Tube/solid state rackmount bass recording preamp.
Combining an Ampeg inspired tube preamplifier and a VCA based compressor unit based on THAT4301 chip.
It is a quite advanced project utilizing a potentially fatal high voltages. If you are in any way unfamiliar with high voltage circuits or are uncomfortable working around high voltages, PLEASE DO NOT RISK YOUR LIFE BY TRYING TO BUILD THE DEVICE.
USE AT YOUR OWN RISK: AS THE PROJECT AUTHOR I EXPRESSLY DISCLAIM ALL LIABILITY FOR INJURY OR PROPERTY DAMAGE RESULTING FROM THIS INFORMATION! ALL INFORMATION IS PROVIDED 'AS-IS' AND WITHOUT WARRANTY OF ANY KIND.
Project background/topology:
At some point i realized i need a proper bass recording device for my small home studio, preferably analogue, tube preamp with versatile sound capabilities. The first version of the BassEngine preamp, much simpler as the new one, was a good testing bed and provided lots of data that shaped the final rackmount version of the device. The main circuit, a multi stage tube preamp is based on a device produced by a well known and respected bass gear manufacturer. I adapted it to suit my needs, added a more advanced VCA based compressor unit, series/parallel FX loop, transformer isolated output, a FET buffered tuner output and a few other tricks.
Below you can see the block diagram of the preamp:
The compressor (THAT4301 based circuit) is placed right after the first gain stage, but before the Gain control. This way the compressor can be used in a few different ways, starting from a common task of taking care of the dynamic range of the signal, ending with using it as a form of a booster to overdrive the next tube stages (Gain control used in conjunction with the compressors MakeUp Gain control).
Controls:
- BRIGHT - high band boost,
- SCOOP - mid frequency notch filter,
- GAIN,
- LOW - bass tone control,
- MID - middle band tone control,
- FREQ - mid band EQ center frequency (6 position rotary switch),
- HIGH - treble control,
- FX ON/OFF - effect loop bypass
- MIX - effect loop dry/wet mix ratio,
- KNEE SOFT/HARD - compressor knee shape,
- THRES. - compression threshold,
- RATIO - compression ratio,
- RESP. - side chain response time,
- GAIN - make up gain,
- COMP ON/OFF - compressor bypass.
Indicators:
- <<< - compressor ON/OFF,
- CMP - gain reduction indicator,
- PEAK - signal clip indicator.
I/O
- IN - main input (front panel),
- IN - (secondary input, back panel),
- OUT - main, transformer isolated line output,
- TUNER - separate buffered signal output for tuner,
- FX SEND,
- FX RETURN,
- POWER - 230VAC 50Hz.
Build:
Construction is mixed, point to point and PCB. Most of the tube signal path is built using point to point technology, power supply, tube holder, compressor and fx loop are built on PCBs.
Tube sockets are mounted on a separate PCB. Slider switches reconfigure the filament for different types of dual triodes (ECC8x/12Axx, 6N2P etc). Each tube has its own separate switch.
Tuner output signal is taken directly from the input, buffered with a FET transistor and sent outside.
PSU supplies following voltages:
- +250V DC plate voltage for the tube part,
- +6.3V DC, tube filament voltage, relay supply,
- +/-15V DC, bipolar power supply for the compressor and FX loop circuit.
This PCB holds the compressor, FX loop and peak detector circuits and is mounted directly to the front panel. There are 3 trimpots for adjusting the signal levels and balancing out the VCA chip. Since the solid state part is powered with bipolar power supply i decided to avoid electrolytic caps in the signal path (hence that huge MKT 10µ/63V capacitor). To save the space, the PCB is designed in mixed through-hole/SMT technique.
