Jump to content

Input impedance


JustaBass
 Share

Recommended Posts

1 hour ago, Gareth Hughes said:

Piezo pickups typically used on double basses work better with input impedances higher than 1megohm, so they’re just making it more enticing to upright players. As far as I’ve heard/experienced, a higher input impedance doesn’t make any difference for electric basses.

 

Well it can make in theory degrade the sound as you have less signal current flowing in the cable, probably not noticeable on a shortish good quality cable though but better to use the right input for the job just to be sure.

  • Like 1
Link to comment
Share on other sites

Current doesn't determine the signal level, voltage does. Input impedance doesn't affect the signal voltage flowing in the cable. Losses occur when the amp input impedance is too low, not when it's too high. When the amp input impedance is less than some ten times the output impedance of the bass what's known as loading occurs, which results in signal loss. https://www.electronics-lab.com/article/input-and-output-impedances-of-amplifiers/

This being the case the higher the source impedance the higher the input impedance must be. Power amps tend to have an input impedance around 10k ohms, because the consoles that plug into them have an output impedance of 600 ohms or less. Instrument amps have an input impedance of 100k ohms or more, because passive pickups have an output impedance of 5k ohms or higher.

 

 

  • Like 1
  • Thanks 1
Link to comment
Share on other sites

I think I have an analogy that's just about right enough to help understand impedance. Imagine you tie one end of a skipping rope to a ring drilled into a wall, and wiggle the other end of the rope. You want to see what frequencies you're pumping out with your arm, and you do so by measuring and plotting the up and down movement of this ring over time.

 

Now, imagine that wall is made of jelly, that ring is going to be wibble wobbling up and down like mad when you're really yanking that skipping rope. That wall has a nice low input impedance, and it's really easy to measure what the ring is doing, which is great. However, what happens when you make the signal small? Your small movements are going to be lost in all the noise of external factors, and in particular your high frequencies are just not being seen in the movement of the ring. Also another problem with the jelly is that there will be some reflections as the ring starts to move the skipping rope, creating an echo (not really a problem in musical instruments, but thought I'd mention it).

 

Next up is the brick wall. The ring is barely moving now, but when it does it's a much more accurate representation of your output frequencies of your arm. This is the high input impedance scenario and the movement of the ring captures all your nuanced wiggles on the skipping rope in exquisite detail. What's the catch? Well if you know that your signal is nice and strong then a brick wall is a waste as the small movements aren't as easy to measure.

 

Here's where we have to really stretch the analogy: you and your arm are the instrument, the skipping rope is the signal going down the cable if not the actual cable, the ring and wall are your preamp, which in this case is included inside your amp. Your nice strong signal would be when you're using a magnetic pickup, and the small weak signal is the piezo pickup. I should also add that this analogy totally breaks down when you consider microphones, with their very small signals yet small impedances, but oh well.

 

Coming back to your amp, the obvious question in your situation is "why would an amp have both? Wouldn't the high impedance work in both piezo and magnetic pickups?" As Bill says, there's definitely a case where you can have your input impedance too low for the instrument, but the case against making it too high is not so obvious. Many preamps just offer the one input impedance for all situations ~5MΩ (e.g. the Fishman Pro EQ is 10MΩ). I suppose if you're not careful you could cause clipping with an instrument that is too hot, but in all my years using an ultra high impedance input for both unbuffered piezo and magnetic I've never had a problem. I just have a lower gain for the magnetic pickups.

 

There's an SoS article about impedance which is excellent. It talks about the history of the values chosen, as well as a little bit of the maths involved. Hopefully my analogy above can be understood by someone with no mathematical background and is correct enough to not upset anyone who does actually fully understand the science behind it.

  • Like 1
Link to comment
Share on other sites

While on the subject: The reason speakers are very inefficient, on average only converting 2% of the electrical energy they receive into sound, is impedance, specifically the impedance of air, which is very low. 8 ohms doesn't sound like it's a high impedance source, but it's a lot higher than air, so nearly all of the energy put into a driver is lost as heat. A horn loaded speaker acts as an acoustical transformer, just like the output transformer in a valve amp. The horn raises the impedance of the air mass that the driver operates into, increasing efficiency as much as tenfold.

  • Like 4
Link to comment
Share on other sites

21 hours ago, Bill Fitzmaurice said:
21 hours ago, Bill Fitzmaurice said:

Current doesn't determine the signal level, voltage does. Input impedance doesn't affect the signal voltage flowing in the cable.

I think were both right but just arguing over the semantics of "signal" which to me means voltage and current since to transmit the signal we need some current to flow.

 

11 hours ago, chyc said:

 the case against making it too high is not so obvious.

 

The case is that the cables effective parallel capacitance affects a higher current (for a given voltage) less than a lower one, not a dramatic effect except in extreme conditions admittedly like perhaps where you have very long and/or poor quality cable.

Edited by bassman7755
Link to comment
Share on other sites

1 hour ago, bassman7755 said:

I think were both right but just arguing over the semantics of "signal" which to me means voltage and current since to transmit the signal we need some current to flow.

You can't have the one without the other, but in this case the current level is far too low to be of any consequence and has no influence on the result.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...