single supply VU meter driver

I got some of those cheap non-standard "VU meters" on Ebay and I found a driver circuit online. Here it is:

http://objectivesounds.co.uk/articles/driving-vu-and-other-ac-meters/

So, I want to try to run the "improved version" on a single 12v heater supply. How would you go about it? That's what I did, but for some reason, it won't simulate on SPICE. Returns floating point calculation errors. Please, have a look at the schematics attached, and let me know what you think.


Spice errors:

Error: -1.#INF, 4.715e+006 out of range for *
Note: starting Gmin stepping
Error: -1.#INF, 4.715e+006 out of range for *
Warning: Gmin step failed
Error: -1.#INF, 4.715e+006 out of range for *
Warning: Gmin stepping failed
Note: starting source stepping
Error: -1.#INF, 4.715e+006 out of range for *
Warning: source stepping failed
Error: -1.#INF, 4.715e+006 out of range for *
Note: starting Gmin stepping
Error: -1.#INF, 4.715e+006 out of range for *
Warning: Gmin step failed
Error: -1.#INF, 4.715e+006 out of range for *
Warning: Gmin stepping failed
Note: starting source stepping
Error: -1.#INF, 4.715e+006 out of range for *
Warning: source stepping failed
doAnalyses: No such parameter on this device

run simulation(s) aborted

Try this one:

Same error on spice.

But you are right. The half voltage reference  should be on + input of the opamp. How did I overlooked it???

About cap across the output... I added that ground reference so that it never gets  reversed. Wouldn't it work the way I drew?

Aso... should I make c4 10uF bigger, maybe?

I just can't understand why spice can't just run it... maybe I'll just solder it and see how it goes. What do you think?

rafafredd said:

About cap across the output... I added that ground reference so that it never gets  reversed. Wouldn't it work the way I drew?

Click to expand...

It probably works but isn't necessary because the meter and cap are connected after the full wave rectifier.

Aso... should I make c4 10uF  bigger..

Click to expand...

Maybe, you will see what the meter will show at low frequency.

... maybe I'll just solder it and see how it goes. What do you think?

Click to expand...

Yes. You can start first with schematic on figure 4. It works for sure.

LTSpice simulates it... well... kind of.

It may depend on your mechanical meter, but I once tried to run mechanical VU meters in a console from a single 12V supply to avoid running bipolar power up to the meter bridge. The LED bar graphs had no problem with a single 12V rail. I got complaints from a critical customer, the first customer for that console series with VU meters, that the meters were under reporting transients. 

It should be hard to see on an average responding mechanical meter, but I changed the meter driver boards to run from +/- 15V rails and the customer was happy. When the customer is happy, I am happy.  8)

If you only want to see the meter wiggle 12V will work, if you want them to look like other properly driven VU meters use enough rail voltage to cleanly pass the full audio signal without clipping.

JR

Thanks, Jojhn...

But as +5vu equals 3.083vpk or 6.175vpp, I though the entire wave for maximum VU display would fit well into +12v supply. Wouldn't that be the case? Why not???

rafafredd said:

Thanks, Jojhn...

But as +5vu equals 3.083vpk or 6.175vpp, I though the entire wave for maximum VU display would fit well into +12v supply. Wouldn't that be the case? Why not???

Click to expand...

My console delivered 20dB of headroom above 0VU (+4dBu). It made a difference in my old console design.

If your audio path is voltage limited to < 12Vp-p you may be OK, mine wasn't

JR .

> as +5vu equals 3.083vpk or 6.175vpp

VU meter is tuned to integrate short peaks 15dB above the indication. At +3VU, that is +18dB above 0.775V, or 6Vrms, or 17.4Vpp.

10Vpp (12V supply with some losses) is missing some of that.

I'm mildly surprised JR's customer noticed. However some drum work is all transients, a too-experienced engineer may know where the needle should be for "thwack" or "THWACK", and feel the big hits are reading short.

Since from AM radio to ADC recordings, the peaks are where the trouble is, I feel the averaging VU is just the best we could do in 1939, true peak meters are now quite affordable, and VU "accuracy" is silly. But they are pretty, and traditional.

PRR said:

> as +5vu equals 3.083vpk or 6.175vpp

VU meter is tuned to integrate short peaks 15dB above the indication. At +3VU, that is +18dB above 0.775V, or 6Vrms, or 17.4Vpp.

10Vpp (12V supply with some losses) is missing some of that.

I'm mildly surprised JR's customer noticed. However some drum work is all transients, a too-experienced engineer may know where the needle should be for "thwack" or "THWACK", and feel the big hits are reading short.

Click to expand...

I was surprised too, but the customer is always right... even when...

Since from AM radio to ADC recordings, the peaks are where the trouble is, I feel the averaging VU is just the best we could do in 1939, true peak meters are now quite affordable, and VU "accuracy" is silly. But they are pretty, and traditional.

Click to expand...

I am definitely not a fan of mechanical VU meters but we offered them as an expensive option on our 24 bus consoles. (I included a red peak LED with each VU so you could see transient peaks).  This customer as I recall was a university (in Canada) with a respected recording program.  I was not in a position to argue with the customer, and after I increased the rail voltage for the meter drivers he was very happy.  Apparently he had other true VU meters in the room to compare the console meters to.

I recall having to spend that time in Toronto, in Feb [=cold]. I was glad to return to the relatively balmy MS.  8)

JR

PRR said:

> as +5vu equals 3.083vpk or 6.175vpp

VU meter is tuned to integrate short peaks 15dB above the indication. At +3VU, that is +18dB above 0.775V, or 6Vrms, or 17.4Vpp.

10Vpp (12V supply with some losses) is missing some of that.

Click to expand...

Have I missed something?

0VU is usually +4dBu so +3VU is +7dBu and 15 dB above that is +22dBu which is nearly 28V pp.

But also, in many cases this may be moot. Original true VU meters included an internal bridge rectifier and were specified to be driven direct by the audio via a 3K6 resistor. The circuits shown by the OP (and many others on line) do the rectification directly (ni 3K6 resistor) and drive a 500uA meter movement. Sometimes a capacitor is palced across the rectifier bridge to mimic some sort of ballistics.

So in terms of indicating overloads, it is not the rail voltage that matters but the current drive of the op amp and rectifier. Since 50uA represents +3VU. 15dB above this is less than 3mA. Should not be a problem for an op amp on 12V supply.

So, have I missed something?

Cheers

Ian

Hello

ruffrecords said:

Have I missed something?

0VU is usually +4dBu so +3VU is +7dBu and 15 dB above that is +22dBu which is nearly 28V pp.

But also, in many cases this may be moot. Original true VU meters included an internal bridge rectifier and were specified to be driven direct by the audio via a 3K6 resistor. The circuits shown by the OP (and many others on line) do the rectification directly (ni 3K6 resistor) and drive a 500uA meter movement. Sometimes a capacitor is palced across the rectifier bridge to mimic some sort of ballistics.

So in terms of indicating overloads, it is not the rail voltage that matters but the current drive of the op amp and rectifier. Since 50uA represents +3VU. 15dB above this is less than 3mA. Should not be a problem for an op amp on 12V supply.

So, have I missed something?

Cheers

Ian

Click to expand...

I think you are right Ian, maybe the statement here is that it's useless to set Vu at +4 "historical " ref level if the line out involved is not capable of it, whatever dim or pad is used to technically move the needle. If you try to reach 0Vu audio will overload
Still you can set your Vu meter to read 0Vu at line out nominal level (minus 8dB in when calibrating)

PRR said:

> as +5vu equals 3.083vpk or 6.175vpp

VU meter is tuned to integrate short peaks 15dB above the indication. At +3VU, that is +18dB above 0.775V, or 6Vrms, or 17.4Vpp.

Click to expand...

Are you sure of that ? 300ms is not short peaks integration...

Best
Zam

As I said in my first response "it may depend on your mechanical meter" that the OP characterized as "cheap non-standard VU meters".

Real "true" VU meters depend on being driven very specifically with rectification performed in series so everything matters (as I learned the hard way). 

If all the OP wants is to wiggle some meters, 12V supply will work for that,,, If he wants accurate metering his money would be better spent on LEDs. (sorry).  Of course any mechanical meter could be driven from circuitry that would mimic true VU ballistics, but it would take an investment in bench time, and probably a lot more complexity. 

JR

ruffrecords said:

The circuits shown by the OP (and many others on line) do the rectification directly (ni 3K6 resistor) and drive a 500uA meter movement.

Click to expand...

The circuits posted by the OP actually are V-to-I converters with a 3.3k sensing resistor, so both voltage and current limits apply. 3mA in 3.3k computes to 10V, so the opamp runs out of juice for the 1.2V diode drop and whatever is needed to drive the internal resistance of the meter (635r in the sim), not mentioning the two 150r resistors.
I would think decreasing the current-sensing res to let's say 1k would solve this issue.

BTW, adopting current-drive plays tricks with the scale and ballistics. Proper VU ballistics rely on adequate value of the series tresistor providing critical damping of the movement. The historically-correct metal-oxide rectifier basically ignored anything below -30VU (-26dBu). Current drive (that contributes to compensate the diodes threshold) linearizes the bottom part of the scale, allowing valid indication of -40VU and below.

PRR said:

VU meter is tuned to integrate short peaks 15dB above the indication. At +3VU, that is +18dB above 0.775V, or 6Vrms, or 17.4Vpp.

Click to expand...

I'm puzzled... Integration is a matter of Amplitude times Time. I would think a VU-meter could display something of shorter duration than 15ms and larger amplitude.
Or did you just mean that a VU-meter is supposed to work correctly as long as the amplitude of the signal is not higher than +18dBu, and should not be submitted repeatedly to higher than +18dBu?

abbey road d enfer said:

The circuits posted by the OP actually are V-to-I converters with a 3.3k sensing resistor, so both voltage and current limits apply. 3mA in 3.3k computes to 10V, so the opamp runs out of juice for the 1.2V diode drop and whatever is needed to drive the internal resistance of the meter (635r in the sim), not mentioning the two 150r resistors.
I would think decreasing the current-sensing res to let's say 1k would solve this issue.

Click to expand...

Yes, changing the gain is quite simple. Changing the current sense resistor to 1K means 0,5V rms input gives 500uA rms current through the meter so we just need to add an attenuator at the input to obtain the required sensitivity

BTW, adopting current-drive plays tricks with the scale and ballistics. Proper VU ballistics rely on adequate value of the series resistor providing critical damping of the movement. The historically-correct metal-oxide rectifier basically ignored anything below -30VU (-26dBu). Current drive (that contributes to compensate the diodes threshold) linearizes the bottom part of the scale, allowing valid indication of -40VU and below.

Click to expand...

A true VU meter feeding through a 3K6 resistor is an approximate current drive so a true current drive, although not the same, is not too different either. I am not sure how much worse the ballistics of cheap modern VU meters are , but I expect they would benefit from the improved current drive of the op amp circuit.

Bottom line is if you have a true VU meter the only way to drive it is via 3K6 resistor buffered by an op amp that can swing far enough.

If you use a cheap 500uA meter or similar, you really have nothing to loose by trying the V/I converter method.

Cheers

Ian

ruffrecords said:

A true VU meter feeding through a 3K6 resistor is an approximate current drive so a true current drive, although not the same, is not too different either. 

Click to expand...

The value of 3.6k is essential in ensuring critical damping, that is the guarantee of the 300ms for 99% and less than 1% overshoot required by the norm. Current-drive will make a genuine VU-meter overshoot significantly.

I am not sure how much worse the ballistics of cheap modern VU meters are , but I expect they would benefit from the improved current drive of the op amp circuit.

Click to expand...

Cheap modern VU-meters are mere objects of decoration, utterly useless in the context of digital audio. I wouldn't fry one brain cell over attempting to improve them.

Bottom line is if you have a true VU meter the only way to drive it is via 3K6 resistor buffered by an op amp that can swing far enough.

Click to expand...

Agreed!

abbey road d enfer said:

  Cheap modern VU-meters are mere objects of decoration, utterly useless in the context of digital audio.

Click to expand...

I will say all VU meters (old, new, expensive or cheap) are more or less useless for modern music monitoring. There are three areas in signal level they show, IMO; too low, ok, and too hot.  I can live with that if they are attached to a tube based device or analog tape recorder.

from:
http://tapeop.com/tutorials/54/meters/
"European audio engineers had another nickname for VU meters, 'virtually useless'."

> Have I missed something?

This heat wave. Beastly sweaty, my abacus beads are slipping. Go with your numbers.

moamps said:

I will say all VU meters (old, new, expensive or cheap) are more or less useless for modern music monitoring. There are three areas in signal level they show, IMO; too low, ok, and too hot.  I can live with that if they are attached to a tube based device or analog tape recorder.

from:
http://tapeop.com/tutorials/54/meters/
"European audio engineers had another nickname for VU meters, 'virtually useless'."

Click to expand...

If you trust your ear for the global balance "real" Vu still usefull for 2tk main out ! they give you a great and quick view of your global energy coming out...

your link state a strange assertion at "Vu disadvantage"

The 'volume unit' standard assumes that peaks will not exceed 10 dB above the reference level, which is inappropriate for almost all modern recording.

Click to expand...

this assumption don't detail that 10dB above ref level is above line level not 0Vu level, which is 8dB over 0Vu (+12dBu line level @ 1k sinwave calibration status) it end at +22dBu...
0Vu<=>+4dBu assume you system can handle +22dB peak

I daily use a "real balistic" Vu (plasma embedded no mechanical needle) at console analogue output, in a digital modern recording "tape" (a computer...)
so as you say it's really a matter of opinion,  as practice and  "workflow"

Ok I admit I have a PPM (needle) in parallel 

Best
Zam