Moddning av Lepai TA-2020 (t-amp)
Jag håller på att uppgradera min Lepai t-amp. Jag orkar inte översätta så jag copy/pastar in från ett annat forum direkt.
"I can´t find these values in any store can I use 0.047 µF + 0.01 µF = 0.057µF = 57n ? "
Funkar det eller blir det fel, alltså kan man parallellkoppla kondensatorer utan problem även när de används i en typ av eq. ?
Det här är alltså det jag försöker göra, från http://www.diyaudio.com/forums/class-d/90500-lepai-t-amp-ta20... .
Either bypassing the tone control section or correcting it is also highly recommended. As djnemesi and col already figured out, the tone control is a 'James' Baxandall variation. From simulations it appears the component values that are installed as well as the ones printed on the PCB, are both horrible. With both controls set to neutral, the frequency response is nowhere near flat, it has a huge bump in the midtones. And to top it off, the components are cheap and inaccurate.
The first attached image shows the responses for bass/treble controls set to 0, 2.5, 5 (flat), 7.5 and 10 for the values that are installed. The curves correspond reasonably to what I measured when taking some extra loss of bass into account. The second image shows curves for the printed values, not much better.
I would suggest the following replacement values (component numbers as on the webpage). This will give the much more balanced characteristics from the third image. As soon as I have the right components, I'm definitely going to do this fix:
R1 (installed 10k, marked 15k): 20k
R3 (installed 1k5, marked 1k): 2k2
R4 (installed 1k, marked 1k5): 10k
C1 (installed 10n, marked 472): 5.6n
C2 (installed 47n, marked 393): 56n
C3 (installed and marked 10n/103 near bass pot): 1n
C4 (installed and marked 10n/103 near treble pot): 10n
There is an additional resistor not in the original James schematic at the treble potmeter (installed somewhere around 300Ω, marked 100). What this one does, is for some reason preventing that the treble control can be set all the way to 0% or 100%. It can be safely replaced by wire links or by any other small value (the marked 100Ω is OK).
Use metal film resistors and your preferred type of film capacitors. Even for C4 which stays the same value you should remove the existing components and replace them by something of higher quality. You can also replace the potmeters although for my needs they are acceptable.
