"Li'l 4x4"
by Fred Nachbaur, Dogstar Music ©2001

5: Dynamic Range Compressor Option

A compressor is a very useful addition to any amplifier used for guitars (electric or electro- acoustic). This is especially a boon for amps with modest output power, to help prevent overdriving and distortion on hard chords, while maintaining a usable level of output at lower playing velocity. A compressor works by sampling the output voltage, and rectifying and filtering it to a DC voltage which, in turn, is used to control the gain of one of more stages of amplification. Typically, the attack response is made very rapid, but the decay time constant is much longer to allow a gradual return to higher gain after the loud transient has passed.

There are pentodes specifically intended for such gain control applications. These are termed "remote-cutoff" pentodes, and have a control grid in which the spacing of the windings varies. They can therefore operate over a wide range of grid voltage, with transconductance (and therefore voltage gain) varying with changes in control grid bias. An example of such a tube is the 6BA6 and its relatives 4BA6, 12BA6 etc., which were once widely used in radio receivers with AVC (automatic volume control) because of their remote-cutoff characteristics.

However, after briefly experimenting with a 6BA6 in the Li'l 4x4 circuit, I opted against it for the following reasons:
Li'l 4x4 guts
"Li'l 4x4" Compressor option

This is all there is to it! The circuit's input connects to the output of the amplifier, i.e. the hot side of the speaker. The peak AC voltage at the speaker is full-wave rectified and doubled, resulting in a DC voltage of up to about 12 volts at full output. Note that capacitor C26 will very rapidly be charged up to the peak value of the output signal, but will only slowly discharge into R44, the "compression level" control.

This DC voltage is controlled by the compression level control, and is applied directly to the control grid of the instrument preamp tube V1 (pin 1), via resistor R45. This reduces the gain of the input stage, compressing the total dynamic range. As the control voltage drops again by discharging into R44, the gain is allowed to increase again.

With the compression level control turned to minimum, there is essentially no change in operation from before. At maximum, you'll get rapid gain reduction on even moderately overdriven chords, and greatly increased sustain on held chords and notes. The price at high compression levels (or is it a bonus?) is a mild distortion due to the input stage operating near cutoff.

Addendum, 10 August 2001

On doing some more poking around, I decided to give a try to the 6BJ6 remote-cutoff pentode. This one, like the 6BA6, was quite extensively used as RF/IF amplifiers in radio sets with AVC. However, as it was primarily used in commercial-grade equipment, it appears to be a cut above the 6BA6 in terms of overall quality, especially with regards to microphonics. In fact, it's almost as clean as the 6AU6 in that department.

Serendipitously, the 6BJ6 works as a direct plug-in replacement for the 4AU6/6AU6 in the Li'l 4x4 circuit. The pinout is slightly different, i.e. the G3/IS and K connections are reversed, but since these are tied together anyway it makes no difference. Another bonus is that its heater current demand is half of that of the 6AU6 (0.15 A instead of 0.3 A) so it's easier on the heater supply. I simply replaced R6 (2.7 ohm, 2 watt) with 0.47 ohms, 1 watt; there was no noticeable or even measureable increase in hum as a result.

The overall gain is somewhat less than that of the *AU6, but there's still plenty of headroom for most electric instruments. If this is an issue for you, decrease R39 to 470 ohms and increase C22 to 4.7 uF to decrease the local feedback. The tube has a sweet, clean sound, perhaps a bit more restrained than the *AU6, so there should be no noticeable increase in distortion as a result of decreasing the local negative feedback.

As a compressor, it runs rings around the *AU6 in terms of distortion. With the circuit shown, providing a maximum DC control voltage around 12 volts, the total available compression is somewhat less, but it remains clean all the way to the bottom. At the maximum setting of the compression control, the signal voltage required to drive the amplifier to just before the clipping point is 2.8 times as high as with compression set to minimum. This corresponds to a maximum compression level of about 9 dB. at 4 W into 8 ohms, as compared to about 12 dB. using the 4AU6.

With a higher-powered output stage (or with a higher speaker and OPT impedance) this ratio would increase correspondingly. What's more, it will remain low in distortion down to much lower gains than sharp-cutoff pentodes such as the *AU6 family. That being said, I find that for most of my uses even the 9 dB of compression is more than adequate, and I usually set it around 6 dB for my style of playing.

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