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I am designing a color organ and am having trouble with my pre-amp stage. My circuit so far: enter image description here

For low frequencies, my system works pretty well. My input voltage is about 200mv pk-pk, and my output voltage from the emitter amplifier is about 4v pk-pk.

My problem is that when I increase the frequency, the gain is reduced drastically. When I reach the cut-off point of about 3KHz for my high pass filter, the output voltage from my amplifier is only about 800mv pk-pk and is not enough to turn on the mosfet.

I am still learning about most of this stuff, so what am I doing wrong? Perhaps there is a better way to do this?

Any help is appreciated, thanks!

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The issue is that your amplifier has a non-zero output impedance (as real amplifiers are wont to do), and the loading of the two filters (LP and HP) are causing the output voltage to droop.

To fix this, adjust your filter values so that the frequencies stay the same but the impedance to ground is higher. For example, try increasing the resistance in the filters by a factor of 10 or 100 and decreasing the capacitance by the same amount.

This modification will load the buffer amplifier less, and give you you a much flatter pre-filter response.

Because I like LTSpice so very much, here are the relevant simulations (I ignored the MOSFET, it shouldn't change things too much. It will, however, change things a little bit and you should check that):

The original circuit: Original simulation

The frequency response. Not so good. Original response

Increasing the filter impedances (increasing resistors, decreasing capacitors), but keeping the original filter frequencies (which now need to be adjusted, by the way, now that the loading on the buffer stage has changed. I'd suggest changing C3 to ~47nF to pull the filter corner back down). Improved simulation

As I said above, you'll need to re-adjust the filter corners. Due to the impedance changes (the filter impedances were interacting with the output cap and other things), the filter corners have moved. Improved response


More edits:

After inspecting the transient analysis, it's apparent that your transistor amplifier is clipping (you don't want this, you'll get icky harmonics in the filters, and it will lead to a not-so-great color organ).

You'll want to adjust that first stage so you don't get so much distortion. (Or go the easy route and use an op-amp).

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  • \$\begingroup\$ Thank you so much for your in depth analysis! I'll add some more impedance to my filters and see if that does make it better. I was aware of the clipping, but since it was only on the negative cycle, and because of lack of time, I chose to ignore it. I would love to use an LM741 or something, but I think it needs a negative Vcc. I am not sure how to implement that because I only have 5v and ground. How could I get -5V? \$\endgroup\$
    – Gigaxalus
    Feb 22 '16 at 3:27
  • \$\begingroup\$ Stay away from the LM741, it's a old, not so great op-amp. You could use a rail-to-rail op-amp (which doesn't require a negative supply) and bias the input up to 2.5V (and capacitive couple the output), or you can create a negative supply. If all you have is a +5V rail, there are switched capacitor inverters (660 is the most common part) that will give you a -5V rail without too much fuss. But they need a lot of capacitance. \$\endgroup\$
    – uint128_t
    Feb 22 '16 at 3:34
  • \$\begingroup\$ Ok thanks for the info and all your help! I will look into it. \$\endgroup\$
    – Gigaxalus
    Feb 22 '16 at 3:37

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