[LAU] checking passive input frequencies on non Hi-Z connection

Tim termtech at rogers.com
Sat May 20 15:12:32 UTC 2017

On 05/18/2017 07:15 PM, Wayne DePrince Jr. wrote:
> ahoy all,
> i have been testing out a Zoom H5 
> <https://www.zoom-na.com/products/field-video-recording/field-recording/zoom-h5-handy-recorder> 
> which (along w/ the H6 
> <https://www.zoom-na.com/products/field-video-recording/field-recording/h6-handy-recorder> 
> for that matter) is advertised as NOT supporting passive instrument 
> connections on the TRS inputs.  to quote the manual: "Direct input of 
> passive guitars and basses is not supported."  thus i assumed that i 
> would need a DI box/effect box/etc. to record well any directly 
> connected passive electric guitar or bass.
> however, when i connect my electric guitar (Lace Sensors and Alumitone 
> pickups) and electric bass, both passive, to the TRS connections on 
> inputs 1 & 2 of the H5 with gain only at 5 of 10, i get a good sound 
> and strong signal, /seemingly/ the same as when they are connected to 
> an interface (in this case a Zoom U-44 
> <https://www.zoom-na.com/products/production-recording/audio-interfaces/zoom-u-44-handy-audio-interface>) 
> that supports Hi-Z connections.
> i first contacted Zoom support and they replied:
> "There is no issue connecting a passive guitar to the 1/4" input on 
> the H5.
> There may only be an issue with level or noise, but if you are 
> satisfied with the audio quality then there is no problem."
> now while i do not yet have a good understanding of Lo vs. Hi-Z, i 
> have read various 
> <https://www.gearslutz.com/board/newbie-audio-engineering-production-question-zone/951585-instrument-level-line-level.html> 
> posts 
> <https://www.gearslutz.com/board/low-end-theory/833242-direct-guitar-recording-problem.html> 
> about it. they seem to indicate that the passive guitar signal would 
> lose some frequencies with this kind of connection and thus this setup 
> is undesirable.
> but again as it "sounds the same", i had some questions before 
> investing in a DI box or getting a different interface:
>   * would the fact that the pickups are Lace Sensors and Alumintones
>     have anything to do with the passive signal being picked up fairly
>     well by the non Hi-Z input?
>   * is there a way to check (preferably in software) if frequencies
>     are actually missing or if the signal quality is indeed reduced
>     when compared to one from a Hi-Z input?
>   * is it possible that the H5 has good enough preamps/whatever that
>     the gain on the inputs is enough to make the passive signal OK?
> so to summarize: is this an acceptable solution for recording a 
> directly connected passive instrument (in this case an electric guitar 
> and bass) even though the H5 does not officially support it?
> thanks, w
> PS: for those wondering the H5 works great w/ GNU+Linux as an audio 
> interface
Hi. According to the Zoom HS specs, the input impedance on Mic/Line In is
  2 Kilo Ohms or more while Inputs 1/2 are 1.8 Kilo Ohms or more.

This is terrible for a guitar pickup.

The pickup's coil and the input impedance of the device it is plugged into
  form a low-pass filter. The higher the input impedance the better.
That's why guitar inputs are always 1 Mega Ohm or more.

But it is bad for another reason as well:
Most guitar pickups have between 6 and 12 Kilo Ohms resistance.
When fed into your 1.8 Kilo Ohms input, it forms a voltage divider
  which will severely lower the overall output of your pickup.

Get a DI box.
If you want the maximum possible frequency response and you
  want the most out of your pickups, get an active DI box.
It may introduce a bit more noise due to the active circuit.

On the other hand, if you want convenience of no batteries
  and just plug and go, get a passive DI box.
It may introduce a bit of noise due to non-linearity of the
  passive transformer, and the frequency response may not
  be as full or flat as an active DI box.
Mine is a Radial Engineering DI box. I am happy with it.
Even though the sound is slightly coloured by the
  passive transformer, it is far better than no DI box at all.

As for measuring the differences, that's a tough one.
I was able to see clearly a difference simply by using
  my oscilloscope and plucking strings.
With the guitar plugged into a 10 Kilo Ohm mixer input,
  I could clearly see less high frequency content (harmonics etc.)
  than with the guitar-to-scope alone. (The scope probe has
  1 Mega Ohm input impedance.)

A more scientific method might involve simultaneously
  recording the un-loaded pickup signal and the loaded
  signal and then comparing with software.

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