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Atomicplayboy 5th September 2009 03:18 PM

Can I have some comments on this design?
3 Attachment(s)
Hi there,

I'm about to get the ferric chloride out and make a few boards up, and before I do I'd appreciate any comments on the design and layout atached. in return they are placed in the public domain, for what they're worth ;)

It's intended for 100W into 8R. The DC rails will be +/-50V

I think the design is fairly conservative - it's based on a dev board I made with the ability to try out a lot of different ideas; my conclusion was to keep it simple. It fits on a standard 100x160 eurocard and the only external components are the output power transistors (possibly some ballast resistors for them) and OP transistor d-caps (these will be wired- point-to-piont with their own ground).

I want to use low voltage-high performance transistors in some places, and I'd be really interested in any comments about start-up / fault etc conditions that might over-volt them. I'd like any suggestions for more suitable transistors for the LTP too...

Also If I've done anything silly ;)


destroyer X 6th September 2009 05:20 PM

Maybe transistor input capacitance will reduce your audio quality
But your circuit seems very interesting.

Have you built it?

I will be looking forward by your testings into the real thing.



tnsguy 6th September 2009 05:57 PM

I think Q15 and Q17 should be PNP?
They are for cascode, right?


Atomicplayboy 6th September 2009 07:56 PM


Thanks Tom, yes you're right (I spotted that right after I submitted the post ;)
Will work much better with PNP...
Also the bias adjust pot should be 1k, not 500R.

-- Ben

Atomicplayboy 6th September 2009 08:07 PM

Hi Carlos,

The three transistors on each side of the LTP are after a mic amp I did with multiple supermatched pairs on the front end; depending on testing I may only fit one or two of them. It almost seems like something for nothing in terms of noise though.

I have built a fairly rough prototype of it (with PNP transistors in Q15/17...) and it works; step response and FFT looks good, but once I get the PCBs done I'll take it into work where I have an AP and let you know.

nigelwright7557 6th September 2009 08:57 PM

Why have you used multiple transistors in parallel ?

My amps work fine with one transistor where you have used 2 or 3.

Atomicplayboy 6th September 2009 10:05 PM

The reason for multiple transistors in the LTP is noise reduction.

Noise in this type of design is pretty much dominated by the input transistors; specifically by their base spreading resistance. Fitting three in parallel reduces this resistance by 2/3, and so noise should be reduced by 1/sqrt(3). Doesn't quite work that well in practice, but the reduction in noise is useful, given the cost.

The other places where there are two transistors in parallel are option fits - I was not sure whether I would want a TO92 or TO126 package in these positions so I've dual-pitched them to give me some options on transistor choice when I'm testing.


Atomicplayboy 6th September 2009 10:22 PM

...specifically, if I need to increase the current from the Q20/Q26 source, I'm a bit concerned about running out of safe operating area with TO92 devices.

Pingrs 6th September 2009 11:35 PM

On a practical note. I'd make your pad drill marks much smaller (15 thou, say). You've almost no annulus to solder to, and they would easily lift with rework.


destroyer X 6th September 2009 11:52 PM

I will be reading the thread... waiting you to do it real world
thank you Atomic


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