Looking for Feedback on this No Feedback Amp.

[Bigun]

phew, that's good.

Now for a crazy idea.

You would get more swing, a lot more, if you went with a Balanced Bridge output. A kind of cheat for having single supply rails. It probably has good PSRR.

 

[Bigun]

cancel that. Doesn't help.

 

[MJL21193]

phew, that's good.

Now for a crazy idea.

Not so crazy - I considered a kind of bridged super symmetrical idea for higher power, mirroring the circuit to operate on a V- side of a split supply. Still pondering...

For now, for the application I have in mind, I'll leave it as is. I just need to get the time (and motivation) to puzzle out a board.

 

[MJL21193]

With some outstanding help from alex mm , I have a board layout to try.
Copied, pretty much exactly from Alex's layout, I have this to etch and try out:

 

[Bigun]

I can see that Alex has done you proud.

 

[MJL21193]

I had some sort of mental block about this amp - I couldn't put together a layout. Maybe lack of motivation, maybe lack of free time or maybe just plain lazy.

Alex has done me a great service, I owe him.


Now, to find the time to etch this board...

 

[Bigun]

You gotta do it, you just hafta do it !!!

 

[MJL21193]

You gotta do it, you just hafta do it !!!

Yes! YES! Ahahahahahaaaa! (ripped from DX - deep yet somewhat condescending laughter )
Heheheheheeee!
First step - copper bottom pdf:

 

[MJL21193]

Next, silkscreen top pdf:

 

[Bigun]

Originally posted by MJL21193 Yes! YES! Ahahahahahaaaa! (ripped from DX - deep yet somewhat condescending laughter

Indeed, I am sometimes looking over my shoulder expecting to see him!

 

[MJL21193]

Well, you know what he looks like, right?

Alrighty then, I did it. A little rough but it will do for the first complete prototype:

Silkscreen is printed on the other side. I'm taking tomorrow off so I may do a bit more with it - drill holes, add parts, etc.

 

[Bigun]

How's it going with the construction MJL21193 ?

I am thinking that if I build this amp I should change the input device to a CFP - it's become my favourite compound device these days - would it work, would it help, did you try this in your sims ?

Perhaps a JFET/BJT CFP - what are your thoughts on this ?

 

[juma]

I don't want to rain on your parade but JFET/BJT CFP is a rather disappointingly sounding gain block (dull, lifeless). IMO it can only be used as a unity gain buffer in guitar preamp - something like this:

 

[Nelson Pass]

You can improve it a bit by putting some resistance in series with
the emitter of the BJT, say 47 ohms.

If you want lower distortion, use another 2SK170 as a current
source to the -9V rail instead of the 820 ohm, and then maybe
load the output with 10K or 20K to ground.

 

[Bigun]

Nelson,

Many thanks ! - I've been struggling with the sims on this configuration for an LTP - I'll give it another try with this change.

So far I have trouble creating an LTP with gain when I use the JFET-BJT CFP.

 

[juma]

You can improve it a bit by putting some resistance in series with
the emitter of the BJT, say 47 ohms.

If you want lower distortion, use another 2SK170 as a current
source to the -9V rail instead of the 820 ohm, and then maybe
load the output with 10K or 20K to ground.

Tried all that and it's wasted effort - B1 still sounds better.

The shematic presented above is just for illustration - JFET/BJT CFP looks attractive but results are sonicaly disappointing.

 

[Bigun]

Why do you think this is the case ?

 

[juma]

Because I listened to it - it seemed like a good idea

And why does it sounds bad when it looks good - well, universe is full of secrets - ever met a hot looking girl but behaving like a b..ch ?

 

[nelsonvandal]

I don't want to rain on your parade but JFET/BJT CFP is a rather disappointingly sounding gain block (dull, lifeless). IMO it can only be used as a unity gain buffer in guitar preamp - something like this:

I disagree, I find JFET/BJT CFP input stage the most alive and least dull of the input stages I've tried.

 

[MJL21193]

How's it going with the construction MJL21193 ?

I am thinking that if I build this amp I should change the input device to a CFP - it's become my favourite compound device these days - would it work, would it help, did you try this in your sims ?

Perhaps a JFET/BJT CFP - what are your thoughts on this ?

I'm still stalled. I haven't done anything since.

The CFP (bjt type) works fine in simulation, though there doesn't seem to be any measured improvement - THD the same.
What does make an improvement to this circuit (at least in simulation) is another "gain" block at the input - a high beta, NPN emitter follower at the input. I was playing with this before and may (will) include it in future testing.

As for FETs, I'm still watching and learning. I have some included on my next Digikey order - Zetex ZVN/ZVP to tinker with.