# Designing bandpass filter for audio amplifier

I am a 3rd year engineering student currently working on a project where I need to basically make an ultrasensitive metal detector.

Basically, I need to create an alternating magnetic field by connecting an audio amplifier (which is also connected to a function generator generating 1 kHz sine wave) to a solenoid. long story short, I need to put a bandpass filter in between the audio amplifier and a solenoid. I need some tips on how to do this. I tried to use filter design calculators available online, but to me, the major problem seems to be that I need a filter that can handle high current input as well as high current output (~1.5 A, I_rms) with minimal voltage drop. The resistors on the bandpass filters I made would just burn due to high power input. I guess this leads to a more general question:

How do you create a bandpass filter that can handle high power input with minimal power loss?

So far, I've focused on passive filter using RLC, but answers involving active filters are also good to me, though I don't really have any experience with it.

Setup: I have two solenoids:

1. Transmit coil which will generate alternating magnetic field.
2. Gradiometric pick up coil (or receive coil) which will sit inside the transmit coil.

The pick up coil generates minimum voltage thanks to the gradiometric design. However, once metal is inserted inside the pick up coil, either the eddy current or the magnetization will induce voltage on the pick up coil, which will be measured by DAQ.

The problem is that total harmonic distortion (THD) is much more amplified on the pick up coil (Ideally, there should be none). I believe this is due to the fact that the voltage induced in the pickup coil is proportional to the derivative of the current. THD doesn't really show in the transmit coil (below noise level).

• Do you really need high current going through the filter? That seems unusual. The receive coil doesn't need to handle several amps, does it? Commented Jun 15, 2022 at 14:33
• Wait a second. I couldn't understand this: I need to put a bandpass filter in between the audio amplifier and a solenoid. Why? You are applying a 1 kHz to the PA so you want that 1 kHz signal to be amplified. So the output frequency will still be 1 kHz. Then why do you need a BPF? Audio PAs are not supposed to generate harmonics unless they are designed to. Commented Jun 15, 2022 at 14:35
• Oh, the solenoid connected to the audio amplifier is the transmit coil, which needs high current to create a strong enough AC magnetic field. Sorry for the confusion. Commented Jun 15, 2022 at 14:37
• Solenoid has several meanings. 1) A coil wound on a cylinder, where the length is large compared to the diameter. 2) A coil with a metal plunger. I don't think that either applies to your situation. A metal detector would typically have coils with a large diameter and minimal length. Commented Jun 15, 2022 at 14:37
• It's difficult for me to explain this succinctly. Sorry for all the confusion. The total harmonic distortion seems negligible in the transmit coil (which generates the magnetic field) but if you check the voltage induced in the pick up coil (due to changing magnetic flux), it is highly distorted, indicating that small amount of THD in the magnetic field is amplified in the pick up coil. Commented Jun 15, 2022 at 14:41

The problem is that total harmonic distortion (THD) is much more amplified on the pick up coil (Ideally, there should be none).

Metal detector designers "parallel tune" the receive coil to avoid this so, if you can estimate the receive coil's inductance you can trial a few capacitors. Not only will you get reduced distortion but you will amplify the received signal be up to ten times.

You should use ceramic capacitors but, unfortunately at your low frequency you may find it difficult to obtain C0G/NP0 dielectric in the values you require. This means that you get a bit of tuning drift with ambient temperature but, that might not be a problem.

Metal detectors that I'm aware of also parallel tune the transmit coil because you can get a larger voltage across its terminals for a smaller drive current from your drive circuit.

In other words, no real reason to use a filter as you are saying.

I have designed two commercial metal detectors.

• "I have designed two commercial metal detectors." Ha, I couldn't ask for a better person to answer this question. Yeah, I will try your solution. Thanks a bunch! Commented Jun 15, 2022 at 14:48
• By the sound of it you are using two receive coil in an inductive balance configuration maybe? Commented Jun 15, 2022 at 14:52
• The receive coil is wound counter-clockwise on the top part and the clockwise on the bottom part to minimize the "feed-through" voltage induced by the magnetic field. However, since it's not perfect, I still get some voltage even without any metal (and consequently, some inductance). It's a clever method where the presence of metal will be detected via the presence of the third harmonic generated by the metal. This is why there shouldn't be any higher harmonics in the absence of any metal. In any case, thanks for dropping by! much appreciated. Commented Jun 15, 2022 at 14:59
• thanks for the reminder. I didn't know I was suppose to do that. Commented Jun 24, 2022 at 10:31