Musical Fidelity A1 Integrated repair/upgrages

[spendorspain]

Great! I’m looking forward to see your tutorial on the A1 soon, Zen Mod.

Please, I’d be very grateful if you could give advice regarding the new power transformer on my A1 because I’m still in doubt about its VA size.

I’ll probably buy the Nuvotem 0120P1-2-018K which is a 120VA unit with one 230Vac primary and two 18Vac secondaries. If I’m not wrong, to obtain the required 18v-0v-18v ac supply I have to join the two 0v output cables (black and orange) and solder both to the “0V” input on the A1 pcb. The other two wires (red and yellow) are soldered each to one of the two “~” inputs on the pcb. Just to be sure, this is the correct wiring, isn’t it?

I have a doubt regarding the size. The original A1 transformer is around 100VA, so I think the 120VA will suffice and its 104mm diameter is perfect for the box I have. I will use the amplifier with a pair of small 4 ohms loudspeakers in a smallish room and I listen usually nearfield at low volume, so the limited 20W power is not a problem, but I’ve read that the A1 is not at ease with a 4 ohms load. Perhaps in this case a bigger transformer is better, helped by the beefier MJ15003/4 output transistors and a big 0.30ºC/W heatsink to cool all down. Will its higher rating and inrush current stress too much the PSU caps and diodes?

What do you think about it? Is there any advantage in going from a 120VA to a 160VA transformer?

Thanks again in advance

Best regards

Jose

[Zen Mod]

considering that stock A1 ( if you're lucky not to have one with crippled bias ) is cruising somewhere at 700mA/2 x 22V per channel - that's 30W ; say that rest of circ is drawing some 10W max , so you have around 40W of draw per channel

80W for both - my logic is to use at least 200VA xformer if shared between channels

if you are going for one xformer per channel , 120VA is more than fine

to further refine things - look at ,say, F5 manual ( FIRST WATT HOME) and observe how NP use two bridges (one per winding) ..... old trick , also used by Naim boyz

that way you'll have cleaner audio gnd , less spikes and buzz in xformer

for going completely nutz - use discrete bridges , made with either shottky or fast T0220 cased diodes

[jaycee]

The original transformer is only 100VA iirc. You would join the black and yellow wires together and connect this to the 0V point.

I wouldn't bother "going nuts" on this amplifier at all. What I would do is replace all of the capacitors, and replace the output transistors (which are MJ2955/2N3055s) with more rugged MJ15024/5 or MJ21193/4 devices. Also as Mark says, build a replacement preamp and bypass the dreadful one in the amp itself.

[adespade]

Hi, does anyone already have a layout for Mark Hennesey's preamp design
or would anyone be willing to verify mine?

thanks

[adespade]

For anyone interested..this is an updated layout