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.