Double Bass Variable input impedance for DB amplification

[Chris Fitzgerald]

Was reading this thread about a great looking and expensive preamp this morning, and it got me thinking. After having been through all kinds of gear periods and having lugged a bunch of different rigs around over the years, I started thinking about the things that I come back to as always being important in an amplification setup. The two big ones seem to be a high pass filter, and input impedance loading that is "friendly" to the pickup being used. Once you have those things, there are a lot of amps out there that can provide a pleasing enough color to amplify that initial signal.

All of this lead me to reread This thread this morning and set me remembering our little get together fondly, but it also got me thinking about the topic of variable input impedance again. It seems to me that if I were designing the perfect preamp - or of putting together the perfect feature set in a portable rig that I could take with me when I travel - the Grace unit from the first link would have just about everything I could want except for sweepable input impedance (it has a sweepable HPF and tons of EQ control). The fdeck HPFpre has the sweepable high pass filter, but little else. The Summit TD-100, from what what I can remember, was a great sounding unit, but as Phil noted, it's "built like a pig". Even so, I find myself wishing I could try one with my current amps to see what kind of a difference it would make.

Anyway, after all of this rambling, I come back to two thoughts, which I will phrase as questions:

1) How many commercially available units are there out there that have this variable input impedance knob?
2) How hard would it be to design a little box like the fdeck HPFpre that loads input impedance?

It seems to me that if I could carry these two controls with me, I'd be in pretty good shape to find a sound on almost any amp.

 

[MrSidecar]

Chris,

while this might not be the answer that you were looking for, IMO there isn't too much use for a variable input impedance circuit, beyond experimenting what the proper impedance for a pickup is. And maybe not even there.

The reason is this: Too low input impedance for a given pickup has well known negative effects, namely a high pass filtering (hollow, midrange heavy sound without good low frequency response). But too high input impedance does not cause any problems as far as I know (not within reasonable limits, at least- 10MOhm is the highest I tried and it's fine, I can't speak for anything higher, although I doubt it's a problem). Things can get a little noisy. I have read somewhere, but that is nothing I have ever heard myself.

So if there is no "too high" impedance, an amplifier or preamp designer who wants to serve all pickups known to man, chooses maybe 10-15 or even 20 MOhms, and all should be fine.

Unless I am victim to a misconception here.

Best
sidecar

 

[Chris Fitzgerald]

I don't know if you are a victim of a misconception or whether I am, but: in the second link above, a number or people whose opinions I value (myself included ) seem to feel that there was an impedance "sweet spot" with the TD 100 that could be found by sweeping the knob until the sound was optimal. I have found much the same to be true for variable HP filters. As Phil Maneri said somewhere in that thread, once the sweet spot was found, you might as well glue the knob in place at that point and leave it alone....for that particular piece of gear. But for instance if you double on BG, then those pickups would also want their own setting, and of course if you are forced to borrow different instruments with different pickups, there would be different sweet spots. Even on my two basses with the same maker, each having the same make of pickup, the optimal amp settings are different. So unless I wasn't hearing what I thought I was hearing back then, I think there may actually be an "impedance load sweet spot" that would be different for different instruments and situations. Would be curious to hear more about the science behind this, even if I likely won't completely understand a good deal of it.

 

[robgrow]

As an experiment, I once built an FET buffer amp with a variable input impedance (500K to 10Meg as I recall). While it was an interesting experiment, the variable input impedance feature did not make enough difference for me to start using the buffer full time. A friend once asked me to modify his fdeck HPF-Pre to have a 1Meg input impedance instead of the stock 10Meg. We both thought it sounded essentially the same after the mod. Personally I think that input impedance is just one of many variables.

 

[shwashwa]

Dragster™ Load Correction Device

 

[moles]

My old Presonus Eureka mic preamp/channel strips had a variable impedance - selectable, not sweepable, and tailored for microphones...but, it's a start, and possibly mod-able if one wanted to get into that.

Sidecar, from what my ears tell me, higher impedances seem to compress the signal a little bit. Maybe that's *my* misconceptions at work, but from what I've heard out of my own various setups, an argument could be made for tweaking the input somewhere below the 10Mohm mark.

 

[Chris Fitzgerald]

Dragster™ Load Correction Device

Very cool! Exactly the kind of thing I was thinking about, except the FAQ page for the Dragster says that it's range makes it not useful for piezo pickups. I can't help but wonder if that was because it would have been a whole lot harder to make, or that it couldn't have been passive to had a load in the range piezo like to see, or if it was just because it was really only designed for magnetic pickups? But yeah, a little box like that, if it extended all the way up to 10 or 20 MegOhm, would be perfect.

 

[flatback]

when I sent my Focus into Rick for repair, I had a conversation with him about why the headway sounded better then the Focus front end. He looked thru the specs and didnt really find anything except the impedance at the front end. He put a 10Mohm buffer in my Focus (apparently they used to be built that way but people started using PU's that didn't need them.) The Headway had a 20 Mohm buffer and that IS the setting I use.

 

[Joshua]

I've been using a full circle/TD100/IP112 for a couple of years now and love it. Unfortunately for this discussion, it's all I've ever used so I have nothing to compare it against.

While hardly the easiest schlepp, I have the rackmount kit and a 2 space soft rack. The rack is very light and carries with a shoulder strap so it's not too bad...

 

[Jeff Bonny]

Very cool! Exactly the kind of thing I was thinking about, except the FAQ page for the Dragster says that it's range makes it not useful for piezo pickups. I can't help but wonder if that was because it would have been a whole lot harder to make, or that it couldn't have been passive to had a load in the range piezo like to see, or if it was just because it was really only designed for magnetic pickups? But yeah, a little box like that, if it extended all the way up to 10 or 20 MegOhm, would be perfect.

Contact Radial and see if they'll do that. They're very much a customer driven company. I'd contact the sales rep and see if he'll rattle the cage for you. Shamelessly play up the Abersold/Talkbass/univeristy prof thing....Radial likes to use that kinda stuff in their marketing. If they won't do it in house ask what would be involved in doing it yourself. There can't be much to this box and it might very well just be a quick solder job to swap out one or two components.

The [Invalid or Expired Link Removed]looks like it will do what you're asking in steps of 220k ohms, 1meg ohms and 10 meg ohms. But it's also a pro quality DI with other bells and whistles that costs around $225. Although you would save about $40 on the current Canadian/US dollar difference buying it from Long & McQuade who has it at the same price in Canadian dollars as Sweetwater does in USD.

 

[drurb]

Chris,

while this might not be the answer that you were looking for, IMO there isn't too much use for a variable input impedance circuit, beyond experimenting what the proper impedance for a pickup is. And maybe not even there.

The reason is this: Too low input impedance for a given pickup has well known negative effects, namely a high pass filtering (hollow, midrange heavy sound without good low frequency response). But too high input impedance does not cause any problems as far as I know (not within reasonable limits, at least- 10MOhm is the highest I tried and it's fine, I can't speak for anything higher, although I doubt it's a problem). Things can get a little noisy. I have read somewhere, but that is nothing I have ever heard myself.

So if there is no "too high" impedance, an amplifier or preamp designer who wants to serve all pickups known to man, chooses maybe 10-15 or even 20 MOhms, and all should be fine.

Unless I am victim to a misconception here.

Best
sidecar

Yes, once again, I think you are right on target, MrSidecar. I posted similar thoughts a while back.

Take a look here, for example.

IMO, it is best to leave the input impedance fixed, as you say at 10-15 or even 20 Mohms and use the tone controls to make adjustments. A variable input impedance control is essentially another high-pass filter and a poor one at that. See here and the link within to fdeck's post.

 

[geoffbassist]

Really interesting discussion and I'm also someone who pretty much only uses the HPF and a decent input impedence. I find the AI Clarus has really stood the test of time and the Fdec HPFpre has helped a lot on gigs with borrowed amps. I've also wondered a lot about a variable input, but it sounds like we may already have pretty much have what we need.

I think I'm lucky though as the Clarus probably gives my Full Circle everything it needs to get the best from it anyway. Plus, I love having a really simple setup with no batteries

 

[MrSidecar]

A friend once asked me to modify his fdeck HPF-Pre to have a 1Meg input impedance instead of the stock 10Meg. We both thought it sounded essentially the same after the mod. Personally I think that input impedance is just one of many variables.

Rob,

that certainly is true for any pickup where 1M input impedance is sufficient (your typical piezo pickup, like underwood-ish, realist, etc). And other variables could be preamp voicing (non-linear vs. linear frequency response), and maybe harmonic distortion (not to be understood like "fuzz" or overdrive). However, in linear preamps (like, for example, the Ehrlund pre), the input impedance is the crucial factor, and the input impedance (granted, together with preamp voicing, if applicable) certainly is the reason why most bass amps built for electric bass use don't play well with piezo pickups without using a preamp.

Theoretically, the preamp's OUTPUT impedance could be a factor (if it were so high as to require a high input impedance of the following amp input), but that would only be the case in a particularly careless design (as that would defeat one of the preamp's purposes).

Best
Sidecar

 

[Chris Fitzgerald]

Interesting discussion! I'm not dissatisfied with my current sound, but I can't always take it with me and since I'm going to be trying to pick my doubling chops back up in the coming year or two, I should probably learn about this stuff. What got me started thinking about this is the PJB combo that I use these days really only has one flaw to my ear, which is that it doesn't have a built in variable HPF. So when I use the fdeck pre, it sounds different when the deck is out front than when it's in the FX loop. It was the same with the Focus, which I tried the Fdeck right after I bought the pre even though the focus had a HPF built in (I turned it off). In both cases, having the HPF up front gave me the impression of a rubbery thickness on the bottom that I don't care for, whereas having it in the FX loop sounded better to me. If I recall correctly, the input impedance of the PJB Flightcase is 4Meg and the input impedance of the Focus is 1Meg. Of course, as others have mentioned, I could be hearing some other factor at play, but it's interesting that I got the same impression both times.

 

[DoubleMIDI]

Chris, the different sound with the HPFpre into the input or effects return is not due to the different input impedances of the instrument input and effects return but because the amps eq is bypassed by the effects return.

 

[drurb]

Chris, the different sound with the HPFpre into the input or effects return is not due to the different input impedances of the instrument input and effects return but because the amps eq is bypassed by the effects return.

I was curious about this so I checked the PJB manual. The effects/send loop occurs after the tone controls. Therefore, according to the manual, it would appear that inserting a device in the loop does not bypass the tone controls.

 

[DoubleMIDI]

Drurb, maybe I could have explained it a bit better. What I meant is that if you use the effect return, the signal does not go through the eq (5 band graphic eq in your simplified block schematic), whereas it does go though the eq if you use the normal instrument input.

 

[Chris Fitzgerald]

Understood, DM; but in my case, I put the HFPre in the FX loop, so either way all of the circuit is being used, just in a different order.

 

[Chris Fitzgerald]

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.

 

[Francois Blais]

I briefly had an Headway EDM-1 recently and found out that the highest impedance was not necessarily the best.
Depends on the pickup I guess.