Double Bass Variable input impedance for DB amplification

[Chris Fitzgerald]

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

I'm happy with the LaScala/Full Circle/PJB thing and for gigs at home and within driving distance, i can always make that work.But this past summer I did some performances on another continent, having to borrow gear at each stop, and got very mixed results. So just trying to figure some things out. I think I actually got a slightly more realistic and detailed sound from the Focus + VL208 on a pole +mic + pickup, but it was a huge schlep and as I got a little older and more brittle, seemed like the return often wasn't worth the investment. You are totally correct - the simplicity of one cable and one amp set flat is a thing to shoot for, so that you can just focus on playing. For about 95% of my gigs, I'm set.

I'd love to play with a TD 100 again at some point, but as you say, it's more of a schlep than either of us would want. Next up on my list of obvious things to try will be a little PJB preamp, assuming it has the same input impedance as the flightcase and would be a lot more portable.

 

[Jeff Bonny]

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.

Agreed on all counts especially on an uncluttered set of controls. When I say "tonal breakpoints" I'm just curious as to the where changing the impedance makes significant tonal changes in a specific system. If I could do it with a $50 outboard box like the Radial that would be great but from what you're saying I doubt I'd want it built into a preamp.

 

[Ric Vice]

Ok guys, I'll give the 20 Meg Ohm setting a shot with the Ehrlund.

Ric

 

[drurb]

Ok guys, I'll give the 20 Meg Ohm setting a shot with the Ehrlund.

Ric

Please do let us know. MrSidecar determined that the input impedance of the Ehrlund pre-amp is about 5 Mohms.

 

[Chris Fitzgerald]

Just wanted to mention that many things about my experiences are wholly unscientific. I'm perfectly fine with that, like a good agnostic. But it may just be that I prefer a certain kind of sound for whatever subjective reason, and that an input impedance of between 1-5 Mohm gets me closer to that sound. Heck, in recent times it could even be that I've gotten used to the sound of the PJB preamps and like that impedance because I like that sound. I don't know. But I find the whole thing fascinating in a "I'm probably never going to solve this equation" kind of way, kind of like a musical equivalent of Pi. If only the whole impedance thing vaguely resembled Stonehenge!

 

[DoubleMIDI]

Hi Chris,

my misunderstanding. I thought you put the HPFpre into the effects return and plug the instrument in it, but you loop through the HPFpre and put the instrument into the amp.

And you are right, that way there will be the eq in the signal path for both cases and only the input impedance differs.

Sorry!

 

[drurb]

Just wanted to mention that many things about my experiences are wholly unscientific. I'm perfectly fine with that, like a good agnostic. But it may just be that I prefer a certain kind of sound for whatever subjective reason, and that an input impedance of between 1-5 Mohm gets me closer to that sound. Heck, in recent times it could even be that I've gotten used to the sound of the PJB preamps and like that impedance because I like that sound. I don't know. But I find the whole thing fascinating in a "I'm probably never going to solve this equation" kind of way, kind of like a musical equivalent of Pi. If only the whole impedance thing vaguely resembled Stonehenge!

When all is said and done, it matters little how you get there. What matters is that you manage to achieve a sound you like. No one can or should try to argue with your success. IMO, that's what's important, although I sure understand the desire to know why and how things work.

 

[Chris Fitzgerald]

Thanks, Les, I agree.

About input impedance, I was reading about mic preamps and came across this Avalon unit: Invalid Link Removed

Among other claims, it boasts:

• Microphone Dual Impedance Selector
• 10,000,000 Ohm DI-In for "Zero-Loading"

Can anybody explain what that second one means in English?

 

[drurb]

Thanks, Les, I agree.

About input impedance, I was reading about mic preamps and came across this Avalon unit: Invalid Link Removed

Among other claims, it boasts:

• Microphone Dual Impedance Selector
• 10,000,000 Ohm DI-In for "Zero-Loading"

Can anybody explain what that second one means in English?

I'll try. There's a reason that input impedance is referred to as "load" impedance. A device connected to an input is said to "look into" the input's load impedance. The input impedance determines the degree to which the device connected to it is "loaded down" and, therefore, how much current the device must supply. Here's an intuitive way to think about it. Suppose the input impedance was zero. That would mean that the device connected to it would see a dead short! Such an input would sap all of current that the device could supply and likely ruin it as it went up in smoke.

Now, as the input impedance is progressively increased from zero, less and less current would be drawn from the connected device as it is loaded less and less. Given a very high input impedance, the device would, effectively, see no load at all. That's what they mean.

There's at least one important down-side to a very high input impedance. Specifically, it invites noise. In addition to the device you connect to an input, there is another "hidden" source that causes noise to find its way into the cable. Such sources can be thought of as being able to supply very, very little current. A reasonable input impedance effectively shorts out such noise sources by loading them beyond their capacity. So, having a very high input impedance can be detrimental. The idea is to have an input impedance that's high enough to load the device properly, but low enough to short out sources of noise.

 

[DoubleMIDI]

About input impedance, I was reading about mic preamps and came across this Avalon unit: Invalid Link Removed

Among other claims, it boasts:

• Microphone Dual Impedance Selector
• 10,000,000 Ohm DI-In for "Zero-Loading"

Can anybody explain what that second one means in English?

10,000,000 ohms is 10 MegOhms. The DI-in is a bit strange. And a bit more strange on a mic preamp.

I guess the following (without reading the device specs):

The mic preamp will get the mic cable and connect it to its input stage with a 10 MegOhms input impedance (stripping any phantom power by a capacitor) and parallel to an XLR output going to the FOH mixer. Since the output impedance of a mic is typically 600 Ohms, 10 MegOhms could be declared as an (almost) zero load. So the mic works for the FOH mixer as if nothing else loads the mic.
Kind of an active DI-box, but for mic inputs, not instrument inputs.

Does that make sense?

 

[Chris Fitzgerald]

10,000,000 ohms is 10 MegOhms. The DI-in is a bit strange. And a bit more strange on a mic preamp.

I guess the following (without reading the device specs):

The mic preamp will get the mic cable and connect it to its input stage with a 10 MegOhms input impedance (stripping any phantom power by a capacitor) and parallel to an XLR output going to the FOH mixer. Since the output impedance of a mic is typically 600 Ohms, 10 MegOhms could be declared as an (almost) zero load. So the mic works for the FOH mixer as if nothing else loads the mic.
Kind of an active DI-box, but for mic inputs, not instrument inputs.

Does that make sense?

I think that makes sense. I also thought the ad copy was a bit strange. This stuff gets complicated!

 

[MR PC]

The idea is to have an input impedance that's high enough to load the device properly, but low enough to short out sources of noise.

This makes total sense..it seems that the circuit after this proper input has as much to do with what we like or don't like about the performance of a device. Thanks.

 

[peterpalmieri]

Chris,

I'm thinking if you are going to be doing more traveling and using unknown basses and amps maybe it's a good idea to set up a small travel kit with a pickup, a mic and small preamp i.e. AMT, realist clip and a headway, of your choice. You then can eliminate some unknowns including having the impedance thing already worked out.

 

[cacophonic]

10,000,000 ohms is 10 MegOhms. The DI-in is a bit strange. And a bit more strange on a mic preamp.

I guess the following (without reading the device specs):

The mic preamp will get the mic cable and connect it to its input stage with a 10 MegOhms input impedance (stripping any phantom power by a capacitor) and parallel to an XLR output going to the FOH mixer. Since the output impedance of a mic is typically 600 Ohms, 10 MegOhms could be declared as an (almost) zero load. So the mic works for the FOH mixer as if nothing else loads the mic.
Kind of an active DI-box, but for mic inputs, not instrument inputs.

Does that make sense?

10MΩ makes sense for an instrument input, but for a mic input...?

Fortunately Avalon publishes the specs for our convenience:

Input Modes (4): Line, Instrument-DI, Microphone high-Z & low-Z
LINE (transformer): Balanced 6,000 Ω, XLR~TRS, gain -3dB~36dB, max. input 47dB
INSTRUMENT ~ DI: Unbalanced 10,000,000 Ω, TRS, gain 2dB~42dB, max. input 36dB
MICROPHONE HiZ (transformer): Balanced 6,700 Ω, XLR, gain 20dB ~ 60dB, max. input 23dB
MICROPHONE LoZ (transformer): Balanced 1,760 Ω, XLR, gain 26dB ~ 66dB, max. input 19dB

 

[robgrow]

Most modern professional microphones -- dynamic, condenser and ribbon, have a source impedance of about 150 to 250 ohms (far lower than a piezo pickup). Most modern professional microphone preamps have an input impedance of about 1000 to 2000 ohms, or about 10 times a typical microphone's source impedance. An input with an impedance of about 10 times or higher than the source impedance is called a bridging input, and looks essentially like an open circuit to the source.

 

[DoubleMIDI]

Thanks Rob,

That way it makes sense to have 6700 Ohms input impedance for the older 600 Ohms mics and 1760 Ohms for the newer 150 to 250 Ohms mics.

BTW, having 10 MegOhms instrument input impedance can still be a problem if used as a DI with high impedance piezo pickups like the BP100 (piezo disk) and passive piezo foil pickups. The reason is that both resistors, the one in the amp/preamp and the one in the DI-box are connected in parallel, so if the pickup needs at least 10 MegOhms impedance to sound good even two times 10 MegOhms in for example the HPFpre and the Avalon will add to a 5 MegOhms load to the pickup. Usually there is no buffering of the signal in a DI box (probably to avoid problems when the power to the active DI has gone for whatever reason), so they just grab the signal from the cable that internally goes from instrument/mic in to though/out. Since this is hidden into the DI-box the avatar musician might not know about it (like me some years ago before I opened up an active DI-box).

 

[Jay Corwin]

This thread is giving me college flash backs.

 

[Champagne]

Thanks, Les, I agree.

About input impedance, I was reading about mic preamps and came across this Avalon unit: Invalid Link Removed

Among other claims, it boasts:

• Microphone Dual Impedance Selector
• 10,000,000 Ohm DI-In for "Zero-Loading"

Can anybody explain what that second one means in English?

I had a u5 here on demo. It was very nice. My live sound buddy wound up buying it. For my needs, it was redundant to what I already had. I demoed it primarily for electric bass di duties. Sounded good and was convenient for tone shaping.

Also, higher isn't always better as far as impedance goes. You have a workable zone and the difference between the bottom end vs the top end of that zone is not dramatic in my experience with the exception of ribbon mics, old ones from the 60s and new ones from today's age really open up with high impedance. If the phenomena was more dramatic we would see more amenities on our gear to geek out with.

 

[Chris Fitzgerald]

Revisiting this old crazy thread because of recent experiences with impedance and high pass filtering.

Aside from the Summit TD-100, I'm wondering if a sweepable impedance circuit would be a difficult thing to add to a potential custom preamp box in the future. @shwashwa recently posted a picture of a really great custom preamp by Broughton Audio that basically has all of the controls he wanted baked into it by request, and a lot of people are over the moon about this builder's work. If this is a feature that could be easily built into a custom preamp along with sweepable HP filter and other requests, it might be worth the trouble at some point.

 

[Azure Skies]

Revisiting this old crazy thread because of recent experiences with impedance and high pass filtering.

Aside from the Summit TD-100, I'm wondering if a sweepable impedance circuit would be a difficult thing to add to a potential custom preamp box in the future. @shwashwa recently posted a picture of a really great custom preamp by Broughton Audio that basically has all of the controls he wanted baked into it by request, and a lot of people are over the moon about this builder's work. If this is a feature that could be easily built into a custom preamp along with sweepable HP filter and other requests, it might be worth the trouble at some point.

It is possible and I've contemplated doing it. I haven't been taking on custom work, but on occasion I'll do a small batch build of a single design where there is interest. A single knob impedance pedal in a 1590A enclosure would be fairly simple to accomplish.