Another digital amp up and running

mgalusha · « **Reply #20 on:** 1 Nov 2003, 11:52 pm »

Quote from: _scotty_

PeteG, you should email Lars and ask him how to set up your input
for about 20kohms to 40kohms. Your tube preamp would like to see this
sort of input impedance or higher. 2k as an amplifier input impedance is 1/10to 1/20 the industry standard. Also the Zap amp should be set for the industry standard 26db of gain if it isn't already. I don't think it should take to much for the Zap people to correct this oversight in their kit.

Scotty, I think this was meant for me and not Pete.

Actually I did email Lars and received a response this morning on how to change input impedance. The new resistors will bring it up to 18K, which will be fine in my setup. The only bad part is everything is surface mount and damn small. Thankfully I have a good magnifying lamp on the bench and some small soldering tips.

Mike G.

JohnR · « **Reply #21 on:** 1 Nov 2003, 11:58 pm »

Mike, why is it 2k by default? Is changing the resistors going to change the gain (just wondering what the circuit looks like)

JohnR

mgalusha · « **Reply #22 on:** 2 Nov 2003, 12:12 am »

John,

LCAudio doesn't supply a schematic but I have to change out 7 resistors per board to change the input impedance. I too wonder why he designed the circuit with such a low value.

From what I can tell there is a 10k resistor from each input to ground (it's balanced) and then there are 2 2k55 resistors on each half of the input stage. The 10k resistors are replaced with 47k and the 2k55 are replaced with 22k. In addition I have to change the feedback resistor from 100k to 1M.

I think the combination of the 10k/2k55 sets the gain along with the impedance. Although the ratio between the 10k/2k55 and the 47k/22k is certainly not the same. Lars did not mention any gain differences but that doesn't mean it there is no difference. I'll know more once I get some new resistors in place.

Mike

KevinW · « **Reply #23 on:** 2 Nov 2003, 04:07 am »

Quote from: mgalusha

I would like to know if you are running the RC (Zobel) filter across the outputs tha ...

I'm not using it because we've solved the oscillation problem in a different manner. I also don't like the way zobel networks have a tendency to "smear" the details in sound.

mgalusha · « **Reply #24 on:** 2 Nov 2003, 05:16 am »

Quote from: KevinW

Quote from: mgalusha
I would like to know if you are running the RC (Zobel) filter across the outputs tha ...

I'm not using it because we've solved the oscillation problem in a different manner. I also don't like the way zobel networks have a tendency to "smear" the details in sound.

Thanks for the info Kevin. I received a reply from Lars and the RC circuit show for the 2.0 version is built into the 2.1SE's. I didn't use the RC/Zobel when I built them but had considered it given the possibility of the modules "burning down" as listed on LCAudio's site on the 2.0 upgrades/fixes page.

Mike G.