Universal FET buffer / preamplifier

[veteran]

Thanks Tom! I was planning to make a few changes on the PCB and your feedback is very valuable

[Bonsai]

Veteran, a lot of people would look at your circuit and wonder why you have placed the JFET's on the output.

The input bias current on the LM4562 is about 10nA, which is very good, although still about an order of magnitude higher than GP JFET's (AT ROOM TEMPERATURE)

So, one option here is to place the JFET in front of the LM4562 (to get the 'JFET Sound' and the generally lower input bias current advantage) and then to buffer the LM4562 output with a class A stage - a simple emitter follower loaded with a suitable current source would do the trick. You could also bias the 4562 output into class A so the whole thing was class A. You could also then take feedback from the class A output stage, or for a bit more harmonic content, just around the 4562.

This would allow you to drive loads better, and easily use your pre-amp to drive headphones (with a suitable output stage load current of course).

Just my two pennies worth - good luck with your project.
:-)

[AndrewT]

Originally Posted by gootee

...........I believe that the one and only point where the signal and power grounds connect together should be at the power supply's smoothing capacitors' ground, and not before there. Otherwise, the common conductor connecting them both to the supply will have voltages induced by the power ground return currents and those voltages will appear back at the input ground reference, which means that they will be arithmetically summed with the input signal! ..................

If I am interpreting what you have posted correctly then I disagree.

The Main Audio Ground (MAG) is where all the various circuits should take their reference voltage from.
That MAG must not be located on the PSU Zero Volts trace/wire/plate.
A single connection must be taken from the PSU Zero Volts to the MAG. This connection can be very short (the thickness of a PCB for example) or longer, much longer.

The Mag can be located on the speaker output terminal, or on the amp PCB, or on the PSU PCB, or floating in mid chassis (mid air) where all the routes to the user circuits are kept shortish and nearly equal (centroid of the user circuits).

[gootee]

So maybe I should have said "a very short conductor that protrudes from the PSU smoothing caps' ground".

It seems like the farther away the star ground is located, from the PSU caps' ground, the more the audio reference grounds' voltages would bounce.

Or maybe you meant that what you call the MAG only has the low-current audio ground returns attached to it? As long as any significant length (actually, any significant inductance or resistance, regardless of length) of the connection back to the PSU ground isn't shared with any large-amplitude or fast-changing currents, then it should be fine.

[veteran]

After a few changes I measured the preamp again.

This is what I can squeeze out of my sound card:


Only the FET output stage with dc-servo circuit active:


Complete preamplifier with LM4562 on-board. THE+N is higher because of the gain set to x6:

[Jenyok]

Could you a little describe of your scheme ?
"Red line" ?
"R6+R7" (top of scheme) ?
Ku = ? and Ku = formula ?
Changes LM4562 to OPAxxx op amp ?
Changes TL072 to OPAxxx op amp ?

[Jenyok]

Au ???
Mate, are you here ?

[enantra]

Hi veteran we haven´t news of this circuit....why?
i am interested in this preamplifier, also in the selector kit...
thanks

[veteran]

Originally Posted by enantra

Hi veteran we haven´t news of this circuit....why?
i am interested in this preamplifier, also in the selector kit...
thanks

I have a news for you I finished the final version of this preamp. I also added a small PCB with low pass DAC filter and "soft start".

PCBs are in production and should reach me in two weeks from now.

This this the final diagram:
http://www.unisonus.com/PDF/unisonus..._schematic.pdf
http://www.unisonus.com/PDF/unisonus...dimensions.pdf

[veteran]

This is how the final design looks like: