computer speaker amp questions

[hardbop200]

Hey everyone!

(First, I have no real electronics knowledge, I just like to tinker and break things.  That being said, here we go...)

I was gifted a pair of old Altec Lansing computer speakers that I thought would be neat to make into a practice amp of sorts.  I've discarded the original speakers, and don't want to mod the circuit any (see above disclaimer).  Here's what I'd like to do, and I'm just wanting to get your thoughts on if it's possible or not:

1. The amp has a stereo (balanced?) input, I really need it to just be unbalanced so I can run guitar or some noise maker into it.  If I wire hot + common to a 1/4" jack, where do I stick the ground?  (in other words, I need to go from balanced input to unbalanced...)

2. The amp has L and R speaker outs, both powered a 4 ohm speaker each.  Can I just solder these two together (hot+hot and common+common) and run an 8 ohm mono speaker?  True, maybe stereo is better, so I might just leave it the way it is with two outputs, and just not run one side.  Still, I've got a neat little box that I'd like to run my guitar into for picking around at the house, and it is 8 ohm.

I'm sure I've gotten some of my terms wrong, especially on the hot/common/ground thing, but I hope I've been clear enough to make sense.  Please let me know if you need any clarification or if I'm just plain stupid. 

(FYI, I'm working on building some of the layouts on the Ruby, Noisy Cricket, etc., this is just to keep me going in the meanwhile.)

Thanks everyone!

Josh

[sean k]

Go into the circuit board and find out if it's a numbered or ID'd chip then find the data sheet and look up the bridged output mode. You can't do that with the outputs and if you could you'd want a 2ohm speaker as opposed to a 8ohm as you're doubling the current to a speaker but the voltage stays the same so half the resistance is what you would want but you can't do it anyways.

I've got the same type of thing as you that I want to do and I've got to go back and look at bridged outputs myself. Basically one amp pushes the positive part of a wave and the other amp pushes the negaive half and the inputs then are opposite so one inverts while the other non-
inverts and they mix in the speaker and because of this they don't need to be capacitively coupled... but find the data sheet and everything you want to know will be in there.

[hardbop200]

Go into the circuit board and find out if it's a numbered or ID'd chip then find the data sheet and look up the bridged output mode. You can't do that with the outputs and if you could you'd want a 2ohm speaker as opposed to a 8ohm as you're doubling the current to a speaker but the voltage stays the same so half the resistance is what you would want but you can't do it anyways.

Thanks Sean, I will do that!  I appreciate you responding. 

[Transmogrifox]

To help you with terminology, the input to the computer speaker is not "balanced".  It's just stereo.  Think of it as two amplifiers in one box.  The tip goes to amp #1, the ring goes to amp #2, and the sleeve is the common ground between them.

there are many ways I can think you can go about using this as a guitar amp, but here are three ideas:
1) Ignore one of the stereo channels and pretend it isn't there.  Use a 1/4" to 1/8" stereo adapter jack.  When you plug in the guitar cable, the sleeve will by default ground one of the inputs, and the signal on the tip will be routed to one of the two internal amps.  The speaker wires to the unused amp could just be taped off, chopped off, or ignored.  The active amp could be connected to about any speaker with impedance larger than 4 ohms.  With 4 ohms you would get the greatest power output.  You may wish to purchase an audio transformer for impedance matching if you want to maximize power into a specific speaker load.
2) Use a stereo 1/8" cable between the adapter.  Cut the cable in half.  Twist/solder the tip and ring wires together on the amp input side.  Find the "hot" (ungrounded) of the two on the guitar side (you can do this by experimentation), and twist that in with the two from the amp side.  Twist the shielded grounds back together and use some electrical tape to repair the cable after you have things connected right.  From there, drive two speakers separately, or in bridged mono mode if that particular amp supports that option.
3) Do number (2), but use audio transformers on each speaker output.  You can bridge the secondary sides of the transformers to get the doubled output, or you can parallel them for driving lower impedances.

You can identify the wires in a stereo cable as follows:
The braided shield is ground.  Two wires inside are your tip & ring.  Identifying which is tip and which is ring can be determined with a multimeter, or just by experimentation.  Input some music from a guitar or cd player.  Connect a speaker to on of the outputs and find which wire makes it go dead when you cut it. 

I would guess old Altec Lansing speaker system doesn't do bridged mono mode.  These probably give all the juice they can give from a single supply....in other words, each speaker output probably already behaves like a bridged mono connection.

[Minion]

I Can see a potential impedance Issue ... the Input of the Amp is probably (10k - 47k) but your pickups Like a much higher Load than that , so you might experience Tone and/or signal loss ... one way to solve that is to put a pedal or preamp in front of the amp .....


Cheers

[Transmogrifox]

I Can see a potential impedance Issue ... the Input of the Amp is probably (10k - 47k) but your pickups Like a much higher Load than that , so you might experience Tone and/or signal loss ... one way to solve that is to put a pedal or preamp in front of the amp .....


Cheers

The DIY way is to build a buffer.  Actually it's highly probable that the input uses an op amp, and the impedance issue could be solved simply by replacing the 10k shunts to ground at the input part of the circuit board with something larger, like 470k.