Tag Archives: DI Boxes

Is The Problem Voltage, Or Voltage Transfer, Or…?

If you’re going to fix a problem, you have to know what the problem actually is.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

voltagetransferWant to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

If you’re going to troubleshoot (and if you’re in the business of show production, troubleshooting is inevitable), there are two basic rules:

1) You have to know what the device is supposed to do.

2) You have to know how the device does what it’s supposed to do.

There are many layers of doing 1 and 2 effectively. The deeper you go, the more problem solving you can do. Gaining the knowledge required to peel back more and more layers is a long process. Decades long. I’ve had my hands in pro-audio since I was a teenager, and with about 20 years under my belt I’m finally starting to feel like I get what’s going on. In part, that’s because I’m getting more and more acquainted with the oceans of material I still don’t know. When you start to realize just how deep the rabbit hole is, you’ve been falling down that hole for a good while.

The above is a basic, foundational statement for this article, which is a follow-on to the opening “case study” from my previous post. After having a potential issue discussed with me, I ended up finding an alternate route to a solution. I took the different path because I had a suspicion that the problem wasn’t the voltage level of a pickup’s output. I figured that the real bugbear was that the voltage from the pickup was being transferred poorly, and also that the pickup’s bottom end was being lost. I considered this assumption as possible because I have a notion (not a truly detailed one, but a notion nonetheless) about how high-impedance pickups work. That is, I know that they can be reasonably modeled as a voltage source in series with a capacitance. This all comes together to form a device with a rather high output impedance in pro-audio terms. The issue with high-impedance outputs is that voltage transfer becomes non-trivial, and the issue with capacitors in series with voltage sources is that they create a high-pass filter.

Modeling Voltage Transfer With DC

For audio folks, what we’re interested in is voltage transfer. Even when amplifiers and loudspeakers are involved, and we become interested in power transfer, we achieve power transfer by way of voltage transfer. In many ways, effective voltage transfer is invisible to audio humans. It just sort of happens for us, because a lot of our gear is built to play nicely with a lot of other gear. At times, though, we’ll encounter gear that was NOT actually built to interface nicely with our existing equipment, and that can throw us for a loop. In the case of a high-impedance pickup interfaced with pro-audio inputs, we can get into a situation where we’re PILING on the gain, only to end up with a relatively weak signal. If we don’t know how the device does what it’s supposed to do, then we can start to assume that the voltage from the pickup is too low.

But that’s not the case. Piezo pickups – probably THE example of a high-output impedance device – make plenty of voltage. When mated to, say, a basic DI box, the problem is that the voltage doesn’t transfer. The input impedance of the mic pre is too low.

Before I go any further with this, I need to say something:

IMPORTANT – Audio circuits are NOT direct current. They are alternating current. Modeling an audio circuit via a DC example is not an entirely accurate picture of what’s happening. DC examples are simple to read and easy to “construct,” but they neither show the entire picture nor all the details of what’s happening.

With that in mind…

At a very basic level, the underlying issue with voltage transfer is that voltage drops when it travels across resistors. If we mentally model an audio circuit as a voltage source across one resistor (output impedance), and then have the remaining voltage travel across an additional load resistor (input impedance), we start to get a basic idea of how things can play out.

In our simplified, DC, everything-in-series circuit, the voltages across each resistor add up to the total voltage in the circuit. As such, the proportionality between the resistors representing output and input impedance matters a lot. If the output impedance is high in relation to the input impedance, a good deal of voltage will drop before ever getting a chance to drive the input. In the reverse case, only a small amount of the total voltage drops across the output impedance, allowing a healthy voltage transfer into the next part of the audio chain.

If I take a quick jaunt over to PartSim, I can build a quick ‘n dirty example circuit. This one represents one of my EV ND767a mics plugged into one of the preamps they usually “see,” which are on an M-audio Profire 2626. At a continuous level of 94 dB SPL (1 Pascal), an ND767a is rated for 3.1mV of RMS voltage output. That output can be modeled as being in series with a 300 ohm resistor. The mic-pre of the Profire can be modeled as a 3.7 kilohm resistor.


In this example, 0.23mV drops across the output impedance of the microphone. If you do the math to figure out the decibel loss, you find that about 0.67 dB was lost before the signal hit the mic pre. Even with this being a DC example, that number tracks very well with the output of the bridging calculator at Sengpiel Audio.

The above is an example of equipment that’s designed to interface nicely. What happens when a piezo pickup gets plugged into a basic DI box? That’s probably something like a 1 megohm output impedance being mated to a 50 kilohm input. The piezo can develop plenty of electrical potential. One volt RMS is +2.2 dBu, or definitely within the “line level” range. The voltage isn’t a problem at all, but the transfer of that voltage is a big deal.


Immediately, 26 dB of voltage is dropped. If the DI box steps the voltage down even further (as is apt to happen), then the signal arriving at the console pre might be 46 dB down from the original voltage supplied by the pickup. The voltage arriving at the preamp is no higher than what you would get from a “hot output” dynamic mic in front of a not-too-loud source.

But Why Does It Sound So Bad?

Now then.

If the only real downside of our “not enough input impedance” situation was voltage loss, it wouldn’t be so bad. We’d have to run our preamps a little hot, but that’s hardly a dealbreaker.

The real awfulness comes about when the AC circuit issues enter into play. As I mentioned earlier, a piezo pickup in an audio circuit naturally tends to create a high-pass filter on its output. The high-pass filter becomes less audible as the load (input) impedance goes up. The problem, then, is that a too-small load impedance causes a very marked loss of low-frequency information. The pickup sounds “clanky” or “nasal,” because all of its really usable output becomes restricted to the high-frequency part of the audio passband.

Here’s a simplified model of a piezo pickup connected to a 50 kilohm DI box. I haven’t tried to fully represent the output impedance of the pickup, so the voltage numbers won’t be right. I used a 650 pF capacitor to represent the pickup, because the simulation of the circuit with that capacitance seems to basically represent what I’ve observed in the field.



At 1 kHz, the signal is about 13 dB down from the maximum level. At 200 Hz the signal is down 27 dB. Good luck correcting that with any bog-standard EQ you have handy.

Compare that with what happens when the load impedance is 1 megohm, which is what some of my active DI boxes are rated for:


Yes, there’s still a highpass filter in effect. Even so, it’s rather less terrifying. The filter’s 3 dB down point happens at about 250 Hz, and you’re only about 8 dB down at 100 Hz. That’s hardly perfect, but it’s manageable. (A DI box or preamp with a 10 megohm input impedance basically makes the low frequency loss a non-issue.)

Once more, I need to emphasize that these are simple models. They won’t exactly represent what you run into during the course of setting up an actual show.

But they do show that the voltage generated by a troublesome audio source is not necessarily the root of a given problem. Poor voltage transfer and circuits that mess with frequency response (when presented with a small load impedance) may be what’s really hurting you.

Your Pickup Probably Isn’t Broken

A guest post for Schwilly Family Musicians.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

Want to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

In some cases, trouble with “drop in” or “stick on” pickups isn’t really a problem with the pickup at all. Here’s why.

Electric Guitar: Mic Or DI And Why?

Mic the cab if the specific sound made by the cab is “make or break.” Otherwise, you can go direct if you have the right tools.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

Very reliable, but not the only valid technique.

So, my last article was precipitated by a question asked by Dee from The Black Smoke Gypsy Band. The group was debating whether to go direct with their electric guitars (in some way) or just mic the cabs. The question had inspired a bit of back and forth in the band, and Dee wanted to know what the right answer was.

Of course, I answered him with what I knew to be the most correct answer in pro-audio: “It depends.”

That’s also the most frustrating and infuriating answer.

The conversation didn’t stop there, though. A tech can’t just throw “It depends” at someone and walk off. You then have to talk about what might work for the questioner, and why.

Micing A Cab: Reliable and (Relatively) Simple

In my article about looking at electric guitar rigs as a kind of vocal (or any other acoustic instrument), I got at the idea that sticking a microphone in the area where “the noise comes out” is a simple and effective choice. All the questions of exactly how an instrument makes the sound it does are back-burnered. You just figure out where enough, decent-sounding level is present, and stick your transducer there.

A transducer is a device that converts one form of energy into another, corresponding form of energy. Mics transduce sound pressure waves into electrical signals. Speakers transduce electrical signals into sound pressure waves.

With the electric guitar as we have come to know it, the sound almost always comes out of a loudspeaker that’s mounted in a cabinet. That cabinet may also house an amplifier (a “combo”), or it might be part of a “stack” with a separate amplifier “head.” In any case, you don’t necessarily have to spend a lot of time philosophizing. The sound comes out of the speakers, so you mic the speakers.

Now, I don’t want to downplay the possible complexities of micing a guitar cab. Indeed, a lot of ink (and sometimes blood) has been spilled on all the intricacies that you can get into when micing a guitar rig:

Should you pick the best-sounding loudspeaker and put the mic up close?

If the mic is close to the cone, what area of the cone should it be closest to? (The dust cap area usually has more high-frequency information than the cone edges.) Should the mic diaphragm be parallel to the baffle? At an angle? Which mic should you use anyway? Should you use more than one mic? Should you try to time-align those mics, or should you pull one back slightly so that phase effects cancel out the frequencies you dislike?

…or, should you pull the mic back far enough to get the whole cab? (This rarely happens in small-venue work, but hey, you never know.)

Things can get very wooly.

For a sound reinforcement human, a lot of the time you end up making choices that are based on simple utility, and not “the very best sound possible, ever.” You close-mic a cone because you need maximum separation between the guitar and everything else making noise on stage. You pick the cone you do because you can get the mic stand in the right place easily, and because the mic setup will be the least in the way of the guitar player (and everybody else). You go for a placement that’s somewhere between the dust-cap crease and cone edge, figuring EQ will fix anything you don’t like.


Why mic a guitar cab? Why make that choice over other choices?

In the end, it comes down to this:

You should definitely use a mic for electric guitar if the specific sound produced by specific speakers in a specific cab is a crucial and non-replicable part of the guitar player’s sound.

See, electric guitar players can be incredibly choosy about their sound. Pretty much everything has an effect. There are folks who will spend days (if not more) trying to figure out which material for a pick has the best sound when used with their setup.

I’m dead serious.

In some cases, a critical, irreplaceable part of a guitar player’s sound is a certain brand and make of loudspeaker, with a certain amount of “miles” on it, with a certain amount of power flowing through it, that has been bolted into a specific kind of cabinet. If you were to even do something like replacing that speaker with a brand-new unit of the same model, their tone just wouldn’t be there.

You have to mic that. There’s no way around it. All other tricks and tactics are unacceptable.


There are plenty of guitar players for whom this is not the case. There are lots of folks who like the sound of their cab just fine, but who aren’t “married” to its very specific effect on their overall sound.

So, to restate, you definitely want to mic an electric guitar rig if you are unable to get a sound that’s acceptable via some other method.

(Whether or not a guitar player having “their sound” is a good/ bad/ selfish/ team oriented/ stupid/ smart/ pleasant/ unpleasant/ crazy/ sane/ impossible/ doable thing in the context of the band in a particular venue is a whole other question, by the way.)

Going Direct: Might Be Easy, Might Be Hard

There’s a lot of mythology that goes around in guitar circles regarding the practice of going direct. It usually boils down to “going direct sucks.”


If you want to hear a direct-in guitar that sounds good, just come on down to Fats Grill when Blues 66 is playing. Leroy’s guitar sound has a workable bottom end, a plenty-usable midrange growl (that I sometimes add to a bit, depending on the night), and a top end free of annoying hash and sizzle (that I sometimes low-pass anyway, again, depending on the night). He has no amp – just a POD HD…something…could be a 500.

It sounds like guitar to me, anyway. Nobody’s ever complained about it. The same thing was true for a band I used to work for, called Puddlestone. We ran the guitar processor through a cab-sim DI. It sounded fine. Great, even.

The point is that going direct with an electric guitar is entirely possible. You just have to use the right tools, and know which part of the signal chain you want to pull the line from.

So, where do you want to go to get that signal split?

When running an electric guitar direct to the console, you should take your signal at some point that is post the devices that have the greatest contribution to the essential components of the player’s sound.

This actually holds true for micing, because (as I said), the loudspeakers and cabinet may be an essential component of the player’s tone. If they are, then you have to get your signal “post” the loudspeaker. That means a mic.

In other situations, though, you have a number of different possibilities. For some folks, the essential components of their sound are created through processing. This processing may happen through stompboxes, or a rackmount processor, or both. When that’s the case, you need to take your split from a point that’s downstream of the processing chain, usually straight from the output of that process chain.

For other players, an essential component of their sound is the power amplification itself. There are coveted guitar amplifiers that produce a signature tone by driving, a power tube (or tubes) into saturation. If that saturation is an essential to the guitar player’s sound – not just a nice extra, but a critical piece of the puzzle – then you need to find a way to take a split from the output of the power amplification section. You can do this with a DI that has a 20+ dB PAD (Pre Attenuation Device) included, along with a parallel output to feed a cabinet. The parallel output is very important, because:

If you take a split from a point post the power amp, you must be very careful that an appropriate load is still being presented to the amplifier. Failure to do so can mean a costly repair.

Transformer-coupled amplifiers must have a minimum load present, and that load must be able to dissipate the power from the amplifier. Otherwise, the output transformer can be cooked by an internal arc, or other components can be wrecked by “flyback” voltage. When it comes to putting a DI on a power-amp output, you’re connecting a device that probably is NOT going to be seen as a proper load. Most modern, solid-state amplifiers don’t exhibit this behavior, because they don’t use output transformers, BUT ASSUME NOTHING.

When in doubt, parallel connect a suitable load to the amp.

Now – there’s one more thing about going direct.

Remember up there where I said that you have to use the right tools? This is an essential of getting a direct signal that actually sounds good. A generic, run of the mill DI box is not, in itself and without help, a sufficiently good tool for this job.


It all comes back around to those loudspeakers in guitar cabs.

Your average guitar loudspeaker starts significantly rolling off the high frequency information in the signal after about 3 – 4 kHz or so. For example, here’s a frequency response graph that I made from data provided by Eminence for their “Red, White, and Blues” loudspeaker. After about 3 kHz, the frequency response is “goin’ downhill in a decent hurry.”

This rolloff is a critical component of what we know as “the electric guitar sound.” The problem is that a half-decent, bog-standard DI doesn’t roll off the high end. Even cheap ones can be essentially “flat” to 20 kHz. The basic DI actually preserves high-frequency information that the basic guitar speaker just chucks out the window (to one degree or another). This is why a lot of people think that direct-fed electric guitar “just sounds bad – all fizzy and high endy.” That’s actually what a guitar signal is, right up until it gets pumped through a loudspeaker.

Whaddya gonna do?

Some guitar processors have a dedicated direct out. These direct outs almost always have “cab simulation” applied, and so they sound reasonably like a miced rig without any other intervention. The fancier they get, the more they take into account the effects of different speakers loaded into different cabinets. Some even simulate different mics. Some even add a bit of “room” sound to the mix. Most importantly, though, they low pass the raw signal in a way that’s similar to a loudspeaker.

If all you’ve got is a basic DI that’s pulling a signal from somewhere, there’s still hope. If you can find an EQ to insert on the guitar channel, one that has a sweepable low-pass filter, you may be able to “build your own” cab simulator. Just twist that frequency selector knob until you get the frequency in the neighborhood of 4 kHz, and then tweak a bit more until your taste is as satisfied as possible. From there, you can do more shaping with other EQ bands.

To close, I’m going to leave you with some bullet points about when it’s good to attempt a proper DI solution. Remember that there’s an assumption here: You’ve already determined that any particular cabinet is not an absolutely crucial part of the guitar player’s sound:

  • If the amp and cab sound terrible and/ or painful. (It can happen to anyone. I’ve heard some ostensibly all-tube rigs that sounded dreadfully screechy.)
  • When you need to run high monitor levels with the guitar without worrying about mic feedback. (This does happen, just not that often.)
  • When you can’t get good separation between the guitar rig and everything else on stage. (This can happen when a guitar rig is run a bit quietly, and then a bunch of guys who want to be disproportionately loud with everything else get on stage.)
  • When you can’t get even a half-decent mic placement for some reason.
  • When you don’t want to chew up stage space with mic stands.
  • You think it would be cool to try something different, and the player is on board with that idea.

Micing can be done well, and direct input can be done well. You just have to figure out what’s appropriate, and then execute your chosen strategy in the right way.