VU METER BOX REBUILD AND REVIEW OF JLM VU METER BUFFER CIRCUIT KITS

35+ YEAR-OLD DIY VU METER BRIDGE REBUILT WITH JLM DIY BUFFER KITS

by Mark King for proworkshop.com

DIY QUAD VU METER PACKAGE ORIGINALLY CONSTUCTED IN 1979

Back in 1979 I had virtually no money to spend. I was playing in a band and taking college courses so recording was very low on the financial priority list. There was no internet and raw hardware components were hard to find and expensive.

RACK PANEL ON LEFT, CHASSIS ON BENCH, NIBBLED OUT HOLES LET METERS MOUNT

I wanted to build some rack mount equipment so decided I could make a low cost two-rackspace enclosure by attaching a standard rack panel to an aluminum chassis. After building the prototype of the rack mount packaging I needed to build something into it so I decided to build a quad VU meter bridge.

ORIGINAL BREADBOARD VU METER DRIVER CIRCUIT FROM 1979

ORIGINAL 741 OP AMP INTEGRATED CIRCUIT ON BREADBOARD DRIVERS

I used Radio Shack breadboard to make the circuits required to drive the VU meters. Each meter had an adjustable trim pot to set the 0-VU level point. Since the op amp is not in the audio path I could use inexpensive 741 DIP type parts because the sound quality was not important to the meter.

ORIGINAL BUILD RCA PHONO CONNECTOR ON EACH METER

On the original build each meter was connected via an RCA/phono type connector. This allowed me to do lots of testing as I was prototyping the circuits.

ORIGINAL POWER SUPPLY BREADBOARD

COMPLETE ORIGINAL POWER SUPPLY SECTION

I have no data about the power supply design, I don't remember how I came up with the original layout, there are no regulators in it.

I've been carrying this original VU meter package around for a long time. Back in the 80's most of my recording system was unbalanced Tascam and Fostex equipment. It was very difficult controlling ground loops with that equipment but we ended up making some good sounding recordings with it anyway. It's time to refresh this piece of equipment with new guts.

BALANCED OUTPUTS ADDED AND NEW 2.1mm 12-VOLT DC POWER INLET JACK

Now it is 2017 and it is time for this old horse to get updated to balanced inputs. This time, instead of breadboarding my own driver circuit I took the easy road and ordered VU Meter Buffer Kits from Joe Malone's JLM Audio in Australia. Joe is known internationally for the great studio gear his company builds and the wonderful kits they provide. I ordered four VU Buffer Kits from his web site without difficulty and within a couple of weeks the package arrived. I was excited to build this but had many more pressing tasks so this one moved to the back burner and sat for a year.

Recently I built a quad attenuator package for our API quad microphone preamp, I included four VU meter outputs because I intended to mate that up with a quad VU meter box that comes after the attenuators. I wanted the VU meters to be calibrated to dBu which is the level I use in our system. I'd been watching for something used to pop up on eBay but no such luck for any reasonable price. I finally decided to go dig out my old VU meter bridge and install the JLM kits.

BUILDING THE CIRCUIT BOARDS
Each JLM kit comes in a small zip lock bag and includes everything needed to build the kit, the circuit board and components are all provided in the complete kit. I assembled them one at a time. I thought about batch processing them but that would have taken up more space. Sometimes you learn something from building one kit, then you can apply that learning to all the others. I did learn something, be very careful about polarity. I accidentally soldered one of the balanced input connectors backwards, fortunately I saw it immediately but then I had to confront the ugly unsoldering tool and kill half an hour trying to remove the poorly installed part without destroying it or the circuit board. Having that little problem early in the process made me extra hyper vigilant as I added other polarized components.

There is one 100 ohm resister and one 1-meg ohm resister in the kit so I started with these two, the other remaining resisters are only two values, 3.3k and 47k. I installed all the resisters before moving to the other circuit board components.

STARTING TO POPULATE A JLM VU BUFFER CARD

JLM does not provide build instructions, instead JLM refers you to their internet forum where they have threads about all their kits. The beginning of each thread contains pictures of the circuit board and basic assembly instructions but the information could be presented in a much more clear manner. I could find no photo of the finished assembled unit which would be a welcome reference if you've never worked with one of these before.

My four VU meters have internal diodes, this requires a mod to the circuit which I could not find on the JLM web site. As it turns out it was right in front of me but I was so distracted by the forum build thread method of instructions that I did not see the picture showing the mods.

I knew I needed the mod because my meters have the diodes, I finally had no choice but to email Joe for help. Within 12 hours of my email Joe had sent me the answer I needed. I printed it out and hung it above my assembly bench for easy reference.

I made no more errors during assembly, I went slow, I cranked up some blues by Walter Trout and spent half an hour per circuit board carefully installing the components and checking my solder connections with a magnifying glass to be sure there were no bridges or cold joints.

FINISHED UNITS ARE SMALL AND LIGHT WEIGHT

The quality and layout of the JLM circuit boards is excellent. The solder pads are clean and take my lead based solder just fine. Every component location is very accurately labeled, they've been making these for quite a few years so it's a very mature and well thought out kit like all the JLM products.

COPPER SUPPORT LEGS READY TO ATTACH TO METER

VU BUFFER CIRCUIT BOARD SOLDERED TO BACK OF METER WITH COPPER LEGS

The VU buffer circuit boards are very small and they don't have any heavy components. The assembled kits are so light weight Joe recommends soldering them directly to the back of the meter. My meters did not have any exposed tabs so I fabricated small brackets from solid copper wire. These brackets would keep the board up away from the aluminum chassis where they could short out. I would not use this type of construction on a piece I intended to haul around in a truck for live work but for stationary mounting in our studio this means of attaching and mounting the circuit boards would be more than adequate. Joe has included long mounting points across one side of the circuit board to facilitate soldering them directly to the meters.

NOTE LED ON RIGHT BOARD, SOLDERED DIRECTLY TO CIRCUIT BOARD TO COMPLETE THE BUILD

These buffer circuits include a peak LED. Because my old VU meter box was very limited on front panel space I decided to not mount the LEDs for use, instead I simply soldered the component right to the circuit board. I did not want any instability or oscillation from not having that side of the op amp terminated.

The JLM VU buffers are driven by a single 12 volt DC supply (not provided with the kits). I used a small 12 volt DC supply that I bought from Amazon for $4, it has a 2.1mm connector just like stomp boxes use for external power. I have some of these 2.1mm panel mount female connectors in stock so I mounted one of these to the rear of my VU enclosure to get power into the buffer circuits.

12 VOLT DC CONNECTOR ON THE LEFT, REUSED ORIGINAL POWER CORD HOLE

I completely disassembled the original construction, removed all the meters, repainted the faceplate screws to be black and I added eight TRS female Switchcraft jacks on the back of the enclosure. Four of the jacks are for the four meter inputs, the other four jacks are wired in parallel so the signal can be passed back out to go some place else if needed.

PARALLEL INPUT CONNECTORS ON LEFT, POWER SUPPLY INLET ON RIGHT

After reassembling the enclosure I presoldered all the rear jack connections and mated them with the plug-in cords that feed each meter. The power supply connections were next, I twisted all the wires together, soldered them and then wrapped them in electrical tape for insulation. It's not heavy duty style construction but fine for sitting in my mixing console.

Finally it was all done and ready for testing. I fired up the oscillator, calibrated it to 0-dBu and then stuck the signal in to the meter, nothing. Was it on? Was power getting to the circuit? An improvement I could make would be to add a power present LED indicator but I did not want to do that now, I just wanted to test the box.

COMPLETE AND READY FOR TESTING

I grabbed a tiny jewelers screwdriver and began spinning the 25 turn trim pot that controls level to the meter. Nothing. I consulted the drawing to verify the trim pot control and discovered I was turning the trim for the peak indicator. Armed with the correct information I began turning the correct pot, round and round and round until, low and behold the needle began to move. I turned the trim until the needle on the VU was at zero, then unplugged the signal source to see if the needle would go back down, it did. It worked!

All four channels fired right up and worked perfect. I was very happy, my old friend was back to life with a new heart and I had what I wanted, four VU meters for my API mic preamps. The combination of the VU meters and the output attenuators has made my 3124 API into a great source of microphone audio, great sounding, easy to adjust in every way and now with the addition of the VU meters the API is more useful than ever.

REBUILT VU METER BOX ABOVE API 3124+

I installed the VU meter bridge above the API mic preamps so it's very easy to see the one to one relationship between the preamps and each external meter.

The VU meters in my box originally had internal lights in them which would be nice to have, but 35+ years of use combined with traveling 2200 miles from St Louis to LA and then the even longer journey from LA to Florida have taken a toll on these inexpensive little meters I bought at Gateway electronics so long ago, the light bulbs are non-functional and not replaceable without dismantling the meters. These meters were really inexpensive at the time and they were not made to take apart. In the interest of getting this rebuild done I decided to let the meters stay dark, the overhead lights in our studio make them easy to see without internal lighting.

I am super impressed with the ballistics and how the JLM circuits drive the meters, they are fast and have great response. You can not tell that the original meter units were made over 38 years ago, I think they cost about $15 each which was a lot of money to me back then. This box represented a lot of design work, metal work and hand fabrication. It is wonderful that after all these years it is back in business and doing good and important work in our modern recording studio.

I have quite a few more VU meter mechanisms stored out in our garage. I'd love to have VU meters on the group outputs of our Soundcraft mixing console and I intend to build that one of these days. To support my future project I made a quick return visit to the JLM web site and ordered the remaining 10 VU buffer kits they have in current inventory. More meters, more meters, I just can't get enough.

If you want to add a VU meter to a gadget or some piece of gear you have JLM offers several different kits and they have lots of raw meters to choose from as well. When I receive the next 10 meter kits I think I will lay them all out and assemble them batch style, this means install all the resisters on all the boards, then all the op amps, then all the caps, then all the diodes and so on. This is the most efficient way for me to do batches of things I'm familiar with.

If you like DIY or you just like cool studio signal processing I highly recommend you check out JLM Audio on the internet. I'll provide a link at the end of this article. My kits were all complete and perfect, nice work Joe!

Good music to all!

Click here to visit JLM Audio on the internet

COMPLETED RACK WITH REBUILT METER BRIDGE POWERED BY JLM AT THE TOP