Tranformerless Active DI and Reamp circuits

A couple of days ago I talked about the myths behind active vs passive circuits.  I also said that part two would be out to talk about the newest Corvus Audio Product, the DI+, a transformeless simultaneous active DI and Reamp box.

The transformer is a great device, it can isolate grounds, step up or step down voltages, can change the effect of a load on a circuit.  For many years, the passive DI setup was the most common way to manufacture DI and Reamp Devices.  They are simple, they only have one component, connected to jacks and a ground lift.  The unfortunate effect of these passive DI units is that the voltage conversion is intended for going into a mic preamp.  There are a few problems with this.

-The voltage is stepped down to mic level, only to be amped again throught the preamp, increasing noise
-The mic preamp has a low input impedance
-Mic preamps are rarely color or distortion free
-The final input impedance to the guitar is still relatively low
-The transformer has a high distortion at low frequencies

The big problem is that the transformer overcompensates the voltage drop needed to get the pickup to line level, it has to do this because the larger the turns ratio is, the more the original load's effect load to the pickup is magnified, meaning, if you have a small load, in order to get that load to look much larger to the guitar pickup, you need a large ratio on the transformer.  Since the average of 28K to 47K load of a line level device is still to low for the modest small ratio transformer (the guitar pcikups load would still be too low) a larger reduction in voltage is needed to get the signal voltage low enough to be capable of going into a mic preamp.  A mic preamp's impedance is even lower, usually in the 1K region, however due to the large step down of the transformer, the effective load to the pickup is usually around 470K.  This is still way to low as most guitar amps have a load of 1M.

There is another problem with transformers in general.  From one winding to another, they act like a capacitor, they do not let low frequencies through.  Unfortunately, storing a low frequency charge requires a large transformer, much larger than what it in a typical DI device.  The result is that the cutoff point for the high pass filter is in the audible range and does effect the perceived sound.

One huge problem with passive DIs is that with a mic preamp load they tend to only load the pickup of about 470K, when the parallel out is being used to monitor through the amp, the amp's impedance is now in parallel with the transformer load to the mic preamp.  470K in parallel with 1M in parallel with the volume pot, lets say 500K for a passive pickup is 195K.  195K with the DI versus the 333K if it went straight into the amp.  Try this, plug your DI in to a mic pre, turn it on and plug it into your interface, plug the parallel out into an amp and plug your guitar into the DI, play through it, then remove the XLR cable from the DI, removing the transformer from the mic pre, the tone coming out of the amp will come to life as long as the DI is not plugged into the mic pre.  Even a small load change can have a large effect on the tone.  It is also effect on the tone going to the mic pre as well and consequently, the tone you are recording

To combat the load problem of passive DI's some companies began loading the transformer with an active buffer.  The winding ratio would remain the same so the stepdown voltage would remain the same.  The benefit of adding a DC power supply and active components was that the load to the guitar would be the same when the amp was plugged into the parallel out, the loat would raise to that as to what was typical without the load.  There still remains the problem though of using the transformer, as it is still producing a bandpass filter that is causing distortions at both high and low frequencies.

So why are transformers used even in active devices?  The designers want an easy way to electronically isolate the signal.  That or they like keeping the people ignorant to the fact that transformers are not the only way you can isolate grounds.  An easy way to isolate ground is by not having a ground connected to a jack if you know that jack will always be in a setup where there will be an earth ground connected elsewhere.   You only need one earth ground connection to an active device, no other grounds are needed, the shielding of the cable is actually supposed to be left open on one side and not connected to ground on two ends.  The only reason guitars are connected to both ends is that the guitar itself does not have an earth ground connection and gets it from the amp.  In case of the DI, we know that we will always be using the DI output so if we permanently remove the ground of the parallel out, the guitar amp is not grounding to the DI as well as the computer/interface/mic pre.  This removes the ground loop and is good because the ground shielding from the parallel out to the guitar amp is only connected to the guitar amp, any noise that is present across the signal gets grounded to the amp, the amp is not earthed to another device, and the guitar itself has an earth connection through the recording device connected to the DI.

Still to this day all manufactures are connecting all the jacks to ground and using transformers to isolate the grounds.  This reduces linearity of the signal and increases the cost of the DI dramatically.  All this just because the designer wants to have everything connected to ground, even when the best option is to have everything not isolated and connected to only one earth.  Then you can remove the transformer and get the benefits of the linearity of a signal that has no direct capacitance or inductance.

We can further increase the design by dealing with the noise level and coloration of going through a mic preamp.  Since the guitar pickup's level is only slightly higher than line level, if we use an active buffer, a volume control and an unbalanced to balanced converter, we can attenuate the exact level needed for that pickup to the line level desired, we removed the massive attenuation to mic pre level and we have removed the need for the colorful mic pre.  We now have a line level DI output that can amplify all the way to DC, ultra linear, virtually no distortion.  But we are not done.

I said before that 9v supplies cannot correctly power guitar pickups.  To combat this a pump charge converter wired in a basic configuration or even a voltage multiplier setup, can provide the output voltage necessary from a 9V supply.  All Corvus Audio products that run on a 9v adapter can supply a clean output voltage of 17Vpp, meaning with the DI, no pickup would ever distort the device, and the line input coming from the DI would distort way before with the volume on maximum.

Lets go ahead and take a look at the Corvus DI+

The benefit of this DI is that it allows you to use it as a DI and Reamp at the same time.  There are separate connections for the DI and the reamp, unlike some devices that can do both.  With other devices, if you want to go from DI to reamp, you have to switch cables or switches/settings on the device every time.  The Corvus DI has two separate circuits, one for the DI and one for the Reamp, which outputs are mixed together using a  mixer buffer that goes to the amp output.  There is no parallel output, you have an amp output that is a mix of the buffered raw guitar and the signal from the reamp.  With a simple send routing setup, you can record your DI monitoring through your amp.  You can turn the send in your DAW on and press play, and the DI track from your DAW will instantly come out of your amp, without needing to touch your DI box.

There is one ground lift switch that is to be engaged when you are using both the DI and reamp portions at the same time,  if you are using just the DI or just the reamp, the ground lift should be off.

Here is a little block diagram of the internals of the DI, as you can see the guitar buffer and the Reamp outputs are being mixed together to one amp out.  To top it off there are TWO amp outputs, yes the DI+ has a ground separated Y splitter for running two amps at the same time.  The DI+ can be used as a standalone active buffered Y splitter so you can record, practice or even gig live with two amps at the same time, without needing to use the reamp or the DI features.

The DI output is active unbalanced to balanced using the standard TRS cable, no more XLR.  The reamp input is also active balanced to unbalanced using the TRS cable.  No more dealing with mic preamps, except for the guitar input, all inputs and outputs of the DI are line level, ready to go direct into your converters and outboard gear.

This device will most likely cost $160 USD including shipping to the US and $180 USD including shipping to the EU.  They may actually be cheaper than that when they are available to buy within the week.

Until next time.

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

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.

Black Box Theory

The Corvus Audio webpage has been up for some time.  In that time I never thought to run a blog where I can give out informationals to dispel a lot of myths that go around in the music world as well as give advice, tips and tricks that I learned first hand the hard way.  Over on the Andy Sneap, Ultimate Metal sub forum, it would seem too arrogant for me to post an informational about a topic that may have been discussed before to have a thread that was informational but overall trivial and left a discussion of electronic gurus from all levels of experience to fight out how they understood electronics.

I don't care to write books to make a profit, I always wanted to give the information that I had for free, mostly because my knowledge in the music world came to me at no cost and I have learned additional things a long the way.  The best thing I realized I could do was to make a blog on my very website, hosted by Blogger of course, to allow me to give our such free information.

The first topic is a rather psychological one, one of the belief that visual representations can change perception of aural stimuli.  When I first started building stompboxes this was never something I had in mind, but as time went on I developed this love for the Black Box.  What I mean is, when I first started making stompboxes, I wanted to make them different colors, different graphics and sized to the electronic components.  After hand painting my first two boxes white, I realized that it took too much time without proper equipment, in my case I wanted to power coat, but the company that I got the boxes from offered every enclosure in a textured flat black.  When I first put everything together, I absolutely loved the look.  The problem was that because black was used by a pedal, I couldn't use it again for something I was more proud of, something I felt black really deserved.

Then it hit me, use the same enclosure with the same power coat from the factory for all the pedals.

I realized that this was something different.  To date, no other company has made stompboxes in only one color, let alone black and to make them the same size, with the same simplistic graphic means that the only way to tell one pedal from the other is by the the small labeling noting what the pedal is.  The layouts for all pedals being the same, means they all look the same, a very simple plain, but still aesthetically pleasing appearance.  The user won't be enticed to like or dislike the sound based on its appearance, certain colors effecting the way they perceive the sound.  Down to the overly simplistic programmer or electrical engineering font, the only sense of attitude is from the Corvus Audio font.

The enclosures still have a vibe about them, something that does look good to see, but they have an emotionally neutral vibe, and as far as I am concerned, that is the way I like it.

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