Archive for microphone

RCA BA-72A Preamps

Posted in Racked Modules, Restored with tags , , , , , , , , , , , , on January 22, 2014 by AnalogAudioRepair

A client in Los Angeles wanted a pair of RCA BA-71 preamps racked with the option of using them to run some stereo tracks through.  These use the same circuit as the BA-31 (minus power supply) and the BA-72.  I had a few BA-72A modules in stock and came up with this single space rack mount solution.  First the modules were tested, re-capped and tested again.  Then I built a power supply with 30 VDC for the preamps and 48 VDC phantom power.  The modules were wired with a switchable 20dB pad as well as a switch to change the input transformer from 150 ohms to 600 ohms.  With both the pad and 600 ohm setting, the preamps can take up to 0dBm input without clipping.  Of course the output is +20dBm at that point, so I added 600 ohm attenuators to the output.  Everything fit nicely in the single rack unit and I put collars around the +48 switches, original RCA knobs on the outputs, and a black and silver RCA “meatball” logo on the front panel for good measure!

recapped RCA BA-71 72 31 transistor preamplifier

Recapped Preamp

connecting RCA preamps BA series discrete transistor

Wiring in Progress

Wiring RCA preamp rack BA-72A

Wired Up

RCA BA-71 vintage preamplifier front

Front View

RCA BA-72A Preamp back panel

Back Panel

RCA BA-72 discrete preamp controls


RCA meatball logo on BA-72A preamp

Meatball Logo







4 Channel Mic Pre & Limiter

Posted in Modded, Racked Modules with tags , , , , , , , , 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.

mic preamp/opto limiter

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

4 channel mic preamp/opto limiter

Completed unit

knobs and switches


Neumann CMV-563 Recap

Posted in Repaired with tags , , , , , , , , , , , on December 3, 2013 by AnalogAudioRepair

The Neumann CMV-563 microphone body can be used with different capsules including the famous M7 (cardioid) and M8 (figure eight).  This unit was getting noisy and swapping the tube did not entirely solve the problem. The two critical capacitors are a polystyrene cap which couples the capsule to the tube, and a film cap connecting the tube to the output transformer.  We decided to replace the third cap as well, although it was not original (green chicklet cap in the Before photos). The polystyrene cap requires extra care. A heat sink during soldering prevents damage to the component, then everything in this high impedance area must be carefully cleaned.  Any flux or oils from fingers can cause noise.  The other two caps were replaced with Wima film caps, increasing the size of the output cap to 1mfd. This was a really tight fit, but worth it. In the end, with a fresh NOS tube (not shown) the mic sounds great and the noise floor is much lower.

Before - cheap replacement cap

Before – cheap replacement cap

original Neumann CMV 563 polystyrene capsule coupling capacitor

Original polystyrene cap


Heatsink for Neumann CMV 563 tube microphone capacitor replacement

Heatsink on the new polystyrene capacitor

Neumann CMV 563 vintage tube condenser microphone output coupling capacitor

Original output coupling capacitor

Neumann CMV 563 vintage tube condenser microphone Wima output coupling capacitor

Wima 1.0uf output coupling capacitor

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

Posted in Modded with tags , , , , , , , 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 with input pads and direct outs

RCA Tube Mixer Wiring

RCA Tube Mixer Wiring

RCA Power Supply Mess - Before Servicing

RCA Power Supply Mess – Before Servicing

Ampex MX-35 Line Inputs

Ampex MX-35 Line Inputs

Ampex MX-35 Direct Outs

Ampex MX-35 Direct Outs

Telefunken ELA V624 Discrete Preamp & EQ

Posted in Repaired, Restored with tags , , , , , , , , , , , on June 29, 2013 by AnalogAudioRepair

Bad packing, or should I say NO packing, is to blame for the damage this preamp suffered.  It was shipped to me in a small flat rate Priority Mail box with nothing to protect it.  As you can see, the frame was bent and one of the circuit boards was broken into six pieces!  The frame was easy enough to bend back, but the PCB was like a little jigsaw puzzle that required epoxy and jumper wires to make it structurally and electrically sound again.  While I had it open, I replaced the electrolytic caps and added XLR ins and outs as well as a power jack with regulation.

The ELA V624 is a nice little module from later Telefunken mixers.  It has a mic pre with 60dB of gain, high and low EQ (I believe at 60Hz and 10kHz), and a line input as well.  Both the mic and line have transformer balanced inputs and I wired the unit with an impedance balanced output.  The circuit is fully discrete transistor and runs on +24v DC.  They are not as huge sounding as the earlier discrete modules (V672 Pre, W695 EQ, etc.) but very useable on a variety of sources.  Certainly worth the time spent repairing such unnecessary damage!

Bent Frame

Bent Frame

Broken PCB

Broken PCB

Broken PCB 2

Broken PCB 2



Fixed Frame

Fixed Frame

Fixed PCB

Fixed PCB





Completed, Front

Completed, Front

Completed, Back

Completed, Back

Altec Coke Bottle Mic PSU

Posted in Repaired with tags , , , , , , on November 28, 2012 by AnalogAudioRepair

A client sent me a vintage Altec M11 “Coke Bottle” tube microphone that needed re-capping.  The 518A power supply has a lot of caps, but the new ones are much smaller.  After re-capping and a new tube the mic sounds great!

Universal Audio 1108 & 508

Posted in Racked Modules, Restored with tags , , , , , , , , , , , , , on October 7, 2012 by AnalogAudioRepair

This is a set of vintage Universal Audio (UA / Urei) 1108 discrete transistor microphone preamplifiers with matching 508 passive equalizers.  The preamps are transformer in and out and sound amazing.  The EQs use passive capacitors and inductors for simple but smooth EQ.  Restored and racked with phantom power and gain pots.

Yamaha PM1000

Posted in Modded, Racked Modules with tags , , , , , , , , , , , on September 19, 2012 by AnalogAudioRepair

Yamaha PM1000 discrete transistor console strips cut down, recapped, modded, and racked as mic pres with EQ.

GE 4BA1 Preamp

Posted in Restored with tags , , , , , , , on August 5, 2012 by AnalogAudioRepair

Vintage GE 4BA1 vacuum tube mic preamp.  Recapped (electrolytics inside the old can), re-tubed, and wired up with a restored GenRad regulated power supply.

Neve 5442 Mixer

Posted in Repaired with tags , , , , , , , , , , on August 5, 2012 by AnalogAudioRepair

Neve 5442 8×2 Mixing Board.  Built custom power supply and replaced failed tantalum caps.