Quad 405 Parasitic Osacillation! - diyAudio
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Old 4th July 2005, 02:38 PM   #1
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Question Quad 405 Parasitic Osacillation!

Hi ppl

i have built myself a Quad 405-2 clone. It is very close to working now but there is a problem i do not know how to fix.

The output has a parisitic oscillation on positive cycles ( both channels).
It's frequency is about 4Mhz and it is small but visible on my scope.
It also seems to cause crossover distortion when it's present .

I have managed to get rid of it by whacking a 50 ohm resistor at the base of TR9 ( positive dumper) and the crossover distortion becomes much less (how much is acceptable??).
I think this is a dirty way of getting rid of it though.

Here is a link to the schematic:
http://home.jps.net/~shiloh/

These are the changes i have made to the circuit:
TR9 and TR10 are MJ21194's
TR7 and TR8 are BD244C's
TR1 is a 2N5401
TR2 is a 2N5550
TR3 is a ZTX753

If anyone knows what i can do to get rid of it i would be grateful.

Thanks a lot
Craig
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Old 4th July 2005, 03:27 PM   #2
edl is offline edl  Hungary
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let's solder a 2,2n/200V folie cap between the base and emmiter of the "lower" output transistor (TR10 on the schema)

the Quad405 likes oscillate, kill the speaker, etc... that a real sh*t amp, sorry...
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Old 5th July 2005, 09:51 AM   #3
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Thanks for the reply edl.

Why do you think the Quad 405-2 is a poor amplifier?

Could you expand on why this capacitor is needed and how you obtained its value, i am a bit nervous connecting the base to the negative rail?

I have got all distortion to disappear by using a 70 ohm load, what does this suggest as to its source?

Cheers
Craig
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Old 10th July 2005, 10:14 AM   #4
405man is offline 405man  Scotland
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I have built several 405 clones in the past and used a copy of the Quad PCB layout. I had no problems with oscillation even using 2N5038 output transistors which are very fast with an Ft of 60Mhz. Perhaps you have a problem with PCB layout. You can disconnect the output transistors and run the amplifier into a resistor of about 5k to check the performance of the class A stage. When working properly these amps are capable of very low distortion ( We measured 0.0018% ). I think these amplifiers are much maligned. Operated within their design parameters (i.e. not trying to drive 8 ohm loudspeakers which measure 3 ohms) they perform very well. The 405-2 has a much more sophisticated current limit circuit and addresses many of the problems of the earlier 405. The amplifier is also unconditionally stable and will happily drive an 8uF motor start capacitor as a load. The current dumping principal is a very elegant solution to the design of a high power amplifier and I am sure can be developed further. I shall have a look when I get home and see if I can be of more assistance

Stuart
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Old 11th July 2005, 08:07 AM   #5
tvicol is offline tvicol  Romania
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Craig,

First make shure that you don't have any ground loops.
Please also use bulk cables to connect each 405 module to common ground point.

If still oscilate, add a 100pF between base and collector of Tr8 (BD244C).


Regards, Tibi
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Old 11th July 2005, 08:38 AM   #6
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Tvicol, thanks for you help.
My grounds are ok i think i have used thick wire from each module to a big star ground and my grounding scheme on the modules is as Quad specified in their circuit, the 10 ohm resistor seperates the lower current parts of the circuit from the drivers and dumpers ground.

I will try the 100pf cap later today when i get time i hope it works i am desperate to listen to some music!!

Hi 405man,
any help you could offer into this annoying problem is very welcome .
I think my PCB layout is ok i have avoided parallel tracks and things are quite spaceous, also a big physical gap from the drivers to the op amp.

Any ideas as to what is causing crossover distortion although it is quite small it is visible?

Thanks
Craig
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Old 11th July 2005, 08:45 AM   #7
405man is offline 405man  Scotland
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On some early 405s there is a 1nF capacitor between base and collector of Tr10however if you have visible crossove distortion then I think you have a more serious problem. I would check around the bridge components, when working properly you will not be able to see any distortion.

Stuart
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Old 11th July 2005, 03:59 PM   #8
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Hi

I have tried the 100pf capacitor but it didnt get rid of my oscillation, i think this is because the oscillation is only over the positive cycle.
If take away the signal source i can actually see the oscillation on the offset voltage with my scope set very sensitive, could this mean it is oscillating due to RF inteference?


I have checked my bridge and everything is correct but i can still see crossover distortion, what could be causing this if the bridge is working?
I thought it might be the output transistors i am using being Darlington and needing 1.2V to conduct and leaving the output under class A control too long. But other people have used these transistors in the same circuit without problems.

on the good side of things this amp delivers 112 watts RMS into 8 ohms before clipping

Cheers
Craig
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Old 11th July 2005, 04:11 PM   #9
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I forgot to mention that crossover distoriton only becomes visible at high frequencies about 10Khz but is visible even at low output levels like a few watts.

Same with the oscillation, it is less visible at lower test frequencies.

Cheers
Craig
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Old 11th July 2005, 04:25 PM   #10
405man is offline 405man  Scotland
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I would try a pair of non-darlington output transistors first or add extra diodes between the bases so that the output pair only require 0.6V bias as in the 405-2. I can send you a pair from a 405-2 if you dont have any handy

Stuart
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