Simple method to measure an amp's frequency response ?

Discussion in 'Amps [BG]' started by OldogNewTrick, Jul 17, 2006.


  1. OldogNewTrick

    OldogNewTrick

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    Dec 28, 2004
    Location:
    Germany, EU
    Hi, is there a simple way to measure an amp's frequency response without the aid of sophisticated electronical equipment ?

    I'm using a Dual Showman as bass amp and was wondering how well the fundamentals (30-80Hz range) are handled.

    I have got Duncan Amps TSC 1.3 to help me set the tone controls to near flat, however I'm wondering if the amp's natural response curve is not dropping off below a certain point...
     
  2. billfitzmaurice

    billfitzmaurice

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    Owner, Bill Fitzmaurice Loudspeaker Design
    Yes and no. You can download a signal generator to get the necessary signal into the amp
    http://www.world-voices.com/software/nchtone.html
    and then use a digital multi meter to read and chart the output voltage at various frequencies at the output via a second speaker cable. What you'll find with a Dual Showman, or just about any Fender, is anything but flat response, but that's mostly due to the pre-amp. As I recall from testing one 40 odd years ago it works OK down to at least 30Hz or so. It certainly goes a lot lower than most speaker cabinets will.
     
  3. OldogNewTrick

    OldogNewTrick

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    thx billfitzmaurice.
    I've got nchtone - one more "stupid" question:
    do I measure dc voltage across the speaker leads ?
     
  4. Geoff St. Germaine

    Geoff St. Germaine

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    Dartmouth, Canada
    You should be measuring the AC voltage.
     
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  6. fdeck

    fdeck Supporting Member

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    Click on my sig and download my little program, written for exactly this purpose.
     
  7. Geoff St. Germaine

    Geoff St. Germaine

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    The only thing to watch out for is that he is trying to measure the frequency response of the whole amplifier (not the just the preamp or the speakers as far as I can tell). As such he should not be plugging the speaker outputs of the amp into the inputs on his sound card.
     
  8. fdeck

    fdeck Supporting Member

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    Very good point.
     
  9. OldogNewTrick

    OldogNewTrick

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    thx guys.
    I'm indeed trying to determine the frequency response of the whole amp. Measuring the AC voltage across the speaker terminals should be pretty easy. Plug the data into a spreadsheet, make graph and ready is.
    Could this really be so easy ??? :meh:

    The amp has no output after the preamp stage, so I cant measure the preamp alone.
    And yes, plugging the speaker leads into my soundcard, looking intendly at the screen will cause in short order :smug: :eek: :bag: :bawl:

    Measuring the speaker response will be my next step. Using fdeck's little programme :)
    I just want to know for future projects that if it sounds like this, the response curve should look like that.
    And since most musical instrument amps, definitely vintage tube amps, provide anything but a flat responsonse curve, I need to test amp and speaker separately.
     
  10. Espidog

    Espidog

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    UK
    Hi OldDogNewTrick!

    Are you planning to measure the output signal with the amp feeding a loudspeaker, or a dummy load? Each method will produce different results.

    A good dummy load will give you a plot of the amp's "naked" response, because the load's impedance will remain nearly constant throughout the audio frequency range.

    A loudspeaker is a reactive load. Consequently it has an impedance "curve" - i.e. its impedance changes according to frequency. Every type of loudspeaker has its own unique impedance curve, and this curve is further affected by what sort of cabinet it might be loaded into. So, measuring your amp with a speaker attached will give you information about the amplifier's frequency response only when working into that specific speaker and cabinet arrangement.

    Both sorts of information are useful!
     
  11. OldogNewTrick

    OldogNewTrick

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    Espidog, thanks, you're definitely right, should have thought about that.
    So a 3,9 Ohm 25 W resistor should do the trick, right ?
     
  12. Espidog

    Espidog

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    I'm not sure what the wattage of the Dual Showman is. My researches seem to suggest that it must have changed over the years - I've seen it quoted as anything from 60 to 135 !!

    For safety's sake, why not use two 8-Ohm 25 Watt resistors in parallel - or even four 16 Ohm ones (if you can get them) ?

    Anyhow, just make sure you don't let the beast run at or near full output for more than a few minutes, or those resistors will get very hot indeed. :eek:

    Have fun!
     
  13. Bassman62

    Bassman62

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    The Silverface 70s 'Dual showman Reverb' was 100 watts, finaly, as with 70s the Bassman 100 it was re-rated to 135 watts, I was told that they just increased the voltage somewhere, maybe someone can explain how.
     
  14. fdeck

    fdeck Supporting Member

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    Given the frequency dependent impedance of a speaker, you might need to measure the response curve under the actual load. Wear earplugs. Perhaps, borrow a direct box that has a speaker level input, and run the output to your PC sound card. It is still prudent to start out at a very low signal level. In any event, the direct box might not be a waste of money if you are using an amp that doesn't have a DI output.
     
  15. WOOFMAN

    WOOFMAN

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    DOD use to make a 31 band frequency analyzer with a build in noise generator. This is how I do it. Quick simple, and somewhat accurate.
     
  16. OldogNewTrick

    OldogNewTrick

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    @ Espidog
    It's a 100W 70's Silverface, but then I wasnt planning on running full blast, while measuring voltages accross a resistor....

    @Fdeck
    I dont have anybody on hand, who has a DI box with speaker in, so that's out.
    But I will measure the frequency response against the actual load also.
    And I will have to organise a sack full of earplugs to hand out to family and neighbours.
    Me, should have started wearing them 30 years ago... ;)

    @ Bassman62
    yeah, I'd like to know that too... however, pretty certain a different output transformer, other than that prly only subtle changes.
    However, I'm not going to mess around with mine, will stay stock and to original specs.

    @WOOFMAN
    dont want to buy something to use just once...
    DOD = Dep. of Defence ??? Thought they were messin with other stuff...


    So, will organize now a dummy load of 4 parallel 16Ohm/25W resistors and start measuring.

    Will post the results for both tests in a couple of days !
     

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