Having now almost all 907 series service manuals available thanks to @JimEGR kindness, I decided to address a bit some chronology regarding the 907 series.
I will start again by saying that from a technical perspective "X-Balanced" and "Alpha" amps are just a different marketing name, all of them being balanced amplifiers. Alpha series benefited indeed from some more qualitative parts compared to previous X-Balanced series. But the topology and the way how they work are identical.
This time I won't cover any changes regarding power-amp section, as I mentioned (I believe) all of them in my earlier postings. So there is no reason to repeat the same thing. But, if anybody wants to have all the information in a single thread, no problem - feel free to add my relevant earlier postings also to this thread.
Instead, this time I will focus on an area usually overlooked by many people - the power supply unit (PSU).
Believe it or not, the main filtering caps (being for the power amp or another section of the amp) are in the circuit and they inevitably influence the sound. And before jumping to say that this is a nonsense or whatever, just ask yourself the question "how does the current flows in the circuit when rectifying diodes are not conducting ?". Rectifying diodes are conducting only during the charging pulses, otherwise the circuit can be seen as "open" from their perspective.
After the main caps, any bypass cap and eventually any following voltage regulator has also "something to say", as it will play its part on the final sound.
This is why the PSU is important - because whatever bias/idle and dynamic current is required in the circuit to drive the "load", it will only come from the PSU.
And the general golden rule is "the better the PSU, the better the sound".
First 907 X-Balanced model was AU-D907X, which enjoyed a dual-mono topology for power-amp section and a "shared" closed-loop (CL, aka with feedback) voltage regulator for phono and tone/flat-amp modules. When I say shared it means it's only one voltage regulator which supplies power to both L and R channels. This CL-type regulator was somehow in-line with the older models, as the older AU-919 for example also had a feedback voltage regulator (albeit a more complex one).
The topology was a "centralized" one, this being the only voltage regulator and then having just decoupling caps on phono and tone/flat-amp modules.
The same "centralized" topology is to be found also on first Alpha 907, AU-α907 from 1986. But this time instead of using CRD's they used JFET's with resistors for current limiting - doing the exact same thing as the previous CRD's. For anyone wondering., CRD = Current Regulation Diode
All Alpha 907 models don't feature anymore the older AU-D907X's dual-mono topology for power-amp section, instead the main caps are connected in parallel - even if there are separate secondary windings and rectifiers. for each bank of caps The effect would likely be lower PSU impedance as seen by the amp sections, but the price to be paid for that is the existence of some cross-talk between R and L channels. This was also discussed on Hyperion's thread related to his AU-α907MR, so more details can be found there (including a tweaking/mod).
As a general rule, for all 907 Alpha models the main open-loop voltage regulator is shared between R and L channels.
First change on PSU came with AU-α907i, where the main voltage regulator was changed to an open-loop type (no feedback, classic zener + series pass transistor/darlington). But this time the "centralized" topology was dropped and individual voltage regulators were added for phono and tone/flat-amp modules.
The exact same topology was maintained also for next AU-α907L Extra.
Later series AU-α907DR, AU-α907KX and AU-α907XR simplified the regulator for tone/flat-amp, having a single transistor instead of a compounded darlington. The main open-loop voltage regulator was preserved like before, and that goes until late AU-07 Anniversary (most likely also NRA series).
Then on AU-α907MR and AU-07 Anniversary the voltage regulator for tone/flat-amp module was dropped entirely and replaced with a simple RC filter. Some will maybe say that on MR and later series there are current mirrors, and so there was no need for a local regulator anymore. That could be true or not, at least partly. Because no current mirror or CCS is "immune" to the ripple present on the supply lines. So having a local regulator for sure cannot hurt and can only make things better.
For AU-α907NRA is not clear (no SM), but I have no reason to suspect that anything was changed compared to MR series from PSU perspective.
If anybody wants to address in a similar way other sections of the amps, or feel that I missed something - please feel free to add (to this thread).
Regards
I will start again by saying that from a technical perspective "X-Balanced" and "Alpha" amps are just a different marketing name, all of them being balanced amplifiers. Alpha series benefited indeed from some more qualitative parts compared to previous X-Balanced series. But the topology and the way how they work are identical.
This time I won't cover any changes regarding power-amp section, as I mentioned (I believe) all of them in my earlier postings. So there is no reason to repeat the same thing. But, if anybody wants to have all the information in a single thread, no problem - feel free to add my relevant earlier postings also to this thread.
Instead, this time I will focus on an area usually overlooked by many people - the power supply unit (PSU).
Believe it or not, the main filtering caps (being for the power amp or another section of the amp) are in the circuit and they inevitably influence the sound. And before jumping to say that this is a nonsense or whatever, just ask yourself the question "how does the current flows in the circuit when rectifying diodes are not conducting ?". Rectifying diodes are conducting only during the charging pulses, otherwise the circuit can be seen as "open" from their perspective.
After the main caps, any bypass cap and eventually any following voltage regulator has also "something to say", as it will play its part on the final sound.
This is why the PSU is important - because whatever bias/idle and dynamic current is required in the circuit to drive the "load", it will only come from the PSU.
And the general golden rule is "the better the PSU, the better the sound".
First 907 X-Balanced model was AU-D907X, which enjoyed a dual-mono topology for power-amp section and a "shared" closed-loop (CL, aka with feedback) voltage regulator for phono and tone/flat-amp modules. When I say shared it means it's only one voltage regulator which supplies power to both L and R channels. This CL-type regulator was somehow in-line with the older models, as the older AU-919 for example also had a feedback voltage regulator (albeit a more complex one).
The topology was a "centralized" one, this being the only voltage regulator and then having just decoupling caps on phono and tone/flat-amp modules.
The same "centralized" topology is to be found also on first Alpha 907, AU-α907 from 1986. But this time instead of using CRD's they used JFET's with resistors for current limiting - doing the exact same thing as the previous CRD's. For anyone wondering., CRD = Current Regulation Diode
All Alpha 907 models don't feature anymore the older AU-D907X's dual-mono topology for power-amp section, instead the main caps are connected in parallel - even if there are separate secondary windings and rectifiers. for each bank of caps The effect would likely be lower PSU impedance as seen by the amp sections, but the price to be paid for that is the existence of some cross-talk between R and L channels. This was also discussed on Hyperion's thread related to his AU-α907MR, so more details can be found there (including a tweaking/mod).
As a general rule, for all 907 Alpha models the main open-loop voltage regulator is shared between R and L channels.
First change on PSU came with AU-α907i, where the main voltage regulator was changed to an open-loop type (no feedback, classic zener + series pass transistor/darlington). But this time the "centralized" topology was dropped and individual voltage regulators were added for phono and tone/flat-amp modules.
The exact same topology was maintained also for next AU-α907L Extra.
Later series AU-α907DR, AU-α907KX and AU-α907XR simplified the regulator for tone/flat-amp, having a single transistor instead of a compounded darlington. The main open-loop voltage regulator was preserved like before, and that goes until late AU-07 Anniversary (most likely also NRA series).
Then on AU-α907MR and AU-07 Anniversary the voltage regulator for tone/flat-amp module was dropped entirely and replaced with a simple RC filter. Some will maybe say that on MR and later series there are current mirrors, and so there was no need for a local regulator anymore. That could be true or not, at least partly. Because no current mirror or CCS is "immune" to the ripple present on the supply lines. So having a local regulator for sure cannot hurt and can only make things better.
For AU-α907NRA is not clear (no SM), but I have no reason to suspect that anything was changed compared to MR series from PSU perspective.
If anybody wants to address in a similar way other sections of the amps, or feel that I missed something - please feel free to add (to this thread).
Regards
Accordingly to Hyperion, he found it to be an improvement. I always walked the dual-mono path in my diy amps in the last 15 years or so, for me this is the "default" (whenever possible/achievable).
