lm3886 stereo amp question

[PurpleTheory]

Hey guys,
I'm trying to build an lm3886 stereo guitar amp, basically it'll be 2 of these from ggg (http://www.generalguitargadgets.com/pdf/ggg_lm3886_amp.pdf) each powering a seprate speaker. Just had a couple of concerns I wanted to run by you guys before I ruin something:

First off, I've been working on the cab today and was wondering if it matters that I put 2 speakers of different wattages in the cab, both 8 ohms, but different wattages. One is 150 watts and the other is 75. Will this be a problem?

Also, on the back of the cab i put a plate with both jacks to the speakers on it, which will connect to the amp, but then I started wondering wether each jacks need to be isolated to prevent grounds from connecting, or does it matter?

Thanks for any help and to the guys that helped me before on this project.
Matt

[didier]

First off, I've been working on the cab today and was wondering if it matters that I put 2 speakers of different wattages in the cab, both 8 ohms, but different wattages. One is 150 watts and the other is 75. Will this be a problem?

no problem

Also, on the back of the cab i put a plate with both jacks to the speakers on it, which will connect to the amp, but then I started wondering wether each jacks need to be isolated to prevent grounds from connecting, or does it matter?

its okay for the grounds to connect.
see this schematic: http://mark.rehorst.com/LM3886_amp/LM3886_amp_schematic.png

just out of curiosity:
are you using 2 seperate power supplies for the two poweramps? or just one that is capable of running the two?
what power are you using?

[mojotron]

What brand/model transformer have you guys used?

[PurpleTheory]

Cool, thx for the help guys.

just out of curiosity:
are you using 2 seperate power supplies for the two poweramps? or just one that is capable of running the two?

I haven't actually built it yet so I haven't quite decided, my original plan was just to use one, but is two is better? I saw some old transformers at a local hardware store that are 35v 2A (i think), would one or two of those be suffiecient? What is the ideal transformer for the job?

What brand/model transformer have you guys used?

I would be interested to know this as well. Also, I noticed that a lot of people who have built gainclones have used toroid transformers, are those better for any reason?

[R.G.]

The optimum supply voltage for an LM3886 amp varies depending on whether you drive an 8 or a 4 ohm load. For a 4 ohm load, you really should drop the power supply voltage a bit to keep the power dissipation down on the device. Otherwise, it may go into thermal shutdown more often that you'd like - which is ever, yes?

Read the app notes on the LM3886 on the net.

A transformer putting out 44Vct at 2A is just about right for an 8 ohm load. For 4 ohms, you want something like 36Vct to 38Vct at  two to three amps.

You definitely want centertapped. If the 35V transformers are not centertapped, that give you a single power supply amp, which will be easier to heat sink as the tab on the LM3886 can be attached to ground, but you'll need a big capacitor from the output terminal to the speaker, perhaps 1kuF at 63V. You will need to AC couple the input as well, although that's easier.

Toroids are about half the size and weight of a corresponding E-I transformer and cause much less magnetic interfernce to be emitted.

I use 100VA toroids with dual 22V secondaries.

[didier]

you can use either of the options i said...

just remember that when you use one trafo it will have to be rated at double the power and same voltage.

for one LM3886 i used a 80VA 48Vct toroid which does the job perfectly.

so you either buy two of those rated at 80VA or higher
or you buy one rated at 160VA or higher

you will save some cash buying just one... (100VA @ Mouser: about $50; 200VA @ Mouser: about $70; if i looked good...)