Double Bass Preamps With Variable Input Impedance

[iAsh]

Are there preamps available that have variable input impedance either switchable or sweepable? How do they work for you and with what pickups? Are you using it to get a good sound that on paper is a mismatch or happier running it more closely to pickup spec?

On paper impedance matching seems like a cut and dried issue but in practice there seems to be a lot of voodoo and things that shouldn’t work but do. Sweepable input impedance seems like a useful feature I don’t recall having seen on a preamp.

 

[scott reed]

Yes. Switchable and sweepable models are available. Especially useful if you use more than one bass, amplifier or
pickup.

 

[Wasnex]

The answer is yes. So you have to decide whether you want a sweep or discrete impedances.

Pretty sure you can also find devices that do both. If you haven't already read this thread, you might want to get started: Sarno Black Box

The Black Box stock sweep is 33K to 1 Meg. Brad Sarno built the OP of the thread a custom box with a switch that allows higher impedance.

Check out these posts as wel:
Sarno Black Box
Sarno Black Box

 

[Drew Tanner]

I’m a happy Sarno V-8 user. In addition to the aforementioned Black Box, Brad Sarno will also add the sweepable Vari-Z (variable impedance) to his V-8 preamp. (Black knob on the left side of the unit, in the photo below.) Important to note that the V-8 is a true preamp, whereas the Blackbox is a buffer. For Double Bass use, he offers the Vari-Z on either model with an expanded range of 33k to 10 Meg.

On paper, my Fishman Full Circle should prefer 10 meg, but I tend to find that a bit too thick on my bass. I typically dial the Vari-Z closer to the 3-5 Meg range, depending on the room and which cabinet I’m using. The caveat here being that I also use that sweepable impedance control as a stand in for a HPF, which likely factors into my preference for the lower impedance settings. (Heresy alert: Even with a Grace Felix, Fdeck HPF pre, or Noble, I find a 10 Meg impedance to be a touch to heavy sounding for my ears. This may well be due to my particular bass, strings, technique, etc. YMMV.) To your question, Felix is also switchable with fixed settings for 10k, 332k, 1Meg, 10Meg, and 20Meg. I find myself wishing there was a 5 or 4.7 Meg setting available.

 

[iAsh]

25988799[/URL], member: 209686"]The answer is yes. So you have to decide whether you want a sweep or discrete impedances.

Pretty sure you can also find devices that do both. If you haven't already read this thread, you might want to get started: Sarno Black Box

The Black Box stock sweep is 33K to 1 Meg. Brad Sarno built the OP of the thread a custom box with a switch that allows higher impedance.

Check out these posts as wel:
Sarno Black Box
Sarno Black Box

Lately I’ve gone back to trying to make a mic work so this piezo related question was supposed to be a more abstract one than it turned out to be...I certainly didn’t think I’d be dropping $300 within an hour of posting. But man, that’s one helluva great looking preamp! Offering the double bass variable input impedance mod at no additional cost iced the deal.

As much as I prefer the sound of a mic every time I go there I always come back to the real world practicality of a pickup. The Black Box seems like a good investment for me. Thanks for pointing me towards this one.

25989037[/URL], member: 337184"](Heresy alert: Even with a Grace Felix, Fdeck HPF pre, or Noble, I find a 10 Meg impedance to be a touch to heavy sounding for my ears. This may well be due to my particular bass, strings, technique, etc. YMMV.) To your question, Felix is also switchable with fixed settings for 10k, 332k, 1Meg, 10Meg, and 20Meg. I find myself wishing there was a 5 or 4.7 Meg setting available.

The Noble is a really great preamp but I found what might be a similar heaviness running a Lifeline or half an Underwood into a very similar Tonecraft 363. It’s funny that I didn’t even think of the Felix. It’s just never appealed to me. I guess I like to solve one problem at a time and those “do-it-all” boxes make me feel like there will be compromises at some stage in the signal path.

 

[DoubleMIDI]

There is an easy way to lower the input impedance of a buffer/preamp. Make a short cable with plug and jack and solder a resistor from hot to shield.
The total impedance is a result of that parallel resistances of the amp impedance and the parallel resistor in the cable.
As an example:
If your input impedance at the buffer/preamp is 10 MegOhms fixed, then one more 10 MegOhms buffer from hot to ground in the cable gives 5 MegOhms total. If the additional resistor in the cable is 20 MegOhms, the total impedance drops to 6.67 MegOhms.
The larger the difference, the lower the influence of the higher resistance (impedance). You can find formulas for the value of parallel resistances in the web or any high school physics book.

Impedance is a frequency dependent resistance. For our application we can assume that it is a fixed resistance for the interesting frequency spectrum in that part of the electronic circuit.
The HPF effect comes from adding the piezo, which partly acts as a capacitor (often of a lower value) as well, not from the capacitors in the input stage of the buffer/amp.

Some buffers use a capacitor to block DC if there would be any in the input (like the HPFpre). Some don’t. For a passive piezo input it is not needed, but if you want you can put a plastic foil capacitor of about 22nF (for a 10 MegOhms resistor, double value for half resistance and vice versa) in series with the resistor in the cable if there is one in the buffer/preamp. Be sure to isolate well.

For those asking for the inverse way:
There is no way to enlarge the input impedance externally without dropping the signal value by a large amount.
The internal resistor for the input impedance needs to be replaced by a larger value. Cutting one side, best the signal side, and soldering a new one over it (to the hot side of the cut) is enough, but not recommended with todays SMD technology without proper tools and knowledge.

 

[LaFaro01]

The Grace Design Felix also offers switchable input impedances in the range from 10 kOhm up to 20 MOhm, if you "ignore" the Mic Input with 8,1 kOhm.. and it works really fine, as I noticed trying it out with my Ehrlund PU
https://gracedesign.com/products/instrument-preamplifiers/felix

 

[BobDeRosa]

Call me a hack, but this is a sincere question. For what kinds of gigs do you need such sophisticated and expensive sound reinforcement? I don't play any classical, but for the jazz trio and acoustic rock gigs I do, I get by fine with my Fishman Full Circle through an Aguilar head and even through a Fender Rumble. I sometimes use a preamp/DI, but nothing that has cost more than $150 or so. What am I losing by not using the fancy stuff mentioned here? Am I being naive or just too easily satisfied?

 

[AGCurry]

Call me a hack, but this is a sincere question. For what kinds of gigs do you need such sophisticated and expensive sound reinforcement? I don't play any classical, but for the jazz trio and acoustic rock gigs I do, I get by fine with my Fishman Full Circle through an Aguilar head and even through a Fender Rumble. I sometimes use a preamp/DI, but nothing that has cost more than $150 or so. What am I losing by not using the fancy stuff mentioned here? Am I being naive or just too easily satisfied?

Bob, Bob, Bob! You're obviously the kind of guy who's generally satisfied with a $15 bottle of wine.
Get with the program, man.

 

[DoubleMIDI]

don't play any classical, but for the jazz trio and acoustic rock gigs I do, I get by fine with my Fishman Full Circle through an Aguilar head and even through a Fender Rumble. …
What am I losing by not using the fancy stuff mentioned here? Am I being naive or just too easily satisfied?

You (luckily) have equipment that works well together, take your own equipment to your gigs and are on the „sounds good enough for“ side , not on the „best DB sound I can get“ side.

Nothing wrong with it and it keeps you more money in your pocket.

 

[logdrum]

You (luckily) have equipment that works well together, take your own equipment to your gigs and are on the „sounds good enough for“ side , not on the „best DB sound I can get“ side.

Nothing wrong with it and it keeps you more money in your pocket.

The Fishman FC is known for it's adaptability to sound similar whether it is plugged to a guitar bass amp ( many are in the 1 Mega ohm range anyway ) or an acoustic bass preamp that has a 10 Mega ohm, input/ The BP-100 will sound klanky (although some people like that sound ) unless it's 5 Mega ohm or better still 10 Mega ohm.

 

[BobDeRosa]

You (luckily) have equipment that works well together, take your own equipment to your gigs and are on the „sounds good enough for“ side , not on the „best DB sound I can get“ side.

Nothing wrong with it and it keeps you more money in your pocket.

Exactly!

 

[Zbysek]

Broughton HPF DI

 

[Antonio Abete]

Broughton HPF DI

Looks great but unfortunately it's out of stock

 

[Chris Fitzgerald]

There is an easy way to lower the input impedance of a buffer/preamp. Make a short cable with plug and jack and solder a resistor from hot to shield.
The total impedance is a result of that parallel resistances of the amp impedance and the parallel resistor in the cable.
As an example:
If your input impedance at the buffer/preamp is 10 MegOhms fixed, then one more 10 MegOhms buffer from hot to ground in the cable gives 5 MegOhms total. If the additional resistor in the cable is 20 MegOhms, the total impedance drops to 6.67 MegOhms.
The larger the difference, the lower the influence of the higher resistance (impedance). You can find formulas for the value of parallel resistances in the web or any high school physics book.

Impedance is a frequency dependent resistance. For our application we can assume that it is a fixed resistance for the interesting frequency spectrum in that part of the electronic circuit.
The HPF effect comes from adding the piezo, which partly acts as a capacitor (often of a lower value) as well, not from the capacitors in the input stage of the buffer/amp.

Some buffers use a capacitor to block DC if there would be any in the input (like the HPFpre). Some don’t. For a passive piezo input it is not needed, but if you want you can put a plastic foil capacitor of about 22nF (for a 10 MegOhms resistor, double value for half resistance and vice versa) in series with the resistor in the cable if there is one in the buffer/preamp. Be sure to isolate well.

For those asking for the inverse way:
There is no way to enlarge the input impedance externally without dropping the signal value by a large amount.
The internal resistor for the input impedance needs to be replaced by a larger value. Cutting one side, best the signal side, and soldering a new one over it (to the hot side of the cut) is enough, but not recommended with todays SMD technology without proper tools and knowledge.

If I had the chops to do this, I would do it to see what the difference would be on the Noble. It couldn’t cost more than a few $, and would be a worthwhile experiment. Would be even better if it made a positive difference and could be reduced to a single male to female jack adapter with no cable in between.

I have been very happy with the Full Circle through the Noble, and my grad student helped me find the range on it this semester when he helped guide me to find the most natural sound while sitting in the seats at a concert. The end result was to keep the bass knob all the way counterclockwise, engage the HP filter (which I was doing anyway), and adjust the treble to taste. The result was confirmed by the recording and I just go with that as the default setting at this point.

This basically is an affirmation of the maxim that I have observed for years: in order to have the best sound reach the audience, it’s usually better to have the sound a little brighter and carved out on the bass side on stage. For me, it’s always been a matter of calibrating my ears to that sound on stage and learning to live with it because I know it will produce the most natural sound in the mix.

EDIT: In real life, my go-to perfectly acceptable bottle of wine is $8. I would not be missing much if that was all that was available moving forward.

 

[Jefenator]

I'm not adverse to trying something with sweepable or switchable input impedance.

OTOH if it's going to be a "set & forget" kind of control, I'd honestly be happier if it were set internally - on the circuit board. (I feel much the same way about EQ.)

There might be a sweet spot waiting out there for me between the 500K or 1M on most bass amps and the 10M on my fDeck. Until I get a decent opportunity to try that, the fDeck makes my pickups sound better almost every time and I really like the affordability (and convenience; I'd rather replace a 9V every 6 months than fuss around with an extra power hookup every time I play out).

 

[DoubleMIDI]

If you just want to experiment, get a few resistors (10,6.6,5,3.3,2,1.5,1 MegOhms), Open one connector of your TS-cable and bend the resistor wires, so that one connects to shield and the other one to hot. You can use small (non-conducting) claps to keep the wires in contact or use a second person to do that (but don’t touch the hot side).

An adapter is rather long, specially with the cables plug in it and can damage your amp input if an impact happens by accident.

You can use a normal cable and just solder the resistor in. I just thought the short plug-jack cable uses less space, several with different resistors can be made and carried (a color coded cable would be cool) and no higher risk for the amp input than with a normal cable.

But these resistance values (1/4 watts standard would be fine) are often hard to get in the electronic parts shop round the corner.
Also get used to the color coding scheme of resistors or get a multimeter with a resistance measurement range up to at least 20 MegOhms.

 

[CryingBass]

I do not have any experience with this unit but I think I might pick one up to see what it is capable of.

Cloud Microphones Cloudlifter Zi 1-channel DI and Mic Activator with Variable Impedance

 

[iAsh]

25989763[/URL], member: 221502"]Call me a hack, but this is a sincere question. For what kinds of gigs do you need such sophisticated and expensive sound reinforcement? I don't play any classical, but for the jazz trio and acoustic rock gigs I do, I get by fine with my Fishman Full Circle through an Aguilar head and even through a Fender Rumble. I sometimes use a preamp/DI, but nothing that has cost more than $150 or so. What am I losing by not using the fancy stuff mentioned here? Am I being naive or just too easily satisfied?

Apply the law of diminishing returns to what everyone else hears and you might not be missing much. If you genuinely like your sound then maybe all’s well, save your time and money. But my question is how many great preamps have you used? It’s easy to be happy with a good Kay if you’ve never played a master instrument. But as you play one great instrument then several more you might start to feel constrained by the limitations of a Kay. Maybe you don’t need more than a Kay to get and keep jobs but now knowing the difference are you satisfied with it?

There are two reasons I have expensive preamps: Too many choices and buying into the My Bass Only Louder fantasy for too long.

MBOL? I laughed when I heard that one. More like MBOLBS. When you start to amplify an acoustic instrument it starts to become an electric one. When the volume of the amp completely overtakes the acoustic sound of your bass you’re playing an electric bass. I’m still enthusiastic about chasing a more satisfying amplified sound but hunting unicorns is no longer on the agenda.

Too many choices is what’s killing me now. When the best system available was an Underwood straight into a GK combo I was happy. Now that there are more choices I’m not satisfied without exploring (within my budget) all the options that appeal to my sensibilities. When is curiosity satisfied? When is it “good enough”?

Actually there’s a third reason. If I like a preamp I rarely sell it to buy another. Over time a collection forms. As the collection grows I become a preamp connoisseur. When my collection can’t do something that would be useful like variable input impedance I ask around. Discovering the Sarno Black Box I buy it partly out of connoisseur curiosity. It’s like collecting Hotwheels or manual typewriters.

I have no contempt for people who are satisfied not exploring the latest technology. Any I might have had is zeroed out by a genuine appreciation that they’re satisfied. There are advantages to not being overly obsessed by equipment and lots of great players aren’t.

 

[CryingBass]

I was a Marcus Berry kind of guy in the past. Does that count?