Why so few higher powered Class D stereo amp boards?

[Millstonemike]

I had been hesitant to go the full 24v because of the 'warnings' on the product page, even though you've even previously tried to asswage my fears Mike... but you're absolutely right... it seems that these amps can take it just fine.

I've been doing some near full-output temperature testing on the TDA7492 amp: https://www.parts-express.com/tda749...2x50w--320-606 and with the video player preamp faceplate feeding the source material it's peaking at around 120 or so degrees on the heatsink. That's pretty chill for a legit' amplifier producing real output. I like that little amp a lot more now. It's not until the very last three or four clicks on the preamp that it starts sounding harsh, but by then the woofers are going pretty much berserk at that point so it's a wash. Of course, I can get the preamp turned up to full and the ND91's are still alive, so the preamp determines a lot in this equation, but I think the amp is really doing a phenomenal job putting out some real power in a tiny size.

TomZ

Are you measuring temp with the unit open? There may be a significant difference when it's buttoned up. Did you mount the amp at the top of the table radio - where heat collects?

I would mount the amp so the heat sink fins and channels are vertical at the bottom of the cabinet. That creates a passive airflow over the heat sink as it heats the surrounding air.

See my link (since TT won't allow pic uploads - always a "corrupted file error").

[tomzarbo]

Are you measuring temp with the unit open? There may be a significant difference when it's buttoned up. Did you mount the amp at the top of the table radio - where heat collects?

I would mount the amp so the heat sink fins and channels are vertical at the bottom of the cabinet. That creates a passive airflow over the heat sink as it heats the surrounding air.

See my link (since TT won't allow pic uploads - always a "corrupted file error").

Hey Mike,

I have the amp fastened to the top of the cabinet using 1/4" (roughly) brass standoffs. It is positioned towards the rear of the cabinet as well. It was the best place to locate it to keep it out of the way placement-wise considering the other components in the space. I agree though, on the bottom would be better thermally speaking. Here is a link to my current "Bantam Table Radio" project and how it's mounted. (the radio is upside down in the pic.) http://techtalk.parts-express.com/fo...26#post1423326

That temperature measurement is with the amp mounted permanently inside the cabinet. The entire bottom of the center portion of the cabinet is open (there is a screen covering it but it has airflow) and I have installed a soffit vent right next to the amp's heat sink fins to allow any hot air to escape.

I can't post any pics of this at the moment until the forum gets fixed, but you can see the heat sink fins right next to the vent on the rear of the cabinet. After it's been running for awhile at high volume you can feel a bit of warmth coming from the vent area. Hopefully this will be sufficient to keep the amp cool when it's running at warp-factor 10 for long periods.

I'm thinking that 120 degrees max would lie within it's normal operating temperature? I think you mentioned before that the chip itself would shut down around 150 degrees or so, and the heat sink would not probably get that hot, so maybe at high volume I'm getting closer to maxing it out than I think. Hopefully, adding the soffit vent right next to the heatsink fins will keep things 'cool enough.'

Have to say though, it really sounds good. If anyone remembers hearing the 'Bantams' from DIY NY earlier this year, this is them -- just closer together. With this preamp, the TDA7492 amp really does the speaker justice at 24v and I couldn't be happier with how it sounds.

I originally mentioned wanting to do two versions of this radio, this one, and another with a built-in subwoofer, possibly taller and deeper. I think I'll just stick with this setup for now as it just works so well. I think for a radio, this has bass enough to satisfy nearly anyone.

I ended up using some black aluminum rigid grill-type material to make a covering for the open underside of the enclosure just to keep anything from getting in there and from any wires from peeking out. I covered the edges with a layer of black electrical tape to conceal the sharp edges. I was able to cut it easily with scissors.

Hopefully soon I can post some pics of the last few detail items.

TomZ

[Millstonemike]

Hey Mike,

I have the amp fastened to the top of the cabinet using 1/4" (roughly) brass standoffs. It is positioned towards the rear of the cabinet as well. It was the best place to locate it to keep it out of the way placement-wise considering the other components in the space. I agree though, on the bottom would be better thermally speaking. Here is a link to my current "Bantam Table Radio" project and how it's mounted. (the radio is upside down in the pic.) http://techtalk.parts-express.com/fo...26#post1423326

That temperature measurement is with the amp mounted permanently inside the cabinet. The entire bottom of the center portion of the cabinet is open (there is a screen covering it but it has airflow) and I have installed a soffit vent right next to the amp's heat sink fins to allow any hot air to escape.

I can't post any pics of this at the moment until the forum gets fixed, but you can see the heat sink fins right next to the vent on the rear of the cabinet. After it's been running for awhile at high volume you can feel a bit of warmth coming from the vent area. Hopefully this will be sufficient to keep the amp cool when it's running at warp-factor 10 for long periods.

I'm thinking that 120 degrees max would lie within it's normal operating temperature? I think you mentioned before that the chip itself would shut down around 150 degrees or so, and the heat sink would not probably get that hot, so maybe at high volume I'm getting closer to maxing it out than I think. Hopefully, adding the soffit vent right next to the heatsink fins will keep things 'cool enough.'

Have to say though, it really sounds good. If anyone remembers hearing the 'Bantams' from DIY NY earlier this year, this is them -- just closer together. With this preamp, the TDA7492 amp really does the speaker justice at 24v and I couldn't be happier with how it sounds.

I originally mentioned wanting to do two versions of this radio, this one, and another with a built-in subwoofer, possibly taller and deeper. I think I'll just stick with this setup for now as it just works so well. I think for a radio, this has bass enough to satisfy nearly anyone.

I ended up using some black aluminum rigid grill-type material to make a covering for the open underside of the enclosure just to keep anything from getting in there and from any wires from peeking out. I covered the edges with a layer of black electrical tape to conceal the sharp edges. I was able to cut it easily with scissors.

Hopefully soon I can post some pics of the last few detail items.

TomZ

TOm,

I went to your build thread to see the amp's position in the cabinet. Didn't see the vent so that's better than I thought.

Just to make sure ... when you post 120^o, is that Celsius or Fahrenheit? When I quoted the datasheet's auto shutdown temp, 150^o, that's in Celsius. Operating temp range goes up to 85^o C (185^o F).

And your right. The temp in the chip will be hotter than measured at the heat sink.

[neildavis]

Any opening will significantly lower the heat at the amplifier heatsink. It's all about "thermal resistance"--if the thermal resistance to ambient air drops to near zero, the temperature will rise quite a bit.

I've been using heat pipes to draw the heat to an aluminum plate in the back of the chassis. The 4mm flat pipes available at Digikey fit into the heatsink fins of some of these amps. You can buy a tubing bender from Home Depot to help shape the heat pipe, but with the flat pipe you just need to wrap it around some tubing--the flat heat pipes are easy to work with.. I mix up some of the white thermal paste with 5 minute epoxy (about 1 to 1 mix) for DIY thermal epoxy. You can see the heat pipes in this picture, but this was before using the epoxy:



Next time I'll buy a shorter heap pipe . I also used some thick silicon thermal pads to heatsink the power supply to the mounting plate. The back panel gets warm, so I know the heat sinking is working.

[JRT]

I’d like to see more digital amps that are rated, even at 50 watts, but with less than 1% distortion. Many of these digital amps are rated at at 10% for their advertised power output.

10% is a good point on the curve to indicate that the amplifier is well into clipping, and is good for assessing headroom. At half of that power level, at signal levels -3_dB below the signal level associated with 10% THD, the amplifier should be operating below clipping and producing much less nonlinear distortion.

To avoid the nonlinearities associated with clipping, I would argue for 0_dBFS signal levels on tweeters not exceeding a threshold of -6_dB below the signal level associated with 10%_THD, and amplifiers for other drivers not exceeding a threshold of -3dB below the signal level associated with 10% THD.

Although in music the average power at high frequencies is much lower than it is in lower midrange, the brief crests can be higher. For tweeters, signal crests can reach +3_dB above 0_dBFS, for peaks centered between samples at frequencies near 1/2 sample rate. For midrange and below, crests can extend very much longer than high frequency crests, but do not rise significantly above 0_dBFS in level.

[tomzarbo]

TOm,

I went to your build thread to see the amp's position in the cabinet. Didn't see the vent so that's better than I thought.

Just to make sure ... when you post 120^o, is that Celsius or Fahrenheit? When I quoted the data sheet's auto shutdown temp, 150^o, that's in Celsius. Operating temp range goes up to 85^o C (185^o F).

And your right. The temp in the chip will be hotter than measured at the heat sink.

Hey, we've got pics again!

Mike, that was

Fahrenheit. So the chip can get to 185 degrees F before it cuts out? I don't think I'm getting close to that if I had to guess on the chip itself.

Here's a pic of the vent and you can see the little heat sink behind it. Hopefully it does the job.

Thanks for the input and all the guidance too.

TomZ

[tomzarbo]

Any opening will significantly lower the heat at the amplifier heatsink. It's all about "thermal resistance"--if the thermal resistance to ambient air drops to near zero, the temperature will rise quite a bit.

I've been using heat pipes to draw the heat to an aluminum plate in the back of the chassis. The 4mm flat pipes available at Digikey fit into the heatsink fins of some of these amps. You can buy a tubing bender from Home Depot to help shape the heat pipe, but with the flat pipe you just need to wrap it around some tubing--the flat heat pipes are easy to work with.. I mix up some of the white thermal paste with 5 minute epoxy (about 1 to 1 mix) for DIY thermal epoxy. You can see the heat pipes in this picture, but this was before using the epoxy:



Next time I'll buy a shorter heap pipe . I also used some thick silicon thermal pads to heatsink the power supply to the mounting plate. The back panel gets warm, so I know the heat sinking is working.

Neil, that is a cool way to get rid of some heat. (I wasn't trying to be funny there)

I like how that works, and I can totally understand how it would do the job. I solder a wire holding it 2-3 inches from the iron, and in a few seconds it's getting pretty dang hot. That looks to be a great way to move heat around to where it can go away easier. I'm going to use that in the future I think. I'm glad I have smart friends to help me with all these projects... or I would have probably burned my house down by now.

TomZ