Buffered IGC. Advice needed.

[Dxvideo]

Hi all,

Last week Nordic recommended a buffered inv.gainclone solution for my iPod...

Which is Pedja Rogics JFET buffered inverted gainclone.
Then I decided to make that. Because Nordic said a lot of good things about it. He said "the JFET buffer makes too much difrerence". I will use 2SK170 as J2..
Do you agree with him? Do some transistors on input make that much difference? I started to make a PCB for it. And its a bit complex to put them alltogether on one PCB and I dont like many PCBs in one enclosure.
Has anybody tried that circuit before? And is it worth to build?
Thanks a lot.

[Nordic]

Hi there it wont matter much which transistors you use I think, as long as they are largely similar. I think pedja said one could even use 3 of the same type... but just read his site to see if there is any adjustments...

If you read the thread in the solidstate section on the buffer, you would have seen comments by T. who claimed this circuit combined with Pedja's own PSU is one of the best sounding chipamps...

There is a stripboard layout that is quite easy to build and adapt to your component sizes as well as showing right orientation for the fets...

Or if you want I can forward you some basic regulated boards in eagle format... you can always omit the regulator parts and build Pedja's...

[AndrewT]

Hi Dxv,
It's not so much that the buffer improves the signal.
I believe it's more down to preventing the unbuffered signal being ruined by all the wrong (varying) impedances that exist from source to amplifier.

To all our experts,
what does R1 do? Is it there only to attenuate RF?
If so, then I think it would be better before the buffer rather than between the stages.

What does C2 do?

[Dxvideo]

Hi Nordic,

I think I already finish a combined PCB for all.

But I always afraid complex PCBs because of RF interferences etc. I know its too much complex to have a quick examination.
May you have a look pls?
Thx.

[Dxvideo]

Hi Andrew,

I read Pedja's site a bit. As my understanding, the buffer is for impedance immunity.
And the C2, R1, C1 trio creates a pi filter to reducing ultrasound interferences I guess.

But for all matters I am not sure. Pedja tells more detailed in his site. But my poor English doesnt enable me to get more information.

http://www.pedjarogic.com/gc/index.html

Pls have a check here.

Thx.

[Nordic]

very nice PCB. good luck...

please send me a copy of the files if your test unit works... I think it will be a great amp...

[Dxvideo]

Thank you.
But you say; "good luck". Meant will work if youre lucky?
By the way I use LM3886 for this project as you can see. Its for I have two spare LM3886 and I dont want to loose them...

[Nordic]

I think there is always some luck involved with us beginners...

But it is like horserideing, if you fall off, you must just get on again immediatly... worst case, scenario, you can desolder your parts and build the veroboard version, which works just fine...

I build both channels on the same board, so it is still small and neat..

[Nordic]

Just a question you did pull down the mute pin with at least a resistor?

[justblair]

What cap do you intend using for C3?

The shape looks like its for a stacked film resistor. I had a bad experiance of these in my build. (DISCLAIMER: I've only built one, so I'm not an expert). I found that the stacked films I used introduced noise to the signal path and limited my sound.

My suggestion would be to use good quality axial caps such as sonicap, mundorf supreme or similar (there are a few, sonciap VII bypassed by Vishay mkp1837 sounds very good) for this cap. This can be done on the underside of your board perhaps.