Variable Impendance

[Nutmeg II.]

I would like to build a DIY Variable Impendance circuit. Is there a schematic anywere or does some body know houw to build one?

[Geoff_T]

Originally posted by Nutmeg II.
I would like to build a DIY Variable Impendance circuit. Is there a schematic anywere or does some body know houw to build one?

Hi

What exactly do you want it to do? The capacitor and inductor are variable impedance devices. Their impedance (resistance) varies with frequency.

[Nutmeg II.]

Ups! Sorry meaned variable rise-time! Like on the Groove Tubes Vipre.
Is there a way to build a stand-alone circuit?

Thanks sofar!

[Geoff_T]

Originally posted by Nutmeg II.
Ups! Sorry meaned variable rise-time! Like on the Groove Tubes Vipre.
Is there a way to build a stand-alone circuit?

Thanks sofar!

Hi

Well, you can make a variable impedance on the input by either having a transformer with multiple taps or adjust the value with a resistor across the input.

Rise time is when you look at a square wave on an oscilloscope and it's the vertical line of the square wave. It takes a finite time to rise from nothin' to somethin' and you can measure it by jacking up the sweep rate on the scope to magnify the error and measure it on the graticule on the screen versus the time in microseconds per division. You need a fast scope to do this!

Anyway, the rise time goes to pot when the high frequency response is cut with a treble pot or roll off aspects of the circuit.

Then, instead of a sharp vertical line, you get a curvy one as the treble rolls off and it takes longer for the voltage to rise.

So, while not quite the same, you can mimic the effect with slight treble cut.

[Nutmeg II.]

Hi Geoff,
thanks again for the reply!



OK, I see it now too.
A lowpass will round out the edges of a squarewave to the extrem of a sinewave.

Similar with the effect of slow swellrates.
But a highpass will not act like a faster swell rate I bet!

And does a slow swell rate effect the right part of the wave too (before it falls)?

What is the differens in the two: risetime <> lowpass?

[Geoff_T]

Originally posted by Nutmeg II.
Hi Geoff,
thanks again for the reply!



OK, I see it now too.
A lowpass will round out the edges of a squarewave to the extrem of a sinewave.

Similar with the effect of slow swellrates.
But a highpass will not act like a faster swell rate I bet!

And does a slow swell rate effect the right part of the wave too (before it falls)?

What is the differens in the two: risetime <> lowpass?

Hi

Square waves are excellent quick tests of equaliser functions. The "treble bit" is the vertical line and it will overshoot or wilt alarmingly with boost and cut.

The "bass bit" is the horizontal line at the top. If you boost, it slopes up, if you cut it slopes done. You can get other wonderful effects with mid frequencies that might increase some of the treble and some of the bass, as vice versa.

You can't really improve a rise time with a low pass because, by virtual of letting the low frequencies though it will stuff your high frequencies.. depending on where the slope starts.

I can really recommend playing with a scope and seeing what an equaliser can do. You might try feeding the input signal into one trace and the output into another. If you sync the scope off the input signal you will be scared to death by the phase shift (the apparent drifting of the output trace) as you pile on more EQ!

[Nutmeg II.]

Originally posted by Geoff_T
...I can really recommend playing with a scope and seeing what an equaliser can do. You might try feeding the input signal into one trace and the output into another. If you sync the scope off the input signal you will be scared to death by the phase shift (the apparent drifting of the output trace) as you pile on more EQ!

Hi,
Better not doing it or I will never ever EQ again!

But still my question is not answered jet. Is there a circuit that will make the swellrate/risetime variable?

[Geoff_T]

Originally posted by Nutmeg II.
Hi,
Better not doing it or I will never ever EQ again!

But still my question is not answered jet. Is there a circuit that will make the swellrate/risetime variable?

Hi

"Swell rate" reminds me of church organs and the pedals that adjusted the volume... which I suppose was air pressure, not being an expert on such things. On that topic, how many folk know that SSL originally started out by modifying such organs for electrical control via solenoid valves (I presume)?

Like I think I mentioned above, the rise time is the vertical (trebly) part of the square wave and you can slow it down by rolling off the high frequency response.

[Nutmeg II.]

OK, my last try then!

Regarding to what you say the neat five position rise-time knob on the GT Vipre every body rave about is a clever marketing thing and is a high cut filter for real?!?

[Geoff_T]

Originally posted by Nutmeg II.
OK, my last try then!

Regarding to what you say the neat five position rise-time knob on the GT Vipre every body rave about is a clever marketing thing and is a high cut filter for real?!?

Hi

I can't answer that because I don't have an intimate knowledge of the amplifier's circuitry so could only respond in general terms about rise times.

I don't want to second guess what they do but, not that I'd offer that option on a unit I made, if I did that would be basically what it was...

I suppose you could email the manufacturer?

[Nutmeg II.]

OK I'll give up!

[ciminosound]

I read in a review of the Vipre that the Rise Time control changed the amount of negative feedback to the cathode of the first stage. I've never seen the inside of one or a schematic to confirm this.

[Nutmeg II.]

Originally posted by ciminosound
I read in a review of the Vipre that the Rise Time control changed the amount of negative feedback to the cathode of the first stage. I've never seen the inside of one or a schematic to confirm this.

Heard that too!

But I would like to muil a non tube device that cold be patche into any channel...

[Peter Simonsen]

please go back and carefully read what Geoff_T wrote...!!! You really cant get much closer than that... .

I read in a review of the Vipre that the Rise Time control changed the amount of negative feedback to the cathode of the first stage. I've never seen the inside of one or a schematic to confirm this.

Again please see what Geoff_T wrote...

to extend on that..."normally"..when, and/or/if you apply negative feedback to a circuit One of thing things you are doing is to extend the freq responce..."bandwith" of the circuit = more high freq´s...when you cut back on the negative feedback...well...!!!..huuraayyy..!!! you do the opposite...;-)

Kind regards

Peter

[gyraf]

Good point, Axis.

Jakob E.

[Peter Simonsen]

Originally posted by gyraf
Good point, Axis.

Jakob E.

Thanks Jakob...btw just call me Peter...the "axis" is just an ´ol network name thingy, that goes way back... I´m the same "axis/Peter" as the "axis.dk/Peter" over at recording.org..

<hmmm does that make sence...!!!

Kind regards

Peter

[ciminosound]

Sure negative feedback effects bandwidth it also effects gain and the distortion in a circuit.

The use of negative feedback has much more of an effect on a circuits performance than a hi-cut tone control....

[Peter Simonsen]

Originally posted by ciminosound
Sure negative feedback effects bandwidth it also effects gain and the distortion in a circuit.

The use of negative feedback has much more of an effect on a circuits performance than a hi-cut tone control....

Ohhh yes sure no doubt about that, but I was simply staying close to what I thought was the topic in hand.."hence" my answer regarding the effect of high freq responce when, or not you apply negative feedback in a circuit.

Kind regards

Peter

[Tim Farrant]

It would be an interesting test for someone who owned a Vipre to measure the effect of the Slewrate control on the units freqnency response. I have to agree with Geoff that the only way of limiting the rise and fall time of a circuit is to introduce a time constant. This is easily done with a resistor and capacitor. The resistor could be made variable.

Nutmeg, you could try this; a 10k pot in series with a 100ohm resistor in series with the signal line and a capacitor of 1nF (that 1000pF) across the signal line after the pot/100ohm (a bit like an L pad). Varying the pot from end to end would result in a filter (assuming a relatively low impedance driving circuit) variable from 1.6Mhz to 16kHz.

Obviously you will also loose some signal level as you vary the pot unless it feeds into a high impedance (>1 meg ohm) input such as the non-inverting input of an opamp.

Something to experiment with perhaps....

Cheers
Tim.

[Nutmeg II.]

Thanks for the thint Tim!

Would be nice to get know how the viper effects the signal!

[Bernd G]

I vaguely remember from playing around with the VIPRE's impedance control the low-pass filter effect. The question I guess that we all have is whether there is any noticeable OTHER change in the sound such as added distortions etc. that would warrant further investigation.
Any thoughts ?

[Tim Farrant]

After thinking a bit more on this subject, I must point out that slewrate is not only an effect of frequency response but it is also about "level".

An amplifer that has a slewrate of say 2V/uS could be deemed "fast enough" if the maximum output level it had to achieve was only -10dBu (0.245V RMS). The same amplifier however when asked to deliver +20dBu (7.75V RMS) may introduce distortion because of the "time" it takes to go from zero to that level, and therefore could be deemed "not fast enough".

Does this make sense ? The slewrate spec has a "time" factor, which is constant and therefore to be meaningful the level at which the slewrate is measured should be specified.

Tim.

[Nutmeg II.]

Aha! Thanks for the hint!

[René Hatt]

Hi

I'm not a specialist on that topic, but as far as I know the introduction of this Variable Input Impedance has its roots in a matching problem: Some mics (especially ribbon mics) have got a different Output Impedance than the average condenser and therefore need a mic preamp that offers a different Input Impedance (Impedance more or less is "resistance for AC").

I once heard that the matching is perfect if the Input Impedance is six times higher than the Output Impedance of the previous unit.
I nearly sold my ADL 1500 because of mismatching Impedances! This unit has an unusual 600 Ohms Input Impedance and sounds absolutely terrible (is "pretty useless" as Fletcher stated once) if the previous unit has got the wrong Output Impedance.
Does anyone know more about that?

[shikawkee]

Originally posted by René Hatt
Hi

I once heard that the matching is perfect if the Input Impedance is six times higher than the Output Impedance of the previous unit.
I nearly sold my ADL 1500 because of mismatching Impedances! This unit has an unusual 600 Ohms Input Impedance and sounds absolutely terrible (is "pretty useless" as Fletcher stated once) if the previous unit has got the wrong Output Impedance.
Does anyone know more about that?

I don't know about that but I tested the ADL DI pretty thoroughly and it performed very well. Mind you, I was only plugging in a variety of different instruments to test, not other gear.

[Magic Genie]

I've been wondering about this.

I think I like the preamp to sound as "fast" as possible, so that all of the air and clarity of "being there" remains.

I think this is why I love my Martech MSS-10.

WOuld I be gaining anyting by getting the Vipre or some Variable Impedance pre?

[shikawkee]

Originally posted by Magic Genie
I've been wondering about this.

WOuld I be gaining anyting by getting the Vipre or some Variable Impedance pre?

Maybe. You may find that some microphones respond to impedance variations while others don't. Ribbons seem to like a change at times as do some dynamics. It's worth having the loading options to try different settings IMO. I dig it with my UA 2-610. YMMV.