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Help matching impedances

PatrickPatrick wrote 10/15/2019 at 22:51 • 3 min read • Like

Hey there!

I'm looking for some help in understanding impedance matching.  I'm hoping one or more of you technically savvy electronics folks will jump in and give me a simple circuit recommendation.

Specifically I want to try to build a passive "pad" that goes between the output of a Fishman Prefix Plus built-in guitar pre-amp and the input of an IRig HD2 guitar-audio-to-usb converter.  My issue is that, as it stands, I have to turn the volume control of the pre-amp as close as I can to it's minimum value (like 0.01, which is iffy), and set the gain of the Irig input hard to it's minimum value to get any kind of decent sound.  Anything else results in lots of distortion.  I'm trying to get a clean guitar sound.

https://www.fishman.com/products/series/prefix/prefix-plus-t-onboard-preamp/

https://www.ikmultimedia.com/products/irighd2/index.php?p=specs

I get the feeling that by using the extreme lowest setting on the pre-amp I am not in the sweet spot of the pre-amp design.  I feel like it makes the sound tinny ... I'm always lacking bass when I use the device (compared to just plugging my guitar into the Yamaha Stagepas PA).  It's just my noob idea that the amp was probably designed to be used in the middle, rather than at the extremes of its gain settings.

So, I'm thinking about making a passive "pad" that basically consists of two resistors. 

input    O------- R2 --------+----------------O output
                             |
                             R1
                             |
ground  O------------------- +----------------O ground

My basic, very limited, understanding is that such a passive pad consists of a voltage divider created by resistors R1 and R2.   I really have no idea what values to use, although I sort of think their ratios should be about 9:1  where the value of R2 is about 9 times greater than the value of the "shunt" resistor "R1".   Rather than just guess, I thought I would ask the community.

The specs for the devices are copied from the user manuals  Here is what I see:

FISHMAN Guitar Pre-amp

IRIG HD2 audio-to-usb converter

I cannot find the actual input impedance specification for the IRIG HD2.  There is a reference in the user manual that it is a "high-Z input".  I am guessing it is simllar to the input impedance for the IRIG2 which IS specified as  380kOhms.

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So, given those specs, what should be the values of R1 and R2?

Any other thoughts on what I'm trying to do here?   I'd sure like to get a good sound and a feeling of control instead of a tinny sound and a pre-amp that is turned down to 0.01 ....

Thanks in advance for any replies.

- Patrick

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Ken Yap wrote 10/16/2019 at 00:23 point

Let's look at it from the viewpoint of the preamp. The impedance that it sees is R2 + R1 // input impedance of converter. Since that last term is quite high, effectively it's R2 + R1. You want this to be at least a few times higher than the output impedance so as not to load it down much. This means 10kohm and above should be fine.

From the viewpoint of the converter, the impedance seen is (R2 + output impedance of preamp) // R1. As the converter is high impedance, this contribution by the output impedance is negligible.

So try something like 10k and 1k, which will divide the voltage by 11. If that isn't enough attenuation, decrease R1. 10k and 100 will give you a attenuation of 101.

One last thing, that converter may have a +20dB boost control, like many sound cards do. You might want to check that this isn't enabled and making it more sensitive than needed.

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Patrick wrote 10/16/2019 at 02:31 point

Thanks Ken!   There's no attenuation (boost control) switch on the box.  As per your suggestion,  I rigged up a 10K/1K combo and it worked pretty darn good ... it put it right in the middle of the range of the pre-amp, so that the pre-amp volume knob can now be set to about "5" on a scale of 1 to 10 without distorting.  It sounds at least as good as before, maybe better.  I think i'm going to make it semi-permanent, maybe with a bypass switch.  

In all cases, thanks for answering!

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Ken Yap wrote 10/16/2019 at 02:40 point

It may be a software boost switch. Is there a driver and control panel?

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