Stereo "real" VU with phase check with bypass possibly?

[TheGuitarist]

I know this has been discussed before, but i couldn't find anything specifically like this.

I've seen this unit at work, and i'm sure it wouldn't be that hard of a project to make.

Basically its a 2ru box, with 3 VU meters, one each for left and right and then one for phase check. Would a unit like this be a complicated build? Would it degrade sound quality? If it does i suppose a bypass switch would cure that. Also, any suggestions for VU meters for such a project? Sifam vintage series? I'm not too sure with VU meters and which have good balistics. Hairball vu meter maybe?

http://www.proaudio.com/product_info.php?products_id=964

I found that, but at nearly US$650... seems like they're 100% profit or more.

 

[abbey road d enfer]

Would a unit like this be a complicated build?

The VUmeters are relatively easy, the phase correlation meter may be more difficult because it requires an electronic circuit that is not, to my knowledge, available on the market.

Would it degrade sound quality?

VUmeters, passively bridged across an audio line will introduce some THD. The amount is dependant on the impedance of the line and its characteristics (purely resistive or inductive). The only way to avoid that is to install a balanced-input buffer. The possibility to adjust the gain of this buffer is a bonus, since it allows to calibrate the deviation with the actual operating level.

Also, any suggestions for VU meters for such a project? Sifam vintage series? I'm not too sure with VU meters and which have good balistics.

IMO, SIFAM is the only manufacturer today offering DIYers a comprehensive choice of models and guaranteed ballistics.

 

[TheGuitarist]

Would a split work? Like a through on a DI, the untouched signal passes through untouched but then a split gets sent to the buffer, then meters.

Oh wait... that would also halve the signal impedence wouldn't it, so i would need to passively or actively split it, which may degrade the sound quality. Or are there any Xformers/something that will leave the signal clean.

I did just find this in my searches though:

http://www.groupdiy.com/index.php?topic=9301.0

It has a pcb layout and shcematic for a phase meter i THINK.

 

[doubleroger]

A buffer will not degrade your signal: you use it before the VUs, it's not on your real signal path. As you use an opamp like TL072, with a high input impedance, it does not cause any problem. You can do it on breadboard easily.

The phasemeter. I've done the led one in the aforementioned thread. Sucked. It was my first DIY too, so that might explain. The main problem was that all the lights went off as soon as there was no signal. I like to see a needle right in the middle in that case...
I finally got a good deal on a neumann card + a needle meter on ebay. I don't know about other designs.

 

[Gold]

Canford Audio in the UK sells a Sifam correlation meter with a driver card. You can buy just the driver card if you want.

 

[alexc]

Just in case you go for vu buffers - JLM Audio makes some quite good kits.

Cheers

 

[abbey road d enfer]

Would a split work? Like a through on a DI, the untouched signal passes through untouched but then a split gets sent to the buffer, then meters.

A passive split won't work, because it does not provide real load isolation. An active split uses a buffer. You need a buffer.

 

[TheGuitarist]

Well i suppose if i really wanted to be anal, all i would have to do to check if the signal was being degraded at all is just rerecord the clean signal and then the metered signal and flip the phase. But if you guys reckon the sound quality won't be touched with a buffer, hey, who am i to argue.

Canford Audio in the UK sells a Sifam correlation meter with a driver card. You can buy just the driver card if you want.

Just in case you go for vu buffers - JLM Audio makes some quite good kits.

Cheers

Thanks for the tips! But ouch, 200 pounds is just a BIT over what i'm willing to pay... maybe 150 pounds over?

 

[audiox]

D&R has used a relatively simple phase meter circuit in their consoles. You can replace the discontinued led driver with buffer and moving coil meter. Parts cost (without the moving coil instrument) is only a few euros.

http://www.d-r.nl/dnrsite/pdfmanuals/Phasemeter-Man+Serv%20(old).PDF

 

[doubleroger]

A VuMeter buffer is not on the signal path. All it can do is change a little bit the load on the output of your console / converter / whatever. That's why you use high impedance opamps for buffers, so the load change becomes neglectable.
So, no degradation of the audio.

 

[TheGuitarist]

Cool, i've been looking at some schematics and i sort of get how it works now, thanks for clearing that up.



A few euros ay... i think that may be the way to go, as long as it works well. Might even just keep the LEDs, look kinda cool, 2 vu meters and then the phase with LEDs underneath.


Does anyone know what ker refers to in the capacitor section of the BOM?

Obviously poly is film, and rad or axi are electrolytics with radial or axial... but whats ker...

 

[audiox]

Does anyone know what ker refers to in the capacitor section of the BOM?

Some languages use K instead of C.

 

[PRR]

> the phase correlation meter may be more difficult because it requires an electronic circuit that is not, to my knowledge, available on the market.

I was going to say that, but the $630 box he points to says "A phase button for the pair of meters places left plus right on the left meter and left minus right on the right meter. Comparing these two readings gives a quick check for correct phasing."

That's just a differential amp, or even just bridging the VU movement across L and R buffers.

So it is <$10 for the magic, and $620 for box, meters, connectors, manual, user hand-holding....

"left minus right" is a useful reading, will quicky show mono or super-stereo, but there are much more sophisticated ways to waggle phase. As discussed in the 2005 thread.

 

[TheGuitarist]

So if i added a third VU meter, would it be just as easy as feeding it the left and right signals with say the right signal with just the signal cables backwards to flip the phase. Crude, but kinda handy, nice, easy and cheap.

Unless of course someone wants to suggest a better way of doing this or would the one audiox suggested do it fine.

 

[SSLtech]

As PRR said: read the 2005 thread... What you're describing isn't a useful phase meter; it's FAR too amplitude-related.

It's all been covered and rediscovered.

Keith

 

[abbey road d enfer]

Well then, what's the purpose of a correlation meter?  (yes, I'm fussy about using the correct terminology - the thingys in a mixer are not phase-meters)
Vinyl mastering is such a niche issue today, that I don't think it is the primary concern of The Guitarist (correct me if i'm wrong).
I think, as always, we should know more about the intended use of this meter, what information is expected from it and moreover, what corrective measures could be applied to a presumed problem.
Designers of correlation meters have tried to achieve the ultimate mouse trap, which would indicate correlation down to -60 dB below and perfect indication irrelevant of spectrum.
When correl meters were used on vinyl lathes, that was way over what was actually needed, because phase inversions create groove width problems only at high levels and low frequencies.
When large studios still existed in my country, I've never seen a correl meter in a music production studio, only in mastering or duplicating labs. Nobody ever complained about this, or requested a correl meter added to the set-up (although some mixers had a L-R meter).
Today, in view of digital release (CD, DVD, mp3), why does one want to be informed about phase? And what a correl meter would indicate that a good listening test would not?
I wouldn't use a correl meter to check the phasing of snare bottom vs. top, or overheads vs. spot mics. A listening test with phase inversion is much quicker and IMO more reliable.

 

[TheGuitarist]

Mostly for things like overheads, because my control room is not completely isolated from the tracking room, its sometimes hard to tell if the overheads are in phas with themselves, or the rest of the kit, and the inbuilt protools phase meter shows me when its way out or if i'm getting close. Doing this would be quicker than recording a sample, listening for phase issue, running over, changing some positioning, etc etc. I just like analog things

 

[abbey road d enfer]

OK, I had not considered the impracticality of a listening test. I think you don't need a sophisticated correl meter to do that.
I would use a meter displaying a 1st-order low-passed (L-R) signal. This is quite easy to DIY, just taking the signals from the balanced-input buffers that feed the L and R meters, into a differential low-pass filter.
EDIT: resistors to meters should read 3.6k

 

[PRR]

You've low-passed the main meter outputs; did you really want to do that?

3.9K vs 3.6K... I forget which is right, but the 300 ohm of the matched-600 line was included in the original plan from 1939.

 

[abbey road d enfer]

You've low-passed the main meter outputs; did you really want to do that?

3.9K vs 3.6K... I forget which is right, but the 300 ohm of the matched-600 line was included in the original plan from 1939.

As usual, you're absolutely right. I've tried to edit my post but couldn't replace the attachment