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Old 15th July 2011, 03:30 AM   #1
nitrion is offline nitrion  United States
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Default Yamaha Power Amp Circuit Theory Help

Does any circuit junkies want to take a stab at what the following power amp circuit does on a Yamaha RX-V1500 receiver. I blew all the output transistors by accidently shorting the B-C on the top NPN while debugging a DC bias issue (I think the root cause of that was a faulty cap in the pre-amp circuit). I would like to understand the cirucit better so I can slowly replace the transistors, instead of all at once. However, I am debating that.

I'm having trouble figuring out what the highlighted section does, or really how the circuit works at all. I know the last transistor Q249 is the overcurrent detection circuit. The last pair Q227C and A are the power transistors. Q221C/A feed the power transistors. Normally they have a +1.0 and -1.0V at the base. I think this is created simply by the voltage drops across all the base to emitters of the transistors.

POWER AMP:
Click the image to open in full size.

Here's where I need help. I *think* the bottom left Q264 is some sort of constant current source. I have no idea what Q214 is supposed to do. Part of that is because I don't really know what kind (voltage, current, amplitudes) of signal the pre-amp sends this circuit. I have included the pre-amp below. I think the only important part is to see the top right PNP transistor Q34 up towards +57V rail.

My concern is that I have removed all the transistors Q264 and Q249, and I have -55V on the base of Q221C. I want to be sure that when I plug that those transistors in again, I don't kill the transistors. I *think* this -55V is coming up from the bottom constant current source and from the lack of diodes within the 4 transitors, but not sure.

PRE-AMP
Click the image to open in full size.

Any tips in understanding this circuit are appreciated.
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Old 15th July 2011, 08:15 AM   #2
djk is offline djk
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Q214=bias transistor.
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Old 15th July 2011, 09:01 AM   #3
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Q214 is a thermal compensation circuit, without this the amplifier would go into thermal runaway within minutes.

This bias circuit or atleast the transistor belonging to it is mounted to the heatsink and thus lowers the bias current as the amp heats up to keep it thermally stable.

A typical bias (quiescent) current through the output devices would be between 30mA and 70mA, the manual should tell what the correct value is.

However if you dident know what this circuit does, you shouldent even have taken the lid off.

Take the amp to a service center and have it repaired properly, your messing around have damaged it way beyond your abilities.
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Old 15th July 2011, 01:55 PM   #4
nitrion is offline nitrion  United States
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I *should* have taken the lid off, since I quickly found the root cause of the original problem (a leaking cap on the pre-amp) and could have fixed it for about $0.50. Unfortunetly I was reaching into a very difficult spot to reach with a DMM probe and knew damn well what I was doing, that if I slipped on that lead it was over. And I slipped. One of those things that you know is stupid while you are doing it, but do it anyways. Looking back, I made a mistake. I will fix the problem, just for a few extra parts than expected. I already have the amp working better than it did since before the dc bias protection previously shut down the entire amp. Couldn't have done that with the lid on.

I have already identified every aspect of the power circuit except for one section that I asked for assistance with, so I think I have more than enough knowledge to fix this.

Thermal stability is not something I was originally thinking about, so its an interesting theory - although the transistor is not attatched to the heatsink. Yet you fail to mention how temperature change will change the current through Q219 and how that will affect Q224. I calculate Q267 constant current source to be pulling about 4mA. Not sure how to calculate how much current is going through B-E of Q224 since I don't agree with my simulation.
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Old 15th July 2011, 03:01 PM   #5
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Just search "thermal runaway" here on the forum and you'll find more info than on a well stocked library.
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Old 15th July 2011, 11:04 PM   #6
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The japanese OEM's are notorious for using thermistors for the thermal circuit (Vbe) . One of Q214's biasing resistors is most likely one and is on the heatsink (small component with a gob of white grease).

Nitrion , I had to read twice to understand. Your "pre-amp" is commonly known as the input pair/long tailed pair- (LTP -Q26/31 ) + the voltage amplification stage (VAS - Q34). A true "preamp" is usually separate and just switches between sources + may provide 2-5X voltage amplification / and- or a headphone output.

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Old 16th July 2011, 01:17 AM   #7
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"Yet you fail to mention how temperature change will change the current through Q219 and how that will affect Q224."

How could we?

Your schematic does not show them.
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Old 16th July 2011, 11:31 PM   #8
nitrion is offline nitrion  United States
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djk: You're correct. I originally pasted the schematic for the wrong amp channel and started referring to the actual one that is dead. Therefore I don't expect anyone to understand what I am referring to! sorry

I have done some reading on the thermal bias circuit which has helped using some of the keywords suggested, which I was unfamiliar with before. I also took it to a colleague at work who helped explain it. Although we both have a hard time to calculate the exact voltage bias shown in the schematic of that thermal circuit. That "current follower" transistor has an abnormally high hfe so its been very hard to find a replacement. So there is a bit of risk here. Still working at it...

I call the other circuit the pre-amp only because the schematic calls it that (not shown in the image).

Thanks,
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