4002 Data

[bassduke49]

Yeah, you're getting all of this, right Paul??

Sure. It will all fit onto pp. 152-173.

The book is 144 pages, though.

[cjj]

As to the lo-Z XLR out on a 4002 and trying to do it without a separate winding on the pickup, you would have to use a step-down transformer right off the pickup coil, custom wound to match the coil impedance and have an output impedance of 600 ohms. The transformer output winding would be the + and - of the XLR, with ground coming from the ground circuit.

Yes, XLR connections are "balanced" or what is called "differential" in most non-audio applications. For this, the ground has nothing to do with the signals and is just for shielding purposes. What counts is the "difference" between the signal levels and the circuitry in an XLR amp input actually subtracts one from the other.

Why? It's a LOT quieter, with respect to noise and interference! If some nasty noise signal gets picked up on the cable, is gets on to both wires and will have about the same level on both, lets say +2 volts. So, when the subtraction is done, the noise gets cancelled out: 2 - 2 = 0. Since the audio signal is opposite polarity on the two wires, +1 on one, -1 on the other, it does not: 1 - (-1) = 2.

Now, in the case of the 4002 pickup, with it's separate coil, we have another really cool thing going on besides generating a differential (balanced) audio signal. Since the nominal impedance of an XLR type circuit is 600 ohms, this coil has a LOT less wire/turns than a coil that measures at 7k. A 7k coil will have nearly 12 times as much wire, and therefore a lot higher inductance. The inductance of the coil will have a big effect on the frequency response and therefore the sound of a pickup.

With fewer turns, a pickup's output response will be flatter and have a much better response, especially in the higher frequencies. Ideally, a coil with one turn would be almost perfect! Of course, with fewer turns, you get a smaller voltage out of the coil/pickup (which starts making a balanced connection mandatory). With 1 turn, the signal would be so small it would be hard to even tell it was there - in other words, you'd need a good pre-amp. This is something that can be done with active pickups.

Another way to deal with this is to use a transformer as John suggested. But with a low number of turns on the coil, the transformer would be reversed to step the voltage up. But then you have to worry about the effects that the coils in the transformer will have (dang, nothing's free, huh?)...

[aceonbass]

The secondary windings on Sepp's 4002 measured at 700 Ohms. Scott Pope however has told me that the output was so low as to be almost useless. I thought it'd be neat to be able to bypass the tone controls and go straight to a board for recording, but I guess a 15K signal into an XLR jack would blow out the board?! I'm thinking it may also be possible to tap a signal directly off of the pickups and running each channel to a mono switching jack like RIC uses on a ROS jack. I'd have to use a mono switching jack because if I shorted them together in a standard mono jack, the channels would bridge and affect the tone and switching in the rest of the bass when I was plugged into the standard mono jack. This way I'd still have an unaltered, pure signal that I could record with.

[cjj]

The secondary windings on Sepp's 4002 measured at 700 Ohms. Scott Pope however has told me that the output was so low as to be almost useless. I thought it'd be neat to be able to bypass the tone controls and go straight to a board for recording, but I guess a 15K signal into an XLR jack would blow out the board?! I'm thinking it may also be possible to tap a signal directly off of the pickups and running each channel to a mono switching jack like RIC uses on a ROS jack. I'd have to use a mono switching jack because if I shorted them together in a standard mono jack, the channels would bridge and affect the tone and switching in the rest of the bass when i was plugged into the standard mono jack. This way I'd still have an unaltered, pure signal that I could record with.

The balanced audio specifications that cover XLR type signals don't really define a signal level, so whether or not it would "blow out the board" would depend entirely on the board. Most professional audio equipment (the main thing that uses XLR connectors) is expecting signals in the +4dBu range. That's 1.23 Volts RMS, or 3.47 volts peak to peak, which is probably a lot higher than you will get out of a guitar pickup. Passive guitar pickups are more in the range of -20 dBu, which is 0.077V RMS or 0.22V p-p. Hi-gains and hum buckers, with more windings may be in the range of -10dBu (0.24V RMS, 0.69V p-p), but still far below +4dBu.

Of course, another thing that uses XLR connectors and balanced signals are microphones. These have nowhere NEAR +4dBu outputs, sometimes as low as -50dBu (2 mV) and have to go into a microphone pre-amp before they can connect to equipment that wants to see +4dBu signals.

Now, I would never think that the XLR output from a 4002 would be at a +4dBu level and would assume it needed to go into some sort of pre-amp as opposed to going directly to a board. The same thing will be necessary with normal pickups, some sort of amp will likely be needed.

Another thing the pre-amp does is to provide impedance buffering. If you took your standard pickup output and ran it into a 600 ohm XLR input, it would be just like hanging a 600 ohm resistor across the output. And as we've seen before, this may not be a great thing to do, it will likely load down the output so much that you'll get even lower signal levels into the input.

Now, the 600 ohm rating is a "nominal" rating and this can vary quite a bit. Most of the time, impedance matching is not really done in audio applications. The frequencies are so low that things like reflections from mismatches are negligible due to the relatively short cables used. What is generally done is what is called "impedance bridging" where a low impedance output drives a higher impedance input. In this case, the driver generally has enough output current to not be loaded down by the input - hence 7k-15k pickups driving 500k-1M inputs. Now, with XLR type units, the impedances are usually much closely matched, 700 ohms vs. 600 ohms is well within the usual range...

[cassius987]

I once wired up just the "XLR coil" of my 4002 pickup to see what it would sound like. It was very, very quiet but the tone was fine. When I used a preamp to get it loud enough it didn't sound very different than the main coil to me except it was a lot better at picking up string noise artifacts. I also tried wiring it in parallel with the main pickup but again, the effects were negligible. I finally opted to leave it out altogether.

When you consider how often we use external buffers that come with balanced line-level outputs, and then think about the limitations of a passive pickup to drive a balanced line-level signal effectively, I think the 600 ohm coil goes from seeming like a really great idea to something more interesting than useful. Wal basses also had XLR out but I guess they used onboard preamps to get a hot enough signal. And in that case there may still be some external buffering required. Really all a bass player has to do to accomplish this themselves is just use one of the aforementioned buffers to drive said signal while sending an unbalanced out to the rest of the "normal" signal chain. Or you may want all of the signal chain in what you send to the board in which case the best opportunity to take a balanced signal out is probably the preamp stage of your amplifier.

[cjj]

When I used a preamp to get it loud enough it didn't sound very different than the main coil to me except it was a lot better at picking up string noise artifacts.

That's doing exactly what I was talking about before, lower number of turns will give better high frequency response, which is what most string noise is going to be. So, depending on just what you are aiming for, better hign frequency response may not be such a great thing...

[cassius987]

When I used a preamp to get it loud enough it didn't sound very different than the main coil to me except it was a lot better at picking up string noise artifacts.

That's doing exactly what I was talking about before, lower number of turns will give better high frequency response, which is what most string noise is going to be. So, depending on just what you are aiming for, better hign frequency response may not be such a great thing...

String noise artifacts drive me nuts, especially with roundwounds, so I quickly grew tired of that wiring scheme.

[aceonbass]

So the bottom line here is that an XLR output with the full output of the pickups won't put out too hot of a signal into a recording device/console/mixer? I'm thinking I'd like to be able to plug into the XLR input in a digital recorder with a signal that bypasses the controls in the bass, but I've never looked at input impedances and such in the past. I just plugged instruments into the 1/4' jack, and mics int the XLR.

[cjj]

You really have to look at the specs on whatever device you are connecting to in order to know if there will be problems. In general, most stuff can handle inputs that are too high, they just clip the signal and distort horribly. But obviously, connecting your AC mains to the input ain't gonna be a good thing. But, it's best to know what you are dealing with and make sure you don't exceed the input specification just to be sure you don't cause any damage...

[johnallg]

Isn't part of the audio XLR spec that it is always 600 ohm? Whether it is line level (+4dBu) or mic level (-50 to -55dBu), it still matches the 600 ohm balanced spec. The 4002 XLR with the lower output winding I would think was chosen to go into a board at mic level.

[cjj]

Isn't part of the audio XLR spec that it is always 600 ohm? Whether it is line level (+4dBu) or mic level (-50 to -55dBu), it still matches the 600 ohm balanced spec. The 4002 XLR with the lower output winding I would think was chosen to go into a board at mic level.

Yes, nominal impedance is 600 ohms. And yes, a mic level input (which will have a built-in pre-amp should work fine. Just don't expect it to go into an XLR line level (+4dBm) input and have enough signal to work with...

[aceonbass]

I finally concluded my testing of the 4002 wiring in my 4003 bass. In a nutshell, there is no benefit to the circuit in a 4003. At least not mine with a pair of HB1's on the neck circuit and a RIHS on the bridge circuit. I decided to try and get to the bottom of the volume drop when backing the tone controls off more than 20%, so I started eliminating components to narrow it down to the cause. First I removed the 10M resister. No change. Then I replaced the .01uF cap with a piece of straight wire. Still no change in the volume drop issue, although there was a minor change in tone. It turned out that the volume drop was caused by the fact that the pickups were connected directly to the tone pots (with integral .1uF tone cap)->selector switch->volume pots->output jack. I'm not sure why the tone control volume drop occurred, but it definitely had something to do with the 4002 specific signal flow, not the caps or the resistor. So I pulled everything apart and started from scratch with the circuit I had before, a modified version of the 4004 VVT wiring formerly found on Joey's Bass Notes site, which is actually the same wiring as a Fender Jazz bass. The pickups are wired directly to the volume pots->selector switch->tone pot (with integral .1uF tone cap for HB1's and .047uF tone cap for the RIHS)->output jack. I like the sound of the HB1's with the .01uF cap. Scott Pope had some technical reasons why in the 4002 positions, a .022uF tone cap wouldn't be best (I think he suggested a .068uF cap), but I was already liking the "kick" the tone had with the .1uF cap, so I kept it. I prefer this circuit in general to the standard 4003 wiring because It seems the tone controls do more when wired this way. It certainly seems to work well on 4004's. Another idea I tried to borrow from the 4002 was inspired by the bridge pickup having separate windings going to a lo-Z jack for recording. I made a separate mono output jack assembly that was wired directly to the same volume pot lugs as the pickups themselves. The goal was to have a separate combined pure pickup signal that would be unaffected by the electronics. This turned out not to work as I had expected. It turns out that the signal taken directly off of the same pot lugs the pickups were soldered to was actually effected by the volume and even the tone controls. Not having a degree in electronics, I have no idea why this would be. I'd still like to have a pure signal to send to a separate output if anyone knows how it can be done. I'm done experimenting for the night! zzzzzzzzzzzzzzzzzz

[jps]

I think the only way you can have separate "direct" pickup outputs will be with a buffer preamp between the pickups and the volume controls to isolate the signal from the pickup and the input to the volume control. Your direct feed would tap off before the preamp. Having two different windings on the 4002 bridge pickup did the same as the Hi Ω and Low Ω signals were isolated from each other that way while using passive electronics.

[ken_j]

... I made a separate mono output jack assembly that was wired directly to the same volume pot lugs as the pickups themselves. The goal was to have a separate combined pure pickup signal that would be unaffected by the electronics. This turned out not to work as I had expected. It turns out that the signal taken directly off of the same pot lugs the pickups were soldered to was actually effected by the volume and even the tone controls. Not having a degree in electronics, I have no idea why this would be. I'd still like to have a pure signal to send to a separate output if anyone knows how it can be done. I'm done experimenting for the night! zzzzzzzzzzzzzzzzzz

I think the only way you can have separate "direct" pickup outputs will be with a buffer preamp between the pickups and the volume controls to isolate the signal from the pickup and the input to the volume control. Your direct feed would tap off before the preamp. Having two different windings on the 4002 bridge pickup did the same as the Hi Ω and Low Ω signals were isolated from each other that way while using passive electronics.

Jeff is correct. Dane you can't think of the wiring as a one way street, electrons follow the easiest path. You could isolate the circuit mechanically with a switch to cut out the rest of the circuit. It may be possible to do this the the mono jack (I think its a Switchcraft #13) I would need to study it a bit.

[aceonbass]

I was using a #13 Switchcraft switch for the "Direct Out". Since I was pulling a signal off of two different sides of the circuit, I didn't want them shorting together when I wasn't using the jack, so I used the switching jack to short the the two sides of the circuit together only when it was plugged in. The odd thing (among many) was that when it was wired like a 4002, the volume controls didn't affect the output signal with this setup. Only the tone controls worked, which was halfway to what I was trying to accomplish. The one improvement I was able to make was the way the tone cap circuit is wired via the 4002/Fender method. Hmmm...I wonder if wiring the pickups through the switch first, then to the volume, then tone, then out would make my direct signal idea work? Ken, if you know how to wire the #13 jack to do what I want, I'd be interested to try that.