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Old 1st April 2011, 03:43 AM   #1
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Default Planning LM1875 Stero Build. Advice plz!

First off this is a great forum and I want to say thanks in advanced to the community. I am planning a stereo LM1875 build using the chipamp kit. For power I'm looking at an Avel 160VA 18V+18V Toroidal. I plan to drive some FR88ex 8ohm 15w full range drivers used as computer speakers. My questions concern specific features and basic build choices.

1. As I plan on connecting this to a computer/external sound card, do I need a preamp?
2. How difficult would a headphone jack (turns off both speaker channels) be to implement and would it affect the driver sound quality?
3. For Audio in I plan on simple RCA jacks. For audio out are banana plus (amp and speaker) preferred over bare wire binding post?
4. What would be an appropriate gain setting (via the R3 gain resistor) to get the most out of a volume pot? (any recommendations 10k vs 50k? brand? How is this actually hooked to the audio signal chain?)
5. Lastly, I may decided to hook up a micro controller (arduino) to power a led style vu meter. Can power for this be pulled off the d/c side without affecting the amp/audio?

This is my first stab at a proper amp (built a CMOY a few years ago) so thanks for all the help!

Last edited by bobsagat666; 1st April 2011 at 04:09 AM.
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Old 1st April 2011, 11:04 AM   #2
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
go to ESP and download his articles on Amplifier and Pre-amp and other Audio gear.
Go to his articles on Audio signal processing.
Finally look at some of his projects.

Look at Decibel Dungeon.
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regards Andrew T.
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Old 1st April 2011, 09:12 PM   #3
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Thx ill check it out
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Old 2nd April 2011, 12:08 PM   #4
johnr66 is offline johnr66  United States
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The transformer will work, but is way overkill at 160 VA.
1. No pre amp needed. The IC with standard recommended components give it plenty of gain.

2. Should not affect the sound quality. Not sure if they make those style of jacks that open the speaker connection when the headphone jack is inserted or not. Check around for that, otherwise use a speaker switch.

3. I'd use binding posts. It shouldn't matter. Use what you want.

4. The input biasing resistor will cause much attenuation of pots 25k or more, so, I'd use a 10k. People tell me that that resistor can be a higher value without affecting the amp, but it provides bias current for the input, so I'm inclined not to monkey with it. You can change the gain of the amp via the neg feedback loop, but going too low or to high of gain will make the amp unstable.

5. Don't know about your micro controller, but most have high high input impedance and should not affect the sound.
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Old 3rd April 2011, 03:58 PM   #5
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Quote:
Originally Posted by johnr66 View Post
The transformer will work, but is way overkill at 160 VA.
1. No pre amp needed. The IC with standard recommended components give it plenty of gain.

2. Should not affect the sound quality. Not sure if they make those style of jacks that open the speaker connection when the headphone jack is inserted or not. Check around for that, otherwise use a speaker switch.

3. I'd use binding posts. It shouldn't matter. Use what you want.

4. The input biasing resistor will cause much attenuation of pots 25k or more, so, I'd use a 10k. People tell me that that resistor can be a higher value without affecting the amp, but it provides bias current for the input, so I'm inclined not to monkey with it. You can change the gain of the amp via the neg feedback loop, but going too low or to high of gain will make the amp unstable.

5. Don't know about your micro controller, but most have high high input impedance and should not affect the sound.
Awesome thx. Yeah I was thinking the 160va transformer might be a bit much.
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