TDA7294 Amplifier. Best place to add a 9v fan on the circuit??

[guidoilieff]

Hi. I made this amplifier. Everything is ok but it gets too hot even with the heatsink.

http://www.videorockola.com/proyectos-electronicos/amplificadores-audio-potencia/construya-un-amplificador-de-90-watts/
http://www.mediafire.com/file/lu8xwbidnx1c5zy/tda_90w_mono.zip


I want to add a cpu fan but don't want to add an extra transformer. Where can I hook up the V+ of the fan on the board without adding noise to the signal?



Thanks

[vigilante397]

My work firewall blocks both of those links so I can't look at the documents until I get home, at which point I'll see if I can actually answer your question. But I have a quick question for you:

You said it gets too hot, how hot is too hot? Looking at the datasheet for the chip thermal shutdown is at 145 degrees celsius, is it getting to there? Also, how big of a heatsink are you using?

[guidoilieff]

My work firewall blocks both of those links so I can't look at the documents until I get home, at which point I'll see if I can actually answer your question. But I have a quick question for you:

You said it gets too hot, how hot is too hot? Looking at the datasheet for the chip thermal shutdown is at 145 degrees celsius, is it getting to there? Also, how big of a heatsink are you using?

Well, I don't have any tools to measure temperature, but almost too hot to keep your fingers on the heatsink. The aluminium heatsink is 5x8x1cm which I know it isn't big enough but that's the biggest I can fit on the enclosure I made. My solution is adding an external fan mounted on the back panel.

Here's the schematic:

[vigilante397]

Okay, seeing a schematic definitely helps. The first thing I'm seeing that looks promising is that 18VAC. Where is that coming from? Is that another transformer tap? How much current load can that handle? If it's a fairly beefy transformer you could run another rectifier off of it (best to isolate it from the mute and standby voltage divider network) and either put a 9V regulator or a voltage divider there to get down to 9V. Though I guess this also begs the question, do you already have the 9V fan? How much current is it going to be taking?

Also, with regards to the heatsink, I would expect it to get hot. You said almost too hot to keep your fingers on the heatsink, and while this isn't an exact number it makes me think it's still within operating range. Never hurts to be safe though

[guidoilieff]

Okay, seeing a schematic definitely helps. The first thing I'm seeing that looks promising is that 18VAC. Where is that coming from? Is that another transformer tap? How much current load can that handle? If it's a fairly beefy transformer you could run another rectifier off of it (best to isolate it from the mute and standby voltage divider network) and either put a 9V regulator or a voltage divider there to get down to 9V. Though I guess this also begs the question, do you already have the 9V fan? How much current is it going to be taking?

Also, with regards to the heatsink, I would expect it to get hot. You said almost too hot to keep your fingers on the heatsink, and while this isn't an exact number it makes me think it's still within operating range. Never hurts to be safe though

Its only a 4amp transformer. Pretty heavy. My guess is that the fans draw less than 0.5amps.

I think the 18v is a typo error, it should say 28v. Its the ac before the rectifying diodes.


Yeah, actually is painfully enough to keep your fingers but it burns.

[maartendh]

Have a look at the ESP website, there is some good information about chip-amplifiers there and their use as an amp for guitar.

TDA 7294 in a guitar amp is quite good (have one myself), but 90 Watts is a bit optimistic. Probably less gain (ratio between R2 and R3 here is 56, better would be about half of this) and a lower supply voltage (about 30 volts + and - each at max) will make your amp last longer and also will prevent that your the temperature safety in this chip will kick in during a gig.

If you need more than 60 watts with the TDA 7294, you should build a second amp.

Maarten.

[PRR]

That door-trim is much too thin for a 90 Watt audio amplifier. Go in your attic or thrift store and look at heat sinks in commercial amplifiers of similar power. (Or look how thick the metal is on a CPU heat-sink.) (A very large CPU sink with fan may be a better sink for this project than your thin aluminum.)

Use a Case Fan. They are usually 12V, not 5V like CPU fans, which gets you closer. A typical PC case fan is 12V 0.15A. The numbers are usually printed right on the fan label.

A simple series resistor is ample to drop voltage to a small DC fan.

Given 37V, needing 12V, we must drop 25V. At 0.15A, the resistor is 25V/0.15A or 167 Ohms, and must waste-off (heat up) like 25V*0.15A or 3.75 Watts. Round-up the resistor, 180 Ohms may be a common value. At-least double the Watts for long life: 7.5W which means buy a 10 Watt part.

Mostly, wall-power amplifiers turn to 120/240V AC fans right off the wall power. You can't find these in every PC scrap pile, but they tend to be simpler fans with more tolerance for old oil and dust, and no power-waste resistor.
Quiet Muffin Fan, 115V 120V AC 80mm x 38mm Low Speed, for DIY Cooling, Amazon, $17.99

[guidoilieff]

That door-trim is much too thin for a 90 Watt audio amplifier. Go in your attic or thrift store and look at heat sinks in commercial amplifiers of similar power. (Or look how thick the metal is on a CPU heat-sink.) (A very large CPU sink with fan may be a better sink for this project than your thin aluminum.)

Use a Case Fan. They are usually 12V, not 5V like CPU fans, which gets you closer. A typical PC case fan is 12V 0.15A. The numbers are usually printed right on the fan label.

A simple series resistor is ample to drop voltage to a small DC fan.

Given 37V, needing 12V, we must drop 25V. At 0.15A, the resistor is 25V/0.15A or 167 Ohms, and must waste-off (heat up) like 25V*0.15A or 3.75 Watts. Round-up the resistor, 180 Ohms may be a common value. At-least double the Watts for long life: 7.5W which means buy a 10 Watt part.

Mostly, wall-power amplifiers turn to 120/240V AC fans right off the wall power. You can't find these in every PC scrap pile, but they tend to be simpler fans with more tolerance for old oil and dust, and no power-waste resistor.
Quiet Muffin Fan, 115V 120V AC 80mm x 38mm Low Speed, for DIY Cooling, Amazon, $17.99

Haha yeah I know. A have a much bigger heat sink but it wont fit.


I meant a case fan. I have a few but the one I like is a Yate Loon made in taiwan 12v dc 0.12A. Its heavier than the rest, noiseless and waay older.
That quiet muffin fan is gorgeous but too expensive in Argentina. Found a suspicious generic one that may be worth it.


A rectifier and a 12v regulator would waste 3.75w too?

https://www.jameco.com/Jameco/Products/ProdDS/889305.pdf
(A L7812 with a to-220 package says "Power dissipation Internally Limited" and "Thermal resistance junction-case 5 °C/W")

I don't know what unit is that... centigrades or watts?

[PRR]

> "Thermal resistance junction-case 5 °C/W"

Junction to case... and then what? Where you going to put that heat? In most cases you have to put it to AIR. This number "assumes" an Infinite Heatsink. Even a VW Beetle or air-cooled Porsche finned-head is not an infinite heat sink.

Next line: The naked TO220 package "junction to AIR" is 50 degrees Centigrade per Watt. So one Watt of internal heat makes it 50 degrees hotter. At 25 deg C room temperature it becomes 75 deg C, about as hot as you can touch for a few seconds. You can run some hotter (to 150 deg C!), but really 1 or 2 Watts is all you can dissipate in a naked TO220.

Jameco heatsink Part no.: 1115643. 24.4°C/W. This plus the 5°C/W of the TO220 package makes 30°C/W. Now 2 Watts is not too hot. 3 Watts will work for years but burn your finger. Still quite shy of 3.75W. (The regulator will throw about 5%-10% *more* heat because it takes some power to run its internal bits.)

Jameco Part no.: 696626. 2.6°C/W. So about 8°C/W including the TO220 package. This can go over 6 Watts safely. It is also a more costly part. Yet sometimes you need the beefy part. A one-chip Power amp certainly needs a very heavy heatsink. The heat is large and very concentrated. An old-school power amp would spread heat over two transistors you could space-apart on a light sink. And some power chips have higher losses than a couple transistors.