modulay

Too much gain on Amek 2500
« on: September 07, 2008, 02:13:59 PM »
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

Too much gain on Amek 2500
« Reply #1 on: September 07, 2008, 03:51:27 PM »
> This is the schematic:

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

modulay

Too much gain on Amek 2500
« Reply #2 on: September 07, 2008, 04:14:16 PM »
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

Too much gain on Amek 2500
« Reply #3 on: September 07, 2008, 06:19:52 PM »
> 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

Too much gain on Amek 2500
« Reply #4 on: September 07, 2008, 07:13:30 PM »
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

Too much gain on Amek 2500
« Reply #5 on: September 08, 2008, 03:42:12 PM »
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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