really basic LM3875 amp for workshop

[chris737]

Hi All,
I need to build a minimalist stereo amp for my garage/workshop. It will be driven by an iPod. Audio quality does not need to be spectacular.

I already have:
LM3875T chip amps
35-0-35VAC transformer

By my calcs the transformer should give me around +/- 45VDC. According the the TI data sheet, the absolute maximum value of the combined supply rails for the 3875 is 84V, so I'm a few volts over. I'd prefer to use transformer I have, so I'm wondering if there's a way to pull the voltage down? Failing that, can I derate the output power of the 3875 with a higher supply voltage?

If the answer to the above questions is "no", then I guess it would be cheaper to buy 2 new chip amps with a higher Vs rating than to buy a new transformer? In that case, can anyone recommend a higher rated chip?

Edit: I also have a stack of LM338T regulators too.

Regards,
Chris

[jackinnj]

best advice with any of these national semi hyped opamps is to keep the leads as short as possible, you can connect the resistors directly to the appropriate pins -- the data sheet is clear in this regard and should be heeded.

You can reduce the output of the power supply by placing a bunch of rectifiers in series with the output. 1N4002's are cheaper than resistors for the same purpose.

no regulator is necessary for the LM3875, 3886 etc.

[chris737]

Thanks Jack,
I'm aware that a regulated supply is not required for the 3875, but I thought it might be one way of reducing the supply voltage. I have a large heatsink I pulled out of an old home theatre amp, so I have plenty of room to mount the 338's if I were to use them.

[GloBug]

That would take a lot of diodes to drop that much voltage.
Your transformer makes around 50v.

Here is a link where a guy regulates a suitable voltage.
Roman Black's gainclone amp

Keep looking for another transformer, that's what I did.

Something in the 18v, 22v or 25v secondary range works with these chips.

[!]

Well, I wouldn't use 1A diodes, and using a resistor you can then have an RC filter.

What type of workshop and how large an area? I found 5W-10W/channel was plenty for my workshop or garage and that power level allowed me to reuse some salvaged power brick/wall wart AC/DC plug packs. Getting the transformer out of the amp case and hanging a heatsink on the back, the fairly sealed little amp case (project case about 2" H x 3" D x 5.3" W, doesn't get very warm inside.
Example, http://www.electronics123.com/s.nl/it.A/id.2343/.f

I wonder if using different higher voltage chips or even LM3875 is overbuilding for a workshop. You're increasing amp size and heat generation in an area that might be subject to debris so you may not want many ventilation areas in the amp casing?

[chris737]

Thanks,
interested to hear that 10W is ample in a garage.

After sleeping on it, I've decided to pull a 25-0-25V 160VA toroid out of a 2x50W chip amp I made a few years ago and replace it with a 300VA toroid, as I intend to make it into a 4x50W amp. I will use the 160VA tranny to power my garage amp, thereby solving my over-voltage problem.

My workshop is very dusty, so I don't plan to have ANY vent holes in my amp casing. My plan is to have the heatsink mounted externally.

I have just ordered a 2x50W chip amp PCB from Elliot Sound Products and have managed to find a pair of LM3876s to put in it. Hopefully it will be straightforward from here!

Thanks to all respondents for the help.

[GloBug]

Ya that will work out for ya. In this day and age you may as well put a LM3786 in it from a price and performance aspect.

We have had an older model (1990s) Pioneer receiver in the workshop aka "the shed" for the past 4 or 5 years. It's nice to have decent sound quality and low end response out there while working on stuff.

I guess it is approx 20' x 40' post and beam type structure.

[chris737]

Till now I've had a Denon 5.1 amp in the workshop that I was given due to a blown rear channel. Front L & R worked well for a while then just started cutting out for no good reason. I gutted it for useable parts and threw the rest out.

Shop is 20x25 brick.

[Bazukaz]

I see you have already solved your problem.
In the past I used to simply unwind some windings from trafo's secondary if the voltage is too high. With toroids usually its quite straightforward. However maximum power of trafo may be somewhat reduced(or may not - depends on secondary's diameter) as higher current will flow at full power.

Regards,
Lukas.