7 amp aleph

[MEGA-amp]

Does the Aleph have a limit to its bias current where stability becomes an issue?

The plan:

-/+30v rails
24 fets
7 amps of course (only because i'm limited to 450W dissipation with my monoblocks)



-john

[Nelson Pass]

Now there's an interesting question.

450 watt dissipation means maybe 10 to 20 devices, or 5 to
10 devices in parallel off each rail.

If the 450 watts is with a 25 or 30 deg temperature rise, then
we can talk as few as 10 total, although 16 is more like it.

You can build very large Alephs with stability, since the dominant
pole is the capacitance of the devices themselves, but as the
number of parallel devices piles up, the Cgd starts getting out
of hand, and the distortion at high frequencies gets worse.

I recommend maximal sinking, minimal devices, and maximal
current through the input diff pair and the npn transistor used to
control the current source. If you are using the original Mosfet
input circuits, that means 20 mA per input device to drive the
capacitance.

All very do-able.

[jacco vermeulen]

Interesting for moderate efficient electrostats.
Provided the 900w diss. is manageable and worth it, regular passive cooling becomes a hurdle and a burden beyond 500-600w imno.

[MEGA-amp]

Thank you for the input Nelson. Based off the Aleph Power and Dissipation cheat sheet attached I was able to dissipate close to 500w with each block without concern, between 400-450 would be a little more safe, not sure if that sheet is 100% accurate though. Right now I have an Aleph with 4 outputs and 0.16R source resistors running at +/-20v rails which comes out to 7.3 amps, and it sounds great. I can only listen to it for an hour though because the coils in the supply start to smell and it gives me a headache; with a low speed window fan I can listen as long as I want, but its noisy. Probably need Solen's 10ga or 12ga coils to make it work reliably and of course a few more fets.

Another question: (based off the attached sheet) How am I getting 148C junction temperatures with a fet thats dissipating 21watts?


-john

edit-edit: I think I can figure out the input diff pair current. By removing R19 (ref Aleph2), is this how we achieve max current for the npn controlling the current source?

[Stefanoo]

I take the opportunity to ask for few advice:

I posted this project long time ago...and obviously it will take time since i'm gathering experience on amps by doing other things and first i'll make the Aleph p1.7

This project is pleasently abicious (aleph AX).

Since Nelson mentioned that the Cgate could be an issue,and rises up by paralleling output devices causing therefore bad high frequency responce.

I am actually planning on buildin g an AX version with 8 devices per quadrant (32 devices per channel).
Bias set up to 1.3A per device; dissipation on each IRF 38W.
Tot dissipation 1200W per monoblock (600W per side)
By using a 0.05deg/W the temperature should rises up 30deg.

Current set up on the front end by the CCS is 27mA (so 13.5mA per each 9610 in the front end)

RMS power

100W@8ohm
200W@4ohm
400W@2ohm


First question:

- Nelson mentioned that the current on the input 9610s should be as high as possible....but...does this current fix the output DC offset? does it? So it cannot be arbitrary and not any value can be chosen, correct?

- Assuming that i can handle the heat with this heatinks, aren't the output devices gonna be to many that they will cause a bad high frequency response?

- Is there any general advice on the project that you guys can give me?

any hints/tip is very welcome at this point.

[kropf]

Originally posted by Stefanoo
First question:

- Nelson mentioned that the current on the input 9610s should be as high as possible....but...does this current fix the output DC offset? does it? So it cannot be arbitrary and not any value can be chosen, correct?

Choose whatever current you like, then change (lower) the drain resistors on the differential pair so that the current multiplied by the resistance is about 4.5V.

Jeremy

[MEGA-amp]

Hey Stefano,

These are the outputs I am planning to use, they do offer a Ciss of 830. Which should allow you to parallel many fets with no problems. (correct me if i'm wrong)


FQA19N20C

[Nelson Pass]

Originally posted by MEGA-amp
Another question: (based off the attached sheet) How am I getting 148C junction temperatures with a fet thats dissipating 21watts?

I think I can figure out the input diff pair current. By removing R19 (ref Aleph2), is this how we achieve max current for the npn controlling the current source?

The semiconductor is going to experience something like 1 to 2
deg C per watt, so you would get a junction temp like that if
your heat sink is over 100 deg, which seems to be the case on
your sheet.

Regarding the npn current, you are looking to reduce the
resistor values all around that part of the circuit to get the
current (and thus the speed) up.

[Nelson Pass]

Originally posted by Stefanoo
Nelson mentioned that the current on the input 9610s should be as high as possible....but...does this current fix the output DC offset? does it? So it cannot be arbitrary and not any value can be chosen, correct?

- Assuming that i can handle the heat with this heatinks, aren't the output devices gonna be to many that they will cause a bad high frequency response?

- Is there any general advice on the project that you guys can give me?

Let's remember that by the standards of most of the guys over
in Solid State, the distortion of the Alephs is too much anyway.

The 9610's are power transistors - you can crank them right on
up as long as you heat sink them. As you do, the Drain resistance
on them needs to go down proportionately, and as it does, you
will see the open loop gain decline and the bandwidth increase.

Could be quite good. Remember not to use the IR parts on the
9610's.

[MEGA-amp]

Awesome, thanks for the help Nelson.

Also for Stefano, I plan on using these for the input pair as well, they are a cross reference for the 9610, and as Nelson has stated, the Fairchild P channel devices are slightly superior.

FQP3P20