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Old 30th November 2004, 11:06 AM   #11
AKSA is offline AKSA  Australia
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Thanks Per-Anders, you answered the questions very well. Jennice wrote:

Quote:
Item 6:
Are you referring to R13?

Item 7:
Do you mean a 10R resistor between the driver and power transistor?

Item 8:
Why? I thought the point was to lower the AC impedance (?).
Item 6 refers to the Long Tail Pair, Q2 and Q3. Their emitter degeneration resistors are R2 and R3. R13 merely sets the constant current flowing through Q5 which supplies the VAS.

Item 7: Yes, that's exactly what I mean here.

Item 8: It's purpose is quite complex. It is a charge suckout cap, whose purpose is to charge up as the voltage across it increases. This happens as one output device hands over to the other; with increasing current, the voltage across this cap starts to increase. C5 quickly and predictably soaks up the charge across its terminals, pulling charge out of the base region of the inactive output device, switching it off without creating a spray of switch-off artefacts. The result is a cleaner handover, or crossover by another name, and this gives less distortion because it removes the HF artefacts which plague Class AB amplifiers. Note that the value of this cap, and the value of the parallel resistor R15, is related more to the geometry and electrical characteristics of the emitters of the drivers and the bases of the output devices. All this is taking place at audio frequencies; in fact it's the time constant of these two components which is important. I'm suggesting a time constant of 0.044mS, which corresponds to a corner frequency of 22.7KHz, actually beyond the audio range and not strictly related to it.

I hope this answers your questions.

Cheers,

Hugh
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Old 30th November 2004, 01:02 PM   #12
Jennice is offline Jennice  Denmark
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Thanks AKSA,

I think I see your point.

However, lowering the resistors in the long tail pair results in higher gain, which may lead to oscillation if the phase runs away at HF. I had to increase the values in one of my amps to solve this problem, despite caps in the voltage gain stage.

Jennice
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Old 30th November 2004, 01:40 PM   #13
Eva is offline Eva  Spain
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I've tried both ceramic up to 100nF, film up to 1uF and 1uF electrolytic capacitors for 'C5' applications in test circuits

All capacitors reduced cross-conduction across the output devices when leaving clipping at 10Khz [no cross-conduction at all with 1uF]. Some linearity improvement at zero-current crossing was also appreciated at the VAS base current waveform

Ceramics and films reduced phase margin and caused ringing when leaving clipping and also at zero current crossing when the amplifier was underbiased. The 1uF film also caused serious driver cross-conduction when recovering from clipping [too low impedance at HF]

The loosy cheap 1uF 100V electrolytic won because it effectively worked as a RC network at high frequencies [I think ESR was about 0.5ohm] , so it had little or no effect on stability and produced very little cross-conduction across the drivers [and none across the output devices]

NOTE : Paralelling 100nF ceramics with the 100uF 63V rail decoupling electrolytics also caused lots of ringing in the power supplies during clipping recovery and even at zero current crossing. Measurements show that placing 100nF ceramics everywhere is usually a bad practice. I prefer ESR resistive impedances at HF because capacitive impedances here allways produce resonant systems
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Old 30th November 2004, 01:45 PM   #14
Jennice is offline Jennice  Denmark
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Eva,

Very interesting observations!
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Old 30th November 2004, 01:49 PM   #15
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My experience comes from mosfet amps and I didn't have those problems (didn't notice them ) but your observation Eva, seems to be reasonable.
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Old 30th November 2004, 06:46 PM   #16
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Default THanx

I will try some of this stuff out and let you know how it goes. Thanx guys.
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Old 30th November 2004, 08:00 PM   #17
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Default ASKA

Do you mean 1150R from collector of Q2 to collector of Q3??? This is in reference to ITEM 2
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Old 2nd December 2004, 04:09 AM   #18
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Default getting better

I decreased all of the the resistances in the Input stage and there was substantial improvement. 20k is still kinda ugly though. Will providing more current to the VAS help with this also?
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Old 2nd December 2004, 07:19 AM   #19
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Hi sauuuuuce,

Hugh's advice re stabilisation capacitors was good.

Your amp currently has substantial VAS C.dom stabilisation and thus I wonder why R20 and C7 are fitted, you could try temporarily removing these.

You could also try 100nF between the base of Q5 and the +37V rail.

Good luck ............ Graham.
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Old 20th November 2007, 04:10 AM   #20
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Quote:
Originally posted by AKSA


3. Q3 base to output resistance is 10K, but Q2 base to ground is only 1K. These resistance paths must be equal for zero offset with matched 1% transistors. Solution: Increase R25 to 10K.

Cheers,

Hugh
Hello Hugh and everybody

I know that in the blameless amp input mister Self do match those resistances, but is that only apply to that type of input ?

Thank

Bye

Gaetan
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