• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Double Bass How can I eliminate feedback?

M

MatthewT

Aug 9, 2011
13
0
#1
Hi,

I play an Upton plywood double bass and use an Underwood pickup with wedges underneath each of the pieces on the bridge (as opposed to on top of the pieces, which I believe was making feedback worse due to more direct exposure to the body). I usually go through a Peavey kb60 keyboard amp combo, but not without a decent amount of feedback at greater volume levels. I have read that the Underwood pickup likes amps with input impedances of 500kohms to 1Mohms. The Peavey has a high gain input at 220kohms and a low gain input at 44kohms. Are the low input impedances of the peavey causing more feedback than an amp with an input impedance closer to the underwood's favorable range (500kohms to 1Mohms) would cause? Also, what are some "real" (not electric-like) sounding pickups that can handle volumes higher than the underwood can handle? All feedback is appreciated (except from my bass).

Thanks
 
#3
Wow! Didn't get an answer within half an hour, so bumped? There's no "on-demand" section here. :)

The search function is a wonderful tool-- especially for topics that have been discussed here at length.
Check this out.

No, the non-optimal input impedance of your amp is not contributing in any substantial way to your feedback problem. You would do well to have one of this .

Virtually all piezo pickups do well looking into impedances of 1-10 Mohms.

Where do you live? If you're local to Upton, you could have them install an RS-2 (Revolution Solo 2, Rev Solo, etc.). If you're not local to them, you can install it yourself. I've never had any feedback issues with mine and, yes, it's a "real" pickup.

You can read all about the fdeck pre/HPF and the RS-2 by-- you guessed it-- using the search function. :)
 
#4
MatthewT said:
Hi,

I have read that the Underwood pickup likes amps with input impedances of 500kohms to 1Mohms. The Peavey has a high gain input at 220kohms and a low gain input at 44kohms. Are the low input impedances of the peavey causing more feedback than an amp with an input impedance closer to the underwood's favorable range (500kohms to 1Mohms) would cause? Also, what are some "real" (not electric-like) sounding pickups that can handle volumes higher than the underwood can handle? All feedback is appreciated (except from my bass).

Thanks
Click to expand...


Matthew,
If you're having feedback problems I don't believe that impedance matching is going to solve your problems. Three tools that effect feedback are. lo cut filters, notch filters, and phase reverse switches. Purchasing a HPF/Pre
Invalid Link Removed is a ideal way to solve a lot of these problems. I like his new Series 2 model because of the additional features.

Ric
 
#5
FYI on a couple points.

If using a piezo pickup unbuffered, it can do weird things to the tone - suck out the bass, make the mids sound "quacky", that sort of thing. If you're over-compensating for this with extreme EQ settings to make it sound "good" again, a buffering preamp like the FDeck DRURB mentions (and one of several other options) might help - a little - by allowing you to set your EQ's a good bit more normally. Some of those heavy boosts can certainly create a feedback hotspot. But there is no real guarantee that a preamp would help.

Another thing to try, just for giggles - pull one element of the Underwood out of the bridge wing and just let it dangle; see if the output of the bass (without changing settings or volume levels on the amp) gets any better. If it sounds fuller and louder, you might be fighting a battle with phase cancellation, which is occasionally something that happens with the paired transducer pickups like the Underwood. If it is an issue with your setup, you might again be EQ-ing or boosting gain to compensate, thereby inviting more feedback into your happy home.
 
#6
Good points. I should say, for the record, that my suggestion for the use of the fdeck had nothing to do with the feedback issue. It was a suggestion to help the overall tonal balance given that the OP is running the piezo into a non-optimal load. Loading a piezo optimally can make a very substantial difference in terms of tonal balance.
 
#8
sixandeightstringer said:
FYI on a couple points.

.....a buffering preamp like the FDeck DRURB mentions (and one of several other options) might help - a little - by allowing you to set your EQ's a good bit more normally. Some of those heavy boosts can certainly create a feedback hotspot. But there is no real guarantee that a preamp would help.
Click to expand...

Mark,
This is a very good point, changing the input impedance generally improves the tone quality . The HPF does have two other controls that in the right situation are very effective however. It's got a high pass filter that can "take the boom out of the room" and a phase switch. If the phase switch works it really works, it's a all or nothing proposition though.

sixandeightstringer said:
Another thing to try, just for giggles - pull one element of the Underwood out of the bridge wing and just let it dangle; see if the output of the bass (without changing settings or volume levels on the amp) gets any better. If it sounds fuller and louder, you might be fighting a battle with phase cancellation, which is occasionally something that happens with the paired transducer pickups like the Underwood. If it is an issue with your setup, you might again be EQ-ing or boosting gain to compensate, thereby inviting more feedback into your happy home.
Click to expand...

Interesting idea, I owned a Underwood and never once considered that the elements could actually be out of phase with each other. This makes sense especially if you consider that the Realist, Full Circle, Ehrlund, Revolution Solo, and Bass Max are all single element pickup's. One interesting thing, here Don Underwood actually built a custom Underwood pickup for Glen Moore that had two elements in each side, so Glen could throw the pickup out of phase.

Ric
 
#9
Ric Vice said:
Interesting idea, I owned a Underwood and never once considered that the elements could actually be out of phase with each other. This makes sense especially if you consider that the Realist, Full Circle, Ehrlund, Revolution Solo, and Bass Max are all single element pickup's. One interesting thing, here Don Underwood actually built a custom Underwood pickup for Glen Moore that had two elements in each side, so Glen could throw the pickup out of phase.

Ric
Click to expand...

Well... you have to determine what are the wavelengths of the low-bass frequencies as they propagate through wood (the bridge) and then consider those in relation to how closely the Underwood elements are spaced. I haven't done the calculations (anyone care to?) but I'm betting it's not the low-bass frequencies that are going to suffer from cancellation.

Addendum: In air, the wavelength of a 100-Hz tone is about 11 feet. Sound travels about ten times faster in wood than it does in air. That means that, in wood, the wavelength of that 100-Hz tone is about 110 feet! So, to cancel that 100-Hz tone across the two Underwood elements, they'd have to be placed about 55 feet apart. I wouldn't worry about a few inches. :)

Check out these sites and play with frequencies and speed of sound:
Invalid Link Removed
Speed of sound in different media
 
#10
Folks, isn't the simple answer to this question to use a high-pass filter and a phase switch? I find these two things get rid of most of my feedback. You try the high-pass filter in the 40 Hz range, even try it a little higher and see if that adversely changes your sound, and you play a few notes with the phase switch in either position to see which one gives you the loudest low notes.

The FDeck has both and is easy to use and inexpensive. I have the same functionality on my Acoustic Image Ten2 so I don't generally use the two together.

I know that's not everything and it's even oversimplified but, hey, that's where I'd start - phase issues can cause feedback, and the ability of your rig to produce sounds that are lower than the musical notes you play is another.

-S-
 
#12
Ric Vice said:
Mark,
Interesting idea, I owned a Underwood and never once considered that the elements could actually be out of phase with each other...
Click to expand...

It's actually one of those things that comes up fairly often when speaking to people about pickups... lots of anectdotal evidence around the web, too - do a quick google search for underwood pickup phase and you'll see what I mean. Some people seem to believe that installing the pickup with one end dangling should be the standard install!

In my experience, it's far more common for the pickup to sound just fine than to exhibit phase issues, but when I have a customer with pickup problems with the Underwood, I always suggest a quick experiment of pulling out one of the elements to see if there's a difference.
 
#13
SteveFreides said:
Folks, isn't the simple answer to this question to use a high-pass filter and a phase switch?...but, hey, that's where I'd start - phase issues can cause feedback, and the ability of your rig to produce sounds that are lower than the musical notes you play is another.

-S-
Click to expand...

Well, not really. The ability of the rig to pass very low frequencies, even infrasonics, is not a substantial contributor to feedback problems. A phase-switch, hit-or-miss as it is, can be very useful. A high-pass filter of the type on the fdeck unit is not likely to filter the part of the spectrum causing the feedback until you get to its upper edges. Actually, a notch filter centered higher is likely to be more effective.

sixandeightstringer said:
It's actually one of those things that comes up fairly often when speaking to people about pickups... lots of anectdotal evidence around the web, too - do a quick google search for underwood pickup phase and you'll see what I mean.
Click to expand...

That's one of the problems with anecdotal evidence of this kind. :) See post #9 above.
 
#14
In my experience, one of the most important ways to prevent feedback is to find the pickup that best fits a particular bass and be sure that it is installed properly. Assuming that, then go down the line from there. Do you get a big, full sound acoustically? Then, go to amplification.
All of the above suggestions are good ones. Because your amp is inapropriate for DB, I would at minimum use an Fdeck HPF to properly load the pickup. If you intend to keep the amp, all you can do is attempt to fix it with EQ.
Other suggestions would be to put the amp either off to the side or in front of you and to stuff a towel under the tailpiece to dampen sympathetic vibrations.
Toad's thread on this subject would be helpful if anybody can find it.
 
#18
One thing that could account for the reported phase-cancellation effects of the Underwood would be if the transducers, themselves, were not matched in terms of their phase response. I think it's clear that it's not a matter of cancellation coming about as a result of spatial separation on the bridge (see post #9). If the transducers were not well-matched, then that would indicate poor design/quality control. No good explanation, but the reports are what they are. :)
 
#19
Just asking, here, for the sake of my own personal edification:

I understand your point about the wavelength; but I wonder if we're looking at the source of vibration incorrectly - and thereby coming to a hasty conclusion. The point of origin of the sound (hence, the vibration/wavelength) is not at an element of the pickup, but rather at the string(s).

Could it not be that the source of the vibration/wavelength, meaning - the string - could in some cases be equidistant (enough) from both elements of the pickup to cause some manner of phase issues? I don't know whether the two elements of the pickup in question are wired in phase or out of phase with each other, but it seems logical.

Again, pure curiosity here. Thanks!
 
#20
sixandeightstringer said:
Just asking, here, for the sake of my own personal edification:

I understand your point about the wavelength; but I wonder if we're looking at the source of vibration incorrectly - and thereby coming to a hasty conclusion. The point of origin of the sound (hence, the vibration/wavelength) is not at an element of the pickup, but rather at the string(s).

Could it not be that the source of the vibration/wavelength, meaning - the string - could in some cases be equidistant (enough) from both elements of the pickup to cause some manner of phase issues? I don't know whether the two elements of the pickup in question are wired in phase or out of phase with each other, but it seems logical.

Again, pure curiosity here. Thanks!
Click to expand...

The propagation that drives the bridge mounted pickups is through the wood on the bridge. Now, if one used the string as the point of origin and assumed that the propagation was through air-- the only other path to the pickups-- then the 1/2 wavelength distance would be about 5.5 feet at 100 Hz.

I think it is far more likely that any cancellation comes about via non-uniformity in the phase responses of the pickups. As I mentioned, though, that would have to be one heck of a mismatch.
 

Latest posts

Top Bottom
Back