Beginner's Gainclone, HiFi LM1875, The Amplifier Board

Beginner's Chip Amplifier, HiFi LM1875, The Amplifier Board

This is for the purpose of an accessible hifi chip amplifier, it is in reference to Commercial complete Gainclone kit for a beginner? - diyAudio and it is by request. Here's LM1875, at low cost, and easily made by following the "play by play" photographic format (starting at post#24). The LM1875's "only 5 pins hookup," and the absence of spike system noise, can give you 25 watts per channel of high fidelity, at a bargain price, and with an easy time of building your own amplifier.

But, first, let's have a look at how "not" to use the LM1875: If you're planning the maneuver of pushing inefficient speakers with great force, then try something else. Our introductory LM1875 project here, has not been paralleled to withstand 4 ohm speakers.

LM1875 works great with efficient 8 ohm or 16 ohm speakers: Since this thread starts from Gychang's post and since he is famous for the full range hifi genre speakers, then I think that the laid-back hifi sounds of LM1875 could be a perfect fit. Any reasonably efficient 8 ohm speaker is suitable.

Circa 2012 update
Suitable transformer voltage is actually 18+18vac or less. These transformers are affordable: Antek - AN-0518 Monobloc/dualmono build is recommended for current control and wider imaging. For good longevity don't give an LM1875 chip access to more than 1a~1.5a worth of transformer. That's why Monoblocs really make sense.
Input load resistor should actually be 10k (not 15k).
Due to lack of NFB-shunt cap, consider adding high quality output cap assembly, in series to the speaker, for long lasting speaker protection. Adjust capacitance value to suit speaker size for enhanced headroom. Output caps strongly recommended if using irreplaceable speakers with Any chip amplifier kit or project. (click this link to see older nearfield design). The 2012 edition is a slightly refined version of the circa 2008 original, and suited to studio mixer/desktop use.

Circa 2013 update
371882d1379517833-beginners-gainclone-hifi-lm1875-amplifier-board-lm1875_daniel_2013-ii.gif
With its gain of 34X, a computer sound card or digiplayer can drive this amplifier easily, without straining the source device. Now for 2013, it does have the NFB-shunt cap so it can't amplify DC. The parts paralleling shown is for high end quality at low cost. Output caps are no longer shown on the schematic but they are fun to use for bass and headroom enhancement. Tone option: To compensate for a peakish speaker, add just ONE of this model 4.7u from V+ to V- (rail2rail cap) at the amplifier board's power circuit to cause quieter midrange with higher resolution. Mains fuse and speaker jack fuse are suggested. Each monobloc produces up to 25 watts of power to an 8 ohm speaker.

Power Supply
Here are CRC power supplies for these monoblocs:
338750d1364389626-beginners-gainclone-hifi-lm1875-amplifier-board-lm1875_power.gif

Option: You can use 3300uF 35v or 50v caps, such as Nichicon FW or Panasonic FC.

Transformer Voltage
Example transformer voltage is 18+18vac
Click on best match for your application to see a hookup diagram.
Your mains is 120vac and your transformer is 18+18 or less?
Your mains is 230vac and your transformer is 18+18 or less?
Your mains is 120vac and your transformer is 36+36 or less?(dual primaries in series)
Your mains is 230vac and your transformer is 9+9 or less?(dual primaries in parallel)
(please assure that the transformer output is 18+18vac or less)
And see Decibel Dungeon for a briefing on how to power your audio amplifier, including construction and safety.

Transformer Amperage
Greater durability can be had via good design and a right-sized transformer, or you could use a lot of fuses.
Monobloc/Dualmono:
♦ 36va*0.67=24 watts, which is perfect; however, we might want to adapt to dual secondaries and twin bridge rectifiers to maximize the little 1 ampere transformer. Use a mains fuse.
♦ 50va*0.67=33 watts, which is a bit too much; however, that's safe if given a Center Tap transformer (just one bridge rectifier) And the CRC power supply. Use a mains fuse and speaker jack fuse.
Stereo:
♦ 100va, instead of breaking chips, you may choose to use mains fuse and rails fuses for right channel amplifier board and rails fuses for left channel amplifier board and speaker jack fuses for each speaker. No matter if stereo or monobloc, a minimum of 7 fuses is mandatory if the transformer is greater than 50va.

Before powering on the new amp
Here's AndrewT's checklist:
Have you powered up via a bulb tester?
Keep this bulb tester in circuit until AFTER you have completed all tests and proved you have everything wired up correctly.
Have you powered the transformer alone and checked you have it wired correctly?
Have you powered up the transformer and rectifier and checked you have it wired correctly?
Have you powered up the transformer, rectifier and smoothing capacitors and checked you have it wired correctly?
Have you powered up the transformer, rectifier, smoothing capacitors and one channel of amplifier and checked you have it wired correctly?
Have you shorted the amplifier input and checked you have near zero output offset?
Have you measured the output noise with the input short in place?
Have you removed the input short and re-checked the output offset and output noise?
Have you connected your source and re-checked the output offset and output noise?
Have you tried moving the volume control and switching the source off and on and checked the output offset during all these changes?
Now you are ready to connect a dummy load.
I would also like to suggest putting a capacitor in series with the speaker for protector. It costs less than replacement speakers. And you get the Bonus of fun bass efficiency tuning for maximized headroom (a lot more power), by simply reducing the capacitance size to match what the speaker can do and roll-off what it can't do. This makes your amplifier output only what the speaker can use. Cheaper: If installed at speaker ground of a stereo build, two speakers can share the protector.

Avoiding destruction
It may be very difficult to get up and running without the NFB-Shunt Cap, so here's a photo of the old 2008 project re-fitted for greater durability and so it won't amplify DC. This added part, NFB-Shunt Cap is the electrolytic cap in the upper-left corner pictured. Whatever else it may do, it is absolutely certain to give you better odds at building a working amplifier. So, use it.
338772d1364394794-beginners-gainclone-hifi-lm1875-amplifier-board-lm1875.jpg
 
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For example, here's an LM1875 that I built on veroboard (phenolic, no pads) quite a while back.

For now, I'd like you to see that the 5 pins are quite useful and well located.
Here we have the input on the left side, and the power circuit is down the middle, and the speaker output is on the right side.

The optional items were deleted from this photograph, so you can have a clear view:

Edit: Note that the ground stripe that's down the center doesn't connect to the LM1875 chip.
 

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danielwritesbac said:
Parts list (per each channel):

LM1875
A size TO220 Heatsink Insulator

Phenolic board (veroboard)

Resistors
27k
15k
820R
470R
2.2R

Capacitors
2 of 470uF 50v electrolytic
2 of 100nF ("104") ceramic

Whoops! Where's the input filter cap?
I forgot to list the 2.2uF capacitor.
*This can be a "made for audio" electrolytic or a "poly" capacitor.

Application note:
Since LM1875 is a lower powered amplifier, you can use a poly input filter cap of a smaller value, so that it produces rich bass, but not lower than "about" 40hz. That "conserves" the power of your LM1875 so that it will play as loud as a larger amplifier. This poly cap can be as small as 0.47uF.

A friend sent me an e-mail about the missing cap, and he gave me some more goodies for our "optional components" section, which is coming up, right after the assembly demonstrations.
Thanks man!
 
danielwritesbac said:
. . . poly cap can be as small as 0.47uF.

And, he just emailed me again. The forum regulations prevent me from simultaneously quoting the contents of his email and giving his name.

I DO wish that I could give him credit. But, the content is most important, and here it is.
I don't agree that a low power amp should settle for reduced performance.

Therefore, choose your input filter cap size, based upon the capabilities of your speakers.
Simply put, that's "smaller with smaller" and "larger with larger"

We're aiming for efficiency, not limitations. ;) So, I hope to have cleared that up.
 
Edits said:
Hi, you give me reasons to cook my LM1875. I'am watching!;)

I think that's in reference to the awesome high fidelity capabilities of the LM1875, versus the slight cruelty that its not a 500 watt amplifier. Hey, it sounds so good that you really want to turn up the volume.

At the very first post, I dropped a hint about this.
danielwritesbac said:
. . . 88db efficiency or higher.

You can get high output, but not with voltage.
Do it with speaker efficiency.

See this example: http://www.partsexpress.com/projectshowcase/indexn.cfm?project=MagnaCumLaude
And consider that this style can be made with Pioneer B20 as the midrange, Pioneer's 3/4" cloth dome tweeter, and 95db (or so) prosound/DJ woofers available in your local market.
See also "Harbeth Monitor 40" for a rather spendy example of the same style speakers.

Yes we can do very loud hifi, as long as we remember that speakers are the output.
 
I would go further.
For a 25W amplifier I would put my money on >=93dB/W/m 8ohm speakers. Not 4 to 8ohm.
For a chipamp (with very limited peak current ability) giving that 25W I would try to find speakers that have a gentle to medium reactance, certainly not severe.

Personally, for best performance >=97dB/W/m 8ohm should satisfy most listeners. Way better than 88dB/W/m
 
Thanks man!

I posted the 88db as a sort of minimum suggested speaker efficiency.

The approximately 95db concert style speakers will come a lot closer to a concert realistic performance--especially if high output is desired.

Then, given efficient speakers, your LM1875 can put a lovely hole in the ceiling, like in this photo:
 

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At this point. . .

We've addressed the concerns about:
Power output
Ease of construction


Thanks for all the help!


Next up, we need to discuss voltage vs speaker impedance--as applied to transformer selection. And we need to address a small error (about 2v) in LM1875.pdf file from National Semiconductor. There are two authentic LM1875's and at least one clone. We're going to attempt a safe voltage selection (compatible with all versions) that will also create the advertised power output. That's coming up next.

After this next discussion, then we'll build it. :D
 
Let's say that we have no idea what our speaker impedance is or how to select a transformer. That's okay! We have a "basic" transformer selection here, based on real-life applications.

Monoblocs? Use one like this for each chip:
http://www.alliedelec.com/Search/ProductDetail.asp?SKU=227-2060

Stereo format (two chips running from one transformer? Okay, use just one, just like this.
http://www.alliedelec.com/Search/ProductDetail.asp?SKU=227-2065

The actual "middle of the road" transformer selection corresponds to these transformers, with their 18-0-18 vac.

After our power supply board gets done with it, we get approximately 26-0-26 volts DC. That's the maximum for use corresponding with 4 ohm speakers--and the LM1875 chip isn't harmed.
 
Important information regarding reliability under certain operating conditions.

Daniel has asked that this post be amended following reports of possible problems with the higher voltage under certain operating conditions.......
Updated info from Daniel,

"Using a higher voltage transformer as originally outlined here will result in an amplifier that appears to work perfectly for months and then, later, a broken chip outputting one rail into the speaker."

A 20-0-20 (40vct) is unsafe for LM1875 when speakers have impedance dips/peaks and unsafe for every power surge and during conditions of high line voltage. It takes about 6 months for LM1875 to break when running from that transformer voltage.

P.S.
There are other transformers mentioned on that thread, and the 18+18vac transformers are still doing just fine--Especially durable are the monoblocs made with inexpensive low amperage (1a to 1.5a when secondaries are in series like any center tap) 18+18vac (36vct center tap) right-sized transformers that will sag before the chip can break.


Original content for reference.
Let's say that we know for sure that we'll use our LM1875 with 8 ohm speakers. Okay! Then we can give the system a little "push" with a 40vct (20-0-20) transformer.

Here's an example of that:
Stancor Waldom - P-8566 - Power Products - Transformers - Allied Electronics

I know that's a big thing, but you'll get excellent results with your 8 ohm speakers, and the LM1875's inbuilt limiter won't trip.

Caution: The 40vct (20-0-20) transformer selection is theoretically incompatible with LM1875's that run 4 ohm speakers. Authentic LM1875's will trip their limiters, causing the audio to go off and on.

I know this because its that very transformer that runs my home system.

After our power supply board gets done converting the 20-0-20 AC then we have approximately 30-0-30 DC--near maximum for LM1875, but safe for use with 8 ohm speakers.
 
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Staying cool.

Now that we have an aggressive 30 watts of hifi, and now that we've allowed exuberance without clipping, let's have a look at how to keep this cool.

Two things:

1)
An example of the heatsink:
http://www.alliedelec.com/Search/ProductDetail.asp?SKU=619-0079
Just in case. . . then "too big" is so much better than too small. This one does two LM1875 chips.

2)
An example of the cool-running rectifier is in the photo below. We'll explore this more thoroughly in our power supply thread. For now, I'll say thanks to Mr. Mark Houston, and thanks to diyaudio.com member Puffin. They did the research on this although its also seen in 1970's era hifi receivers and tuners.
 

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Wow, my favourite :)
Well, as said, max +/-30vdc, put big trafos... they're will not clipping.
Daniel, you're really great in simplify thigs..
this potentialy vascinating newbiw to built their diy amp.
I hope your next thread will be :
' how to deal with. Mr Andrew ( for beginner )'.
its much better to do right since beginning, than just like i'm doing now stripping all things to improve my mains wiring, and put some fuses. ;)
No offence,
Thankyou...
 
eketehe said:
. . . I hope your next thread will be :
' how to deal with. Mr Andrew ( for beginner )'.

What to do with him? That would be. . . listen closely, look stuff up, and say "thanks" often. I asked him for help on this project and he's been fantastic.

Your post has reminded me that the power specifications are incomplete.
Please also include:
1). A wooden amplifier enclosure. (safety at the workbench)
2). Heatsink insulators, TO220 size and flat. (no volts at heatsink)
3). AndrewT's safety earth grounding. (safety during operation)

And a power supply much like this one: http://www.electronics123.com/s.nl/it.A/id.347/.f Although, we'll be building something nicer at our upcoming power supply thread (where further power discussions can take place).

I have been working on presentation materials for the build of this amp, but I have also been working on presentation materials for the power supply board's (seperate) thread.

And also, thank you for your contribution of the centerpoint reference (330uF) for caps at the amplifier board itself. I specified 470uF here, because of their greater availability. Anyway, thanks again!
 
Eka,
' how to deal with. Mr Andrew ( for beginner )'.

Every time I do anything related to this diy project now, my left ear wishper:"..check lightbulb!..", and my right ear :"Check Safety earth"..., thanks to Andrew:cool: Precious things to learn!
You know quite well how our people here deal with safety:whazzat: I don't think there should be forum titled "Obituary, about those who died in diy mission":devilr: :D