The "Forum Amp" design thread

[davidallancole]

Hi Taylor,

I was wondering if the schematic you show in post #220 above is your final schematic?  Are you open to any comments about the parts values and bias scheme?

[Taylor]

Uh, well, I'm open, but at the same time, the PCBs already have been designed and are being fabricated. So I can't really do anything to change them now.

[bassmannate]

Hey, Taylor,

What kind of speaker cab are you using for bass with this thing?

[Taylor]

I have a TL606 cab loaded with an EVM 15L, I think. This is a very nice cab for it's time, but these days there are better speakers which are lighter, go lower, have better sensitivity, etc. and much better cab designs.

Some day when I get a lot of free time I'd like to build one of Bill Fitzmaurice's designs, like the Jack. His stuff would be great to pair up with my little amp design, because they are horn designs so the sensitivity is through the roof compared to direct radiator cabs. You could get a lot of volume and great frequency response with just 20 watts through one of his cabs.

[therecordingart]

How are the boards coming along?

[Taylor]

Still being manufactured. There was a bit of a delay because of a misunderstanding with the factory. Should be available pretty soon though. I'll post here when they're available.

[Taylor]

PCBs are now available:

http://musicpcb.com/pcbs/tiny-giant-amp/

[azrael]

Hm, but where to buy TDA7240As....?

Looks great Taylor! I think I'll be getting some of these.

How would one adapt this for use with bass?

[Taylor]

I will have a bunch of info on parts sourcing on the site in the next day or two. There are plenty of sources for the chips, and I'll also have parts kits which include the TDA and everything else on the board for pretty cheap.

I use mine with bass without any modifications. It's a full-range amp - the chip is designed to reproduce full spectrum audio for car audio systems, so it doesn't have a guitar-specific voicing.

The "audiophile" thing to do for more bass would be to increase the power supply reservoir cap. It's already bigger than the datasheet suggests, but you could make it really huge like 4700uf or even bigger probably. Not sure if this makes a difference in real life.

A good thing to do would be to pair it up with a nice EQ - I like a couple of bands of semi-parametric EQ. But the amp itself can handle and dish out lows quite well.

[defaced]

Hm, but where to buy TDA7240As....?

2 bucks a piece on Mouser.  They've got 400 of them right now, so get in while the gettins good [/sarcasm]. May as well get the 2R2 resistors while you're there too. 

This is a cool project.  I'm going to have to find a use for some of these. 

Taylor, would it be possible to parallel the boards/chips for more power into higher loads?  Google and the data sheet was of no help in telling if the TDA7240A plays well if paralleled. 

[Taylor]

Yes, Mouser has them. Last time I looked all they had were the backwards-bending ones (with H at the end of the part number) but now they have the ones that stand upright, which is what you want. You can also find them on ebay for pretty cheap, and for a dollar each here, but unless you're getting something else from there the shipping might not make it worthwhile:

http://www.packetradio.com/catalog/index.php?main_page=product_info&cPath=24&products_id=1003

You can't bridge the amps because they're already bridged. I think it's possible in theory to parallel them, but when I read up on it, it seemed that it was very important to match the output resistors or else one of the chips will pull current from the output of the other one and they could get damaged.

I want to parallel them too, so I will be researching and experimenting with that to see if it works in reality.

[Strategy]

This project is ideal for me. I play keyboards, bass, and guitar and have been pondering how to have one amp that can serve all my purposes. I think it might be fun to build just one amp but then have options of different cabs for different situations and instruments. Great 2nd hand speaker cabinets with lots of character are the kind of things that are always popping up around Portland.

- Strategy

[waltk]

would it be possible to parallel the boards/chips for more power into higher loads?  Google and the data sheet was of no help in telling if the TDA7240A plays well if paralleled.

The chip is already in a bridge configuration, so you can't parallel two chips into one speaker.  You could easily just build two boards, hook up a separate speaker to each, and then put a splitter in front of them.  In fact, you might not even need a splitter because the input is fairly high impedance.

Edit: Ooops, Taylor already answered this.

[Taylor]

Right, as Walt says, and I forgot to mention, the easiest way to get more power is to plug 2 of these into 2 speakers. Double speaker area gives you 3db boost on its own, so adding a second speaker and power amp would give you quite a bit more volume.

[zambo]

Taylor that board is lookin sweet! Do you have the full kits available yet?

[Taylor]

Thanks zambo, I will have the kits all ready to go tomorrow.

[therecordingart]

Since these are full range I could use them to power a small pair of speakers, right?

[oldschoolanalog]

Anybody have any thoughts about using 2 of these to make a true stereo chorus amp (a la Roland "JC")

Discuss...

[Taylor]

Since these are full range I could use them to power a small pair of speakers, right?

You mean like stereo speakers? Yes, definitely - that's what the chip was designed for. But I must admit I have no knowledge of stereo equipment in terms of in and out impedances, so I can't say with certainty how the opamp buffer would work for non-instrument signals.

Anybody have any thoughts about using 2 of these to make a true stereo chorus amp (a la Roland "JC")

Discuss...

Yeah, that kind of thing was one of the reasons I decided to do this simple power amp design - that way people can easily incorporate it into projects like that, or mini-Leslie clones, etc. I think it would be really cool to see these built into a cab with outward-angled speakers to do a stereo spread thing.

[zambo]

I think you are a better person for designing this and should therefore pm me with a price for a full kit in us dollars or chickens or bushells of wheat.  It may be a little bit before i can buy one but i am for sure interested. I just fired up my tube pre and want to make the mini hybrid wreck! 20 watts of love!