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Baldin 26th December 2007 01:38 PM

Omitting "emitter" resistors in power stage
I'm building on a power amp.

It will use a CFP, and I'm thnking on omitting the usual "emitter" resistors (0.1 ohm) (here it's actially collector resistors.

Thay are quite bulky, will generate distortion.

Guess they will help stabilise bais curent (and make it easier to ajust ... measure voltage drop over these resistors)
They will also limit the current somewhat and might also act as fuses in a fault situaion.

Any thoughts?
Anyone tried this?
Any comments to sound or measurements?

Seems that the AKSA amps do not use these!

Kind regards Baldin

PS. A Merry Christmas to all :santa2:

linuxguru 26th December 2007 01:51 PM

You can omit them in Class-B output stages, but they're pretty much mandatory in Class-A or Class-AB Emitter-Follower output stages for bias stabilization. Anything from 0.1 to 0.33 ohms should be sufficient.

AndrewT 26th December 2007 02:28 PM

a single pair of CFP output devices can be used without emitter resistors. It's more usual see the CFP without.

If you require to double up the output devices, or more, then you must use emitter resistors for each device.

I think the CFP drivers always have emitter resistors.

Jan Dupont 26th December 2007 05:53 PM

I go with AndrewT

Shaun 26th December 2007 06:10 PM


The emitter resistors need not be bulky. In my present amp there are 3 parallel output transistors in CFP configuration. Each emitter resistor is made up of 2x0.22R 0.6W resistors in parallel. A subsequent iteration replaced these with 0.1R 1W resistors. At these low power ratings one is still able to use non-wirewound types.

Greg Erskine 26th December 2007 08:24 PM

Re: Omitting "emitter" resistors in power stage

Originally posted by Baldin
Seems that the AKSA amps do not use these!
Hi Baldin,

AKSA amps do have emitter resistors. They are mounted under the PCB and therefore don't appear in most pictures you see. AKSAs are EF topology though, and Hugh normally uses 0R47 resistors.


thorstenlarsen 27th December 2007 05:04 AM

Hi !
The NAD 3020 doesnt have emitter resistors, as far as I remember. It uses double emitter followers at the output.
Thorsten (another one )

Baldin 27th December 2007 08:01 AM

Thanks all.

It's a class-AB with only one pair of output devices.
I think I'll put them in for safety and stability ;)
No fun having the amp blow up :xeye:


AKSA 27th December 2007 11:26 AM

Greg is correct.

I do not use the CFP configuration in any of my amps.

I do however use 0R47 output emitter resistors.

The higher this resistor, the more stable the bias, and the lower the quiescent current for a given crossover disjunction.

By all means try the CFP. Very elegant, but very tetchy, and can be difficult to prevent oscillation in Class AB.

The double emitter follower is an excellent start point.


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