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Amplifier Troubleshooting.
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Old 18th May 2008, 05:31 PM   #101
5th element is offline 5th element  United Kingdom
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Quote:
Originally posted by AndrewT
A heatsink temperature of 70degC and a device temperature of just under 100degC is asking for the grey smoke.

The temperature derated SOAR is reduced by 60% when Tc=100deg for a 150degC device. i.e. a 150W device is reduced to just 60W and with supply rails around 60Vdc the maximum current is reduced to around 1A per device. If secondary breakdown limitations are also brought into the calculation then the permissible device current is much lower for 60Vce (maybe another 10% to 30% less).
If the peak current is a one off non repetitive event then the 100mS SOAR current can rise considerably, maybe two to three times the DC value. The 10mS current can go even higher.
But you are measuring these currents & voltages after bringing the output stage upto temperature over a period of minutes. 1kHz test signal over 5minutes brings each device to it's peak current about 300000times. A long way above the single short term event.
Yes I looked at the device parameters and saw that myself and then was a bit puzzled. It seems high output power requires the devices to be cool, yet by nature they are going to heat up in a situation like this, limiting the overall output they can provide.

It seems this amplifier isn't capable of delivering the quoted output power with anything other then huge heatsinks.

The temperature thing just seems to make more and more sense. If I play a musical track with heavy bass content, lets say a bass drum periodically plays every second and whilst this is going on there are other instruments adding to the mix.

Now if the bass drum is on its own, no clipping noise is detected, however if a large amount of other bass information bas preceded it, or say the bass drum has beaten 4 times per second in comparison to the usual 1 per second. The additional power and strain this has on the FETs causes them to momentarily heat up, at this point the next bass drum hit to happen AFTER the heavy bass usage, clips. But give it a second or two and there is no clipping any more. So in other words once the FETs have cooled back down, the amount of current required can pass and the clipping disappears.

So if I describe this again

Drum beat - no clip
Drum beat - no clip
Heavy bass passage + drumbeats - clipping
Drum beat - clipping
Drum beat - less clipping
Drum beat - no clip
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Old 18th May 2008, 05:33 PM   #102
AndrewT is offline AndrewT  Scotland
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what you describe could be associated with a collapsing supply rail voltage.
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Old 18th May 2008, 05:48 PM   #103
Mooly is offline Mooly  United Kingdom
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Amplifier Troubleshooting.
Quote:
Originally posted by AndrewT
what you describe could be associated with a collapsing supply rail voltage.
I agree ! You need to do more involved measurements than are possible with a meter. Sorry but I think that's the bottom line here.
I still think the design is pushing the envelope for a dual pair of MOSFET's. I would have thought the double die type would have been more suitable with a 16 amp IDS rating. MOSFETS do inherently limit at high drain currents, it's not a design feature, it's to do with the physical properties.
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Old 18th May 2008, 06:28 PM   #104
5th element is offline 5th element  United Kingdom
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Quote:
I notice you say you have the drivers on the heatsink. Is this specifically mentioned in the construction notes as lateral FET's require no thermal compensation.


This is not mentioned, however the picture Slone provides has them connected to the sink.


Quote:
The other thing that keeps haunting me is that you are using an unmounted speaker as a load ( Have I got that right ). It's only meaningful to make measurements with a defined load- usually either 8 or in your case 4 ohms resistive. Which for 250 watts takes some doing, electric fire element in a bucket comes to mind (Really )
I can't do a lot about this at the moment, but I could probably rig something up if absolutely required.


Quote:
With the protection circuit out of the way is it loud enough or are there still distortion problems.
It's decent I suppose. The main issue is there is between 10-30dB of gain through the bass due to the open baffle compensation, so the amps have to work hard in comparison to say the upper midrange and tweeter that have no power Eq at all.


Quote:
Going back to power again 250 watts RMS, thats 8 amps in the load (RMS).
Right just to clear this up.

8Arms into the 4 ohm load.

Am I correct in saying that this workload is shared by each rail, such that 4A is provided by the P channel and 4A is provided by the N channel?

If this is the case what current draw per rail at the amplifier supply should I see to allow this?

Quote:
At the risk of you saying I have'nt studied all the posts whats actually the problem now the protection isn't operating. Is it just the heat issue.
Haha, I was thinking of mentioning this, but decided not to, to help keep the atmosphere pleasant, after all it's not that hard for me type a sentence summing the problem up.

What I do not know currently, (but it seems realistic) is if the temperature is the limiting factor at the moment. There could be nothing wrong with anything except the FET temp. getting too high and thus the max output is curtailed. When playing bass heavy music (at just before clipping) for 30 mins the heatsinks are about as hot as they were with 0.15V bais per FET.
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Old 18th May 2008, 06:34 PM   #105
5th element is offline 5th element  United Kingdom
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Quote:
Originally posted by Mooly


I agree ! You need to do more involved measurements than are possible with a meter. Sorry but I think that's the bottom line here.
I still think the design is pushing the envelope for a dual pair of MOSFET's. I would have thought the double die type would have been more suitable with a 16 amp IDS rating. MOSFETS do inherently limit at high drain currents, it's not a design feature, it's to do with the physical properties.
I mentioned earlier if either of you cared to read it that the rails did/do NOT droop by anything appreciable, we're talking a couple of volts per rail. Unless you were meaning something else.

Alright I need more involved measurements, what do I need to do this?

And what capabilities do these things need to have?

I can connect the amplifier to the sound card and do a host of stuff with the ARTA suit. I can probably also get an oscilloscope program too, but as I said earlier the bandwidth would be horrible.
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Old 18th May 2008, 06:49 PM   #106
AndrewT is offline AndrewT  Scotland
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250W into 4r0 requires 44.7Vpk and 11.18Apk.
Vpk=sqrt[ P * Rload * 2].
Ipk=Vpk / Rload.

All of that 11.2Apk comes alternately from each supply rail.
It's the same argument when we were considering the power available when 3.3Apk was flowing to the load.

PS,
we are reading it, it being what data you are able to gather with just a DMM and no load resistor.
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Old 18th May 2008, 07:24 PM   #107
5th element is offline 5th element  United Kingdom
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Quote:
Originally posted by AndrewT

we are reading it, it being what data you are able to gather with just a DMM and no load resistor.
So you're telling me measuring the rail voltages with a DMM, whilst increasing the input isn't enough to see if the PSU is having issues?
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Old 18th May 2008, 07:50 PM   #108
AndrewT is offline AndrewT  Scotland
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Hi,
an interesting experiment for you to try.

Switch your DMM to 200Vac and measure the DC voltage on each half of the PSU. Do this with the amp idling and normal bias. Do again with the bias turned right down to minimum. return the bias to normal and measure again when you are sending near maximum signal to the test load.

Tell us the three Vac readings and the three Vdc readings.

Then we'll try part 2 of the experiment
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Old 18th May 2008, 08:08 PM   #109
Conrad Hoffman is offline Conrad Hoffman  United States
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Several posts back, it sounded like the amp was heating up and the current was going up all out of proportion to the signal level you described. It should have been blowing you out of the room at that point. It almost sounds like the amp is trying to drive a short circuit. Look for something loading it down- output network wired wrong, speaker problem, etc. IMO, this self correcting business with the FETs is a tangent. There is some fundemental wiring/hookup problem, or maybe a bad part.
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Old 18th May 2008, 09:31 PM   #110
5th element is offline 5th element  United Kingdom
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Quote:
Originally posted by AndrewT
Hi,
an interesting experiment for you to try.

Switch your DMM to 200Vac and measure the DC voltage on each half of the PSU. Do this with the amp idling and normal bias. Do again with the bias turned right down to minimum. return the bias to normal and measure again when you are sending near maximum signal to the test load.

Tell us the three Vac readings and the three Vdc readings.

Then we'll try part 2 of the experiment
Amp Idling with bias set.

+rail = 64.9
-rail = -64.9

It's latish in the night and as people are turning things off the rail voltages keep rising by a small amount, as the rails increases due to the lower demand. It's only small mind you, but each rail is always within 0.1V of each other. If I turn my 2kW heater on the rail voltages drop by almost a volt anyway I'll carry on.

Amp Idling with zero bias.

+ rail = a change of 0.3 volts
-rail = a change of 0.3 volts.

As the mains voltage is fluctuating I just measured the difference between nominally and zero biased.

Amp driving 55hz sine wave into paralleled XLS just before clipping.

Both lines dropped by 5V down to 61.5V, still 1.5 V higher then the 60V they are supposed to have.
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