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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

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.
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:
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
AndrewT is offline AndrewT  Scotland
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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
regards Andrew T.
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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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File Type: jpg lm1875 makes hole in ceiling.jpg (91.3 KB, 5194 views)
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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:

Stereo format (two chips running from one transformer? Okay, use just one, just like this.

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.

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:

An example of the heatsink:
Just in case. . . then "too big" is so much better than too small. This one does two LM1875 chips.

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 member Puffin. They did the research on this although its also seen in 1970's era hifi receivers and tuners.
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File Type: jpg rectifier propeller.jpg (31.5 KB, 5186 views)
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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,
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Old 19th May 2008, 09:21 PM   #19
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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: 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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' 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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