Active vs Passive Hi Pass Filter for Audio

[AltAudio]

Hello, I am attempting to build a hi pass filter for audio signal sourced from CD walkman, averaging 600mV. I took the suggestion presented by MikeMl and downloaded TI's Active Filter Design Application - FILTERPRO - TI Software Folder active filter app tool. Very impressive however, depending on the desired stop band attenuation, it can get somewhat involved. That being stated, I've also seen the basic passive formula - 1/2π*RC, and have worked out the desired band pass frequency of 11kHz could be theoretically attained with R=240kΩ and C=62pF. The filtered signal would then hit an op amp(as I foresee) which inverts and steps up to apx 4V. Is this possible to do passively or is the input signal too weak?

Many thanks...

 

[MikeMl]

Only you know if 600mV is too weak?

If you think you need a gain of 4, you can either build a single-pole passive high-pass C-R filter followed by a gain of 4 opamp, or you can build a two-pole active high-pass filter using the same opamp with a gain of 1 to ~100. Or if you need steeper roll-off, you can build a four, six, eight, ... pole filter using two, three, four, ..., opamp stages.

What exactly are you trying to do? Gain required? Cut-off frequency? How steep a transition from no-pass to high-pass?

 

[crutschow]

The problem with a 1st order (single RC) filter is that the rolloff into the stop-band is quite slow (6 dB/octave) and thus may not give the rolloff you want. That's why higher order filters, such as are designed by Filterpro, are typically used for audio filtering applications.

 

[AltAudio]

I see. Well I've put 12.5kHz hashmarks in a recording and want these and only these markings to be stepped up and outputted via transformers to electrodes. It's kind of a simulated bpm that I've introduced via hard disk sequencer that I mixed into the audio track. So in essence I don't want anything lower than the hashmark frequency to make its way to op amp and transformer portion of circuit.

 

[ronv]

Filters

Here is a site Mikebits gave me with a great passive calculator.
Butterworth Tee LC High Pass Filter Calculator

Is there no signal above your hash marks?

 

[MikeMl]

Here is a comparison of a simple C-R followed by a gain of 4 to an 2 pole active filter, gain of ~4, MFB configuration, Chebycheff w/ 3db ripple.

 

[AltAudio]

Correct ronv, there is no signal above hashmarks.

 

[AltAudio]

MikeML: so the green line is the 2 pole Chebycheff circuit? I think I prefer that design. I'm using Radio Shack's LM386 low voltage audio op amps, is this adequate for the 2 pole design? By the way, which application is that?

 

[MikeMl]

The solid green line is the voltage at node 2P (in db relative to the input V(IN)). The dashed green line is the phase relative to V(IN).

The **broken link removed** is an audio power amplifier; not an OpAmp. If you are constrained to only using RatShack, they carry a **broken link removed**. It will work ok at 11kHz, but it needs a minimum power supply voltage of +5V and -5V, or 10V single-ended. If you can buy from other suppliers, there are much better OpAmps available.

The simulation was done in LTSpice. I used FilterPro to get the values.

 

[AltAudio]

MikeML: I have 2 LME49740 Audio Operational Amplifiers as well. Would these suffice in building the hi pass filter you referenced?

 

[MikeMl]

MikeML: I have 2 LME49740 Audio Operational Amplifiers as well. Would these suffice in building the hi pass filter you referenced?

I have never seen these, but looking up their specs on **broken link removed**, they look ideal.

 

[AltAudio]

Better yet, which op am model do you recommend to accomplish the task at hand?

 

[AltAudio]

Oh I didn't see your reply before I submitted mine. Thank you.

 

[MikeMl]

Better yet, which op am model do you recommend to accomplish the task at hand?

Depends on cost, power supply voltages available, power supply ripple, load resistance, the phase of the moon, which stores you have available, how long you want to wait for shipping, etc, ....

 

[AltAudio]

Mike ML: I'm going with the 2pole MFB design you presented using the LME op amps. Forgive my inexperience, but as I'm looking at the diagram I will be sending the audio signal to the - input of opamp(inverting) and the + input to ground, this leaves me 14 leads for which I am not sure as to configure correctly.

 

[MikeMl]

+supply to pin 11, - supply to pin 4. You only need one of four opamps in the package. Pick one, and follow the pin numbers. The other three unused ones, ground both + and - inputs (six pins to be grounded). Three unused outputs will be left open.

 

[AltAudio]

Ahh makes sense. Was wondering why I would need four. Thought maybe they got chained together somehow, but this is relatively simple. Many Thanks...

 

[AltAudio]

Now to purchase the caps/resistors.

 

[AltAudio]

Ok, got the components from Mouser and hooked up the Chebychev 2 pole configuration. The O-scope registers the frequency of the signal right around 11.7KHz, so it appears to be working. However, the output voltage is woefully low, < 100mV. The audio signal input is apx 600mV and the source is 9v battery. How can I output apx 5V?

 

[MikeMl]

You do realize that as drawn, the circuit requires TWO 9v batteries (+9V and -9V), right? .

What is the "load" that this circuit drives?