Guitar preamp x stereo boombox - diyAudio
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Old 2nd December 2013, 04:31 PM   #1
ATAUDIO is offline ATAUDIO  Austria
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Default Guitar preamp x stereo boombox

Soo,
my artist friends now ask me to hook an electric guitar (a bass, really) to the boombox...
I havent found much suitable circuits, so I have made a tentative on my own.
It should work with any electric guitar to any home stereo or boombox (BTW, like Aura NS6-255-8A 6" in a 12 L ported box) line input.

Just do not abuse of your home stereo with your rock guitar! It won't last long
Supply is the usual 9V battery, should last about one day of operation.
Gain is 50, if somebody wants more, just change R6 to a bigger value.
I have use quad op-amps, since I have some good ones at home, so I have used one stage just for the voltage splitter. Of course a dual op-am (i.e. TL072) would do, just use two 1k resistors for a rough voltage spitter.
Any comment or suggestion is welcome...
Attached Files
File Type: pdf Pre1.pdf (15.8 KB, 62 views)
File Type: pdf Pre1PCB.pdf (26.1 KB, 19 views)
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Last edited by ATAUDIO; 2nd December 2013 at 04:34 PM.
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Old 3rd December 2013, 02:16 AM   #2
JMFahey is offline JMFahey  Argentina
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Nice design
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Old 3rd December 2013, 08:01 AM   #3
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Quote:
Originally Posted by JMFahey View Post
Nice design
Seems a shame to waste an opamp though

Might be a nice idea to use it to add an extra balanced XLR output as well?
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Old 3rd December 2013, 11:48 PM   #4
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Line 6 Pod or just dont use an amp intended for music, it will sound pretty horrible, just buy a cheapo little used crate amp or something and torture that instead... hehe
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Old 4th December 2013, 07:30 PM   #5
ATAUDIO is offline ATAUDIO  Austria
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Thanks to all for the kind answers.
Some small improvements.
I added a poor man limiter (or soft clipping) using blue LEDs. They should start to act around 3Vpp , that is almost the value that saturates my boombox.
We'll see if it works.

There would be many nice things that could be done with the remaining op-amp, but that would inevitably complicate the circuit (that is already too much in my opinion) so I decided to do nothing.

BTW I am not too concerned for my boombox. If you have a look on the thread/ pictures you'll see that is done exactly for rough use. It survived already a dance summer camp....outdoors in the Alps ....and then this is DIY audio.....
Attached Files
File Type: pdf Pre1_8.pdf (17.2 KB, 24 views)
File Type: pdf Pre1_8PCB.pdf (30.9 KB, 5 views)
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Old 7th December 2013, 04:26 PM   #6
ATAUDIO is offline ATAUDIO  Austria
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Prototype done!: )
And tested with generator and scope:
The input is correctly amplified about 50x starting from about 15 hz to well after 10K, no instability or problems shown.

Clipping with the diodes happens at about 5.5 Vpp, when the diodes are fully bright.

Also I have done a test connecting it to my subwoofer amplifier, and using a small speaker driver as input ( I do not have an actual guitar by myself).
The thinghy is unbeliavely stable, and the noise floor is fair even with the cheap LF347 of the test setup... Consider the usual test setup, no shield/box, cables everywhere.
Even with a 100K series to the input speaker-microphone, it works very fine as a voice amplifier, just the hiss raises, of course.
So, it is time to close it in a nice box and give it to the rockers!
Attached Images
File Type: jpg proto.jpg (22.8 KB, 50 views)
File Type: jpg 100sig.jpg (38.8 KB, 50 views)
File Type: jpg 1ksig.jpg (37.0 KB, 51 views)
File Type: jpg 10Ksig.jpg (38.5 KB, 49 views)
File Type: jpg clipping.jpg (38.3 KB, 49 views)
File Type: jpg LedClip.jpg (25.1 KB, 14 views)
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