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Old 4th August 2003, 06:49 PM   #11
Mad_K is offline Mad_K  Norway
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Quote:
Originally posted by mikelm



Talking about heatsinking, what material are you using for heat tranfer isolation washers ? It looks like ceramic.

if it is where did you get it from ?

cheers

mike
I you are talking about the white thick ones from an earlier pic of my BGC; they are aluminium oxide washers for to3p (to247) -These are std item that should be available by any big electronics supplier. Since I was tweaking alot I reverted to std silicone washer (There is also another reason which I won't say out loud though (

They do get hot, sometimes very hot (like when drinking cognac with friends), but they can take it
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Old 4th August 2003, 07:18 PM   #12
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Quote:
Originally posted by carlosfm


Without anything they get hot, but it won't burn your fingers.
What you used may be sufficient, as I said, even a small heatsink will do.
I used one of those very small and cheap heatsinks for the 78/79xx regulators for each pair of MUR860s and it works fine.

If they don't burn your fingers, they're probably below 45 celcius. They can run at that temp longer than you live.

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Old 4th August 2003, 08:32 PM   #13
ALW is offline ALW  United Kingdom
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I'd leave the caps off though, if it were me.

In my experience they almost always sound worse, and provide a better path for HF noise from mains supply / transformer to the PSU.

Try proper snubbers, instead, but put them at the transformer, where they belong.

http://www.hagtech.com/pdf/snubber.pdf

Peter seems to have totally misunderstood a snubber - electing to 'leave the resistor out'.

Without the resistor, it's not snubbing...

We won't touch on the star-point at the bridge instead of the caps - maybe that's history?

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Old 4th August 2003, 09:25 PM   #14
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Nevermind. Figured out it is cathode.
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Old 5th August 2003, 06:33 AM   #15
mikelm is offline mikelm  England
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Quote:
Originally posted by ALW
I'd leave the caps off though, if it were me.

In my experience they almost always sound worse, and provide a better path for HF noise from mains supply / transformer to the PSU.

Try proper snubbers, instead, but put them at the transformer, where they belong.

Peter seems to have totally misunderstood a snubber - electing to 'leave the resistor out'.

Without the resistor, it's not snubbing...
funny thing is, on simulation there is huge oscillation in the secondary after the diode swiches off, but in real life on the scope I have yet to see any visible oscillation at all.

despite this I am aware that a cap across the secondary ( I use quite a large 1uf - 4.7uf without a damping resistor ! ) brings a huge improvement in sound even though on the scope I cannot see any difference.

In theory the high value cap brings down the frequency of oscillation......even though it not visible.

could it be that the oscillation is happening in the current domain ?
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Old 5th August 2003, 07:00 AM   #16
Nuuk is offline Nuuk  United Kingdom
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Quote:
despite this I am aware that a cap across the secondary ( I use quite a large 1uf - 4.7uf without a damping resistor ! ) brings a huge improvement in sound even though on the scope I cannot see any difference.
Another one of those arguments that is quicker to prove or disprove by trying it than talking about it!

So could I ask if that is a cap between the two secondary rails or between a single secondary and the zero volts rail. And presumably, it comes before the rectifier bridge?
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Old 5th August 2003, 08:36 AM   #17
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Talking If it has a hole, it's for a heatsink

Quote:
Originally posted by janneman

If they don't burn your fingers, they're probably below 45 celcius. They can run at that temp longer than you live.
Jan Didden
Jan, I always think this way:
If a 78xx can't give 1A without heatsink why would a MUR860 give the 8A without any heatsink?
The same for a BUF634, don't expect them to deliver the 250ma they say on the datasheet without heatsink.
As simple as that.
I tested the MUR860s without any load and they got slightly warm, so I suppose that when you turn up the volume on your amp they may become quite hot.
It's not a matter of reliability.
If these things were made to be used with a heatsink, why not use it?
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Old 5th August 2003, 08:49 AM   #18
UrSv is offline UrSv  Sweden
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Default Re: If it has a hole, it's for a heatsink

Quote:
Originally posted by carlosfm


Jan, I always think this way:
If a 78xx can't give 1A without heatsink why would a MUR860 give the 8A without any heatsink?
The same for a BUF634, don't expect them to deliver the 250ma they say on the datasheet without heatsink.
As simple as that.
I tested the MUR860s without any load and they got slightly warm, so I suppose that when you turn up the volume on your amp they may become quite hot.
It's not a matter of reliability.
If these things were made to be used with a heatsink, why not use it?
I always look at it this way. When a 78XX gives 1 A with a drop of 10 V it dissipates 10 W all the time. When a MUR860 gives 8 A for a little while with a drop of 0.6 V it dissipates 4.8 W peak and significantly less constantly. When running an IGC it is even FAR less. Thus I do not feel the need for a heatsink. However, I do feel the need to mount them securely and that can be done on a piece of aluminium...
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Old 5th August 2003, 09:22 AM   #19
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Default Re: Re: If it has a hole, it's for a heatsink

Quote:
Originally posted by UrSv

I always look at it this way. When a 78XX gives 1 A with a drop of 10 V it dissipates 10 W all the time. When a MUR860 gives 8 A for a little while with a drop of 0.6 V it dissipates 4.8 W peak and significantly less constantly. When running an IGC it is even FAR less. Thus I do not feel the need for a heatsink. However, I do feel the need to mount them securely and that can be done on a piece of aluminium...

As Mad_K said, sometimes they get very hot.
I tested them and got to the same conclusion.
UrSv, you're talking of a power amp, not a transistor radio or a preamp.
To be certain, you need to test, not just come with theories.
What makes you think your IGC doesn't need current?
I see you're one of those guys who think the power op-amps (like the LM3875) don't even need a heatsink, just put it on the bottom of a thin case and that's it.
Put Ben Harper playing at half the volume and you'll see the diodes getting hot and the power op-amps getting really hot.
And connect a pair of 4 ohm speakers to your Gainclone and you'll see how it gets hot.
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Old 5th August 2003, 11:15 AM   #20
ALW is offline ALW  United Kingdom
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Quote:
funny thing is, on simulation there is huge oscillation in the secondary after the diode swiches off, but in real life on the scope I have yet to see any visible oscillation at all.
I've certainly seen it, by triggering on the DC sawtooth and then viewing the sinusoid from the transformer on a 'scope. It can be quite high in frequency though, and may not last that long so takes some careful 'scope work to view. Unless you measure the transformer characterstics, you won't know what you're dealing with. In a good TX with little stray C, the frequency may be close to the 'scope's bandwidth?

It will vary dependant upon the real-world charcteristics of the transformer though, but as I say, Peter's implementation does little to damp any of this, but does provide a better path for the oscillation or noise (if present) into the equipment being powered. That's not what I want to do in a PSU, generally.

As for a cap sounding better, that may be - the frequency will be lower and may be more readily filtered. Surely it'as better to eliminate it entirely though - using a simple RC isn't exactly difficult.

The C part isn't critical, it just has to have low Z at the resonant frequency.

Andy.
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