Altec 436 Mods

The 436 does seem very limited (pun intended) to those of us whose expectations have been shaped by more modern compressors. It's a simple device designed to tackle a simple task: to take the fairly weak output from a mic preamp, smooth out any gross variations in volume and amplify the signal up to a level sufficent to feed a telephone line, studio-transmitter link or power amplifier in a PA system.

Although the manual makes no reference to doing so, you could run a mic directly into the 436, it has so much gain. And because of its high gain, and high output level, input and output attenuators are most definitely needed to interface it to typical +4dBU line-level system. On Dave's 436, I added a stepped bridged-T attenuator after the output transformer that gives 0 to -27dB (plus "off") in 3dB steps. I also added a switchable 12dB attenuator after the input transformer, just to give some measure of input level control.

But even with the input attenuator switched in, the average input level still needs to be below -10dBU or so if you desire full range on the threshold control--that is, 0dB compression when the control is turned up all the way. So, with a +4dBU (nominal) input, a 10 or 20dB pad may be needed even when the internal attenuator is switched in. I didn't want to make the switchable attenuator any greater than 12dB, figuring that any really big mismatches could be better handled by an external attenuator before the input terminals, preventing overload of the transformer.

If you play around with the sidechain time constants in your 436C, you'll soon find out why Altec choose those particular values. You can't reduce them much below stock values without causing instability at certain settings of the controls. I was only able to reduce R9 from 270K to 180K, and R12 from 33K to 27K. This equals nominal minimum attack and release times of approximately 30mS and 200mS, respectively--still pretty slow compared to what a modern compressor can manage! I also implemented a variable attack control by including a 100K linear pot in series with R12. Of course, this pot can only slow down the attack by an additional 0 to 100mS.

For what it's worth, adding a balance control to the 6BC8 variable-gain stage seems to help in minimizing the "roughness" at short attack and release settings. I used a 250-ohm trimpot wired between the cathodes, with the wiper going to the GR meter. I tried 100 ohms originally and found that it didn't give enough of a range to keep the "sweet spot" near the middle of the pot, where it should be.

The balance adjustment is made by applying a common-mode signal to the grids, adjusting the threshold control to some median value of compression (say, 10dB) and trimming the pot for minimum signal at the 436's output terminals--or, if preferred, minimum negative DC voltage at C4. An easy method for providing a common-mode test signal is to connect a 1uF, 100V cap between either side of the heater supply and the junction of R9 and R12.

Also, I replaced R8 with a 100-ohm trimpot to allow calibration of the GR meter.

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