Heatsink Mounting

[linguist]

When mounting a heatsink to the Transistors of an amp say around 200W Max, Is it advisable to have the Transistors & thermal washers on one side of a sheetmetal chassis & the heatsink on the other side of the steel chassis?
So the chassis is sandwiched between the transistors & the heatsink--see attached drawing..

With it done this way how efficient is it compared to having the Transistors mounted directly to the heatsink.

Would it be better if the chassis was aluminium sheet?

Cheers

 

[Sceadwian]

The thermal conductivity of steel is about 1/7th that of aluminum, and 1/10th that of copper. So it's pretty bad efficiency wise, you're effectively crippling your heatsink
The transistor should be mounted directly to the heatsink with only as much thermal compound in between them to avoid air gaps, if electrical isolation is required get isolated transistors or isolate the heatsink itself.

If the case is aluminum you're MUCH better off, probably much so because the case itself will supplement the heatsink.

I can guarantee you won't be dissipating 200 watts of power in a 200 watt amp, so you need to first figure out how much thermal energy is going to be produced, the temperature the transistor must be maintained at and the thermal constants of the heatsink itself.

True heat management is not easy.

 

[linguist]

Thanks Sceadwian,

I thought it may be an issue, that's why I asked the question.

Thanks again

Cheers

 

[audioguru]

I can guarantee you won't be dissipating 200 watts of power in a 200 watt amp, so you need to first figure out how much thermal energy is going to be produced, the temperature the transistor must be maintained at and the thermal constants of the heatsink itself.

Some fools make a class-A amplifier that has low output power but enough heating for their entire neighbourhood.

 

[linguist]

audioguru,

I just built a mono amp, the power amp section at this stage just as a test amp for listening, is this one a neighourhood heater as well?
I'll attach the schematic, it looks ok to me.

I'll attach a sim of a crossover circuit & how I was planning on hooking up the speakers I have available at this stage.

Can I do things this way, is the crossover circuit a starting point or do you see it's not correct for what I have. I don't fully understand exactly what I need with the crossover circuit.
I understand I need a low pass & high pass filter but where is best to set them up for this application & speakers I have at hand?
Where would you start?

Cheers

 

[audioguru]

Your amplifier is class-AB, not class-A so it won't heat your entire neighbourhood.
But it uses diodes to set its idle current instead of having a trimpot so with some diodes and transistors it will be pretty hot and with other diodes and transistors it will be cool but will have crossover distortion.

Woofers and tweeters have datasheets with recommended crossover frequencies and slopes.
With only a capacitor as the crossover parts for your tweeters then they might be quickly destroyed by powerful low frequencies.
I calculate a crossover frequency of 11.8kHz for your tweeters which is much too high.

I didn't look at the lowpass filter for your woofers.

I don't know why you are attenuating the tweeters so much. Maybe because your woofers have poor efficiency?

 

[kinarfi]

Tiny hijack about heat sinking, tab mounted directly to heat sink with compound is best, but if you need isolation, using thermal pads or mica, what the best type of isolator.
Thanks,
Jeff

 

[linguist]

Thanks for the information Jeff.

audioguru,

It's taken me a while to try & get some data for these speakers, I only have the data for the woofers & nothing at all on the tweeters from the supplier--manufacturer.

I have attached it.

On a different page of the datasheet it says the Frequency response of the woofer is 35Hz-4kHz, but on the sticker on the rear of the speaker it says 35Hz to 6kHz.

To be honest I need to learn a lot more about speakers & in general the crossover circuits before I could attempt to answer anything you ask about it.
I wouldn't be talking from experience but from out my - you get it!

That's why I'm asking you

I have been reading about it all, reading, understanding & doing in a short period of time is a little different.

I need to learn about this, so:

Looking at the data I have for the speaker--driver parameters.

Where they have Impedance at Fo (Zo) 63.4Ω
I take it that this is the speaker Impedance at X Frequency, Is this tested at 1kHz or is this the Impedance at their recommended crossover frequency?

Instead of trusting these figures is it best to test the speakers I have to find the Impedance at the Crossover frequency as it appears i'll have to test the tweeters anyway.
If I understand correctly I need to know the Impedance at the crossover frequency to be able to start with anything--Correct?

One thing I read said to have the crossover frequency at around 600Hz another thing I read said to have the crossover frequency at 2kHz.

I understand that this is a personal preference thing as audio is, but:

Where to start?
Obviously I want to avoid any damage to start with.

Cheers

 

[audioguru]

On a different page of the datasheet it says the Frequency response of the woofer is 35Hz-4kHz, but on the sticker on the rear of the speaker it says 35Hz to 6kHz.

Those statements mean nothing because they do not say in which size and type of enclosure (maybe the entire trunk of a car?) and do not sat plus and minus 3dB (or plus and minus 12dB?).
Good speakers have a graph showing their frequency response in a recommended enclosure. All woofers have severe trouble at 3khz to 6kHz with the upper frequency depending on thier design. You want the crossover network to attenuate the troubled frequencies.

Where they have Impedance at Fo (Zo) 63.4Ω. I take it that this is the speaker Impedance at X Frequency, Is this tested at 1kHz or is this the Impedance at their recommended crossover frequency?

Fo is the frequency where the speaker resonates in free air without an enclosure. An enclosure raises the frequency a lot. The datasheet for your woofer says that Fo is at 34.8Hz. It will be about 60Hz to 80Hz in a sealed enclosure and will be the frequency where the lower frequencies have reduced level dropping at 12dB per octave.

Instead of trusting these figures is it best to test the speakers I have to find the Impedance at the Crossover frequency as it appears i'll have to test the tweeters anyway.
If I understand correctly I need to know the Impedance at the crossover frequency to be able to start with anything--Correct?

Speakers are inductive. The increased impedance caused by inductance ruins the design of a passive crossover network. Good speakers use a simple Zobel network (resistor in series with a capacitor) is used to make the total impedance flat and that is required.

One thing I read said to have the crossover frequency at around 600Hz another thing I read said to have the crossover frequency at 2kHz.

Without knowing if the speaker system is 2-way or 3-way and without knowing the spec's of the speaker drivers then the statement is useless.

Where to start?

Buy good tweeters that have a detailed datasheet and look at every article and project in this link: **broken link removed**

 

[linguist]

Thanks audioguru,

Yes I see what you mean with the data available for the speakers, it's useless & of no real use, I was looking at some better branded speakers & the available data for them. Like most things these days the speakers I have are cheap rubbish.

I have been looking into the Zobel network already & also different types of enclosurers.

Unfortunately I have to use the speakers I have at the moment until I can upgrade but I suppose for learning purposes it may work out ok.
If I can sort this mess out & get some descent results it will be worth the effort.

Later on I want to add another channel to the amp & the I will buy some quality speakers to go with the setup.

I will read Rod Elliot's site, it's becoming the bible.

Thanks again,

Cheers