JLM Meter Buffer

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Holger

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Jun 11, 2006
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I often used Joe Malone's little VU buffers for my builds.
Today I'd like to share my findings what they are doing.
I used a 1kHz sine tone for testing. The first picture illustrates the output connected directly to the input.

direct.jpg


Then I connected a little Sifam AL 15. The dirt/distortion introduced by the meter is clearly visible.

unbuffered.jpg


This picture shows the VU buffer result. No fake, it's a different screen shot...

buffered.jpg



 
I've always thought it's a pity to connect a VU-meter directly across an output, particularly if it's a transformer-balanced output. A radical means of ruining the performance of an otherwise well-designed product.
 
Wow - thank you for posting your findings. I wonder if the results would be as dramatic when the meter is hung across something with a more robust drive capability, as opposed to the output of a soundcard?
-Ben
 
plumsolly said:
Wow - thank you for posting your findings. I wonder if the results would be as dramatic when the meter is hung across something with a more robust drive capability, as opposed to the output of a soundcard?
-Ben

Nope.
The distortion from the VUmeter's diodes is always there.

Just for the sake of it, I've tried some time ago with my Studer 269 [hehee sometimes we hope things will be _not_ so precisely like skilled people says],
the result is that now I know I've to make a 16 ch buffer if I still want to add this big nice Modutec-stuffed additional meterbridge.
 
Holger's not talking about hanging a VU meter directly across an output, rather the JLM Audio VU meter buffer circuit. The diodes are isolated by a TL071 configured as a balanced/unbalanced receiver and buffer. This should not be introducing the harmonic distortion shown.

http://jlmaudio.com/VU%20Buffer.htm

My question is, are you using Joe's PCBs or just the circuit (which isn't his)?

Simply put, I would suspect the build or connection and not the design.
 
skipwave said:
Holger's not talking about hanging a VU meter directly across an output, rather the JLM Audio VU meter buffer circuit. The diodes are isolated by a TL071 configured as a balanced/unbalanced receiver and buffer. This should not be introducing the harmonic distortion shown.

http://jlmaudio.com/VU%20Buffer.htm

My question is, are you using Joe's PCBs or just the circuit (which isn't his)?

Simply put, I would suspect the build or connection and not the design.

Erm.. the "bad" Holger's plot _is_ of a VUmeter directly connected across an output.
[And so my results out of the Studer.]

 
Thanks 1954U1.
My not very scientific but real world findings were about the fact that the JLM VU buffer does exactly what it is designed for.
Pic 1: output measured directly, Pic 2: meter across the output, Pic 3: meter buffered via Joe's PCB.
 
plumsolly said:
Wow - thank you for posting your findings. I wonder if the results would be as dramatic when the meter is hung across something with a more robust drive capability, as opposed to the output of a soundcard?
-Ben
THD introduced by non linear loads are directly related to the actual output impedance (not the rated load impedance) of the the stage it is bridged to; the lower the impeadnce, the higher is its capability to drive the non linear part, hence the lower the distortion. On a very low output impedance stage, such as the direct output of an opamp, THD can be negigible. But in real life, most output stages have a protection resistor and a DC-decoupling cap, which increases the source impedance.
Most soundcards have an actual output impedance of 100-600ohms. A typical pro audio mixer has an impedance of 22-100 ohms. It is obvious that THD will be smaller on the latter. A transformer output introduces a significant leakage inductance to the output impedance. The mix of inductance and switching non-linearities is a very bad combination.
 
abbey road d enfer said:
I've always thought it's a pity to connect a VU-meter directly across an output, particularly if it's a transformer-balanced output. A radical means of ruining the performance of an otherwise well-designed product.

but also very practical.

Let's have a famous example, the LA2A. you would need to add a meter buffer *and* PSU regulation/filtering to accommodate it. That's a pointless endeavor when you can just tell people to keep the meter at gain reduction setting at critical times.
 
I haven't seen anybody mention the standard 3.6K resistor usually added
to a "VU" meter circuit and how much this reduces the impact of the distortion.

On the other hand, I haven't built any gear without meter buffers for 25 years.
 
I agree with the last two posts.

The resistor in series with the meter is theorectically there to reduce this very distortion as I understand it.

As for the LA2A anecdote, most people whom use LA2A's in my humble experience use them in GR mode for most of the time anyway. It is true that the most practical answer is purely to flip the meter to GR if one really feels that there are certain moments when your recording cannot have any meter distortion without your client threatening to rip your ass out and wear it as a hat.

Holger, its an interesting issue for us to discuss. Thanks for posting this data.
 
Also with the 3.6k resistor added, the distortion is still there.
Its not so loud [the distortion], but anyway, in my example [the Studer 16ch, but also with the 1176, same thing], I dont want to go through months spent in recapping, rewiring, studying,
and at the end having at least -74dB of THD added only for some little diodes bridges..
I mean, if I want distortion I want to decide where and when..

Its also true that for e.g. a standalone compressor you can always use the GR meter only for reference..
 
The 3.6k resistor is there not so much for reducing distortion than making the scale correct. If you omit this resistor, the meter will have nominal deviation at ca. 0dBu instead of +4, but the scale will be compressed, giving too small useful range. The 3.6k resistor, in combination with the internal resistance of the meter, combine to reduce the distortion of the rectifier.
 
Arguments for the buffer:

It eliminates distortion (which IS -as has possibly not been fully grasped by some- less of a problem with super-low source impedances).
It means that you can meter with impunity.

Arguments against the buffer:

It may require more power supply rails
It completely removes the benefit of galvanic isolation

-That last one right there is the deal-breaker for me most of the time.

If you're using an LA3, LA4, 1176 or LA-2a, you probably meter in GR most of the time, and only occasionally check the output during setup. -In that case, why bother?

Keith
 
Arguments against the buffer:

It may require more power supply rails
Yes needs +9v to +36v at about 10mA. We run it on higher rails with just a series resistor.

It completely removes the benefit of galvanic isolation


-That last one right there is the deal-breaker for me most of the time.
When using the buffer with a device that has a output transformer and you want to keep the galvanic isolation just connect the buffer to the primary side of the output transformer which can be unbalanced or balanced and use the att/gain trimpot on the buffer to cal out the output transformer step up or down ratio level difference. That way the output transformer secondary is still fully floating. The only draw back is the VU meter will not reflect any losses in the output transformer under widely different loads like 600ohm to 10k which could be a db or two over that range of loads.

If you're using an LA3, LA4, 1176 or LA-2a, you probably meter in GR most of the time, and only occasionally check the output during setup. -In that case, why bother?
yes we tell owners of the above to just leave the meter in Gain reduction whenever recording.
 

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