Amp oscillation - Page 3

Bonsai · 13th March 2007, 11:01 AM

Continuing.

Point #4 re input diff amp degeneration resistors. Looking at the circuit again, you can probably take these up to 100 or 150 Ohms. This will reduce the loop gain . . . but that's ok if the result is you get a clean amp that does not suffer from marginal stability problems.

kubeek · 16th March 2007, 05:01 PM

Could someone explain me how exactly does the VAS stage in this amp work?

I ask because I just found out that on one channel, which is happily working with no distortion or oscillation into 2ohms+2uF in parallel, I swapped the MJE340 from ECB connection to BCE. How is it possible that the amp still works?

AndrewT · 16th March 2007, 05:50 PM

Hi,
the cascode is carrying most of the voltage across the VAS.
and it swings most of the voltage when signal passes through.
Are you sure MJE is inserted incorrectly?
To220 and To126 have reversed lead connections.
Check the PCB traces and find which voltage is on which leg.

The Zener holds the cascode base at 3.9v away from supply rail voltage. This places about 3.3v across the signal To92 VAS transistor. The VAS operates as a common emitter amplifier.
The cascode operates as a common base amplifier.
This dual transistor arrangement is effectively two amplifiers working together in cascade mode and the odd name cascode has been invented to cover exactly this arrangement.

13th March 2007, 11:01 AM	#21
Bonsai diyAudio Member Join Date: Aug 2006 Location: Taipei, Taiwan	Continuing. Point #4 re input diff amp degeneration resistors. Looking at the circuit again, you can probably take these up to 100 or 150 Ohms. This will reduce the loop gain . . . but that's ok if the result is you get a clean amp that does not suffer from marginal stability problems. __________________ bonsai http://www.hifisonix.com/

16th March 2007, 05:50 PM	#23
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, the cascode is carrying most of the voltage across the VAS. and it swings most of the voltage when signal passes through. Are you sure MJE is inserted incorrectly? To220 and To126 have reversed lead connections. Check the PCB traces and find which voltage is on which leg. The Zener holds the cascode base at 3.9v away from supply rail voltage. This places about 3.3v across the signal To92 VAS transistor. The VAS operates as a common emitter amplifier. The cascode operates as a common base amplifier. This dual transistor arrangement is effectively two amplifiers working together in cascade mode and the odd name cascode has been invented to cover exactly this arrangement. __________________ regards Andrew T.

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16th March 2007, 05:01 PM	#22
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	Could someone explain me how exactly does the VAS stage in this amp work? I ask because I just found out that on one channel, which is happily working with no distortion or oscillation into 2ohms+2uF in parallel, I swapped the MJE340 from ECB connection to BCE. How is it possible that the amp still works?

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