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Old 26th July 2008, 03:05 PM   #1
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Default AVR7000, emitter resistor value, increase number output devices?

I got my hands on a harman kardon AVR 7000 with a small defect.

One of the channels was not working. So after i got the service manual i started checking for faulty components and found out that a zenerdiode in the voltage supply line for the VAS had died.
After i replaced it the channel worked again.

All that was left was to check if the bias and offset of the channel were still OK. Being a bit crammed inside the amp i accidently created a short wile measuring the offset voltage. Almost the complete output stage including the emitter resisters blew.

Now im left with a few questions.

- original emitter resisters are dual 0.27ohm 5W types. I can't find them anywere. Does anyone know were to get them? Or if i really can't find them, would it be better to use 0.22 or 0.33 as a replacement? And how should i adjust the bias current?

- The amp has 5 channels each output-stage has 2 output BJT's in paralell (so 4 in total). Will the repaired channel sound different if i were to change the emitter resisters to another value?

- how nescesarry is it to match the output devices? And should i match the paralel devices or the push-pull devices? Any good method of doing so?

- In the service manual i found that the output print is the same as for the AVR9000. The difference is that the AVR9000 has 3 output devices in paralel and maybe (i'm not sure) a little higher voltage rails. Since i'm buying new transistors anyway it would be a small effort to just put that 3rd device in for each channel. Or is this a bad idea?

A lot of questions, hope someone can help.


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Old 27th July 2008, 12:39 PM   #2
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The higher value resistor will give better bias stability and would probably be a better choice. The value needed for the emitter resistors is determined by a combination of power supply voltage, the thermal resistance from junction to heatsink and how much positive feedback one can live with.
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Old 27th July 2008, 01:21 PM   #3
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I read somewhere that this amp has only limited global feedback. So you're saying if i would increade the emitter resister i would increase global feedback?

I read in a thread (AKASA amp thread?) that the voltage drop over the emitter resister is fixed (or should be the same for every amplifier(AB)) Is this true?

I included the schematic of the amp, im not sure if this is allowed, if not i will remove it and mail it to people who might want to help me out.

What is with the rail voltages betweeen ()? Is this the no-load value? Or is this the voltage for the AVR9000?

btw, as far as i know HK never made a AVR9000
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Old 28th July 2008, 12:05 AM   #4
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No, I'm talking about bias current stability. Higher resistor values will reduce gain in the positive feedback loop transistor power dissipation -> temperature -> vbe decrease -> more current -> more power dissipation.

What will happen too is that the protection circuit operation will shift - higher resistor values will give earlier protection. Lower resistor value will provide less protection as the current needs to be higher before it activates.

It looks like the gain is limited at low frequencies (resistors loading the VAS), so the feedback will be lower at low frequencies than otherwise. This will give higher distortion at low frequencies - but some say this sounds better instead of the rising distortion with frequency that is otherwise seen.
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Old 22nd August 2008, 06:57 PM   #5
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Thanks for your help!

I found a repair centre/ harman kardon dealer in the netherlands they sell spare repair parts even to individuals without a company. it's called CE-repairs in Mijdrecht.

Prices are ofcourse pretty high (6 euro for the emitter resistor, 10 euro for the end-transistor) but they do supply everything.

So the question that remains is, would it be usefull to order some extra resistors and transistors so that i can upgrade this AVR7000 Amp-board to a AVR9000 amp-board? Or would i need a bigger torroid and bigger caps for it to make really sense?
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