1. Introduction

Modeled after another late 1950s legend, the ALT-436C is an iconic single channel, vari-MU tube compressor with fixed attack times. Being the third revision of a compressors series, which were heavily modded by sound engineers working on pop records of the era, the ALT-436C has added Threshold and Release Time controls. Loved by producers in the Beatles era, this initially inexpensive and now legendary unit is extremely helpful for bass guitars, vocals, string instruments, and even synths! Back in the day pop music wasn’t so cold and sterile, and this unit is one of the reasons why. Explore it thanks to the Antelope Audio FPGA technology!

2. Features

  • Up to 30 dB of thump-free gain reduction.
  • Maintains uniform level of reproduced sound.
  • Preserves optimum dynamic range.
  • Does not cause overmodulation.

3. Original Hardware Specifications

Maximum Input Level
+20 dB maximum
Up to 56dB
Frequency Response
30Hz – 15kHz, +/- 1.5dB
Maximum Compression
Harmonic Distortion
Less than 2.5% at 30dB of compression, 25Hz – 10,000kHz

Noise Level
74dB below rated output (-111dBm equivalent input noise)
Attack Time
50 milliseconds
Release Time
Adjustable from 0.3 seconds to 1.3 seconds
Adjustable from 0dBm to +16dBm
Compression Ratio
2:1 at 0dBm threshold
4:1 at +16dBm threshold

4. Layout

1. Volume Control Knob

Adjust the input level to provide the desired average compression as indicated on the Compression Meter.

2. Release Time Control Knob

Controls the length of time between the signal falling below the compression threshold and the compressor returning to 1:1 ratio (effectively stopping compression). Adjustable between 0.3 seconds to 1.3 seconds.

3. Compression Meter

Displays the compression amount in dB.

4. Threshold Knob Control

Adjusts the output level at which compression commences, as well as the compression ratio. Here are some examples:

  • At maximum clockwise position, compression starts at approximately 16dBm output level. Increasing the output level by 20dB at this point results in a 5dB output level increase, thus giving a 4:1 compression ratio. At this setting, maximum compression before distortion is 22dB.
  • In the maximum counter-clockwise position, compression starts at zero level. An 20dB increase in input level results in an output level increase of 10dB, making for a 2:1 compression ratio.

5. Power Switch

Used for bypassing the compressor. Doubles as the BP button in the AFX Rack.

6. Output Gain Meter

Visualizes the output gain of the compressor.

7. Output Gain Knob Control

Adjusts the output gain of the compressor, letting you make up lost gain.

5. Contacts

Having difficulties with the ALT-436C or anything else Antelope? Get in touch with us by the following means:
European Direct Support Line
+44 2039578550
10:00 a.m. – 10:00 p.m. (EET), Monday – Friday
USA Direct Support Line
+1 734 418 8661
7:00 a.m. – 6:30 p.m. (EST), Monday – Friday
Online (live chat and e-mail)
Antelope Audio Support Page

6. Glossary

Adjusts how quickly the compressor starts to work after the signal exceeds the compression threshold.
Controls how soon the compressor will stop after the signal goes below the compression threshold.
Controls how much compression is applied. For example, a 3:1 ratio means that every time the input signal crosses the threshold by 3dB, output signal will be attenuated 1dB.
The Threshold is the level above which the compressor considers the signal too loud and starts applying compression.
Knee and Attack have a lot in common, but they are not the same. Like Attack, Knee controls how the compressor will react once input signal passes the threshold. Hard Attack makes the compressor engage immediately while Soft Knee shapes its attack time to make it slower and smoother.
Also known as “variable gain” and probably the earliest compressor design in existence, Vari-MU is a registered trademark of Manley. The technology works by constantly re-biasing a vacuum tube, smoothly changing its gain to achieve compression. This technique is also at the core of the Tubechild 670 model.