just fyi, the high and low input on the polytone refer to volume level, not impedanceSo the question that I keep coming back to after reading a bunch of threads containing info about input impedance is this:
If higher input impedance is really the best for all sources, why do so many units - from mic preamps to instrument preamps to combo amps - offer a choice of input impedance settings? Heck, even my ancient Polytone amp had a Hi and Low input choice. If higher is always better, why bother with low at all?
EDIT: Double MIDI - when i use the HFPre, I put it in the FX loop, so either way the whole circuit is being used, just in different order.
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Double Bass Variable input impedance for DB amplification
- Thread starter Chris Fitzgerald
- Start date
So the question that I keep coming back to after reading a bunch of threads containing info about input impedance is this:
If higher input impedance is really the best for all sources, why do so many units - from mic preamps to instrument preamps to combo amps - offer a choice of input impedance settings? Heck, even my ancient Polytone amp had a Hi and Low input choice. If higher is always better, why bother with low at all?
EDIT: Double MIDI - when i use the HFPre, I put it in the FX loop, so either way the whole circuit is being used, just in different order.
Chris-- The specifics of your Polytone aside (see shwashwa's post), you are quite correct that many amps offer low- and high-impedance inputs. The low-impedance inputs are often labelled or referred to as
High-impedance inputs run in the range of 1-10 Mohms or so. Those input impedances are what typical active devices such as pre-amps and active effects devices should "look into." The optimal loading for those devices runs in the Mohm or tens of Mohm range.
Now, as it turns out, piezo pickups, although passive, are devices that are optimally loaded by high impedances in the Mohm (or tens of Mohm) range. The statement about higher being better is one that is restricted to those types of pickups and is not a blanket statement referring to all kinds of devices. I agree with what MrSidecar posted. For piezos, the idea is to have the input impedance be high enough so that the lowest desired frequencies are not rolled off. When the input impedance is too low, the low-frequency response is lost, leading to the "honky" midrange-heavy sound most of us despise. See here.
MrSidecar was careful to say "within reasonable limits." Once the input impedance is high enough, increasing it to substantially greater values can invite noise and unnecessarily reduce levels.
So, why not have a variable impedance knob? Well, the reason is that, functionally, it would basically be a control that allows the user to non-optimally load the piezo pickup. The control would basically be a poor high-pass filter. A manufacturer might include one because users want it. That's the marketplace. If people want that and enjoy it, they should be happy and go in peace.
Manufacturers have dreamed up all sorts of tone-shaping controls that people seem to like.Here's my take. Typical piezo pickups are best loaded by impedances in the 10 Mohm range. Such loading maximizes their low end. A well-designed high-pass filter, such as that included in the fdeck device, can then be used to rid the system of pesky infrasonics that, despite the effective high-pass filtering of the input loading, can find their way in. The high-pass filter can also be used to tame "boominess."
After that, it's a matter of the use of well-designed tone-shaping controls.
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Drurb - thanks for the detailed answer. I think I get it. in the case of the Focus and the PJB amps, then if I hear the deck unit as sounding better in the FX loop because I find it too gooey/thick when placed out front, what I'm really hearing is that I like the slight HP filtering of the less-than-10Meg input impedance combined with the HP filter better than I like just the HP filter. Does that make sense?
I've recently starting using one the radial P-Z deluxe preamps. The input has two selections - 6.8k / 10M ohm. It also has a three position high pass filter circuit. Though not sweepable, it does has some nice options on it. You can also invert the phase for both outputs individually which is cool. It's a nice unit with a lot of features. Maybe not exactly what the OP is looking for, but nonetheless a nice preamp.
Very interesting discussion Chris. I haven't had to deal with this issue since the Ehrlund Pickup is so compatible with my particular instrument. The Headway EDB-2 is a perfect match, for the Ehrlund as it has three impedance settings 1 Meg Ohm, 5 Meg Ohm, and 20. I just set it to 5 meg and forget it.
In my particular setup, it's a combination of the EDB-2's Pre Amp and variable input impedance that works it's magic. Just to clarify some of the information here, it was my impression that if you insert the FDeck into the effects loop it negates the impedance loading of the device. Sorry, perhaps I missed something in the discussion? One thing I did discover lately, is that an Ampeg B-15N had three different impedance settings, depending on which of the three inputs you plugged into. Channel 1 is set at 5.6 Meg or 1 Meg while Channel is set at 2.2. That may contribute to why these amps sound better with pizeo pickups. Sorry to geek out, I just thought it was an interesting bit of trivia.
Ric
In my particular setup, it's a combination of the EDB-2's Pre Amp and variable input impedance that works it's magic. Just to clarify some of the information here, it was my impression that if you insert the FDeck into the effects loop it negates the impedance loading of the device. Sorry, perhaps I missed something in the discussion? One thing I did discover lately, is that an Ampeg B-15N had three different impedance settings, depending on which of the three inputs you plugged into. Channel 1 is set at 5.6 Meg or 1 Meg while Channel is set at 2.2. That may contribute to why these amps sound better with pizeo pickups. Sorry to geek out, I just thought it was an interesting bit of trivia.
Ric
Exactly. My curiosity is to why you would choose the 5 Meg Ohm instead of 20 if "higher is better", as the prevailing wisdom always seemed to be.
Nope - it's in the FX loop, then the pickup loads at 4 Meg Ohm (PJB input) instead of whatever the deck normally is. To my ears, it always sounds better that way, and I was just trying to figure out why.
I'd like to try a sweepable impedance device just to hear where certain tonal break points are. What should work (or not) on paper is sometimes skewed by "rogue" elements of a particular system.
It's revelatory to plug in a piezo to the Headway and flick back and forth between the impedance settings and to do some playing and listening. That function isn't the only magic in the box but (especially if you spend some time playing and listening to how the sound changes with the impedance shift) there is a change in bass response and the thickness of the sound. For me a piezo sounds best into a 20Mohm impedance and then futzing with the eq and HPF after...most bang for the buck easist way to get the best of them. The Stat B does not care what setting it is on ...I think that it really matters for Piezos and for them it REALLY matters. I'd rather deal with the added bass at 20 Mohms and the better response (then eq) then the thinner sound of 10Mohms or less. My Focus has a 10Mohm imput now and I really prefer the Headway at 20. 2 cents.
Yes, but I don't understand why you'd prefer have the pickup loaded by the lower impedance. So far, as I understand it, you've compared the chains:
Pickup=> fdeck=> PJB
and
Pickup=> PJB=> fdeck in loop
How would you compare either of those to the case when the fdeck is absent?
What I described is hardly theoretical. I don't believe there's substantial unexplained variance (rogue elements). Sure, it would be fun and enlightening to play with a variable impedance control. I'm not sure what you mean by "tonal breakpoints." As I described, you'd be sweeping a rudimentary high-pass filter. Besides just experimenting, my point was that, in my opinion, and based on my understanding of and experience with these devices, there's no substantial practical reason for a manufacturer to include a variable impedance within the "piezo range," nor would it be very helpful in achieving one's desired sound, given the availability of a quality high-pass filter and a good set of tone controls. One could add all manner of knobs that would affect tonal balance in all kinds of ways and some will report liking them. I favor a straightforward complement of controls that provides for maximum utility while being few in number.
It's the same feature as on the P-Z deluxe I posted earlier. It's switchable between two set values. I think the OP was tasking about a sweepable input range,
Theoretically, I don't, either. In practice, it just sounds better.
Well, when the deck is absent, the amp sounds too thick with too much artificial low end, or what I think you have called "subsonic information" in other threads. When I don't have the fdeck, I have to cut a lot of the lowest bass band. This effect is exaggerated when I have the fdeck in front of the amp input with the filter off; naturally I can control some of this with the filter itself. With the HPF in the FX loop, it's the best sound of all, with the least amount of artificial rubbery bottom. I have a drummer friend I play with a lot - real child prodigy guy, was touring with Joe Henderson and Getz when he was a teenager, and he's a real bass tone nerd - and he agrees with the assessment. Says the sound is clearer and more organic.
I owned a brand new EDB 2 recently, and preferred the middle position on the impedance selector..is that 5 meg? Sweet sounding box. Anyway, the unit I had developed a serious hiss mid gig, using the 18v power supply..like a component had drifted way of spec. Dunno. Had to send it back. Maybe I'll try it again sometime, I must of got a bad one.
Low impedance inputs on BG amps are typically labeled "active" as buffered signals have low output impedance. "Passive" inputs are typically 500-600Kohm on solid state amps and 1-2Mohm on tube amps. If an amp has no input higher than 100Kohm it's likely a cheap POS solid state amp.
Otherwise a good post, I think you got your labels temporarily mixed up.
Chris I love reading your posts and while I certainly envy your abilities I more often envy the fact that you just decided to go with the PJB gear, stop worring about it and play. So all this gear talk has me a bit worried
That being said I had a short stint with the Summit Audio TD100 very cool piece of gear and for your purposes might work out when travelling with unknown gear, I decided it was one to many items to integrate long term. I think there is one for sale in the bass guitar classifieds, give it a shot or use one of the online retailers 60 day return policy. I found a specific setting that worked best and did the set and forget. But all my other gear was constant.
I do agree best case would be to see it integrated into something like the Fdeck HPF, that's my go to unit if I am plugging into someone else's amp.
That being said I had a short stint with the Summit Audio TD100 very cool piece of gear and for your purposes might work out when travelling with unknown gear, I decided it was one to many items to integrate long term. I think there is one for sale in the bass guitar classifieds, give it a shot or use one of the online retailers 60 day return policy. I found a specific setting that worked best and did the set and forget. But all my other gear was constant.
I do agree best case would be to see it integrated into something like the Fdeck HPF, that's my go to unit if I am plugging into someone else's amp.
Yup, thanks for the catch! I corrected the post.Low impedance inputs on BG amps are typically labeled "active" as buffered signals have low output impedance. "Passive" inputs are typically 500-600Kohm on solid state amps and 1-2Mohm on tube amps. If an amp has no input higher than 100Kohm it's likely a cheap POS solid state amp.
Otherwise a good post, I think you got your labels temporarily mixed up.
Hmmm... in that case, I suspect that your original conclusion about why you like the fdeck better in the loop might be right on target. That is, you're losing a bit of the low end to start.
