Tuesday, January 15, 2013

Active vs Passive

There is a pretty interesting phenomenon that is still taking place in the world of modern music, musicians are paying large amounts of cash for passive components, usually because there is this thing going around the music world that makes musicians think that active components are bad.  Even worse the typical op amp is regarded by audiophiles and choosy gear snobs as the most unmusical device there is on the planet.  None of this can be further from the truth, but the reality is, a lot of companies are still using transformers in things they can use well designed op amps in so they can charge an arm and a leg and the buyer feels it is justified.

Same goes for active circuits in general, I hear it all the time, designers staying away from, even bragging about not using active circuitry, as if the active circuit was this evil thing that sucked all the life, vibe and character out of the tone.  I recently heard this about a passive, foot switchable gain adjustment that was in pedal form, as in, it sits between your guitar and your amp.  Now the designer mentioned about not having active circuits just pure raw tone, so let analyze with a little bit of electrical engineering math.  Now a typical gain knob in an amp is 1M, so lets just say that this gain knob stompbox is indeed 1M.  Now if we use that in conjunction with the guitar's volume pot is in parallel with the gain knob, is also in parallel with the amp's 1M input.  At complete maximum, both the volume of the guitar and the gain control,  you would have an  input impedance to the pickup of only 166Kohms.  Now the typical humbucker output impedance ranges from 12K to about 20K, meaning that this 166Kohm input impedance simply is not high enough.

So what happens to the sound.  For starters the pickup cannot deliver the current being drawn from it by the load.  With a very high quality low capacitance cable, with a high powered humbucker, the low pass filter being created is just above 20KHz with a 6m or 21ft cable.  Obviously with a longer cable or cheaper cable, you are now attenuating high frequencies.  Naturally with a guitar that has a 500K volume pot straight into an amp that has a 1M input impedance, you could use up to 60ft (~19m) of cable before you would even begin getting into audible attenuation of the high frequencies.

Now what would happen if a well designed op amp based buffer was used.  The pickup would be seeing roughly  4-17Gohms, yes you heard right, 17 GigaOhms with a low noise FET input audio op amp.  This is much higher than the guitar amp's impedance but the relative loading difference between the two in terms of how the pickup operates is <0.1%.  The large plus is that the output impedance of the opamp is also much lower than the pickup, meaning it can deliver the current needed.

So for the most part why are active circuits hated by the tone snobs?  Well here are a few myths and points made:

Noisy
More complex
Robs tone
Op amps distort badly

The big reason true bypass is still a big buzzword is because most pedal companies make horrible input buffers and suck the tone out of the guitar.  Typically you won't see a pedal that has an input impedance higher than 470K and even that is relatively high.  Figure the zones even less, then when you figure that the load is in parallel with the volume pot, you can get effective input impedance that are almost as low as the pickups output impedance (YIKES!!!).  This does give the buffer a bad name and is one of the big reasons some really do hate active circuits.

"They are more noisy"

Sure, any semiconductor adds more noise and generally requires additional passive components which add their own noise as well.  Semi conductor noise and passive component noise is amplified by the amplifiers being used, the more the gain, the worse the noise is amplified.  One thing to keep in mind that if you have clean amplification, no clipping, the signal to noise ratio stays the same, the noise ratio only increases in clean circuits only when additional circuitry is added, both passive and active.  Transistors and op amps of yesteryear may have been noisy, but today, with high grade FET input audio transistors and op amps, their noise level reaches that lower than even passive components such as resistors.  As for as buffers are concerned, with a gain equal to or less than 1, the noise being added to the circuit are humanly impossibly to hear, even when put into a high gain amp that has a cumulative gain >6 Million.

"They are more complex"

Again another true, the more you add to a circuit the more that could go wrong.  In most cases with a well designed active circuit, the benefits outweigh the very slight increase of failure probability.

"They rob tone"

Now this can be true with a bad design.  Remember when I was talking about how impedance could effect frequency response?  If too high a load is used on a semiconductor the small amounts of capacitance in the board or circuit could cause a high pass filter that could get into the audible range.  Again this is bad and has a lot to do with horrible sounding active circuits.  Also, poorly designed input buffers can just like the guitar pickup example have negative effects on the previous stage.

"Op amp distortion is awful sounding"

This comes down to again the design and the quality of the chip.  There have been articles all over the place that talk about how "swapping op amps in your pedals can make the tone better is a myth" and for clean headroom based applications, including distortion pedals that use diode clipping this is completely true.  However, there is a condition that happens with cheap op amps.  For those who don't know an op amp is an amplifier that can amplify DC voltages hand have superior input and output impedance compared to a transistor.  The op amp has two inputs, an inverting and non inverting, the output of the amp is the difference of the two inputs.  One main signal input usually goes to the non inverting (+) and a feedback loop sends a small amount of the output signal to the other input, the inverting input (-).  When the input signal (+) exceeds the supply voltage to the amp, the input voltage continues to increase, however, the output can only go as high as the input, it clips off.  Now the two inputs are not "virtually" the same and the gain dramatically bucks around like an angry bull, swinging dramatically making all sorts of weird output signals as long as the input signal stays above the supply voltage.  This is not a problem with more expensive audio op amps as they distort just like a transistor or vacuum tube will.

9v battery adapters and supplies aren't typically enough to power pedals as most guitar pickups, line level signals, mic preamp output signals etc can all exceed that by merely a few volts causing distortions, this is why EMG pickups distort slightly when ran on 9v and goes away with 18v.  In all pedal deign, a good designer can implement a voltage multiplier which can turn a 9v adapter into just about any voltage desired.  As a matter of fact all new Corvus Audio pedals that run off of 9v adapters can give a clean output of 17Vpp, meaning you will almost never distort the signal unless you were doing some crazy stuff that I wouldn't recommend, but even if you did, the op amps will distort just like a transistor, nice and musical, not as musical as a 12AX7, but still musical non the less.

It all comes down to good design, as good design  with high quality components and forward thinking means that certain active designs can far surpass their passive counterparts.

In part two, we will be going over the design philosophy of the Corvus Audio DI+, the transformerless active simultaneous DI and Reamp unit.

Until next time.

As always, check out Corvus Audio for new products and be sure to like us on Facebook.

No comments:

Post a Comment