saint gillis

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #20 on: August 05, 2020, 11:16:49 AM »
First of all here's the schematic :
http://sub-continental.com/diy/amek/amek_sch.jpg


saint gillis

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #21 on: August 05, 2020, 11:22:11 AM »
I don't understand the ICs voltage charts, are they the voltages at the corresponding pins without the ICs on the sockets?

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #22 on: August 05, 2020, 01:07:45 PM »
First of all here's the schematic :
http://sub-continental.com/diy/amek/amek_sch.jpg

Thank you for the Schematic.
I'll try to learn how its work  :)

Ipin

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #23 on: August 05, 2020, 01:21:53 PM »
I don't understand the ICs voltage charts, are they the voltages at the corresponding pins without the ICs on the sockets?

Yer, on the sockets without all the ICs on, maybe I have to more into your schematic first to find out what happened. I just wonder how its happened? 
Sorry for the questions  ;D   
Thank you Saint Gillis

Ipin

L´Andratté

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #24 on: August 05, 2020, 02:06:52 PM »
Saint gillis, just noticed the caps on the input are not back to back like they ought to be :)

ipindalieson
standard approach: inject a signal (could be sine wave) and trace it (with an oscilloscope) along the signalpath, where does it vanish? Is it present at the input, the output of the tl072´s, at the output of 5534, etc.
That gives an idea where to look---
Also you need to check the soldering at least thrice ;)
That´s how it is, always a stupid mistake or a couple that needs to be found!
"Why not get an assistant to work the faders? Real vintage! And maybe cheaper... ;)"

moamps

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #25 on: August 05, 2020, 03:31:10 PM »
There is no need to have two electrolytic capacitor in series at input. Amek used it most probably because they used caps rated to 40V only. One 10uF/63V would be enough. Or you can try to use MKP 4u7/63V instead.
C12 is backwards in redrawn schematic, also why it is 220uF? Could someone calculate -3dB point when it is loaded with 10k pot?
@ipindalieson
Put in the socket only 072 and measure DC voltages on it and report back. Measuring voltages on sockets without ICs do not say too much.

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #26 on: August 06, 2020, 06:58:34 AM »
Saint gillis, just noticed the caps on the input are not back to back like they ought to be :)

ipindalieson
standard approach: inject a signal (could be sine wave) and trace it (with an oscilloscope) along the signalpath, where does it vanish? Is it present at the input, the output of the tl072´s, at the output of 5534, etc.
That gives an idea where to look---
Also you need to check the soldering at least thrice ;)
That´s how it is, always a stupid mistake or a couple that needs to be found!


Thank you L´Andratté  :)

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #27 on: August 06, 2020, 06:59:29 AM »
There is no need to have two electrolytic capacitor in series at input. Amek used it most probably because they used caps rated to 40V only. One 10uF/63V would be enough. Or you can try to use MKP 4u7/63V instead.
C12 is backwards in redrawn schematic, also why it is 220uF? Could someone calculate -3dB point when it is loaded with 10k pot?
@ipindalieson
Put in the socket only 072 and measure DC voltages on it and report back. Measuring voltages on sockets without ICs do not say too much.

Thank you moamps  :)

saint gillis

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #28 on: August 06, 2020, 08:32:53 AM »
There is no need to have two electrolytic capacitor in series at input. Amek used it most probably because they used caps rated to 40V only. One 10uF/63V would be enough. Or you can try to use MKP 4u7/63V instead.

This is exact! I didn't thought about that when I made the boards, it's not a problem though..

C12 is backwards in redrawn schematic, also why it is 220uF? Could someone calculate -3dB point when it is loaded with 10k pot?

That's right, it looks like a mistake, or maybe I've measured the polarities on my first board and found the output of the IC slightly positive, I don't recall. In fact with the 10K pot anything between 47µ and 220µ will give a decent low pass filtering and phase shift...

12afael

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #29 on: September 15, 2020, 04:07:57 AM »
it seem that the gain cap needs to be low ESR.  I´m not being very scientific but I have seen low voltage, non polarized and many in parallel in that position.

Would be great to hear an explanation why.
heavy metal is the law!!!


Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #30 on: November 08, 2020, 10:17:40 AM »
Your version is a bit of an overkill with the additional output gain trim and the extra expensive differential output driver IC.

I have built almost a hundred channels for myself and my friends since I came across the schematic on Jakob's website a very long time ago. It's a very nice sounding mic preamp, cost only a few dollars (buy BC560C/2SA970GR in bulk quantity, then hFE matched them 10 pcs for one channel) and also very easy to troubleshoot/repair.

What I had done back then was to replace the 1KC pot with a 12 position MBB rotary switch with calculated resistor values that follows the reverse log (or for simplicity, a linear) pattern to cover the wide 60dB input gain range.

I also didn't change the output buffer (NE5534) after TL072, but simply add another 47R resistor to connect RING (TRS) or pin 3 (XLR) to the common ground for an impedance balanced output instead. However, if you insist on using NE5532, the 2nd opamp can be used to invert the signal for a differential output.

it seem that the gain cap needs to be low ESR.  I´m not being very scientific but I have seen low voltage, non polarized and many in parallel in that position.

Would be great to hear an explanation why.

Really? at the time I just used what I had in stock, a Chemicon SME (general purpose electrolytic) 2200uF/6.3V, and there was no problem whatsoever. Only for the recent ones that I built I used a low ESR Panasonic FC/FM because dude is so obsessed with them  ;D

saint gillis

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #31 on: November 08, 2020, 12:06:37 PM »
Sure you can just make a simple impedance-balanced output with a single resistor!
Using a 2nd 5532 to make a balanced output is not a good idea, because if the negative pin of the xlr output is shorted to the ground you will get some horrible distortion.

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #32 on: November 08, 2020, 04:24:04 PM »
Using a 2nd 5532 to make a balanced output is not a good idea, because if the negative pin of the xlr output is shorted to the ground you will get some horrible distortion.

Is that so? Then I guess it was silly for Jakob to do that with his GSSL output section either, not to mention a ton of other designers who also uses the same balanced circuit since the day it was invented.

And yes, I literally meant that as a sarcasm  ::)

saint gillis

Re: Cheap Nice and Funky Amek 2500 preamps
« Reply #33 on: November 09, 2020, 09:28:43 AM »
Hahaha yes... that's how I understood what floating meant..


 

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