Archive for transformer

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

Controls

RCA meatball logo on BA-72A preamp

Meatball Logo

 

 

 

 

 

 

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Gates Sta-Level

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

A client gave me a beautiful original Gates Sta-Level to bring back to life.  It had all original components but had not been turned on in over ten years.  After testing all the tubes, I brought it up on my Variac with just a solid state rectifier installed.  The power supply filter caps were good, but not great so they got replaced first.  With the new PSU caps and the tubes installed the unit was working ok, but had a very strange frequency response.  Above 1kHz it was fairly even, though not quite up to spec.  Below 1kHz it increased steadily to about +12dBm at 30Hz!  It turned out the feedback caps from the 6V6 output stage to the 12AU7 were leaky.  I replaced those and the coupling caps and things are now back to factory spec.

Vintage Gates Sta-Level

Original Gates Sta-Level

Gates Sta-Level original guts

The guts – dead stock!

Gates Sta-Level power supply re-cap

New Power Supply Caps

Gates Sta-Level capacitors

Replaced Film Caps

 

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

Controls

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

Before

Before

Fixed Frame

Fixed Frame

Fixed PCB

Fixed PCB

Recapped

Recapped

XLR Box

XLR Box

Completed, Front

Completed, Front

Completed, Back

Completed, Back

Transformer Saturation Box w/UTC & Davens

Posted in New Build with tags , , , , , , , , on June 29, 2013 by AnalogAudioRepair

This is a neat box I built for Joel Hamilton at Studio G Brooklyn.  It is a simple circuit consisting of UTC transformers and Daven attenuators.  The idea is that a hot audio signal can be sent in, saturating the input transformer.  It is then attenuated by the Daven and sent back out through a second transformer.  Joel likes to use it on inserts of the SSL so he can pump a track through a high output compressor like a Sta-Level or Collins 26U and then get some extra transformer tone before returning it at a reasonable level to the board.  Since it is built into a 3 space rack case, we decided to get extra large knobs and dial plates for the Davens for a great vintage “black box” look!

Chassis drilled full of holes.

Chassis drilled full of holes.

Inside

Inside

Close Up

Close Up

Front panel

Front panel

Telefunken V72S Transformer Replacement

Posted in Repaired with tags , , , , , on February 26, 2013 by AnalogAudioRepair

This is a rare Telefunken V72S – the “S” is a higher gain version.  Unfortunately this one had a bad input transformer.  The frequency response of the transformer looked like this: /   A rebuilt original was too costly for my client, but Sowter offers a replacement that works just fine.  The hard part is removing the original and mounting the new one!

GATES Discrete Microphone Preamps

Posted in Racked Modules, Restored with tags , , , , on December 29, 2012 by AnalogAudioRepair

This is a pair of Gates broadcast discrete transistor mic pres.  These microphone preamplifiers were used in Gates radio boards and have transformer balanced inputs and outputs.  They have 37 dB of gain and I racked them with a level control ranging from 20-37db.  The position of the level control is immediately after the input transformer so there is no need for a pad on the input unless you are overloading the transformer!  These units were re-capped and wired for 150 ohms in and 600 ohms out.  This was a super simple and clean rack job so they only have XLR in and out, level control, and a power switch for the outboard +30v power supply.