I am looking for advice how to eventually improve Yamaha AX590, AX592, AX596, AX870 and AX890 amplifiers by modding them.
All of these amplifiers have basically the same power amp structure. And having read Douglas Self’s book on audio power amplifier design, I suspect some potential benefits in modding these amplifiers. I will try to enumerate my modding candidates along the specific circuit of the AX596:
- Input differential pair: Include Q103/Q105 collectors GM degeneration resistors for improved linearity ot the input pair (along with a suitable Ic)?
- Input differential pair current source: Split R123 and add a midpoint capacitor to V+ in order to improve PSSR?
- Input differential pair current mirror: Approximately match both Q107 and Q109 and reduce the values of R113 and R115 to max. 47 Ohm ... 100 Ohm?
- Dominant pole compensation: Is performed by C105/R117/R199 at base of Q113? A Cdom between - Base of Q113 and Collector of Q115 might be a better option, along implementing an output-inclusive compensation (Self, 6th edition, Page 339)?
- VAS: Quite low Ic for Q113? Increase it to fit the best possible value? Or exchange VAS type altogether?
- VAS: Cancel/omit the (Self: cruel – sic!) open loop gain reduction which is performed by Q115 collector’s loading down to ground by R201?
- Bias voltage generator: Insert a trimmer in Q117 R network (for all AX59x, the AX870 has one)?
- Output stage drivers: Fit a parallel bypassing capacitor along FR101 in order to improve switchoff behavior of Q119A and Q119C?
- Output stage power devices: Decrease Q121A, Q121C, Q123A and Q123C emmiter resistors R111 and R135 values to 0.1Ohm instread of 0.22Ohm, in order to improve linearity of gain in the crossover region?
I use several of these AX59n, AX870 and AX890 amps for multichannel projects. And it seems, that these inexpensive mods could more or less easily be done on the printboard. But is it really worth to switch on the soldering iron in order to perform some of them? And if so, which ones? And maybe I overlook some candidate mods?
Thanks for any advice. And also thanks for an eventual link pointing to an AX890 service manual – it seems nowhere to find in the net.
YamahaAX.jpg
All of these amplifiers have basically the same power amp structure. And having read Douglas Self’s book on audio power amplifier design, I suspect some potential benefits in modding these amplifiers. I will try to enumerate my modding candidates along the specific circuit of the AX596:
- Input differential pair: Include Q103/Q105 collectors GM degeneration resistors for improved linearity ot the input pair (along with a suitable Ic)?
- Input differential pair current source: Split R123 and add a midpoint capacitor to V+ in order to improve PSSR?
- Input differential pair current mirror: Approximately match both Q107 and Q109 and reduce the values of R113 and R115 to max. 47 Ohm ... 100 Ohm?
- Dominant pole compensation: Is performed by C105/R117/R199 at base of Q113? A Cdom between - Base of Q113 and Collector of Q115 might be a better option, along implementing an output-inclusive compensation (Self, 6th edition, Page 339)?
- VAS: Quite low Ic for Q113? Increase it to fit the best possible value? Or exchange VAS type altogether?
- VAS: Cancel/omit the (Self: cruel – sic!) open loop gain reduction which is performed by Q115 collector’s loading down to ground by R201?
- Bias voltage generator: Insert a trimmer in Q117 R network (for all AX59x, the AX870 has one)?
- Output stage drivers: Fit a parallel bypassing capacitor along FR101 in order to improve switchoff behavior of Q119A and Q119C?
- Output stage power devices: Decrease Q121A, Q121C, Q123A and Q123C emmiter resistors R111 and R135 values to 0.1Ohm instread of 0.22Ohm, in order to improve linearity of gain in the crossover region?
I use several of these AX59n, AX870 and AX890 amps for multichannel projects. And it seems, that these inexpensive mods could more or less easily be done on the printboard. But is it really worth to switch on the soldering iron in order to perform some of them? And if so, which ones? And maybe I overlook some candidate mods?
Thanks for any advice. And also thanks for an eventual link pointing to an AX890 service manual – it seems nowhere to find in the net.
YamahaAX.jpg
