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Old 24th March 2008, 01:13 PM   #1
bartek is offline bartek  Australia
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Default Different pre amp idea

Hey Guys I've just been floating around the forum the last few weeks and have already learned a lot but couldn't find the answer to my question. Its my first post and I've still got to learn a lot so be easy on me

Basically I want to make an integrated amplifier and the was thinking of making a decent quality headphone/preamp circuit and (using a select switch) choosing to send the output from the headphone/preamp straight to the headphone jack or choosing to send the headphone/preamp output to the speaker amplifier input.

I was thinking of using a Jung multiloop circuit for the headphone/preamp like the Pimeta described on Tangent's site (except only amplifying two channels and using cheaper opamps) and a basic LM3886 chip amp for the speaker amp.

...Is this a good idea or should I just put the select switch after the volume pot and send the signal to the head amp OR the chipamp?....
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Old 25th March 2008, 01:15 PM   #2
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If you have a volume pot after a switch that selects between two outputs, then you need two volume pots?

As far as using "cheaper opamps" - why?
They're dirt cheap now!!

Use good opamps.

The circuit is overkill now, with the loop feedback after the buffer.
It would probably work and sound just fine without the extra loop... but no matter...

Not sure why one needs the current source pull down at the output of the opamp - presumably to pull it into "class A" - given the vanishingly low distortion figures of the latest crop of opamps... dunno, maybe there is some justification still... guess it won't hurt.

Although it is heresy to some, my feeling is that you can likely still hear differences depending on the choices of opamps, fets, and especially buffers... maybe even the resistors.

More heresy, you might actually like listening to it more if you use carbon comp resistors not metal film...

Thing is... you do not really need a preamp to amplify things that are already at line level on their outputs, assuming the gain of the amplifier (chipamp?) that you are using drives to full power (or loud enough) with the line output of your source! You'd only want a volume pot, and perhaps it can be argued a buffer before the pot, not a gain stage.

If you use a gain stage, you'll just be knocking down the gain with the pot... it's ok, but something to think about.

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Old 25th March 2008, 03:30 PM   #3
bartek is offline bartek  Australia
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Thanks for your reply bear.

I forgot to mention that I was planning to omit the class A bias, and you're right about using good opamps but pricing in Aus can be unreasonable... it will be cheaper to order everything from the US than from the shop down the road...

The idea was that if I already had a decent headphone amp circuit I could use it as a preamp for the speaker amplifier when needed; as you pointed out though the gain might be too high so perhaps i will try reducing the gain of the headamp/preamp somewhat and also tweaking the speaker amp.
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Old 25th March 2008, 03:42 PM   #4
Pan is offline Pan  Sweden
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Or slap a pot on the LM3886 with an extra resistor as a pad. Then add a 50-100ohm resistor in paralell with the speaker output for feeding the cans.


/Peter
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