Cheap Nice and Funky Amek 2500 preamps

[saint gillis]

  Hello, I made a quad Amek 2500 mic preamps rack, so here is the layout if some of you want to etch some single side PCBs :
http://sub-continental.com/diy_eng.html#amek

 

[aviel]

Thanks! mind sharing the schematic you used as well as sound samples?

 

[saint gillis]

Hello, you can find the schematic on gyraf website's for instance : http://www.gyraf.dk/schematics/Amek_2500_micamp.gif
And the output stage is published on the webpage.
I don't think I will put samples but t's a quite renowned preamp and you can find opinions about it if you make a search..

 

[gyraf]

Very nice!!

We will need to know board dimensions in order to be sure to print it out to correct scale (more than 5% err makes it difficult to mount DIL ic's)

For sound examples I think we got the 2500 in 1982, for Karl Pitterson's recording of Steel Pulse's  "True Democracy" album: https://www.youtube.com/watch?v=h1L3tdNKnuY

Jakob E.
ed:link

 

[saint gillis]

The file on the website is at the true dimensions (106,93 x 57,4mm  /  300px/inch).
The drawing is quite basic, you can add a bit of blank space at the back or on the sides to add mounting holes. I think I added two mounting holes at the back, so the boards hold between them and the pots.

 

[gyraf]

*like*

 

[aviel]

awesome!
i really have to learn to etch on my own

 

[saint gillis]

I was minding, between the two extremities of the gain pot, the offset is quite low (something like 4mV max) :
http://www.gyraf.dk/schematics/Amek_2500_micamp.gif

If we use a stepped gain switch, can we remove the coupling 2200µ capacitor ?

 

[gyraf]

There are two current sources Q11/Q12 that set DC operating points for the transistor arrays - and I think you'd prefer to keep their two DC points different.

But that's theorizing: Try it, and report back?

Jakob E.

 

[saint gillis]

I worked on a preamp of the same topology , and I tried almost everything on my breadboard, unfortunately I didn't noted it all on a paper but I'm almost sure I tried the amek with or without gain cap and that it sounded the same...

 

[gyraf]

Might very well be so.

Maybe the big cap allows for wider variations in the transistor parameters?

Jakob E.

 

[saint gillis]

Maybe... also I'm not sure but I think there was still some few milivolts across the pot

 

[ip_dalie]

  Hello, I made a quad Amek 2500 mic preamps rack, so here is the layout if some of you want to etch some single side PCBs :
http://sub-continental.com/diy_eng.html#amek

Hi Saint Gillis.

I've finished it finally. Thank you for sharing the documents.
I found the 1M resistors are not in the BOM but in the layout, and I used SSM2142 that you suggested to me for replacing the THAT1646, It's hard to find THAT ICS here  ;D

I have some questions,
I forgot that the preamp needs +/-16V, but I have spare PeterC PSU that supplies +/-15V, can I use it? or maybe do some modification on it?
What current does the amps need? I have 5A trafo, is it enough for 4 channels?

Thank you 

 

[Ricardus]

I've been wanting to build some Amek preamps. I've worked on a lot of Amek consoles over the years and it would be cool to have some in my toolbox.

 

[mjrippe]

I have some questions,
I forgot that the preamp needs +/-16V, but I have spare PeterC PSU that supplies +/-15V, can I use it? or maybe do some modification on it?
What current does the amps need? I have 5A trafo, is it enough for 4 channels?

Your +/-15v will do just fine.  I don't know the current draw of this circuit but I would bet the PSU can do +/-1A with good heatsinks.  Should be fine for four channels.

 

[ip_dalie]

Your +/-15v will do just fine.  I don't know the current draw of this circuit but I would bet the PSU can do +/-1A with good heatsinks.  Should be fine for four channels.

Thank you Mjrippe! 

Ipin

 

[saint gillis]

Yes I found 21mA per rail, so yes you might even not need big heatsinks..
Thanks for the 1Ms, I'll add them to the bom.
Nice looking job!

 

[ip_dalie]

Yes I found 21mA per rail, so yes you might even not need big heatsinks..
Thanks for the 1Ms, I'll add them to the bom.
Nice looking job!

Thank you very much Saint Gillis 

Ipin

 

[ip_dalie]

Hi Guys,

I tested the preamps today, and no sound, just humming from both channels.

Here is my ICs voltage chart (both channels are nearly the same)

Without NE55322:
1. 14.5
2. 14.7
3. 14.7
4. -14.9
5. 14.4
6. 0
7. 0
8. 15.0

Without TL072
1. 14.8
2. 10.0 (dropping till 7.4)
3. 10.0 (dropping till 7.4)
4. -14.9
5. 7.4
6. 7.4
7. 14.8
8. 15.0

Without SSM2142
1. 0
2. 0
3. 0
4. 0
5. -15.0
6. 15.0
7. 0
8. 0

I checked all the soldering and it's all fine, nothing short.
Does anybody know what happened here? Is it bad ICs? Or Transistors? Or Caps? Because its happened on both channels
Thank you

Ipin 

 

[Ricardus]

Seems like this would be an ideal 500 series circuit.