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Old 28th January 2007, 08:05 AM   #381
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You have to use low Gts transistors. With IRFP240 and IRFP9240 your amplifier will work more in class B than A A/B
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Old 28th January 2007, 10:54 AM   #382
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Hi,
what is a low Gts transistor?
Which parameter do I read?

Why is low Gts necessary?
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Old 28th January 2007, 11:26 AM   #383
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Sorry, wrong shortcut Vgs - (gate threshold voltage).
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Old 28th January 2007, 02:37 PM   #384
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Quote:
Originally posted by veteran
You have to use low Gts transistors. With IRFP240 and IRFP9240 your amplifier will work more in class B than A A/B

Quote:
Originally posted by AndrewT
Hi,
what is a low Gts transistor?
Which parameter do I read?

Why is low Gts necessary?

Low Vgs is not at all necessary.

You just have to use the LM4702 as an opamp (tie the source and sink together) then drive your output stage with something like a BS107(small signal MOSFET) in common drain which will present a very easy load for the 4702. You can then use a VBE multiplier, or diodes + pot, or whatever other means of obtaining your bias voltage/current you like. This approach allows you to almost limitless output device possibilities.

I have been running a couple circuits now with this approach with very good success.

This link describes the output stage I am currently using:
http://www.diyaudio.com/forums/showt...32#post1105232

I just made a PCB with no input stage and drove the circuit with an LM4702. I now have two channels running, and they sound great.

I have found that using the LM4702 in this way seems (to me anyway) to get more out of it. I was not very impressed with either the data sheet application nor the later app note circuit, but that may just be me.

Here is what I will try next which I think will be better than using Darlingtons like SAP16...

Cheers!
Russ
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Old 4th February 2007, 02:49 AM   #385
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Here's a comparison -- I use a Sumo Polaris in the lab -- so I thought I would compare them from 10kHz to 100kHz --

Click the image to open in full size.
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Old 15th April 2007, 07:30 PM   #386
lgreen is offline lgreen  United States
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Default Questions

Hi, working on my LM4702.

Questions-

1. What darlingtons have people in the USA been using? (other than the Sankens, I cannot get my hands on any of those fancy schmancy Sanken SAP16N and SAP16Ps here in the USA, and even if I could, wondering what else would be ok to use).

2. Can the LM4702 drive 2 pairs of darlingtons per channel rather than the single pair I have been seeing on this thread?

3. If I use the Vbe multiplier, will this help provide drive current to the darlington pairs, making it possible to parallel some more output devices?

4. For a BJT amp, what are people using or suggesting for the Vbe multiplier xistor?

- thanks
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Old 15th April 2007, 08:36 PM   #387
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I have used the On Semi and Texas Instruments Darlingtons -- paralleling Darlingtons isn't generally recommended.

The Sanken devices have been obsoleted and are replaced with a "somehwat" similar device -- Perfusion in the UK has the new devices listed. I found that their service was very good when I ordered some of the SAP devices from them about a year ago.
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Old 15th April 2007, 09:00 PM   #388
AndrewT is offline AndrewT  Scotland
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Hi,
use the 4702 to drive discrete darlingtons arranged in dual EF.
You can tack on as many output devices as the driver EFs will allow.
That's the beauty of discrete design.

Integrated darlingtons usually perform worse than discrete. I suggest you forget they even exist.
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Old 16th April 2007, 03:14 AM   #389
dfdye is offline dfdye  United States
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Man, this is making me wish I had more free time to get my output stage finalized. . . . .

Crud.

I guess I will have to wait until the summer after teaching responsibilities end. . . . .
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Old 16th April 2007, 08:40 PM   #390
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Quote:
Originally posted by AndrewT
Hi,
use the 4702 to drive discrete darlingtons arranged in dual EF.
You can tack on as many output devices as the driver EFs will allow.
That's the beauty of discrete design.

Integrated darlingtons usually perform worse than discrete. I suggest you forget they even exist.

Complementary Darlington a la Leach, perhaps:

Click the image to open in full size.

Dr. Leach places diodes for the VBE multiplier on the heatsink -- I may be able to adapt my driver boards to do this.
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