AMEK2500 preamp Troubles

Hello of people!
I make a AMEK2500 von Olaf Matthes
(http://www.akustische-kunst.org/audio/micpre2500.html)
I have there a question: "OUT -" with R26-27 and C13 what is that?
AND I already made this Preamp and I have approx. 12V on OUT (!?!??), as can be.
Excuses my English.

"OUT -" with R26-27 and C13 what is that?

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Do you now what a balanced (symmetrisch) out is, i.e. on XLR? Out- is pin 2.

And I already made this Preamp and I have approx. 12V on OUT+.

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After C12 you should read very close to 0 V. What do you have at IC3 out?

Samuel

i know balanced
but R26-27 and C13 are not connected on schematic (ground only)

IC3 pin 6(out) = 19V +
most be 0V

But R26-27 and C13 are not connected on schematic (ground only)

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It is an "impedance-balanced" output. There is no signal at the cold out, but the impedance to ground is equal to the one seen at the hot out--that's what counts for good CMRR. Many mid-priced gear uses this output and it works IMO well.

Are the voltages at R5/R6 OK? About 0 V as well.

Across R11/R12 you should get about 0.65 V.

Samuel

It works best if the next stage has some input iron, not an opamp.

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In what sense? About any output has best CMRR if it runs into a good transformer...

I would put it the other way round: to not degrade CMRR of good input iron you better don't use any cross-coupled stuff.

The impedance balanced out provides no CMRR into an unbalanced in, that's clear.

Samuel

i have R14,16, C7 C8 breaked
pin 2 on the IC 1,2 solderingon the Ground.

it is 19 V on the OpAmp outs
unbeleaveble

thanks
I control wether I had soldered everything correctly on your plate.

thus I have 4 errors found - now function preamp. but the Noise is rather loud. That is white noise.
I switched off OUT - completely, since I have anyway unbalanced OUT.

and i have noise about -44db (in Wavelab)

that is to much

-44db noise at what gain, and at what input source impedance?

These are important specs when evaluating a mic pre!

Jakob E.

with max gain (i have rotary swith - max is 1.5 om)
input impendance is Shure sm57 (about 200 om)

And how much gain is that, in dB's?

Try measuring the actual gain when set to max. Then short the input and measure output voltage at maximum gain.

These two figures should add up to some 115dB's worth of input noise figure..

Jakob E.

i m short in to ground
gain is on max (66db)
in wavelab is noise -42 - 44 db
output voltage cant measure, that i have no osciloscope.

If wavelab is referring to a prosumer-level, then my guess is that your pre is working as it should..

Jakob E.

I did not understand the latter post.
I compare the Noiselevel with AMEK2500 preamp and my self-developed Tubepreamp (with Transformers)
with Tubepreamp is noiselevel about 90db in wavelab and I can the noise not hear.
With the AMEK2500 is very noise to hear. (44db)

i do that
gain (amplification level) is identisch by both preamps

by tube is noise 85-90 db
by amek 44 db

what can the cause be?
what do I have to still examine?

You measure wrong.

85dB distance-to-noise is simply not possible (theoretically) at 65dB of gain.

A 200 Ohms source has an equivalent noise of around -127dB.

Which means that at 65dB Gain and a 200 Ohms source, the theoretical maximum distance to noise is 62 dB's - very-very good designs can come within 2-3dB from that.

But there's no "going over" this figure..

Jakob E.

probably I measure wrongly, sorry
but I cannot hear white-noise into headphones with tubeamp
with the transistor preamp can I hear.
(it is now no 66db amplification)

85dB distance-to-noise is simply not possible (theoretically) at 65dB of gain.

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Perhaps he is looking at a FFT-spectrum?

Samuel

http://kaa-band.narod.ru/amek2500.png

that is FFT spectrum

So you get: 66dB gain + about 70dB noise - about 14dB headroom (guessed, depends on soundcard being used) --> -122dB EIN

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No, no, no! For noise, you cannot conclude frome FFT spectrums to full-bandwith amplitudes! There is no simple relationship between them--they are proportional (if one rises, the other does as well with the same amount), but that's about it.

It doesn't matter what and how he measures, as long as he has another pre-amp that measures much better under equal circumstances, there is something wrong.

Did you terminate the input pins (2 & 3 on XLR), i.e. by shorting them or with a small resistor? Or same thing the other way round: is the noise audible with a mic connected?

Samuel