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Old 7th February 2016, 08:53 PM   #1
bendio is offline bendio  Switzerland
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Default Newbee: LM3875TF P2P

I am new to DIY Audio or in fact DIY electronics. Nevertheless, I want to build a ChipAmp using the LM3875TF Amps. Since I couldn't get hold of an PCB in Switzerland - and because it should be more fun - I decided to go either on stripboard or P2P.
I looked at the schematic on the data sheet of the chip and, mainly, to the instructions given by audiosector for their kit.
Based on that, I came up with the attached schematic. Now I would be really really grateful if someone more experienced could have a look at that. I am wondering whether having one place where all the different grounds go together (star ground?; signal and power ground?) is correct.
I also could not get hold of Caddock MK132. I have a carbon resistor (Yageo), 1/4 W, 5% or a metal foil (Royalohm) 1/4 W as possible replacements. One of these any good?
Thanks-
Benjamin
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Old 7th February 2016, 09:19 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by bendio View Post
I am wondering whether having one place where all the different grounds go together
(star ground?; signal and power ground?) is correct.
Use power supply bypass capacitors on pins 1 and 4, 10uF//0.1uF on each, mounted close to the IC.
Also you need a 22uF (or more) capacitor in the nfb loop between R3 and ground, to minimize DC offset.
See the data sheet for details. http://www.ti.com/lit/ds/symlink/lm3875.pdf

Point to point can be good if carefully done, but a single "star ground" is not really the best method.
That's too long a discussion for here, though. Use high quality resistors (Dale RN65C, etc.) for the nfb loop.

Check out this forum. Chip Amps - diyAudio

Last edited by rayma; 7th February 2016 at 09:44 PM.
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Old 8th February 2016, 03:25 AM   #3
tonga is offline tonga  United Kingdom
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Hey Bendio,

While I am by no means an expert, I built a point to point LM3875 last week so I can at least tell you how I did it

I used the same circuit as yours pretty much, albeit with the inclusion of an input resistor because I didn't use a pot. For grounding scheme; I made power star ground directly on the capacitor leads (which were soldered together) and here I connected the speaker return and positive and negative power ground from the diode bridges. I made signal ground at the ground tab of the input RCA and here I connected R2 and R3. I connected signal ground to power star ground with a wire.

I used vero board for the rectifier diodes and omitted the two bypass capacitors altogether and mounted the 1500uF caps directly onto the IC power pins. It is easiest if you remove the NC pins apart from pin 11 and run a wire to that from the negative supply pin(4) - this makes it easier to connect the caps directly to the chip. I used a physical layout similar to this Mick Feuerbacher Audio Projects (although I am neater!) and that is where I got the ground scheme from. I built each channel on its own heatsink which I scavenged from an old Cambridge amp and I mounted the RCA's and speaker terminals on it such that I could make all the connections to them using the leads from the resistors.

I would not worry too much about the resistor types for a first go - keep it cheap in case you blow it up I used 0.6W Vishay metal films 1% and it sounds pretty good to my ear. I've just built up Audiosector's kits with Caddocks so I'll compare the sound of both approaches.

I'll try and post a picture later today if I can.

Good luck, take your time, measure the offset (with the input shorted) before you attach the speakers and use a bulb tester first time you crank it up.
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Old 8th February 2016, 01:13 PM   #4
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I also used p2p to build by setup, but not sure I would do it again like that.
Using a not too big stripboard can make your life lot easier. Specially if you are new in DIY. As Reservoir caps are about 10cm from the chip, I only used 100nF smoothing caps on the chip. Also as rayma mentioned, used a cap after R3.
Many schematics suggest to use inductor+resistor on the output, but I also left it out.
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Old 8th February 2016, 09:27 PM   #5
bendio is offline bendio  Switzerland
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Thanks Rayma for your suggestions. If I keep power ground and signal ground at two physically different points (like tonga did) - where would these capacitors connect to?
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Old 8th February 2016, 09:38 PM   #6
bendio is offline bendio  Switzerland
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Hi tonga,
thank's for all the information you provided!! It would be great to see some pictures!
Did you include some kind of "safety earth connector" as suggested by Decibel Dungeon (Building a Gainclone chip amp power supply.)?
And is it correct to connect the earth from mains to the chassis and then have a connection to power ground??
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Old 9th February 2016, 01:41 AM   #7
tonga is offline tonga  United Kingdom
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Hi again,

For the purpose of my test rig I did not employ a safety earth - the circuit is just mounted onto the heatsinks and is a very temporary proof of concept that only I have access too. However I do (and everyone must) use safety earth when making a more permanent home for my stuff. If you look at Peter Daniel's instructions for building his kit you'll see that he connects safety earth (from the mains) directly to the chassis he uses, he then connects power ground to the same point (this is his test set up). You may need to observe specific rules about how you achieve this depending on where you live.

Here is a picture of my build. Click the image to open in full size.
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Old 9th February 2016, 01:52 AM   #8
tonga is offline tonga  United Kingdom
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..and here is a high res close up of one channel. You'll notice that I did not remove the NC pins..I did when I built the other channel and that made it easier. Oh, and if you us terminal blocks like these always keep checking the wires are held nice and tight - they have a tendency to compress over time and become loose and if you lose a rail I think you'll get some nasty DC at the output!
Click the image to open in full size.

EDIT: apologies to all if the image size is too big, Mods please remove if that is the case!

Last edited by tonga; 9th February 2016 at 01:57 AM.
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Old 9th February 2016, 09:57 AM   #9
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I connected the earth to the chassis as it gives safety, but didn't connect the earth to the ground. But on my headphone amp that originally only had 2 pin connector it collected some noise from the air. And it goes away if I touch the its chassis. So I connected the house to the earth and it is silent. I never really understood why it is good to connect the earth to the ground. Those have separate functions and you only generate ground loops.
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Old 9th February 2016, 10:44 AM   #10
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Quote:
Originally Posted by mrWagner View Post
I never really understood why it is good to connect the earth to the ground. Those have separate functions and you only generate ground loops.
Fuse blows if transformer secondary shorts to primary (transformer fails).
If no connection, fuse will not blow and you have the mains at the amplifier output.
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