Double Bass Input impedance, pre-amps and adjustable bridges

[victorgator]

Hello all. Thanks for all the great advice on this forum.

I currently have a Realist pickup on my carved upright plugged directly in to a Genz Benz GBE600. I have found that the passive input impedance on the GBE600 is 500k ohms. I'm experiencing quite a bit of feedback, especially around the "D" string tones. I know that I've read here that the pickup impedance must match the amp's input impedance, but I can't figure out if mine already match, or if they don't, which pre-amp would work for me in order to match the impedance levels.

I'm also looking at buying a K&K Rockabilly as I heard someone playing with that system and loved the sound. Maybe I wouldn't need to buy a separate pre-amp to match the impedance levels if I run an active pickup as the GBE600 has an active switch that is set 150k ohms active input impedance?

Lastly, has anyone else experienced muted pickup sounds when using an adjustable bridge? I formerly played with a Barcus Berry B3150 String Bass Pickup Piezo Transducer Preamp, which sounded awesome when I had a non-adjustable bridge. However, when my local luthier installed an adjustable bridge, the Barcus Berry sounds hugely muted and no longer functional. The Barcus Berry works fine when I clip it back on a bass without an adjustable bridge on it. Just wondering if anyone thinks I might have the same problem before I plop money down on the K&K.

Thanks for helping through my lack of electronic knowledge.

 

[SAlcott]

I'm having good results with a Realist and the FDeck preamp/HPF.

 

[agedhorse]

A couple of things...

The input impedance is >500k on your GBE-600 which matches well with most pickups the only exception that I am aware of is the very small piezo-ceramics crystals, multiple crystals wired in series, and the small area Piezo-Kynar films (not common). If you have the opportunity to try a buffer before you buy it, that's IMO a better idea than just going out and buying something because you think you need it or read about it on the internet.

There is no difference in impedance between the passive and active inputs, the active input has lower gain and provides an adequate impedance to an active source since the line is already terminated at the source end by a low impedance to ground.

It's very unlikely that the input impedance has anything to do with the feedback you are experiencing. Do you have a luthier that specializes in installing and performance tuning of pickups? He may be able to identify the real cause for less money the experimenting and making assumptions that may or may not be accurate. He will also have the various options to try out before committing to buying as his reputation is also on the line.

 

[walkin]

Hi,

Anyone knows if the input impedance of my swr Babybaby Blue, 800k ohms, will matches well with a full circle ?
I'll install this pickup next week, for the first time on my Dbass.
Do i need to use a preamp ?
thank you for advice.
Olivier

 

[engedi1]

In my opinion and ime, 100% of piezo pickups need a preamp to work well with most amps. Impedance problems cause weird tones with no punch and lots of feedback. Even with a preamp, the realist is famous for its feedback at anything other than lower volumes. You get much more volume from the full circle. I myself have been using an erhlund which sounds amazing, but still needs a preamp. I would recommend fdecks preamp as it is really cheap and also is a high pass filter, killing too birds with one stone. It is like, $50 or something. It might solve your problem right there. That and having a luthier making sure it is installed correctly.

 

[walkin]

In my opinion and ime, 100% of piezo pickups need a preamp to work well with most amps. Impedance problems cause weird tones with no punch and lots of feedback. Even with a preamp, the realist is famous for its feedback at anything other than lower volumes. You get much more volume from the full circle. I myself have been using an erhlund which sounds amazing, but still needs a preamp. I would recommend fdecks preamp as it is really cheap and also is a high pass filter, killing too birds with one stone. It is like, $50 or something. It might solve your problem right there. That and having a luthier making sure it is installed correctly.

Thank you for your advices but the fdeck aren't shipped outside the UScont.
My Best

 

[engedi1]

Good point. There are many other preamp. The erhlund preamp is what I use with a pickup or mic, it is excellent, but a little pricier. Made in Germany so they will have no problem shipping you one. Radial pz pre is awesome, but is quite pricey.

 

[drurb]

A couple of things...

The input impedance is >500k on your GBE-600 which matches well with most pickups the only exception that I am aware of is the very small piezo-ceramics crystals, multiple crystals wired in series, and the small area Piezo-Kynar films (not common). If you have the opportunity to try a buffer before you buy it, that's IMO a better idea than just going out and buying something because you think you need it or read about it on the internet.

Not really. The majority of piezo pickups discussed here, not merely the special cases you mention, benefit from an input impedance >= 1 Mohm. As you likely know, a piezo pickup and the input impedance of the amp form an effective (R-C) high-pass filter. I discussed this in some detail here. The fact is that most of the piezo pickups we discuss in these threads only achieve a good low-end response when they are loaded with an impedance between 1-10 Mohm. Any higher than that is overkill. Any lower than that often results in the "mid-rangy" nasal sound that most of us abhor. So, if one wishes to use one of the commonly described piezo pickups with an amp having an input impedance in the 500 kohm range, there is good reason to go out and buy a pre-amp having a more optimal input impedance. Some things you read on the internet are true. This is one of them. Oh, and I figure you likely also know, given your background, that it is not a matter of impedance matching (in the true, maximum power-transfer engineering sense) but of optimal loading of the pickup. The input impedance of the amp should not match the internal impedance of the pickup.

Another issue that can plague users of piezo pickups is one that I have been discussing here for years. Specifically, that such pickups can produce nasty infrasonics that rob and waste power and that can cause rather extreme cone excursions. (This occurs despite the overall effective high-pass filter response that I discussed above). Many manufacturers, out of a misguided sense that "wideband is better," design their products so that they will happily pass frequencies substantially lower than 10-15 Hz. That's another reason to have a pre-amp that incorporates a high-pass filter that will get rid of infrasonic energy.

We're fortunate to have the fdeck pre/HPF available at such a reasonable price. In contrast to your recommendation, I would advise just about everyone using or planning to use one of the popular piezo pickups to run and out and get one of those units, sight-unseen and sound-unheard. I think it represents the largest bang for the buck one is likely to encounter when it comes to electronics for amplification of the DB.

In my opinion and ime, 100% of piezo pickups need a preamp to work well with most amps.

Well, almost 100%

 

[agedhorse]

Then we will have to agree to disagree.

Piezo pickups do not have a single source impedance, nor do they form a (pure) "RC" high pass filter. The system can BEHAVE like a high pass filter when the impedance ratio falls below a critical value, this number depends on a variety of things like crystal area, geometry, mounting method, impedance, etc.

Piezo pickups are in fact a reactive source, their source impedance varies from high to low as frequency goes from low to high. It's this varying impedance that makes the effect of loading so difficult to make universal predictions about.

I necer said anything about impedance MATCHING, what I did say is that in general pickups match up well with input impedances greater than 500k. The main reason for this is to swamp the mechanical and electromechanical effects of a piezo crystal under vibration. Thses are modeled as additional LCR elements, and as such can generate additional negative artifacts that are also attribued to "piezo badness". Specifically, stored electro-acoustic energy in the electromechanical LRC (model) filters.

Infrasonics are already protected against in all of our amps with asymmetrical 4th order filters.

Again, I suggest that folks TRY before assuming that they MUST have a preamp or buffer. Often, it's completely unnecessary. Sometimes, the improvement justifies the additional expense and hardware.

 

[drurb]

Then we will have to agree to disagree.

No surprise there.

Piezo pickups do not have a single source impedance, nor do they form a (pure) "RC" high pass filter. The system can BEHAVE like a high pass filter when the impedance ratio falls below a critical value, this number depends on a variety of things like crystal area, geometry, mounting method, impedance, etc.

Piezo pickups are in fact a reactive source, their source impedance varies from high to low as frequency goes from low to high.

Impedance, by it's very nature is reactive and varies with frequency. You know that. So, the above simply reinforces the definition. That they are a reactive source squares with this fine post from fdeck to which I linked in my post. It sure seems that Francis is correct (no surprise there either). Just look [Invalid or Expired Link Removed] and/or here or even this fine post from fdeck.

I necer said anything about impedance MATCHING, what I did say is that in general pickups match up well with input impedances greater than 500k.

That's why I said I figured that you know it's not matching in the engineering sense. You're not the only one reading this, so when you use the word "match" with regard to impedance, it makes sense to clarify. The truth is that input impedances of around 500 kohms do not "match" all that well with most piezo pickups.

Infrasonics are already protected against in all of our amps with asymmetrical 4th order filters.

Of course, you could have said a 24 dB/oct high-pass filter and then readers here would understand. I do my best here to try to promote understanding. By the way, what's the effective high-pass cutoff of your amps? In any case, nice to know it's been addressed.

Again, I suggest that folks TRY before assuming that they MUST have a preamp or buffer. Often, it's completely unnecessary. Sometimes, the improvement justifies the additional expense and hardware.

Sure, no harm in trying... right before you order the fdeck, receive it and connect it to find that the response is improved over what you'll get with an input impedance around 500 kohms. Of course, a player wouldn't know how much better the pickup will almost certainly sound until the near-optimal load is provided. In real life, just about every piezo pickup discussed here will benefit from that loading. Yes, that's the nature of their "crystal area, geometry, mounting method, impedance, etc." The experiments have been done hundreds of times across dozens of players, pickups, and amps. No amount of asserting otherwise will override that. Both experience and the science are in harmony on this one.

 

[engedi1]

I have heard that genz benz amps are better in the high pass filter than most amps, but we have to consider most players aren't using genz, only a percentage. For the rest, buffering and high pass filtering make a night and day difference.

 

[drurb]

I have heard that genz benz amps are better in the high pass filter than most amps, but we have to consider most players aren't using genz, only a percentage. For the rest, buffering and high pass filtering make a night and day difference.

That doesn't solve the optimal loading issue. That buffer would still be a plus with any amp having around a 500 kohm input impedance when used with the vast majority of the piezo pickups discussed here.

 

[engedi1]

That doesn't solve the optimal loading issue. That buffer would still be a plus with any amp having around a 500 kohm input impedance when used with the vast majority of the piezo pickups discussed here.

Yep. My point exactly. Buffering is good, maybe even essential. I don't own an fdeck, but hope to get the newest model when it come out of beta.

 

[drurb]

Yep. My point exactly. Buffering is good, maybe even essential. I don't own an fdeck, but hope to get the newest model when it come out of beta.

Indeed. I was responding to:

I have heard that genz benz amps... For the rest, buffering and high pass filtering make a night and day difference.

So, not just for the rest... even for G-B amps if their input impedances are down around 500 kohms, the buffering stage with a higher input impedance will be beneficial in most "piezo" cases. Seems we agree.

 

[lowfreqgeek]

So, not just for the rest... even for G-B amps if their input impedances are down around 500 kohms, the buffering stage with a higher input impedance will be beneficial in most "piezo" cases. Seems we agree.

The stated impedance on GB amps is "greater than 500k" and is in fact closer to the "optimal" impedance for most pickups - per a response from Agedhorse in another thread.

Indeed, in my experience with a Full Circle and my ShuttleMax, the pickup's response is not significantly improved when running a high-impedance buffer rather than straight into the GB amp. As to why GB specs it ">500k", I can't say. Regardless, it *is* greater and seems to work well for me and many others without any additional buffering. That holds true for the Shuttle and ShuttleMax amplifiers, but I can't really address the GBE series amps.

 

[drurb]

The stated impedance on GB amps is "greater than 500k" and is in fact closer to the "optimal" impedance for most pickups - per a response from Agedhorse in another thread.

Indeed, in my experience with a Full Circle and my ShuttleMax, the pickup's response is not significantly improved when running a high-impedance buffer rather than straight into the GB amp. As to why GB specs it ">500k", I can't say. Regardless, it *is* greater and seems to work well for me and many others without any additional buffering. That holds true for the Shuttle and ShuttleMax amplifiers, but I can't really address the GBE series amps.

I don't see how "greater than 500k" is closer to optimal. It's not even specified. Of course, "greater than 500 k" could be 1M! It could be 10 M! It would be much more transparent to say what it is. Yes, I saw his post in that other thread. I won't even get started. The impedance chosen is what some panel of players preferred. Oh yes, and the "triboelectric effect." Please.

 

[lowfreqgeek]

I don't see how "greater than 500k" is closer to optimal. It's not even specified. Of course, "greater than 500 k" could be 1M! It could be 10 M! It would be much more transparent to say what it is. Yes, I saw his post in that other thread. I won't even get started. The impedance chosen is what some panel of players preferred. Oh yes, and the "triboelectric effect." Please.

Yes, it could be 1M, or 10M or something else. As I said, I don't know why it isn't specified more precisely.

But why is the preference of a panel of random players any less valid than the preference of some random guys on the internet? You can tell me why one impedance is better than another all day long and I'll understand every word you say and even agree with your analysis. Then we'll plug in and try it and find that our tonal preference is 180 degrees out of phase.

 

[drurb]

Yes, it could be 1M, or 10M or something else. As I said, I don't know why it isn't specified more precisely.

But why is the preference of a panel of random players any less valid than the preference of some random guys on the internet? You can tell me why one impedance is better than another all day long and I'll understand every word you say and even agree with your analysis. Then we'll plug in and try it and find that our tonal preference is 180 degrees out of phase.

The collective judgment of a random panel of players engaging in unknown procedures is hardly justification for claiming that the value of the parameter chosen is closer to optimal than those chosen by other designers. It may be that we have different tonal preferences. That is NOT the point here. What is desired by most is to achieve the best low-end possible from the piezo pickup while avoiding other ill effects. Tonal preferences can be addressed with tone-shaping circuits. Now, assuming the goal I mentioned, it's not a matter of preferences and listening panels. On the other hand, if one wants to mess around with input impedances to achieve different tonal effects, then one is free to do that. I wouldn't recommend it and I've explained why at some length in other posts. As for "random guys on the internet," that's sure not the basis of my views on the subject. The input impedances that are regularly discussed here were not determined by a vote among internet guys. I suggest that you go back through the threads here on the topic. They actually contain a wealth of information contributed by people who are well-educated on the subject and, mostly, who are not aligned with any amplifier company.

I have no trouble accepting that, for many piezo pickups, there is no substantial advantage in increasing the input impedance beyond 1 Mohms or so. If some value in that region is what characterizes the G-B amps, then using a pre-amp to provide a higher impedance would likely not be of much benefit in many cases. 500 kohms, the value bandied about before, is a different matter. I don't believe that there is, in general, for the population of popular piezo pickups used, some magic value of input impedance or that listeners in practical settings can discern the effects of small changes. It is also true that setting the impedance too high, e.g., 20 Mohms, can produce ill effects.

 

[agedhorse]

Durb - you are experssing your opinion/experience and I am experssing mine. No need to get accusatory towards me for suggesting that folks TRY a particular design approach rather than assuming that it won't work, especially when it costs NOTHING to try. Why does this offend you?

In a "former" engineering life, I designed more than a few piezo ceramic and one piezo film pickup system, I have a lot of engineering time working with the transducer electro-mechanics and I am stating my opinion based on this specific experience. You may have a different personal experience but that does not invalidate the experience that I wish to share with players asking this question in general, and with a product that I have engineered in specific.

 

[Ric Vice]

Several, years ago I had a long conversation with Don Underwood, who as we all know, designed one of the first modern bridge wing pickups. He was a very intelligent engineer, he worked on the pickup designs for the Baldwin Electronic Harpsichord. Walter used Don's pickup to set the input impedance of the MI-100-8 at 1 meg ohm. That's one of the reasons that a Woods sounded better with a piezo pu. When I used an Underwood with the Woods or the first GK200MB's (also set at 1 meg ohm) I didn't find the need for the HPF/Buffer Pre Amp. Euphonic Audio and Acoustic Image also set their amps to 1 meg ohm.
There is a Shuttle 3.0 10T at Webster University where do the summer jazz camps. When I use my Ehrlund Preamp the amp really sounds remarkable.

Ric