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Old 1st October 2003, 06:43 PM   #1
Borc is offline Borc  Slovenia
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Default Leach Amp different emitor resistors

I intend to change emitor resistors on output devices from default 0,33 Ohm to 0,22 Ohm (Mills group buy). I removed protection circuit so this is not an issue.
I would like to have some help on the new optimal bios for output devices. Is there any mathematic formula to determine it.
For the same voltage drop on emitor resistors this will be 67,5mA
(from default 45mA), but I think this is too much.

Another Q.
Anybody tried with 1 uF capacitor across common emitor resistor on drivers. Any changes?

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Old 1st October 2003, 07:07 PM   #2
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0.22 ohms is still quite reasonable.

One method of testing cross-over distortion is to use a 2 channel oscilloscope. Channel 1 at the bases of the output stage (doesn't matter which side, NPN or PNP) and channel 2 at the output. Put the two traces on top of each other. Adjust so the output signal is 1-3 V rms and the frequency 10 kHz. Trim so the channnel 1 looks as much as possible like channel 2. Very easy test with simple means. You also use your ears but then I'll guess the test frequency must be 1-3 kHz, so you can hear any harmonics. Cross-over distortion sounds like a busted speaker (burned voice coil).
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Old 1st October 2003, 07:49 PM   #3
Borc is offline Borc  Slovenia
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Thanks Peranders, interesting method!
If i understand correctly:

1. - CH1 HOT to emitor of driver (as I'm without base resistors on output devices) on PNP or NPN side and CH1 COLD to ground
2. - CH2 HOT to the output and CH2 COLD to ground.
3. -Set the CH2 sinal to 1-3V rms and 10 kHz
4. -Trim Iq, so the signal between bases is as much like the output signal.
5. - output load is connected (can be pure R or must be speaker ?)

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Old 1st October 2003, 08:10 PM   #4
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Both probes (recommend this) grounded.

CH1 at the output of VAS stage, better because you see more distortion but this is a sensitive node(!), a probe is strongly recommended and you may experience disturbance (hopefully not).

The result is better with a real load, at least a pure resistor, but I suspect a speaker is better. This type of distortion is strongly dependent of the load.

When the two curves are alike the distortion is less than a couple of 1% and this is before feedback. If you apply 30 dB (or whatever you have) feedback the distortion (in theory) will be 30 times lower.

A capacitor across the emitter resistor of the drivers can be good because you create low impedance in both directions which is good when is comes to turn-off times of the main power transistors, even more important if you have MOSFET's. I would choose 100 nF, which good HF properties. 100 nF/63 V polyester is quite alright to use.
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Old 1st October 2003, 08:23 PM   #5
Borc is offline Borc  Slovenia
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Thanks,
Procedure corrected now.
Regards
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Old 1st October 2003, 11:17 PM   #6
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Most of what is being said is nonsense. The optimum drop across a voltage driven complementary bipolar output stage is 15-25mV across each emitter resistor. Somewhat more or less will still work. Caps? Across .22 ohm?
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Old 2nd October 2003, 12:55 AM   #7
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"Caps? Across .22 ohm?"

A cap across the emitter resistor for the drivers was suggested, it is more like 150R.

I first saw this done on the ML3.
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Old 2nd October 2003, 01:34 AM   #8
AJT is offline AJT  Philippines
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hi djk,
if i want to parallel say, 8 output power transistors, can i increase the emitter resistors from say 0.22, to .5 ohms, reason being a higher emitter resistor promotes better current sharing among the output transistors, please comment....thanks
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Old 2nd October 2003, 02:51 AM   #9
jam is online now jam  United States
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Mr. Curl,

Could you please explain your reasoning for setting the bias at 15-25 mls as being optimal.

Another question, would you attach the junction of the driver emmiter resistors to the output or use a single emmiter resistor for the drivers not hooked to the output.

Thank you.

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Jam
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Old 2nd October 2003, 05:57 AM   #10
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This is derived from an article published by HP in about 1971. This is the best area of operation for lowest distortion in the transition between class A and AB.
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