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Old 18th May 2008, 02:34 AM   #1
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Default 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
Click the image to open in full size.
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:
Click the image to open in full size.
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:
Quote:
Originally Posted by AndrewT View Post
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.
Click the image to open in full size.

Last edited by danielwritesbac; 18th September 2013 at 05:04 PM. Reason: 2A Fuses, same as the "K50" kit.
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Old 18th May 2008, 02:42 AM   #2
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There are kits for LM1875. Have a look:

http://www.electronics123.com/s.nl/it.A/id.425/.f

http://www.siliconchip.com.au/cms/A_103236/article.html

Mini LM1875 design Wow!

Finally made a LM1875 amp


And, we can build our own on veroboard (phenolic board), with parts available anywhere. . .
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Old 18th May 2008, 06:07 AM   #3
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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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File Type: jpg lm1875 simple five.jpg (98.1 KB, 8597 views)
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Old 18th May 2008, 06:11 AM   #4
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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

This is about $6 worth of parts.
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Old 18th May 2008, 06:56 AM   #5
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Default LM1875 in Technicolor?

Okay. I just got a request for "Technicolor"!

I can't wait to see John's reaction to this one.
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Old 18th May 2008, 09:39 AM   #6
Bluto is offline Bluto  United States
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Hey Gychang -

Hear that??!!

Famous!!

Someday I'm gona be famous too!

Bluto
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Old 18th May 2008, 10:57 AM   #7
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Quote:
Someday I'm gona be famous too!
"Fame is but a fruit tree
So very unsound.
It can never flourish
Till its stalk is in the ground."

Nick Drake. (Song 'Fruit Tree' from the album 'Five Leaves Left')
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Old 18th May 2008, 12:55 PM   #8
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Quote:
Originally posted by danielwritesbac
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!
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Old 18th May 2008, 01:03 PM   #9
Edits is offline Edits  Indonesia
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Hi, you give me reasons to cook my LM1875. I'am watching!
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Old 18th May 2008, 02:17 PM   #10
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Quote:
Originally posted by danielwritesbac
. . . 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.
Quote:
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.
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Old 18th May 2008, 02:35 PM   #11
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Default Re: Beginner's Gainclone, HiFi LM1875, The Amplifier Board

Quote:
Originally posted by Edits
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.
Quote:
Originally posted by danielwritesbac
. . . 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/projects...=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.
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Old 18th May 2008, 02:55 PM   #12
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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
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Old 18th May 2008, 03:26 PM   #13
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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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Old 18th May 2008, 04:30 PM   #14
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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.
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Old 18th May 2008, 08:44 PM   #15
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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/Pro...p?SKU=227-2060

Stereo format (two chips running from one transformer? Okay, use just one, just like this.
http://www.alliedelec.com/Search/Pro...p?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.
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Old 18th May 2008, 09:02 PM   #16
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Default 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.

Last edited by Mooly; 3rd December 2012 at 05:24 PM. Reason: Updated info from Daniel
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Old 18th May 2008, 11:23 PM   #17
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Default 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/Pro...p?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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Old 19th May 2008, 02:40 PM   #18
eketehe is offline eketehe  Indonesia
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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...
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Old 19th May 2008, 09:21 PM   #19
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Quote:
Originally posted by eketehe
. . . 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!
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Old 19th May 2008, 11:16 PM   #20
Edits is offline Edits  Indonesia
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Eka,
Quote:
' 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 Precious things to learn!
You know quite well how our people here deal with safety I don't think there should be forum titled "Obituary, about those who died in diy mission"
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Old 19th May 2008, 11:30 PM   #21
Edits is offline Edits  Indonesia
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Hi Dan,
Can not wait the sequel! Compiled those parts, but still miss the 27K feedback resistor. Can I substitute with 20k, 22k? or 33k?
Is there any "Design guide" for LM1875 like the one for LM3886?
Quote:
The actual "middle of the road" transformer selection corresponds to these transformers, with their 18-0-18 vac.
That is the one I'll try.
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Old 20th May 2008, 02:51 AM   #22
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Quote:
Originally posted by Edits
Hi Dan,
Can not wait the sequel! Compiled those parts, but still miss the 27K feedback resistor. Can I substitute. . .
There are many designs possible. This thread is about just one design, which does include a 27k resistor. Feel free to use the PM/Email feature!

The resistor values that I've used are widely available in "variety packs" from retail storefront electronics hobby shops.
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Old 20th May 2008, 03:34 AM   #23
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Default Heatsink.

I had a knife out, scoring some plain veroboard so that it would snap to appropriate sizes, got curious and unpacked the box with the new heatsinks. . .

WOW!! OMG! That heatsink (above) just arrived in the mail. Its whopping huge! Some metric versus U.S. conversion error? Anyway, about half that large will do it.

Its so tall that I'll need to go to the hardware store and buy a metal cutting blade to fit my "Junior Sabre Saw" (we call that a JigSaw, here in the U.S.). Although that's quite inexpensive, if you do it, use a dust mask and eye protection.

You can also get heatsinks from these guys: http://www.apexjr.com/Sinks.htm

At "established" (old) computer shops, you can find good-size "Slot CPU" type heatsinks, probably for free. You can also find suitable heatsinks inside defunct equipment that's sold at "thrift" stores and second-hand stores, also at little to no cost.
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Old 20th May 2008, 06:39 AM   #24
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Default Building it

Here's a photo showing LM1875 with the pins slightly spread and fit into veroboard (plain phenolic), for easy soldering.
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Old 20th May 2008, 06:51 AM   #25
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This photo shows the pin spacing. Its a view from the bottom side of the board. Note that this is plain phenolic board, without any copper on it.

There's at least 1/8" of pin sticking through the board. And there's plenty of room to get a hook or loop of copper wire around any of the pins, without worries about any connections too close to any other.

See photo.
(click photos to enlarge)
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Old 20th May 2008, 11:22 PM   #26
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Next step, we get some nice 24ga or 22ga copper wire (or copper core wire) and do like this picture.

These 3 wires are about an inch longer than the edge of the board. Each has a small hook so it can make solid contact with the LM1875's pins.

These little hooks are dipped into the flux jar before soldering. That makes for instant success with the soldering.

Make the end of your soldering iron, bright, silver, clean, by wiping it with either a wet sponge or #0000 steel wool. That helps to make the connections nice and clean.
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Old 21st May 2008, 12:26 AM   #27
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Okay, here's our 27k feedback resistor.
That's its name because its one of the two resistors that make up the NFB. That stands for Negative Feedback Loop, and it sets your gain.

This 27k resistor is red, purple, orange.

After setting the resistor in place (as shown), I looped it around the pin of the LM1875 (as shown).

Because the connection is smaller than my fingers, I used Hemostats, although needle-nose pliers will do in a pinch.

Then I painted the area with flux (for success).
Lastly, I cleaned the tip of the iron and soldered.

I'm proud of this picture. Click on it.
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Old 21st May 2008, 12:56 AM   #28
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The partner to the feedback resistor is an 820R (820 ohms).
This one is gray, red, brown.

Connect as shown.
(click photo)
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Old 21st May 2008, 03:33 AM   #29
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Before we get much further, I need to say:
The design on this thread, and its schematic, are free for diy, non-profit use.
Okay!

Next, we'll hook up the speaker + wire. And, that step completes the connections to the chip.
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Old 21st May 2008, 04:26 AM   #30
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Its time to hook up the main ground lead.
This is thicker than any other wire.

The first of the power caps help hold the ground lead in place.
They're 100nF, which is 0.1uF, and they probably have the number "104" written on them.

You can use the plain 3 cent ceramic caps or something that looks prettier. See photo.

To hook up caps on veroboard, just hold your wire firmly against the pin/leg of the capacitor. Bend the pin/leg of the capacitor over the wire (it holds the wire). Apply flux, and solder into place.
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Old 21st May 2008, 04:36 AM   #31
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It wasn't talent. It was tools.

See these two posts:
http://www.diyaudio.com/forums/showt...01#post1509701
http://www.diyaudio.com/forums/showt...79#post1509879
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Old 21st May 2008, 05:15 AM   #32
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Its time to attach the onboard power capacitors.

Notice the orientation in the photo below.

You can use 470uF or 330uF.
A 50v model is recommended.

See that the centerpoint between the two power caps does connect to the ground (0v) lead.
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Old 21st May 2008, 05:29 AM   #33
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Here's some details about the power circuit.
(click the photo)
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Old 21st May 2008, 06:57 PM   #34
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Its time to do the input circuit! Whee!

The first, and most important part of the input circuit, is the 15k input impedance resistor.
This one is brown, green, orange.

It runs from the input + to the 0v (ground), thereby presenting a load.

Now, in the photo below, the "input star ground" has started to form. Have a look at the photo below and see that the NFB and the input will ground at the same point (marked in green).
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File Type: jpg lm1875 on veroboard input impedance 15k.jpg (96.6 KB, 2129 views)
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Old 21st May 2008, 08:21 PM   #35
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The following photo will show the hookup from Power ground to Input ground.

Compare this with the photo above. We have added just the one wire to make this connection.

This will complete the "input star ground" See photo--its marked in green.

EDIT: Notice that only one end of the 15k resistor connects to ground. The other end (marked yellow in the photo) is your signal + line.
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Old 22nd May 2008, 12:39 AM   #36
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Here's a bit clearer picture.

Notice the input star ground.

Green = Ground
Yellow = Input signal
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Old 22nd May 2008, 01:16 AM   #37
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Optional Component

This post is about an optional component called the "groundlift" resistor. It can be from 2.2 ohm to 8 ohms.

What it does it to make your input circuit "less attractive to hums" because a bit of added resistance prevents, "point of least resistance."


Compare the photo above with the photo below.
Notice the optional component has been added--the groundlift resistor.

This one is 2.2R (2.2 ohms) and its red, red, gold, as shown.
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Old 22nd May 2008, 02:15 AM   #38
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Hey, let's add our input filter cap and its compensation resistor.

The input filter cap blocks DC from entering the amplifier.
This particular input filter cap, I spec'd for 2.2uF.
*See previous discussion at post #8 and post #10.

Use either a "made for audio" electrolytic or any "poly" cap.

And, because source equipment generally dislikes a capacitive load, we have a compensation resistor. This compensation helps prevent the "endless search" for the "perfect" input filter cap. So, putting a resistor here is so much easier.
This resistor is 470R (470 ohms) and this one is yellow, purple, brown.

These are in-series with the input signal line.
See this photo and the next photo.
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Old 22nd May 2008, 02:19 AM   #39
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Here's the "color" marked photo, showing the input filter cap and its compensation resistor.

Ground is shown in green
Input signal is shown in yellow
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Old 22nd May 2008, 03:12 AM   #40
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Optional Component, RF blocker
This is a tiny capacitor, 330pF (says "331" on it), that is added onto (parallel) the 15k input impedance resistor.

The RF blocker is a noise reduction component, thereby increasing the efficiency of the amplifier.

And. . .


Optional Component, input cable load
This is a 1M (1 megaohm) resistor.

It goes from signal input + to signal input - (as a load).

This resistor presents a tiny load, and its helpful that your input cords shall have a resistive load at the amplifier.
So, the 1m resistor is also a noise reduction component.
This one is brown, black, green.
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Old 22nd May 2008, 03:23 AM   #41
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Here's the "color" marked photo showing the RF blocker and the 1M resistor.

Green is ground
Yellow is input signal +
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Old 22nd May 2008, 03:42 AM   #42
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Here's the speaker hookup.
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Old 22nd May 2008, 04:16 AM   #43
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Optional Component, Output Zobel

Because a speaker wire is like a big antenna, and it leads directly into the NFB, then an amplifier could get some noise in through the speaker leads. A speaker output zobel helps block some of that. This is a noise reduction component.

In addition, the Speaker Output Zobel serves to enhance the stability of the amplifier.

This zobel shouldn't be within the audio band, but rather, just above.

For this, I've selected:
10R (10 ohms) and this one is brown, black, black,
plus
10nF, which is 0.01uF and probably has "103" printed on it.

Zobel. That's a resistor and capacitor, "RC," as a load.
This load is upon the speaker output.

See picture.
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Old 22nd May 2008, 04:28 AM   #44
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And now, the complete amplifier in "dazzling technicolor"

Hey, don't worry! I'll put a nice clean photo up after this one.
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Old 22nd May 2008, 04:34 AM   #45
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Complete.

Please do trim the connection wires short.
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Old 22nd May 2008, 05:05 AM   #46
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Here's an example on how to connect the external components (such as the power supply board). Tie the connections up tight, prior to soldering. See photo.
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Old 22nd May 2008, 06:13 AM   #47
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Quote:
Originally posted by Edits
Hi Dan,
Can not wait the sequel! Compiled those parts, but still miss the 27K feedback resistor. Can I substitute with 20k, 22k? or 33k?
That is the one I'll try.
Okay! Now that its done, we can mod it if you like.

Its cross-compatible with the AudioSector resistor values, so 22k with 680R can work for the NFB. That's gain 32.

If your source/preamp is really strong or if you've used less than the recommended voltage, then you can use:
20k with 680R = Gain 30
22k with 820R = Gain 28
20k with 820R = Gain 26

*There may be slight differences in DC offset with different gain settings, and the chipamps do vary between individual samples.
Do check with your meter, at the amplifier's speaker output.

**I set a rather high gain of 35 (that's 27k with 820R) to help modern sources with digital volume controls that shouldn't be run "maxed out" because they are often op amp chips.

The National Semiconductor Overture Design Guide spreadsheet does work with with LM1875. Dial in LM1876, which is two LM1875's and an extra circuit crammed onto one LM1876 chip. The resulting designs will work for LM1875.
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Old 22nd May 2008, 02:13 PM   #48
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Daniel you have made an excellent and helpful article .

Why don't you upload it in a website or a blog .It will be very helpful for us .

Please keep it up .

Somak
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Old 24th May 2008, 05:32 AM   #49
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hi Daniel,

personally i like my input cap to be poly film, i dont know about anybody else, but my guitar sounded much better.
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Old 24th May 2008, 10:22 AM   #50
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Quote:
Originally posted by casiomax
hi Daniel,

personally i like my input cap to be poly film, i dont know about anybody else, but my guitar sounded much better.
Oh yes! You can use Polypropylene, Polyester, Ceramic, Electrolytic, and botique types. And, you can use them solo or in combination.

One combination, was recommended by "that very helpful guy who earth-grounds everything very thoroughly," and that suggestion was 4.7uF + 0.47uF.
I haven't tried it yet, because I was pretty happy with just 0.47uF part.

So, do what ever sounds good.
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