Aleph 5 need help

[DFroehlich]

Hello
I'm in the process of building an aleph 5 and have some questions:
I have to run them with a less bias current, because I have only 2 Heatsinks with 0,35 K per watt, so I think the most dissipated power would be something around 90 W per channel. I have two 300 VA Transformer with 2 x 25V primaries. I want to run the amps with 1,3 A current.
After some ersearch in the threads, I think there are two options of getting less quietcent current: replacing R19 with a pot and adjust the current to 1,3 Ampere total or replacing the 1 Ohm resistors with another value.
Now to my question: Is it possible to run the amp with only 8 instead of twelve output transistors and run them with 1,3 A total and leaving R 19 and the 1 Ohm resistors as they are ?

Excuse for my bad englisch, hope you get the importent things

[wuffwaff]

Hi,

a few answers

for 90 watts dissipation you can use 4 instead of 6 Fets per channel (22.5 W/Fet).

You can leave the 1 ohm source resistors in but you will have to adjust R19 (it is always better to set bias with a pot because otherwise both channels will be different because of the tolerances)

It would be better to use a lower power supply voltage cause 1.3A of bias will give only 27 watts into 8 Ohms. Here a supply voltage of around 22 volts is enough (using 18V transformer)

You wont get much power into lower impedances. 4 Ohm power at 50% ac-current-gain is only 13 watts!

If you would use a 22V supply instead of a 33V supply you could up the bias from 1.3 to 1.95A for better power into lower impedances. You could also try to get the optimum for your speakers by setting power supply voltage/bias so to get max. power at you speakers impedance.

William

[DFroehlich]

Hi,
thank you William for the quik reply. That's what I tought, but I wasn't sure.
I tought The aleph' s have a ac-current gain of 63 %, don't they.

I have the transformers laying around and want to use them (because of no money for new ones) I think I can live with the less power and my speakers are in the 6-8 Ohm range ( accuton keramik bass-midrange and ribbon tweeter with an active subwoofer - diy of course).
Is it possible to let R 19 in the circuit and change to a pot later on if the bias is that different in the both channel ? My question is : Is it possible to let everything like in the production aleph 5 except to let 2 output transistors out and have only 1,33 A bias instead of 2 A ?
May be the question is a little bit stupid, but this becomes my first aleph, and I don't want to blow it .
Dirk

[wuffwaff]

Hi,

yes it is possible although it really is neccesary to use a pot for R19 to set the bias. My Aleph 5 had 2,6A bias with the original resistors. It is also neccesary to change r21 because the ac-current gain will be incorrect using two fets and a different bias.
This means you´ll have to buy 4 pots (2x 1000R for R21, 2 x 100k for R19) . Set R19 to 20k for starters and R21 to 500 Ohms.
This way your bias will probably be a bit low but that´s better as too high. Ac-current-gain can be set as described here many times. You can go as high as 60-65% but listen for the changes in sound!

William

[DFroehlich]

Hi,
thank you William for the very helpfull replies.
So I will install the 2 pot' s per channel. Here my calculations:
When I run the aleph with 1,3 A current instead of 2 A, then I have 2/3 of the production one, so a decrease of 1/3. it seems to me, that I have to increase R21 with the same 1/3 to get the same ac current gain like the production A5. So instead of 453 R it will end in the range of 600 R.
Am I right, or do I understand it just wrong ?
Maybe it is better to try things out, but I only want to know in wich range the R21 will end up, so I can trimm it from this point to the best sounding value.

[wuffwaff]

Hi,

yes it will end up a bit higher as before. A 2k pot (10-turns) will do nicely.

William

[DFroehlich]

Yesterday night, i put together one chanel of my aleph.
It now have 4 output fet's and pots for R19 and R21.
The big problem is, that R11 (Aleph 5 schematic) smokes when I turn the amp on with R19 at lowest setting (it's a 200 k poti).
The voltage across R11 is around 30 V and that's to much (should be 5 V).
in addition (or may be this is the reason for the wrong measurements on R11) Q2 gets very very (untouchable) hot

Is anyone there who had similiar problems and know how to fix it?

My idea is to see if the second channel have the same problem, but may be there are some aleph experts who had the same problems.
I use boards from brian gt (the old ones,with seperate output boards)

Thanks Dirk

[rchua77]

Hi Dirk,

From my limited knowledge, I'm sure there are better people around more well versed than me.

R19 should have no effect on R11, I'm using the Aleph schematics from the Pass manual as a reference. What I hazard guess is that the zener is non functional as you are getting the supply voltage across R11. Alternatively, there could be a solder jump around that region.

I hope this helps as the limited knowledge I can give.

[jh6you]

Or, most probably, the zener, Z5, is attached upside down. . .

I believe you have good soldering skil. . .

Regrads
jH

[DFroehlich]

rchua77 and jh6you,
thanks for the quick reply.
I' ve cheked the whole board double and tripple, the zeners are all located correct So maybe Z5 isn't working.
I have to make some more measurements tonight. I hope there is nothing wrong with the fet's.