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Old 1st November 2013, 08:14 PM   #1
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Default quad 405 clone oscillation

Hello.I bought assembled quad 405 from ebay
QUAD405 Clone MJ15024 Stereo 100W 100W Assembled Board Heatsink 2pcs | eBay
I used 50v symmetrical supply .
After connecting power supply it sings for about a minute , getting hotter and then oscillating and current goes from 200ma to 2.5A
It is a plug and play amp and says can work with 50v
So what can be a problem.I heard many good things about this amp and want to run it.Thanks
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Old 1st November 2013, 10:59 PM   #2
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Try decoupling the power supply paying attention to the ground point for the speakers, power supply and input.
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Old 2nd November 2013, 08:57 AM   #3
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I have built hundreds of amps for many years and i have not met an oscillation because of gnd connection
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Old 2nd November 2013, 09:22 AM   #4
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You buy this version?
LJM QUAD405 Clone MJ15024 Stereo 100W 100W Completed Board Heatsink | eBay

If yes, then PCB kit that is not done properly. For example, the operational amplifier is not properly decoupled with 0.1uF close to the power pins. And how was routed GND on PCB, is not the best choice. If we add to these deficiencies and other assembly errors (very common in chinese electronic kits), the result is that where you are now.

By the way, you compared the value components on the PCB with the value from circuit diagram Quad405?
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Old 2nd November 2013, 09:41 AM   #5
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Yes this version.One pcb works and oscillates after warming in 1 minute the other plays with white noise when plugging.I thought it can work because it was sold hundreds of kits .I have not compared values yet.And i replaced 100uf caps with Panasonic hfz caps because it was damaged in shipping.
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Old 2nd November 2013, 11:23 AM   #6
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When they were building the first clones in the late '70s, early '80s, the only problem area for oscillation was the current source (Tr1), the active resistance for the Tr2 amplifier transistor. Adding a 100n disk capacitor in parallel with C5 (100u) usually solved the problem.
References from the first schematics in the following document:
http://www.keith-snook.info/amplifie...0evolution.pdf

Good luck, Peter
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Old 6th November 2013, 06:17 PM   #7
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Hi Umut,

Just curious, have you resolved the oscillation problem yet?
The reason I am asking is that I am also thinking about getting those clone kits myself.
I am looking at the unassembled version of the kit with the plastic final transistors: 2pcs Quad 405 125W 125W Power Amplifier KIT | eBay
The PCB looks the same as yours except for the final transistors.

The other reason for oscillation in those early clones were the usage of faster final transistors. Looking at the specs of the MJ15024, that transistor surely can not be accused as a fast device, assuming that you do not have fakes!
Still, you could try increasing the value of C19, shown on Issue 4 and Issue 5 drawings in the Quad-405 Evolution document.

Looking at the PCB, the other disturbing fact is that the input signal is run right next tot he output line! While this amp does not have huge amplification, about 31dB, but still, one does not make such elementary mistakes!

Peter

Last edited by orbanp; 6th November 2013 at 06:41 PM.
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Old 6th November 2013, 07:31 PM   #8
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Hi.Now i saw a problem with solder of 120pf cap and fixed it.Now the other channel also started to play.It sounds good at startup and oscillated after about a minute too.Now both amps have same problem.I have not added 100nf cap yet.There is 100uf 50v panasonic hfz cap instead of 100uf 6v.May it cause a problem?I really want to play this amp.Is it possible to find a better clone? For example quad 405-2 clone? I am also looking for a good price (I saw a board about 170euro in a web site but i do not want to pay such a money)
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Old 6th November 2013, 07:44 PM   #9
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mj15024g transistors are cold but it still oscillates. One of tip42c is hot.
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Old 6th November 2013, 11:12 PM   #10
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Hi Utmut,

You have to be pragmatic fixing these amps.
Put those 100n bypass capacitors onto the opamp power supply pins. solder them up on the back of the PCB.
Put a 100n parallel with that 100u (C5) for the current source. again, solder it on on the back side of the PCB.
The 100u 6V or 100u 50V should not make a difference, they are nearly identical, there is little bit of difference in the ESR, but that should not matter.
You have to stop the final stage oscillations. Increase that 1n (C19) capacitor. alternately, you can put a 220pF capacitor between the collector and base of Tr8.
Hopefully you have a scope to watch the output for parasitic oscillations.

The 405-2 is very similar to what you have, you can modify your circuit to become an 405-2. Read through the following page about the evolution and modifications of the Quad amps: QUAD 405 Amplifier Information and Modification

Peter
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