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Old 23rd March 2012, 11:05 PM   #11
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This is the circuit. The 16 Ohm is the speaker. All of the right side resistors are subject to change. The 600 Ohm is the laptop. I am going to measure its actual impedance soon. I want to put a pot in series with the 600 Ohm for volume control. Is there a better way to do this?

View image: Screen Shot 2012 03 23 at 6 57 37 PM

My amp is a tube amp, by the way. I am not opening up my amp. I don't think it's necessary as I am deducing that the distorting is coming from the computer because I am giving it too much voltage. Do computers sense for too high of an input and add additional input impedance as a safety feature?
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Old 24th March 2012, 12:13 AM   #12
sek is offline sek  Germany
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Quote:
I was told by a professor that 1V, 600 Ohms was the standard for audio input.
It probably was, back in the days.

These 600 Ohm stories really seem to stick...

Your circuit suits the demands for impedance matching between interconnected devices. In essence you employ an L Pad, assuming the input impedance of the computer to be around 600 Ohm.

An amplifier and a line level input are not to be impedance matched, as they don't possess a common impedance level, nor a predefined impedance ratio. Instead the source impedance (of the amplifier) can be assumed as sufficiently low (in the single digit ohm range) and the input impedance of the sink can be assumed as sufficiently high (in the multiple kilo-Ohm range). By extension the input impedance is large compared to the output impedance. Thus, simple voltage divider rules apply. No point in optimizing the network for an impedance level of, say, 600 Ohm.

A voltage divider does the trick, all you need to know is the estimated output voltage level (full scale) of the amplifier and the acceptable input voltage level (full scale) of the line input. No impedance calculation involved, it's a simple voltage ratio problem.

Regarding galvanic isolation, it's neither required (as safety is not an issue, no dangerous voltages are involved) nor helping (because a transformer can easily overload a laptop computer line input, too).

Quote:
The 600 Ohm is the laptop. I am going to measure its actual impedance soon.
Here's your contradictory assumption. You'll find that the actual input impedance is far from 600 Ohm. Amplifier and Speaker together then form a low enough source impedance.

Cheers,
Sebastian.

PS: I know some people who wrecked their MacBook Pro's audio circuitry by applying too large voltage levels, particularly DC. The full scale input level is probably below 1V. Be careful.

Last edited by sek; 24th March 2012 at 12:18 AM.
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Old 24th March 2012, 12:28 AM   #13
sek is offline sek  Germany
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Originally Posted by Littlebilly91 View Post
I don't think it's necessary as I am deducing that the distorting is coming from the computer because I am giving it too much voltage.
That's right. Your actual input drive level is simply ill-defined due to the interaction of the amp, the transformer and the laptop input impedance.

The System Preferences contain a Sound setting. This one has an Input tab with a level meter. Use this to check levels and overload conditions.

Quote:
Do computers sense for too high of an input and add additional input impedance as a safety feature?
How would they? They can automatically adjust recording levels in software. They have no means to foresee electrocution, though.
Computers just aren't built like that...

Quote:
Originally Posted by Littlebilly91 View Post
I am not opening up my amp.
What amp exactly? I find it unlikely that it shouldn't have an auxiliary output or an effect send/return loop.

Last edited by sek; 24th March 2012 at 12:33 AM.
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Old 24th March 2012, 03:47 AM   #14
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Thanks for all the advice. I stuck a 56k resistor in and the voltage is around 50-100mV. Things seem to be working much nicer. I want to build a peak detector so I know what level the input is at before I plug it in. I know how to do that, but my next question is if the voltage before and after the computer is connected is different (due to the computer's impedance which I found to be a little over 1kOhm), what is the best way to account for that difference? I believe this would mean that my circuit must have low impedance, but in order to keep the voltage across the laptop low, I had to use a the 56k resistor, which made the output impedance fairly large.
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Old 24th March 2012, 04:04 AM   #15
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If it works, it works. If it doesn't that's a different matter.

If you want an explanation of how your circuit is working, then you have to post EXACT details of what you've got. It's no good saying 'I got a 56k resistor in there'. We're not clairvoyant. What else have you got? How is it all connected?

Draw a diagram, photograph it, and post the photo if you can't do any better.
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Old 24th March 2012, 04:12 AM   #16
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Sorry about that.
View image: Screen Shot 2012 03 24 at 12 06 48 AM

I would be using a voltage buffer to connect it to a peak detector, so I have no impedance concerns for that part. It would be ideal to be able to tell if the voltage clears some threshold whether the load is attached or not. As it is, that can't be done because the load changes the voltage.
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Old 26th March 2012, 07:11 PM   #17
cbdb is online now cbdb  Canada
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due to the computer's impedance which I found to be a little over 1kOhm
Seems rather low. Sure you didnt measure the input resistance? Acording to Apple its "greater than 20k ohms" and will take up to 3 volts.

iMac (Mid 2011): External features, ports, and connectors

The macbooks will be the same as the Imacs

If your trying to record the output of a keyboard (an instrument) you DONT NEED the power amp or any circuit, just a cable!

If you are using the amp as an effects box, thats a different story.

Last edited by cbdb; 26th March 2012 at 07:14 PM.
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