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Old Jan 30, 2006, 10:28 AM   #1
joecool85
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Will I hurt my PB recording like this?

Using my 12 PB - It doesn't have a mic jack, only a line in.* If you use a mic straight to that you get basically no sound (duh).* So, I've been using my little gem 1/2watt guitar amp as a preamp.* I plug the mic into that, and then use the speaker out on that into my laptop.* I monitor the levels on my laptop making sure I'm not peaking too much.

Here is the schematic for the little gem:


Maybe I should keep the volume down on the little gem, and just crank my amp :-)* Right now the way I do it is put my amp at a comfortable jamming volume (reasonably loud) and then just adjust the volume on the little gem till the meters on my software are near peaking.* That is normally 3/4- almost full volume on the little gem.
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Old Jan 30, 2006, 11:43 AM   #2
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Strictly you should never, ever, ever plug the speaker level output of ANY amp into ANYTHING other than a speaker, you run the risk of damaging the audio input if the volume jumps accidentally. That said, I'm not sure 1/2 a watt is going to carry much power...

You should really get a pre-amp as described in many threads in this forum, USB or FireWire are standard and a single or stereo channel pre shouldn't cost much money.
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Old Jan 30, 2006, 01:28 PM   #3
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Yeah, you're right. I probably should buy a preamp. But I probably won't. I just tested and my little gem is giving me roughly 2.5-3.5 volts when mic-ing my dean markley amp. I checked the specs for my laptop and the linein will take 2Vrms and 5.65Vpp. So, I should be all set.

*edit**
On that same note, where could I get a cheap preamp?
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Old Jan 30, 2006, 04:55 PM   #4
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You could just buy a speaker DI box, this will convert speaker levels to line or mic levels. Ask at your nearest guitar shop.

Or get a preamp.
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Old Jan 31, 2006, 08:45 PM   #5
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Stop!

I really would stop doing that, if I were you. I blew out 2 channels on my TASCAM 4-track going from a speaker-out to a line-in. The voltage from a speaker-out can be about TEN TIMES (as I discovered) that of a line-out. Regardless of how you set the levels on your stuff, I would say you're eventually going to blow the line-in on your PB (because the circuit can't handle the voltage), and you may well take the motherboard with it. Get a pre-amp.
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Old Jan 31, 2006, 09:10 PM   #6
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Quote:
Originally Posted by joecool85
where could I get a cheap preamp?
might as well get a whole audio interface, the a/d conversion will be better than what's on board your laptop (and then you could go in either the usb or firewire port).

check here for an idea of what's available.
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Old Jan 31, 2006, 09:36 PM   #7
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No offense, but learn to read. I checked the voltages, and they are well within what the laptop is suppose to take in.
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Old Feb 1, 2006, 02:25 AM   #8
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Quote:
Originally Posted by joecool85
No offense, but learn to read. I checked the voltages, and they are well within what the laptop is suppose to take in.
why are you being less than kind to people who are trying to help you?

so long as you're checking voltages, check your impedances. i think you'll find a high output and a low input, which is the opposite of what you want. that's why you shouldn't proceed with your setup.
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Old Feb 1, 2006, 04:56 AM   #9
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typical speaker impedance is between 2 and 32 Ohms for musical instrument application, the impedance of your PB input is waaaay lower than that, so you are driving that poor little amp very hard with such a light load, voltage is only part of the picture.

As I said earlier, keeping the gain down on the amp will protect you, but it's a dangerous situation, plus as Zim helpfully pointed out, a good interface will sound much better that the poorly powered input that Apple use on the PowerBooks.

Have a look through this forum, there are plenty of threads about pre's and interfaces.
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Old Feb 1, 2006, 06:05 AM   #10
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Well, the input impendance is actually extremely high, not low. I just tested it out at 47.5K, in other words, 47,500 ohm resistance. It's not driving that "poor little amp" too hard. Thats actually barely a load at all.

And Zim, that comment I made to beatsme was because I had just stated the voltages, and then he was saying that speaker voltages can be ten times what the input on my PB is. If he had read what I had posted, he would have noticed that the 1/2watt amp I built is right within the safe range.
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Old Feb 1, 2006, 07:00 AM   #11
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Quote:
Originally Posted by joecool85
Well, the input impendance is actually extremely high, not low. I just tested it out at 47.5K, in other words, 47,500 ohm resistance. It's not driving that "poor little amp" too hard. Thats actually barely a load at all.

And Zim, that comment I made to beatsme was because I had just stated the voltages, and then he was saying that speaker voltages can be ten times what the input on my PB is. If he had read what I had posted, he would have noticed that the 1/2watt amp I built is right within the safe range.
47K Ohmage? What kind of speaker works at that load?

Seriously, you'd have a hard time getting any instrument amp to drive a 47K Ohm load, a headphone amp maybe.... The original post mentions a 16 Ohm load, so I'd imagine a 47K Ohm load would be a bit much...

Either way, your set-up is not correct and you should still get a proper pre-amped interface, your choice (and your PowerBook).
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Old Feb 1, 2006, 12:32 PM   #12
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Lets get this back into perspective, you original post asked if this could hurt your PB. The answer is Yes, it could. Putting Speaker level signal into a Line level jack is like putting a Mini disk into your slot-loading PB optical drive. It might work, but chances are sometime you are going to have a problem...
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Old Feb 1, 2006, 01:22 PM   #13
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Quote:
Originally Posted by joecool85
Well, the input impendance is actually extremely high, not low. I just tested it out at 47.5K, in other words, 47,500 ohm resistance.
impedance and resistance aren't the same thing. since:

I = E / R

where E is energy in volts and R is resistance in ohms, the bigger value of R you get, the lower your impedance.
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Old Feb 1, 2006, 02:31 PM   #14
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Quote:
Originally Posted by zimv20
impedance and resistance aren't the same thing. since:

I = E / R

where E is energy in volts and R is resistance in ohms, the bigger value of R you get, the lower your impedance.
Volts/Ohms = Amps = Current, usually denoted I.

Impedance, usually denoted Z: http://en.wikipedia.org/wiki/Impedance

B
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Old Feb 1, 2006, 04:43 PM   #15
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Quote:
Originally Posted by balamw
Volts/Ohms = Amps = Current, usually denoted I.

Impedance, usually denoted Z: http://en.wikipedia.org/wiki/Impedance
holy crap! this is what i get for trying to think at 2:30 in the a.m.

i stand embarrassingly corrected.
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Old Feb 1, 2006, 05:49 PM   #16
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Quote:
Originally Posted by WinterMute
47K Ohmage? What kind of speaker works at that load?

Seriously, you'd have a hard time getting any instrument amp to drive a 47K Ohm load, a headphone amp maybe.... The original post mentions a 16 Ohm load, so I'd imagine a 47K Ohm load would be a bit much...

Either way, your set-up is not correct and you should still get a proper pre-amped interface, your choice (and your PowerBook).

Maybe you just don't understand how amplifiers work. As a rule, amps put out more power at lower ohms, but its harder on them. The higher the resistance, the lower the power output, and the easier it is on the amp.
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Old Feb 1, 2006, 09:47 PM   #17
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for the record

Quote:
Originally Posted by joecool85
No offense, but learn to read. I checked the voltages, and they are well within what the laptop is suppose to take in.
I read the original post, typed my response, and put my machine to sleep on MONDAY (phone call + other things to do). I didn't post my response until Tuesday night, but when I wrote it, the post wherein you mention the voltage had yet to appear.

Don't be so snarky
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Old Feb 2, 2006, 06:10 AM   #18
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Quote:
Originally Posted by joecool85
Maybe you just don't understand how amplifiers work. As a rule, amps put out more power at lower ohms, but its harder on them. The higher the resistance, the lower the power output, and the easier it is on the amp.
...er thats the point I was making, that high a load will result in a very low performance from the amp, my mistake was reading output, not input originally...

The fact remains that a speaker output is designed to drive a speaker not a line input, and there is the potential to damage the input and, incidentally, the output transformer of the amp too, although in this case that wouldn't be an issue, but try it with a valve amp and watch it burn.
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Old Feb 2, 2006, 06:53 AM   #19
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Well, I wouldn't use it with any other amp quite frankly. The lm386 just seems to work real well as a preamp. Regardless, the thing is powered off from a 9volt powersupply...

Since the lm386 chip can run at as low as 4volts, and I've run one on 4.5volts just fine, I'm going to build one just for this using 3 AAs (4.5volts) and that way the max will be in the 3-3.5volt range.
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