Questions about GGG LM3886 amp

black mariah · January 25, 2007, 05:07:06 AM

Would there be any adverse effects to hooking up a regular 10k pot in place of the 10k trimmer so I can have level control anytime I want it?

Is there a ready-made power supply that would work? I don't want to bugger about with transformers and stuff just yet, but that "bipolar" thing is throwing me off. I have no clue what that means in this context.

mydementia · January 25, 2007, 12:30:39 PM

I almost have all the parts for this build - you're going to need a transformer... I got this one: http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=122-615

Check out Brian's site (chipamp.com) for excellent power supply/amplifier kits - he just started offering a Mono kit (for guitar use). Also, check out Joecool's ssguitar.com for more information on building these beasties. I'm still trying to figure out how much heatsink I need... and then how big an enclosure...

Good luck!
Mike

Oh - Brian's schematic has an optional 25k pot after the input to ground for volume control...should be just fine in the GGG project too...

mojotron · January 25, 2007, 01:40:21 PM

Quote from: mydementia on January 25, 2007, 12:30:39 PM
I almost have all the parts for this build - you're going to need a transformer... I got this one: http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=122-615
...

I'm wondering why everyone uses that transformer?

Ok, maybe this is a silly idea, but I was thinking of using 2 IBM (16VDC, 2 pronged, 32VDC together) laptop power suplies - each isolated from ground - connecting the (-) from one to the (+) of the other - like you would with batteries - and using the common connection point as the circuit ground - which would be isolated from the output using Marshall-style in/out jacks. I used one of these with Ruby-like circuits and the power was very clean, I figured I could use 2 together to power an LM3886 amp - thoughts?

black mariah · January 25, 2007, 01:42:53 PM

Why do I need a transformer?

Why wouldn't something like a wall wart work? Hey, don't look at me like that! I'm a n00b!

Seljer · January 25, 2007, 01:45:52 PM

Quote from: black mariah on January 25, 2007, 01:42:53 PM
Why do I need a transformer? Why wouldn't something like a wall wart work? Hey, don't look at me like that! I'm a n00b!

most wallwarts really don't handle the power you'd need for an 50watt amp

mojotron · January 25, 2007, 03:18:12 PM

Quote from: Seljer on January 25, 2007, 01:45:52 PM
Quote from: black mariah on January 25, 2007, 01:42:53 PM
Why do I need a transformer? Why wouldn't something like a wall wart work? Hey, don't look at me like that! I'm a n00b!

most wallwarts really don't handle the power you'd need for an 50watt amp

These laptop power supplies are rated at I think 70W - with 2 that would make a 140W supply. I could run an amp, and my toaster off of that!

black mariah · January 25, 2007, 04:00:32 PM

Alright, did some more reading. I think what I'll do is just buy the mono kit from chipamp.com and get a transformer from Parts Express. Figure $20 in parts for each of the preamps, $30 for a chassis... with any luck and a bunch of planning I might get out of this for under $200.

The Tone God · January 25, 2007, 04:11:31 PM

Quote from: mojotron on January 25, 2007, 03:18:12 PM
These laptop power supplies are rated at I think 70W - with 2 that would make a 140W supply. I could run an amp, and my toaster off of that!

I have mentioned this before but most of those switching supplies are designed for static loads. Many can't handle dynamic loads that amps demand. Also while you think that 70w input = 70w output you are not taking into account efficiencies. I would say you are looking at getting more like 30-35w from one of those supplies.

Andrew

RDV · January 25, 2007, 05:22:27 PM

There ARE no shortcuts.

Build a nice supply, and you'll have a nice amp.

Screw around and you'll get smoke and silence.

RDV

black mariah · January 25, 2007, 05:55:10 PM

Quote from: RDV on January 25, 2007, 05:22:27 PM
There ARE no shortcuts.

Build a nice supply, and you'll have a nice amp.

Screw around and you'll get smoke and silence.

RDV

Shuddup! I don't care about your FACTS and REALITY!

R.G. · January 25, 2007, 08:28:57 PM

1. Power amplifiers are misnamed. They are really power supplies that let some of the power out in a very carefully controlled way to the speakers.

2. Power amplifiers are not perfectly efficient, as was mentioned. A 100W Class AB solid state power amplifier will eat about 130W of DC power at its maximum power drain power level. A 100W Class A solid state power amplifier will eat 400W of DC power continuously, on a good day. A 100W Class AB tube amp will eat about 350W continuously, perhaps 450W max. You have to know what you're driving with the power supply.

3. Power supplies are not perfectly efficient. A power supply putting out 100W of DC will be eating at least 106W from the AC power line if it's a very simple transformer/rectifier. Things get worse from there.

4.

QuoteOk, maybe this is a silly idea, but I was thinking of using 2 IBM (16VDC, 2 pronged, 32VDC together) laptop power suplies - each isolated from ground - connecting the (-) from one to the (+) of the other - like you would with batteries - and using the common connection point as the circuit ground - which would be isolated from the output using Marshall-style in/out jacks. I used one of these with Ruby-like circuits and the power was very clean, I figured I could use 2 together to power an LM3886 amp - thoughts?

That will work OK, but you will be very limited in the amount of power the amp will put out - not enough voltage. The LM3886 wants about +/- 30Vdc for most performance. And you may or may not get bleedover of the switching frequency from the power supplies into the amplifier. It is difficult to get switching power supplies to run analog amps cleanly.

5.

QuoteWhy do I need a transformer? Huh Why wouldn't something like a wall wart work? Hey, don't look at me like that! I'm a n00b

You need a transformer for two reasons. (a) to isolate the power supply voltage from the seriously deadly AC power line and (b) to change the voltage to just the right voltage for the amplifier. Mess up on (b) and your amp will not produce the right power. Mess up on (a) and you're... dead...

The LM3886 will produce about 50-60Wrms into 8 ohms with a +/-30V power supply. With a +/-16V power supply it will put out about 12-15W. So getting the voltage to it correct is really important.

Then there's that getting dead thing again. You'll probably want to not do that.

mojotron · January 26, 2007, 12:48:27 AM

Wow, excellent information Andrew, Ricky and RG - and from the experts too - this forum is great - Thanks.

Well, those IBM laptop supplies worked so we with 386 based designs @16VDC and for powering teams of effects - but ya, could be a problem with 3886 amps - a whole nother beast power-wise - the whole average vs. peak load rings a large bell. I think that they work really well for lower power situations because they can supply a lot more power than needed.

I told ya, might be silly... I asked in a moment if ignorant weakness.... Ricky's post remins me all too much that in the past some of my science projects were not that great for the reasons given... mostly lacking in science (and I know better

) Great solid advice. I just happen to have a pile of these supplies: Kids - don't try this at home with your dad's stuff!

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Questions about GGG LM3886 amp