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Old 21st October 2008, 11:45 PM   #1
danbli is offline danbli  Sweden
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Default LM3875 and preamp... and more...

Hello!

I'm gonna build myself a gainclone with a LM3875 and I already have everything sorted out for it.

My question is... Do I need a preamp for it? I'm gonna use it with a CD player, DVD player and my TV. Could I just make an input selector and use the sources directly to the poweramp for a start before I made a preamp??

I've tried today to make a test with TDA1524A and I think it sounded crap... I know that a lot of people of you doesn't have the tone controls, but sometimes i find them useful.

If you think I need a preamp I would appreciate some schematics for some simple ones.


/Daniel, Sweden
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Old 22nd October 2008, 01:24 AM   #2
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A properly built gainclone presents a high input impedance, so the need for a preamp is negated substantially, however:

- just wiring a pot up (a "passive preamp", if you will) presents a varying source impedance to the amplifier

- some sources will give an output that is too low in voltage swing to provide full power from your amplifier....ie. if your amplifier is is driving 8 ohm loads at 50 watts, then the voltage at the output is 20 volts RMS (I think. AC signals always confuse me a little). If the gainclone is configured with 30dB of gain, then the input signal will need to be about 0.7 volts RMS in order to obtain this output power.

Many CD players can provide 2 volts on their outputs, in which case a preamp is somewhat unnecessary.

All that said, I think a basic preamp CAN help enough to justify its cost. If you want tone controls, check out project 88 at http://sound.westhost.com/projects-2.htm . If you want something simpler, you should be able to work out how to adapt the gain stages shown for use without the tone controls. If you want to play with something discrete rather than opamp based, have a look at project 37a, which I currently use.
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Old 22nd October 2008, 05:59 AM   #3
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Default Re: LM3875 and preamp... and more...

Quote:
Originally posted by danbli
Could I just make an input selector and use the sources directly to the poweramp for a start before I made a preamp??
Yes, you could. You can still add a preamp later on.

Quote:
Originally posted by danbli
I've tried today to make a test with TDA1524A and I think it sounded ****... I know that a lot of people of you doesn't have the tone controls, but sometimes i find them useful.
If you have always used tone controls, it takes some time to get used to the sound of a flat frequency response. But after a while you may come to the conclusion that tone controls sound ****.
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Old 22nd October 2008, 07:27 PM   #4
danbli is offline danbli  Sweden
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Default Thanks!

Thanks for your answers!

I go for just the gainclone amp at first at maybe builds a preamp later.

I ordered the rest of the parts today so now I know what todo this weekend.


/Daniel
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Old 22nd October 2008, 07:52 PM   #5
lineup is offline lineup  Sweden
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if your cd-player has some form of volume control
then you will not need one preamp

for example myself, i can control the volume with my CD-Player remote control
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Old 23rd October 2008, 10:42 AM   #6
AndrewT is offline AndrewT  Scotland
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get the chipamp working using a passive attenuator.

Next project is a buffer stage between the attenuator and the power amp. keep the buffer an attenuator very close (cm) and use long cables from buffer to power amp.

Third project is a 4times (+12dB) pre-amp to replace the buffer.
You can use the redundant buffer on whichever of your sources has the highest output impedance and/or lowest ability to drive current into your attenuator.
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Old 23rd October 2008, 11:24 AM   #7
danbli is offline danbli  Sweden
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Default Volume control

Would a 10K Potentiometer do for volume control with just the GainClone? Before I built a preamp?

I was thinking about a preamp like this to maybe build after the GainClone is finished. http://sound.westhost.com/project97.htm

/Daniel
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Old 23rd October 2008, 11:49 AM   #8
AndrewT is offline AndrewT  Scotland
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Default Re: Volume control

Quote:
Originally posted by danbli
Would a 10K Potentiometer do for volume control with just the GainClone? Before I built a preamp?
as an interim attenuator, it will show and teach you a lot. Go for it.
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Old 26th October 2008, 04:58 PM   #9
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Thumbs up Lm3875 passive!

Hi danbli! I have an integrated amp LM3875 using a 25k potentiometer "passive attenuator".and I have to say one thing to you...GO for it!. you do not need a pre-amp or tone controls.just make sure you take your time,do not rush it,that way you will avoid common mistakes. Good luck.

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Old 26th October 2008, 08:21 PM   #10
danbli is offline danbli  Sweden
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Hi!

Thats sounds good to me... Do you have any pictures and schematics of your amplifier...?

/Daniel
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