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Old 9th July 2009, 06:47 AM   #1
akis is offline akis  United Kingdom
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Default A guitar (and more) amp for my son

Continuing here from another thread which was in danger of being highjacked.

The electric guitar has 4 octaves. With a standard tune it goes from 82Hz (the bottom E) to 1280Hz (24th fret, top E). Its top, open E, is at 320 Hz, the 12th fret is 640 Hz and the 24th fret is 1280 Hz.

The middle C on the piano is 261 Hz and the piano has another 4 octaves after that, so the piano goes up to 4176 Hz.

I presume a screetching violin, a keyboard/synth, a microphone and all those "effects" pedals - all those could exceed 5 KHz.

Typical guitar amp speakers (8", 10", 12") do not go above 5KHz.

The intention is to allow other musical instruments to plug into the "guitar" amp, so after looking at my 12" speakers (up to 5KHz says the bumph), I decided to buy a set of mid-range drivers that go up to 11 KHz (says the bumph). Those boys (and girls) could plug a violin, a bass, a keyboard, a mic maybe - I had to allow for more than plain guitars and 5KHz sounded a bit limiting.

So I thought a split at around 4KHz was the proper place, maybe 4.5KHz.

The reason for using the LM3886 was that is is quick and easy. It can do 68Watts on a 4R load or 50 Watts on a 8R. So if I have two LMs I will be able to do over 100 Watts, maybe 135 Watts.

In the future I may build a discreet post amp, but I want the design to be mine, so that will take some time :-)
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Old 9th July 2009, 07:22 AM   #2
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A newbie question would be : " Why should I bother of having a system that is flat to 20 KHz when I see that most of the musical instruments produce sounds ranging from 20 Hz to 5 KHz?" They indeed produce other harmonics beyond the fundamental note , not to mention the ambient which reflectes the waves adding more "color".
Voice and guitar , which usually have gentle "attack & decay" behaviour ,are well reproduced by tube amps and fullranges , because of the inherent characteristic of the devices , it is known that even and odd harmonics are created differently from SS devices (odd..1st , 3rd...)and from vacuum ones (even : 2nd , 4th ..)and they are perceived by ourselves as screetching or mellow.
Another usage of tubes well known from the guitarists is their "bad" behaviour when made to work saturated or overdriven , so the original waveform is clipped and coloured.
Ok. Lesson 1 finished.
Lesson 2.AD and DA with thousand types of distortions in the palm of your hand....!
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Old 9th July 2009, 08:35 AM   #3
akis is offline akis  United Kingdom
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I have 4 speakers in total. All are 8 Ohm. The 12" units go up to 5KHz and the 5.25" drivers go up to 8KHz. I was wondering what is the best way to wire them to the two LM3886s.

On the schematic above I had the idea of splitting the signal at 4KHz (ish) so that one LM drives the two 12 " put in parallel at 4 Ohms so I get 68 Watts RMS. The other LM can drive the two 5.25" again in parallel so I get another 60 Watts.

However I can see a problem with this design: 90% of the signal will be in the 16Hz - 2000 Hz range so the grunt of the work will be done by one LM3886 only while the other one will be sitting almost idle.

Alternatively I could wire it so that each LM3886 drives 2 speakers, a 12" and a 5.25" without a crossover. What would happen if say the 5.25" receives signals below 300 Hz (its bottom end) or if the 12" receives signals above 5KHz (its top end)?
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Old 9th July 2009, 08:52 AM   #4
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You can tailor yourself a simple 6 dB/oct crossover with and coils and capacitors you ought to have on hand. 1mH and 22uF could be used. You can even try series crossover , the cap in parallel with the 12'' unit in series with the coil paralleled to the 5'' unit.I would have each amp driving a single loudspeaker or two same units paralleled , and focusing on the preamplifier/tone controls/active crossover section which could be easily made with two common tubes in classical Cathode Follower configuration ...or does 200-300 volts scare you?
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