Changing speaker level to line level

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Racer X

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Good Morning Electronic Geniuses,

I'm not to bright,but maybe you can help me, I need to change a speaker signal into a line level signal.

   I know it just takes a couple of resistors to do it, but I need to know how to figure out the correct values

   I have a 200 watt stereo with 8 ohm speakers and I want to pad the levels down to 69 - 194 mV. I also have DMM that doesn't read mV.


   I think I have the actual wiring part down, it would be a resistor in the + line with another smaller resistor across the plug with + to + and - to - . I hope I explained that clearly enough.

  So, does anyone know a simple formula that I can use to figure this out? Do I need to measure the speaker voltage? is that DC?

Thanks for your help,

Rex Racer
 
regebro asks a good question. But in answer to yours, I remember having to do the same thing for my car stereo when i purchaed a power amp. I don't remember what they are called but you can get them at places like best buy or probably even radioshack.
 
re. why?

I dont have a line-out on my stereo and I want to record a large pile of cassettes to minidisc. I have a headphone out but I'm under the impression that the impedence mismatch will give me inferior results.
 
Have you tried going from your line out from the cassette to the line in on the minidisk
 
My cassette player doesnt have a line-out only a headphone out, which does work but wouldnt a high impedence line level signal be a better matchup to my minididsc and give me better recording results?
 
What kind of stereo do you have? Is it one of those all in one jobs? Does it have a tape loop? Are you a recording musician? If so, what other equipment do you have?

I ask these questions because whithout knowing what you got, it is hard to help you.
 
Yes, I high-level line signal would be better than the headphone signal. But the speaker signal is not a high-level line signal... :)

The headphone out is a better bet than the speaker out, but both sucks, to be honest. The best thing you can do is to borrow a casette deck with line outputs from a friend and use that.
 
There's an easier way to convert your tape collection, especially if you have a high-speed internet connection: Napster. I've been in many arguments with people about the right and wrong of Napster, but I feel perfectly justified downloading music I already bought on cassette. If you have an optical output on your soundcard and input on your minidisk that should turn out nicely.
 
RacerX (BTW Speed Racer is the Man!), why don't you simply get a Y cord adapter consisting of a stereo 1/4 plug and 2 RCA male plugs. Plug the 1/4" plug into the headphone out and the 2 RCA male plugs into the L&R of the minidisc in.
This would be a whole lot easier that using resistors or capacitors to produce a line level output.Not knowing what Ohm resistance to use can result in damage to your system.
 
Wait a minute. No, it isn't that complicated- you do not need a lot of hardware to do this. MisterQCue is right: you *can* do this with a simple wye adapter from the headphone driver output. The quality difference between picking off the headphones, padding down the speaker outs, or picking off a (nonexistent) line out will be pretty minimal: after all, the source _is_ an integrated cassette player of unknown quality.

We don't know much about your cassette deck/stereo thingus, so we'll have to do this feeling in the dark. Most consumer electronic headphone outs are designed to drive 150ohm-and-up headphones. Less expensive equipment simply bridges the headphone output right across the power amp, and can drive down to 8 ohms. None of this matters to you, because the input impedance of the line inputs on your MD deck will probably be 10kohms or up. So as far as the cassette deck is concerned, your MD deck is essentially an open circuit (a bridging input).

For this, I'd run out of the headphone outputs with a simple wye cord, and I'd start with the volume all the way down. Watch the metering on the MD deck, and slowly bring up the volume. You'll find that that headphone output will probably be perfectly fine for this noncritical kind of dubbing. I'll bet that you'll get decent levels to the MD deck at about 1/4 volume on the stereo thingus. Try a dub, listen to it- if you like it, you're done.

What you are giving away with this simple technique is signal-to-noise ratio. Because the gain structures are wrong, you'll be running the cassette deck/stereo thing well below its optimal gain setting, so your noise floor will be higher than it might be in the best case. This is probably moot, because the source is a cassette, whose noise floor will probably be well above that _anyway_. Let your ears be the judge.

If you're *bound and determined* to do this the hard way, and for some reason you want every dB of signal-to-noise you can get, you'll need to build a pad circuit. The bitch about building an accurate pad is that you have to know the driving impedance of the headphone driver, and the input impedance of the MD deck. This would let you perfectly optimize the gain structure, and push your noise floor down so that it is 30dB below the tape noise, instead of only 20dB below the tape noise... (;-)

We don't know any of that stuff, so what the hell- let's improvise. Who needs perfection? This is home audio! I'd build a quick-and-dirty ~20dB L pad for that, based on a ~150 Ohm driver and a ~10 Kohm line in: I'd put 600ohms (1/2W) in series with the headphone out, and 30 ohms (1/2W) from tip to ring across the MD deck input, AFTER the 600. This is rough, but it would let you run the gain a little higher for better signal to noise, while still providing a reasonable impedance to terminate the headphone driver (headphone amps have to be able to stably drive up to 600 ohms, since many headphones _are_ 600!). This has the headphone driver seeing ~630 ohms: close enough. And the MD deck is bridging across 30 ohms. No prob. That'd probably let you run the gain at normal listening levels. 20dB ain't much pad, but that might push the noise floor down by 5-10dB or so, and that might be enough to let you like the sound better.

But why bother with the complexity? I highly doubt that you need it. Don't sweat the impedance mismatch in this case: the bridging input renders it pretty much moot. Run the output level down, dub your tapes direct, move on with life, and worry more about more vexing things!

[Edited by skippy on 10-31-2000 at 16:54]
 
Hey, thanks for all the replys,

I 've done the stereo jack to stereo jack/y cord thing from the headphones before and I wasnt to thrilled with the results. I cant do the napster thing because I only have a crappy web tv set up and not a real computer.

My stereo is a medium high end Sony rack system that cost a grand about 6 years ago(for that kind of dough you would think it would have a line out)

Skippy,you were correct with your 10K guestimate for my MD line-in. The speakers that came with this stereo are 8 ohms. By padding down the speaker level with a resistor in series and one across the jacks at the MD that would give me the optimal impedence match for the best signal to noise ratio. Just like you said.

I was just trying to find out the formula for figuring out the resistor values. I know this sounds pretty anal retentive and all but I'm probably going to spend 30 to 40 hours recording all these tapes and I never want to do this again. So I figure I might as well do it the best way that is possible with what I have. I also have a stack of vinyl that will proably get the same treatment at a later date.

I think a couple of bucks for a few resistors and 1/2 hour of soldering might be worth it if the end result can be improved measurably.


So what do you say surely one of you guys must know how to figure this out. Cause I sure dont,


Rex Racer
( Speed, you dont know I'm you're brother)
 
I was following this thread and I'm confused as to problem -- this is not a big deal at all -- there is no reason why the headphone out won't work for you!

Just connect a 1/4-inch stereo OUT to either RCA left/right or 1/8-inch stereo IN (whatever your MD needs.) You need to adjust the overall volume (volume control on your stereo) for the best signal level and minimum noise that you send into your MD without distorting.

It's been a few years since my electronics courses, but if I'm not mistaken, speaker loads require AC resistance calculations (which are far more complex - load varies with frequency), not simple DC resistance calculations.

Even better -- RENT A CASSETTE PLAYER FOR A WEEKEND - it'll cost you $20 bucks tops!!!!! ;)

Bruce Valeriani
Blue Bear Sound
 
Yeah, what Bruce said. If none of your friends have a casette deck, rent one. It will without a doubt be easiest, and give you the best sound, and won't cost you much...
If you don't have $20 go beg on the subway. :)
 
The formulae won't do you much good unless you know the source and load impedances: my claim is that the ~20dB swag above will work every bit as well as laboriously calculating out a pad value, since you don't really know any of the levels or impedances with any certainty. In any case, they are undoubtedly on the Web, as well: let's have a look. How about http://www.hut.fi/Misc/Electronics/circuits/speaker_to_line.html , for one.

The author doesn't list the L-pad formulae per se on this particular page, but he probably has them on his site somewhere- he has the balanced T pad for mike lines, looks like. I don't know anything about him- that was simply the first hit I got in an AltaVista search, and there are undoubtedly others. But the blob on this page looks like a perfectly reasonable L-pad circuit for the case of the unknown drive and load, and it's no less a swag than mine. He simply bridges (rather than trying to terminate the driver), so his calculations are based on an idealized 10:1 voltage divider rather than a pseudo-impedance-matching power divider like I did. Anyway, the information is out there, for whatever that's worth. If nothing else, cruise to the library and check out a copy of "Audio Cyclopedia", by Tremaine (a copy of which should be on every true nerd's bookshelf).

Like the other folks above, I don't understand why you are so hung up on padding down the speaker output. To my way of thinking, that would be less desirable than using the headphone output, since (as Bruce Valeriani pointed out) they present a complex impedance, and the voltage across them (which is what you'll sample with your bridging pad) will have phase wierdnesses imposed on it *by the speaker itself* interacting with the amp's feedback loop. *That's* where you have to start to sweat the impedance mismatch. If you're that concerned about quality, the speaker signal will potentially be less clean than the unloaded headphone driver.

What, precisely, did you not like about the headphone-out dubs previously? The speaker load is going to be unpleasant to cope with, and if the speaker is disconnected, you have essentially the same situation as the headphone out...

I'm starting to fall into the "borrow a deck" camp, myself- despite how much I like to solder. This is going to take up far too much of your time.

[Edited by skippy on 11-01-2000 at 08:32]
 
like I said in my original post, I'm not to bright. Maybe this is an ill concieved idea.

I got a lot of tape noise with my headphone out on my first forays into cassette to mini disc. I think I am relly just spoiled by the crispness of my other MD recordings(live, CD and me)

I had found a site that showed how to make a line level/lineout from a speaker load on a guitar amp and I figredit couldnt be that difficult to figure out for this. I think common sense is starting to prevail and I may do the borrow someone elses equipment thing

Thanks to all for your time, replys ,links, words of wisdom and advice


Rex Racer
 
I was afraid of that. The basic problem is the source format here: cassette tapes, even in the *best* of circumstances (perfectly recorded levels with good noise reduction) just barely give you 65-68dB signal-to-noise. *Maybe* 70dB, on a good day when the phase of the moon is right and the bias is jes' *perfect*... If you're used to digital sources, which beat that by well over 20dB in most cases, you're *going* to be disappointed: there's no way you can miss that 20dB difference in the noise floor.

Anything one generation old off of a cassette is going to suffer badly in S/N, regradless of whether you pick off the headphone out or the line out: the bulk of the noise is going to be coming right off the tape due to the narrow track/low speed format, and not from running the reproduce electronics at lower-than-optimum levels. Optimizing the gain structure with your existing gear will buy you a little, but probably nowhere near enough to be worth the effort.

Snag a buddy's deck and run from the line outs. But go into it with the understanding that the noise floor is gonna be very high, by comparison with your live or CD-sourced material. It is, unfortunately, the nature of the beast... Best of luck!
 
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