quick q? can i use 25k pot and 2x30v 160va trans w/ lm3886 chipamp


2010-10-25 4:19 am
hey again guys,

just some quick questions now i have the 3886 chipamp on its way.

1) for a 1.4vrms source (gamma2) is a 25k pot suitable? if not, what resistance am i best advised to get?

2) is a 2x30v 160va trans suitable if i limit the amp to only 2x8ohm speakers? I see in the instructions that 'one user' successfully used 2x30v, but i want to know if this is safe w/ 8ohms while still leaving head room... reason being that i want to use the same traffo box and wire it in parallel to my headphone amp that requires 2x30v (beta 22)

I plan to use DPST switches before both amps PS boards to kill the power to the amps (i will also use a single switch at the traffo)


most probably no to everything.

A 230:30+30Vac transformer fed with maximum mains voltage will almost certainly take a chipamp outside it's rated 84V limit.

The source voltage determines the necessary gain of the power amp.
The pot value is determined by the input impedance of the power amp and the current capability of the source.

Why switching the power amps independently? switching in a short across the power amp inputs would put them into an effective mute mode. Fuses in the supply rails and a mains fuse in the primary circuit followed by that DPST to isolate the transformer.


2010-10-25 4:19 am
If i'm understanding you correctly, b/c the headphone amp i am connecting to has many boards (can be as high as 6ch) so it would probably be more onerous to connect switches that way at the board power inputs...

also the sigma22 PS board will have the led constantly on, dont see a point - i'd rather be able to physically turn both amps on and off as I so choose...

ok... so 2x25v 160VA should be safe... what value pot would be most compatible for most low power sources? (its just a DAC) using RCA outputs?? I can always try cheap dual gang pots of different ranges to test it before going for an ALPS or similar, but what ranges would u suggest 10k? 25k?

probably sounds like a compromise but traffos aint cheap in oz and I see little point in duplicating traffo boxes complete w/ fuse and master switch at a cost of at least $70ea when I can save money and space and use 1 for 2 things with no real detriment.



2010-10-25 4:19 am
haha, this is tru... i'm souring on the whole idea of a 2 for the price of 1 traffo... I'm certain i can safely spec 2x25v for both the beta22 and lm3886 chipamp, but dropping the voltage on the beta 22 will kill off its amazing voltage swing (for solid state) advantage to the point where i may as well mod my existing m3 amp...

600ohm phones...

w/ a 2x30v traffo the beta22 would see 43Vp-p and 15.2Vrms, with a 2x25V that drops significantly to 33Vp-p and 11.7Vrms... I could probably mod my m3 to 20Vp-p and 7.2Vrms up from its current 13Vp-p and 4.5Vrms

the beta only needs a 80VA traffo anyway.... hmm, decisions decisions...