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 ?
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)