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Old 5th August 2012, 06:37 PM   #111
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Quote:
Originally Posted by jacco vermeulen View Post
As is Rds(on), relevant for switch mode stuff, but is of no consequence for conventional audio amps.
This is what the author says in the article - Rds(on) increases as the device gets hotter.
But this is completely useless in a linear circuit.

He says its the lowering of Vgs(on) that's important for an audio amp.
This is what causes thermal instability.

and this is why the F5 has a thermistor in parallel with Vgs.
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Old 5th August 2012, 08:06 PM   #112
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Quote:
Originally Posted by Pointed View Post
In our location, it's hard and expensive to find matching numerous IRF240 MOSFET's,
so that why I take my focus on your idea. I got IXTN15N100 and PM50502C from industrial PSU board.
Can you help me prototype an amp with those mosfets.

Big thanks
Pointed,

Recall you have access to n channel devices only - so one option is building a Class A Aleph style amp.

But for a Class A amp, having large heat sinks is critical.

Minimum they will need to be 4 (h) x 8 (l) x 2 " (t) 100 x 200 x 50 mm

Can you get a hold of heats sinks like this ?
.
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Old 6th August 2012, 02:25 AM   #113
Pointed is offline Pointed  Viet Nam
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Quote:
Originally Posted by Uunderhill View Post
Pointed,

Recall you have access to n channel devices only - so one option is building a Class A Aleph style amp.

But for a Class A amp, having large heat sinks is critical.

Minimum they will need to be 4 (h) x 8 (l) x 2 " (t) 100 x 200 x 50 mm

Can you get a hold of heats sinks like this ?
.
Yes, I have 4 nos, 1860cm2 each heat sink block; I intend to build Class A monoblock unit like this
Attached Images
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File Type: jpg IMG-20120801-00011.jpg (83.0 KB, 165 views)

Last edited by Pointed; 6th August 2012 at 02:33 AM.
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Old 6th August 2012, 02:58 PM   #114
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Quote:
Originally Posted by Pointed View Post
Yes, I have 4 nos, 1860cm2 each heat sink block; I intend to build Class A monoblock unit like this
Pointed,

The Heat Sinks look Great - I have a correction to make - between your 2 MOSFET options.


IXYS IXTN 15N100 and Hitachi PM 50502C

Vgs(th) ...... 2 to 4 v .............. 2 to 4 v

I cont@25 .. 15A .................. 50 A

gm (typ) ... 28 A/v .............. 40 A/v

Qg (max) ... 280 nC .............. not listed

Ciss ........... 8nF ................ 10.3 nF


I would suggest the 15N100, since Qg will be lower.

Last edited by Uunderhill; 6th August 2012 at 03:02 PM.
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Old 6th August 2012, 06:06 PM   #115
gl is offline gl  United States
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Hi Uunderhill,

Thank you for the heads up. I am a big fan of Rods and indeed I had read that article already.

His comments are generally about class AB operation with vertical mosfets. Class A mitigates much of this - which is a big reason Mr. Pass uses it.

Graeme
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Old 6th August 2012, 10:46 PM   #116
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Quote:
Originally Posted by Pointed View Post
I intend to build Class A monoblock unit like this
Pointed,

Here are a few articles that will help you

1) DIY Op Amps

2) Using HEXFETs in High Fidelity Audio

3) A person in Sweden used "Bricks" for the output stage of an Aleph.
Aleph 5, start up problem.

4) Solid State Power Amplifier Supply Part 1
Except the article has 3 serious omissions
- the author should have included discussions on simple soft start ideas to control inrush current.
- a discussion between case earth and circuit ground is missing.
- the idea of star grounding is missing.

I have not heard an Aleph using IRF 9610's for the differential pair.

Later Aleph's used jFET's in the differential pair - which are known to sound softer than MOSFET's,
and jFET's also have a very good noise figure.


However, a single jFET does not have the current to drive a "Brick" off the differential pair - so I think you have 3 options

1) Stay with the Aleph that uses IRF 9610's - the schematic is ready to go.

2) Use 3 jFET's in parallel for the differential pair - bias current will need to tripled.
I haven't prototyped this.

3) Add "Bricks" to the Op Amp described in Mr. Pass' article.
Use jFET's for the differential pair, a Bi Polar for the driver stage and then the Bricks.

.

Last edited by Uunderhill; 6th August 2012 at 10:49 PM.
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Old 8th August 2012, 06:22 PM   #117
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Here is the schematic of my modded F5.
Its very thermally stable and reaches full bias in about 15 minutes.

However, the high value for Zout is holding this amp back.

Mr. Pass's equations, to calculate Zout, indicate the options to lower Zout are

- decrease gm of the output MOSFET's
- increase F.B. - which will make the amp more thermally unstable
- decrease the 0R5 source resistors - which will make the amp more thermally unstable

Quote:
Originally Posted by gl View Post
You could also parallel more output transistors.
As GL has alluded to, it seems the typical method to lower Zout is to parallel a bunch of MOSFET's in the output stage.
But that means matching MOSFET's - I'm not thrilled about this idea.

I need a Free Lunch here to lower Zout - any suggestions ?


Thermal run away occurs because Vgs(th) lowers with an increase in Temperature.
Vgs(th) lowers about 1.5mV per oC.

Other than a thermistor, is there a way to thermally stabilize Vgs ?
.
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File Type: jpg img_0798_2.jpg (209.0 KB, 104 views)
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Old 8th August 2012, 06:27 PM   #118
flg is offline flg  United States
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Choosing the right thermistor, or a network to correct it's suitability is important for that theory to be sucsessfull.
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