Several moons ago I designed a PCB for one of my experimental amp designs, and at some point braved ordering parts and sending the pcb design off - today the PCB arrived in the post and I've spent a fair few hours putting it together and doing the very first, low power, tests.
The principle is to take advantage of an opamp as both input stage and to close the feedback loop.
The limited voltage output of standard opamps means using a voltage amplification stage to magnify the opamp output somehow, and I've selected a class B CFP stage with gain of about 5, using 4 small-signal transistors.
The output stage is a pair of Exicon lateral MOSFETs, avoiding the issue of thermal compensation for the bias - less complexity with a new design seems a good idea.
R6 and R14 form the feedback network for the top CFP pair, setting the gain, and the bias voltage is amplified along with the signal by the CFP sections. The bias arrangement is a bit rough and ready I think, but emulated OK.
The implementation is nearly all surface mount, on a board 3" by 1.5".
So far I have got the amp basically working, with the minimum of bias needed to prevent instability, about 15mA are taken overall I think. The bias pot I chose was 11-turns which turned out to be a wise choice, but I had to hunt around for a small enough screwdriver to adjust it!
Unloaded it seems to work fine so far (using siggen + 'scope), I need to arrange some form of heatsink before going any further. Its always a relief when a new board basically works without modification.
This design predates my opamp-tripler circuit (see this posting: https://www.diyaudio.com/forums/sol...stacking-voltage-operation-4.html#post5954865)
I will be building that on a PCB that's also just arrived at some point (parts need ordering). It will be interesting to compare the two techniques for leveraging an opamp as integral part of a power amp.
In retrospect I should have use DIP8 for the opamp in this design to allow experimentation with different opamps.
I think the choice of the discrete transistors is fairly non-critical in this arrangement which is a bonus as good audio transistors are getting harder to source. Great opamps at bargain prices are too good a resource to ignore!
[ oh yes, and I know the input network is DC coupled, I think I forgot to fix that ]
The principle is to take advantage of an opamp as both input stage and to close the feedback loop.
The limited voltage output of standard opamps means using a voltage amplification stage to magnify the opamp output somehow, and I've selected a class B CFP stage with gain of about 5, using 4 small-signal transistors.
The output stage is a pair of Exicon lateral MOSFETs, avoiding the issue of thermal compensation for the bias - less complexity with a new design seems a good idea.

R6 and R14 form the feedback network for the top CFP pair, setting the gain, and the bias voltage is amplified along with the signal by the CFP sections. The bias arrangement is a bit rough and ready I think, but emulated OK.
The implementation is nearly all surface mount, on a board 3" by 1.5".

So far I have got the amp basically working, with the minimum of bias needed to prevent instability, about 15mA are taken overall I think. The bias pot I chose was 11-turns which turned out to be a wise choice, but I had to hunt around for a small enough screwdriver to adjust it!
Unloaded it seems to work fine so far (using siggen + 'scope), I need to arrange some form of heatsink before going any further. Its always a relief when a new board basically works without modification.
This design predates my opamp-tripler circuit (see this posting: https://www.diyaudio.com/forums/sol...stacking-voltage-operation-4.html#post5954865)
I will be building that on a PCB that's also just arrived at some point (parts need ordering). It will be interesting to compare the two techniques for leveraging an opamp as integral part of a power amp.
In retrospect I should have use DIP8 for the opamp in this design to allow experimentation with different opamps.
I think the choice of the discrete transistors is fairly non-critical in this arrangement which is a bonus as good audio transistors are getting harder to source. Great opamps at bargain prices are too good a resource to ignore!
[ oh yes, and I know the input network is DC coupled, I think I forgot to fix that ]
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