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Old 3rd May 2011, 03:52 AM   #1
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Question Potentiometer Value :

I am building Peter Daniel's LM470 kit , with a Plitron 22 + 22
300 VA XFMR . Driving a pair of efficient two - way , 8 Ohm
6 1/2" Woofer speakers . Want to use the " Goldpoint Mini - V ".
How many " Ks " should I order ?

David V. Webber
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Old 3rd May 2011, 04:23 AM   #2
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I'm not familiar with the kit. What is the input impedance of the amp? What is your signal source?
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Old 3rd May 2011, 07:41 AM   #3
rabbitz is offline rabbitz  Australia
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Peter's LM4780 has Zin=22k.

I've used 10K and 20K with Peter's LM4780. I found it's actually better with an active pre (or buffer) after the volume control but that's a personal choice and depends on the output impedance of the source.
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Old 3rd May 2011, 02:17 PM   #4
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Default Questions :

The input wold most likely be a CD Player , presently an AMC CD8b .
But , also , various other inputs through the Parasound P/HP-850
Pre - Amp .

I don't know the values of the amp , but here is a schematic :

http://www.audiosector.com/lm4780%20amp.pdf




David
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Old 3rd May 2011, 05:17 PM   #5
AndrewT is offline AndrewT  Scotland
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10k pot has a maximum output impedance of ~2k5.
The input impedance of the 4780 as shown is 22k. R0=22k sets Zin.
That gives a ratio of 22:2.5 for Zin:Rs of ~ 9:1, OK.
Quote:
I don't know the values of the amp
! and you have started building it.
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Last edited by AndrewT; 3rd May 2011 at 05:19 PM.
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Old 3rd May 2011, 09:14 PM   #6
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Talking P.S. Question :

Where can I get a soldering iron " Pointy " enough to do
those " almost touching " LM4780 pins ?? !! David
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Old 4th May 2011, 05:06 PM   #7
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Is 100k POT too high for LM3886? with Rin 15kOhm.
According to the datasheet, Rin too high will cause the chip to oscillate when input is floating.
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Old 13th May 2011, 11:18 PM   #8
AndrewT is offline AndrewT  Scotland
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For Rin=15k, I will recommend 10k pot.
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Old 15th May 2011, 03:36 PM   #9
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Quote:
Originally Posted by DaveSail View Post
Where can I get a soldering iron " Pointy " enough to do
those " almost touching " LM4780 pins ?? !! David
Use Gel Flux from the radio shack. That will cause the solder to flow where it should go. . . rather than ball up into a mess.
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