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Old 23rd July 2007, 01:55 AM   #1
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Default Aleph 3 Bible

Hello all,

After successfully constructing my JLH amp, I have decided to go on to a more ambitious project - an Aleph 3. I have a few quick questions if anyone has the time. I'm sure these have all been answered before, but not in the one place. Therefore I've called it the Aleph 3 bible for future reference, as pretty much every question that can be asked is below:

1) Resistors. As far as I can tell, all the resistors (except the 5W 0.47R power resistors) can be 0.5W metal film. Have I made an oversight anywhere here?

2) Capacitors. The schematic in the service manual does not show any bypassing of the feedback (etc) electrolytics. Should this be done? Why did Mr Pass not do it if so?

3) Heatsinking. What kind of heatsinking do the small input mosfets need, if any?

4) Power transformer. I have a 300VA, 2x25v, 6A transformer in my collection. Is this sufficient (for two channels), or should I really fork out the extra $$ for a 500VA one? I know there's no such thing as too big a transformer, but realistically is it worth the extra $$ considering the RMS power consumption is about 250W for 2 channels?

5) What is that LED in the schematic for? What kind does it need to be? Please tell me it's not just a power indicator

6) I have pretty much any fet available to me except the damn IRF244s used in the original. Some that spring to mind are IRFP044, IRFP044NPBF, which were used in ZV4, and also IRFP048, IRFP054 and IRFP240s to name a few. Of these, I assume the IRFP044NPBFs are the most preferred?? These have a far lower RDS than the straight IRFP044s. Any problems here? Will the circuit need any modifications? What is the absolute best substitute (in your opinion Mr Pass)? I seem to be able to get any fet from RS, except the IRF244!

7) Zener diodes. Will it be ok to use 1W for all of these?


Your advice is appreciated. Many thanks.

Greg.
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Old 23rd July 2007, 03:49 AM   #2
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Default Re: Aleph 3 Bible

Quote:
Originally posted by gregjpeters
1) Resistors. As far as I can tell, all the resistors (except the 5W 0.47R power resistors) can be 0.5W metal film.

2) Capacitors. The schematic in the service manual does not show any bypassing of the feedback (etc) electrolytics. Should this be done?

3) Heatsinking. What kind of heatsinking do the small input mosfets need, if any?

4) Power transformer. I have a 300VA, 2x25v, 6A transformer in my collection. Is this sufficient (for two channels), or should I really fork out the extra $$ for a 500VA one? I know there's no such thing as too big a transformer, but realistically is it worth the extra $$ considering the RMS power consumption is about 250W for 2 channels?

5) What is that LED in the schematic for? What kind does it need to be? Please tell me it's not just a power indicator

6) I have pretty much any fet available to me except the damn IRF244s used in the original. Some that spring to mind are IRFP044, IRFP044NPBF, which were used in ZV4, and also IRFP048, IRFP054 and IRFP240s to name a few. Of these, I assume the IRFP044NPBFs are the most preferred?? These have a far lower RDS than the straight IRFP044s. Any problems here? Will the circuit need any modifications? What is the absolute best substitute (in your opinion Mr Pass)? I seem to be able to get any fet from RS, except the IRF244!

7) Zener diodes. Will it be ok to use 1W for all of these?
All good questions.

1) Yes, no big deal

2) Bypassing is optional and will not hurt.

3) Usually none, but it does help to put the input devices in
thermal contact with each other to reduce drift, and one way is
for them to share a small sink.

4) The 250 VA will work, but it will be working at rating, given the
typical 80% power factor of such a circuit. Bigger VA transformers
will run cooler and make less mechanical noise.

5) You asked me not to tell you...

6) I'm sure they will all work. There are subtle differences, but
nothing to get excited about.

7) Yes 1W is OK

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Old 23rd July 2007, 11:05 PM   #3
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Default Thanks Nelson.

Most useful.
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Old 24th July 2007, 12:41 AM   #4
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Greg-

One downfall of the Alephs is that they can run incredibly silent with the right preamp. Any mechanical humming from the trafo will likely be audible at your listening position if the trafo is too small. I just re-built a 'mini' with a 225VA transformer drawing a total of four amps. I can hear it. It will soon be replaced with a 400VA transformer.
IMHO--spend the few extra bucks for a much bigger transformer, and you will only have the buy it once.

JJ
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Old 8th August 2007, 02:23 AM   #5
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Default FETs and mounting

All,

Just wondering if the IRF9610s are still regarded as the best for the input fets, or is there a better modern alternative?

Also, I've decided to mount my power FETs separately to the PCB using some wires that I expect to be shorter than 12" long, but probably around 6". Is this likely to cause any problems like oscillation, etc?

Yourt advice is appreciated.

Cheers,

Greg.
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Old 8th August 2007, 02:49 AM   #6
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Default Re: FETs and mounting

Quote:
Originally posted by gregjpeters
All,

Just wondering if the IRF9610s are still regarded as the best for the input fets, or is there a better modern alternative?

Also, I've decided to mount my power FETs separately to the PCB using some wires that I expect to be shorter than 12" long, but probably around 6". Is this likely to cause any problems like oscillation, etc?

Yourt advice is appreciated.

Cheers,

Greg.
9610 is fine. If your looking to feel better Nelson says the Fairchilds might be a bit better, you can dig a bit for that info. Other than that there are a couple of jfet variations kicking around - like the Babelfish.

I'd keep the wires as short as possible and mount the source and gate resistors at the fets,

I can't help with the yourt advice, I don't know what that is....
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Old 8th August 2007, 03:44 AM   #7
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Default Re: Re: FETs and mounting

Quote:
Originally posted by mpmarino
9610 is fine. If your looking to feel better Nelson says the Fairchilds might be a bit better, you can dig a bit for that info. Other than that there are a couple of jfet variations kicking around - like the Babelfish.
Any IR P channel part not made by IR will give better performance,
as it is a "common source" usage. Parts by Fairchild, Harris,
Samsung, etc will work fine - IRFP9510 Through 9620 are all
good candidates as long as they have similar specs.

They are best matched to <10 mV for Vgs at Ids = 10ma.

Also, I have used JFETs 2SJ74 / 2SJ109 (dual) very successfully
in the First Watt version the Aleph J. This is my favorite, although
it does not have the lowest measured distortion. It does, however
have the very lowest feedback factor for the performance you
get.

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Old 8th August 2007, 04:47 AM   #8
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Default Re: Re: FETs and mounting

Quote:
Originally posted by mpmarino

I'd keep the wires as short as possible and mount the source and gate resistors at the fets

Thanks mpmarino. I can appreciate the need to keep the gate resistors near the FETs, but is it _really_ necessary to do it with the drain resistors also? I have a feeling you're going to say yes...so now I'm off to redesign the PCB slightly....bugger!
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Old 8th August 2007, 11:06 AM   #9
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This was advice I'd seen time and time again from those more knowledgeable than I. I was just passing it along
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Old 8th August 2007, 11:02 PM   #10
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Default IRFP044NPBF

Ok, no worries. Thanks.

I'm hoping to use the IRFP044NPBF, but its PD is only 120W. Is that likely to be a problem? I notice the 244s originally used were only 125W, so I expect not? All the other substitutes seem to have 160-180W PDs. What do you suggest?

Cheers,

Greg.
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