Overdesigned VU Meter Buffer Amp
« on: January 23, 2020, 10:41:00 PM »
I've been lurking around this forum for a long while now, and I have learned a lot from just reading everything. I really couldn't find a particular sub forum for this, so this seemed the most appropriate even though it isn't technically a monitoring system or mixer. At the end of the day it really is just an over designed buffer amp for a pair of VU meters, but I am actually very pleased with how it turned out.

I have a MC624 and its meter outputs are tapped right after the input buffer and I figured it was high time I actually properly buffered my VU Meters, which I hadn't been doing for a good year or so. Right, wrong I dunno nor do I really care. I figured if anything it wouldn't hurt. I went surfing the web to see what was available and JLM has pretty nifty looking ones, and looking at them they appeared to be reasonably simple unity gain buffers. So, that is what I opted for initially. Just a simple unity gain buffer built around a couple of TL072s running off a single 28VDC rail. While it worked reasonably well it lacked in a couple of areas for me. First, was trying to locate a suitable AC-DC adapter and even finding a reasonably 28VDC one was not all that easy. Second, I am not often working with very hot signals in the digital domain and wanted to be able to amplify the signals. The end result is my VU Meter Buffer mk2. It works better than I expected it to.


Re: Overdesigned VU Meter Buffer Amp
« Reply #1 on: January 24, 2020, 09:46:39 AM »
I did a quick search for MC624 and did not get a good idea of what that is.

24v is marginal but probably OK for a unity gain buffer. What are the rail voltages of the source circuitry?

True VU meters are typically 600 ohm loads while TL07x op amps are only rated to drive 2k ohm loads.

Good luck... errors from marginal rail voltage*** and inadequate drive impedance may not be noticeable in typical use.


**** I learned this leasson the hard way trying to drive true VU meters from a 12V meter bridge supply. The customer noticed the difference and the customer is always right.  :-[
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


Re: Overdesigned VU Meter Buffer Amp
« Reply #2 on: January 24, 2020, 10:43:22 AM »
The MC624 is a monitor controller from Sound Skulptor. It is built around OPAx134s. Excellent device with lots of lovely relay clicking.

I guess I should have clarified in the original post. This is my second take on doing this. My first unit used a pair of TL072s to drive the meters and ran off a single 28V power rail. While it worked the meters didn't "feel" right.

So, in my second design for the buffer I again opted to keep it single supply and I wanted a single 12V input. However, I wanted to run it off a higher supply voltage and used a LM2577 to create a 34V rail that powers the buffer. Then on the buffer itself I'm using an OPA2134 for the input stage, a NE5532 to do the gain, and a pair of NJM4556s to drive the meters. The NJM4556 is a rather uninteresting device but does have the banner spec of having 70ma output capability.

Now the meters feel like they're moving much more correctly lol.


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