Can I have some comments on this design?

[Atomicplayboy]

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

Cheers,

[destroyer X]

But your circuit seems very interesting.

Have you built it?

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

regards,

Carlos

[tnsguy]

They are for cascode, right?

Tom

[Atomicplayboy]

Hi,

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.

Cheers,
-- Ben

[Atomicplayboy]

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]

Why have you used multiple transistors in parallel ?

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

[Atomicplayboy]

Hi,
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.

Cheers,

[Atomicplayboy]

...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]

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.

Cheers.

[destroyer X]

thank you Atomic

Carlos