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LM1875 integrated Chip Amp with external PSU

Bauhausler

Bauhausler

Rational Subjectivist
Here's something I've been working on. All parts are from Radio Shack except the LN1875s, the fan and a few hardware store items.
It makes 8 WPC continuous into 8 Ohms. The purpose here was not to make the ultimate chip amp. I've made much fancier ones than this. I wanted to design and build something that could be easily duplicated from readily available parts and without special skills.
The amp circuit is straight from the NatSemi datasheet.
I added 2 inputs with switching, a volume pot, power LED and fan.
To keep it compact I outboarded the PT to a separate enclosure made from PVC plumbing pipe. The interconnect cable with the PT is an automotive trailer wiring harness.
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Looking at it again, why not go one step further and cover the umbilical cord with something that is more coherent to the fit and finish of the rest of the work? Fitting some techflex on the cord would be a very nice touch IMO.
 
Thanks to all for the encouragement. I purposely kept this simple and used only commonly available parts and materials. I'd like to cover the cord but probably won't. Spiral cord wrap is widely available but it's stiff. Nylon woven wrap would be ideal but is not available in small quantities to casual hobbyists. The goal was to avoid anything that would be challenging to source. I've made far fancier and better chip amps than this in the past and will again. The challenge here was to un-audiophile the process. There's a reason for that but I can't say right now...
 
The grounds are even starrier now since I added the wiring for the LED and fan after taking these pix. When I first built it I had the grounds daisy chained together compactly. All grounds were very short. In testing there was a trace of overshoot on square waves. I referred to the datasheet and they strongly recommended separate grounds for everything, so I reworked it. Now the square waves are textbook perfect.
So I guess layout does matter.
I fooled around for hours to get these component positions so it would be very short signal paths and short point-to-point wiring.
 
djnagle said:
That turned out great Ross. I like the blue. Good job on braiding the four wires.
Click to expand...

Thanks. The color is actually darker than it photographs. Closer to royal blue.

As for sound, I think it's fine. When I get bored with it I'll circulate it around to the usual crew to get opinions on the sound.
 
Updates on testing

I finally got the thing on the bench. The bandwidth is 3 hz - 100 khz. That 3 hz number is lower than necessary. Since this will typically be used to drive small speakers with limited bass you could take the 2uf input capacitor down to .33uf or even smaller. Due to the limited power output it's an advantage to block out bass input signals that the speakers aren't going to reproduce anyway. This retains the amp's voltage headroom for the frequency range that the speakers can put out.
To see how the thermal performance was I ran it at full output into a 4 Ohm load for a while, a pretty brutal test that far exceeds any music signal use. After 5 minutes both chips went into thermal shutdown to protect themselves, as they should. At 5 Watts output into 4 Ohms it ran without heating up to shutdown. This gives me confidence that it has sufficient thermal capacity for full music output, as it would clip a music signal long before the average level exceeded 5 watts long term.
I would say in fact that the fan is superfluous. With enough vent holes in the top and the bottom plate notched out fro air flow the amp will cool itself adequately in normal use.
I've been using this on my desk to drive a pair of JBL Control 1 Monitors. I measured 100 db SPL peaks and higher when listening to music far louder than I normally would. LOTS of volume despite the amp's small size and modest technology.
 
Way cool.

And inspiring, as I've started an LM3886 based build and haven't waded in these waters before.

Where do you source the Sil-Pads? I'm not finding these readily available in hobbiest quantities. Especially sized for 11 lead TO-220 package.

Must not be looking in the right places ...:dunno:
 
Stuart Pedaso said:
Way cool.

And inspiring, as I've started an LM3886 based build and haven't waded in these waters before.

Where do you source the Sil-Pads? I'm not finding these readily available in hobbiest quantities. Especially sized for 11 lead TO-220 package.

Must not be looking in the right places ...:dunno:
Click to expand...

You can use oversized mica insulators from TO-247 transistors I think. I buy Bergquist material on That Auction Site if needed. Mouser and Newark have it for sure. I've bought mica from a scrapbook/stamp craft store and made my own before as well. Peel it down to .010" or less.

IN place of an insulating washer through the metal tab I usually use a Nylon screw and a metal nut. I get nylon screws at the hardware store. It means fewer parts to keep on hand.
 
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Check out ChipAmp.com They have board sets for the LM1875 and the LM3886 chips. I've used em before and they work and sound great!!
 
I've built chip amps with boards from Tech-DIY as well. LM3875 and LM4780, dual mono and parallel.
For this amp I want a build where it could all be made with common parts. The perf board is from RatShak,as is everything else but the chips.
I think of this circuit board as 'cursive' because it's hand-wired rather than printed. It's easier than it looks. I have another board I'll put up pictures of later. I put labels on the edges and labeled all of the holes, then colored the lead holes with a Sharpie. Makes stuffing easier.

Tech-DIY has boards for dual mono, parallel or bridged LM4780. The LM4780 is basically 2 LM3886 in one package. Very powerful.
 
Still running strong

I've had this little amp running for about 2 months. Right now it's on my desk driving my 1978 Rogers LS3/5A speakers. An odd match for a scratch built amp from Radio Shack parts, but it works.
Oh, boy does it work. This sounds really really nice. The project paid off.
I really don't want to but I have to take it apart. This is the prototype and I need to inspect the board to make another in a different case. If things go as planned, that will be written up in a magazine as my Econowave speaker implementation was.
 
V 1.1 circuit board

I've built another circuit board with a few changes:
1) .1" pad-per-hole board instead of bare perf board
2) 4700uf filter caps instead of 2200uf to squeeze out a little more headroom
3) much neater looking ground wiring
4) 1uf input cap instead of 2uf, which was overkill
5) power supply leads run on component side

I plan to try to mount the rectifier diodes on the board as well.

This will get housed in a u-shaped heat sink fabricated from overlapped 6" sections of 2" X 2" X .062 aluminum solid angle. The PT will be bolted directly to the back. The volume pot and source selection switch will be on the front. I hope to finish the amp this week, but the metal is still on order at the moment.
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