1963 CBS Audimax II Vintage Tube Compressor Limiting Amplifier. Rare.




Item History & Price

Information:
Reference Number: Avaluer:525599Number of Channels: 1
Features: Vacuum TubeModel: Audimax II 443
MPN: Does Not ApplyCountry/Region of Manufacture: United States
Brand: CBSEffect/Enhancement: Compression/Limiting
Form Factor: Rack MountableType: Single-Effect Processor
Original Description:
CBS Audimax II model 443

Vintage classic rackmountable tube compressor amplifier.
Really nice condition, sound quality and performance.  
Cosmetically very nice, a few minor paint marks, see pics.
Top of line Freed audio transformers.
GE JAN 6386 tube.

This unit listed here has vastly improved functionality and sound over the original control circuit design.
It now performs a variety of compression as well as peak limiting outstandingly and sonicall...y is full range and with a really lively sound signature.

The original proprietary CBS automatic control system has been converted to a user controlled system with variable attack, release and threshold controls which are installed on the unit's front panel. See pics.

From left to right, the red knob is threshold, the white knob is attack and the grey knob is release.

For faster attack or release times, turn the knobs clockwise. With the attack set almost fully CW, the release about 3 o'clock position, the unit is closely emulating typical factory fixed average timings on the Gates Sta-Level, GE Unilevel, RCA BA25 and Collins 26j and 26u series compressor's RC networks, all of these units mentioned also employed the same configuration of 6386 control tube with a 6AL5 signal rectifier modulated by the amp's push/pull output.

Very rapid attack and recovery times can now be quickly dialed in and the tube era Audimax absolutely comes alive like it never could before with the original elaborate but sluggish timing circuit CBS had adopted to try and differentiate their product from other manufacturers competing in the broadcasting market.

The threshold knob increases the amount of gain reduction when turned clockwise.

Like all of these era classic legacy compressors, once you have your basic settings, you just keep increasing the input gain while the control amp maintains a constant output voltage and things in general start getting bigger, bolder and the attitude factor ramps up.

With the attack and release time both set almost fully clockwise, the unit is a effectively a peak limiting loudness monster and super ballsy and responsive sounding. I've owned and used many different classic tube compressors over the past two decades, and with these modifications, this unit easily stacks up technically and sonically against all the similar era far more popular and celebrated compressors and limiters. The Freed audio transformers in particular sound magnificent with excellent frequency response, low end and detail.

The variable threshold also allows it to perform gain reduction at lower input voltages that wouldn't have been sufficient to trigger compression action in the original control circuit, further adding to the unit's applicability in differing set ups.

The conversion also clears up the original audio input path in a major way. Unlike the original CBS control system, this couples the input signal, as well as routes the dc control signal, through the input transformer secondary, directly to the control amplifier grids.  With these circuit modifications, this now rare, and overlooked vintage tube compressor is 100% suitable for pro recording and mixing duties and will also effectively perform peak limiting type compression handling extremely fast attack and release times if desired. The meter that used to drift slowly now tracks the gain reduction at a brisk pace. It's a huge difference in performance and sonics over the original configuration.

Also, with the modifications, the tubes V1 and V2, that drove the input modulation source for the CBS control system, are eliminated, and the power supply can now run the beefier, higher plate and filament power consuming 12BH7 output tube.  You can still directly swap in the original 12AU7 output tube if you desire. The 12BH7 provides more output current and headroom and is presently installed on the unit.

The function control on the right front panel gives readouts of the plates of the 6386 and output tube. Now that the 12BH7 is installed, the readings on V4 are higher than the original factory range on the meter. The function control also has a bypass switch.

The original CBS green memory plug in module is included in the sale with the unit in the event you'd want to reconfigure it back to the factory configuration. For present usage, you would not have it plugged into the unit now it's been modified, but I've plugged it in just to show in pics for this auction.

An all tube based unit, it has really good signal to noise ratio, very minimal hum and a flat full range frequency response with excellent low end response and clarity and details in the higher frequencies.

Buyer of the unit will also receive a PDF copy of the original manual.
There's a few pages missing, but all the key documents, service procedures, table of readings, parts list and schematics are provided.
A clear, readable scan of original schematic is included.

Additional tech notes about this unit offered for sale:

The original wirewound and carbon power supply resistors have been replaced with wirewound and metal oxide equivalents.

The original style multi-section canister filter cap was left on the unit, the leads disconnected, and high quality Nichicon electrolytics installed.

 Also, a three prong AC plug is added. The original unit had an unpolarized 2 pin ac connector.
In addition to adding a safety ground. the 3 pin connector ensures the hot side is both fused and correctly polarized.  
Check detailed pictures.

This unit should give years of trouble free service beyond having to periodically replace a defective or worn tube as with any tube based circuit. The modern Japanese electrolytics handling the B+ last many years. Being a 50+ year vintage item, there is no return guarantee offered and the unit ultimately is being sold as is, though it is in excellent working condition and ready to plug in and play.

My goal is buyer satisfaction, and always to sell you an item that's accurately and honestly represented. If you have any questions about setup of the unit after purchasing, I'm happy to help. This thing has been gone through carefully and performance tested and works excellent. Check my feedback history. I'm confident the buyer of this unit will be very happy with the sound quality and performance of this rare compressor.

Sale through eBay Canada is listed in Canadian dollars. Shipping to the U.S. with tracking is $75, to Canada $50 and Europe and Asia is $175. The Canadian dollar is real low against the U.S., so take advantage of the low Canadian dollar!





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