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Instrument input impedance question


DiMarco
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Guys I am puzzeled.

I own a couple of preamps:

Trace Elliot Transit-B
Two Notes Le Bass
Orange Bass Butler

And I own a couple of amps:

Ampeg SVT4-pro
Trace Elliot Hexa Valve
Trace Elliot RAH600SMX
TC Electronic RH750
Mesa Boogie Bass Prodigy 4:88 (sold that one)

Now here's the thing...

The Bass Prodigy sounded amazing with passive basses, harsh with actives.
The Ampeg sounds great with any bass, but loses treble and heft when adding any of my pre-amps in the signal path.
The Orange Bass Butler therefore sounds weak on its clean channel through the SVT.
That same Bass Butler's clean channel sounds fantastic through the RH750 and Hexa Valve.

Is something funny going on with the instrument input impedances on amplifiers or something?
I really love my SVT4-pro but having a weak sounding Bass Butler through it (which sounds ACE through it's two D.I. outputs!) makes the amp a no no for any live playing.

Can I fix this with a buffer or something?

Any help is appreciated.

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The input impedance of amps and devices is going to be similar. The output impedance and output level of passives versus actives is different, as is the input sensitivity, output impedance and output level of other devices, depending on where they're supposed to go in the signal chain. You just have to experiment as to which works where.

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I had lots of issues with compressors and fuzzes. Then a friend of mine - a maker of fuzzes and overdrives - told me to try the units with a hi-Z (passive) and a lo-Z (active) basses. Yes, that was it! Now I have rigs for hi-Z, and lo-Z instruments.

Like @Bill Fitzmaurice said, a thorough test drive is in order.

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Thanks lads, mixing and matching it is... It kinda bothers me though.
A lot of people have just one amp and will never find out how much mixing instruments, pedals and amps can have such a big impact on how strong the tone is. They might simply think the pedal not working well in their chain is simply a bad pedal.

So far the two old Trace Elliot amps and the TC Electronic make everything I plug into them sound good (the TC sounds a bit colder and more hi-fi). The Ampeg and Mesa are more picky on which instruments or pedals they like. I sold the Mesa just because it sounded harsh and ugly with active basses. When I eleborate on that in my mind, the latter two amps bring a lot of character to the game. Maybe throwing 'character' pedals like the Bass Butler on top of that is proving contra productive and these pedals are just meant to be used with 'pedal platform' amplifiers that allow you to build your tone on the effects board or with the instrument's active pre.

Maybe I should stop looking at the situation as a problem but rather as just having a large number of options.

The Ampeg by itself sounds really good, much better then the TC RH750. But the TC+Bass Butler sounds really good, too. Choices choices...
The rare and vintage Trace Elliot Hexa Valve beats both, but I only bring that amp out on rare occasions.

 

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Impedance matching usually doesn't make that much difference. What causes problems is level matching. Something made to work with a low level input signal isn't going to be happy if you plug in something with a high level output signal, and vice versa.

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In general, where the OUTPUT impedance of the SOURCE is less than 1/10th the INPUT impedance of the LOAD, the effects of loading can be ignored. If this is not true, it's possible for issues to occur. 

This is independent of level, a different parameter entirely.

[edit for garbled response, clearly reversed]

Edited by agedhorse
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is this comparable to the mic vs high-z switching on audio interfaces?

The Bass Butler pre sounds really weak and spongy through the SVT-4 pro when hooked up to the instrument input.

 

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I think you've got that backwards. The load input impedance should be at least ten times the impedance of the source.

This goes into greater detail than most of use need to know: http://whirlwindusa.com/support/tech-articles/high-and-low-impedance-signals/   The gist of it says ' In order to preserve signal level and frequency response, it's important to drive equipment with a source signal that is lower in impedance than the destination equipment's input impedance. If the input impedance of a device is not significantly higher than the source impedance, the signal will be reduced or “loaded down” and its signal to noise ratio and frequency response will suffer. '

Quote

is this comparable to the mic vs high-z switching on audio interfaces?

It depends. On a console the mic channel needs an additional gain stage because low-Z mics have very low output compared to a a high-Z guitar or bass pickup. But both have lower output than a low-Z line level signal.

Edited by Bill Fitzmaurice
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1 hour ago, Bill Fitzmaurice said:

I think you've got that backwards. The load input impedance should be at least ten times the impedance of the source.

This goes into greater detail than most of use need to know: http://whirlwindusa.com/support/tech-articles/high-and-low-impedance-signals/   The gist of it says ' In order to preserve signal level and frequency response, it's important to drive equipment with a source signal that is lower in impedance than the destination equipment's input impedance. If the input impedance of a device is not significantly higher than the source impedance, the signal will be reduced or “loaded down” and its signal to noise ratio and frequency response will suffer. '

It depends. On a console the mic channel needs an additional gain stage because low-Z mics have very low output compared to a a high-Z guitar or bass pickup. But both have lower output than a low-Z line level signal.

Yeah, I did kind of get that switched around ;)

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11 hours ago, agedhorse said:

In general, where the OUTPUT impedance of the SOURCE is less than 1/10th the INPUT impedance of the LOAD, the effects of loading can be ignored. If this is not true, it's possible for issues to occur. 

This is independent of level, a different parameter entirely.

[edit for garbled response, clearly reversed]

Happens to us all.

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Thanks for the responses guys. It is hard to find this kind of info if you don't know where to look.

Do bass amp manufacturers state the instrument input impedances and is there a difference between passive and active signal inputs other then the level?

Thanks again.

 

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12 minutes ago, DiMarco said:

Thanks for the responses guys. It is hard to find this kind of info if you don't know where to look.

Do bass amp manufacturers state the instrument input impedances and is there a difference between passive and active signal inputs other then the level?

Thanks again.

 

Some manufactureres do state the imput impedance of their amplifiers, some do not.

Usually active signals are low impedance although that does depend on the design of the onboard preamp. If the volume control(s) are before the last opamp in the preamp, the output impedance will be between a few ohms and a few hundred ohms. If the volume control is after the last opamp, it will be determined by volume potentiometer.

In a passive bass the potentiometers, capacitor(s) and pickups determine the output impedance and it will vary depending on the position of the potentiometers. In  general the output impedance of a passive bass will be in the 10,000-100,000 Ohms region.

Edited by Chienmortbb
added section for clarity.
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If we are looking at pedals of vcourse the other issue is buffers. Some have them and some do not. in the world of 6 and 7 strings, many favour the "true bypass" method where the input signal  is wired directly to the output when the pedal is ,or bypassed. Boss pedals usually have a buffer but many boutique pedals favour the true bypass route. Running a few pedals (many guitarists use more than a few) means the signal passes thrugh lots of pedals through lots of switches. If all the pedals are true bypass and the guitar/bass is passive it can relly add lots of capacitance and screw your signal. An active bass can usually handle this better.

On the other hand you don't want your signal to go through many extra electronic buffers that could add noise. I am doubtful that all pedals use the best components so the fewer pedals the better.

 

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5 hours ago, DiMarco said:

Thanks for the responses guys. It is hard to find this kind of info if you don't know where to look.

Do bass amp manufacturers state the instrument input impedances and is there a difference between passive and active signal inputs other then the level?

Thanks again.

 

Some but certainly not all manufacturers provide these specifications.

Depending on the circuit topology, the input impedance may be different or the same between passive and active inputs. On the Subway amps, the input impedance is ~1M in both passive and active positions.

The specifications for the Subway amps are located in the back of the respective owner’s manuals. 

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