[modulay]

Hi, me again.

I've built an Amek 2500 based preamp and it works and sounds fine.

This is the schematic:

http://www.dmlab.jexiste.fr/Amek%20M2500/amek2500micpre_schematic.pdf

But I have some problems with gain levels.
I think the preamp gives too much gain (not 20-60 dB), including at minimum levels.

I've made two records (voice and guitar amp) using a LA MX2 preamp and two TBONE SC400 condenser microphones for comparison.

MX2 preamp gain was adjusted at mid-level (aproximately) and Amek 2500 gain was adjusted at MINIMUM...nevertheless, preamps out signal levels are similar.

You can donwload audio files here:

http://www.mediafire.com/download.php?8pfawnkvvjh

[PRR]

> This is the schematic:

Has no component values, can't even guess the gain.

[modulay]

Ah,sorry PRR.

These are component values:

Ri: 1k
R1, R2: 6.8k
R3, R4: 56k
R7, R8: 3.3 k
R9, R10: 2.4k
R11, R12: 270 ohm
R13, R15, R18: 1M
R14, R16: 6.8k
R17: 82k
R19, R20: 18k
R21: 5.6k
R22: 3.6k
R23, R25, R27: 10k
R24, R26: 47 ohm
R28, R29, R30: 100 ohm

C1, C2, C4, C6: 100p
C9, C16, C18: 100n
C3, C5: 4.7u
C7, C8: 10u
C10: 1u
C11: 150p
C12, C13, C14, C15, C17: 100u

T1 - T10: BC560
D1 - D4: zener 5.6V
D5, D6: 1N4148

This is theorical gain expression:

Av = [(R14 + R16) / (10 + Rgain +1)] * (R23 / R19)

It gives an approximate gain range of 17-57 dB (with a 1kohm gain trimpot), but I think the circuit gives more gain. The audio files was recorded with Amek gain at minimum level (17 dB, in theory)

[PRR]

> This is theorical gain expression:

Yes.

The most likely error is 100K where should be 10K, 56K instead of 5.6K, etc. Check your pot by replacing it with 1K fixed resistor.

[modulay]

Hi, PRR, thanks for the response.
I've measured all resistors involved on gain level (in the circuit, each one paralleled with other impedances) :

R14:                           6.75k    -    OK
R16:                           6.75k    -    OK
Rgain:             2 ohm to 1.2k    -    OK
R10ohm:                10.2 ohm    -    OK
R19:                           17.9k    -    OK
R20:                           17.9k    -    OK
R21:                           5.58k    -    OK
R22:                           3.72k    -    OK
R23:                           9.98k    -    OK

All of them seems to be ok.
This is so funny.

[modulay]

All seems to be ok.
I've doubled pot value and now signal is 6dB lower at minimum level.
I'm going to put a 4k7 pot for getting signal about 12 dB lower
¿do you think is a good idea?

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