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promitheus 24th July 2002 08:50 PM

jFET input ,full symmetrical class A circuit
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This is a circuit I found at this sight for headphone amps:

I really like this circuit and I need some help from someone to modify it into a discrete op amp.
I was thinking of using it in an I/V and filter circuit of a DAC.

The input and driver parts I think will work perfect. The output stage is a high current one for driving headphones.

Using only one pair of bjts instead of 4 is a good idea?
Should the emitter resisotors be smaller like 4R7 or 10 Ohm ?

Could I just avoid the servo?
I donīt have so much space for it.

Also it has no overall negative feedback. Can you just apply it from output to negative input like an opamp ?

Nelson Pass 25th July 2002 04:06 AM

All the things you want are do-able as you speculate
until you get to the op amp part. The gain of the circuit
is set at a low figure, and you will want to increase the
open loop gain so as to have some leftover for feedback.

To get this, you can take the 200 ohm resistors off the
diff pairs, and consider less resistance off the Emitters
of the 2SA1015 and 2SC1815, and maybe also adjusting
the values off the Drains of the input diff pairs, using pots
to get proper bias and offset values.

Jocko Homo 25th July 2002 05:16 AM

Why do you want to muck it up with feedback? Probably works great without it. You do not need feedback, or op-amps, to make I/V or post filter stages.

As for the output stage, that will depend on what load it will see.


promitheus 25th July 2002 05:51 AM

Thanks a lot Nelson. I will give it a try.
Will your active crossover be using something like this? I would guess probably something simpler. :-)

Jocko , what would be the best for a stage like this?
You know a lot on this subject. What is the dream I/V and filter stage ?

I will be using PCM1704 btw

jcarr 25th July 2002 05:58 AM

I wonder how the author intended for the DC servo to work when someone makes the optional balanced version of the amp?

jonathan carr

Jocko Homo 25th July 2002 01:48 PM

Glad to see you are not still using TDA1541s [joke].

Assuming you are using 8X oversampling, you can get away with just a capacitor across whatever load resistor is in your circuit for a filter. Yes, higher order filters will get rid of more stuff, but you will find that there is not all that much to get rid of.

As to what type of circuit, this is one place I prefer BJTs in the input stage. Try a common-base scheme, with the emitter at ground reference. There are several ways to go from that point, but a simple resistor in the collector path works. I would suggest something biased at 5 mA, or higher, as a starting point. That should get you going.


promitheus 25th July 2002 02:08 PM

What is this information based on ?

Simpler design I would guess. You are right about the filter.
Not much to get rid of anyway above 20KHz.

I donīt know what exactly you mean but I can search.
So a common base circuit with one BJT is enough.
Where should the filter be , at the output of this bjt?
I understand a capacitor in series with the output load.
After that would a buffer stage be a good idea.
A buffer like with 2 jFETs or 2 BJTs.

mlloyd1 25th July 2002 02:51 PM

Have you looked at Borbely's "all FET I/V converter and filter for DAC" solution? Might not be to your liking but might have some useful info otherwise.

promitheus 25th July 2002 03:02 PM

Yeah I have seen it. ITīs very good but not quite my taste.
I am looking at the Nelson Pass solution that works with a balanced DAC with positive and negative input.

Its also a very simple circuit something like what Jocko is talking about.

Jocko what do you think about it?

paulb 25th July 2002 03:02 PM

Just a could always ask Kevin directly, he's an active member here.

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