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Old 10th November 2008, 06:49 PM   #41
AndrewT is offline AndrewT  Scotland
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oscillation?
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Old 10th November 2008, 07:19 PM   #42
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How can I test for this? Would I need an oscilloscope?
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Old 10th November 2008, 07:34 PM   #43
AndrewT is offline AndrewT  Scotland
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search for testing for oscillation methods.
Someone suggested an AM radio and another couple posted tiny but simple circuits that respond to the VHF coming from the output.
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Old 10th November 2008, 08:12 PM   #44
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Ok I will look it up.

I have reseated the chips on bigger heatsinks but they are still getting very hot (the left more than the right).

If they are oscillating would the sound be coming out clear? Or should it be distorted? The amp outputs a very clean sound.
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Old 10th November 2008, 08:49 PM   #45
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Is the IC electrically isolated with mica or silicone washers from the heatsink?

Do you have a Boucherot cell at the output (R5 and C5 in the datasheet)?

What is your speaker impedance? Did you read pages 6 and 7 of the datasheet? What is the thermal resistance of your heatsinks + thermal grease + mica or silicone washers?
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Old 10th November 2008, 09:06 PM   #46
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Quote:
Originally posted by pacificblue
Is the IC electrically isolated with mica or silicone washers from the heatsink?

Do you have a Boucherot cell at the output (R5 and C5 in the datasheet)?

What is your speaker impedance? Did you read pages 6 and 7 of the datasheet? What is the thermal resistance of your heatsinks + thermal grease + mica or silicone washers?
Yes they are isolated.

Not sure to be honest but here is a link to the instructions for the kits I got. See the bottom for the schematic.
http://www.quasarelectronics.com/kit...c-kit/3050.pdf

I did read those pages of the data sheet, but being new to this a lot of it went straight over my head. My speaker impedance is 4 ohms (although I am going to purchase some 8 ohms speakers soon).
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Old 11th November 2008, 04:59 AM   #47
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Your kit has a Boucherot cell aka Zobel network.

The instructions that came with your kit say in the introduction that it is supposed to work from 25 V into 8 Ohm. For that a 1,4 K/W heatsink per IC is recommended. To give you an idea of the dimensions, I have a slightly smaller1,5 K/W heatsink here. It is 75 x 100 x 40 mm.

Working with 4 Ohm speakers will bring the IC into the thermal shutdown zone even with a bigger 1 K/W heatsink.

You got three options.
  • - Buy a 2 x 15 V transformer and two 1 K/W heatsinks.
    - Add fan(s) to the heatsink(s).
    - Or buy those 8 Ohm speakers now.
While the IC is internally protected against immediate failure, driving it at its limits will shorten its lifetime significantly.
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Old 11th November 2008, 05:25 AM   #48
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1875 has good performance. I have just biult an amp with two pieces of 1875. I think the datasheet from NS is very useful.:
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Old 11th November 2008, 06:33 AM   #49
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Quote:
Originally posted by pacificblue
Your kit has a Boucherot cell aka Zobel network.

The instructions that came with your kit say in the introduction that it is supposed to work from 25 V into 8 Ohm. For that a 1,4 K/W heatsink per IC is recommended. To give you an idea of the dimensions, I have a slightly smaller1,5 K/W heatsink here. It is 75 x 100 x 40 mm.

Working with 4 Ohm speakers will bring the IC into the thermal shutdown zone even with a bigger 1 K/W heatsink.

You got three options.
  • - Buy a 2 x 15 V transformer and two 1 K/W heatsinks.
    - Add fan(s) to the heatsink(s).
    - Or buy those 8 Ohm speakers now.
While the IC is internally protected against immediate failure, driving it at its limits will shorten its lifetime significantly.
Thanks for the info. The ICs are getting very hot even with no load and no input connected. I am assuming this is very bad? What could be causing it? I read somewhere last night (can't find the link) that having the output ground connected to the ground bus on the PCBs and not straight to star ground will send the chip into oscillation, could this be the reason why they are getting so hot?
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Old 11th November 2008, 08:05 AM   #50
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One step at a time. First step is to provide adequate heatsinking. Afterwards you will see, if your ICs still run hot. Until then don't bother thinking about oscillations.

The LM1875 draws 100 mA of idle current. With your 27 V power supply rails that means it has to dissipate 5,4 W without even driving a load.

Nice to touch is something between 30 C and 40 C. Somewhere between 40 C and 60 C people start to perceive as too hot to touch.

So let us assume you want to achieve 40 C at the heatsink. Your room has 20 C. The difference is 20 K. 20 K / 5,4 W = ~3,7 K/W. You need a heatsink with less than 3,7 K/W even with no load connected to perceive the temperature as comfortable.

That is not yet counting in the heat from the transformer. And you have two of those ICs in one case, which means the ambient temperature inside the case is above 20 C. The heatsink temperature will be accordingly higher.

An IC does not need as low a temperature as 40 C, although it doesn't hurt either. You should try to maintain it below 70-80 C on worst case conditions. That is too hot to touch, but a good compromise between safe operation and heatsink price and size. And you won't always be listening at worst case level.

So if you connect 8 Ohm speakers, the 1,4 K/W recommendation per IC from the instruction sheet should give good results.

By the way. Did you mount some feet below the case since you posted the photos? You should at least allow fresh air to enter through the slits at the bottom and to leave at the top to improve heat disposal.
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