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Double Bass Smart tech people: How complicated is a high pass filter?

#41
dbas said:
Yes, you are correct that a source such as peizoelectric film has an inherent "reactive" resistance. What you may be missing is the purpose of the little preamp circuit attached to your pickup, and what it's function is. I don't know the level of electronic skill here in this group yet but the term "feedback" on the input amplifier (often called gain but it can be a gain of less than one too), is designed to remove the reactive component of your transducer and produce a linear response to the stimulus of the transducer. That linearization of the preamp, and how well (or badly) it deals with the dynamics of a real source is the frequency response of your preamp. Sort of like what you need to take into account when designing a power amp that has to deal with the back EMF produced when your 18" sub is pushing 400 Watts of a low B string and the cone has to go back to the rest position. In that case it is how to design for good damping factor of the amp.

A pretty good description aout the peizo film transducer can be found here:

[Invalid or Expired Link Removed]

- D
Click to expand...

Well, you'll find that we have geeks here who have considerable EE training and skill. Now, feedback is not synonymous with gain at all. Which type of feedback are you discussing? Do you mean the feedback around say, an op-amp circuit that controls the gain, or are you discussing the "linearizing" negative feedback typically employed to reduce distortion? Neither is, of course, the same as gain.

I don't understand why you say that feedback is designed to remove the reactive component.

The linearization of a pre-amp and its transient response are certainly related but they are not captured in measures of frequency response.

Finally, "back EMF" is controlled by, and damping factor is a function of, the ratio of the impedance of the load to that of the output of the amplifier. I don't quite see the analogy.

Thanks for the informative link!
 
#44
dbas said:
Yes, you are correct that a source such as peizoelectric film has an inherent "reactive" resistance. What you may be missing is the purpose of the little preamp circuit attached to your pickup, and what it's function is. I don't know the level of electronic skill here in this group yet but the term "feedback" on the input amplifier (often called gain but it can be a gain of less than one too), is designed to remove the reactive component of your transducer and produce a linear response to the stimulus of the transducer.
- D
Click to expand...

On a pickup, a preamp is usually used for impedance matching andin some instances to add gain. The Kynar film transucers some of us have been playing with, for examp[le, have very low output- not enough to drive the typical bass amp to full output.
 
#46
RE:
sorry to hijack the thread, but another tech question as long as electronics guys are viewing this thread. how difficult would it be to build a box that has an in, an adjustable impedeance control, and an out at 0 db gain, or perhaps adjustable gain?

This kind of question is confusing. What do you want to do?
Are you thinking like a box that can take a low Z mic or a pickup or
a high Z mic and more less provide a line level output irregardless of the source?
 
#47
i'm thinking of an impedance buffer that i can use to drive a regular bass amp input designed for bass guitars... one that would have variable input impedance to accomodate various pickups i have. i think that different piezos benefit from different impedance loads since not all are the same. is this correct? is it safe to say, that if you're thinking piezo, that higher is better (like 10 meg ohm should cover them all), or should you try to match impedance with each different piezo pickup that you own?

dbas said:
RE:
sorry to hijack the thread, but another tech question as long as electronics guys are viewing this thread. how difficult would it be to build a box that has an in, an adjustable impedeance control, and an out at 0 db gain, or perhaps adjustable gain?

This kind of question is confusing. What do you want to do?
Are you thinking like a box that can take a low Z mic or a pickup or
a high Z mic and more less provide a line level output irregardless of the source?
Click to expand...
 
#48
shwashwa said:
i'm thinking of an impedance buffer that i can use to drive a regular bass amp input designed for bass guitars... one that would have variable input impedance to accomodate various pickups i have. i think that different piezos benefit from different impedance loads since not all are the same. is this correct? is it safe to say, that if you're thinking piezo, that higher is better (like 10 meg ohm should cover them all), or should you try to match impedance with each different piezo pickup that you own?
Click to expand...
According to my understanding, the higher the better, with 1 Meg being sufficient for most pickups to drive the corner frequency below 40 Hz.
 
#49
Peizo's are high impedance devices. In the sensors world, you'd want to see something like 100Meg input impedance for any randomly chosen device like it. 100 Meg is a high enough value to accomodate mostly everything of this class of device. Certainly for peizo sensors used for audio pickup, unless there are some I don't know about. It is easy to lower an input impedance by placing some smaller value in parallel with the larger but in general you won't need to for any "pick up" that is passive. The transimpedance amplifier is another usually better solution.
 
#51
dbas said:
Peizo's are high impedance devices. In the sensors world, you'd want to see something like 100Meg input impedance for any randomly chosen device like it. 100 Meg is a high enough value to accomodate mostly everything of this class of device. ...
Click to expand...

Perhaps, if you can attach the amplifier directly to the sensor. But practical considerations mean that's not always possible. If you have 6 inches of coaxial cable between sensor and amplifier, you're going to have some serious problems with noise when that cable flexes, as well as some serious rolloff at the top.
 
#52
shwashwa said:
i'm thinking of an impedance buffer that i can use to drive a regular bass amp input designed for bass guitars... one that would have variable input impedance to accomodate various pickups i have....
Click to expand...

I can see it being usefull to have a few switched resistors in the front end at (say) 250K, 500K, and 1M. Maybe. Anything higher than that you want in the instrument, as close as possible to the pickup.

Building a source follower like the circuit fdeck has posted is so simple that it's very reasonable to build one into every instrument. I'm working on making Kynar film transducers with the preamp right at the transducer.
 
#53
A while back, I posted this "theory" of piezo sensors, leading to why I think that 1 MegOhm is probably sufficient for most pickups.

http://www.talkbass.com/forum/showthread.php?t=215867&highlight=piezo

There is an additional reason why a higher input impedance leads to lower noise, but I think it's irrelevant at the signal levels we are dealing with.

Getting up into the tens or hundreds of Megohms is where you would want to put the preamp directly on the sensor if possible.
 
#54
mje said:
I can see it being usefull to have a few switched resistors in the front end at (say) 250K, 500K, and 1M. Maybe. Anything higher than that you want in the instrument, as close as possible to the pickup.

Building a source follower like the circuit fdeck has posted is so simple that it's very reasonable to build one into every instrument. I'm working on making Kynar film transducers with the preamp right at the transducer.
Click to expand...
Nah, you really should have a couple glowing tubes out there on your bridge ;)
 
#55
mje said:
My understanding is that a transimpedance amplifier is a current-to-voltage amplifier; a piezo produces high voltage and very little current. Why would we use it in that instance?
Click to expand...

mje--

I'm not sure from where dbas is coming. A transimpedance amplifier is just a fancy name for a good ol' op-amp amplifier circuit with R(in) set to zero. I have no idea why this would be a good choice.

Note in edit: See this too.
 
#56
DRURB said:
mje--

I'm not sure from where dbas is coming. A transimpedance amplifier is just a fancy name for a good ol' op-amp amplifier circuit with R(in) set to zero. I have no idea why this would be a good choice.

Note in edit: See this too.
Click to expand...

Yeah. Might make sense for an optical pickup- there's an idea- but iit doesn't seem to be something you'd choose for a piezo.
 
#57
mje said:
Yeah. Might make sense for an optical pickup- there's an idea- but iit doesn't seem to be something you'd choose for a piezo.
Click to expand...
Funny you mention that, because I designed a transimpedance amplifier for my day job, and it's for an optical detector. In this case, the preamp and detector are both cooled to -200 degrees C, to reduce noise. In the case of a pickup, the transimpedance circuit would impose a funny frequency response curve.
 

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