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:
- The remote-cutoff pentodes appear to be much more microphonic than sharp cutoff pentodes such
as the 6AU6 family.
- The required control voltage is quite high, even as high as 50 volts, which would have made
the circuitry fairly complex.
- An acceptable range of gain control can be afforded using the sharp-cutoff pentode at much
lower voltage (less than 10 volts or so), even though the total amount of available compression is
less; but still adequate for most guitar applications.
- The resulting circuit is simple enough that you can tack it on to see if you like it without
much trouble. If you don't care for it, undoing it is very simple.
"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.