aleph 2 help

[slr 5000]

hello to everyone

i have just finnished some kk pcb aleph 2 and have read about leaving r19 out of the circuit to raise the bias

in an aleph 2 this is the 1k is this correct ? and the 56k2 r 17 still gets installed ?

how crucial is the 56k2 value can i use 56k instead without hurting my amplifier

cheers from jason

 

[MEGA_amp]

R19 is referencing Pass Labs schematic, It's the 56.2K resistor. It trims DC bias. I just leave it out altogether, bias will generally increase by 500-600mA overall.

 

[Nelson Pass]

Yes, it's used for cases where the circuit does not have
quite as much heat sinking as you might want. You
can trim the bias down arbitrarily without screwing around
the ratios of other values.

 

[slr 5000]

hello fellow diy,ers

thankyou for you advice mr pass and mega

i have left r19 out and the mono blocks are running great so far
both are around .18 mv offset and the bias is .6v across the source resistors

one side of one mono block is .5v though for some reason ,how might i correct this ?
the output devices are well matched by tech diy store and the irf920 are also matched triplets so what could cause this

other than the small bias difference on one mono block they are playing great and sound magnificent
they are built as aleph 2 but run on aleph 5 voltage they run between 55 to 63 c temp depending on outside temp and are dead silent when switched on

cheers jason

 

[Nelson Pass]

You can try trimming the resistor value coming off the Drain
of the input transistor to the - rail for that much offset.

 

[slr 5000]

hello nelson

i am very greatfull to you for being able to build this amp ,the more i am listening to it the more it is impressing me

any hints on how much to trim that ressistor ?
a starting value perhaps

cheers jason

 

[Nelson Pass]

Try a larger a larger value. If the original is 392 and gives
you +.5V, then try 475 ohms.

 

[slr 5000]

thankyou mr pass
i will give it a try

cheers jason

 

[gianc]

Same question for Aleph 5

I would like to try high bias on my Aleph 5, currently I have 221K for c19 and heat sinks are not that hot. When you say to leave out r19 does that mean I can snip one side of r19 and it open, or are you supposed to jumper r19 as I have read in another post (Top ten ways to a better Aleph).

 

[MEGA_amp]

leave it open

 

[gianc]

Thanks,

I took out resister r19, getting .64 across 1 ohm resistors. Heat sinks temp seems to have stabilized at 114 F (46 C).

 

[lifefordota]

do you trim down r 21? r21 is actually increase the current gain, therefore a better low impedance output when bias is increase?

 

[2 picoDumbs]

Changing AC current gain will have a greater effect on the sound signature of the amp.
To keep the amp close to stock it would be better to just adjust the bias point.

If keeping it close to stock is not important to you, then tweak away.

Increasing current gain will improve damping factor, but the damping factor is already good.

The only reason to tweak it would be to match the current requirements of your speaker load, assuming it was difficult to drive.

 

[lifefordota]

I wonder is there any optimum r21 value to maximise its ability .

Or in other word, is it good to always maximise the AC gain?

 

[lifefordota]

I wonder is there any optimum r21 value to maximise its ability .

Or in other word, is it good to always maximise the AC gain?

 

[2 picoDumbs]

From a technical specification stand point, more AC gain is better.
From a sonic stand point, less AC gain is probably better.

Basically you want the highest bias point your heatsinks can handle then only add a small amount of AC current gain to cover your speaker load etc. Keep the AC current gain in the sweet spot.