(re)searching for a better preamp

[carlosfm]

I've been fiddling lately with a TPA6120A2, and also with an AD815.
These two chips are current feedback.
I've been very satisfied with my OPA627+BUF634 for a long time, but I just thought... why not try something different?
I'm opening this thread but the designs are not finished yet, so this is work in progress.

Tests with the TPA are here, beginning at post #85:
PowerPad and the ultra extreme headphone amp TPA

Now I'm testing the AD815, and I may test one more CF chip.
I opened this thread because I feel that P-A's thread was going off-topic (sorry P-A) and it's also easyer to make a seach.

In the end I will listen to them all.

[carlosfm]

Talking about using input and/or output signal trafos...

Originally posted by analog_sa
You'll have to measure the dc with the secondaries in circuit. It will probably be very low.

It is.
0mv.

Originally posted by analog_sa
In any case if you don't notice appreciable reduction in bass the cores are not saturated.

I was testing and measuring, but didn't listen to it yet with much attention with/without the trafos at the input.
From the sounds I've heard coming out of this chip, let's say it's worth the trouble.

Originally posted by analog_sa
Is the pot connected directly to the primaries?

Yes.

Originally posted by analog_sa
This may be less than optimal as at low volume settings the primaries will be seeing higher driving impedance and as these transformers likely have lowish primary inductance there will be less bass and higher distortion at low volumes. Ideally you should drive the primary directly from an opamp output but this effectively means an extra stage.

That's what I don't want to use, an extra stage.
I'm trying to avoid that at "any" cost.

Originally posted by analog_sa
How big/heavy are the cores?

The wire is thin (of course), but the trafos are quite big, 2cm sq.
I've seen smaller...

[carlosfm]

Well... with signal trafos on the input it makes a buzing noise.
Bzzzzzzzzz...
With the trafos on the output, no problem.
It seams that analog_sa is right, the trafos don't work well with a pot before it.
Input caps it is.
0.47uf poly + 100k to ground.

There is ~400 mv DC on the output, so I used output caps.
This happens because I can't match input impedances.
I would need 200R ~ 220R from NI to ground and this attenuates the signal too much.
And I don't want to use an input stage.
I may try the signal trafos on the output instead of the caps, but for now I'm gonna let this thing -in.
It's playin' now, but it's late, I can't crank up the volume.
Promising...

[carlosfm]

I had an idea and now...
No DC-offset at the output, with direct output (no caps, no trafos).

[Nuuk]

WARNING WARNING THREAD HIJACK IN PROGRESS

Sorry Carlos but you do promote my 'public service' so I just thought that I would use your thread to advertise my new Gainclone pre amp on Decibel Dungeon.

No rocket science, just some sound basic advice for newbies wishing to build a pre amp to go with their GC's.

HIJACK OVER

[carlosfm]

Nice job, nice page, Nuuk.
You keep doing a great public service for the DIY community.
Oh, thanks for getting in, I was starting feelin' lonely in this thread.

[Nuuk]

Oh, thanks for getting in, I was starting feelin' lonely in this thread.

I guessed that you were lonely Carlos when I saw that you were threatening to build a valve buffer in another thread!

[janneman]

Originally posted by Nuuk
WARNING WARNING THREAD HIJACK IN PROGRESS
Gainclone pre amp on Decibel Dungeon.
HIJACK OVER

Nuuk there is a pic of a pot with in, out, 0V connections. I think the in and 0V connections are the wrong way around.

Jan Didden

[Nuuk]

Nuuk there is a pic of a pot with in, out, 0V connections. I think the in and 0V connections are the wrong way around.

Thanks Jan, I'll check it out. I did it all that rather quickly and I appreciate somebody more knowledgeable checking it over.

(And I'm honoured to have such an educated audience! )

[carlosfm]

Originally posted by Nuuk
I guessed that you were lonely Carlos when I saw that you were threatening to build a valve buffer in another thread!

Like a friend said, "the tube is on her way to Portugal".