Quad 405 Parasitic Osacillation!

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It does the same with a square wave and only becomes noticeable on my scope when the input frequency exceeds roughly 10Khz.

Is this due to the slew rate of the negative cycles being too slow?
I assume it also has somthing to do with load current as it becomes more severe as the output is increased or the load is reduced to say 4 ohms.

As a test i allowed the oscillation back into my amp by removing my 100 ohm resistor and cap and it still does this triangulation but not as badly.

I really have no idea what might cause this at all?
Is this a known problem with the 405 or is it common for amplifiers to do this ?

aaarrrggh

Thanks
Craig
 
Quad 405

Hi Craiq405

I have been a quad repairman since 25 years and i have had many times oscillations with these amps.
First, do not put resistors like 50-100 ohms on any transistor base. The transistor will not be able to do his work ie put some current in the circuit.
The best way to stop the oscillation is to put good output transistors.
I have had great results with 2N3773 and MJ15003 (Motorola)
If you put 2SD424, be sure it is not a fake and you will have to put 1000pf cap on base-collector of TR10.
Don't use too fast output transistors in Quad 405.

I have also made modifications on Quad 405 and the most significants : a good power supply.
:no current protection circuit (a little risky but with good out tr i have had no problems: this one make difference)

Some options are on the net too.

When in good order, a Quad 405 have a nice clean sound!

Good luck and have a good day.
 
Hi Dragon2

How did you get rid of oscillations? i have tried the 1000pf cap but it didnt seem to cure my problem.

I am using decent output transistors (Mj21194).

Could you give me a few tips on what caps you cured oscillation with and where they went.

Did you ever get triangulation of the output wave, specifically just on negative swings?

Thanks
Craig
 
Quad 405-2

Hi Craiq405

I have read the thread
I think that your slew problem come from the 100 ohm-330pf on TR7 base-collector.
I have get my Quad 405-2 schematic.
What kind of cap did you put on TR10 base-collector? You must use a ceramic cap at that place with shorts leads.
Do you use protections modules?
You can try higher value like 2200pf.
The original Quad 405-2 output transistor is a slow17556 (Quad own number on the transistor case)
Can you post a photo of your amp? The layout is very important for this amp.
MJ21194 are very good transistors but a little speedy for that amp.
I never had the slew-triangulation problem.
What kind of resistor did you use: R35-R36=0.18ohm
Do you use air cored inductors?
Hope this help.

Have a good day

Hi Kalok

Sorry but i never had to do that.
The finish seems to be anodization on most model.
Have repaired some models (lates models) with a grey sticky finish. I woul try to clear with acetone and to paint hte case but to anodize the heat sinks.

Good luck

Have a good day
 
Quad 405 layout

Hi Craiq405

Here is original 405 layout.
Unfortunately, i don't have the 405-2 but as an example.

Hope this help!

Have a good day!
 

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Hi Dragon thanks for your help.

I have actually begun a new thread on this subject called 'Triangulation' on the solid state forum.

In answer to your questions.

I used a mica cap i think, it had a flat rectangular waxy outer case. I will try and obtain a ceramic in the values you mentioned.

I did not include the current sense resistors as i have not included the current limiting circuits or the output protection modules. All i have is 2 x 0.1ohm resistors per pair of output transistors to share current.

I have used ferrite cored inductors? is this ok?

I dont have a digital camera so i cant post a photo, sorry.
But looking at the layout of the 405 you posted mine is very similar in component locations, and the the track layout is better spaced on my PCB :)

Thanks for your help
Craig
 
Do you have any resistors on the bases of the output transistors? Try 10 to 15 ohm resistors connected directly in series with the base. Mount the resistors as close to the base as possible and see if that doesn't knock it out. I'd be wary of tacking capacitors in. The 405 is a brilliant piece of engineering. Good luck and enjoy.
 
Craig

I thought you had solved this problem ?? it's simply too much open loop gain - a problem that is adressed by some but not all of the replies you have had.


Dragon2

The 440D inductor is wound on a 19mm x 5.4mm ferrite core - maybe you should get a magnet as well as a soldering iron. And ceramic capacitors especially for C8 - not a good choice try polystyrene or teflon - better still try placing C8 in the correct position in the "feedback" bridge.

Some (lost here for a polite term) people have "created" home made air cored inductors for those in the 405 which are huge and pick up radio transmissions - why bother when all but L2 have resistors across them to damp their Q ?

regards

Keith
 
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