Impedance match Fishman guitar pre-amp to IRIG2HD

Question

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

Nominal Input Level: -20dBV
Input Impedances: 20MOhm
Output Impedance: "Less than 3.5kOhm"
Nominal Output Level: -12dbV

IRIG HD2 audio-to-usb converter

Maximum Level: from 307mvpp to 8.36Vpp
Gain Control Range: 28.7 dB
Input Impedance (guess): 380 kOhms

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.

---

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 ....

Answer 1

The common approach is to use the matching T attenuator circuit. The impedance is not critical, as its a line system instead of a microphone. The simple resistor network I would recommend for this:

^{simulate this circuit – Schematic created using CircuitLab}

This gives about 16db (15.5 or so) of attenuation, to get back to nominal guitar level. You could use a passive DI w/pad, and a xlr to 1/4 inch, but this is much less bulky.

Answer 2

What you are looking for is attenuation, not impedance matching. To avoid loading down the source you want the input impedance of your divider to be higher than it, and to maintain output level the attenuator's output impedance should be lower than the destination. A simple voltage divider will work fine.

The guitar preamp has an output impedance of "Less than 3.5kOhm" so chose a value about 10 times higher for R2. Make R1 1/10th of that and you are good to go. Or better yet, use a log taper potentiometer of ~35k (eg. 25k, 50k) which divides by 10 at about half 'volume', and then you can easily adjust the attenuation.

Answer 3

Phrases like "nominal output level" are a bit ambiguous, what you really want to know is the "maximum" output level of the pre. But let's assume it has about 20dB headroom above the nominal level, that means the max would be around +8dBV which is about 2.5Vrms. So you are never going to overload the input to the USB converter, which can handle 8pk-pk, which is about 2.8Vrms. (Presumably at minimum gain setting - the gain control is probably just an attenuator anyway).

The Fishman has op impedance less than 3k5 (probably to match with guitar amps which are usually around 1M ip impedance) and your guess of the input to the converter is 380k, which sounds reasonable. I wouldn't expect less than about 47k.

So I don't think you should need a matching network. Just connect them together, turn the Fishman to max, and use the input trim on the USB converter to minimum. That should work out fine. If needed turnthe volume of the Fishman down a touch, but you shouldn't need it all the way down.

I wonder why it isn't working. Are you sure there isn't another issue? A bad cable or something? (Maybe the IRig people lie about their USB interface or the Fishman nominal level is a lot more than 20dB less than max. If you have a scope that would tell you.)

(Actually I have an acoustic with a Fishman and I DI it into a Zoom recorder all the time with great results. I think that this should work for you too.)

If you really do need a pad, a simple two resistor network with input resistance (i.e. the sum of the two resistors) at about 40-50K should work. Perhaps a 47k log pot with would be the easiest. (ACW pin to the pre, wiper to the USB and CW pin to ground of course.)