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Old 11th September 2011, 03:00 PM   #1
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Default Questions about building a Pre-Amp

Hello all!

I have decided to add into my design a Pre-Amp. I would like this to incorporate volume control as well as bass and treble cut circuits. If possible, I would like it to somehow share the 300VA 24V transformer that I am using for the LM3886 Mono amp it will preface. Any ways to make this happen? Also, any recommended chips / diagrams to look into? This will eventually become a practice amp for an electric guitar.

Or, even more helpful, an online resource that explains the theory of pre amps so that I may understand them more .

Thanks!
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Old 11th September 2011, 04:42 PM   #2
Minion is offline Minion  Canada
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Well , a 24v Transformer isn"t going to work for yer amp but a "24v 0v 24v" or "2 x 24V" will , hopefully that was just a typo error in yer post ......

You could build a Preamp running off of a Single supply (+35v DC) and use a Dual opamp like a NE5532 with one opamp as a Buffer with the tone controll after the buffer and the second opamp with a bit of gain .....

Personally I would go for an active tone controll as I find Passive tone controlls next to useless ......

What is going to be your audio source ??
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Old 11th September 2011, 04:47 PM   #3
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Quote:
Originally Posted by Minion View Post
Well , a 24v Transformer isn"t going to work for yer amp but a "24v 0v 24v" or "2 x 24V" will , hopefully that was just a typo error in yer post ......

You could build a Preamp running off of a Single supply (+35v DC) and use a Dual opamp like a NE5532 with one opamp as a Buffer with the tone controll after the buffer and the second opamp with a bit of gain .....

Personally I would go for an active tone controll as I find Passive tone controlls next to useless ......

What is going to be your audio source ??
Yes. Typo. Lol. It's a 24 x2 toroidal 300vA from antek. Active sounds much better, but i say that just from the sound of it. I know nothing about either and would like to learn so that I may make an informed decision on the subject. The source is an electric guitar. Mono.
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Old 11th September 2011, 05:17 PM   #4
Minion is offline Minion  Canada
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There isn"t a "Theory" to preamps , a preamp is basicly what it sounds like , it is used to bring up the incoming signal up to a level that is high enough so the poweramp can reach it"s full output power .......

But in a guitar amp it should also incorporate all the features that a guitarist would need like gain , Tone controlls , FX loop ect .....

You can use your Favorite distortion/overdrive pedal as a preamp which is basicly what I did for my first few guitar amps ...... So you can just use a Poweramp with a pedal or pedals in front of it .....


This site has a lot of pedal projects maybe you can find something interesting to build there .....

DIYstompboxes.com - Index


Good luck
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Old 11th September 2011, 07:29 PM   #5
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Click the image to open in full size.

Thanks for the replies. I think I have decided on the LM1036N based off of the attatched circuit. However, it has a Vcc of 9-16 Volts. The transformer I am using has a 2x24VAC, which is, before being rectified, already too high. Do I need a second transformer / recitifier board, or can I drop the voltage somehow to run it off the same board? Noob questions are always fun lol.
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Old 11th September 2011, 07:51 PM   #6
Minion is offline Minion  Canada
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Location: Cowican Bay , vancouver island
You can use a Voltage regulator like the LM317 to regulate the +35v DC down to between 12v-16v DC , as long as the current draw for the preamp is pretty low then there shouldn"t be a problem regulating down to that voltage , you might need a small heatsink for the regulator ....

You could also use a different transformer or even a dc adapter .....


I just finnished building a 50w guitar Head , it has a 2 channel tube preamp (4 tubes in total) and have relay channel switching via footswitch and a 3 band tone controll for each channel , a FX Loop and a 50w into 8 ohm power amp all squeezed into a 4U aluminum case ......

It sounds great and is loud as hell but also took me allmost 8 months to design and buld ......


Cheers


PS: there are guitar preamp Kits you can buy that sound good and are fairly easy to build which might be an option you might want to look into ....
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Old 11th September 2011, 08:50 PM   #7
! is offline !  United States
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That chip is spec'd for 45mA max current, and is a single power rail design. 24VAC from the transformer, using same rectifier bridge you use for the chipamp is fine, results in 33VDC on one of the amp power rails.

Take that 33VDC and ground to a LM317 regulator circuit set to 16V output = 17V drop x 45mA = 0.8W. I'd put a small heatsink on the regulator, but some people wouldn't.

http://www.national.com/ds/LM/LM117.pdf

You could use a different transformer or DC adapter but it would still be nice to regulate the output, so it would just add more parts, size, expense without benefit. Regulator then 45mA preamp isn't going to dirty up the amp power rail enough to matter, especially considering it was otherwise unregulated.
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