Simpson VU Ballistics Variation

[MrG]

Adding here, I multimetered the hot to cold on both meters when disconnected and they're nearly identical...

EDIT: more additions. As I read through some of the linked threads here again (with Geoff Tanner for example), I'm wondering if these meters already have the 3.6k resistor built in.

Additionally, I'm thinking that my "balanced" feed is not balanced accurately, and maybe it's better if I revert to how I was running these unbalanced in the first place. I'll try these things and do some more testing and report back here. Sorry for all the messages.

Thanks,
MG

 

[MrG]

Ok folks - sorry for the blasts as it's seeming like this thread is dead.

Just to conclude, I measured the impedance while loading the meters with a 1k tone at 1.23v, and the one meter was 3.9k, and the other 2.9k. So I'm assuming these have the appropriate resistors internally.

I ended up adding nearly 1k to the one meter which matched it to the other meter's response, and I think this is the best it'll get unless I open them up, which I may do at some point.

Thanks for all of your help here.

Best,
MG

 

[abbey road d enfer]

Ok folks - sorry for the blasts as it's seeming like this thread is dead.

No, this thread is not dead, but my conclusion after all is that something is damaged in one of the meters. It may be the rectifier, shorted turns, damaged spring or a loose magnet.

 

[MrG]

Thanks for this, Abbey. I wasn't trying to be rude or short at all, just figured I didn't want to wear out my welcome and you've already been very generous.

Ok, good to know. If you were me, would you attempt to open it and see about a repair carefully? I could continue to limp it along.

The only strange thing is that neither meter read exactly 0 at +4 dBu. So maybe they're both broken, OR both modified internally?

I'm on the hunt for another pair of vintage Simpson's so if I do try to fix these and fail, I could potentially still have the others.

Thanks!
Mark

 

[blakeyboy]

One fault possible is to lose a germanium diode in the bridge rectifier, either short or open.
To check this isn't the case, connect to the teminals a multi meter set to ohms or diode test.
Try different ranges on the multimeter until it's driving the vu mater needle to around the middle or so.
Then swap the leads over- I.e. swap polarity. If the needle is driven up the scale to a different point, you have a problem diode or two. Trying this with the needle just moving up the scale, and then with the needle moving almost to maximum will show up different conduction curves in the diodes, which can happen, or is produced when someone changes a diode when fixing the bridge.

You can do this test easily if you have a variable voltage dc supply available, with a resistor in series to help the sensitivity of the test.

3k6 is the standard, but isn't that accurate- I fit every Neve with a 3k resistor and a 1k trimmer, and I set the needle to read 0VU with 1.228v going in ( after checking the mechanical zero first! ) Then you can trust them.

 

[MrG]

One fault possible is to lose a germanium diode in the bridge rectifier, either short or open.
To check this isn't the case, connect to the teminals a multi meter set to ohms or diode test.
Try different ranges on the multimeter until it's driving the vu mater needle to around the middle or so.
Then swap the leads over- I.e. swap polarity. If the needle is driven up the scale to a different point, you have a problem diode or two. Trying this with the needle just moving up the scale, and then with the needle moving almost to maximum will show up different conduction curves in the diodes, which can happen, or is produced when someone changes a diode when fixing the bridge.

You can do this test easily if you have a variable voltage dc supply available, with a resistor in series to help the sensitivity of the test.

3k6 is the standard, but isn't that accurate- I fit every Neve with a 3k resistor and a 1k trimmer, and I set the needle to read 0VU with 1.228v going in ( after checking the mechanical zero first! ) Then you can trust them.

Thanks a lot for these insights, blakeyboy. Just a few quick questions:

-Given your last statement, and that the two meters do function very similarly with a slightly different resistance added, might they be perfectly fine, just not totally matched (as I believe you're saying is fairly normal)?

Trying this with the needle just moving up the scale, and then with the needle moving almost to maximum will show up different conduction curves in the diodes, which can happen, or is produced when someone changes a diode when fixing the bridge.

Regarding this point, can you clarify if this is a correct/desired behavior, or if you're saying this will be indicative of a problem component? I don't know whether you're saying it would be ok if someone changed a diode.

Thanks for everything here,
Mark

 

[blakeyboy]

Hi Mark
The difference in resistance from 3k6 is often very small- often only third a dB on the scale at most,
so I don't worry about any changes.

If you have a changed diode or two in the bridge, you will have a different forward voltage drop, inevitably,
and this will be asymmetric- positive going peaks will drive the needle a different ammount to negative going peaks,
and since the reading, on fast changing material, is the mechanical sum of the negative and poistive drives on the movement, the needle'behaivior will cahnge, and not for the good.....

 

[MrG]

Thanks again blakeyboy. Since you posted this, I recently got a bench supply with variable output, so I’d love to try this test as properly as possible, and learn the theory behind it.

Would you kindly share a bit more info about what I should send into the units for calibration? They’re completely passive at the moment, so I don’t know what amount of DC to apply to them to do this test.

And also, I think I understand what you mean now about the diodes - I believe you’re saying a negative voltage will read at a different level than a positive voltage if a diode is damaged.

If I wanted to repair these, how would I most safely open them and work on them? Is there a method you could suggest using to keep them clean, or simply just clean your area as much as possible?

Thanks as always,
MG

 

[MrG]

I just wanted to post an observation which I think makes sense. Regarding these meters which (to remind readers) may have a diode issue, when I first hooked them up, I did so unbalanced, hot to one post, ground to the other. The meters performed the best/most naturally that they have the whole time in this way. After thinking about @blakeyboy 's suggestion of flipping polarity of voltage to test deflection, I'm thinking maybe that a cheap/bad fix for the issue of diodes is that if you're only sending both meters one polarity of signal, they deflect similarly (maybe by luck of picking the right pole?) due to the test you mentioned blakeyboy.

Is this logical thinking? I just keep coming back to why they behaved so well when I ran them unbalanced, and I've been having to sort issues ever since.

And on that note, is unbalanced connection to them best to do via + on one post, and GND on the other, or should I strap positive across both posts? I recently got a pair of great VUs where someone had strapped the positive across both terminals, so I wanted to be sure.

Thanks so much,
MG

 

[ccaudle]

Is this logical thinking?

No. Audio is an AC signal, using just one leg of a balanced connection, the signal will still go above and below "ground" level. You need to know details of the output driver circuit to know how to correctly connect a balanced output to an unbalanced input. The only universally safe way (safe in the sense of not risking damaging the output, but also safe in the sense of having a predictable output level) is to connect a balanced output to a balanced input.

should I strap positive across both posts

What are you calling "posts"? Do you mean terminals of the meters? Can you be more explicit in what connection you are describing? Hot leg to one meter terminal, and also hot leg of the same channel to the second terminal of the same meter? That would be the same signal on both sides of the meter, no signal difference so no meter deflection.
Hot leg of one channel to the first meter terminal, and hot leg of the other channel to the second terminal of the same meter would show the difference between the signals, essentially a L-R meter, or a side meter if thinking of it in mid-side terms.

Or are you not referring to the meter terminals at all, but to the input terminals of the buffer circuit you were originally using, the one based on an SSL design?
It is not clear looking at your earlier posts whether you are still using the buffer circuit, or if you removed that and are trying to drive the meters directly from the line level output of your audio interface. If you are familiar with drawing schematics that would be the most precise way to describe the connections you are asking about.

 

[MrG]

My apologies, ccaudle, and thanks for calling it out. You are, of course, correct in that I was being far too vague/casual. I will be more specific here, and also respond to a few of your helpful points. Thank you very much for that also!

First to summarize, I am discussing the differences between wiring the electronically balanced stereo outputs of a Grace M905 monitoring controller, into a passive box with two vintage Simpson VUs, fully balanced (pin 2 to one terminal, pin 3 to the other, ground floating), with a 3.6k resistor and a trimmer in series on one leg of the circuit, to one terminal per meter.

My theorizing in my previous post was regarding the fact that when I first got these meters - without doing a bunch of research - I hooked them up unbalanced, with pin 2 to one terminal, pin 1 (ground) to the other terminal of the same meter, and jumpered pin 3 (cold) to ground, and they behaved the most linearly and also similarly between the two meters that they have throughout my modifications of the configuration. I then went on to research, hook them up balanced, etc.. Responses below!

No. Audio is an AC signal, using just one leg of a balanced connection, the signal will still go above and below "ground" level. You need to know details of the output driver circuit to know how to correctly connect a balanced output to an unbalanced input. The only universally safe way (safe in the sense of not risking damaging the output, but also safe in the sense of having a predictable output level) is to connect a balanced output to a balanced input.

Definitely hear you and understood. I will follow up more below on that. What I was toying with was simply the idea of the postive and negative sides of a balanced cable being sort of in contrast with eachother voltage-wise.

What are you calling "posts"? Do you mean terminals of the meters? Can you be more explicit in what connection you are describing? Hot leg to one meter terminal, and also hot leg of the same channel to the second terminal of the same meter? That would be the same signal on both sides of the meter, no signal difference so no meter deflection.

I answered this one above and yes sorry - terminals of the meters.

Hot leg of one channel to the first meter terminal, and hot leg of the other channel to the second terminal of the same meter would show the difference between the signals, essentially a L-R meter, or a side meter if thinking of it in mid-side terms.

This is very interesting, and I did not know this. I bought vintage Weston meters that had wires cut off, with one wire incoming to one meter, and then jumpered over to the other terminal of the same meter. When I saw this, it prompted my question, but of course it is not correct for my application.

Thanks again, truly. It's a great reminder that thoroughness is welcome on this forum (when I have to temper that in other facets of my audio work!).

Best,
MG

 

[ccaudle]

jumpered pin 3 (cold) to ground

The wiring diagrams in chapter 19 of the M905 manual show that pin 3 should be left open when connecting to an unbalanced input. That would indicate that the Grace output stage is dual amplifiers with no cross-coupling. If you jumpered pin 3 to ground at the output of the M905, the output amp for the cold side is driving into a short circuit, with only the build-out resistor to limit current. The specs indicate 300 Ohm balanced output impedance, so presumably driving full output through 150 Ohms to ground. Not recommended.

If you leave pin 3 from the M905 open as recommended, when using just pin 2 and pin 1 the level will be 6dB lower than when connecting from pin 2 to pin 3.

the idea of the postive and negative sides of a balanced cable being sort of in contrast with each other voltage-wise

Depends on the output circuit. With your M905 then yes, the hot and cold leg will be complements, when one side increases the other side decreases. They each go above and below 0V, which is why the meter has a diode bridge, it provides a conduction path no matter which terminal is positive with respect to the other.

Have you used test tones at various levels with your DAC set to a known calibration just to see if each meter is responding accurately? If you are using the DAC in the M905 it has variable output level line-up, so you will have to decide if you are setting full scale digital to 18 dBu, 20 dBu, 24 dBu, etc. and work backwards to see what 4 dBu would be relative to 0 dBFS. You can then use digitally generated sine waves at various levels to check whether the meter is responding as you expect.

And of course verify that the meter is not electrically damaged as mentioned previously. Since 4 dBu is about 1.23V RMS, which is 1.74V peak, you could use a AA alkaline battery to put 1.5V DC through each way and confirm that the meter registers the same in each polarity. Should be simpler than messing with your variable bench supply if you have a battery holder lying around, or just tape some wires on a AA or AAA battery, you're just trying to do a quick check, it doesn't have to be durable.

 

[vmsa]

I think this paper that I wrote a few years ago might be useful. It was about identifying meters, and during my research, I discovered why VU meters are different ( in ballistic or dynamic response) from other audio meters. I hope you find it interesting. You can access the paper at this link: https://www.researchgate.net/profil...o-Evaluate-the-Ballistics-of-Audio-Meters.pdf

 

[abbey road d enfer]

Thanks for this paper. Very refreshing after days of wrestling with insanity.