Folsom's great little 7297 Chip Amp

sorinv · « **Reply #260 on:** 23 Mar 2016, 08:41 pm »

I am having a hard time finding a transformer for the Antipole power supply. Hammond costs $25 to be shipped to Europe, which is ridiculous.

Anybody using toroidal transformers, like this one?

I am planing to build the power supply in a different chassis than the amp, so transformer would not be close to the audio path.

Any other suggestion of a good transformer for this?

Thanks

Folsom · « **Reply #261 on:** 23 Mar 2016, 09:02 pm »

Sorin, that should be fine since it's 16v output with plenty of amperes. I might try a James OA type in near future. They're $60 shipped to USA, so maybe less to you. That's for 100VA, 16v out. They seem higher quality than most transformers.

matt_garman · « **Reply #262 on:** 25 Mar 2016, 08:28 pm »

FWIW, I just ordered an AnTek AS-1218 (18V/100VA) to go with my Folsom tda7297... which I finally got working this morning! I swapped all three transistors on the board, and now it's singing nicely. I think at some point I failed to heed Jeremy's warning, and connected "live" power that killed one or more of those transistors.

Sounds good, although it's too soon to do any kind of detailed writeup or comparison with other amps. But at first listen it's certainly good enough to warrant a proper case.

A question for Folsom: I think I'm going to use this Modushop Galaxy 2U chassis (since I already have one). The sides are a "quasi" heatsink, which I'm thinking are probably enough to keep the tda7297 cool. Question is: do you think using a thermal pad between the tda7297 and heatsink is acceptable? Example: t-Global Technology H48-6-45-25-6. My thought is that it would simplify the heatsinking a bit, as the pad is thermally conductive, but not electrically conductive.

Thanks again for your help getting my 7297 running!

Folsom · « **Reply #263 on:** 25 Mar 2016, 08:48 pm »

The heatsink side is probably ok. It doesn't have good convection but the 7297 doesn't exactly run hot.

I'd use the mica I specified. It's just for a TO-3 transistor but it's square. The pad you have there is so thick that it may hurt overall power ability.

matt_garman · « **Reply #264 on:** 26 Mar 2016, 08:25 pm »

Here's the chassis layout I'm thinking about. The goals are compactness and reasonable cost. This is designed around the Modushop Galaxy 2U 230x230 chassis. Internal dimensions are 80x210x230 (HxWxD). I found on ebay a heatsink with dimensions 150x60x25 (DxHxW) for $7. In the schematic below, I'm calling for the AnTek AS-1218 transformer. The Hammond Folsom recommends would also fit, however, I'm a little concerned about magnetic leakage in something this compact. A toroid "should" be better in that regard.

I don't think it really does anything, but that heatsink "feels" like it separates the amplifier from the transformer.

Another approach would be to remove the heatsink entirely, and move the transformer to the left side, and the amp+psu board to the far right. Then the side of the chassis itself could be used as the heatsink.

Folsom, how much heat does that big D45H11G transistor generate? How about using a thermal pad to bridge that to the chassis?

Thoughts?

Note: that drawing is mostly to scale. I used Front Panel Designer to get the dimensions/scale right, took a screenshoot, then used Microsoft Paint to make the colors and annotations.

Folsom · « **Reply #265 on:** 26 Mar 2016, 08:57 pm »

The D45 doesn't make enough heat to be concerned with. If the heatsink is inside it needs some airflow from top to bottom (holes). But it also doesn't need a pad or anything for electrical conductive isolation.

limits · « **Reply #266 on:** 5 Apr 2016, 02:59 pm »

Hi all...thanks, Jeremy, for your work on designing the boards for this, and your helpful build suggestions. Here is mine...need to add the heatsink yet, but ran out of time last night. This shouldn't be too big of a deal, as I dry fitted earlier--just didn't want to mess with the grease

Folsom · « **Reply #267 on:** 5 Apr 2016, 07:10 pm »

So close!

Odal3 · « **Reply #268 on:** 5 Apr 2016, 07:48 pm »

Looking nice.

I didn't get hold of any good mounting hardware and fitting the heatsink without getting a reading on my multimeter took a few tries.

Folsom · « **Reply #269 on:** 5 Apr 2016, 08:26 pm »

Quote from: Odal3 on 5 Apr 2016, 07:48 pm

Looking nice.
I didn't get hold of any good mounting hardware and fitting the heatsink without getting a reading on my multimeter took a few tries.

If it's too hard you can use a deeper isolation washer if you drill out a slightly bigger hole for 1-2mm before the smaller hole you need for mounting. Also you could maybe take the chip tab and remove a little material on the side that goes against the heatsink to help it not touch the bolt - very carefully.

limits · « **Reply #270 on:** 5 Apr 2016, 10:46 pm »

Hi Guys...starting the troubleshooting process...ahem...

I powered up the amp and no smoke but...no sound. I am getting 19.2 VAC out of the transformer and 25.2 VDC through the antipole and to the amp board. Does this sound ok?

That's about as far as I have gotten. Nothing appears wrong. Oh, except for the no sound part. I didn't power anything thing up until then...that's what you get for being overly confident...ugh...

Folsom · « **Reply #271 on:** 5 Apr 2016, 10:52 pm »

Let's see a close up of the board. If you measure from any connection that's a ground (speaker output won't work) to the positive leg of one of the OSCONs what do you get?

90% of the time the mute/stdby is preventing turn on, but we'll see.

limits · « **Reply #272 on:** 5 Apr 2016, 11:10 pm »

Ok...so I will take a pic of the amp board...the bottom of it, right? Positive leg of one of the 10 caps to the voltage input ground to the board ok?

Folsom · « **Reply #273 on:** 5 Apr 2016, 11:13 pm »

I think there's 12, but yes.

Top pic too.

limits · « **Reply #274 on:** 5 Apr 2016, 11:59 pm »

Well crap. Measured 1.7 volts positive leg to ground. Until the amp chip tab touched metal. It made its first sound. Breifly (pop)...right...12 caps...I said 10, but I ended up soldering in 14, as I put 2 in the alternate location. And fixed

This one is fighting me a bit...

Folsom · « **Reply #275 on:** 6 Apr 2016, 12:18 am »

Are you certain you didn't mix the TL431 and BC337?

You didn't drill the holes for the incoming DC did you? or any others?

BTW the two accidental mounts, it would be preferred to put a capacitor that's 35v, and 47uf+ in one of them. I sent an email and I think I got everyone but perhaps not.

I'm not seeing any major problems, yet. Pics are so-so however.

limits · « **Reply #276 on:** 6 Apr 2016, 12:35 am »

Hi. Thanks...really don't want to tie up your time. Sorry about the bad pics...phone...snow...April...

no...on both drilling out the holes and switching the transistors. I did notice an iffy solder joint on the middle leg of the the431. Will try to reflow that. And replace the amp chip, right (based on the pop)? Assume that one is done. First good news of the day... Mr. Fat finger has another 7297. Things are looking up...

limits · « **Reply #277 on:** 6 Apr 2016, 12:38 am »

Hi. Not sure what the reference to the accidental mount cap and location is though

Folsom · « **Reply #278 on:** 6 Apr 2016, 01:19 am »

I don't know for sure about replacing the chip. I didn't hear the pop. I'm wondering why it would pop touching something? The Antipole's bottom isn't touching the case anywhere is it?

The popping makes me wonder if the ground connection to the amp isn't working, and then it found ground through the chassis, which doesn't make a lot of sense...

Folsom · « **Reply #279 on:** 6 Apr 2016, 01:35 am »

Here's a photo of the caps I sent an email about, as they add stability because the inductance of the wires to the antipole is a little too high for the KMR section (resulting in some turn on noise, at least).