FS: THAT1512 preamp pcbs

Ordered some more once again, anybody interested let me know!


(also, Rob's issues have been resolved  )

Since i’m about to stuff a few of these, i’d be curious to know what Rob’s issue was about. Is it documented anywhere?

Thanks!

BT

volker: I seem to be missing a BOM for this project (both pre and PSU). Any chance you have one lying around?

Send me a message with your email and I'll get it to you.

Rob Flinn said:

Hey Volker.

I'm having some issues with RF noise building these mic amps.  I'm trying to PM you but you inbox is full.

I'm having some issues with HF oscillation in that at anything other than minimum gain the output signal is swamped with RF hash.    Has anyone else had any issues like this ?

Can I ask you how you wired the gain switch.  Other than resistor values there is nothng about this.  Are they in a ladder network or wired so just the 1 resistor is across the gain ?

Also was there any build thread for these mic pres on the forum ?

Click to expand...

Hey,

just had a readthrough of the design notes of the 1512:

http://www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf

and this one:

http://www.thatcorp.com/datashts/dn140.pdf

aand this one:

http://www.thatcorp.com/datashts/dn138.pdf

Lots of great info, even for uneducated noobs like me. If you study the various schematics of the proposed implementations, one thing that comes to mind is the part describing the RF suppressing capacitors  (470pf from both + and - through a 47p to chassis ground, installed as close to the inputs as possible, I guess right at the input connector, with the 47p directly to chassis or star ground? I believe it was stated that these should be 1% caps (polystyrene is what I believe my kit from Dantimax was supplied with).
There's also another 470p between the rails just before the 1512 inputs shown in some examples for further RFI protection. They are also not on my (Dantimax) boards. The Dantimax pcb's seem to have a U-pad on the line inputs, where THAT propose to use L-pad instead, as it does not affect the gainrange of the pad when using unbalanced inputs.

There's also a whole section about configuring gain with these chips. Basicly you want a series resistor of 5R followed by a 10k 2,5 % revlog taper potmeter, followed by a cap for the highest resolution of the rotation of the potmeters last 30 degrees or so, and a minimum gain of 0 (good for unity line signals). Or you could use a 5R resistor with a 5k pot for finer control over a broader range of the upper rotation of the gain knob (high gain area), but minimum gain is heightened and unity gain is no longer possible without exterior attenuation. Not a problem for mic pre usage, but might be for line level.

Disclaimer: I am not commenting on the boards Volker sell here; I have no idea how closely they resemble the proposed circuits in the THAT documents (or the Dantimax implementation), altough I have a hunch they are pretty similar. I do not however, from the crappy upside-down picture in the original post, spot any of the mentioned RFI caps? Hard to tell, the pic is pretty low res.

Some more info from the mod would be appreciated  8)

Rob's issues had nothing to do with the pcbs, they were the result of his measurements but actually non-existant.

Don't diss my 10 year old Nokia  ;D, you can't see the RFI capacitors because they are connected directly to the XLR connectors, since these are not placed on the pcb. There is one additional 220p cap (the little blue one)  across the two input pins of the THAT1512.

By the way your inbox is full, I sent you an email through the forum regarding your request.

I can confirm that the issues I was having were down to my measuring proceedure.  I am very pleased with these mic amps.  They sound great & have the advantage of being able to fit 8 in a 1U case if you use an external PSU.

Hey Volker! Sign me up for 3 and a power supply, will pm my email!

Hi there!

If there is some PCBs available I would like some! I tried to PM you but your inbox is full. Just PM me or reply to this thread.


Thanks!

I would like to order 1 board, plus 1 psu board if still possible