LM1875 integrated Chip Amp with external PSU

[Bauhausler]

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.

That's great news. Sometimes finding one small problem is more involved than a major repair. the little issues can be hiding in plain sight.

Before finishing the metalwork you might drop in on the online discussion here. I'll be posting some build suggestions from another reader who's already finished.

Good luck.

 

[Andy_H]

Well, I finally got this thing all wired up last night. (I have a tendency to take forever to finish any project, and I get sidetracked easily.)

But when I powered it on, it just buzzed for a few seconds and then blew the fuse. I powered down, unplugged it, put in a new fuse, it happened again. So obviously something's not wired correctly - maybe something shorting to the chassis? I just read the comments on the Make website and saw where somebody there was having the same problem. So when I get home tonight I will re-check the insulation of the chip tabs (I did use nylon screws, mica insulators, and heat sink compound) and any other possible shorts and post back here.

 

[Bauhausler]

Well, I finally got this thing all wired up last night. (I have a tendency to take forever to finish any project, and I get sidetracked easily.)

But when I powered it on, it just buzzed for a few seconds and then blew the fuse. I powered down, unplugged it, put in a new fuse, it happened again. So obviously something's not wired correctly - maybe something shorting to the chassis? I just read the comments on the Make website and saw where somebody there was having the same problem. So when I get home tonight I will re-check the insulation of the chip tabs (I did use nylon screws, mica insulators, and heat sink compound) and any other possible shorts and post back here.

Sounds like a power supply short to ground. This could be a short of a PCB component to the chassis. Or a soldering short of a power supply component on the board to one of the board grounds. Looking at your pic it looks like the diodes are oriented correctly. It's also possible that one channel has a shorted chip from the problem that you had before. If you can't find any physical shorts, try disconnecting the + and - supplies from the chip that had the previous problem and powering it up again. If it doesn't blow the fuse, then the problem is in that chip or its connections. It's not a bad idea to test the power supply separately with both channels' + and - disconnected to somewhat isolate the problem.

 

[Andy_H]

I didn't really have a chance last night to do anything other than a quick look. I can't see anywhere that anything on the board or any wires are shorted to ground. The chip tabs are connected to the chassis as follows: metal nut on top, (chip tab), layer of thermal grease, TO220 insulator, (chassis), nylon screw holding it all together. This seems like it should insulate the tabs but maybe I did something wrong here?

I am confident the diodes are oriented correctly as I got the initial power supply testing to go OK. I would also think that everything on the board is wired correctly as the testing of that part was resolved as in my previous posts. If I had to guess (with my limited knowledge) I would think I inadvertently shorted something when connecting the rest of the wires. Since the previous testing of the circuit ended up OK, wouldn't the chip be OK after all (i.e., not shorted or otherwise damaged internally)? Of course I am totally guessing here. I will try to find time later today to get into it deeper and figure it out.

 

[Andy_H]

OK, I finally got off my lazy ass and made another attempt to get this thing working. I lifted all the components out of the chassis, including removing the speaker terminals, and powered it on that way. The fuse did not pop, so apparently the short was to the chassis and not on the board. I began putting it back into the chassis one piece at a time, applying power after each addition to check. It became clear to me that the speaker terminals were the problem. None of my local Radio Shacks had the binding post assemblies in the original plans, so I substituted these. When I started to put things together I realized they wouldn't be insulated from the chassis and I thought I could fix that by putting plastic washers on either side of the hole, but I failed to realize that this would not prevent the metal center of the binding post from shorting to the chassis. So I reassembled it with the binding posts not screwed into the chassis, tested with a music source, and it works! Phew. I was getting frustrated. Now I just need to either get some different binding posts or else cut out a section and mount them to plastic like the RCA jacks. Either way, I'm getting closer to having this thing put together (and actually functional).

 

[Andy_H]

Success!

I finally got it finished and operational on Sunday. I was able to find the binding posts at my local Radio Shack after all - don't know how I overlooked them before. I took it to work for the short holiday week and used it as a headphone amp, and I've given it some auditioning back home in my main setup. Mainly I'm just pleased that it works at all, since it's by far the most complex device I've built in my short time working on electronics, but I am also really, really impressed by how nice it sounds - it's very clear and articulate and easily drives my JBL 4425s. You designed a winner here, Bauhausler. :thmbsp: Many thanks for your advice on getting this all together.

I of course need to take more time to evaluate it, and may have to do some tweaking still. I get some hum with no signal, probably a ground issue, but it doesn't seem to detract from the music. Also, I didn't do the best job on the metalworking and I think I may modify the case a bit. For one thing I think I would like to add wood side panels and I might change the top to a metal screen instead of plexiglass but I'm still deciding how I want to do it. I'll post pictures once I get it figured out.

 

[Bauhausler]

Andy;
Congrats on getting it all working. This is a pretty involved build with a lot of critical and tricky steps. Building an entire stereo amp from scratch is not a trivial thing. I'm glad you seem to have found the bug that was causing the short.

Regarding the hum, yes it is probably a ground loop issue somewhere. But headphones are extremely sensitive, and it might not be possible to get the noise to zero on headphones. Most amps that take headphone drive off of the speaker terminals use around a 150 Ohm resistor in series with the signal for each channel (about 4 X the headphones' impedance, typically). This limits current and also reduces PSU noise to the headphones by about 10 dbs. If you haven't done this it might be all you need.

I think you'll find this amp surprises a lot of people. Back in the spring I took a prototype to a Southeast Michigan Audio Club beer/listening session to show off. I wasn't planning to play it but people asked to compare it to the SE 6B4G tube amp that was in the main system so we swapped it in for an audition. People at SMAC events still come up to me and remark on how good the chip amp sounded.

 

[Andy_H]

Thanks! A few more details about the headphone connections and the hum: The phone jack is connected to the chip outs on each side through a 150 ohm resistor per side and the ground is connected to the star ground loop. My phones are Audio-Technica ATH-M45 which are 60 ohms IIRC. Based on the 4x rule should I increase the resistor values perhaps? The hum happens on the speakers as well as the headphones. I just listened to it again and am wondering if there is an issue in my wiring of the selector switch or the input jacks. The hum is pronounced with the switch in the neutral (neither input selected) position and not affected by the volume position. Switched to input #1 the hum is less and it reduces when I turn up the volume pot. Switched to input #2 the hum is more pronounced (about the same as with the switch in neutral) but it increases when I turn up the volume. All of this is with no input signal.

The other quirk I observed is that the channel balance is not perfectly synchronized at very low volumes. In other words, if I turn down the volume pot with a source playing the right side will fade out before the left side. This is actually an issue when listening through headphones because it happens right at my threshold for comfortable listening levels for most CDs - the channels start to go out of balance at that level. Would increasing the values of the headphone jack resistors help here? I also wonder if I may have damaged the internals of the potentiometer because when I was cutting the shaft I accidentally spun the knob all the way past its stop. I suppose it wouldn't be too difficult or expensive to replace the pot and try some different value resistors on the headphone out for the sake of investigation.

 

[Bauhausler]

For 60 Ohm headphones you could use a 220 to 270 ohm resistor but it won't make a huge difference over 150 Ohms. Maybe leave the 150 Ohm in place and put 47 or 68 ohms in parallel with the headphones for each channel (from signal to ground, at the headphone socket). That will give more attenuation while damping impedance variation & giving flatter response.

The volume pots rarely track exactly at the low end, and cheap ones are worse. All of mine have one channel lower than the other at very low volumes. If you can, lower the volume of the source component at bit to get the amp volume control position up into a useful range.

If you used a 3-pole selector switch the input jacks should be grounded only through the switch and not through the jack body to the chassis. It does sound like your hum problem is in the input section since the vol control position affects it. It sounds like input #2 might not be having its neutral grounded through the switch when it's selected. That would cause the hum to increase as the volume is turned up.

You could try jumpering the input jack grounds to the star ground point with a cliplead & see if that affects the hum. If it does, then the switch wiring is not properly connecting the selected input's ground to the star ground point.

Thanks! A few more details about the headphone connections and the hum: The phone jack is connected to the chip outs on each side through a 150 ohm resistor per side and the ground is connected to the star ground loop. My phones are Audio-Technica ATH-M45 which are 60 ohms IIRC. Based on the 4x rule should I increase the resistor values perhaps? The hum happens on the speakers as well as the headphones. I just listened to it again and am wondering if there is an issue in my wiring of the selector switch or the input jacks. The hum is pronounced with the switch in the neutral (neither input selected) position and not affected by the volume position. Switched to input #1 the hum is less and it reduces when I turn up the volume pot. Switched to input #2 the hum is more pronounced (about the same as with the switch in neutral) but it increases when I turn up the volume. All of this is with no input signal.

The other quirk I observed is that the channel balance is not perfectly synchronized at very low volumes. In other words, if I turn down the volume pot with a source playing the right side will fade out before the left side. This is actually an issue when listening through headphones because it happens right at my threshold for comfortable listening levels for most CDs - the channels start to go out of balance at that level. Would increasing the values of the headphone jack resistors help here? I also wonder if I may have damaged the internals of the potentiometer because when I was cutting the shaft I accidentally spun the knob all the way past its stop. I suppose it wouldn't be too difficult or expensive to replace the pot and try some different value resistors on the headphone out for the sake of investigation.

 

[Andy_H]

Reviving this thread with a question (can't get a PM to you, Bauhausler; your inbox is full):

I have been really happy with the Squelette amp that I finished back in November. I've been using it in my office every work day and it's been great. I decided to build another copy of your circuit but in a different enclosure and as a straight-up power amp. (I have a thread here, BTW.)

But I do have one question about building it without the volume pot. In the Make article it says "… you can build the Squelette as a plain power amp by replacing the 100k volume pot with a 100k resistor to ground for each channel." I'm not 100% clear on how to hook the 100k resistor into the circuit. From looking at it, it seems like I could achieve this by using a resistor each to connect I8 and AD8 (left & right inputs) to the star ground loop, but I don't want to smoke anything by hooking it up wrong. I would greatly appreciate it if you could please advise as to the correct way to connect the 100k resistors. Thanks in advance!

 

[Bauhausler]

Reviving this thread with a question (can't get a PM to you, Bauhausler; your inbox is full):

I have been really happy with the Squelette amp that I finished back in November. I've been using it in my office every work day and it's been great. I decided to build another copy of your circuit but in a different enclosure and as a straight-up power amp. (I have a thread here, BTW.)

But I do have one question about building it without the volume pot. In the Make article it says "… you can build the Squelette as a plain power amp by replacing the 100k volume pot with a 100k resistor to ground for each channel." I'm not 100% clear on how to hook the 100k resistor into the circuit. From looking at it, it seems like I could achieve this by using a resistor each to connect I8 and AD8 (left & right inputs) to the star ground loop, but I don't want to smoke anything by hooking it up wrong. I would greatly appreciate it if you could please advise as to the correct way to connect the 100k resistors. Thanks in advance!

That's correct. If you omit the volume control, for each channel connect the RCA jack directly to the channel's input (I8 and AD8) and connect a 100K resistor from there to the star ground point.
If you think about the volume pot, when it's turned all the way up the input is connected directly and the resistance of the pot is just 100K to ground.
Without a volume control, the circuit will have an amplification factor of about 20:1, meaning it will be driven to full output into 8 Ohms with 8/20 of a volt, about 0.40VAC. Most line level source components like CD players, etc have an output level of around 1.0 to 2.0 volts, so you'll need some way to attenuate that such as the component's built-in volume control.

 

[Andy_H]

OK, cool. Thanks for the super-quick response. My just-beginning-to-understand-a-bit-about-how-electronics-work self is pleased that I had the right idea. :nerd: Although I also was suspicious that creating a connection from the input to the ground would create a short circuit. Guess I still don't understand this stuff all that well. :scratch2:

Anyway, I wanted to build this one as a power amp because I have a few different preamps I can try it with, so the source volume control won't be an issue.

 

[Bauhausler]

OK, cool. Thanks for the super-quick response. My just-beginning-to-understand-a-bit-about-how-electronics-work self is pleased that I had the right idea. :nerd: Although I also was suspicious that creating a connection from the input to the ground would create a short circuit. Guess I still don't understand this stuff all that well. :scratch2:

Anyway, I wanted to build this one as a power amp because I have a few different preamps I can try it with, so the source volume control won't be an issue.

You have the right idea. It is a 'short' to ground but a very high impedance one. The source component probably has an output impedance of no more than 1000 ohms. Shorting that to ground wit ha 100,000 ohms resistor reduces the available signal voltage by 1% (1,000/100,000). If you used a resistor of 1,000 Ohms to ground instead, the signal would be reduced by 50% and the source component would have a hard time sending current through the low resistance it was driving.
The 100K resistor is there to provide a ground reference for the source component. The input to the chip is actually AC coupled through the 1uf film cap, so the chip never 'sees' the 100K resistor at the amp input. If you left that 100K resistor off, then any DC leakage from the source component could charge up the input side of the 1uf film cap and develop a net DC static voltage there. That could cause noises ot spurious inputs on switching transients, as the DC would be 'seen' by the amp as a low frequency signal. The 100K safely drains off to ground any stray electrons and keeps the input side of the 1uf cap safely at 0V DC while allowing the AC signal current to flow through it.
This circuit actually requires a lot of different electronics concepts to fully understand. If you're getting comfortable with this, you probably have the mental; tools to fool around with other analog IC circuits, of which there are many.