Yamaha PM1000/PM2000 un buffered VU meters

JW · Jul 1, 2018

I'm wondering if someone can point me in the right direction here. I recently noticed in trying to trouble shoot noise in my DLA2A, that the meters I am using are the culprits. They are nicely made VU meters from a Yamaha PM1000 console. I have big carling switches for the meter switches. I'm using shielded cable. But. Now do I need to buffer the VU meter? Because when I disconnect the meter from the circuit, a lot of high end hash/noise goes away.

Anyway, I've of course built my studio around these meters (as well as neon bulbs) having a Full 'nother PM1000 console, a dual 1176 using them, and then 8 more of similar ilk in a PM2000 console. The concern duders is that all these VU meters are adding noise to my whole system and need to be buffered. Proof in the LA2A (?). But additionally, I always turn the meters off on my PM2000 console before the final sum/mix, because it definitely sounds worse to have the VU's on. Again, they're unbuffered, which is our concern. Which is to ask, what would be a prudent course to take? I'm guessing I can find some little boards with a chip, but then I have to worry about the power supply for said chip in all this gear. Is there a passive option? Where is a good place to start learning about what buffers do and why they're needed?

PRR · Jul 1, 2018

Audio is AC. This type meter is DC in its heart. It would just jiggle. To make it swing we add a rectifier.

Rectifying AC to DC is a nasty process, like turning live birds into supermarket chicken.

I assume your "noise" is not hiss or radio but distortion.

Yes, live in the >0.1% THD world or put buffers everywhere.

boji · Jul 1, 2018

This helped me. Also easy to diy, but yeah need to allot some power.

http://www.jlmaudio.com/AT51%20DIY%20VU.pdf

Matt C · Jul 1, 2018

I'm in the middle of installing VU buffers in my PM2000. I used the circuit in the article linked below, built on strip board (I think it's the same circuit in the JLM kit). You can power it from one of the PSU rails going to the lamp or peak LED circuit, I think it's 12V. It's a little tricky finding a good way to mount them with everything else in there but not that bad. In the case of the PM2000 it might also be simpler to use the meter's existing diodes and resistors since it's sort of a pain to bypass them.

http://www.audiotechnology.com.au/PDF/TUTORIALS/AT51_DIY_VU.pdf

JW · Jul 2, 2018

Awesome you guys. Thanks so much for the info and that page w/ schematic.

Matt, a couple questions for you. Are you doing all 8 in the PM2K? Did you make sure there's enough current in the lights or peak indicator circuit to power 8 TLO71's (which don't draw much anyway) Wait, now I'm remembering you are powering LED's now with the meter light circuit (?), so you probably have plenty of power. I haven't gone LED for the switch lights yet. Anyway . . .

Also, how close does the circuit have to be to the meter?

In my DLA2A and 2 1176 my mind goes right away to cheap off the shelf wallwart DC supply that I can strap to the AC and somehow mount inside the unit as a quick and cheap solution hopefully. Hmmmm. Thinking out loud.

Matt C · Jul 31, 2018

Yes I'm planning on doing all 8 main meters. probably won't bother with the FB/echo meters but who knows. Yeah I just assumed with the switch to LED meter lights that the power rail could handle a few extra TL071 chips.

I'm planning on mounting little circuit boards directly behind each meter, but that is a bit of a hassle (the main reason I haven't finished doing this yet). I'm totally guessing here but I don't think it matters if the buffer is physically close to the meter itself. You could probably just make a single board with 8 buffers, plop it down in some empty corner of the chassis, and run wires to each meter.