| DJG |
Hello!
This is my first post on diyaudio and I am looking for a Pure class A amplifier with Mosfet. I have searched in slovenia, but no luck.
This amp should be about 100-140W / 8Ohm
And it has to be with mosfets!
If any of you has an schematic or PCB, could it be posted here? Please?
thx,
DJG |
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| Nelson Pass |
Try an Aleph 2, documented at www.passlabs.com
and elswhere on this forum. |
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| skaara |
Jao kje si mona:P
Check the passlabs section of this forum..
btw,
U should use the search function for this question:DD |
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| alaskanaudio |
For something challenging and somewhat complex, but with excellent sound quality look at the link below. It is has a fully class “A” output stage and is also a good foot warmer.
I designed this back in back in 1992 or so. My newer versions of this use a similar circuit arrangement. This project is perhaps a lot more complex than most DIY’er care for.
http://www.audioamps.com/diyproject...mps/1r1/1r1.htm
John Fassotte
Alaskan Audio |
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| DJG |
Now, that is a complexed amp!
Those Aleph are really good, I shall choose eno of those - if anyone has any more schematic of A class Mosfet amplifier... Pretty please!
thx,
DJG |
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| DJG |
Hello!
I have a question about these Aleph's.
How do I wire the inpit signal, because there are XLR +/- and RCA U.
This might be a dummy question, but what do they mean?
I just want to give in a normal signal, over chinch connectors.
thx,
DJG |
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| dshortt9 |
| The RCA in is the regular everyday connector. Not sure what you mean by chinch connector. |
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| annex666 |
| quote: | Originally posted by DJG
Mosfet's rule in any respect |
Power - yes,
easy of use - yes
linearity - no
That's what most purists will say including the much respected Rod Elliot. |
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| DJG |
| quote: | Originally posted by annex666
Power - yes,
easy of use - yes
linearity - no
That's what most purists will say including the much respected Rod Elliot. |
What do you mean - not linear?
Aren't Mosfet's the most linear elements?!
So, RCA is that connector, which I should connect with the signal(250mV)?
What about XLR?
thx,
DJG |
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| annex666 |
I will try to explain how MOSFETs work - this will give you an idea of why they are (inherently) not incredibly linear devices.
MOSFET is an acronym for Metal Oxide Semi-conductor Field Effect Transistor. (They are usually created from aluminiun and aluminiun oxide I believe)
A MOSFET relies on the field created at the gate to limit the amount of electrons that pass from source to drain - it is literally a field dependant resistor (note that it is a field created within the device). As field strength is proportional to the square of the distance from the source as the potential difference at the gate is changed in a linear fashion the field will not change with perfect linearity and therefore the device is not inherantly linear.
I hope this has cleared things up?
This is my understanding from a physics point of view, if anyone knows better why these devices are not linear I would be happy to listen to your idea. |
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| DJG |
Thx, annex666, for the post.
What about RCA and XLR?
thx,
DJG |
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| annex666 |
The inputs are designed for balanced (idealy) signals, but can be used for unbalanced signal as well.
The input stage of the amp should be a differential stage (as this is the way to reject commond mode noise on the signal line) - in this case to use your RCA connectors simply connect the signal line to the + (i.e. no-inverting input) and tie the return to the ground and - input (inverting input).
I've not seen the schematic, but this should work. |
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| annex666 |
...also while I'm thinking about it as this amp has a balanced input why not make a bablanced line driver to run it?
This will give much better noise rejection from unwanted signals on the cable and is quite simple - read this;
http://sound.westhost.com/project51.htm
for more information. The line driver only requires power for the op-amps and you can steal this from the power supply in your source equipment (CD player etc). Also ESP is a great source of all kinds of hifi electronics information - I highly recommend it!
Hope this helps? |
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| jgwinner |
If a MOSFET isn't linear, a BJT is?
(I'm actually wondering, not trolling, although it initially came over that way). |
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| paulb |
| quote: | Originally posted by jgwinner
If a MOSFET isn't linear, a BJT is?
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Nope. There's no linear, man, just different degrees of nonlinear.
:cool: Just like saying Class A amps are more linear than Class B. Just like saying that any continuous function is approximately linear if you restrict its input range enough. |
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| DJG |
Thank you, Anex666.
Your post made things clear!
You can see the schematic on the first link on this post.
thx,
DJG |
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| alaskanaudio |
dshortt9 the link you provided above has a very nice presentation of a good number of Aleph project diagrams.
I'm sure that all Aleph fans greatly appreciate seeing these diagrams in such nice format. The person who did this work deserves a pat on the back. Great job!
John Fassotte
Alaskan Audio |
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| jgwinner |
| quote: | | Nope. There's no linear, man, just different degrees of nonlinear. |
Exactly! It was as if this was leaning to 'MOSFET's don't make good amps because they aren't linear'. Then we should chuck all our amps :D |
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| skaara |
| its more linear than a mosfet..? |
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| Narcisse91 |
| quote: | Originally posted by skaara
its more linear than a mosfet..? |
It's non-linear in a different way. IME, the biggest non-linearity problem with BJTs is how they act around 0V on the base. The slew rate on the output voltage doesn't always match the input voltage right around that point (similar to how the output of a diode is when you ramp the voltage from 0V to 10V). The biggest issue (again, IME) with FETs (and tubes) is transconductance, where the output voltage may follow the input voltage well, but the output current is changing at a non-constant rate.
Those may not be either components biggest problem, but that's what I don't like about each, based on my experiences with them. |
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| mefinnis |
If I may be so presumptuous as to quote the man himself (NP) ...... "everyone wants to suggest better capacitors, better resistors, or better wire ...... no-one finds me a better output device". (or words something like that!)
cheers, mark |
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| skaara |
| Nelson is doing magic with mosfets:)) |
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| Nelson Pass |
I don't like to get into arguments about Mosfets vs Bipolar
because they both have their strengths and weaknesses.
Having said that, it is my experience that people arguing
the superior linearity of BJT's over Mosfets are using cases
where either the bias or the source impedance is low. For
example, the typical big Bipolar device has about 1/3 the
distortion of a Mosfet as a follower - if the current isn't
very high and if the source impedance is just a few ohms.
As the source impedance and the current rise, the Mosfet
will equal and ultimately surpass the Bipolar.
The argument is a red herring anyway, since we have already
decided that we don't base everything on THD numbers
anyway.
:cool: |
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| Narcisse91 |
| And they're all inferior to tubes. |
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| HBarske |
| And the earth is a disc... |
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| Nelson Pass |
| and a vinyl one at that. |
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| Narcisse91 |
| quote: | Originally posted by HBarske
And the earth is a disc... |
I forgot this was the solid state forum :) |
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| topicreader |
Hi!
Are you sure that you want to build really 100W Class A amplifier?
The 100W A-class amp. dissipate heat more than 200W. Of course its mean only one channel. You will need huge dimensions heatsinks too.
If you knew these information than I please excuse.
Bye |
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| mefinnis |
Well, this is really a little conservative. If you use one of NP's designs you will gain some efficiency due to the active current source, but even then you are at 300W / channel for 100W output. So, now you need 1.5 kVA in transformer and 0.05 C/W in heatsinks.
"Standard" SE Class A will probably be around 20% efficiency, so you will be up to 500W/ch ..... ouch!
Does anyone have any push-pull schematics at 100W .... this might be the only chance ;)
And here comes the "broken record" bit ...... exactly why do you need 100W ??
cheers, mark
PS: If price, heat and space are not a concern ...... build Aleph2 monoblocks !! |
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| alaskanaudio |
I propose that we close this thread since we are drifting away from its orginal intent of providing circuit diagrams for potential use of the original requester.
I suspect that this need has been met.
John Fassotte
Alaskan Audio |
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| Aud_Mot |
Hope it is not too late to suggest:
http://www.borbelyaudio.com/
Erno Borbely is some one who has made many contributions to Audio DIY.
Have a look.
Aud_Mot |
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| DJG |
So, because this is DIYaudio I am asking all of you - Does anybody have PCB for Aleph 5(in jpeg, gif or tiff format) that would send it to me for FREE?
thx,
DJG |
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| promitheus |
They are better looking.
Both are very good.
I ordered my boards so I used the pdf file from Mark Finnis. |
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| mefinnis |
I have provided positive images which I used for home PCB manufacture. No question the PCBs I made do not compare to professional grade milled / laser-drilled / silk-screened PCBs.
If the later is what you want then the above website is really very good indeed. My only thought about these PCBs is the close proximity of the FETs and heat dissipation. Other than that they look absolutely fantastic.
One small advantage of DIY, and this sort of community, is the willingness of people to help each other. So, if someone piped-up and said, "hey, we have a supply of great heatsinks, but the PCB needs the devices to be at 'x' mm spacing" ..... then you would likely find an appropriate TIFF appear in the not too distant future!
cheers, mark |
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| skaara |
| DJG, you must pay for pcbs.. |
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