By Vasily Ivanenko
SECTION 6 — AF amplifier plus line out
It’s fun and useful to listen to your signals from Jovian reception adventures. Along with electromagnetic Jovian bursts, you may also hear bad noise like RFI or hum which might not glare apparent on your monitor.
I designed and built an AF preamp with a buffer, splitter, active AF gain control, a tone control and finally, a discrete component PA for speaker drive. The enclosure also contains 4 extra DC ports to provide DC voltage to other modules via short, shielded RCA cables.
Josh M pointed out that I really have to act carefully to avoid mixing up my AC and DC ports!
Above — Input to tone control output. This PA goes in a metal box since the last thing we want is RF from the AM BCB band getting detected and amplified. The input 39R and 0.22 µF capacitor low-pass filters all signals above 18.6 KHz including offensive RF.
To provide capability to drive a sound card with an isolation transformer, I split the input with 1 path going to the AF preamp and the other to an output RCA port.
1/2 of a TLO82 op-amp generates the virtual ground and it’s ground referenced to a single point along with each of the 2 AF boards. Yay! No hum was detected in this module.
Many of the ideas for this module came from Douglas Self. I refer to his book Small Signal Audio Design: 2nd edition whenever I fancy making some AF circuitry.
The active volume control gives very low noise at lower volume setting levels. R1 and R2 set the maximal gain and can be manipulated with the standard formula: Vout /R2 = -Vin/R1.
Speakers, cabinets and rooms color sound. I feel a tone control is necessary to allow pleasing sound in whatever speaker you ply in your listening room. This is essentially a 1 control Baxandall circuit developed by Douglas Self and modified for single supply.
Above — PA stage with its own virtual ground to use up the other half of the 5532 op-amp. Giving the PA board its own virtual ground made bench testing this board easy.
I’ve written about this circuit before and developed it for Regen #5. Click and scroll to read about it.
In order to get a power amp output signal swinging as close to the DC rails as possible ( between 0 and 12.3 VDC ), you must depart from a complimentary pair of emitter followers. In these, the collector-to-emitter saturation voltage plus VBE keep you from getting close to the rails. Thus, I’ve adopted the topology shown — a common-emitter configuration for the output pair.
Above — a 12-bit FFT of my PA into a 8 Ω resistor. Drive is here set to give 877 mW output power and each vertical division = 10 dB. 877 mW sounds loud and all harmonics lie under 60 dB down.