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Old 29th August 2014, 04:13 AM   #1
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Default Laptop to Guitar Amp cable. . . Resistor values?

I have a request to connect a laptop's headphone out jack to the input of a bass guitar amp. Well, the laptop is stereo 1/8", the amp is mono 1/4", so this needs a 2-resistor "Y" adapter inside the 1/4" plug. What value resistors should I use for this?
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Old 29th August 2014, 10:25 AM   #2
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Anywhere from 4k7 up to 22k will be fine. Jack out is <16r so no loading issues.
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Old 29th August 2014, 11:54 AM   #3
JMFahey is offline JMFahey  Argentina
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Agree but personally I use lower values, 470R to 1 K, since the EPH out can drive that anyway and so I can I *forget* about poor cables or whatever.
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Old 30th August 2014, 05:54 AM   #4
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OH! I was thinking output impedance of a guitar ~220K; however, there's also output impedance of pedals/effects, which have op-amps and near 0R, except for whatever they happen to use for output series resistance.

Assuming that the guitar amp has a tube/mosfet/jfet buffer at input, it may handle what the heck ever, right? In that case the lower values, like JMFahey's post, may be more important per noise reduction?
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Old 30th August 2014, 07:16 AM   #5
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Considering noise I assume the noise of notebook output will be pre-dominant. So there is not much you can gain with a lower impedance divider.
Generally the lowest impedance divider is the best, giving best suppression of eventual disturbance by badly screened cables, sound degration caused by cable capacitance of longer cables etc.
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Old 30th August 2014, 09:39 AM   #6
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Quote:
Originally Posted by voltwide View Post
Considering noise I assume the noise of notebook output will be pre-dominant.
Some can pass RMAA quality control (but this particular laptop has not been proofed). For a typical computer, the major noise output is on groundside, which is possible to filter. Generally, computers like to drive 12k and smaller figures. Generally they're capable of 0.9v max, beyond which they clip/distort themselves (so that max settings are counterproductive).

That is just another way to say that it is an almost overwhelmingly unsuitable headphone source, unless its particulars are addressed, and even then, it needs some help. For hi-fi output that's a fair challenge!

However, today's usage is that the laptop is serving as a virtual instrument tone generator for a midi keyboard, per piano practice needs, using GarageBand software; there are a plentiful amount of guitar amplifiers present and the MacBook Air's internal speakers (squeakers?) are insufficient to reproduce piano.
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Old 30th August 2014, 09:46 AM   #7
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Quote:
Originally Posted by danielwritesbac View Post
For a typical computer, the major noise output is on groundside, which is possible to filter.
hmm, I do not grip what you are talking about.
I talked about residual audio noise of poor headphone drivers inside the laptop.
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Old 31st August 2014, 02:23 PM   #8
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I think what you're looking for is Figure 1. The "Summed Output" plugging into the bass amp.

Why Not Wye?

The laptop's headphone output is very low impedance (we hope), and the bass amp's input impedance is very high (I expect), so I'd think the resistor values shown would be fine.

Of course the resistor values could be multiplied by two, or by whatever. Or use Figure 2. The whole thing's a voltage divider in disguise.
.

Last edited by bentsnake; 31st August 2014 at 02:33 PM.
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Old Yesterday, 01:48 PM   #9
DF96 is offline DF96  England
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Some laptops may use an unfiltered Class D headphone amplifier. This means feeding lots of HF hash into your guitar amp. Some amps will ignore it, especially if they start with a low pass filter. Some will be troubled by it. Using large resistor values with a longish cable will reduce the HF.
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Old Yesterday, 04:39 PM   #10
JMFahey is offline JMFahey  Argentina
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Agree.
In general I consider headphone outputs noisy/dirty , doubly so because they expect a headphone directly connected so between their inductance, high frequency rolloff and user ears rolloff , designers can save a few cents by being unsophisticated (read it as crude ) and yet get away with it.

Of course, some do care, I consider that a bonus

Cheap MP3 players are usually very dirty, just scope them. Ugh!

EDIT: you might add a small ceramic capacitor after the mixing resistors, maybe even inside the plug, to rolloff supersonic garbage.

As an example, 2 x 1K resistors would yield a combined impedance of 500 ohms.

That with a 22nF cap would be 3 dB down at 16KHz,way above anything expected at a Bass Amp input.
Or use a 10nF one if you intend to play Music at a PA amplifier.

Last edited by JMFahey; Yesterday at 04:47 PM.
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