The impossibly rare 438C (NOT the common 436!!) microphone preamp + compressor circuit, with an additional mic/line mod and the elusive EMI rs124 compressor... all in one box! Possibly the coolest thing listed on reverb to today. Selling for pretty much what it cost me to buy and pay a tech to modify and restore. The 438 is identical in every way to the 436 with the added preamp stage and extra tube!

* Note that these are the "hero" primary on screen outboard compressor used in the Academy award nominated Bob Dylan Biopic, "A Complete Unknown" featuring Timothee Chalamet in 2025 (as shown in last pic).

If you’ve read the “Recording The Beatles” book or know the history of Abbey Road, you probably are aware of this elusive and highly sought after vari-mu compressor. It has had the full conversion and restoration by a world class tech at Custom Audio labs in 2023 (first pic) and features the identical topology of the coveted RS124.

In addition, I recently had it further modified (second pic) with a mic/line switch on the front (see second pic) in January of 2024 to be able to be used as a microphone OR line level source!

The 438C is identical to the 436C in every way... with one steep advantage: This is a compressor amplifier just like the 436 but it has an extra microphone preamp stage that the 436 does not have, differentiating it from the 436 by being at mic level when stock, instead of line level like the 436's. Can be used like a 436C or mic pre! This one has been modded to do both.

These are far more rare, although they look completely identical up front but have an extra tube and an extra transformer in the back. In fact, from the front, the only tell is the 8 and the 6 In the model number. I spent 4K on the donor unit, 1st mod was 1200, second mod another 400, and after Reverb fees, taxes and now income tax, I am breaking even on this unit at asking price so price is very firm. No returns, sold as is. Ask questions or come try it out in person before purchasing please.

The preamp section was built to handle even the most gain hungry ribbon mics such as the RCA44 and the Altec 639, boasting well over 90db of surprisingly quiet gain. The unique mic/line toggle mod lets you now send a line level to this unit or you can send a microphone, giving you the added benefit of having the full RS124 modified compressor section built right in for tracking or mixing like the ultimate racked channel strip! Recording a vocal that goes from medium to scream without needing to ride it. Or a dynamic snare drum take that goes from soft ghost notes to power punk. Nothing tames while simultaneously giving you instant Beatles tone and character without ever sounding over impressed, no matter the setting. You are somehow able to drive any source into oblivion until your monitors explode without clipping the digital realm, mostly because of how well the copper-core Peerless transformer beautifully rounds the tops of the waveform instead of snapping them off like other high end compressors.

As far as we know, we have not seen a 438C with the full elusive RS124 conversion completed. As you can see from the photos, almost every component has been removed and replaced with an entirely new topology of the Abbey Road EMI 124 schematic (actual printed elusive schematic will be provided with the sale).

We stayed true to the EMI schematic with two slight changes to improve them:

First, we doubled the value of the two signal rectifier coupling caps to improve low frequency gain tracking. Secondly, we had initially assembled and tested the stepped output pots like the EMI schematic calls for (as shown sitting on the schematic sheet in the 4th pic) but we realized that an ordinary 1K audio pot will give you about 3dB more output than the stepped switch! So this particular pair comes with variable sweep output instead, allowing you drive them a little harder than typically, which provides slightly hotter output at the very top. Other work includes deep cleaning the chassis (already incredibly clean and well preserved), all new components and wiring, VU calibration, new bulbs, Nuetrik Pigtails added, output mod, bias mod, new pots and NOS tubes, etc.

How to use the biasing pot mod (little blue box with flathead):

Only do this when the tube is replaced. Never touch them unless you replace the tube! Tubes were more consistent back in the 50’s so input pots and resistors on the way input trimmers reduces second order distortion. All you need is a volt meter by looking at cathodes across pot (.6-2Volts) or watch plate with no signal and no limiting (input turned down). Balance that pot so it’s within a tenth of a volt. Hit it with 1Khz signal between top and bottom of plate. Plates should read 112-118 with full compression. It’ll rise up to 120-140.

Quite unique and unimaginably rare. You certainly will not see another like this pop up ever again. If you’re looking for a massive soundstage that’s larger than life, a warm colorful output and a clean noise floor with tons of headroom, look no further.

Any questions please ask. Thanks for looking ~ JML