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Old 2nd November 2011, 04:16 PM   #1
tca is offline tca  Portugal
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Default Yet another bulb lamp amplifier: the bulbamp

Hi,
I just want to share with you this little amplifier.

Click the image to open in full size.

I use it, with a smaller output cap (470u), as a guitar amp (it needs a buffer in this case).

Cheers.
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Old 2nd November 2011, 04:30 PM   #2
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Using a bulb amp leads to compressor as the bulb resistance changes with current. Using a current source in stead of the bulb it improves it by orders of magnitude. A simple regulator chip and single resistor configured as current source. It does not add much to the complexity and t is probably even cheaper than a bulb.
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Old 2nd November 2011, 05:58 PM   #3
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Quote:
Originally Posted by Nico Ras View Post
Using a bulb amp leads to compressor as the bulb resistance changes with current.
That is a nice thing to have if you want to use this amp as a guitar amplifier


Quote:
Originally Posted by Nico Ras View Post
... cheaper than a bulb.
Two lamps costs 1.5 Eur (auto/car lamps).
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Old 2nd November 2011, 06:16 PM   #4
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I'm questioning the gate bias resistor. Might it not be a better idea to replace the 10 meg resistor with a voltage divider pot which is connected between +9 and Common Ground? This would permit you to accurately adjust the optimum operating point of the MOSFET and reduce the very high input impedance.
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Old 2nd November 2011, 06:20 PM   #5
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Quote:
Originally Posted by Frank Berry View Post
I'm questioning the gate bias resistor. Might it not be a better idea to replace the 10 meg resistor with a voltage divider pot which is connected between +9 and Common Ground? This would permit you to accurately adjust the optimum operating point of the MOSFET and reduce the very high input impedance.
I've done a version just like you said, a few days ago and it works great...

Click the image to open in full size.

For a guitar amp it is a good thing to have an high input impedance.


Thanks.
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Old 2nd November 2011, 06:25 PM   #6
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I was afraid that the very high input impedance might lead to unstable operation.
It could also increase the possibility of RF and other interference on the input of the amplifier.
Good job!
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