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Old 26th March 2001, 04:41 PM   #1
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Hi

This is my first post. I've designed and built an amplifier based around Douglas Self's "Load Invariant" amplifier. My problem is that it is (relatively) very noisy! I wonder if anyone can suggest high voltage, low noise input transistors. I'm currently using MPSA42/92. The supply rails are 62V.

I must say that this amp is very beefy, and quite easily drives the 3ohm voice coils of my 12" Earthquake Magma sub. It also sounded very good with my first speaker project... miles better than with my Elektor Mini Crescendo.
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Old 26th March 2001, 05:10 PM   #2
mlloyd1 is offline mlloyd1  United States
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Shaun:

Congrats on the project and your first post!

Look at these devices: PNP -> 2SA872 or 2SA872A, NPN -> 2SC1775 or 2SC1775A. For PDF datasheet, go here:
http://semiconductor.hitachi.com/2s.html
Look for the part numbers. The "A" suffix means 120V breakdown, instead of 90V.

Good general purpose, high gain, low noise. Available cheaply at most parts houses that stock Japanese semiconductors (e.g. Dalbani, etc.).

These are small signal (less than 20mA Ic) devices

I've used 'em for YEARS. They work well.

Lower on the choice totem pole (to me anyways - lower gain, lower breakdown voltage, lower fT, etc.) are the ON Semiconductor (was Motorola) MPS8099 and MPS8599. One is PNP the other is NPN; I don't remember which. Data at the ON Semi web site.

By the way, are you sure your noise isn't due to some non-audible high frequency oscillations?

Michael

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Old 27th March 2001, 03:46 PM   #3
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I can't confidently state that it's not caused by hf oscillation. I've scoped it, and the signal looked free from oscillations. However, I must concede that development is not complete. I have a friend who has some distortion measuring equipment, and he is building up a pair of the modules for evaluation.

Thanks for your help.
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Old 7th May 2001, 09:25 PM   #4
rborer is offline rborer  Brazil
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Would anybody happen to have the Spice models for these Hitachi devices (2SA872/A, 2SC1775/A)?

RBorer
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Old 8th May 2001, 07:45 PM   #5
mlloyd1 is offline mlloyd1  United States
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Just for grins, I did a quick search via http://www.google.com for "spice model for 2SA872"
The only hit was here: http://www.labcenter.co.uk/index.html?/support/
Maybe you can download one of the trial programs and find the models in the libraries?

If you found them elsewhere, I'd like to know. I'd like the models myself :-)

Michael
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Old 9th May 2001, 05:36 AM   #6
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Hi fellas.
send me a mail at hoffmeyer@aub.dk , and I'll send You the Spice models this afternoon.
They are part of the japaneese bipolar library from Orcad Pspice.
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