Cheap Nice and Funky Amek 2500 preamps

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SIXTYNINER

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Sep 15, 2005
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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:

Jakob E.
ed:link

Hey "Hyper" Jakob
the song at the youtube link sound really not bad at all ,
(..it is the original one or remastered ?)

do you remember if the 2500 was the first console available with full parametric eq on each channel ?

about the 2500 pre project it can also be improved with some upgrade / mod
like those for the other Amek consoles (Einstein ,Hendrix , Mozart, etc..) for sale at "AML" ?

cheers
 

Matt Syson

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Dec 17, 2005
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The 5543 is the main component to provide common mode 'rejection' (otherwise it is largely an unbalanced input). Strictly, the resistors, and even capacitors should be matched but they never were in production. Other implementations would include common mode trimming of low and high frequency aspects, so resistive for low frequency and capacitive for high.
A TL072 is not a good choice for driving low impedances although they do 'give up gracefully' simply increasing in distortion (leading to the supposed advantage of being able to 'drive' a preamp.
Matt S
 

MidnightArrakis

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Sep 25, 2015
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140
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.



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
Would you happen to have a chart and/or a list of the "calculated resistor values" when using a 12-position MBB switch? I would like to include that into a new and modified schematic update of this circuit.

Also.....could you also include either a sketch or a schematic of how you included the additional NE5532 as a differential output? That would be really helpful.

THANKS!!!
 

k brown

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Jul 6, 2021
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San Francisco
A question about this schem - why are the two Rs to gnd at the input to the 5532, different values (5.6k and 3.6k)? I've never seen a diff amp set up this way.
 

Matt Syson

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Dec 17, 2005
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198
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France
As the TL072 outputs are not at 'ground' level at DC the resistor values for the Differential stage will reduce the DC offset for the NE5532(4) stage. The extra 'buffer' is essentially 'free' and allows 'any old value' of level pot to be used although using a 2(or 1) K pot as the level control would allow the THAT1646 to be properly driven (I think, without having rechecked the datasheet.
The ferrite beads are available as a '1 pass through' device which are the same footprint as resistors.
Matt S
 

jensenmann

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Oct 10, 2004
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Karlsruhe, Germany
A question about this schem - why are the two Rs to gnd at the input to the 5532, different values (5.6k and 3.6k)? I've never seen a diff amp set up this way.
The summing resistors are 18k for both inputs. The shunt resistance to ground is basically the same at both inputs because R25 is parallel to R26 which results in the same 3,6k as R24 is.
 

jensenmann

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Oct 10, 2004
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Karlsruhe, Germany
I am still waiting for a super accessible version of this project. Maybe something 500 series.
Check our member Sahib´s project called Micpre One:
 

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