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Old 26th December 2007, 12:38 PM   #1
Baldin is offline Baldin  Denmark
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Default 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
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Old 26th December 2007, 12:51 PM   #2
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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.
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Old 26th December 2007, 01:28 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
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.
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Old 26th December 2007, 04:53 PM   #4
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I go with AndrewT
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Old 26th December 2007, 05:10 PM   #5
Shaun is offline Shaun  South Africa
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Baldin

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.
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Old 26th December 2007, 07:24 PM   #6
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Default Re: Omitting "emitter" resistors in power stage

Quote:
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.

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Old 27th December 2007, 04:04 AM   #7
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Hi !
The NAD 3020 doesnt have emitter resistors, as far as I remember. It uses double emitter followers at the output.
Thorsten (another one )
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Old 27th December 2007, 07:01 AM   #8
Baldin is offline Baldin  Denmark
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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

Baldin
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Old 27th December 2007, 10:26 AM   #9
AKSA is offline AKSA  Australia
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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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