Double Bass Impedance again: Dynamic Mic into a Bass Amp...

[Don Hergert]

I've been learning again today... Maybe this will be helpful to others who may be new double bass mounted dynamic mic users...

I accidentally made a discovery. As I was testing my double bass with its onboard mic with cables and adapters for a Sunday gig -- I tried one of these little adapters that I had in my junk bag:



It was another "DUHHH!!" moment.

There have been lots of great discussions here in TB/DB about a proper impedance match between piezo based pickups, pre-amps and amps, but I had never put 2 and 2 together and realized that dynamic mics also can have an impedance mismatch when trying to feed directly into an amplifier...

When my onboard DB mic didn't work properly with my amp, I figured it just needed more headroom, and so duhhh, I just plugged in an old battery powered pre-amp that I had sitting around. And it worked nicely enough, providing everything that I thought it needed, but it also provided more than I needed, including unnecessary EQ and gain controls and other settings that my amp already provides.

Really all I needed was a better impedance match. Plugging in one of these Impedance Matching Transformers in-line directly from the mic to the amp allows full power of the mic to work with the amp, and in doing so eliminated the need for a battery-powered pre-amp and all of its really unneeded settings.

I'm not an electronics person, but from what I've read, the mic I'm using has an output impedance of 350Ohm @ 1kHz, with a minimum terminating impedance of 1kOhm. While I cannot find specs for input impedance for my amp, I've read that most guitar and bass amps expect a minimum of 1MOhm and go up from there steeply. So my mic was a definite mismatch in that regard.

I'm not aware if this also applies to condenser mic users, but if so, impedance matching transformers may also help.

Again, I'm not an expert at this, so if anyone here has more wisdom to share about it, I will definitely appreciate it!

 

[Ric Vice]

Not only do condenser microphones have impedance matching issues with bass amplifiers, they also require 48 volts fed to the microphones capsule, through a phantom powered preamp or a amplifier i.e. Acoustic Image, Euphonic Audio, Genz Benz with it's
own built in phantom power. I use a Radial SB-48 Phantom/ DI for just this purpose.

 

[Don Hergert]

... they also require 48 volts fed to the microphones capsule ...

Yup, phantom power was one of a few major reasons that I opted to go with an onboard dynamic mic. I have an AT LDC and two Octava LDCs that I've used with my modest sound system, and while I do really like condensers for studio work, I've almost completely abandoned them for live work, mostly because of background noise. The phantom power concern is an added complication, and the feedback concern exists, but is shared to a lesser degree with dynamic mics.

Now that I've started using the impedance matching transformer, I'm especially happy not to be worried about phantom power as it might have mandated another signal chain device to worry about. This is not to say that dynamic mics don't also have some of their own concerns, but personally I really prefer a simpler approach.

 

[Wasnex]

I've been learning again today... Maybe this will be helpful to others who may be new double bass mounted dynamic mic users...

I accidentally made a discovery. As I was testing my double bass with its onboard mic with cables and adapters for a Sunday gig -- I tried one of these little adapters that I had in my junk bag:

View attachment 3697419 View attachment 3697420 View attachment 3697421 View attachment 3697422

It was another "DUHHH!!" moment.

There have been lots of great discussions here in TB/DB about a proper impedance match between piezo based pickups, pre-amps and amps, but I had never put 2 and 2 together and realized that dynamic mics also can have an impedance mismatch when trying to feed directly into an amplifier...

When my onboard DB mic didn't work properly with my amp, I figured it just needed more headroom, and so duhhh, I just plugged in an old battery powered pre-amp that I had sitting around. And it worked nicely enough, providing everything that I thought it needed, but it also provided more than I needed, including unnecessary EQ and gain controls and other settings that my amp already provides.

Really all I needed was a better impedance match. Plugging in one of these Impedance Matching Transformers in-line directly from the mic to the amp allows full power of the mic to work with the amp, and in doing so eliminated the need for a battery-powered pre-amp and all of its really unneeded settings.

I'm not an electronics person, but from what I've read, the mic I'm using has an output impedance of 350Ohm @ 1kHz, with a minimum terminating impedance of 1kOhm. While I cannot find specs for input impedance for my amp, I've read that most guitar and bass amps expect a minimum of 1MOhm and go up from there steeply. So my mic was a definite mismatch in that regard.

I'm not aware if this also applies to condenser mic users, but if so, impedance matching transformers may also help.

Again, I'm not an expert at this, so if anyone here has more wisdom to share about it, I will definitely appreciate it!

I have used Shure A95U line matching transformers quite a bit, except I used them the other way around. I.E. I plugged a bass in the 1/4 side and plugged the XLR side into the mic input of a mixing console. My point is line matching transformers work both ways.

Per the Shure datasheet (see attached), the output impedance of the A95U is 41K with 150 ohm source impedance on the 75-300 ohm input tap. So I believe your 300 ohm mic would translate to 82K on the output. This suggests you're not really matching the impedance here.

However, the transformer does a nice job of converting the signal from balanced to unbalanced and vice versa. Also the transformer increases the voltage available on the output, which is probably the most important factor. Keep in mind a mic level signal is weaker then instrument level, so increasing the voltage is essential.

The Audio Technica CP8201 specs 250 ohms and 50K nominal. Invalid Link Removed

Basically I believe impedance matching is important when you are using a transformer and want maximum power transfer. With microphones something called impedance bridging is used where the input impedance should be at least 10 times the source impedance. Per the following article, impedance bridging ensures maximum voltage transfer. Microphone Impedance: What Is It And Why Is It Important? | My New Microphone

 

[LaFaro01]

Basically I believe impedance matching is important when you are using a transformer and want maximum power transfer. With microphones something called impedance bridging is used where the input impedance should be at least 10 times the source impedance. Per the following article, impedance bridging ensures maximum voltage transfer. Microphone Impedance: What Is It And Why Is It Important? | My New Microphone

It's not only important for microphones.. in my believe most problems about sound and power for example with piezos are the consequence of impedance mismatching and it's the same with mics, magnetic pu's and so. You always have to look on input impedance... some like the sound of impedance mismatching and declare it to "their own", but if you want to get power, clarity and dynamics, you better look to chain of signal source and input impedance..

 

[Wasnex]

It's not only important for microphones.. in my believe most problems about sound and power for example with piezos are the consequence of impedance mismatching and it's the same with mics, magnetic pu's and so. You always have to look on input impedance... some like the sound of impedance mismatching and declare it to "their own", but if you want to get power, clarity and dynamics, you better look to chain of signal source and input impedance..

You obviously did not read the article I posted. There are many other articles that explain how impedance bridging works with microphones if you Google "microphone impedance bridging". Check it out .

I totally agree that using the appropriate input impedance is important for piezo pickups, but it's not impedance matching you want. A piezo is largely capacitive, so if the input impedance is too low it will essentially form an HPF. In other words the low frequency cutoff of the pickup will be too high, so the sound will be too mid-rangey. So basically the input impedance of the amp needs to be significantly higher than the output impedance of the piezo for best performance. I believe we are still talking about impedance bridging here.

It also possible for the input impedance to be too high, and this will cause the sound to be underdamped (I.E. loose, uneven, and woolly).

Here is a related post Sarno Black Box

There are some technical papers attached to the linked post that explain how input impedance relates to the piezo's low frequency cutoff. Basically the required impedance relates to the size of the piezo element. The bigger the piezo element, the higher the capacitance.

 

[LaFaro01]

I totally agree that using the appropriate input impedance is important for piezo pickups, but it's not impedance matching you want. A piezo is largely capacitive, so if the input impedance is too low it will essentially form an HPF.

I'm totally with you.. I don't know the use of the word "impedance" in English, but in German it means a "complex resistor", which contains (more or less) inductive and capacitive shares and there you "get" your HPF. It has not to be a "real resistor"..
I just wanted to mention, that looking for a correct input impedance could be important for different reasons

 

[Ric Vice]

Yup, phantom power was one of a few major reasons that I opted to go with an onboard dynamic mic. I have an AT LDC and two Octava LDCs that I've used with my modest sound system, and while I do really like condensers for studio work, I've almost completely abandoned them for live work, mostly because of background noise. The phantom power concern is an added complication, and the feedback concern exists, but is shared to a lesser degree with dynamic mics.

Now that I've started using the impedance matching transformer, I'm especially happy not to be worried about phantom power as it might have mandated another signal chain device to worry about. This is not to say that dynamic mics don't also have some of their own concerns, but personally I really prefer a simpler approach.

Well, that’s certainly an easier way to go in some ways. I'm not generally a fan of a bunch of little boxes that I have to schlep around. In my experience, dynamic microphones are usually large, so I have little desire to mount one of these on my bass.

While I've experimented with small diaphram condensers (I own both the Remic Green LB 5400 and the Remic Red 5400) I really only use them as FOH feeds for the mains, which they work very well for. Running them into an amplifier is still fraught with

problems, so I have the Ehrlund EAP that sounds enough like a microphone on my bass to satisfy both myself and the ensembles I play in. We're very close to having a small condenserwith the feedback rejection capable of playing live through an amplifier

DPA, Remic, and Xlson are working on a solution. All microphones and pickups have bleed, if you listen close enough to the sound coming from your amplifier, it's just a matter of how much you can put up with and how well the microphone cuts through it. Just my take.

 

[Don Hergert]

... So I believe your 300 ohm mic would translate to 82K on the output. This suggests you're not really matching the impedance here. ...

I agree, I was simplistically doing the math a little last evening and came to this conclusion too. Impedance is closer to a match than without the transformer, but as you've suggested, there's something else happening that is also helping, or perhaps even helping more.

Basically I believe impedance matching is important when you are using a transformer and want maximum power transfer. With microphones something called impedance bridging is used where the input impedance should be at least 10 times the source impedance. Per the following article, impedance bridging ensures maximum voltage transfer. Microphone Impedance: What Is It And Why Is It Important? | My New Microphone

Thank you, this is an excellent article. I can't pretend to understand it all, but a couple of points they make do sink in -- one in particular as you mentioned is about impedance bridging, which to my reading sounds like is actually more important than impedance matching with mics. I'll re-read this article a few times in hopes that more will sink in. (I just retired from a systems-analyst / programmer-analyst career where we simply sent all concerns like this to our electronic engineers .)

I also spoke with my son, a pretty experienced electric bass guitar player, last evening about this. These are details he hasn't worried about, since typically the impedance levels between electric guitars/basses and amps have been almost plug-n-play for him. He has switched pickups around just a little, but never worried about impedance with them; he has been concerned about a proper impedance match for amp (head) to speaker (cab), but that's been the extent of it. I explained that with double basses impedance details become a concern since we often design our own pickups, pre-amps and amps by choices we make in purchasing them.

Thanks again!

 

[rickwolff]

Well, that’s certainly an easier way to go in some ways. I'm not generally a fan of a bunch of little boxes that I have to schlep around. In my experience, dynamic microphones are usually large, so I have little desire to mount one of these on my bass.

While I've experimented with small diaphram condensers (I own both the Remic Green LB 5400 and the Remic Red 5400) I really only use them as FOH feeds for the mains, which they work very well for. Running them into an amplifier is still fraught with

problems, so I have the Ehrlund EAP that sounds enough like a microphone on my bass to satisfy both myself and the ensembles I play in. We're very close to having a small condenserwith the feedback rejection capable of playing live through an amplifier

DPA, Remic, and Xlson are working on a solution. All microphones and pickups have bleed, if you listen close enough to the sound coming from your amplifier, it's just a matter of how much you can put up with and how well the microphone cuts through it. Just my take.

Well put, Ric. All mics have some amount of bleed. Sometime back I had tried a promising figure 8 mic placed under the bridge (no, not that one). There was so much bleed from the noise of the audience that the pianist complained about it to me and I turned it off and just went with the piezo.

The ratio of the input from the bass vs the input of 'bleed' is probably the critical factor. IE, if you are getting lots of input from the bass into the microphone you won't need so much gain and therefore won't be amplifying the ambient sounds as much (the bleed).

I did a demo with my Xlson microphone to test this and am attaching a link below. What I have found to be true on the gig ('real world') is that I get so little bleed with the Xlson that it is really negligible. I actually don't hear it through my amp. And even though, as you say, running a mic into your bass amp on stage CAN be problematic, in practice I find I can do that with the Xlson without problems of either bleed or feedback.

 

[Don Hergert]

... I don't know the use of the word "impedance" in English, but in German it means a "complex resistor" ...

I like that. Some of my reading last evening was pointing to this too.

... looking for a correct input impedance could be important for different reasons ...

Thank you... I'd be interested in your feelings about what those other reasons might be.

 

[Don Hergert]

... We're very close to having a small condenser with the feedback rejection capable of playing live through an amplifier

DPA, Remic, and Xlson are working on a solution. ...

Thank you!

This is my next frontier with the Senn e604 mic I'm using. Now that I've changed my signal chain, replacing the pre-amp with the transformer, today I'm going to test with/without the wind muff I've been using which has been affecting the sound output. The mic might just be less prone to noise and feedback without the muff. Sort of a simplistic search for a solution, but it's worth trying.

 

[Don Hergert]

The ratio of the input from the bass vs the input of 'bleed' is probably the critical factor. IE, if you are getting lots of input from the bass into the microphone you won't need so much gain and therefore won't be amplifying the ambient sounds as much (the bleed).

Thank you Rick!

This is part of what I'm after now too, reducing technique noise. And a lot of that is purely technique which I'm also working on, but also a lot of that noise is really louder than what I produce when playing acoustically so the current mic and/or setup still seems to have an affinity for noise. It was suggested that I can reduce that with EQ and it does help, but I'm going to push further with that in testing today.

Great demo, btw, that less treble'y, thicker tone is exactly what I'm looking for.

 

[DoubleMIDI]

Impedance is a frequency dependent resistance.

If there is a single value, it either means that resistance at a specific frequency (usually 1 KHz, at least for speaker drivers) or the same resistance over the whole (interesting) frequency spectrum (but then it might be better called resistance).

There is nothing complex (like two real values for a complex number) in it, it is a one-dimensional curve, so a function of a single variable: y = f(x) or resistance = function_of (frequency).

 

[LaFaro01]

Impedance is a frequency dependent resistance.

If there is a single value, it either means that resistance at a specific frequency (usually 1 KHz, at least for speaker drivers) or the same resistance over the whole (interesting) frequency spectrum (but then it might be better called resistance).

There is nothing complex (like two real values for a complex number) in it, it is a one-dimensional curve, so a function of a single variable: y = f(x) or resistance = function_of (frequency).

Impedanz – Wikipedia

 

[Francois Blais]

Well, as you know, technically speaking, impedance is the vectorial sum of a resistance and a reactance.
It's the reactance which is frequency dependant. (inductance, in the case of a dynamic mic coil)
The resistance component is not.

 

[LaFaro01]

Well, as you know, technically speaking, impedance is the vectorial sum of a resistance and a reactance.
It's the reactance which is frequency dependant. (inductance, in the case of a dynamic mic coil)
The resistance component is not.

quite right..
Electrical impedance - Wikipedia

 

[Wasnex]

I agree, I was simplistically doing the math a little last evening and came to this conclusion too. Impedance is closer to a match than without the transformer, but as you've suggested, there's something else happening that is also helping, or perhaps even helping more.



Thank you, this is an excellent article. I can't pretend to understand it all, but a couple of points they make do sink in -- one in particular as you mentioned is about impedance bridging, which to my reading sounds like is actually more important than impedance matching with mics. I'll re-read this article a few times in hopes that more will sink in. (I just retired from a systems-analyst / programmer-analyst career where we simply sent all concerns like this to our electronic engineers .)

I also spoke with my son, a pretty experienced electric bass guitar player, last evening about this. These are details he hasn't worried about, since typically the impedance levels between electric guitars/basses and amps have been almost plug-n-play for him. He has switched pickups around just a little, but never worried about impedance with them; he has been concerned about a proper impedance match for amp (head) to speaker (cab), but that's been the extent of it. I explained that with double basses impedance details become a concern since we often design our own pickups, pre-amps and amps by choices we make in purchasing them.

Thanks again!

Impedance is not typically a concern with active basses as the preamp buffers the pickups. Impedance can be a factor with passive basses. Today amps frequently have 1Meg input impedance, but some say 220K is preferred for passive instruments. Fore example the input impedance for the Radial J48 is 220K.

 

[Don Hergert]

Well, I just tested some more with the impedance matching transformer. Since it significantly changes the signal chain, I wanted to put it though its paces to make sure I'm prepared for what it does.

I did discover a number of nice things.

1) The transformer adds a huge amount of volume for the mic, so I've re-thought out all my volume and EQ settings on the amp: currently they are approximately Bass 50%, Mid, 25%, High, 25%, Volume 50%, Gain 10%. These settings manage the volume and tone nicely and noticeably reduce noise. I'll probably move these around as needed for any venue.

2) I've turned compression off and will leave it that way. That also significantly reduces noise.

3) While tone is a little thicker with the wind muff on, notes are clearer with it off, so I'm leaving it off.

4) With all this added volume and with the wind muff off, I've found that the angle of the mic to the top table makes a big difference in clarity, noise sensitivity and tone. This is documented, but I had never felt a need to check it out before. The mic is mounted centered on the tailpiece, about 5" from the bridge feet and ~5/8" above the table. I'm pointing the mic at a ~45 degree angle toward the table, and pointing downward toward the tailpiece cable; this seems to provide the most warm, acoustic tone and the least noise sensitivity.

5) The transformer works transparently with wireless, so I have that added flexibility.

6) With some minor pre-amp adjustments I can also use my piezo system with the impedance matching transformer. To switch between piezos and mic, all I have to do is turn off the wireless transmitter on the bass, move the transmitter to another socket, and turn the transmitter back on. So the wireless transmitter on/off switch effectively mutes both of the systems. Very quiet, easy and fast.

7) The transformer also works with any mic, so I can use it very effectively with a voice mic that I carry with the amp. A handy quick mini-PA.

With all this new volume, there is also an increased chance of feedback. Proximity to the amp and the angle of the amp output to the bass is more critical now. That said, I tried up-pointing the amp on the floor next to me while playing, and there was no feedback at reasonable volume. I normally like to put the amp slightly behind me on a stool to my right as I'm playing, that also works well as long as the bass is facing away from the amp.

 

[N. Kentucky Bass]

Impedance may not be the only issue with using a dynamic mic into an amp. The passive signal from a dynamic mic can be a bit weak and often needs a boost. There are several devices out there to do this simply. Fethead, Cloudlifter, etc. Here's a quick review: