Active Pickups

[MikeRivers]

I know there's not much technical information about electric guitar things other than what gets passed around on forums, but I'm trying to find some real, measured data about the output level of active pickups.

 

Now I'll admit that what I'm asking for is pretty vague and probably covers a pretty good range - the brand and model of the pickup, the strings, how hard you're playing the guitar, and so forth. Anyone know worst case or even typical output levels?

 

I've been having a discussion with someone on another forum that unfortunately has degraded because I want data and he says he doesn't care about data, he knows what he hears. The discussion centers around whether the "instrument" input on an interface can handle the output of an EMG pickup without distorting.

 

Having measured the characteristics of the the input myself, I know its maximum input capability. It starts clipping at +18 dBu which is about 17v peak-to-peak.

 

Given that the active pickup system is powered by a 9v battery, it can't have a peak-to-peak output voltage of greater than 9v unless there's an output transformer there that boosts it or the 9v battery powers a DC-DC converter to provide a higher working voltage for the amplifier circuit. Given the space available inside a guitar and the size a transformer would have to be in order to handle that level and the lowest frequencies produced by a guitar string, that's not likely. Nor is boosting the 9v supply likely. So whatever problem he's having, I don't see that it can be as a result of input clipping.

 

He quoted something from EMG that said (paraphrased) that the active pickups may be too hot for some amplifiers and that it may be necessary to turn down the amplifier's input gain control (if it has one). But I looked all over their web site and catalog and the only reference I could find to voltage was the 9v battery, and the only reference to dB had to do with gain of the built-in preamp, dynamic range and signal-to-noise ratio. So they don't seem to publish any data about output level.

 

The only electric guitar that I have handy, when I'm slamming it as hard as I can, measures about 6v p-p. EMG has the 'Aferburner' active electronics that they state adds 0-20 dB of gain to the pickup. You can add 20 dB of gain to quiet playing and get a pretty hot output from some gentle picking, but 6v increased by 20 dB is 60v and you won't get that out of guitar electronics powered by a 9v battery without a pretty sophisticated design.

 

So it could be that his problem has nothing to do with the instrument input on the box, but rather that he's overdriving the electronics in the guitar. Does that happen? Does anyone know? Can you turn the knob up to 11, do the windmill trick, and see square waves coming out the jack?

 

Surely a company with as strong a reputation as EMG wouldn't build a pickup that required a special amplifier. Active pickups are generall a little hotter than passive ones so you know that something's going on, but both active and passive pickups are designed to work with guitar amplifiers, and the input characteristics of guitar amplifiers are probably reasonably uniform across models and brands since there are so many guitars owned by people who aren't system engineers.

 

So what's up with that?

[philbo]

I would think that the input impedance of the amplifier would be a major factor here. The Zin would determine the current load on the pickup or PU preamp (as would, to a lesser extent, cable capacitance).

 

You are right, there is no way a 9V powered active pickup will ever output more than 9V; more likely it would top out at +/-4V peak, then would clip.

 

However, I don't really understand what your question is...

 

The advantage of active pickups lies primarily in:

a) Allowing the use of active EQ right at the guitar, and

b) Allowing a guitar-mounted volume control

[MikeRivers]

 

I would think that the input impedance of the amplifier would be a major factor here. The Zin would determine the current load on the pickup or PU preamp (as would, to a lesser extent, cable capacitance).

 

 

The idea of active electronics inside the guitar is that you shorten the length of the cable that loads the pickup, and you can provide a known, and presumably optimum impedance load for the pickup.

 

I don't really know how they deal with the output impedance of the active electronics, though. The high impedance input of an conventional instrument amplifier is a significant part of the circuit that, with a passive pickup, includes the cable capacitance and the inductance of the pickup. The active electronics may simulate some of that. This is part of what a real ReAmp box does. But the simplest thing is to provide a moderately low source impedance.

 

 

You are right, there is no way a 9V powered active pickup will ever output more than 9V; more likely it would top out at +/-4V peak, then would clip. However, I don't really understand what your question is...

 

 

Oh! Did I forget to ask a question after all that? OK, how many volts does an active guitar pickup put out? This probably isn't a general question, but if anyone knows that spec for a particular pickup system or has measured it, that's what I'd like to find out.

 

The other thing is whether the gain staging is such that it's possible to drive the internal preamp to clipping with too much gain and a too-hard strum. I'd like to think not, but if it's capable of amplifying the pickup by 20 dB and the pickup is putting out 5-6 volts, that's more than the output stage can put out with only 9V power, unless the amplifier actually runs at a higher voltage. A DC-DC converter isn't hard to make, so the output stage could be an op amp running on

[philbo]

 

Oh! Did I forget to ask a question after all that? OK, how many volts does an active guitar pickup put out? This probably isn't a general question, but if anyone knows that spec for a particular pickup system or has measured it, that's what I'd like to find out.   The other thing is whether the gain staging is such that it's possible to drive the internal preamp to clipping with too much gain and a too-hard strum. I'd like to think not, but if it's capable of amplifying the pickup by 20 dB and the pickup is putting out 5-6 volts, that's more than the output stage can put out with only 9V power, unless the amplifier actually runs at a higher voltage. A DC-DC converter isn't hard to make, so the output stage could be an op amp running on

[MikeRivers]

 

I did find this on the web for the EMG P90 pickup, which says (in the 'Headroom' section) it can put out +/-4.5Vpk (apparently they're using a rail-to-rail op amp or similar). They also mention using two 9V batteries in series... So I guess they're out there...

 

 

Thanks for that find. 9V p-p is still about 6 dB lower than the maximum input level of the instrument input of the PreSonus 44VS that I measured, so at least that combination wouldn't be unhappy.

 

I suppose the next question is how much level a typical guitar amplifier can take before clipping the input. A quick perusal of The Tube Amp Book shows that designs go both ways - some have a 1 m? or so pot between the input jack and the first stage (either tube or solid state) which would mean that you can turn down any level low enough not to clip. Other amps have the input going directly to the first stage with the volume control being at the output of the first stage. A tube with 150-250 volts on the plate should give a huge amount of headroom for a pickup. An op amp should be able to go pretty close to the typical

[JeffLearman]

IMHO, with active pickups, you should use a line input, not an instrument input. On a lot of computer interfaces, these two are the same (basically, a hi-z mic input, which is little different from a line input with a wider gain range and higher input impedance). If it clips, just turn down the input level! Ditto for a guitar amp. What's the problem?

[MikeRivers]

The problem isn't mine. I'm just trying to gather measured data, not anecdotes. What's the problem?

 

 

 

I just thought that some folks here might be intersted in some actual data about both sides of the instrument interface. I like ot know why things don't work (if they actually don't work) and I'd like to think that others care, too. Too many people buy stuff they don't need because they don't understand the real problem. I'm only trying to be helpful, but sometimes it seems futile.

[JeffLearman]

It was a rhetorical question. My suspicion is, the "problem" is the guy is simply wrong, and you're right. It's terribly easy to be misled in audio, without doing a double-blind study. Cf Ethan Winer's video, which I bet you've seen:

[video=youtube;BYTlN6wjcvQ]

 

Of course, with that Afterburner 20dB boost you mentioned, who knows.

 

Sorry that I can't offer any real data, not having any active pickups. You see, I don't want anything between my pickups and my amp but wires, because otherwise it robs the sound of all its mojo. ( :wink: )

[MikeRivers]

 

It was a rhetorical question. My suspicion is, the "problem" is the guy is simply wrong, and you're right. It's terribly easy to be misled in audio, without doing a double-blind study. Cf Ethan Winer's video,

 

 

I was there. And as JJ said, if you can't measure what you're hearing, you're measuring the wrong thing.

 

It's hard to say that the guy with the active pickup was wrong because he never said you couldn't use it with the interface, he only said he had problems with active pickups going into other things. I don't doubt that he heard something he didn't like, it's just that I never found out what it was.

 

I was just playing with a Zoom H4n this morning, comparing it to a TASCAM DR-40 that I'm reviewing, and although I don't have any active pickups, I can predict that a pickup like the EMG with 18v power (

[JeffLearman]

Please let us know when you've posted that review. I have a DR40 and a bandmate has the H4, and they sound pretty much the same to me. The feature set seems pretty similar too (compared with the H2n, that is).

[Hard Truth]

Some active pickup sets use two 9v batteries.

 

With both basses and guitars I have not experienced distortion from active pickups that could not be prevented by adjusting the amps preamp gain. When using a direct box with a built-in pad, I have rarely experienced distortion from active pickups that could not be prevented by adjusting the mixers input gain. Once I had to insert a pad before a mixer's input for an aggressively played active bass.

 

Note that distortion can be a symptom of dying batteries in an active instrument.

[MikeRivers]

Well, now that we've effectively changed the topic of this thread:

 

 

Please let us know when you've posted that review. I have a DR40 and a bandmate has the H4, and they sound pretty much the same to me. The feature set seems pretty similar too (compared with the H2n, that is).

 

Oh, I plan on spamming the forum when I post the review.

 

I didn't try to do a shootout between the DR-40 and the H4n. I'll leave that to someone who can write it in 2500 words instead of my usual 10,000. But I did borrow an H4n to see if the TASCAM is really a Zoom killer (as some have asked, but never answered, on various forums). My take on that is that if you're a musician looking for more of the things in your computer that you can put the palm of your hand, the Zoom has the edge - things like a real 4-track mode, metronome, a long list of effects including some amplifier simulators (after all, this is Zoom's bread and butter), and a 1/4" input that works with at least normal pickups. The TASCAM seems to be more suited for versatile field recording that you can do sound-on-sound type recording if you must.

[philbo]

The problem isn't mine. I'm just trying to gather measured data, not anecdotes. What's the problem? The interface in question has inputs designated "Mic" and "Instrument." Nothing designated "Line." The user wants to connect a keyboard and has read some place (perhaps in the manual, perhaps on a forum) that the manufacturer advises against connecting a line level source to the instrument input as there is the potential for damage. I called BULL{censored}! and advised the user that he could do no damage to either his keyboard or the interface by connecting it to the Instrument input. I recommended that he start with the output volume of the keyboard set fairly low and take a bit of time to find the combination of keyboard output level and interface input gain that gave the least noise, and also assure that he had adequate headroom. I have tested (and reviewed) the next model up from this interface which has two "Line" inputs as well as two "Instrument" inputs. I believe that the Instrument inputs are the same on both models. I found that at minimum gain, the Instrument inputs clip at +18 dBu in, which should be ample to accommodate just about anything that he might want to connect, particularly if there's an output volume control. I assured him that the high impedance of the instrument input wouldn't be a problem, at least for a short and properly shielded cable. There's an issue with the fact that the Instrument input is differential - short version: It works better when fed from an unbalanced source. Problem solved, I thought. Red Herring steps in and comments that he's had problems in the past using active pickups. Doesn't say what kind of problem. The only thing I can think of that could cause a problem is a high enough output level from the pickup to clip the input. So I wanted to find out just how "hot" these things really are. Apparently not hot enough to clip this interface, but EMG (pickup manufacturer) cautions users that the pickup at full volume might overload some amplifiers. Now there's some data that actually puts a number to the output level. I'm satisfied that even that pickup will work with this interface, even though that wasn't what the original poster was asking about. Non-Problem solved. I just thought that some folks here might be intersted in some actual data about both sides of the instrument interface. I like ot know why things don't work (if they actually don't work) and I'd like to think that others care, too. Too many people buy stuff they don't need because they don't understand the real problem. I'm only trying to be helpful, but sometimes it seems futile.

 

Ah, now I understand the issue... Since most keyboards have a volume control, you'd simply need to turn down the output level to feed an instrument input. Not that big of a deal; I've done it many times & it works fine.

 

If it is an organ with a continuously variable expression pedal, you'd set it with the pedal all the way down so you're dealing with the maximum output level of the instrument, then adjust the keyboard volume knob. (and maybe lock it down with gaffer tape if anybody else will be playing it! )

 

I do something similar all the time with my guitar synth; it has a line out level, but I feed it to my guitar amp instrument input. Note my amp (a Marshall Valvestate 2000 AVT) has a tube input stage, which is basically immune from damage due to overdrivng the input. I might damge your ears, but it won't hurt the amp.

 

As I mentioned earlier, I'd be a lot more cautious with solid state input stage, especially an FET input stage.

[MikeRivers]

 

Ah, now I understand the issue... Since most keyboards have a volume control, you'd simply need to turn down the output level to feed an instrument input. Not that big of a deal; I've done it many times & it works fine.

 

 

I didn't mention it here because maybe I didn't remember correctly, but it seems that they guy who asked about the keyboard and instrument input said he sometimes uses stomp boxes with his keyboards. Well, those are designed to go into instrument inputs, so as long as he can control the level of his keyboard so that it doesn't clip the stomp box (or that he doesn't have to because it's not a problem) then he has something that looks more like what a guitar amplifier expects as an input source than just a turned-down keyboard. No problem.

 

 

I do something similar all the time with my guitar synth; it has a line out level, but I feed it to my guitar amp instrument input. Note my amp (a Marshall Valvestate 2000 AVT) has a tube input stage, which is basically immune from damage due to overdrivng the input. I might damge your ears, but it won't hurt the amp. As I mentioned earlier, I'd be a lot more cautious with solid state input stage, especially an FET input stage.

 

 

If amplifier designers have any sense, they'll put clamping diodes at the input to protect it. This is pretty much standard design practice for op amp inputs today. And with an input that doesn't clip until the level gets to +18 dBu it's doubtful that the level will ever get that high anyway (but that's why I was asking the initial question here - to find out how hot the output from a guitar with active pickups (which is presumably greater than a passive pickup) can get.

[Zooey]

I found this post from a moderator at DiMarzio's forum:

 

 

Passive pickups generally top out around the 500mv area or about 0.5V, but because EMG's are active, the preamp boosts the output level to around 3-4 times higher than your typical passive pickup. The actual output voltage for EMGs are: EMG 81/85 RMS: 1.25V (1250mV) Peak: 1.75V (1750mV) (note: the EMG 85 has more perceived output as the peak resonant frequencies are more in the mids than the 81 which is more in the highs/upper mids)

 

 

http://www.dimarzioforum.com/forum/index.php?topic=870.0

[MikeRivers]

This should absolutely not overload any guitar input. I guess the guy who had "problems" had some other problem.