Mono VU Meter from a Stereo Bus

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

earthsled

Well-known member
Joined
Dec 1, 2009
Messages
405
Location
Asheville, NC USA
Hi all.

I'm working on a project for a friend - wiring up an ADM meter bridge so he can use it on the outputs of his D to A. My friend wants to use 2 meters in the bridge for his main stereo outputs while using a 3rd meter for a mono sum.

Is there a simple circuit that will allow me to sum the left and right signals for the "mono" meter while maintaining isolated L+R channels?

Thanks in advance!


 
Quick and dirty method, if the L/R outputs are balanced: Connect one side of the meter to the + side of the left out. Connect the other side of the meter, via a 20k trimmer resistor, to the - side of the right out. Adjust trimmer for zero VU reading at desired reference level on the left and right. Downsides: may interfere with common mode rejection and may add very slight distortion.

Proper method: Build a small active summing amp circuit with two balanced inputs (assuming the D-A outs are balanced) and use that to drive the meter.
 
Thanks David! Great solutions.

I'd like to try the active summing amp circuit with two balanced inputs that you mentioned (this is a balanced D-A). Any idea where can I find a layout for a circuit like this?


 
Search for "meter buffer" and "active summing" here. 

At it's simplest, it is a 1 opamp circuit with a trimmer added.  Google "opamp circuits".

Mike
PS: Usually those ADM bridges have buffers on the meters?  You can mod that circuit to make a mono meter.
 
Thanks Mike!

Yes. The meters do have buffer amps. In fact, I have some spare buffers.
I'm very interested in doing a mod that would accept two inputs for the mono meter.

Could I simply have 2 buffers feed a single meter, or would this cause issues? (attached diagram)

 

Attachments

  • VU_buffers.jpg
    VU_buffers.jpg
    59.4 KB · Views: 50
earthsled said:
Thanks Mike!

Yes. The meters do have buffer amps. In fact, I have some spare buffers.
I'm very interested in doing a mod that would accept two inputs for the mono meter.

Could I simply have 2 buffers feed a single meter, or would this cause issues? (attached diagram)

Yes, that's a good solution. But if you already have buffers for the main L and R meters, there's an even easier way. Just feed the mono meter from the L and R buffers with passive summing resistors. Just make sure that the summing resistors are connected directly to the opamp outputs (not after some other resistor), to avoid crosstalk between the L and R meters.
 
Just feed the mono meter from the L and R buffers with passive summing resistors. Just make sure that the summing resistors are connected directly to the opamp outputs (not after some other resistor), to avoid crosstalk between the L and R meters.

Thanks David. This seems like a great solution.

Would these resistor values be appropriate to follow the buffers as you suggest?

n109fig3.gif


(borrowed from Rane's website)

 
I don't know why Rane recommended 475 ohms, that's quite low. Bad call on their part, IMO.

You might look at what value resistor was used in series with the L and R meters, and using resistors that are about double that value. I hope I was clear that you'll be connecting the summing resistors to the outputs of the VU buffer amps, not the balanced outputs.

Out of curiosity, what's the reason for the mono VU meter? Is somebody mixing to mono and stereo at the same time, or will the stereo mix be folded down to mono later on?
 
i might have just been given something like this.  Its 3 ADM meters Left, Right and AUX in a 19"rack.  Crazy.  I should take a pic and see if its what you are talking about.
 
I don't know why Rane recommended 475 ohms, that's quite low. Bad call on their part, IMO.

You might look at what value resistor was used in series with the L and R meters, and using resistors that are about double that value.

I'm really not sure about which value is in series with the L and R meters. Can you recommend a ballpark value? Also, should I keep the 20k resistor between + and - on the output (like in the Rane schematic)?

Thanks for your help on this!
 
Sorry for the delay.

Without seeing the actual circuit it's hard, but I would say try about 7.5k for each of the summing resistors.

You can test things by bringing up a 1k tone and adjusting level so that the right and left meters read zero VU. Then choose summing resistors that make the mono meter read zero. (Or maybe -3. There are different philosophies about this.)

Another way: connect a dual 10k pot so that one section is the left summing resistance and the other section is the right. Set up tone as above and adjust the pot till mono meter reads zero (or minus 3, etc.). Carefully disconnect the pot without bumping the shaft, then measure one of the sections. Choose a pair of fixed resistors that are close to this value. You could fine tune the meter reading downward by adding a third resistor (smaller value, maybe 100-200 ohms, you have to experiment) in series with the ground wire going to the meter.

Another idea, maybe the best: use to 4.7k summing resistors, put a 10k pot or trimmer in series with the ground wire of the meter. Do the tone thing for zero on the stereo meters and adjust the pot for zero (or level desired) on the mono meter.

Whatever you do, check with stereo program (music, dialogue, whatever) to see if the mono meter is reading about the way that you want. Again, mono fold downs can have different levels. A console with a real mono buss might have a mono fader, which the engineer would set as desired. Then the buss could be metered accurately. But without a separate mono buss, you may just "guessing" (by a few db) at what the mono level will be when the stereo mix is folded down to mono.

In the end, resistor values will involve some trial and error. I hope this helps and makes sense!
 
Thanks David. This is great information!

Perhaps I should stay with the idea of having two buffer cards output to the mono meter. The cards already have trimmers on them which would allow me to fine tune the input levels (as you are suggesting). Also, I think it would work to "piggy-back" a second card on the back of one meter using aluminum standoffs.

I'll do some tests this weekend and post the results.


 
Hello,

Sorry for digging.
I am kind of asking myself the same question. I am working on a pretty simple transformer balanced tube stereo circuit. I want to have a single VU meter on the stereo output. Is there a simple way of doing this, or should I have a solid state VU buffer? My circuit is all tube so I have approx 250VDC and 6.3VAC. Best case scenario would be a passive / transformer thing...? Maybe?
 
Tube amps have non-zero output impedance. This means:

VU meter rectifier adds distortion, acceptable in 1949 but high in today's world.

A stereo/mono mix network will allow crosstalk.
 
Ok, so I'll need one buffer per channel I guess, and they'll both drive the same meter?
I was thinking maybe I could find a way of doing a passive thing with a dedicated transformers, but you are right, crosstalk will suffer from this...
 
Your real challenge is to power a chip-amp with 300V.

I have several clever ideas, but working them out and drawing is more work than you using a 30V Zener and a 1W resistor to drop ~~300V to 30V at2-mA-5mA to power say a LM324. (Not an all-around audio chip, but good enough for a meter, and very thrifty.)
 
Maybe my easiest and cleanest shot is to add another power transformer, let's say a 110 > 12V and then a little veroboard with a diode bridge, caps and 7812... So I don't mess with the main PSU?
I am not very familiar with Zener diodes and voltage dividers...
 
I'm recalling a 3 resistor passive network that "mixes or splits with no crosstalk", but don't recall connection points.  Meant for 600 ohm systems, three 600R if I recall.  Modern analysis may have a different feeling about "no crosstalk".  Funny thing is the circuit is so old I can't find trace of it online. 
 
Here it is.  I haven't tried it in practice.  The balance requirements are possibly impractical, I don't know that you can add build-outs and shunts effectively to get perfectly matching Z at all legs without too much loss, at which point you should just build an active mixer to achieve isolation. 

https://groupdiy.com/index.php?topic=64485.msg816553#msg816553
 

Latest posts

Back
Top