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I'm doing a sound system for\with a friend of mine (I've done many, many before)....

In the past I've done either simple L\C or LC crossovers, but I've been looking around for something I can build by Sunday with what I have available (Everything but inductors, and very minimal op amps that aren't really meant for audio), but also something that is A-grade work. So I pondered an RC crossover, but evidently you can't do a second order\two stage setup effectively with those.

For a single stage RC high pass crossover at 120Hz, my R value was going to be 13.2KOhms, and C value was 1uF.

But then I found these and I'm highly interested:
http://www.sentex.ca/~mec1995/tutorial/xtor/xtor6/xtor6.html

http://www.radio-electronics.com/info/circuits/transistor_activehighpassfilter/transistor_highpassfilter.php

The problem is either there are no formulas available, or I'm just having trouble with the math (it's not an excuse to be lazy, I actually have trouble with some math at times due to a brain ailment)

So I was wondering if I could just get a little assistance. The crossover frequency is ~120Hz. I'd like a second order filter for a faster rolloff...

Thoughts?

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Your emitter-follower buffer has a voltage gain that is just ever so slightly less than unity (0.99 or so). You could safely replace the op-amp in a filter design if the op-amp is configured as a unity-gain buffer. One example is a Sallen-Key filter. There are many web-based calculator tools that simplify your effort - one such is here

However, I did run across a web calculator that will do the design as a pair of cascaded RC networks. It is here

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  • \$\begingroup\$ Yeah, I'm aware of the Sallen-Key filters. I just haven't really looked over my Op-Amps to see if they would work. I have a MAX410, a MAX492/494, and an ADA4895-1. The MAX492/494 says it's Unity-Gain Stable, yet didn't seem to say it was for audio amplification, while the 410 said it was, but not unity-gain stable. As for that calculator, I saw that, but I read that you need to have your second stage be higher numbers. Stage 1: R=13.2KR, C=1uF, so I'm assuming multiply all values by 10? Wouldn't that affect the impedance of the signal in a bad way? Thank you! \$\endgroup\$
    – Dominic Luciano
    Commented Apr 11, 2015 at 17:01

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