# Using audio amplifier speaker output to drive an electromagnet

My first time here as a poster so forgive me if I make a misstep...

I am building a coil (22swg enameled copper wire, about 500 ft), some 4in inner diameter of the bobbin, 1" wide, will have ~ 450 turns. I am using lacquer to make sure wire is wound tightly and does not move to avoid heating. I am hand winding it, so it is not the neatest coil... hoping that does not matter so much.

I want to give it a lot of power, and wanted to use an audio amplifier for that. According to some online calculator, the coil I am building should give about 8ohms of resistance, which is perfect for my audio amp as it can do both 4ohm and 8 ohm output (automatically adjusts itself).

The reason I want to drive it with an audio amp is:

1. Building my own amplifier is not easy, and good power supplies are not cheap; the cost of this audio amplifier (used) is lower than some power supplies.

2. I need to drive my coil with some frequencies, rather than just constant signal, so planning to use audio frequencies for that generated with my audio software (sine, square and such).

I am not very educated on these matters so I would like to hear what your thoughts are on this setup:

• Generate frequencies using my audio software and output that to a pre-amp. It will be a mono signal
• Feed from pre-amp to the power amp. I will make sure that all signal strengths are the way they should be (I am aware that pro audio equipment has different line level values than amateur one for instance).
• Connect my coil to the speaker terminal for one speaker. Even though the amplifier has two channels, it can be bridged, to give me more power. I am hoping that since a speaker is essentially a coil, connecting my coil should work too.

I count with dumb luck and increasing volume on the pre-amp very slowly and checking the heat that my coil produces. If it stays only warm to the touch, I am thinking - it is OK.

Am I being too optimistic or can this setup actually work?

Also, given the above info, what would it take to get the maximum power out of that coil and the whole setup? The goal is to produce as strong a magnetic field as possible.

Thank you for any guidance, and if I am too clueless, I will appreciate pointers to articles or books that you think I should study to understand my idea and its shortcomings better.

ps. Power amp specs: 800W at 8ohms in bridge mode or 250W at 8 ohms in stereo mode It can also do 400W in stereo mode at 4 ohms and 600W at 2 ohms in stereo mode. Max power is 1200W at 4 ohms, bridged.

EDIT:

• There would be no ferric core, only a coil.
• The goal of this experiment is to see what the biological effects are of such a magnetic field, using various frequencies, around 200-2000 Hz, with most of them around 300-700 Hz. This would be similar to Rife experiments who used radio frequencies to, allegedly, kill viruses. Apparently, or hopefully, microbes have a frequency at which they (may) die, and it is with that in mind that I am building this coil. Naturally, for a test under a microscope, the field does not have to be very strong, but for a real biological effect, "in vivo", the field would have to be stronger in order to penetrate the tissues.
• I always thought that the stronger the magnetic field, the more power the coil would consume, which means - more amps or watts, right? (probably wrong :) )
• Voltage gain of the power amplifier is 32.5V/V or 30.25 dB.
• Slew rate is >50V/microsecond or >100V/ms in bridged mode.
• Current slew rate is > 32 A/microsecond at 2 ohms
• Geometry of the coil is not critical, I just built a round one, by hand, and used plenty of lacquer to make sure wires are fixed and cannot move (to prevent losses to magnetic field due to heat).
• Recalculate the ohmic resistance of your coil. It is without ferromagnetic core ? What is the voltage supply to the final stage of your amplifier? Is the geometry of the coil critical? – GR Tech Jun 27 '14 at 3:35
• I doubt that your amplifier "automatically adjusts itself" to 4 ohms versus 8. Don't believe the glossy brochures. – Kaz Jun 28 '14 at 4:58

In this case, you are being led astray by what you don't know. A speaker coil has a (more or less) constant impedance because it drives a cone against air resistance. In your case, with no speaker attached, it behaves as an inductor, not a resistor. From the numbers you provide, it will have an inductance of ~ 5 milliHenrys (.005 H). For an inductor, the current (and the magnetic field) will go down as frequency goes up. The relevant equation deals with impedance, not resistance, represented by Z.

                    Zl = 2 x pi x L x f


and this will equal 8 ohms at f = 250 Hz (about). So, for frequencies below about 250 Hz, the total impedance (sqrt(Zl^2 + R^2)) will increase with frequency, until at 250 Hz your current amplitude will be about 70% of its DC value. Further increases in frequency will continue to increase impedance and current will drop further as you increase the frequency. You probably don't need to hear about phase shifts.

The effect of all of this is to reduce power dissipation in your coil as frequency increases. But particularly at low frequencies you do need to keep a very close watch on your temperatures. Dissipating 800 watts in a coil the size you're considering stands a pretty good chance of melting the interior layers. It certainly nay cause the bobbin to melt or catch fire.

• I just wanted to say that I added some more information to answer questions asked by those who answered so far - thank you (and I am eagerly awaiting more answers if the new information helps any). – user46345 Jun 28 '14 at 1:44
• For the frequency ranges you're interested in, you should be fine. The amp will have no problems. If you want the field as strong as possible at the sample, make the coil as small as possible. In general, for an air coil, field strength is inversely proportional to loop area. And I can't stress too strongly that the limit on your field will be temperature rise in the coil. You can get in serious trouble running 800 watts through a coil this size. – WhatRoughBeast Jun 28 '14 at 14:12
• Nice, (yeah think 100W as a max... maybe less) I wonder how you estimated the coil inductance. My hand wavy sort of answer for thin coils is mu_sub_zero *R.. which gives me something that looks like ~10 mH (If I didn't make a mistake.) – George Herold Jul 28 '14 at 14:55
• Hmmm. 22ga wire has a dia of ~.025 in, so a one inch wide bobbin will have ~40 turns / layer, and 11 layers. I used 66pacific.com/calculators/coil_calc.aspx, and running my numbers now I get 30 mH, rather than 5. (450 turns, 4.25 dia, 1 length, .25 depth. I've no idea what I did wrong. – WhatRoughBeast Jul 28 '14 at 16:11

Also, given the above info, what would it take to get the maximum power out of that coil and the whole setup? The goal is to produce as strong a magnetic field as possible.

What you have stated above is contradictory - to remove power from a magnetic field produced by a coil is, inevitably, to reduce that magnetic field's intensity. So producing "as strong a magnetic field as possible" is contradictory to extracting maximum power.

What are you really trying to achieve - forget about power amps and pre-amps and all that sideline information, forget about how you build it and what wire gauge it is; what are you trying to achieve by generating your magnetic field?

If you are trying to produce a remote magnetic field that another device can extract some power from so that it can be activated (like a tag system) then using a power amp is going about it the wrong way - you should be using tuned resonant LC circuits to maximize the current flowing in the coil.

If you are looking to make some form of induction heater it's the same argument - forget about linear power amps - think about tuning and resonating the ccoil with a ccapacitor.