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Old 18th January 2011, 04:39 AM   #9971
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Quote:
Originally Posted by Nikon1975 View Post
I am even more puzzled, I thought that the increase in temperature had the effect of lowering the current. After few hours of use and test I had 630 mV, while yesterday I set it around 590 mV. The DC offset is stable.

I am trying to understand if I am figuring an issue or not. The heatsink is not too hot, I can put my hand on it indefinitely, but inside the chassis I have 50 C.

Thanks,

D.
So for the time being I decided that this is a non-issue, and will live happily until I check the bias again in few weeks. I made a quick check with my limited spectrum analyzer, and I do not really have the same figures that NP has, the second harmonic is still dominant, and distortion is higher on the right channel.

Just to be sure I understand how things works: is it correct to say that the DC offset is the effect of the bias difference between the two mosfet ? If I have 0.6 V on one side and zero offset, can I assume that the other mosfet is also close to 0.6 V ?

I also would like to switch on the amp with a relay. I bought two, just for testing. One is a solid state relay and the second one a conventional relay, both rated 25A. I install them on the live.

Is it normal that I can measure AC also on the coil ?

Any "sonic" reason why I should not have a relay on the main ?

Thanks,

Davide
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Old 18th January 2011, 02:47 PM   #9972
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Quote:
Originally Posted by Nikon1975 View Post
Just to be sure I understand how things works: is it correct to say that the DC offset is the effect of the bias difference between the two mosfet ? If I have 0.6 V on one side and zero offset, can I assume that the other mosfet is also close to 0.6 V ?
...
Thanks,

Davide
Yes and yes.
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Old 19th January 2011, 03:42 AM   #9973
Kilroy is offline Kilroy  United States
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I have my 4 f5 channels done and I have my power supply sorted...

But I guess I've run into the dreaded heat-sink decision roadblock. I had some more creative ideas that ultimately wouldn't have worked out so I've turned to the more traditional solutions. But expense is an issue and from all my research Heatsink USA seems to be the most cost effective for the state-side crowd. I haven't had much luck finding anything on ebay and I don't want the finished product to look like it fell out of the back of a truck...

So do you guys think an 8" slab of this will be sufficient per board?

10.080 inch profile custom cut aluminum heatsink

It looked like it to me but I understand that simply buying a longer piece doesn't necessarily mean it will move enough heat so I wanted to run it by the experts.
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Old 19th January 2011, 04:09 AM   #9974
dazed2 is offline dazed2  Canada
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What about these guys for heat sinks?
Par-Metal
They are in the US as well. I ordered from them.
The 11" profile is $4.5 /in
the 7" profile is $2.5 /in.
Its the same price as Heatsink USA, but you can get 11" instead of 10"
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Old 19th January 2011, 04:19 AM   #9975
udailey is offline udailey  United States
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The par metals extrusions will not dissipate the heat that Heatsink USA sinks can handle. Look at the height of the HS USA vs PM. You dont want the fins ultra close together either, thats for forced air cooling. To tight together and they wont let the air flow up the fins efficiently. Between the two I would go HS USA in a heartbeat.
Uriah
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Old 19th January 2011, 11:42 AM   #9976
AndrewT is offline AndrewT  Scotland
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I see 0.7C/W for 3" long profile for HeatsinkUSA.
A 12" length should be about half of this, i.e. 0.35C/W

I cannot see any spec for Parmetal.

I expect that HeatsinkUSA can dissipate a lot more heat than ParMetal
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Old 19th January 2011, 11:59 AM   #9977
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Speaking about heat, I have the Cviller boards v1.0 from diyaudio store. Is it better to have the mosfet up or down ? From the point of view of heat dissipation is better down, but like this all the other component are also heated.

What do you think ?

D.
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Old 19th January 2011, 01:05 PM   #9978
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I put them down.
The heat of mosfets goes up, flowing through the gap between board and heatsink
.
No problem.

Edit:the board being parallel to the heatsink.
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Old 19th January 2011, 01:49 PM   #9979
udailey is offline udailey  United States
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Doesnt really matter since the air moving over the mosfet is taking away nearly none of the heat. Facing down your other components would get warmer like you said but probably the only one to worry about would be the thermistor and I assume you are going to put it on or nearly directly on the mosfet itself. The sink is taking it away and the air moving over the sink is what really matters. Place them about one third of the way up from the bottom of the sink.
I forget which site its on, but on one of the major heatsink manufacturers sites there is a java or flash based system that will simulate dissipation for you. So you find a similar profile (it has to be very very similar) and then tell it how big your mosfet is and how much heat it will generate then the program shows you how hot it will be at different points on the sink.
I cant find that tool anymore on either Aavid Thermalloy or on Wakefield's sites. However, if you go to Aavid's site you can choose an extrusion and then it will show you the average dissipation at different air velocities (choose natural convection) and also at different ambient temps.
I think this link will work to show you what I mean
Extrusion Profiles
For good information on heatsinks and frankly to answer just about any question about them in this application you can go here for a lengthy but direct discussion
ESP - Heatsink design and transistor mounting
Pay special attention to how to mount the transistors.
Uriah
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Old 19th January 2011, 02:00 PM   #9980
udailey is offline udailey  United States
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That Rod Elliot site is just awesome. The heatsink article, reading it now, is fantastic. For instance check this out.. Bet you didnt think painting your sink with black oil paint would dissipate more heat than black anodized and consider that your heatsink from both places you mentioned, par metals and HS USA, will NOT be anodized.
Table 3 - Emissivity of Various Surface Treatments
Surface Emissivity

Polished aluminium 0.05
Polished copper 0.07
Rolled sheet steel 0.66
Oxidised copper 0.70
Black anodised aluminium 0.70 - 0.90
Black air-drying enamel 0.85 - 0.91
Dark varnish 0.89 - 0.93
Black oil paint 0.92 - 0.96

He also has a heatsink calculator http://sound.westhost.com//heatsink.zip
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Last edited by udailey; 19th January 2011 at 02:06 PM.
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