LM1875 integrated Chip Amp with external PSU

[carbonman]

Thanks for the quick response. Now I have a couple of brands and part #s to take to my local electronics shops. I'll try a local order before getting something shipped from overseas.
BTW your PM box is full.

 

[Andy_H]

I just bought this issue of Make after seeing this thread. I don't expect to get around to the build for a while yet, but I'm looking forward to when I do.

 

[carbonman]

When you accumulate parts, make sure you order appropriately sized film caps. They can be a little tough to find in sizes that work with this layout. The layout is fine - some of the bits aren't stocked everywhere.

 

[Bauhausler]

It takes a little care to find appropriately dimensioned parts. If you order from a big supplier they'll have the dims available on a spec sheet. Here's a link to the comments with part sizes:

http://makezine.com/23/chipamp/

You can sub 0.47uf for the input cap if it fits better with no degradation of performance. Likewise the 0.1uf film bypasses can be 0.047uf if they fit better.

 

[westend]

Very cool! :thmbsp::thmbsp:
A visitor dropped by, yesterday, looking for a simple amplifier. The design criteria of using all easily available parts is the penultimate. :yes: I will refer him to this build.

Way OT, here, and might be a little bit intrusive but could you describe your Bauhaus? I have a period built house, also, and am looking for some exterior design ideas.

 

[Bauhausler]

Very cool! :thmbsp::thmbsp:
A visitor dropped by, yesterday, looking for a simple amplifier. The design criteria of using all easily available parts is the penultimate. :yes: I will refer him to this build.

Way OT, here, and might be a little bit intrusive but could you describe your Bauhaus? I have a period built house, also, and am looking for some exterior design ideas.

Thx for the recommendation. I hope the build goes well for your visitor.

Our house is Art Moderne style, built in 1939. White painted brick exterior with major glass block wall areas. Interior is wet plaster over metal lath (they didn't anticipate wi-fi). No decorative trim. It originally had a large second story summer room with jalousie windows and the building had a flat roof. We're 1000 miles too far north for those ideas so the outdoor room was incorporated to double the size of the master suite and the roof is now peaked & doesn't leak in winter.
The Moderne touches are bourgeois divergences from Bauhaus minimalism. Thick marble floors in the bathrooms, foyer & elsewhere. Oak parquet in other areas.
In 1958 they added a 36' X 24' room with one wall done in 3 10' X 6' windows looking out on the back yard. It's floored in 70# slabs of travertine. When we viewed the house the first time there was a New York Steinway grand in one corner so we call it the ballroom. Technically it's an informal dining room & TV room now but usually it's just heaped full of my audio junk.

 

[Andy_H]

I've been in the process of acquiring parts over the last couple of days, and I have a question about the resistors. I can't seem to find the 830 ohm 1/2 w resistors anywhere. Is this the correct value and if so where do you get them?

 

[Bauhausler]

I've been in the process of acquiring parts over the last couple of days, and I have a question about the resistors. I can't seem to find the 830 ohm 1/2 w resistors anywhere. Is this the correct value and if so where do you get them?

Just use 1K 1/2w. I think that's for the LED dropper, right? That will give 14 - 15ma, which is plenty for decent brightness.

 

[Andy_H]

OK, cool. I was just going off the parts list and I should have taken a closer look at the actual instructions to see where it went. I'll have to take another look at what I've bought once I get home, but IIRC the LED I bought is a whole LED assembly and it already has resistors in it.

Thanks for the quick reply. I'm getting excited about this build. I'm still an electronics novice so this will be the most complex thing I've built thus far.

 

[Andy_H]

Finally got all the parts and got started putting it together, but I ran into an issue when I got to step 1g (testing the power supply circuit). No sparks, pops or smoke, the fuse did not blow; however, I'm reading 0V from P10 to P13. P10 to P3 reads 19.1V which I figured is probably close enough to 18V. I can't see any wiring errors or deviations from the plan, but I'll have to take another look tomorrow after I get some rest.

One thing I am wondering is if perhaps my $5 no-name DMM I found at a closeout store doesn't read negative voltages? I don't know much about meters, but I do know I need to get a better meter one of these days.

 

[westend]

Thx for the recommendation. I hope the build goes well for your visitor.

Our house is Art Moderne style, built in 1939. White painted brick exterior with major glass block wall areas. Interior is wet plaster over metal lath (they didn't anticipate wi-fi). No decorative trim. It originally had a large second story summer room with jalousie windows and the building had a flat roof. We're 1000 miles too far north for those ideas so the outdoor room was incorporated to double the size of the master suite and the roof is now peaked & doesn't leak in winter.
The Moderne touches are bourgeois divergences from Bauhaus minimalism. Thick marble floors in the bathrooms, foyer & elsewhere. Oak parquet in other areas.
In 1958 they added a 36' X 24' room with one wall done in 3 10' X 6' windows looking out on the back yard. It's floored in 70# slabs of travertine. When we viewed the house the first time there was a New York Steinway grand in one corner so we call it the ballroom. Technically it's an informal dining room & TV room now but usually it's just heaped full of my audio junk.

Hey, thanks for sharing that, it descibes what must be a great house. 1958 and a Steinway in that room conjures up a group of partiers knocking down cocktails and maybe spinning some Glen Miller with a few couples dancing. I've seen a few of these rooms of that era and the owners all reported the same use in those days. Great stuff.
My house is a bit older, '1920's and reflects Midwestern practical with a Craftsman look. We had lath and plaster, originally, but that has been replaced during the remodeling stages. I've tried to keep the original interior design elements and plan on doing the exterior in a similar manner. I'm surrounded by contemporary housing so my place is not the usual but it feels good to me.

 

[Bauhausler]

Finally got all the parts and got started putting it together, but I ran into an issue when I got to step 1g (testing the power supply circuit). No sparks, pops or smoke, the fuse did not blow; however, I'm reading 0V from P10 to P13. P10 to P3 reads 19.1V which I figured is probably close enough to 18V. I can't see any wiring errors or deviations from the plan, but I'll have to take another look tomorrow after I get some rest.

One thing I am wondering is if perhaps my $5 no-name DMM I found at a closeout store doesn't read negative voltages? I don't know much about meters, but I do know I need to get a better meter one of these days.

The DC voltages measured will be a little higher than 12.6V X 1.4 because the transformer's winding ratio isn't exact. Also your line voltage might be a little high. The voltage drops down into the correct range under load.

As for the zero reading from P10 to P13, try your meter reversed first. If it's still zero, check the polarity of the caps and the diodes and make sure that the (green) wire from p13 to AD1 is in place. That connects the diodes to the negative filter cap.

I have the board photographed in the article on my desk here.

 

[Andy_H]

Thanks for the advice. Considering it was getting late last night when I was working on this I will check it all out again with fresh eyes , but unfortunately I won't be able to do so till tonight since I have been called in to work this fine Saturday. Oh well, gotta get the money to pay for all this audio stuff, right?

 

[Bauhausler]

Thanks for the advice. Considering it was getting late last night when I was working on this I will check it all out again with fresh eyes , but unfortunately I won't be able to do so till tonight since I have been called in to work this fine Saturday. Oh well, gotta get the money to pay for all this audio stuff, right?

I count 73 solder joints on a PCB the size of a business card. It's challenging but doable if you're careful. Go slow & I can walk you through it. It's worth doing for the sound. I can't believe how good this little circuit sounds, and neither can others who hear it.

 

[Andy_H]

After putting in a 16-hour day at work yesterday and a brief wrap-up this morning, now I've finally got time to get back to this project. I checked and checked again the polarity of all diodes and caps and that the jumpers were all in the right places. Next guess: bad solder joints. I went back and reworked the joints that looked potentially iffy, plugged in and tested again.

Good news! Latest readings are 18.9V from P10 to P3 and -18.8V from P10 to P13. Thanks for your continued "tech support", Bauhausler. On to the next step!

 

[Andy_H]

Well, I finished wiring the amp board the other day, but I hit another snag when I tested AC voltage with the dummy load. I gradually increased input volume with a sine wave test CD and the voltage steadily rose to 9V on the right channel, but I'm getting nothing on the left channel. The right channel dummy load resistor and heat sink clamp got warm, but on the left channel they stayed cool. Obviously the signal is stopping somewhere in the circuit on the left side, but I don't know how to figure out where. I checked once more that everything is in the right place and connected where the plans call for, and found one spot (can't recall off the top of my head exactly where, but it was near the chip) where I missed a connection. I soldered that but still got no signal.

I would guess that there's either 1) a bad connection somewhere or 2) a bad component. Could I have killed the chip by powering up the circuit with it wired incorrectly? When I get home tonight I'll take another look for poor solder joints, but other than that I'm not sure what to do short of re-doing the entire right channel (or calling it a success as a mono amp :stupid?

 

[Bauhausler]

Well, I finished wiring the amp board the other day, but I hit another snag when I tested AC voltage with the dummy load. I gradually increased input volume with a sine wave test CD and the voltage steadily rose to 9V on the right channel, but I'm getting nothing on the left channel. The right channel dummy load resistor and heat sink clamp got warm, but on the left channel they stayed cool. Obviously the signal is stopping somewhere in the circuit on the left side, but I don't know how to figure out where. I checked once more that everything is in the right place and connected where the plans call for, and found one spot (can't recall off the top of my head exactly where, but it was near the chip) where I missed a connection. I soldered that but still got no signal.

I would guess that there's either 1) a bad connection somewhere or 2) a bad component. Could I have killed the chip by powering up the circuit with it wired incorrectly? When I get home tonight I'll take another look for poor solder joints, but other than that I'm not sure what to do short of re-doing the entire right channel (or calling it a success as a mono amp :stupid?

There are a lot of ground connections - 5 on each channel IIRC - and they're all essential. Count the grounds on each side & make sure the dead channel isn't missing one.
Well, you have one working channel, so you can compare it to the 'bad' one and look for differences.
1) short out the signal inputs on both channels
2) leave the load resistors connected but you can take off the heat sinks
3) carefully power up the circuit
4) with the DC voltmeter, connect black to Ground
5) using the Red meter lead, compare DC voltages between channels on the solder side of the board. Start at the chip power pins. Be careful not to short out the closely-spaced pins with the lead tip.

If DC checks out, respond back & I'll figure out how to test the AC signals.

It's unlikely that the chip is dead. They're pretty rugged and have internal protection. I just repaired a fancy pro monitor speaker that had chip amps inside. The PSU caps on one speaker melted down at one point & stayed that way for hours. The chips were fine.

 

[Andy_H]

Good to know that the chip is probably safe. I will check out all this when I get home from work tonight. I'm 99% sure all the ground wires are in place but I do recall having some difficulty with soldering some of them to the ground loop so I'll double check the joints there.

I have a few questions though: What do I do to short the signal inputs? I should do this testing without a signal source connected, right? Oh, and which pin # is the power pin?

Again, thanks for your help.

 

[Bauhausler]

Good to know that the chip is probably safe. I will check out all this when I get home from work tonight. I'm 99% sure all the ground wires are in place but I do recall having some difficulty with soldering some of them to the ground loop so I'll double check the joints there.

I have a few questions though: What do I do to short the signal inputs? I should do this testing without a signal source connected, right? Oh, and which pin # is the power pin?

Again, thanks for your help.

The power pins are AG13, AC13, E13 and I13 (I think. The letters scale fell off of my board). Short the input by connecting signal input (1uf film cap) to ground on both channels. You'll be testing it with no input.

 

[Andy_H]

Got it figured out, and I didn't have to go into as much depth as I asked for advice on, sorry about that. I am learning that you really can't re-check the placement of components enough. What happened was I had placed pins 2 and 4 of the left channel chip one hole off of the plan. I kinda realized this and thought I had offset the connections where necessary to compensate, but it turned out that there were still a couple things connected incorrectly. I fixed this and tried the signal test again and now both channels test out OK. Whew!

I already have the metal pieces rough cut, and I'm thinking I may have this project completed by the end of the coming weekend.