Hi, I got some Aleph PCBs from BrianGT ages ago, now I am planning the build.
I want to build mini aleph monoblocks using no extra heatsinks. Just by using the cases I have to dissipate the heat. I estimate case to have 2*C/watt thermal resistance per side (4 case sides in total if I build monoblocks).
Is it true mini Aleph only draws about 1A with +/-15v power... 30W of heat per channel max? In that case, amps should reach 50*C max. But if I find they are running too hot, can I simply reduce power supplies to +/-12v instead?
I want to build mini aleph monoblocks using no extra heatsinks. Just by using the cases I have to dissipate the heat. I estimate case to have 2*C/watt thermal resistance per side (4 case sides in total if I build monoblocks).
Is it true mini Aleph only draws about 1A with +/-15v power... 30W of heat per channel max? In that case, amps should reach 50*C max. But if I find they are running too hot, can I simply reduce power supplies to +/-12v instead?
If you're running too hot you can reduce either the rail voltage or the bias or both and that will cure the heat problem. (Or get more or bigger heat sinks.) The thing about reducing the rail voltage is that distortion will rise rapidly; I was pushing it a bit by building the circuit with 15V rails. The problem being that the MOSFETs' Gate capacitances increase below roughly 25V or so. That, in turn, makes the MOSFETs harder to drive, particularly at high frequencies. As a direct consequence, distortion increases.
Is it the end of the world? Of course not. It appears that a lot of these amps (and variations thereof) have been built, and people seem to be quite happy with them. It's just a question of being aware of the tradeoffs before you start making changes.
On the other hand, reducing the output bias will also increase distortion a bit. There's no such thing as a free lunch.
Just for the record, the lowest 'optimum' rail voltage for MOSFETs often turns out to be 25V or so. At that point, the Gate capacitance tends to make a dogleg and begins flattening out. Further increases to the rail voltage tend to produce only incremental reductions in Gate capacitance. That's not to say that there aren't other potential benefits, but we're drifting farther and farther away from your original question.
Grey
P.S.: It's no accident that Nelson tends to publish schematics for MOSFET circuits that show rails ca. 20-25V...
Is it the end of the world? Of course not. It appears that a lot of these amps (and variations thereof) have been built, and people seem to be quite happy with them. It's just a question of being aware of the tradeoffs before you start making changes.
On the other hand, reducing the output bias will also increase distortion a bit. There's no such thing as a free lunch.
Just for the record, the lowest 'optimum' rail voltage for MOSFETs often turns out to be 25V or so. At that point, the Gate capacitance tends to make a dogleg and begins flattening out. Further increases to the rail voltage tend to produce only incremental reductions in Gate capacitance. That's not to say that there aren't other potential benefits, but we're drifting farther and farther away from your original question.
Grey
P.S.: It's no accident that Nelson tends to publish schematics for MOSFET circuits that show rails ca. 20-25V...
GRollins said:
Not drifting too far off topic here. I do have a spare 25V transformer that would be good to make use of. Trouble is by the time it's been rectified and filtered it's going to be at about +/-34v. Do I dare try to use this? If I can reduce bias enough will it work well? Bare with me here I'm not even sure how to reduce bias current.
This is the PCBs I have... I would need to change R27 and R28 to adjust bias current is that right?
An externally hosted image should be here but it was not working when we last tested it.
Babowana said:
I have thought that it is because Papa wants to 100% utilise his 18VAC-secondary transformers he still has in stock.
Am I wrong. . .?
Now ask yourself why Nelson bought transformers with those particular voltages...
mr.duck said:
The problem with reducing the current to match your heat sinks with that sort of rail is that you'll be so lightly biased that the amp will leave class A at anything over a zillionth of a watt. Not a pretty picture, particularly with an Aleph. Alephs don't do the class B thing so well.
I'd suggest scrounging up some bigger heat sinks...then you won't have to worry about the heat so much.
Grey
Looking at something like the IRFP240 (Fig. 5), the input capacitance becomes more-or-less flat after you pass the low to mid-20V range. Below that, it's a fairly smooth curve. The FQA19N20C (also, coincidentally, Fig. 5) shows an even clearer flattening at the same voltage range. Curiously, the FQA shows the curve flattening below 1V as well...didn't remember that. Not that running a power MOSFET at 1V is necessarily all that good a game plan.
I'm not saying that you can't get music out and that the music doesn't sound pretty darned good. It's just a question of which compromises you want to make.
On the other hand, you could run a MOSFET at 20A bias at 1V, then cascode for voltage if you were to provide an alternate path for the bias around the cascode device...
Grey
I'm not saying that you can't get music out and that the music doesn't sound pretty darned good. It's just a question of which compromises you want to make.
On the other hand, you could run a MOSFET at 20A bias at 1V, then cascode for voltage if you were to provide an alternate path for the bias around the cascode device...
Grey
You should be fine with that. The 20mA that the front end current source is supplying, is split between two fets. If you want to be perfect, match each at 10mA.
question for all: While running below the optimum 25V, say 10-15V rails, couldn't you just increase diff pair bias to make up for output stage gate capacitance issues?
question for all: While running below the optimum 25V, say 10-15V rails, couldn't you just increase diff pair bias to make up for output stage gate capacitance issues?
Given that 10mA per device is easy to achieve (it's not like you're trying to do whole amps for output devices), I'd say match them at 10mA.
The problem with the capacitance is not just the amount, but the fact that it's non-linear with the voltage across the device...and what fun is a device that can't wiggle with the volts? Music will come out, and depending on your standards you might just be really happy, but it won't be as good as it could be.
Grey
The problem with the capacitance is not just the amount, but the fact that it's non-linear with the voltage across the device...and what fun is a device that can't wiggle with the volts? Music will come out, and depending on your standards you might just be really happy, but it won't be as good as it could be.
Grey
OK. Thanks.
I did matching at 10mA.
Another 2 questions:
- Is there any difference in sound between 1 IRF644 biased at 1A and 2 parallel biased at 0.5A each? I think power dissipation is better with 2 parallel devices. I have one heatsink of 8x25x5cm for each channel.
- Shall I make stabilized power supply? Or a simple C-R-C is enough?
I did matching at 10mA.
Another 2 questions:
- Is there any difference in sound between 1 IRF644 biased at 1A and 2 parallel biased at 0.5A each? I think power dissipation is better with 2 parallel devices. I have one heatsink of 8x25x5cm for each channel.
- Shall I make stabilized power supply? Or a simple C-R-C is enough?
Hi !
I'm building a standard Mini-A using BrianGT's boards (Amp+PSU) with a 2x12V toroid, hoping 15v at the rails. I need about 10W.
I was wondering about some questions :
1° - With 3K3 resistors as R1/R2, a strap is required for R3 and R12. Please confirm.
2° - Could the PSU board be fed with 2 Leds and a single 10K R ? Please confirm.
3° - I'm planning to use 2x(22000µF/10000µF/3K3/10000µF) in this order. I'm right or not ?
Would it be a better solution with 22000µF/22000µF/3K3/10000µF ?
Nice to read from the expert !
I'm building a standard Mini-A using BrianGT's boards (Amp+PSU) with a 2x12V toroid, hoping 15v at the rails. I need about 10W.
I was wondering about some questions :
1° - With 3K3 resistors as R1/R2, a strap is required for R3 and R12. Please confirm.
2° - Could the PSU board be fed with 2 Leds and a single 10K R ? Please confirm.
3° - I'm planning to use 2x(22000µF/10000µF/3K3/10000µF) in this order. I'm right or not ?
Would it be a better solution with 22000µF/22000µF/3K3/10000µF ?
Nice to read from the expert !
korben69 said:
Hi Korben,
R1/R2 and R3-R12 are independent from each other. R1/R2 are the bleeding resistors, the latter are the filter resistors. I would put in some small resistors there, if you don't want to waste too much voltage even 0.22 Ohm will do better then no resistors at all.
You should get Duncan's PSU Designer, then you can see, which capacitor arrangement suits best.
Greetings,
Chris
Sorry for cross posting this but I realised this may be a far better place for answers & opinions, so please bear with me.
I'm gonna make a mini-A using BrianGT's PCBs and I'm planning on using 10,000uF/10,000uF/R33/10,000uF for the psu. I'm gonna make the R33 using three 1R wirewounds in parallel.
1. Are wirewounds OK for this? Will the inductance affect the RC filter time-constant?
For rectifier diodes I wanna use four HFA08TB60's.
2. Should I go for diodes that can handle more current?
I was thinking about using a 225VA toroid.
3. Will 225VA be enough or should I buy 300VA?
4. 2x12V or 2x15V?
I have difficulty in sourcing the 3 ohm resistors (R0) for the amp pcb's. A value of 3 ohm apparently is an American value. In Europe it's either 2R7 or 3R3. How critical is the value of R0?
5. Could 2R7 or 3R3 work too?
6. How 'bout creating ~3 ohm with a 5R6 and a 6R8 in parallel?
Thanks!
I'm gonna make a mini-A using BrianGT's PCBs and I'm planning on using 10,000uF/10,000uF/R33/10,000uF for the psu. I'm gonna make the R33 using three 1R wirewounds in parallel.
1. Are wirewounds OK for this? Will the inductance affect the RC filter time-constant?
For rectifier diodes I wanna use four HFA08TB60's.
2. Should I go for diodes that can handle more current?
I was thinking about using a 225VA toroid.
3. Will 225VA be enough or should I buy 300VA?
4. 2x12V or 2x15V?
I have difficulty in sourcing the 3 ohm resistors (R0) for the amp pcb's. A value of 3 ohm apparently is an American value. In Europe it's either 2R7 or 3R3.
How critical is the value of R0?5. Could 2R7 or 3R3 work too?
6. How 'bout creating ~3 ohm with a 5R6 and a 6R8 in parallel?
Thanks!
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