Modified Dual Differential F5

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I built this a while ago also with cascodes and 40V rails.
I have left the cascodes out of the drawing to keep it simple.
I really like this amp but I am wondering if I can squeeze more performance out of it with a few more mods to the circuit.

What do you guys think?
 

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Official Court Jester
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you obviously slipped when Papa sez something about effects of heavy current feedback net , and benefits of that ;

I think he mention some sort of bootstrapping of input Jfets with that ..... ;)

I can't see that you still have that virtue in your iteration

everything else in F5 , besides specific feedback , is beaten to death .

but - that eeeny weeeny tiny detail is work of ....... :rofl: no one else but this sort of Genius :
 

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I wonder if parralleling the inputs of the original would be better than dual differential.

I really don't understand enough about electronics (I have never read one text book on the subject).

Every thing I have learnt has come through trial and error based on ideas from Nelson Pass, Erno Borbley, Joe Curl, and everyone here.
 
thanh,

The cascode should help. I have found how that you bias the cascode can affect the sound and I like using a bipolar as the cascode device.
One thing you might try is using current mirrors on the differentials which would lower distortion but I have found might not improve the sound, you will have to try and see.

Jam

P.S. Paralleing the inputs should help as long as the input capacitance does not get too high but in this case the cascode woud help reduce non-linear capacitance.
 
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thanh,

The cascode should help. I have found how that you bias the cascode can affect the sound and I like using a bipolar as the cascode device.
One thing you might try is using current mirrors on the differentials which would lower distortion but I have found might not improve the sound, you will have to try and see.

Jam

P.S. Paralleing the inputs should help as long as the input capacitance does not get too high but in this case the cascode woud help reduce non-linear capacitance.

When you say biasing cascodes. Do you mean votage dividing?
I have set it so the Jfets see around 22V.
 
Since it is going to a high Z point, you don't need to have low Z resistors? :D

I presume that is functioning like a follower, so you have current, not voltage gain...

Otoh, the 10 ohm resistors effect the gain of the input jfets, and also provide some separation of the two feedback paths... you might want to put that back in??

You'd have to check to see that the two "feedback" jfets can source enough juice to do that job, since the juice for the original scheme comes from the outputs via the low Z resistors... and I am not sure that qualifies as a "long tail pair" without a single source (emitter) resistor being shared between the two transistors...

But I only guess at most things...

_-_-bear :D :Pawprint:
 
Official Court Jester
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What is the best way to implement dual differential here?

no way - if you want to keep that precious detail , which I already mention .

you can make it with dual differential , but that way it's not F5 anymore , but just one of many similar minimalistic amps

it's really just that twist which differentiate F5 from many others


just in case - even if you can "glue" two more fets there , source resistors are miniscule to even remotely qualify as decent tail for LTP ....... needed to establish dual differential
nail on coffin is fact that these two source resistors must be tied to GND
 
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I am using the exact same feedback as the original.
Aside from the previously mentioned points, this amp depends on specific gains in each stage along with the top and bottom halves for the sound signiture, or sweet spot, that it has. Just the multiple output devices will likely increase gain greatly in that stage. While keeping the feedback resistors the same "may" create the same closed loop gain, it will force the extra gain to be used as more feedback :eek:
Having said that, I do not know what to expect from that input stage and how you would tweek to a sweet spot like an original F5 would have :smash:
 
thanh,

Yes, I mean setting the point where the cascodes sit but 22v might be a bit high try 10 to 15v.

To set the bias point you can try a resistive divider, a zener and resistor, a zener and a current source, a resistor and a current source or a voltage regulator and they all can sound different.

Jam
 
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