Is it simply amp input impedance that ruins passive tone?

D

DiMarco

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Feb 4, 2010
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#1
Quick question based on experiences:

With my active basses this doesn't seem to matter and they sound pretty much the same across different amps, but my passive basses seem to be very picky @ the input impedance of amps. Sometimes they sound weak/whimpy on one amp and awesomely brutal on another.

Is this caused by the amp's input impedance? If so, would a preamp pedal fix the problem?
 
#2
I have always felt that active preamps ruin passive tone:). However, one bonus effect of an active preamp is that the tone should remain the same no matter where you set the guitar volume, with a passive bass there is some treble roll off as you decrease volume on the bass. I believe this is something to do with the impedance loading of the guitar preamp rather than the amp. No doubt others will provide a more scientific explanation. If you prefer active circuitry, as many do, something like a Sadowsky preamp pedal will give you what you desire very effectively.
 
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#3
Active pre-amps tend to have a fairly low output impedance. This “swamps” the input impedance of the amplifier so you get a consistent tone. A passive bass requires the amplifier to have a high input impedance to sound at it’s best. Within reason the higher the better.
 
#8
Which amps have you had trouble with? I have played through a ton of amps with stock passive basses and never had trouble.

I swapped out the input resistor on a few channels of my old Fender PA100, but those things were really designed for mics, not guitars or basses.
 
#11
I'm with two fingers, I have never used an amp where input impedance was an issue with magnetic pickups. Not sure what problems DiMarco is having.

BassmanPaul is also correct, piezo pickups like very high impedances (over 1Mohm, 10Mohm is usually better) which is why buffer preamps are commonly used with them.
 
#12
Ive never measured input impedance on an audio amp, but it should be easy to get it into the megaohm range at audio frequencies(which are very low). It would be very weird if amp manufacturers were putting out products that overloaded the industry-standard form of input.
 
#13
A lot of amps have a 1M ohm input impedance. This combines with the pickup, tone controls, and cable into the amp and affects how the instrument bridges with the amp. A non optimal impedance will rusult in loss of the high end bandwidth. At the very least, it will reduce the harmonics.

An amp intended for piezo pickups often has a higher input impedance, such as 10M, to provide a more optimal impedance match between the piezo pickup and the amp.

With a passive bass, it can help to use the normal input vs the attenuated input on the amp. A boost/buffer pedal will increase the signal level into the amp, this can help as well. These pedals tend to add a bit of high end so you might want to compensate for that with the EQ.
 
#14
1meg usually the standard has more to do with the nature of passive pickups and the tone volume controls on the bass. sometimes something being perceived as " louder" is better. and active pickups will always be louder. likewise a passive pickup that has lower output can actually have better frequency response. the trade off lower output. I think the capicitance of cables has been debunked and the actual resistance depending on length and quality is more the culprit
 
#15
It may not that simple. I often play thru a YBA-1, which is an old non-master tube amp. There are great sounds I get from the Ripper through a compressor acting mostly as a buffer and preamp to send a stronger signal into the amp's input. Plugging straight in and turning the volume up higher sounds good, but is not the same and generally less to my liking even at the same output volume.

I think gain structure matters, along with input impedance and probably other factors.

Otto
 
#16
BogeyBass said:
1meg usually the standard has more to do with the nature of passive pickups and the tone volume controls on the bass. sometimes something being perceived as " louder" is better. and active pickups will always be louder. likewise a passive pickup that has lower output can actually have better frequency response. the trade off lower output. I think the capicitance of cables has been debunked and the actual resistance depending on length and quality is more the culprit
Click to expand...
Cable capacitance is debunked? I missed that--got any links? It would save me a lot of hassle if I didn't have to find the lowest capacitance cables for my passive basses to preserve the higher frequencies.
 
#17
bobcruz said:
Cable capacitance is debunked? I missed that--got any links? It would save me a lot of hassle if I didn't have to find the lowest capacitance cables for my passive basses to preserve the higher frequencies.
Click to expand...

I'm not sure if it's cable capacitance or possibly impedance, but I have a couple of cables that sound a little thin with passive pickups. The cables are fine with active pickups or if I use passive pickups with a pedal in the chain. So, yes, cables can make a difference.
 
#18
Unless your bass has piezo pickups, it's probably not impedence per se so much as it is sensitivity of the amp's input stage. If you don't have an input gain knob right up front, or an "Active Instrument" pad/input you disengage to normalize things, your passive pickup signal through the tone stack to the power stage would be significantly weaker, which will affect the sonic character. As WalterW says, *any* buffered pedal (e.g. one that doesn't do "true pass-through") should help in that situation, even if it's not engaged.
 
#19
My Acoustic 370 has a very low input impedance...under 50k ohms. It snubs clarity and volume of a passive bass. Plug a dummy plug into the 2nd input, and the impedance jumps to around 500k and the clarity appears.

Not a bass amp, but the classic Pignose battery operated practice amp is another low input impedance amp...depending on the year of manufacture as low as 10k but usually 50k. A passive guitar is dull and dark. Plug a buffer in and the amp comes to life, clear and strong.
 
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#20
Thanks for the replies guys, some of the stuff in there is really helpful.
I grabbed a Morley Twinmix pedal which has booster functionality and it seems to clear up my tone. Will further experiment with this.
 
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