it's impedance during the measurement. (a simple version of Δx ⋅Δ p≥ h  )
JLM's buffer has only 47k input impedance.

Awesome.  Thank you-I will look into high impedance buffers. 

I strongly suggest designing the buffer for each channel and placing it close to the gain stage, not using one buffer for all channels. This way you will get much better isolalation of each channel's gain stage from others and from noise inducing by measuring switch and cables. If your VU meter already has rectifier inside (as Sifam do), your buffer can be simple, just non-inverting opamp set for single power supply in this case.

Yes-they will be built in to each channel separately.  Since I will be switching between monitoring two different stages, is that something where I can just use a switch controlling a relay to switch between the two points?

 

[Mbira]

Δx ⋅Δ p≥ h

I'm trying to figure out how you determined what the impedance needs to be.  I googles that formula and I got to the Heisenberg Uncertainty principle.  Is that related....or a joke?  I don't know enough to know....