High output, low volume? Input impedence mismatch?

Discussion in 'Pickups & Electronics [BG]' started by bdplaid, Feb 15, 2013.


  1. bdplaid

    bdplaid

    Joined:
    Aug 31, 2007
    Hi, electronics mavens! Help me get my head around the concept of bass instrument output impedance vs amplifier input impedence - which is where I think I'm having a problem.

    I rehearse through a Roland D-Bass 2x10 combo. Generally a nice little amp. It works nicely with passive electronics in my basses. Get damned loud using those basses, too. But when I bring in an active instrument, especially one with a really hot output, there's no combination of settings that allow any kind of sound volume to be pushed out of the amp that's like when it's being fed from a passive instrument. And note the passive basses are not quiet, either. One in particular has Bart P-bass classic bass pup's that are pretty loud.

    Clearly, when using the hot active basses, the input is overloading. I've switched it from "passive' to "active," which should adjust the input impedance, and also readjusted the input gain level. But even doing this, the volume drops so much I get about 2/3 the max volume out of the amp that i do when using quieter passive basses. It seems like the input gain starts to act like a compressor.limiter, and just maxes out and goes no further. But as I say, with the semi-hot passive basses, this is not the case.

    I suspect there's some kind of impedance mismatch going on, but not sure. I've not quite gotten my brain to absorb that. I've always viewed it as the input impedence needs to be substantially higher than the bass output impedence, rule of thumb being 10X higher. The input impedance on the amp is claimed to be 1 MOhm.

    So all comments and thoughts are welcome on this one.

    Thanks!
  2. Jazzdogg

    Jazzdogg Less barking, more wagging! Supporting Member

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    Jul 29, 2006
    Location:
    San Diego, CA
    Ignore the button that says "Active/Passive" and use which ever setting gives you the sound you want. That button should be labeled -10dB pad." Play with the pad button and the gain knob until you are happy with sound.
  3. line6man

    line6man

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    Jun 20, 2008
    Location:
    Close to Los Angeles, CA
    1M Ohms is the standard for amplifier inputs. Some amps have a lower impedance to reduce thermal noise, and occasionally you will see something higher. If you are using piezo pickups or such, they are usually buffered to an output impedance that jives with the standard 1M input impedance spec of amps.

    Volume has no inherent relation to impedance, and overloading an amp's input usually has nothing to do with signal impedance.

    This does not sound like a impedance issue.
  4. bdplaid

    bdplaid

    Joined:
    Aug 31, 2007
    Jazzman: that's what I've always done, but with this it behaves differently. It's recently come to my attention that some of the active/passive switches on inputs not only pad the signal down, but also change the input impedance. This Roland changes the input impedance from 1Meg to 50K. hence my original thoughts.

    Line6man: good to know . this behaves like an over-saturated signal - goes to clipping and then no further because it can't go higher - but as I said, even when it's padded down and the gain light no longer illuminates, I just can't get the volume back when using an active bass.

    Thanks guys. it's still a mystery.
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  6. walterw

    walterw Supportive Fender Gold Supporting Member

    Joined:
    Feb 20, 2009
    this makes no sense, unless you've got the EQ on the active bass boosted to where the amp is losing all its headroom trying to recreate boosted bass and treble.

    try flattening out the bass's EQ entirely.
  7. bdplaid

    bdplaid

    Joined:
    Aug 31, 2007
    Walter: I'm with you that it makes no sense. the EQ on the bass was totally flat. That bass uses EMG BTC, btw (can I squeeze in more acronyms? :) ). it also works fine with other amps, like my Streamliner 900. as an experiment, I have rewired the bass to bypass the active controls, so now it's passive all the way through. will report back after tomorrow when we have rehearsal, and I play through the twilight zone of bass amps.
  8. Jazzdogg

    Jazzdogg Less barking, more wagging! Supporting Member

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    Location:
    San Diego, CA
    Changes in input impedance are usually perceived as differences in treble.

    FWIW, my passive P-bass (with Quarter-Pounders) puts out a much stronger signal than any of my 9v active basses.
  9. bdplaid

    bdplaid

    Joined:
    Aug 31, 2007
    a question: what happens when the output impedance is the same or larger than the input impedance?
  10. Cadfael

    Cadfael

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    Jan 4, 2013
    Location:
    Germany, EU
    I have a Roland CB-100 and a D-BASS 115 ...

    I only have passive basses but I have also used the D-BASS 115 as active extension cab for my CB-100 - from CB into Line In or the input of the D-BASS! Also my (22 y.o.) BOSS SE-50 caused no problems ...

    I wrote a "Roland Amp Trilogy" two years ago. All in all 500 pages ...
    You will find any infos about the D-BASS in there (but it's written in German!):
    http://www.bossmusik.de/tipps/amps
    (I am NO Roland employee but did this as a Roland fan and "hobby music historian")

    I have never heard of any problems that the Roland amps don't "like" a signal. It's more the other way around. I played my E-Drums via my CB-100 and others play keyboards or guitar via different kinds of Roland amps. They normally take "every signal" ...

    But you only switched the button and didn't use the other input (which is specially for Piezos)?!
  11. bdplaid

    bdplaid

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    Aug 31, 2007
    No, not the one for piezos. just pressed the button and adjusted the gain knob. even with the gain knob flashing hysterically, the output is lower. distorted, but lower. If I use the active bass into the amp in passive mode, it will very audibly clip the input signal - loads of distortion (expected) to the point of limiting (more or less expected).

    FYI: emg says the output impedance is 2K. don't know what the voltage is.
  12. bdplaid

    bdplaid

    Joined:
    Aug 31, 2007
    Cadfael: Thanks for the link . Interesting history of Roland bass amps! I didn't know they made so many.

    However, of interest to me is Page 59, the concept diagram for the active control. If I understand the diagram correctly, that might be the issue - it looks like the D-Bass, in active mode, lowers the input dramatically. however, the specs don't jive with the picture. In any event, I'm thinking this might be an issue where Roland's older concepts doesn't play well with modern electronics. I mean, I think most modern amps, in accommodating modern active electronics, don't do much other than dramatically lower the input gain - minus 10 or more dB. I'm not sure about that, but it's my read of the modern amps I have (G-B, G-K, MarkBass). Roland seems to do more than that, and might be my problem. I know the Roland has a reduced input impedance, for example. Older basses I have of the same period (My Ibanez comes to mind) are active, but there is no perceptible increase in output voltage/volume on the bass. But there is a difference in impedance, the apparent goal being to make it quieter rather than a stronger signal.

    and BTW - I, too, have passive basses that have stronger signals than many active basses. these seem to work fine with the D-Bass. All my issues are with active basses.

    Thanks again,
  13. bongomania

    bongomania Gold Supporting Member

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    Here's the thing about the input impedance changing when you press the button: padding the input with a resistor (which is how it's done) automatically drops the input impedance. Amp brands try to be clever and make it sound like it's a special feature they developed, but the impedance change is just an incidental by-product of using a resistor that way, nothing special or meaningful. The reason amp makers can get away with calling it a "feature" is that active basses don't need to see as high of a z in as passive basses do.

    The input impedance might drop from 1M down to 500K, for one common example.

    Now, the output impedance of 2K is surprisingly high for an active bass. For example an Aguilar OPB has a z out of just 100 ohms. But even 2K still shouldn't have any problem with the a z in of 500K. It should be fine with even 100K.

    So while yes the z in does drop when engaging the pad, I don't think you're experiencing a problem with impedance.

    If I had to guess, I'd say there's something wrong with the pad circuit, a bad solder joint or something.
  14. rubbadubdub

    rubbadubdub

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    Location:
    UK
    I wonder wether there's something funny about emg stuff. I have an odd and possibly similiar issue with an EXB circuit. I get a consistent boost in the treble and bass as I turn the knob up on all of my 1meg input gear apart from when plugging int my TC 1220 eq. With the tc, i get a bass cut near the lower end of the pots travel. I have no idea why but I wonder wether its for a similar reason? My other active basses don't display this behaviour and it makes me very nervous of plugging the offending bass into other peoples DI boxes at gigs. I actually like the bass cut effect but the tc is very sensitive to input level and I dont want to DI after my eq.
    Do you get the same volume drop if you go through a DI box before you plug into the Roland?
  15. line6man

    line6man

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    Jun 20, 2008
    Location:
    Close to Los Angeles, CA
    The signal gets loaded down.

    If you do it the other way around and set the input impedance excessively high, you will get thermal noise. So you want to find a happy compromise between the two.
  16. bdplaid

    bdplaid

    Joined:
    Aug 31, 2007
    Perhaps I'm making a bad assumption, but I *assume* that a circuit like the EMG BTC buffers the signal before it, so at the output all one has to deal with is the EMG. FYI: everything before the BTC is pretty much standard passive vol/blend/tone circuit, all 250K pots. Supply voltage used is 9V.

    Here's what EMG publishes for specs on the BTC:
    SPECIFICATIONS
    Input Impedance (Ohms) 1 MegOhm
    Gain/Attenuation/ Frequency Bass Control +/-12db/20Hz
    Gain/Attenuation/ Frequency Teble Control +/-12db/(Adjustable)
    Input Referred Noise -120dbV
    Output Impedance (Ohms) 2K
    Recommended Supply Voltage 18 Volts
    Current @ 9V/18V (Microamps) 600/740
    Battery Life (Hours) 750
    Maximum Supply (Volts DC) 27 Volts

    Also, an Ibanez SRX active 5-string that's about a year old does the same thing - overloads the input, with resultant *decrease* in output. and that is one LOUD bass.
  17. bdplaid

    bdplaid

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    Aug 31, 2007
    Line6man: thanks, but what does "loaded down" mean?
  18. SGD Lutherie

    SGD Lutherie Supporting Member

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    I always use the passive input on my Hartke LH500, even with my active basses. The active inout reduces the level too much.
  19. line6man

    line6man

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    Close to Los Angeles, CA
    It means that the resistive part of the input impedance diverts the flow of electrons to ground, reducing the output and changing the behavior of the resonant circuit, if the bass is fully passive.
  20. bdplaid

    bdplaid

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    Aug 31, 2007
    ok, thanks, got it!
  21. bdplaid

    bdplaid

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    Aug 31, 2007
    David: Thanks, I tried that too. On the D-Bass, when I dialed the input level back such that it wasn't distorting, I couldn't get the overall volume up. But on my other amps of more recent vintage, this is not the case, and i do as you.

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