I made hard drive speakers by feeding an amplified music signal into the voice coil of an opened hard drive. It's producing music nicely, but I think I'm having problems with the music amplification.

The problem is that my hdd speakers don't behave the same way I see other hdd speakers working on Youtube. Their actuators oscillate back and forth in the middle of their motion range, whereas my actuator only pushes toward one side. Unless I turn the volume really low, this basically means that my actuator will just stay at that side (but still making music). If I apply a tension spring that pulls it toward the other side, then it will move as I'd like, but I don't want to resort to such a method. So how do I amplify it to get it to move like the ones I see on Youtube?

What I'm doing right now is feeding my headphone jack signal into an LM386n-1 op-amp, which outputs to the hdd. I gave the op-amp a V+ of 12V and a V- of ground. Am I right in guessing that my problem is that I need to give it a V- of -12V? If yes, then what is the easiest alternative solution (I don't have a -12V source)?

 

Perhaps placing a capacitor in series will work. Definitely no expert with this type of electronics work. Hopefully someone can verify/deny?

 

Could you be more specific please? Place a capacitor in series with what?

 

Do you have an output coupling capacitor between the IC and the HD coil?

 

I would guess a cap or inductor would be my suggestion. I have tried that a few times. They take a lot of power, I found there is a level that it will play decently at, go over you get a pretty tinny sound, you do something high with a lot of bass, it may jump from end to end clanking into stuff.

I'm not sure how far you got into the drive. it may not be electrical at all either. I make my speakers ever since my first one worked poorly I remove the heads. Hard drive heads if the platters aren't spinning and the device is sealed will actually tough the platters. It's actually the force of the air that raises the heads off so they read. no platters spinning=no wind, no wind= heads crash. You may be able to remove the heads, and leave the fine metal stuff, or adjust them so it's not an issue. Adjusting isn't always possible though as may be too little space depending on how tight the platters are stacked. I usually just take head, thin metal part off with pliers, and throw it out. usually the metal is so weak that even pulling the head off it you'll mess up the metal, and that will then make issue.

 

The hdd connected to ground is ok, the lm386 output should connect to one side of a large capacitor and the other side of the cap should connect to the hdd.

That allows the hdd to swing to both sides of ground and let the 'speaker' swing both ways.

300 to 1000 uF should be in the right range.

 

Thanks so much!! Putting a capacitor in series with the lm386 output definitely got the actuator unstuck from the side. I started at 400uF and went up to 3000uF - it seems so far that the more capacitance I add, the better it works and more it moves. Now, when the music is quiet, the actuator stays at one side and gets closer to the other side when there's more music volume. Why does adding the capacitor have this effect?
Also, it still doesn't really behave as the Youtube ones I described earlier do. When the music is silent, their actuator will be in the middle and oscillate more toward both sides when the music gets loud. How would I make mine do the same?

 

Thanks so much!! Putting a capacitor in series with the lm386 output definitely got the actuator unstuck from the side. I started at 400uF and went up to 3000uF - it seems so far that the more capacitance I add, the better it works and more it moves. Now, when the music is quiet, the actuator stays at one side and gets closer to the other side when there's more music volume. Why does adding the capacitor have this effect?
Also, it still doesn't really behave as the Youtube ones I described earlier do. When the music is silent, their actuator will be in the middle and oscillate more toward both sides when the music gets loud. How would I make mine do the same?

Draw out your exact circuit so we see what you are doing. Maybe there is an error in thought or action.

 

https://www.dropbox.com/s/qcq403bqujygrp3/2014-04-12 21.05.33.jpg

To clarify, I'm using the TIP31c to amplify the music signal even more, because I don't want to increase the gain for the LM386 (I'm simultaneously using it for some music synchronized lights). My HDD acts the same way without the TIP31c, just with less motion and less audible music sound.

 

We thought you were doing it like this. As soon as you add a single transistor, you destroy its ability to drive in both directions.
If you're going to do a current booster stage, you have to go full blown push-pull with your design.

 

I just disconnected my lights and did it that way, making my 386 give a gain of ~800%, and it behaved the exactly the same as with the transistor

 

Exactly the same? Off center? The capacitor should block all the DC current after a second or two. Measure for DC current with no audio signal.

 

Looks like the guys online are using the speaker outputs of standard audio receivers. I don't know what resistance is on an HDD but your simple lm386 is pretty small output vs what your on-line friends may be using.

 

Yeah exactly the same - the actuator is on the left for quiet/silent sections of the song, and goes to the right whenever the song gets loud.

Yeah I think they are all using bought amps/receivers. So can I not match them with my breadboard amp?

 

You only have 2 choices for why it's off center, mechanical or electrical. The capacitor should stop all the DC so that leaves mechanical.

Whether you can deliver the amount of power a, "regular" amplifier delivers is entirely up to how much amplifier you build. I'm sure several of us at AAC have built serious amplifiers, but it takes more time and money than buying one.

 

It's because there is no load.

A normal speaker has a cone and support rubber which is like a spring that holds it in the centre of its travel.

With a voice coil you need to add a spring that holds it near the centre, or some electronic sensro and feedback system that will keep it near the centre.

Otherwise the tiniest amount of bias will make it drift to one side or the other, which is what is happening.

For the "spring" you could add a small nearby magnet that puts a weak pull on the armature to hold it in the centre.

 

Perhaps add a DC bias current to centre the coil?

 

Oooh I see now! Yeah, I realize now it's a mechanical error - the magnet pulls the actuator to the left when nothing is connected. This is on a Seagate HDD. I opened up another Western Digital HDD and the magnet pulls the actuator to the right, so I'm guessing all HDDs are pulled toward one way.
So I guess, also, that the HDD speakers I see on Youtube are receiving music signals amplified so much that it just overpowers that inherent magnetic pull by a lot??

Thanks so much, guys!!

 

Perhaps add a DC bias current to centre the coil?

That's sort of what I was thinking - if a transformer output amplifier is used, the secondary can be strung between the bias generator and the the mid-point terminating the other end of the coil - actually there's various ways of doing it, the other end could go to the middle of a series pair of electrolytics across vcc/gnd, the bias arrangement could be as simple as a pot across vcc/gnd with the wiper to the junction of the 2 caps. No need for a transformer then as the bias pot can also null out any offset on the output of an AF amp chip.

 

Some of the ones on you tube stiffen the arm so it dosnt move, it is claimed it sounds better. I have no idea as personally I have never done this with a HDD, interesting idea though.