This is the input amplifier from a Revox A77 reel to reel (circa ~ 1967) - which also happens to be almost identical to the mic preamp in Trident A-Range mixing consoles from the early seventies.
So far out of the preamps that I've been working on, which is the Neve, RCA and this, I'm happiest with this one. Apart from sounding cool, it also retains a pretty flat response, and could be used as an "always on" sweetener. I was probably just hearing very light distortion, but I did prefer the tone of my guitar with this on running clean, rather than off. It felt a bit more forward and bigger sounding.
It’s very very loud - tons of output level available. This could happily be described as a clean boost as well as an overdrive / light fuzz.
Due to the insanely high output, I think this is a good candidate for some form of High or low pass filter for some tone shaping. If you’re using it to smash another pedal or front end of an amp, some bass cut might be handy. Note that on the highest gain setting (which is fuzzy), it does roll-off high and low frequencies.
The original uses silicon transistors, I've used Japanese 2SD128 germanium transistors, which are usually around 40hfe.
REVOX A77 INPUT AMPLIFIER SCHEMATIC
Note the switching used to set impedance and recording (gain) levels. As I was aiming for boost and fuzz, the resistor switch on the input has been removed on my layout. The fixed resistors controlling gain have been replaced with a pot.
TRIDENT MIC PREAMP SECTION FOR COMPARISION
REVOX A77 INPUT AMPLIFIER FREQUENCY RESPONSE
On the highest gain setting, high and low frequencies roll-off. It's very flat elsewhere.
The output is very high, and the overdrive / fuzz does not occur until the last couple of hundred ohms when resistance to ground is heading towards zero (hence the reverse log pot)
my notes from spice re the schematic above:
- input cap can be 1u without effect response
- 1u output cap only has slight low frequency roll-off, even with a 5k volume pot
- decrease C4 to 10u to roll-off low frequencies
- increase C2 to increase high frequencies
- decrease R8 to reduce fuzz
- C6 can be smaller (down to 10u)
- C5 can be removed, or increased to boost treble from 8k / 10k
- R5 can be removed for a very slight gain boost
There are so many variables, I haven't show the LTspice results here, or it would be a very lengthy post.
REVOX A77 INPUT AMPLIFIER - CLEAN BOOST / OVERDRIVE / LIGHT FUZZ
Work in progress on the bench - as you can see, it's the mojo layout (also know as the unnecessarily large version). Approx 65mm x 40mm on 0.15" vero with 1w carbon film resistors
And it’s boxing day - using my ‘spare no expense’ cardboard box wiring rig to get the spacing right before finishing it off in the enclosure.
It’s not the greatest looking, but I do like the sound. Really really loud by the way.
ON THE SCOPE / FFT
caution re audio levels - start low. This is bench test of my build.
