Amek 2500 preamp gl122 board

[Arthurelletson]

I did a search but couldnt see one of these projects all ready done.

I just drew out the preamp card of the amek 2500. This is the transformer balanced one
I though it might be a good option for a DIY project. It's just one ne5534 chip and a transformer.
Output is currently unbalanced but I was thinking of a carnhill to balance it. The sowter 4057(?) input transformer in not available but I have a 3195 at home that I think would work instead.

In terms of pcb and power, I was going to try and design a board that could either be mounted in a 500 series rack or four in a row in a 1 unit rack.

The schematic is currently just on a bit of paper as I traced it at the studio, but I will draw it up in the next day or so.

Let me know if you are Intrested in this.

Arthur

 

[gyraf]

Interested. I've only seen the electronically balanced module till now.

IMO no real use for balancing output - I would go for impedance balancing for simpleness...

Jakob E.

 

[Arthurelletson]

We have a desk that is full of them, with only one or two electronically balanced channels out of 36. Strangly we don't have a schematic for the ones with transformers. I think that jenssenmann has the same desk, He posted a schematic as well a while back.

I will draw a cuircit up in the next day or so when I get a chance.

Impedance balanced outputs might be simpler for pcb design. (I'm just a sucker for iron.)

 

[jensenmann]

Here are the pictures of my desk (24ch transformer micpres + 12 stereos):
http://www.twin-x.com/groupdiy/thumbnails.php?album=212

and here´s the schematic of the preamp:

Here´s the Sowter chart with 4057 data. If you ask Brian nicely I´m sure he will do them again. Amek is out of business longtime.... http://www.sowter.co.uk/archive/proaudio.htm

If you want to modify the circuit then do something like this:
Change C1 to 100pF (FKP or Mica), increase C4+C5 to as big as possible, 6V rating is enough + add a parallel 0,1uF foil bypass cap (no ceramic!). Increase C6 to 470uF/10V plus 0,1uF foil bypass cap. If you want to change to a more modern chip (this will change the original character) then add 0,1uF bypass caps between rails and ground closest possible to the new chip and skip C2. Definitely use a BJT input chip, no FET. The only chip worth to change the 5534 with in my book is a LME49710. Of course everybody else knows it better (without having it tested it in this particular circuit) but that´s the internet 
If you add an electronic balancing stage then skip R5. If you use impedance balancing then keep R5 and connect Pin3 of the output XLR with a 47Ohm resistor to ground. Stick with R5 if using an output transformer. I hope all that helps.

 

[Arthurelletson]

Wow, thanks for all that info, have you performed any of these mods on your desk? If so what was the found difference? 

I am hoping to use impedance balanced, I am just reading on it at the moment, I would like to have the option of a transformer as well, I will have a look at the channel outputs tomorrow to see what they are doing there. 

Thanks for reposting that schematic, I was slightly amazzed that after I had traced the board, it looked like yours, it ment I had not made any mistakes. (do you have a schematic from the factory?)

I don't think I want to change to sound of these too much as I think that they sound good like they are, I found the page for the 4057, but at the moment I will just prototype with the 3195 as I have one at home, but good to know that Brian can be hospitable.