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Old 16th December 2012, 12:30 AM   #111
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Quote:
Originally Posted by lausar View Post
I appreciate the help Dan but I can't see myself adding all the trimmers and finding the initial time to do all that tweaking, at least not this winter. I have to fiddle with my TT motor and build a new set of SUT's this winter too.
Just 3 trimmers are a faster trip to hi-fi than desoldering to swap parts. However, there are other options.
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Originally Posted by lausar View Post
Can you get me started with a few values for the input section. I know it won't be optimal until I can find the time.
Your preamp has provided a great convenience with its excessive voltage output because that makes it possible to have the preamp see 124K load while the power amp sees 56K load. So, I've taken the post 101 design, guessed, checked it with an engineer and put it onto a K50-looking schematic for you. Since your preamp already has a cap, C1 is a solid copper wire and R2 is located at the power amp's RCA jack.
See attachment. I believe it is "in the ballpark" and can get you "up and running" very fast.
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File Type: gif LM1875-K50-LowGain-LightLoad.gif (17.7 KB, 441 views)
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Old 16th December 2012, 11:19 AM   #112
AndrewT is offline AndrewT  Scotland
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I'm doing my homework...

Understanding and redesigning the input stage of LM1875
Lausar,
use the schematic in the Thread you linked.
Change R3 from 22k to 100k. That should get the output offset down a fair bit.
Change R1 to a film capacitor.
Add xxxpF across R3. Maybe from 330pF to 1000pF to suit your ears and your speakers.

Read the rest of the Thread and pay attention to the grounding and the different parts of the grounding.
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Last edited by AndrewT; 16th December 2012 at 11:22 AM.
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Old 16th December 2012, 12:53 PM   #113
lausar is offline lausar  United States
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Originally Posted by AndrewT View Post
Lausar,
use the schematic in the Thread you linked.
Change R3 from 22k to 100k. That should get the output offset down a fair bit.
Change R1 to a film capacitor.
Add xxxpF across R3. Maybe from 330pF to 1000pF to suit your ears and your speakers.

Read the rest of the Thread and pay attention to the grounding and the different parts of the grounding.
Thanks Andrew T.
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Old 16th December 2012, 09:12 PM   #114
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The attached schematic has nearly the same gain setting and ~100k input load but with your preamp's excessive voltage output put to good use--The amplifier sees significant feedback current and significant input load for higher resolution performance that relates to larger perceived soundfield and good stereo imaging. That's the difference.
Attached Images
File Type: gif LM1875-K50-LowGain-LightLoad.gif (17.8 KB, 374 views)
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Old 16th December 2012, 10:52 PM   #115
lausar is offline lausar  United States
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Default An early Xmas present...

Quote:
Originally Posted by danielwritesbac View Post
The attached schematic has nearly the same gain setting and ~100k input load but with your preamp's excessive voltage output put to good use--The amplifier sees significant feedback current and significant input load for higher resolution performance that relates to larger perceived soundfield and good stereo imaging. That's the difference.
I'll be ordering up the parts from Digikey and Mouser tomorrow but won't be able to get started until after Xmas. I'll keep you posted. It was a fun week cramming all this stuff in.

Thanks again Dan and to everyone else who contributed to the LM1875 threads!
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Old 17th December 2012, 04:52 AM   #116
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Originally Posted by lausar View Post
I'll be ordering up the parts from Digikey and Mouser tomorrow. . .
Some Panasonic for all of the caps at the amplifier board perhaps?

P.S.
The actual 4.7u used across the rails is this model: http://my.mouser.com/ProductDetail/C...hF6LLFVMj9I%3d and it is installed directly from X5 to X7
The rest of the caps can be 50v tolerance for long life and preferably panasonic or nichicon (search diyaudio.com for favorites).
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Last edited by danielwritesbac; 17th December 2012 at 05:04 AM.
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Old 17th December 2012, 01:43 PM   #117
lausar is offline lausar  United States
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Quote:
Originally Posted by danielwritesbac View Post
Some Panasonic for all of the caps at the amplifier board perhaps?

P.S.
The actual 4.7u used across the rails is this model: SEK4R7M250ST Cornell Dubilier | Mouser and it is installed directly from X5 to X7
The rest of the caps can be 50v tolerance for long life and preferably panasonic or nichicon (search diyaudio.com for favorites).
Thanks, I'm a Nichicon KZ and Elna Silmic II believer.
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Old 17th December 2012, 02:22 PM   #118
lausar is offline lausar  United States
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Quote:
Originally Posted by danielwritesbac View Post
Some Panasonic for all of the caps at the amplifier board perhaps?

P.S.
The actual 4.7u used across the rails is this model: SEK4R7M250ST Cornell Dubilier | Mouser and it is installed directly from X5 to X7
The rest of the caps can be 50v tolerance for long life and preferably panasonic or nichicon (search diyaudio.com for favorites).
What's the 4.7uf used for?
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Old 17th December 2012, 02:39 PM   #119
lausar is offline lausar  United States
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Quote:
Originally Posted by danielwritesbac View Post
The attached schematic has nearly the same gain setting and ~100k input load but with your preamp's excessive voltage output put to good use--The amplifier sees significant feedback current and significant input load for higher resolution performance that relates to larger perceived soundfield and good stereo imaging. That's the difference.
What will the gain be and can you give me the formula?
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Old 17th December 2012, 04:24 PM   #120
JMFahey is offline JMFahey  Argentina
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Just arrived (no, didn't read the 12 pages of posts) but curious: Why do you add unnecessary output capacitors in series with the speaker?.
Besides showing them as polarized in post #111.
Wiring them in antiparallel will not turn them into bipolars.
In post #114 you attribute no polarity to them, maybe hinting you'll use bipolars there.
I wonder where you will find 1000uF bipolars but even more what's the perceived advantage of adding them there.
AFAIK the Gainclone idea is to build a very simple very flat and clean "piece of wire with amplification" , and big electrolytics in series with the speaker do not exactly fulfill that task, at all.
Plus, as I said before, you don't need them, that split supply amp has 0V DC at the output.
And a polarized electrolytic is not happy at all (as in exploding with a loud bang) with AC across its terminals.

The gain in this case is (R5/R4)+1=(68/3.9)+1=18.4
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