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Old 28th May 2006, 10:49 AM   #11
AndrewT is offline AndrewT  Scotland
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Hi,
I was thinking of matching the Vbe of the darlingtons not the hFE.

But, I see now that there are only ONE pair per channel and Vbe cross match between NPN and PNP is not quite so important, but still worth doing.

I do not know of the argument that mismatch between hFE across the N & P pair causes an excess of distortion, maybe I need to do more reading.
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Old 28th May 2006, 11:55 AM   #12
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by AndrewT
I do not know of the argument that mismatch between hFE across the N & P pair causes an excess of distortion...
Not really....unless you're using a complementary common-emitter output stage, and then only if you employ an inordinately low amount of major loop feedback.
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Old 28th May 2006, 11:57 AM   #13
mikeks is offline mikeks  United Kingdom
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Default Re: Matching is important for VOS

Quote:
Originally posted by chewrock
Class B elimination of 2nd order harmonic distortion can't occur unless the matching is very close.
Even-order harmonics are only eliminated, in principal, by push-pull action in Class-A, and not Class-B.
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Old 28th May 2006, 03:23 PM   #14
elaar is offline elaar  United Kingdom
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Hi, thanks for all of the helpful replies, much appreciated.

There is only 1 trimmer pot per channel, this must adjust the quiescent current I'm assuming, so how exactly is the DC offset adjusted? or does this change as an indirect result of adjusting the current?


Also, i have replaced the other damaged parts that were affected during the fault (resistors, a few diodes etc..). The trim pots looked okay but it wasn't until i took some readings from them I found they must have also been damaged, 100ohm pots (100222M), yet one reads 180ohms and the other 50ohms (measured from one end to the other (not the slider)). Is this how faulty/overpowered trim pots tend to measure?

Many thanks,
Andy
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Old 28th May 2006, 03:53 PM   #15
AndrewT is offline AndrewT  Scotland
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Hi Elaar,
your pot could be either DC offset or quiescent adjust.

You need to try and figure which before powering up.

DC adjust should probably be set for mid range.
Quiescent set for minimum current (maximum resistance usually).
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Old 31st May 2006, 07:18 AM   #16
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Default Not Theoretical, Been There

My comments about the hFE matching are not theoretical in nature. They are based on actual output device replacements in consumer electronics amplifiers over a period of over 40 years. I screwed up my first amplifier output device replacement in 1969, on an Allied Radio product that a friend of mine had built from a kit. He came to me when he smoked it, and asked me to help him fix it. It took two more rounds of smoke before I figured out that the hFE of the replacement part had to match the hFE of the existing complementary pair.

It was a hard lesson, but I've never forgotten it. Later, another friend brought me a Dynaco Stereo 80 with an output device that was destroyed. He had purchased a spot on replacement part and bolted it to the heat sink, only to find that the channel ran extremely hot and was distorted. The reason? The hFE of the replacement was not a match for the "good" transistor that was still remaining.

Here's the problem with depending on the feedback loop to correct for this. When the hFE doesn't match, the "weaker" transistor has to be driven all the time. It might as well be running in Class A, because the DC Servo or DC feedback has to "push" that side of the rail back to where it is supposed to be.

That means that all of the drive circuits on that side of the waveform are being driven all the time in order to eliminate the VOS. That's fine when the amplifier is at rest, but when a real music signal comes through, now half the waveform is pushing one entire side of the amplifier harder than the other. The result is that the driver circuit goes into overdrive on the "weak" side at a much lower volume than it would normally. In the case of a fairly obvious mismatch in hFE, the driver circuit can be overdriven at normal volume levels.

You can try it if you like, but I've already been there and done that. I wish you a lot of luck with it. Uh, put some nice fuses in your speaker cables, say 3/4 amp normal speed (never slow blow) or less. You know, just in case I'm right.
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Old 31st May 2006, 07:30 AM   #17
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Default I forgot about the Vbe

Uh, I'm sorry, I forgot about your comment on the Vbe. That's a common error to make.

Vbe does not have to be matched in output devices, because it drops like a stone as soon as the devices heat up. At room temperature, you will probably have double diode values of around 1.5 volts from the Darlingtons. But that is so temperature dependent that you can test it with your fingers.

Set the test leads from your forward voltage drop meter on one of the devices. Now, pick up the device and press your fingers onto both of the flat sides. Wait a moment for your body temperature to heat the device. You should see your Vbe start to drop. The longer you hold the device, the farther the Vbe will drop!

Some designers have a "sensing diode" that is mounted to the heat sink in order to detect the temperature of the output devices. As the forward voltage drop of the diode goes down it sends a signal to the circuit to reduce the static bias current. This prevents the bias from running away as the devices heat up.
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Old 31st May 2006, 08:56 AM   #18
AndrewT is offline AndrewT  Scotland
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Hi Chewrock,
I have read your post16, only once so far, and do not believe you have analysed the problem correctly. Unfortunately I do not have the experience to give you a reasoned counter argument.

Post 17 is close to right but
Quote:
drops like a stone
is completely untrue. Just look at the manufacturers' data and curves to see the variation with temperature.
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Old 31st May 2006, 09:43 AM   #19
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Couple of thoughts you may rip to pieces at your leisure:

1. Darlington's have such high gain that a real 'match' is gonna be nigh-on impossible.
2. I don't have 40 years experience doing the amp repair thing, but in a standard complementary output stage using a common collector topology, swamping the emitter with a low value resistance is usually plenty good enough to compensate for gain variations in the output devices. Matched devices are nice, but not very necessary with this topology. mikeks has already more or less said the same thing.
3. Variations in Vbe are significant enough to use for bias compensation, but in output devices who cares what Vbe is doing?
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Old 31st May 2006, 01:25 PM   #20
cpemma is offline cpemma  United Kingdom
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Quote:
Originally posted by elaar
There is only 1 trimmer pot per channel, this must adjust the quiescent current I'm assuming, so how exactly is the DC offset adjusted? or does this change as an indirect result of adjusting the current?
My NAD3020 only has one trimpot per channel, for DC-Offset adjust.

Quiescent current is checked across the emitter resistor as usual and, if not in range, a fixed resistor in parallel with another (lower value) resistor is changed to suit.

Shown here as R641 (Rx) in parallel with R647 on Q609 base.
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