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Old 2nd June 2010, 03:54 AM   #101
guaxian is offline guaxian  China
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Nice!
but I don't like 1875 & 2030A,
powerless @-@
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Old 6th June 2010, 09:49 AM   #102
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Originally Posted by guaxian View Post
Nice!
but I don't like 1875 & 2030A,
powerless @-@
Speakers are the output. The difference between a 25 watt amplifier and a 100 watt amplifier can be made up by a 6db difference in speaker efficiency.

Possibly, those would be slightly larger speakers, but this isn't larger than any other speakers that could make a similar output (that 25w is a big head start). Only the design is somewhat different, but its not less powerful.

An example:
Per each channel this example is 2 of 7" Dayton RS 4 ohm speakers, four of LM1875's arranged as a pair of parallel amplifiers. Each parallel amplifier (2 chips each amplifier) drives 1 woofer. The cabinet size of this speaker is modest, given that the output resistors have reduced the cabinet size requirement a bit. Now, that's just one channel. Twice that much would be sufficient to crack up plaster or drywall. Its physically about 100 watts per channel, except that the speakers mentioned are extremely efficient, and the example mentioned produces 400 watt equivalency (200w equivalency per channel).
In other words, this can make for some fiercely loud audio, more than sufficient for most home use needs.

You can scale to 4 or 5 woofers per cabinet and still have a nice light active speaker that's about the size of a footlocker. Just remember that it takes 2 of LM1875 (as a parallel amplifier) per each four ohm woofer.

This simple equipment can run nonstop without straining (no straining = still hi-fi when cranked up loud) and is scalable to much higher figures, which would destroy some part of your house because of too much force.

So, I can't imagine why anyone would think its powerless.
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Old 10th August 2010, 12:41 AM   #103
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Originally Posted by zymurgn View Post
Rick,

I(Allan) am currently working on that supply list for you, as well as a manual that is thorough yet not too confusing to a novice DIYer such as myself.

In the meantime, Brian and I can answer questions you might have to help you put those boards to use.
hi, any progress on the manual for this amp.? I got the kits and transformer in the mail, and am a bit confused about how to hook up the transformer to the AC and volume control. The boards and transformer seem straightforward.

This will be used to drive some JBL speakers, (http://www.jblpro.com/pub/obsolete/4301.pdf) I've had since the early 80's (rebuilt many times)

Peter
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Old 16th August 2010, 06:09 PM   #104
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Never mind, I figured it out. Autoranging multi-meter solved one problem and an article in Make magazine, sorted out the other with AC wiring. Sounds great....
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Old 11th March 2011, 08:10 AM   #105
jemraid is offline jemraid  United Kingdom
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Default Mark Audio CHP-70's and LM1875

Hi all,

I have an LM3875 Gainclone with [very] standard components and a pair of 160VA 25v toroids.

Having heard a pair of Frugel Horn MK3's with the new Mark Audio CHP-70's driven by a pair of 300B's, [which I can't afford to make] I am going to make a pair of speakers myself.

I tried my GC with them and it was not in the same street as the 300B, CHP-70's are 4 Ohm 18 Watt drive units and I wondered if the LM1875 would be suitable for them.

Looking through the chipamp threads I came a cross a post from Dave [Planet 10] who said he prefered the LM1875 over the LM3875 and the LM3886. Looking at the LM1875 data sheet and if the complexity of the internal components is anything to go by then it wins hands down. It looks to me like a little National Semiconductor mistake, I am into photography and Canon make a cheap plastic 50mm lens that is up there with the 700 all metal ones with edge to edge definition over a full 35mm full frame sensor.

I wondered if anyone has made any side by side comparisons and if indeed the LM1875 would mate with the CHP-70's.

Any opinions would be very welcome.

Jim
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Old 11th March 2011, 09:02 AM   #106
jemraid is offline jemraid  United Kingdom
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Oh blast the 'edits' gone! The CHP-70's are 85 db.
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Old 11th March 2011, 09:23 AM   #107
AndrewT is offline AndrewT  Scotland
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Low impedance and low sensitivity combine to make a bad load for any of the chipamps.
The National chipamps perform better when the load impedance is high and the sensitivity is high.
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Old 11th March 2011, 09:49 AM   #108
jemraid is offline jemraid  United Kingdom
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Dear Andrew,

Blast!! :-)

I will have to make another JLH 10 Watt class A. My first one packed up after 10 years use, I can identify the components but when something goes wrong ......... I'm b0110xed

Thanks - Jim
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Old 11th March 2011, 09:53 AM   #109
AndrewT is offline AndrewT  Scotland
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10W into an 85dB speaker does not allow much overhead for transients.

If you were to build a 95dB 8ohm speaker, then either amp solution will work. The JLH and it's upgrades just that bit better.
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Old 11th March 2011, 11:09 AM   #110
jemraid is offline jemraid  United Kingdom
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Dear Andrew,

When I listened to Colin's [toppsy] CHP-70's in his FH3's he was using a pair of 300B's, max 8 Watts. I did try my LM3875 GC and though it didn't sound as good as the 300B's it wasn't having any problems that I could hear.

I did hear some Fostex FE126en's in the FH3's as well they are 8 Ohm 95 db but the CHP-70's sounded much better to my my ears.

Warmest wishes - Jim
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Last edited by jemraid; 11th March 2011 at 11:12 AM.
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