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Discussion in 'Amps and Cabs [BG]' started by rumblinbass, May 30, 2005.
Does anybody know the standard input impedance of preamps for active and passive pickups?
It's the appropriate impedance for active and passive pickups. Curious why you want to know the number? What are you planning on hooking up to it, or are you trying to build one?
It depends on the circuitry. Guessing anywhere from maybe 500k to 10Mohms. Generally the higher the better for passive pickups. Basses with preamps have a lower output impedance can drive anything, so the input impedance of the amp doesn't matter. Low drives high, high can't drive low.
I think Piezo's pickups need even higher input impedance, around 100M ohms, that's why they always have their own onboard preamp. Normal guitar cord would kill the piezo's tone without a preamp buffering it, cord capacitance at the super high impedance of the piezo pickups essentially looks almost like a partial short circuit at higher freqs.
Actually the average input impedance for a bass amp is somewhere between 100k and 500k, and most piezo pickup manufactors recommend somewhere bewtween 1meg and 10meg. 100meg input impedance would be overkill and would be overwhelmed by input and cable capacitance in most circumstances.
Yes, I am building one. When I started college I decided to major in Electrical Engineering so that I could design/build my owm amp because I couldn't find exactly what I was looking for.
I know that piezo transducers are around 1Meg
I know that active electronics are extremely low...somewhere around 50 ohms...output impedance if a typical opamp circuit is 50 ohms unless deliberatly altered.
Passive electronics are around 47k...I think.
I'm just curious about the input impedance for an amplifier is calculated. Not all PU's are created equal, there has to be a tolerance. Back when I was looking for a new amplifier (I have since acquired a GK700RBII and am very happy with it) a lot of amps had a passive and active input jacks. My GK has only one input jack but a -10dB cut (I know how that works).
If you wish to learn more I suggest you download a spice sim, I use Switcher Cad III from linear tech (www.linear.com I believe) since it's free and fast. You can than use the sim to model a pickup and how it interacts with an amp input impedance, and there by gain a much more intuitive understanding of whats going on.
P.S. On how to decide what the input impedance should be, it's usually put high enough that it has little or no noticable effect on the pickups response.
+1 on the use of Switcher Cad. It's great for choosing the values of EQ components to get the desired curves.
I just finished building my own preamp, but I was looking for compatibility with piezo's for upright bass use, so I used FET op amps and a 1 Meg input impedance.
Here is my suggestion. In most cases with electric bass, passive or active, the bass itself will determine the impedance in parallel with its own output. This is not a transmission line situation where you would have to match the impedance of the bass. The shunt resistor on the input of the preamp is only there to prevent the circuit from going haywire if the input is left open. If you use bipolar op amps, this resistor is also necessary to bias the input. In most cases, a value of around 250k will do the trick.
I downloaded the Switcher CAD software so now I have to figure how to use it.
I am actually designing this preamp specifically for my upright but would still like to use my electric on it as well. So I'm thinking about a switchable input impedance.
Switchable impedance isn't needed, an average pickup will be less than perfectly 'happy' with an input impedance less than 100k, but will do fine with anything higher, all the way up to infinity, I say design for 1 or 2 megs input impedance and you'll be fine.
With piezo pickups, the higher the input impedance, the lower the bass response. This is a result of the piezo being a capacitive device, and forming an RC high-pass filter with the preamps input impedance. I would try designing for more around 5M. Theres no problem running regular passive pickups into this.
I also highly recommend SwitcherCad (LT Spice) for simulation. Ive been using it professionally for several years now. Dont let the good price (free) fool you. Its as good or better than applications costing several $k, and the users group on Yahoo is excellent.
IIRC - aren't the older Fender tube amps running 1 mg ohm ? I've run actuve & passive basses in to Bassman & Showman amps with great results.
1M is more than high enough for anything except piezos.
I looked up a couple of fender tube amps, most had a 136K (two 68k's in series) input impedance on the -6db gain jack and 1 meg on the normal jack.
I guess I will have to check out Switchercad then.
I thought that there were standard input impedances on most preamplifiers. I guess that is probably what gives each amp it's sound/tone. I just want to design a fairly versatile amp because I play both an active bass, an upright, and am working on a Frankenbass project that will have passive pickups.
I think the choice of input impedance has relatively little effect on tone, assuming that the impedance is not simply too low. But for a DIY pre, why not make the input impedance variable, and try different settings? One minor concern is that high gain and high input impedance will cause oscillation with some layouts.
Factors that influence tone are linearity / nonlinearity of (typically) the front end gain stage, and choice of tone contours, including whether the contour is flat or "voiced" when the controls are all centered. It is worth familiarizing yourself with the basic Class-A JFET gain amplifier, and the JFET source follower, both of which have just enough nonlinearity to let you decide whether some "warmth" is what you want for your sound. In my view, simulating this nonlinearity with op amps is not worth the bother, so you should only use an op amp front end if "hi fi" is what you really want.
Beware that with something like a JFET, the parameters of real devices have some variation. You will never guess this from the SPICE model. A lot of commercial bass amps with JFET front end have bias adjustments for this reason.
The beauty of building your own is that you can experiment with those parameters, or even make them variable if you want. You can also have two or more inputs, each associated with a front end designed for a particular kind of bass.
The JFET idea sounds interesting. I did try to look up some several months ago but had trouble finding JFET's at all. I'll start looking again.
I'm sorry, I didn't mean to say that I had a hard time finding JFETs at all - more like I had a hard time picking out a suitable JFET for my purposes or at least audio.
But I do work with a guy with similar interests and he turned me on to a couple that I can start from.
SwitcherCad comes with a model for 2N4393, which I have used for simple preamps. It is an extremely low noise device.
I advise going on a search mission for published preamp designs on the Internet, of which there are many. There is a beautiful piezo preamp circuit at the Jensen Transformer website, and you can use it without the transformer. It is literally a studio quality preamp requiring just a few bucks worth of parts.
What I don't know is if you are seeking any amount of "warmth" from your circuit. The Jensen preamp has negative feedback, and is actually a fairly low distortion design. To get more warmth requires a more simplistic circuit such as a JFET common source amplifier, or even a source follower.
While we're on the subject, if you stray outside the limited number of JFET models supplied with Switcher CAD, then you have to find published models and add them to the model file. This is a royal pain, and I don't think it is necessary.
In my opinion, it is better to familiarize yourself with a small handful of devices than to spend your life searching for the perfect device. For this reason, I think the small sample of model files in SwCad is ample.
Also, if you get really ambitious, there are Spice models of tubes out there. You don't have to build tube preamps to predict how they will behave, and you can even decide if a JFET is a good enough substitute.
I have already found some schematics online. I also have several text books from my college days. I'm not looking for the "perfect" part, I'm just looking for a JFET suitable for audio applications. I'm sure most are, but some are moreso than others. By finding published schematics I can get a list of common parts used in audio applications.
I have looked a Switcher CAD and plan on learning how to use it. I also have another simulation package that has a collection of models as well.
Not in the linked schematic -- I'm seeing 1.68M to ground, paralleled with the grid impedance of the preamp tube.