I constantly try to educate myself on operating principles of various circuits and just the other day I came across an interesting – and very puzzling – one. Take a look at the power amplifier schematic of Traynor 200KB:
http://www.traynoramps.com/downloads/servman/sm200kb.pdf
At first glance the circuit may look kind of simple, just an amp with an ordinary opamp-based voltage amplifier section - and to increase the voltage swing the opamp is “bootstrapped” by modulating its supply rails according to output signal swing. (Why is the PNP side bootstrap different by the way?) Anyway, further looking got me really puzzled:
Take a look at the bias circuit, in fact the whole branch driving the PNP output Darlingtons. What is the idea of this configuration?
What is the circuit around transistors Q15 and Q18 doing? My guess is that it is some kind of DC servo that keeps the inverting input of U5B in about zero voltage.
Q19, Q28 and Q16 must be some additional protection based on the thermistor, right?
Then, take a look at the feedback network: How is the gain defined when the resistor ratio is so odd. (It’s not an error). It also looks like the input is bootstrapped (via R141) but with such odd topology I really can’t be certain.
Often when I run across an odd circuit that I don’t understand I simulate it. This time it didn't help: I ran a SPICE simulation (of a simplified circuit that omitted the combined VI/thermistor protection and OTA-based limiters) and it shows that the amp has only slightly higher gain than unity, which doesn’t make any sense since I firmly believe that the amplifier truly doesn’t have an input sensitivity of about 30Vpeak (which is what the simulation showed me). The preamp couldn’t even deliver it. I’m obviously overlooking some very important point.
How can I find more information about this circuit? Books from Slone and Self discuss only very basic Lin circuit derivatives and most white papers about OpAmp power buffers or such seem to be no help either. What is this kind of circuit called? Has it been patented? Why use it in the first place? I faintly recall seeing this circuit topology before in some insanely complex public address amplifier (can’t recall which but I don’t mean the other Yorkville sound products). Any help is truly appreciated.
http://www.traynoramps.com/downloads/servman/sm200kb.pdf
At first glance the circuit may look kind of simple, just an amp with an ordinary opamp-based voltage amplifier section - and to increase the voltage swing the opamp is “bootstrapped” by modulating its supply rails according to output signal swing. (Why is the PNP side bootstrap different by the way?) Anyway, further looking got me really puzzled:
Take a look at the bias circuit, in fact the whole branch driving the PNP output Darlingtons. What is the idea of this configuration?
What is the circuit around transistors Q15 and Q18 doing? My guess is that it is some kind of DC servo that keeps the inverting input of U5B in about zero voltage.
Q19, Q28 and Q16 must be some additional protection based on the thermistor, right?
Then, take a look at the feedback network: How is the gain defined when the resistor ratio is so odd. (It’s not an error). It also looks like the input is bootstrapped (via R141) but with such odd topology I really can’t be certain.
Often when I run across an odd circuit that I don’t understand I simulate it. This time it didn't help: I ran a SPICE simulation (of a simplified circuit that omitted the combined VI/thermistor protection and OTA-based limiters) and it shows that the amp has only slightly higher gain than unity, which doesn’t make any sense since I firmly believe that the amplifier truly doesn’t have an input sensitivity of about 30Vpeak (which is what the simulation showed me). The preamp couldn’t even deliver it. I’m obviously overlooking some very important point.
How can I find more information about this circuit? Books from Slone and Self discuss only very basic Lin circuit derivatives and most white papers about OpAmp power buffers or such seem to be no help either. What is this kind of circuit called? Has it been patented? Why use it in the first place? I faintly recall seeing this circuit topology before in some insanely complex public address amplifier (can’t recall which but I don’t mean the other Yorkville sound products). Any help is truly appreciated.