Aleph3 questions

[sherwood]

Hello,

This is my first post on this forum. I'm planning to build an Aleph3 with a separate power supply. I've started to collect the parts to build it but I still have some doubts about some components and a question about the original schematic:

1: would a 300 VA- 2X21 toroid enough to feed the power supply?

2: On the original schematic the power thermistor TH2 is connected on lead to the thermistor T1 but I don't see (bad printing?) where is connected its second lead? Is it connected to the tap of the transformer? If not, where?

3: I would like to add a led on my PS block. Do I have to put a resistance between the switch and the circuit? Which value do I have to use? (4.75K like in the channel schematic).


Sorry for these basic questions, I'm a beginner.


Thanks.


Rodolphe

[wuffwaff]

Hi,

let´s try to answer your questions:

1. Maybe, 21V would be ok but 300VA only just so. The Aleph 3 dissipates around 100 watts per channel so 200 in total. Normally one chooses the transformer 2 to 3 times heavier than the load. In this case this would mean 400 to 600VA. If you have it already I would just try and keep a bit more space in case you need a bigger one.

2. Th2 is between AC in and transformer primary (inrush current limiter). TH1 is between chassis and mains earth.

3.For a led you want apr. 10-20mA flowing through. If you connect it between plus and minus (50V) this would mean 50/0,01=5k

William

[Alain Dupont]

sherwood,

Each channel draws 100 watts
so for 2 channels that's 200 watts continu power drawn of the transfo.
You got a little reserve with 300VA, and in the case of a class A it's just enough... The rules are 2 to 3 times the power to be comfortable...
see spreadsheet about Aleph-X

Go for it. It will work fine.

The connection of the thermistor is better seen on the board, because the
Aleph 3 can be wired in 120 or 240 AC
The 2 primaries goes in parallel or serie. In your case France, the primaries are "in serie", and the thermistor is connected right to the second 115 pin.
A strap connects the 0 A to the 115 of the other bobin...

You get about -+25 DC volts or 25 volts between + and ground
for 10 mA of a blue Led that is 2.500 Ohms in serie with the led
minimal value ; and with 4.7 Kohms it will work at 5.3 mA good too...
but maximal value. {could use something in bettween}

Happy building.

Alain.

[Blues]

Two 100W channels on one 300VA transformer will definitely work but might buzz and hum...vibrating windings at 50/60Hz. As what Nelson experienced with two ch of Zenv4 (much like Aleph 3)on one 300VA xformer. You'll be better off with at least one 500VA or get another 300VA for the second channel...as transformer hums are out of synch/step with most music

[sherwood]

Thanks very much William, Alain and Blues: it really helps me. I will go for a 600VA Toroid.

I have one more question concerning the choice of MosFets:


I have found IRFP244 (International Rectifier)on Digikey and IRFP244b (Fairchild) on Mouser. Both seem to be of the same type but I haven't checked point to point their specs. It would be usefull if somebody can make comments on their performance. Which brand would be better to use for my project?

I'm planning to buy 50 to 100 pieces. Too many? How many do you usually buy to have a well-matched set of 8 pieces?


Any suggestion is welcome.


Rodolphe

[wuffwaff]

Hi,

50 is a lot. You only need 4 pairs of two matched fets. This should be easy when you buy 25 pieces.

William

[jacco vermeulen]

Originally posted by wuffwaff
Hi,

50 is a lot. You only need 4 pairs of two matched fets. This should be easy when you buy 25 pieces.

William

Speaking from experiece or do you know of any statistics ?
Rephrased : suppose eight matched pairs are needed, how big would the lot need to be ?

Een haan is geen kip maar als ie uit de oven komt zie ik geen verschil !

[wuffwaff]

Jacco,

speaking out of experience.
If you only need four sets of two you could probably get away with buying 16 fets. I matched IRFP240, IRF510, IRF610, 2SK170 etc. If you buy them at the same time they don´t really differ that much. I got 6 sets of three out of 25 IRFP240 . 8 sets would also have been possible when matching to 0,2V instead of 0,1.

William

[Blues]

with Digi-key, just let them know you want mosfets with the same lot code...matching will be easier this way and less trial and error.

[Nelson Pass]

Originally posted by jacco vermeulen



Speaking from experiece or do you know of any statistics ?
Rephrased : suppose eight matched pairs are needed, how big would the lot need to be ?

Een haan is geen kip maar als ie uit de oven komt zie ik geen verschil !

If you want 4 pair (8 pieces), buy 12 pieces, and don't worry too
much about the matching - just do the best you can.