Solid state amp project questions

Electron Tornado · January 10, 2013, 07:01:37 PM

With respect to ground:

VD = 9V
VS = 3.2V
VG = 0

Right now I think the chip is having "issues". I tried the volume pot as tca suggested and got a much lower volume and a distorted output. I changed the pot back to what I had with the same result. Changed the wiring to the stock Punch circuit, and still the same thing. An audio probe shows I'm getting a good signal up to the input of the chip. Checked the connections to the chip as well.

Too bad. It was working quite well the other night. Hmmm....so the one time I don't buy a spare chip ......

I don't think I did anything to damage it. Ran it at 9V, tested the circuit with speakers of 7.2 Ohms, 6 Ohms, and 3 Ohms (no 8 Ohm available).

tca · January 11, 2013, 04:26:42 AM

Quote from: Electron Tornado on January 10, 2013, 07:01:37 PM
Too bad. It was working quite well the other night.

Don't despair! Get another chip and keep going!

Cheers.

Electron Tornado · January 11, 2013, 10:50:52 AM

Quote from: tca on January 11, 2013, 04:26:42 AM
Don't despair! Get another chip and keep going!

Cheers.

A little frustrated and inconvenienced, maybe, but no despair, just another opportunity to learn. I just need to come up with enough of a list of parts to order a couple of those. Why it failed has me scratching my head, though. I don't think I abused it. Although, I think Paul said it shouldn't have been distorting like it was without the 66k resistance, so I wonder if it was already bad when I bought it.

Electron Tornado · May 10, 2013, 12:45:08 AM

Quote from: tca on January 11, 2013, 04:26:42 AM
Don't despair! Get another chip and keep going!

Cheers.

I tried a TDA7052A using the datasheet and the pre-amp from the Punch. Works great.

Electron Tornado · May 13, 2013, 11:01:45 AM

Quote from: Electron Tornado on May 10, 2013, 12:45:08 AM
I tried a TDA7052A using the datasheet and the pre-amp from the Punch. Works great.

Hmmm...maybe I spoke too soon. It worked great on the breadboard, but now that it's all boxed up, it's got a problem. When working at high volume levels it works fine on battery only, but with an external power supply, it will start to cut out with the volume high. It was tested on the breadboard with external power and worked fine. Here's a link to the datasheet:

http://www.datasheetcatalog.org/datasheet/philips/TDA7052A_AT_CNV_2.pdf

Here's my schematic and a photo:

tca · May 13, 2013, 12:28:03 PM

Quote from: Electron Tornado on May 13, 2013, 11:01:45 AM
Quote from: Electron Tornado on May 10, 2013, 12:45:08 AM
I tried a TDA7052A using the datasheet and the pre-amp from the Punch. Works great.

Hmmm...maybe I spoke too soon. It worked great on the breadboard, but now that it's all boxed up, it's got a problem. When working at high volume levels it works fine on battery only, but with an external power supply, it will start to cut out with the volume high. It was tested on the breadboard with external power and worked fine. Here's a link to the datasheet:

http://www.datasheetcatalog.org/datasheet/philips/TDA7052A_AT_CNV_2.pdf

Here's my schematic and a photo:

Nice cab!

So it works at high volumes with a battery and not with a external power, that is strange. Does it sound like a sagged amp? Check *all* the ground connections and specially the input DC jack, does the battery connection turns off when you plug the external power?

Cheers.

Electron Tornado · May 13, 2013, 02:50:06 PM

Quote from: tca on May 13, 2013, 12:28:03 PM
Nice cab!

So it works at high volumes with a battery and not with a external power, that is strange. Does it sound like a sagged amp? Check *all* the ground connections and specially the input DC jack, does the battery connection turns off when you plug the external power?

Cheers.

Thanks! The cab is a cigar box, the panel on the top is from a previous project, and the grill cloth is some old burlap.

Not sure if it sounds like its sagging or not. I tested it again by disconnecting the battery and using only external power and it still cuts out. It will work fine for a few seconds and then start cutting out. It still doesn't happen when using only the battery. The external power supplies I have been using are a One Spot and a Danelectro wall wart. The battery does disconnect when external power is plugged in.

I also tested the amp with different guitars. It exhibits this problem when I use humbucking pickups, but not when I use single coils. When I built it on a breadboard I tested it with humbucking pickups and it was fine.

Electron Tornado · May 13, 2013, 10:53:59 PM

I think I might have an answer to the amp cutting out. The supply voltage range is given as 4.5-18V, however, the 1.1 watt output power for the TDA7052A is for the conditions of an 8 ohm load and a supply voltage of 6V. Running at 9V, the chip can get mighty hot, while at 6V you have to work to get it warm.

External power is giving 9V, while the battery I was using was only giving about 6-6.5V. With a fresh 9V battery, the chip heats up again. I'm thinking I should step down the supply to 6V. Now, how to drop the voltage....this is a power amp, but can I still do a diode drop a la this: https://www.smallbearelec.com/Projects/FuzzE-One/FuzzE-One.htm

PRR · May 14, 2013, 02:01:09 AM

> I'm thinking I should step down the supply

Or heat-sink the chip. Use a very thin layer of metal-filled epoxy to stick a quarter to the top.

That Small Bear plan is great for very-very small powers but not for big power.

A 3V Zener or five 1N400x series will drop 3V, but that's wasteful.

Electron Tornado · May 14, 2013, 07:50:45 AM

Quote from: PRR on May 14, 2013, 02:01:09 AM
> I'm thinking I should step down the supply

Or heat-sink the chip. Use a very thin layer of metal-filled epoxy to stick a quarter to the top.

That Small Bear plan is great for very-very small powers but not for big power.

A 3V Zener or five 1N400x series will drop 3V, but that's wasteful.

How do I determine how big of a heat sink to use?

Electron Tornado · May 14, 2013, 09:56:03 PM

Quote from: PRR on May 14, 2013, 02:01:09 AM

Or heat-sink the chip. Use a very thin layer of metal-filled epoxy to stick a quarter to the top.

So, I guess super glue ain't gonna cut it?

PRR · May 14, 2013, 10:13:25 PM

> how big of a heat sink

Since it almost works: same as a closet. If a 2x2 closet is almost big enough, a 3x3 closet will be better. In this case, the body of a half-inch chip is almost enough, a 1-inch chunk will be better.

I once tried SuperGlue on a hot 486 CPU. Sink fell off a few minutes after the CPU was turned on.

J0K3RX · May 14, 2013, 11:05:52 PM

Try a TDA7267A You really can't beat this and super simple!! I have built more than 15 of them and always sound great!! It will drive a 4X12 cab also and operates on 5v to 15v... I just use 9.5v and it's super loud! Sounds great with guitar but you will want to put some sort of a preamp in front of it..

http://www.datasheetcatalog.org/datasheet/stmicroelectronics/5908.pdf

Electron Tornado · May 16, 2013, 12:06:35 AM

Quote from: J0K3RX on May 14, 2013, 11:05:52 PM
Try a TDA7267A You really can't beat this and super simple!! I have built more than 15 of them and always sound great!! It will drive a 4X12 cab also and operates on 5v to 15v... I just use 9.5v and it's super loud! Sounds great with guitar but you will want to put some sort of a preamp in front of it..

http://www.datasheetcatalog.org/datasheet/stmicroelectronics/5908.pdf

I'll have to try that one eventually. Have you tried it with a 4 ohm speaker? (Electronic Goldmine has some 4 ohm speakers on sale.) The speaker I used in my amp above was a very old speaker (50's or 60's) that measured about 3 ohms. I put a 5 ohm resistor in series with it and it sounds good. I just need to try a heat sink on the chip.

Electron Tornado · May 16, 2013, 02:24:10 PM

Here's a few more questions:

1. If I want to check how much current is being used, do I take the rms voltage or peak voltage and use the equation P=I*V, solving for I ?

2. If an amp chip datasheet doesn't list specs for different speaker loads, can I use the calcs R.G. showed here (see reply #12) http://www.diystompboxes.com/smfforum/index.php?topic=102512.0 to determine if a different load resistance can be used effectively without abusing the chip?

3. What are the pros and cons of using a resistor in series with the speaker, like I did here?

PRR · May 16, 2013, 11:15:02 PM

> cons of using a resistor in series with the speaker

Waste of power.

Electron Tornado · May 17, 2013, 12:36:26 AM

Quote from: PRR on May 16, 2013, 11:15:02 PM
> cons of using a resistor in series with the speaker

Waste of power.

Yup - powering a transducer and a heater.

Apart from that, I don't see that this chip cares what it sees for the load, as long as the load is the right number of ohms.

Electron Tornado · June 03, 2013, 06:05:05 PM

I was fiddling around with this again, and it looks like I can get some better performance out of it by putting a BJT pre-amp in place of the JFET buffer.

Any pros, cons, or pitfalls in that?

Seljer · June 03, 2013, 06:53:04 PM

Quote from: Electron Tornado on June 03, 2013, 06:05:05 PM
I was fiddling around with this again, and it looks like I can get some better performance out of it by putting a BJT pre-amp in place of the JFET buffer.

Any pros, cons, or pitfalls in that?

the preamp would probably have a low enough output impedance to make the buffer redundant

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Solid state amp project questions