Variable Input Impedance

Eoin · Post by **Eoin** » Tue Jan 04, 2005 7:38 pm

Variable input impedance - mic preamps do it quite a lot... could it be done as a mod for a superlead preamp? Any ideas as to the best approach? What are the various input impedance values for the average superlead pre at factory spec?

Okay, that's three questions. But if I get answers, more questions will invaribly follow, so beware!

Post by **VelvetGeorge** » Tue Jan 04, 2005 7:42 pm

Mic preamps typically use an input transformer. Some of them have taps for different input imp to match various mics.

A standard superlead has a 1M resistor (the one mounted on the jack). You can make the input more sensitive by raising the value. Try a 1.5M or 2M.

George

Necrovore · Post by **Necrovore** » Tue Jan 04, 2005 8:00 pm

What exactly would raising the impedence of the input do for real world applications? would it make your signal appear hotter or something to that effect?

Eoin · Post by **Eoin** » Wed Jan 05, 2005 10:39 am

Necrovore wrote:What exactly would raising the impedence of the input do for real world applications? would it make your signal appear hotter or something to that effect?

Possibly... or the opposite, depending on where you're coming from. It changes the load on the transducer (pickups in this case), which could alter any number of things, level, frequency response etc. or may do very little. All transducers have different impedances. In a mic pre it's used basically to tailor the input impedance to get the best response out of any given mic (normally the input impedance in a mic pre should be 5 or 6 times the impedance of the mic, or higher). I presume guitar pickups have equally varying impedances.

George, do you think as a mod it has any merit in a guitar preamp? Obviously, again, I'm not going to butcher my '76 trying something like this on it. Thinking aloud about custom superlead preamp again.

Post by **VelvetGeorge** » Wed Jan 05, 2005 2:03 pm

The input impedance and pickup intereact together obviously. They define certain parameters such as frequency response signal swing ect.

For any given pickup there will be a quiesent operating point. Basically the condition where it is reproducing signal most accurately. With full frequency content and the largest distortion free peak to peak voltage swing. It's important for microphones to operate at this quiesent point to sound their best. So variable input imp helps accomplish it.

Guitar amps are a little less critical, but you do want a good match between your pickup and input imp. This helps to insure clarity and avoids strange artifacts caused by a mis-match.

So what should you try in your projects? Stick with the double or halve factor. IOW try a 500k and a 2M in place of the 1M. If there's a benefit to swapping you should find it. Really hot pu's should benefit from the lower value and vice versa.

George

Post by **VelvetGeorge** » Wed Jan 05, 2005 2:07 pm

I thought of another analogy: think of your pu as an amplifier and your input impedance as the load it's playing into.

This is somewhat over simplified, but it might make sense.

george

Eoin · Post by **Eoin** » Thu Jan 06, 2005 11:41 am

Hmm... would it work with a 2M pot (or a dual 1M with the top wired through the bottom) wired as a variable resistor in series with a 300-500k resistor? Or would a pot in place of a carbon composition compromise the tone?

Billy Batz · Post by **Billy Batz** » Mon Jan 10, 2005 8:53 am

One of the schematics I have of a TrainWreck shows an input trim control thats what you may be thinking of. Its a 1M pot in series with a 22k resistor to ground.

Eoin · Post by **Eoin** » Tue Jan 11, 2005 2:12 pm

Could be the same thing alright.

Still unsure as to whether there are negative effects from adding a pot to that position that might outweigh the advantages of having it.

Billy Batz · Post by **Billy Batz** » Tue Jan 11, 2005 2:25 pm

Like what? Youd have to be one hardcore audiophile to claim theres an audible difference between types of resistors for ONE single resistor on the input. The variation you get in sensitivity seems to outweigh it anyway. Besides you can put the fixed resistor first then the pot if that makes you feel better about it

Eoin · Post by **Eoin** » Tue Jan 11, 2005 7:27 pm

Well, different compositions apparently display different resistance characteristics dependent on frequency. As the first resistor bleeding to ground in the signal path (after the vol and tone on the guitar), it stands to reason that it should have the greatest ramification on the signal.

I've never tested various composition resistors for their signal altering characteristics, so I don't know how much of that is just bullshit. I'd be interested in hearing a few different opinions and/or facts and figures though.

My general viewpoint is: The Superlead was made to sound the way it sounds with carbon comps. I could decide to "upgrade" all my resistors to new modern ones, but the component values and layout of the design weren't optimised for modern resistors. So if I go down that road, I might as well redesign the amp (which I'll get around to in a few years). While I'd like to customise this preamp/amp that I build to as great a degree as possible, there's no point in altering the things that make it worth building in the first place. Or something like that.

Billy Batz · Post by **Billy Batz** » Fri Jan 14, 2005 8:18 pm

Trust me Eoin. Marshall wasnt doin any optimisation when he was designin' his amps. He copied another amp, made some changes of his own and used used parts that were available like Fender. There's more then one type of resistor in most old Marshalls and if you take 10 old Marshalls at least a few would have a different type or make of the same part for any given part. BTW old Marshalls used mostly Carbon Film not Carbon Comps.

I agree. Any small change in the input is going to be amplified at every following stage. But one resistor to ground on the input. Its not even in line with the signal. Maybe Im wrong but I dont think you even need to worry about that for a second. Even audiophiles arent saying single resistors are chaning the tone of an amp like they are about caps. The most Ive read from audiophiles is a) the noise aspect which gets better going from comp to film to metal film, and, b) even with carbon comps its the cumulative effect of some kind of slight distortion they add they make them sound smoother or whatever people always say about them.

Your givin Marshall and amp manufacturers way too much credit if you think there changing their circuits around based on what type of resistor their using. Besides. I got that idea from a schem of a Ken Fischer amp. Talk about an audiophile. He would be the god of an audiophile religion.