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Old 19th Sep 2021, 8:31 pm   #1
OldTechFan96
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Default Fluctuating Q Current?

Recently I've been using a Hacker GAR600 in place of my HiFi setup. After an hour of use I noticed that the RH O/P pair was warmer that the LH pair.

I decided to check the quiescent current by connecting a meter to the test point on each O/P module.

The Q current for the amplifier should be 15mA. The RHC was drawing 80mA! This explains the warmth. The LHC was close to what it should be.

I was able to adjust both channels to the correct setting but I couldn't help notice that the current measured was somewhat unstable. It would increase and decrease. Not by milliamps at a time, more like +/- 0.5mA or so. Maybe a bit more.

Does anybody know what could be causing this? I do wonder if it could be excessive ripple from the PSU? The AC on the HT line to the O/P modules measures 0.333V. Probably way too much.

Any thoughts would be much appreciated!
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Old 19th Sep 2021, 8:47 pm   #2
kalee20
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Default Re: Fluctuating Q Current?

It does seem a bit strange from a design point of view, that TR203 isn't thermally coupled to TR206 as well as TR205.

Seeing that you were able to adjust the quiescent current to the right value, and that it works, I'd say that the power transistors and drive circuits are OK. I'd look at R207, R208, RP202, and possibly TR203.

I can't see ripple on the supply line having a detrimental effect on quiescent current.

Last edited by kalee20; 19th Sep 2021 at 8:48 pm. Reason: Last sentence added
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Old 19th Sep 2021, 8:48 pm   #3
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Default Re: Fluctuating Q Current?

Dirty pot would be the first candidate. Carbon resistors can go iffy, and beyond that, you're looking for sulking transistors.

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Old 19th Sep 2021, 11:40 pm   #4
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Default Re: Fluctuating Q Current?

Quote:
Originally Posted by Radio Wrangler View Post

......

beyond that, you're looking for sulking transistors.

David
A mental image of a transistor with drooping ears in a boggy place just came to mind. Lovely concept!

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Old 20th Sep 2021, 9:21 am   #5
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Default Re: Fluctuating Q Current?

Aargh! Lockfits! This is a classic dodgy lockfit symptom. As said, check the presets though.
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Old 20th Sep 2021, 9:43 am   #6
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Default Re: Fluctuating Q Current?

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Originally Posted by paulsherwin View Post
Aargh! Lockfits! This is a classic dodgy lockfit symptom. As said, check the presets though.

Funnily enough, I've already replaced all of the lockfits in the pre and power amp sections. I was tracing an intermittent noise issue in the RHC pre amp when I remembered the noise issue with old lockfits. So I replaced them all but the problem still persisted. I tracked the fault down to some stubborn dirt in the bass control. It needed to be submerged for a few days before it cleaned up.

I'll clean the adjustment pots. They look to be sealed so I'll remove them for a better clean.
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Old 20th Sep 2021, 10:47 am   #7
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Default Re: Fluctuating Q Current?

As it changes with time/temperature try some freezer spray - one part at a time and see when the current drops.
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Old 20th Sep 2021, 11:44 am   #8
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Default Re: Fluctuating Q Current?

I've given the two adjustment pots a good clean and work out with Servisol. This has made no difference.

I assume that the Q current should snap to 15mA when the amplifier is switched on?

I ask this because from cold the Q current measures 3mA when the amp is switched on and slowly rises to about 15mA +/- 0.5mA over a few minutes.

I did wonder if temperature influenced things. The O/P pair was given a quick blast of freeze spray. This caused the current measured to fall to about 14mA, then increase to 16.6mA. After 5 minutes it fell to 15.4mA. Both power amps are sensitive to a blast of freeze spray.

Could somebody please explain to me what the attached circuit does? Also, what does 'th.c' mean between the two transistors?
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Old 20th Sep 2021, 11:56 am   #9
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Default Re: Fluctuating Q Current?

A quick blast of D201 and T202 caused the Q current to spike. Assuming 'th.c' means 'thermal coupling', do D201 and T202, T203 and T205 need to be v.close or touching? I noticed that T205 has a heat sink.
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Old 20th Sep 2021, 12:07 pm   #10
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Default Re: Fluctuating Q Current?

TR203 is a circuit called a "Vbe multiplier"

It is a little feedback circuit where the collector-emitter voltage potted down by the resistor and the trimmer is compared against the base threshold voltage as a negative feedback loop.

So the collector-emitter voltage is stabilised as a multiple of the transistor base threshold (say 0.65 for a silicon device) TR203 conducts more or less as needed to maintain this voltage. Think of it as a little, local feedback controlled, voltage dropper

This allows the trimmer to adjust the bias voltage between the bases of the output stage transistors.

Now, the base emitter threshold voltage of a transistor is sensitive to temperature, in fact it's proportional to absolute temperature. So the Vbe multiplier voltage is temperature dependent to a fairly well defined law. This is good, because the voltage it makes is being used to compensate mostly, the Vbe voltages of the transistors in the power stage and drivers... only a little is in the power device emitter resistors.

So the Vbe transistor is normally thermally linked (usually mounted on) the amplifier's heat sink. Just linking it to a single device in the power stage is a bit half-assed, but probably sufficient. Sometimes you might see a metal clip holding things together. Some makers use a dab of thermal compound. I've seen transistors glued together.

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Old 20th Sep 2021, 8:39 pm   #11
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Default Re: Fluctuating Q Current?

Thanks for that detailed reply, Radio Wrangler. It's very helpful as amplifier circuits are not my strong suite.

I've adjusted the components mentioned in post 9 so that they are close and touching via thermal compound.

This has not solved the issue although the Q current reaches 15mA or so quicker than before.

The components in post 2 test fine.

Is the current fluctuation a fault or is it expected in this sort of amp? I think that it is odd that it takes a few minutes for the Q current to reach about 15mA. I'll check CE202 and 204 in case they have drivted out of spec.
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Old 20th Sep 2021, 11:32 pm   #12
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Default Re: Fluctuating Q Current?

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Now, the base emitter threshold voltage of a transistor is sensitive to temperature, in fact it's proportional to absolute temperature.
Actually, it's not! Although it doesn't invalidate the rest of the explanation.

If Vbe were proportional to absolute temperature, you'd expect Vbe to increase with temperature. But it doesn't - it decreases. The transistor actually turns on at a lower voltage at a higher temperature.

The reason is, in the classic Ebers-Moll model, I = Is x (exp(qV/kT) - 1), you think that raising T (temperature) would indeed require a corresponding rise in V (voltage) to give the same current I. But it ignores the fact that the small so-called saturation current Is also has a temperature coefficient, and this dominates.
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Old 21st Sep 2021, 12:00 am   #13
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Default Re: Fluctuating Q Current?

Yeah, that's better. I didn't mention the direction and the offset. It drops proportionately to absolute temperature. -2mV per degree C for Silicon. Diode junctions can be used as temperature sensors.

If the base-emitter voltage of a transistor is fixed, this gives rise to the tendency to thermal runaway as increasing temperature reduces Vbe, and with a fixed base-emitter voltage, that increases base current which increases collector current which increases heating.

In a typical modern audio amplifier, two things operate to prevent the quiescent current running away. The addition of the low value resistors in the emitters of the power transistor networks establishes some degeneration of DC voltage to current characteristic, and the use of a Vbe multiplier slightly over compensates the thermal reduction in all the Vbe, because some of its voltage is expressed across those nice, linear and stable resistors.

Folklore says that Silicon was the fix for the problems of early germanium based amplifiers running away. Silicon made the task easier, but it was the development of suitable circuit techniques to tame the transistors which did the job. Silicon can still run away if not handled appropriately.

Note the phrase 'over-compensates' above. A typical amplifier should show Iq reducing slowly as the overall temperature increases. If the bias Iq needs to be accurate, then you'll get worsening distortion as it gets hotter. Better than a destructive meltdown.

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Old 21st Sep 2021, 10:15 am   #14
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Default Re: Fluctuating Q Current?

I wonder if I am overthinking this 'fault'. Maybe the the gradual rise in Q current after switch on is normal as temperatures stabilise?

Maybe the up and down drift of the Q current is also normal for this sort of amplifier? The slow drift is probably less than +/- 1mA. Or should I be aiming for a solid 15mA?

Am I chasing my tail here or is there a fault somewhere?
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Old 21st Sep 2021, 11:11 am   #15
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Default Re: Fluctuating Q Current?

That doesn't sound like a fault to me.

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Old 21st Sep 2021, 12:46 pm   #16
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Default Re: Fluctuating Q Current?

Thanks for the reassurance, RW.

I'll let the radio play to itself for a bit, check the Q current and make adjustments if necessary.
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