Trying to find 24vdc driver circuit for VU meters

[Govinda Doyle]

Hi there,
I'm trying to find a schematic for a 24vdc driver circuit for some VU meters.
Any help would be greatly appreciated
Govinda

 

[abbey road d enfer]

It's easy, but you must give some indications. Do you want balanced or unbalanced inputs? Do you want high-impedance inputs?
What source to you want to visualize? What type of meter? Is it a real VU-meter with built-in rectifier, or one that needs an external rectifier? What are the characteristics of the meters?

 

[mjrippe]

Discrete Neve circuits run on +24VDC, search for some of those?

 

[ruffrecords]

Nwvw VU buffer.

Cheers

Ianj

 

[abbey road d enfer]

Why is the series resistor (R4) 2.2k instead of the more PC 3.6k?

 

[ruffrecords]

Why is the series resistor (R4) 2.2k instead of the more PC 3.6k?

Not sure. I thought 2.4K was usual. Isn't the important thing that the total resistance, including the meter winding, be a certain value?

Cheers

Ian

 

[abbey road d enfer]

The standard defines the total impedance at 7.5k, which makes me think the movement has a higher resistance than the "standard".

 

[ruffrecords]

TBH I am not even sure of the accuracy of this schematic. It is certainly not a Neve original. I found it on the internet ages ago. I do not recall it being used when I was there in the 70s. In those days VU meters were just strapped across balanced outputs.

Cheers

ian

 

[abbey road d enfer]

TBH I am not even sure of the accuracy of this schematic. It is certainly not a Neve original. I found it on the internet ages ago. I do not recall it being used when I was there in the 70s. In those days VU meters were just strapped across balanced outputs.

Cheers

ian

So it looks like it's a circuit that has been designed for a different type of meter. Anyway the circuit should work, as long as the resistor value is adapetd to teh actual movement.

 

[Brian Roth]

I can't find the paper I recall reading, but here are these from Jay McKnight (Ampex guru from The Day, now Mr. MRL alignment tapes). I think I have other references in my files.

Bri

 

[abbey road d enfer]

I can't find the paper I recall reading, but here are these from Jay McKnight (Ampex guru from The Day, now Mr. MRL alignment tapes). I think I have other references in my files.

Bri

I had actually forgotten that the 3.6k is correct for a 300 ohm source Z, but in the case of a very low source Z, such as the one presented by an opamp output, the resistor must be 3.9k.
Reducing this value increases the damping of the movement, so the schematic posted by Ian must be either for a movement that has less intrinsic damping or less sensitivity, or an attempt to make the meter react faster, at the cost of allowing some overshoot.

 

[Govinda Doyle]

I have two old ampex meters I believe they are a (M401 4 inch meter ME-2) in the Ampex parts ref manual, I cant find much more info..
They have a built in rectifier, i'm trying to put it on the output of a 4 way momentary switch which selects different stereo sources on a console, I have a 24vdc tap which would make it much simpler to power, but im not familiar with a 24v driver circuit for this type of old meter.
so to answer your questions

Balanced inputs
High impedance (can always change this lower if it doesn't work)
sources are mostly DAW sends and a console transformer tap output
real vu meter with rectifier
Characteristics are VU if I understand the question properly

Thanks so much for your reply
Govinda

 

[Govinda Doyle]

Thanks Brian they are super helpful
Thanks Abbey Road that makes a bit of sense there...

 

[Govinda Doyle]

Discrete Neve circuits run on +24VDC, search for some of those?

as far as I know they were just tapped off the transformers.. im wanting to drive it for multiple inputs, out of the 4 inputs only 1 is a balanced transformer, I can strap it off that but then I miss out on the current input selection...

 

[moamps]

The JLM buffer kit runs on single 24V

https://www.jlmaudio.com/shop/vu-buffer-peak-led-kit.html?display_tax_prices=1http://www.jlmaudio.com/VU/VU buffer & Peak LED 131201.pdf
and I think it will fully meet your requirements without the need for any modifications. Just if you really want, you can omit R8, 11, 12, D2, 3 (your VU meter already has a rectifier, so you don't need this one on your PCB). Instead of D2, insert a 3k6 resistor and connect your VU meter between the "+ meter" pins and ground.
If you do not need a peak indicator, you can also omit the following elements: R9, 10, RV2, C3, D4 and RED LED.

 

[abbey road d enfer]

Excellent suggestion, moamps. however I would recommend modifying the values according to this:
R3, 4, 5, 6 = 15k.
R7=10k
VR1=50k (47k)
With the original values, the gain range is huge (+/-13dB) and unity gain is cramped at one end of the trimmer.
Considering the OP needs basically unity gain, the proposed values offer a gain range of -2 to +4, which will make the calibration much easier.
Since the output of the opamp is essentially zero-ohm, the series resistor should be 3.9k instead of 3.6.
Not that it makes much difference in operation, since anyway VU meters are not adequate for visualizing transients that can trespass the 100%fs mark of a digital chain.

 

[moamps]

I forgot to add that pins 6 and 7 of the unused opamp should be connected together, while pin 5 should be connected to V/2 voltage if the opamp is not used as a peak indicator.

 

[FIX]

Try a variation of this. The 2nd op-amp creates a lo-z 1/2 voltage point that can be used as the "gnd" reference for the meter and bias for the opamp. Since the meter has internal rectifiers, you don't need any diodes. The output trim will be enough adjustment.

 

[moamps]

Nice variation. I would just note that the polarity of the input capacitors should be reversed in your circuit. You can use here smaller non-polarized caps too (1uF MKT would be just fine).

 

[mjrippe]

as far as I know they were just tapped off the transformers.. im wanting to drive it for multiple inputs, out of the 4 inputs only 1 is a balanced transformer, I can strap it off that but then I miss out on the current input selection...

The BA-252 is listed as a meter board (unspecified), BA-274 is a PPM meter driver, BA-286 is a VU buffer. But I do not have any of these schematics