[KonaKona]

I was thinking about possibly posting this in power supplies but I think it works better here.

Now in my infinite cheapness, I've decided to build a boombox. I have some speakers laying around from some bose 802/901s, and I took the tape deck out of some fools VW (Hey, he parked the thing in my back yard. There was a birds nest inside, no steering colum, holes in the floor....oh wait that's mine nevermind), and I'll either use a bunch of D batteries or maybe even a car battery for the power.

Now the only thing I'm worried about is this puny little tape deck. Frankly, I doubt it would be able to run those speakers. What would I have to do to get, say two 50-75w channels from a 12v source? I've seen mock ups for a few different kinds of headphone amplifiers (a few small ones and a few fancy audiophile ones) but I don't think that would be of much use here.

Oh, one other thing. The source here is a cassette tape, so obviously I'm not doing some fancy audiophile project. As long as it powers the speakers and doesn't manage to make the tape sound worse, it's fine.

[The_Jizzler]

need more specs on teh speakers. im sure if u hook it all up it will make sound and play fine. but the tape decks built in amp will probably struggle if you start to crank the volume.

[EarthDog]

Make sure the impedence of the amp matches the speakers and vice versa and you should be good to go!!!

[KonaKona]

I'm pretty sure they are 8 ohm, not sure on the wattage. The 802s are 240w and 8 ohms or something like that, but I'm not sure how they are wired. I'll do some searching and see if I can't find out. It also looks like I have 5 of them, if there is any decent way to wire up 5 speakers in two channels then I'll use all 5, otherwise I'll probably just use 4.

Oh, what's the point of a boombox other than being loud?

EDIT: Oh, I'm not sure about the tape deck but the speaker in my bug had a large "20W 2OHM" printed on the back of that. I'm not sure if the previous owner was smart enough to actually match the amp to the speakers though, and it's deffinitly not the stock tape deck.

[EarthDog]

To look retro...probably before you were born retro.

[KonaKona]

Originally Posted by EarthDog

To look retro...probably before you were born retro.

Kinda hard to do that when you are making one though...

Just looked up the speakers, it said 60w at 4 ohms. Sounds about right to me.

[EarthDog]

A boombox itself is retro, its going to happen if you like it or no especially with a tape deck. Just walk out of the house, put it up on one shoulder, and be sure to take a large piece of cardboard with you to your street corner destination for proper breakdance flooring.

On a serious note, looking forward to seeing it.

[KonaKona]

Originally Posted by EarthDog

A boombox itself is retro, its going to happen if you like it or no especially with a tape deck. Just walk out of the house, put it up on one shoulder, and be sure to take a large piece of cardboard with you to your street corner destination for proper breakdance flooring.

On a serious note, looking forward to seeing it.

I don't think it'll look that good. Picture a wooden box with 4 (or 5) 4.5" drivers in the front and a little tape deck in the middle. Might even have a car battery hanging out the back if I can't find some cheap D batteries.

It'll deffinitly be getto enough though.

EDIT: Alright, stupid comment of the day time. Could I find some rail to rail opamp and run the output of the tape deck through them (and maybe some caps as well? God I have no clue how this stuff works....) to get enough power?

Maybe?

Heck I don't even know what a rail to rail opamp is.

[whooping_a_panda]

lm741 op amp
http://www.national.com/ds/LM/LM741.pdf
easier to get version:
http://en.wikipedia.org/wiki/File:Ge...pinout_top.png
note the only pins your really interestin in these is 2,3,4,6,7 where 2 is your inverting input, 3 is your noninverting input 4 is your -power rail 6 is your output and 7 your +power rail, these cost about a buck at rat shack.

inverting amplifier circuit using above op amp with gain equation listed.
http://en.wikipedia.org/wiki/Operati...ting_amplifier

Vout = -Vin * Feedback resistor / input resistor
gain then is defined as: A = -Rf/Rin

its also mentionable that you could cascade two inverting amplifier circuits together to make a non inverting amplifier, that is your output matches the sing of your input just make your second op amp circuit have a gain of A = 1. dont really think you need to worry about sign with an audio output though in this application.

what rail to rail means is that if you had an ideal perfect opamp of the aforementioned model you could gain a very small voltage infinitely into a very very large one think .1 mV into 10's of thousands of volts just by picking the resistors right to suit that equation. well back here in the real world we know that that cant really happen and conservation laws of physics say we cant make nothing out of something or something out of nothing, never sure how that went, so that output power has to come from somewhere. In this case the output signal doesn't come from the input signal to be amplified but rather the rails you attach to the op amp plus and minus whatever volts(this is where your D cells go). this also limits your opamp output to those rails meaning if you use a 10 volt rail you cant get more than 10 volts out of the output no matter what input signal you use. you'll just get clipping/distortion at the points that would reach past 10 volts (your sine sound wave will become square like yo'haircut if you be breaking out that cardboard wit your new ghetto blaster )

[KonaKona]

Originally Posted by whooping_a_panda

lm741 op amp
http://www.national.com/ds/LM/LM741.pdf
easier to get version:
http://en.wikipedia.org/wiki/File:Ge...pinout_top.png
note the only pins your really interestin in these is 2,3,4,6,7 where 2 is your inverting input, 3 is your noninverting input 4 is your -power rail 6 is your output and 7 your +power rail, these cost about a buck at rat shack.

inverting amplifier circuit using above op amp with gain equation listed.
http://en.wikipedia.org/wiki/Operati...ting_amplifier

Vout = -Vin * Feedback resistor / input resistor
gain then is defined as: A = -Rf/Rin

its also mentionable that you could cascade two inverting amplifier circuits together to make a non inverting amplifier, that is your output matches the sing of your input just make your second op amp circuit have a gain of A = 1. dont really think you need to worry about sign with an audio output though in this application.

what rail to rail means is that if you had an ideal perfect opamp of the aforementioned model you could gain a very small voltage infinitely into a very very large one think .1 mV into 10's of thousands of volts just by picking the resistors right to suit that equation. well back here in the real world we know that that cant really happen and conservation laws of physics say we cant make nothing out of something or something out of nothing, never sure how that went, so that output power has to come from somewhere. In this case the output signal doesn't come from the input signal to be amplified but rather the rails you attach to the op amp plus and minus whatever volts(this is where your D cells go). this also limits your opamp output to those rails meaning if you use a 10 volt rail you cant get more than 10 volts out of the output no matter what input signal you use. you'll just get clipping/distortion at the points that would reach past 10 volts (your sine sound wave will become square like yo'haircut if you be breaking out that cardboard wit your new ghetto blaster )

I thought I kept those pictures in the benching lounge?

Alright, I get the last part, but I still don't quite understand the whole inverted input thing. Something about the non inverted input being the source and then the output loops back to the inverted input after it goes through a transistor?

Sorry, I don't do much work with these kind of electronics.

[whooping_a_panda]

the inverting and non inverting inputs are better interpreted as sensors rather than inputs as by properties of this amplifier there is no current flow into or out of pins 2 and 3, Vp and Vn. also by property of the op amp the voltages at node 2 and 3 are equal. which can sound confusing but if you think of the nodes as sensing the input given to them then the op amp does its thing and uses the rail to give an output.

using different configurations with resistors and or capacitors and voltages at pin 2/3 you can use this circuit also as a non-inverting amplfier (this config is usually less stable and is why we use the inverting configuration) as well as a summing or difference amplifier (the latter great for removing signal noise from an output) and also differential/integration amplifiers (these will differentiate/integrate an input signal and give its output, just like calculus).

hope that helps some.

[Roofles]

Originally Posted by whooping_a_panda

lm741 op amp
http://www.national.com/ds/LM/LM741.pdf
easier to get version

Close but no cigar.

But, you are headed in the right direction, but that will not drive a 4-16 Ohm speaker. You need something with a much higher current capacity.

If you want 50W out of your speaker at 12V you need 50W/12V ~= 4A of current to be pushed through your speaker. Also keep in mind with a high current op amp you will need a heatsink.

Check out national.com for some ideas. The LM675 has 4A output with a 10-60V supply.

Secondly, I think you might be aiming a little high with 50/60W from D cell batteries, especially from what I assume are bookshelf or smaller size speakers. Try aiming for about 10W and I think you'll be impressed with how loud that actually is.

National's helpful search (high current output op-amps)

[KonaKona]

Originally Posted by Roofles

Close but no cigar.

But, you are headed in the right direction, but that will not drive a 4-16 Ohm speaker. You need something with a much higher current capacity.

If you want 50W out of your speaker at 12V you need 50W/12V ~= 4A of current to be pushed through your speaker. Also keep in mind with a high current op amp you will need a heatsink.

Check out national.com for some ideas. The LM675 has 4A output with a 10-60V supply.

Secondly, I think you might be aiming a little high with 50/60W from D cell batteries, especially from what I assume are bookshelf or smaller size speakers. Try aiming for about 10W and I think you'll be impressed with how loud that actually is.

National's helpful search (high current output op-amps)

Well they weren't originally bookshelf sized, but I guess they are now. 50W might be a bit much, but they are 60w speakers... maybe 20w each? Total of 80/100w? I guess that is a bit much for D batteries... Wait what if I have 16 of them? Maybe 32? I can always use a car battery if I really need the power.

...ugh, just looked up rechargeable battery prices. I'm gonna have to see how many car batteries I have laying around because I'm not about to spend that kind of cash on batteries.

So if I was going to use a LM675, how would I go about wiring it up? I'm assuming I would need more than just the opamp...

[whooping_a_panda]

sorry, hes right, i should have clarified the part i referenced was largely for an understanding of the what and how of an op-amp circuit as its a widely used part for intro circuits.

though if you're going to draw 4 amps out of D cell batteries... you'd be better off with a car battery, and even better off with a mains line.

reducing you wattage to around 10watts would give you 10 watts at 12 volts you now have a draw of just 833 mA wich is still on the High side for D cells but not unreasonable

http://data.energizer.com/PDFs/E95.pdf graph on the bottom gives you around 18 working hours with a 600mA draw with that battery.

usefull equations to use here,
power equation: P = V * I
ohms law: V = I * R

derivitives of power equation using ohms law P = V^2/R = I^2*R

[whooping_a_panda]

also its to my understanding the wattage of the speakers is what kind of power you can put through them before you ruin them. and since wattage is a number used as a selling point its usually higher than you would want to run it at. if you have a 650 watt psu you dont want to run it at 650 watts and based on manufacturer half the time it wont even make it there.

what does that mean? running 60 watt speakers at 60 watts will probably ruin them.

[KonaKona]

Originally Posted by whooping_a_panda

sorry, hes right, i should have clarified the part i referenced was largely for an understanding of the what and how of an op-amp circuit as its a widely used part for intro circuits.

though if you're going to draw 4 amps out of D cell batteries... you'd be better off with a car battery, and even better off with a mains line.

reducing you wattage to around 10watts would give you 10 watts at 12 volts you now have a draw of just 833 mA wich is still on the High side for D cells but not unreasonable

http://data.energizer.com/PDFs/E95.pdf graph on the bottom gives you around 18 working hours with a 600mA draw with that battery.

usefull equations to use here,
power equation: P = V * I
ohms law: V = I * R

derivitives of power equation using ohms law P = V^2/R = I^2*R

Might be worth trying to wire it up so it can run from a wall socket or batteries, but I think that might be a gigantic pain... Also, I was looking at some rechargeable Ds that were 10000mha, but they were expensive, and also only 1.2v each.

Originally Posted by whooping_a_panda

also its to my understanding the wattage of the speakers is what kind of power you can put through them before you ruin them. and since wattage is a number used as a selling point its usually higher than you would want to run it at. if you have a 650 watt psu you dont want to run it at 650 watts and based on manufacturer half the time it wont even make it there.

what does that mean? running 60 watt speakers at 60 watts will probably ruin them.

Well I guess 10-20w per speaker should be fine then.

I just remembered I was gonna wire up the tape deck to the speakers and see if it could drive them or not. Probably do that soon...

[Roofles]

Checking out the datasheet shows you pretty much what needs to be done to build a complete circuit.

Using the LM675 as an example, its pretty much only a good choice up to 10W of power. (As shown in the THD graph in the data sheet).

Lastly, high power op-amps have fairly low gains 1x-5x, and in fact the chip itself apparently shuts down if set up for a gain over 10x. In this case you'll have to make sure that you have a line-level type signal into the amp, ie. one that doesn't need further amplification before going to the speaker.

[whooping_a_panda]

heres a basic op amp amplifier that will run off a 9V battery, maybe not as loud as you'd want but its an easy design(and cheap to assemble) to get your feet wet with.

http://www.techlib.com/electronics/audioamps.html#LM386

[KonaKona]

Originally Posted by Roofles

Checking out the datasheet shows you pretty much what needs to be done to build a complete circuit.

Using the LM675 as an example, its pretty much only a good choice up to 10W of power. (As shown in the THD graph in the data sheet).

Lastly, high power op-amps have fairly low gains 1x-5x, and in fact the chip itself apparently shuts down if set up for a gain over 10x. In this case you'll have to make sure that you have a line-level type signal into the amp, ie. one that doesn't need further amplification before going to the speaker.

Ah, that's what those are for. I thought it had something to do with the internals of the chip.

So, maybe this thing? http://www.national.com/mpf/LM/LM3886.html#Overview I'd have to use it with two car batteries though....

[Roofles]

Originally Posted by KonaKona

Ah, that's what those are for. I thought it had something to do with the internals of the chip.

So, maybe this thing? http://www.national.com/mpf/LM/LM3886.html#Overview I'd have to use it with two car batteries though....

Yeah that would work, but you'd need 3 car batteries (2x12=24, those are 28V min). I'd recommend starting with the LM675 I think you'd be surprised with ~10W into your speakers.

Plus Digikey has them for pretty cheap and in single quantities. (The LM3886 is only ~$0.50 more though)