Preamplifier Design Phase
The PA-100 preamplifier has volume control, high frequency and low frequency tone control, RF communication, microcontroller housekeeping, and power supply sections. It is non-inverting.
Key features
The schematic below shows the right channel tone control circuitry (the left channel is a clone). U21B was designed to accomadate gain but in actual use it is a simple voltage follower (R37 and R38 are not used) to provide a high impedance buffer between source and tone control circuits. U26B is the heart of the tone control, with bass and treble cut/boost 10k linear digital potentiometers connected between the output of U21B and the output of U 26B (the A and B terminals of the potentiometers U5 and U6, signals IN-RT-A and OUT-RT-B). Part of the genius of Self's design was that it delivers logarithmic response while using linear potentiometers.
Those potentiometers feed the treble and bass sections of the tone control via their wiper and the position of the wiper determines the bass or treble boost/cut.
The treble section consists of U25A and U25B. Signal levels sourced from the wiper potentiometer U6, in conjunction with the time constant set by C22/R24 and potentiometer U8, control treble cut/boost. Potentiometer U8 (between C23 and C24, singals HF-RT-TA and HF-RT-TW) sets the cut/boost engage frequency. C30/R44 sets the "return to zero" treble frequency (the frequency at which boost or cut is gradually removed, set here in the range of 20kHz).
The bass section consists of U26A and U27 A/B. Signal levels sourced from the wiper of potentiometer U5, in conjunction with the time constant set by C35/R47/R46 and potentiometer U8, control treble cut/boost. Potentiometer U8 (between C32, U26A and R46, signals LF-RT-TA and LF-RT-TW) sets the cut/boost engage frequency. C31/C33/R47 sets the "return to zero" bass frequency (the frequency at which boost or cut is gradually removed, set here in the range of 20Hz).
Key features
- 256 step volume control (as implemented in remote control, 64 steps are provided, for a total of 64dB)
- 128 step variable bass and treble cut and boost control (as implemented in remote control, 10 boost steps and 10 cut steps set evenly across the 128 bit range. Each step approximately 1 dB).
- 128 step variable cut/boost bass and treble "engage" frequency. As implemented in the remote control, there are 12 bass frequency engage steps from 40 to 400 Hz, with those frequencies specified as the point the signal is 3dB up or down, while the maximum bass boost/cut level is in force. For treble, 9 frequency steps from 2500 Hz to 16kHz were implemented.
- Remote control via 433MHz band communication
- 68K input impedance
- Toroidal power transformers. Separate digital and analog power supply sections. Isolated digital and analog ground planes.
- High quality Cardas Audio RCA connectors; low noise LM4562 operational amplifiers; metallized polypropylene capacitors used where ever feasible; metal film resistors used in signal paths; Nihicon audio grade electrolytic capacitors.
The schematic below shows the right channel tone control circuitry (the left channel is a clone). U21B was designed to accomadate gain but in actual use it is a simple voltage follower (R37 and R38 are not used) to provide a high impedance buffer between source and tone control circuits. U26B is the heart of the tone control, with bass and treble cut/boost 10k linear digital potentiometers connected between the output of U21B and the output of U 26B (the A and B terminals of the potentiometers U5 and U6, signals IN-RT-A and OUT-RT-B). Part of the genius of Self's design was that it delivers logarithmic response while using linear potentiometers.
Those potentiometers feed the treble and bass sections of the tone control via their wiper and the position of the wiper determines the bass or treble boost/cut.
The treble section consists of U25A and U25B. Signal levels sourced from the wiper potentiometer U6, in conjunction with the time constant set by C22/R24 and potentiometer U8, control treble cut/boost. Potentiometer U8 (between C23 and C24, singals HF-RT-TA and HF-RT-TW) sets the cut/boost engage frequency. C30/R44 sets the "return to zero" treble frequency (the frequency at which boost or cut is gradually removed, set here in the range of 20kHz).
The bass section consists of U26A and U27 A/B. Signal levels sourced from the wiper of potentiometer U5, in conjunction with the time constant set by C35/R47/R46 and potentiometer U8, control treble cut/boost. Potentiometer U8 (between C32, U26A and R46, signals LF-RT-TA and LF-RT-TW) sets the cut/boost engage frequency. C31/C33/R47 sets the "return to zero" bass frequency (the frequency at which boost or cut is gradually removed, set here in the range of 20Hz).
Tone Control Circuitry
The digital heart of the preamplifier is an Atmega 328P, the same Atmel part as used in the Arduino Uno. USB connection is provided by an FT232RL. The HopeRF RFM12B is used in the 433MHz band to communicate with the remote control. The 8 AD7376 10k ohm digital potentiometers control signals in the treble and bass op amp sections. The PGA2311 provides volume control and mute. Buffers U18 perform level shifting for the RFM12B. Buffers U10 keep noisy clock and data signals away from the analog section until needed. U11 is simply an inverting op amp buffer that is needed to keep the preamplifier a non-inverting source.
Digital section, for RF communications and control of 8 tone and volume section potentiometers, volume control
Amveco encapsulated toroidal transformers and simple linear regulators were used in the power supplies. Digital logic was driven from one transformer (+/- 7 VRMS) , analog circuits from another (+/- 15VRMS). Op amps were supplied with +/- 15 VDC; PGA2311 was supplied +/-5 VDC analog supply; the RFM12B had its own 3.3VDC regulator; Logic circuits were driven from a dedicated 5VDC regulator.
Power Supply Schematic