Frequency response vs amplifier output impedance calculator for REW

Discussion in 'Audio Science' started by Biodegraded, Oct 28, 2018.

  1. Biodegraded

    Biodegraded Friend

    Friend
    Joined:
    May 28, 2017
    Likes Received:
    2,445
    Dislikes Received:
    0
    Trophy Points:
    93
    Location:
    Vancouver BC
    I imagine most people who've gone to the trouble of wiring up an impedance measurement jig to complement their FR etc. measurements (if you haven't, see here) have already made some sort of variable-OI calculator to use with it, but cursory googling didn't turn up anything publicly available. Tired of doing the calculations by hand for only a few points on the curve I wrote one myself, and I'm pleased to share it with you lovely people:

    https://www.dropbox.com/s/cxjar2y10pqvfdn/Frequency response vs amplifier output impedance calculator.xlsx?dl=0

    Let me know if the link doesn't work for you, I'm not used to this public sharing business.

    I/O and usage notes are on the first page. The results assume the input measurements were taken with an amp of zero Ohms OI, which is of course not the case but if you're using a sensible modern solid-state unit (and some tube ones) the output will be below 1 Ohm and the results will be close enough. If you're obsessive, you could adapt the spreadsheet for the particular amp you're using. Or otherwise modify/improve it, adapt it for other software (ARTA or whatever), as you wish.

    To show you that it works with at least one example, here are results for the Westone UMPro20 IEM:

    [​IMG]

    The red frequency response curve shows underwhelming mids & lower treble, but the brown impedance curve suggests an amplifier with high output impedance might partially cure that. Calculated results for 3 cases below:

    [​IMG]

    The mids are certainly helped, but even the 120 Ohm result indicates lower through mid treble still won't be great (on my home-made tube coupler more neutral things don't start dropping off until after 5 or 6k).

    Anyway, I hope somebody will find it useful. Please let me know if you find mistakes. Regarding which, the usual disclaimers apply: I will not be liable for any injury or death, etc. etc.
     
    Last edited: Nov 25, 2018
    Lyander, monacelli, Brause and 4 others like this.
  2. Biodegraded

    Biodegraded Friend

    Friend
    Joined:
    May 28, 2017
    Likes Received:
    2,445
    Dislikes Received:
    0
    Trophy Points:
    93
    Location:
    Vancouver BC
    Lazy Sunday morning with nothing productive to do, so...

    Here's HD6XX response calculated for different output impedances vs baseline in red:
    Green: 20 Ohms (eg Hafler amps)
    Blue: 32 Ohms (option on Garage 1217 Project Polaris)
    Pink: 120 Ohms (option on Polaris)
    Purple: 200 Ohms (Creek OBH-11).

    Note my impedance measurement (tan) peaks a bit lower than the Innerfidelity HD650 one. This would make only a fraction of a dB difference though.

    [​IMG]
     
  3. m17xr2b

    m17xr2b Friend

    Friend
    Joined:
    Mar 26, 2016
    Likes Received:
    2,301
    Dislikes Received:
    36
    Trophy Points:
    93
    Location:
    United Kingdomland of fish and chips
    Interesting, I think there's more to this story than just frequency response. I have the Ember at work and besides the bass response there's a softness to the sound going higher on the impedance ladder and also a boominess in the bass besides just an increase in SPL.

    Also I never liked Stratus/Stellaris with HD650 on 120ohm but Teton with 142ohm is spectacular. Thus I wouldn't put too much into what impedance the amp has.
     
  4. purr1n

    purr1n Finding his inner redneck

    Staff Member Friend BWC
    Joined:
    Sep 24, 2015
    Likes Received:
    49,401
    Dislikes Received:
    80
    Trophy Points:
    113
    Location:
    Antarctica
    Higher source impedance also results in less power across the transducer according to the square of the ratio between source impedance and transducer impedance. Probably explains some of the softness.
     
    Lyander likes this.

Share This Page