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Old 1st March 2003, 02:31 PM   #1
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Default JLH Class A 15 Watt Amp Preset Help

Hi Guys
I am just getting the parts together for the 15 watt JLH As in Fig 2 on Geoff Moss’s Site
My nologe of electronics is not as good as I would like but can build this amp My question is about the Pre sets R1 & R2 as to the position they should be set at be four switching the amp on I am told that R1 should be set in the 12 o’clock potion And R2 set fully ante clockwise before setting up to required Iq

I will be using a 18-0 -18 volt 300 VA transformer mj15003 Trans
1.1 CW Heat sinks as these are the largest that I have
Amp will be running into 6 ohms at about 2.8 Iq
Can you let me now if this is ok?
Thanks
Regards
Colin
http://www.gmweb.btinternet.co.uk/jlhupdate.htm
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Old 2nd March 2003, 04:22 PM   #2
Taco is offline Taco  Netherlands
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Do you have a 1 C /Watt sink per transistor? I use 0.6 and they are very hot (2n3055 transistors). For details powering this beast up see the following link:
http://utopia.knoware.nl/users/cfw/amp/
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Old 2nd March 2003, 05:17 PM   #3
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Default JLH Heatsinks

Hi Taco

Yes I have 1 C /Watt sink per transistor mj15003 I just reserved email from TimA saying it will be hot but should be ok

Geoff’s class a amp site
Hear is the email that Goeff sent me

Let's try to put some figures together, based on the information you have
sent.

With a supply rail or 23V and a total quiescent current of 2.5A, the current
in each of the output transistors will be 1.25A and the maximum dissipation
will be 23 * 1.25 = 28.75W. We will use 30W to allow for a small safety
margin.

For reliability, the output transistor junction temperature should be kept
to a maximum of 100degC and I will assume a maximum ambient temperature of
25degC. This gives a maximum permissible temperature rise from ambient to
junction of 75degC. With a 30W dissipation, the thermal resistance, air to
junction, must not exceed 75 / 30 = 2.5 degC/W.

The MJ15003 has a case to junction thermal resistance of 0.7degC/W and the
TO3 mounting insulator will contribute a similar amount. This means that the
heatsink must be rated at 2.5 - 0.7 - 0.7 = 1.1degC/W. At this figure, the
heatsink will be at 30 * 1.1 = 33degC above ambient, or 58degC if the
ambient is 25degC. A little warm (or rather hot to the touch) but not unsafe
unless you have small children or nosey pets around. Personally, if
possible, I like to keep heatsinks to a maximum of around 50degC but this
would need a rating of 0.8degC/W for each output device.

For the voltage regulators, the maximum volt drop will be between 8V and 10V
(when the mains is 10% high) and the current will vary between 2.5A and
3.3A, giving a dissipation between 20W and 33W. We will take a mean of 27W
for the purpose of calculating heatsink size. For the same 75degC rise above
ambient, the maximum junction to air thermal resistance is 75 / 27 =
2.8degC/W. The thermal resistance of the LM338K is 1degC/W and the insulator
0.7degC/W which means that the heatsink must be at least 1.1degC/W. (I'm
glad it worked out the same as the amp heatsink since I usually recommend
the same size for each output device and regulator :-).

Right, having determined that you need six 1.1degC/W heatsinks for each amp,
where can you obtain them? I haven't checked the RS Components website
recently (www.rswww.com) but it might be worth a look. When I last checked
(Jan this year), CPC plc (08701 202530) had some 1.0degC/W heatsinks (120
long x 100 high x 37 fin depth) at £6.55 + VAT for quantities of 5+.
Alternatively, they had some 0.5degC/W ones (125 x 150 x 50) at £13.61 + VAT
for 5+. Three of these should fit on each side of your enclosure and would
provide the necessary dissipation for two channels (including the power
supply). Finally, you could try Distel (www.distel.co.uk) for
surplus/second-hand heatsinks.

Hope this helps,

Regards

Geoff
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Old 3rd March 2003, 10:31 AM   #4
Taco is offline Taco  Netherlands
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Let us know how the sound is, when you're done with building.

BTW. nice avatar the PCB's for your JLH if I see it correctly?
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Old 3rd March 2003, 08:45 PM   #5
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Default JLH Heatsinks & presets

Taco
Thanks for the encouragement still a bit unsure of my Heatsinks 1.1CW though I Suppose I Can use a couple of small computer Fans until I have the funds for the 0.5 cw sinks yep the avatar is the JLH not my boards though will keep you all posted on progress as i may requre some more help along the way
Thanks

Colin
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