I've been looking for a suitable circuit to make a moderate voltage but low power amplifier, suitable for amplifying a line-level sinewave to drive a Rega turntable 24v synchronous motor. The Rega PSU has a rating of only 100mA, so that was a boundary condition for the current output.
This example circuit in Horowitz & Hill (2nd edition, from my student days!), from an Analog Devices application note, looked promising for this application. It uses op-amp supply current sensing to activate the PNP and NPN output devices when additional current is required, a potential divider on the op-amp output to provide the required load whilst protecting it from the larger output voltage swing, and global voltage feedback to set the overall gain. Neat!
Simulating a modified version (non-inverting, zener references) using fairly generic parts looked OK, so I made a 'parts-bin' prototype of the circuit shown below. I only had a limited range of op-amps to hand, so have tried a few - OPA134, TL071, and even a LM741. The only critical component for stability was going to be C1 for the current feedback loop.
Step response was good with the OPA134 but it was fairly critical on C1 (~100pF) otherwise instability loomed; with the TL071 the response was also good, but it was far more tolerant to the value of C1, with anything from about 500pF-5nF being acceptable, so settled on 1nF; even the LM741 gave a respectable response at the output, although the op-amp feedback and output signals were rather less pretty.
The step response at 70Hz (around the target frequencies for turntable motor drive) is shown above, along with that at 5kHz, showing good stability and transient behaviour. The output waveform and spectrum of the running system, driven by a synthesised sinewave and driving the turntable, is show below.
I've now produced a PCB for the 2-channel amplifier, Gerbers attached.
Full details also on my GitHub
This example circuit in Horowitz & Hill (2nd edition, from my student days!), from an Analog Devices application note, looked promising for this application. It uses op-amp supply current sensing to activate the PNP and NPN output devices when additional current is required, a potential divider on the op-amp output to provide the required load whilst protecting it from the larger output voltage swing, and global voltage feedback to set the overall gain. Neat!
Simulating a modified version (non-inverting, zener references) using fairly generic parts looked OK, so I made a 'parts-bin' prototype of the circuit shown below. I only had a limited range of op-amps to hand, so have tried a few - OPA134, TL071, and even a LM741. The only critical component for stability was going to be C1 for the current feedback loop.
Step response was good with the OPA134 but it was fairly critical on C1 (~100pF) otherwise instability loomed; with the TL071 the response was also good, but it was far more tolerant to the value of C1, with anything from about 500pF-5nF being acceptable, so settled on 1nF; even the LM741 gave a respectable response at the output, although the op-amp feedback and output signals were rather less pretty.
The step response at 70Hz (around the target frequencies for turntable motor drive) is shown above, along with that at 5kHz, showing good stability and transient behaviour. The output waveform and spectrum of the running system, driven by a synthesised sinewave and driving the turntable, is show below.
I've now produced a PCB for the 2-channel amplifier, Gerbers attached.
Full details also on my GitHub
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Where's the fun if you don't do it the discrete parts way? 
FWIW, Elektor published a few versions of audio output stages driven from the rails of an opamp - dating from the 1980s, I think. The attached schematic is the simplest Mosfet version but BJT and better versions turned up in their circuit pages occasionally too. Google will find them without much effort.
FWIW, Elektor published a few versions of audio output stages driven from the rails of an opamp - dating from the 1980s, I think. The attached schematic is the simplest Mosfet version but BJT and better versions turned up in their circuit pages occasionally too. Google will find them without much effort.
Attachments
In the PDF attachment is an example therefore.
What I don't understand is the adjustment procedure of the sine wave oscillator.
1) Tr2 and Tr3 are for pre adjustment for 33 rpm speed in such kind, that P2A/B (outdoor adjustment therefore) provide the correct speed in the middle position - but how get I best synchronisation of P2A and P2B ?
2) What is the aim of TR4 ?
Only the function of Tr1 I understand - it is a level adjust for one motor winding half - the adjust is perform in such kind, that a minimum of mech. hum level is present.
Thank you for an advice.
A very similar circuit has been partially analysed by Rod Elliot (state variable filter). My head hurts from the abundance of pots
Tr4 is perhaps to set the gain for best distortion.
https://sound-au.com/articles/sinewave.htm