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4 Channel Mic Pre & Limiter

Posted in Modded, Racked Modules with tags compressor, limiter, mic pre, microphone, optical, opto, preamp, secret weapon, transformer on December 31, 2013 by AnalogAudioRepair

Another fun project for engineer and lover of odd gear, Dan Physics. He dubbed it the Mic RipPre 4. These are funky little portable mic preamps with opto limiters that were designed to run on two 9v batteries. They have Beyerdynamic input transformers and 600 ohm output transformers. I gutted four of them, recycling the XLR connectors and circuit boards, and fit them in a 1U rack case with a +/-15v power supply and +48v phantom power. The only other mod was a control for the amount of limiting. It’s not exactly threshold, nor is it really ratio – just “amount”.

They are pretty clean sounding preamps, no surprise with such a simple circuit. All new capacitors didn’t hurt either. One board needed a new diode in the limiter, but otherwise they were in good shape. Fitting all of the controls on the front panel was the hard part! The original preamps had nice little collars around the toggle switches so I used those to prevent accidents with phantom power.

Stripped down boards

laying out 4 channel mic preamp/opto limiter

Making sure it will fit.

mic preamp/opto limiter control panel layout

Control layout

hand drilling panel for mic preamp/opto limiter

Why I need a Drill Press

wiring 4 channel mic preamp/opto limiter

All wired up

Completed unit

Controls

Lots Of Gear

Posted in Modded, Repaired with tags limiter, Tube amp on September 17, 2013 by AnalogAudioRepair

Last month was super busy with work and a short vacation, so here are a few quick pics. This is the first post done from my phone so I apologize for the brevity.

First is a 1950s Gibson Les Paul tube guitar amp full of NOS RCA tubes. It had been improperly repaired previously (component soldered to the wrong side of a cap) and it had a 6V6 with an intermittent short. Found the fault and replaced the tube with a matching vintage RCA.

Next was a Drip 175 in for servicing. These guys are very particular about the 6BC8 that you choose! Calibrated nicely with a vintage GE tube.

Next is another Allison Labs filter mod with UTC transformers. Followed by another CBS Volumax mod, just because these pieces are so fun.

Vintage Tube Mixer Direct Ouputs (RCA, Ampex, Altec, Etc.)

Posted in Modded with tags Ampex, direct out, microphone, modded, preamp, RCA, tube, vintage on July 30, 2013 by AnalogAudioRepair

Not a lot of great pictures this time, but something I do get requests for. There are quite a few nice tube mixers around that sum two or more inputs into a single output. Often folks would like to use these as individual mic preamps with their own outputs. Your only choice is to use them as a single channel pre, or add some sort of direct out.

The issue is that these preamps use a single tube (or half of a dual triode) to amplify each input, then one or more tubes for EQ and output stages. Of course it would be costly to add identical output stages to each channel, even if the power supply could handle it. So the quick and dirty method is to take an unbalanced signal right off the coupling capacitor from the first stage. The downside is that these outputs can’t drive much of a load, and have a reduced level compared to the main outs. The upside is that they generally work well into the high impedance inputs of most converters and other modern gear. Also, by using a switching jack, the signal can be connected to/disconnected from the mix buss automatically.

I have done this recently with a vintage RCA mixer (MI-C38690) and an Ampex MX-35 and both clients were quite pleased with the results. The RCA also got input pads and a nice panel for all the connectors as well as some power supply cleanup. The Ampex was additionally modded with line inputs on the first two channels.

RCA tube mixer with input pads and direct outs

RCA Tube Mixer Wiring

RCA Power Supply Mess – Before Servicing

Ampex MX-35 Line Inputs

Ampex MX-35 Direct Outs

Zim-Gar Vintage Transistor Amp

Posted in Modded, Repaired, Restored with tags amplifier, discrete transistor, guitar, guitar amp, solid-state, tremolo, vintage, Zim-Gar on May 4, 2013 by AnalogAudioRepair

This is a funny little solid state amp I bought from a fellow on Craig’s List. I bought it “as-is” and it hummed and made noise but passed no signal. At first I thought it was Japanese, but when I took off the back panel there was a tiny schematic marked “Paris Corp. Salina, Kansas”! I replaced the first transistor which took care of the noise and lack of signal, but the tremolo wasn’t working. I rebuilt that section with some new caps and a resistor and dug up a vintage footswitch to turn it on and off. The power supply filter cap had already been replaced so I can only think that the amp has always had some hum (it was certainly not high-end when new). Adding a second stage of filtering killed the hum and made this into something that could actually be used. Honestly it was more work than the amp is probably worth, but a fun project nonetheless!

Zim Gar Front

Zim Gar Panel

Zim Gar Schematic

Zim-Gar Guts

Ampex 601 Tube Preamp & DI Mod

Posted in Modded, Restored with tags 601, Ampex, DI, modded, preamp, tube, vintage on March 22, 2013 by AnalogAudioRepair

This was a fun project that involved removing some tape deck electronics and modding the remaining circuit into a tube mic preamp with DI. As you can see, several of the tubes are not needed when using this as a preamp. A whole bunch of other parts came out too which cleaned up the crowded underside of the chassis a good bit. The headphone jack is now the DI input and the Input/Tape switch selects either the mic or DI. The Line Rec Level pot is now the DI level control and an XLR output jack has been added on the side.

CBS Volumax Limiter Mods

Posted in Modded with tags Audimax, CBS, compressor, discrete transistor, limiter, vintage, Volumax on December 29, 2012 by AnalogAudioRepair

: Lots of holes for lots of mods

: CBS Guts

: Inside the front panel

: Release time caps

: Controls and wires

: Left side controls

: Right side controls

: Front Panel

: Front Panel #2

: XLR Outs

This was a project for a client from Brazil – modding a CBS Stereo Volumax discrete transistor limiter. The Volumax is a two band limiter with transformers in and out. You can do fun stuff like turn off the high or low frequency limiting, vary the HF level, change the release times, and much more. This was a pretty standard mod that also included XLR in and out. Amazing units when you do a bit of work to them!

DuKane 2A81 Compressor Mods

Posted in Modded with tags 2A80, 2A81, compressor, discrete transistor, DuKane, limiter, modded, transformer, vintage on November 28, 2012 by AnalogAudioRepair

: 2A81 and 2A80B

: Dukane Guts

: Modded Circuit

: Modded Front Panel

: Close Up

Here are a pair of vintage DuKane compressor/limiters. They use an all discrete transistor circuit path with a FET for gain control. They come stock with a (useless) noise gate, two inputs, and some controls on the rear panel. These mods remove the noise gate, move the ratio and bypass to the front, and add release time settings. Everything gets cleaned up and boards are re-capped if needed. The results? “A super smash box” according to one user!

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