The nice folks at Everyday Practical Electronics (ex-Practical Electronics) in the UK have sent me a nice little Atlas LCR bridge for saying nice things about them. Payola! Cash for Comments!
Even small LCR bridges are normally big to accomodate all the knobs and range switches and so on. This thing uses PIC magic to do the same job automagically with only two buttons, and small enough to slip into your pocket or toolkit. I am a bit dubious about black boxes like these because they can sometimes return a really berko result with great authority.
I've been trying this one on everything in sight and it hasn't been tricked so far, even told me the cat was 8.2 Henries and 890k which sounds about right. The only con is that it uses AAA batteries rather than AA's.
So while pondering current drive for reverb lines I decided to use this new toy to check with reality (well, its opinion, anyway).
Pulling out the reverb line stash I found the following:
- A large tank marked 460 with broken taut-wire suspensions which may be from an old Fender Twin. This also has a 0.002uF disk ceramic fitted internally across the output which will parallel resonate the sensor at 6KHz.
430 x 110mm overall. - The a Pioneer EAV-201 line I have had for years. Ex-equipment, possibly from a console organ.
300 x 60mm. - A new Belton SN2AA1A1A from Korea via Jaycar. This is the one used in the Silicon Chip “Spring Reverberation Module” article, Jan 2000, and sold as a kit by the normal suspects (Jaycar KC5282). Now all discontinued due to unavailability of these lines (or possible saturation of the reverb kit market). The delay is 22 and 27mS with a decay up to 2.5 Secs.
260 x 60mm - A mini oddity, apparently ratted from some ghetto blaster or possibly some demonic karioki device.
105 x 30mm.
All have two springlines, but the first two also have two different springs in each line.
| Type | Rin ohm | Lin mH | XLin@1KHz ohm | Rout | Lout | XLout@1KHz | nominal |
|---|---|---|---|---|---|---|---|
| long tank | 1.0 | 1.174 | 7.38 | 180.2 | 356 | 2237 | |
| Pioneer | 286.4 | 201.5 | 1266 | 277.1 | 145 | 911 | “1k5/22k” |
| Belton | 0.8 | 1.046 | 6.57 | 49.8 | 76.39 | 480 | “8/500” |
| Mini | 0.9 | 0.2224 | 1.4 | 449.6 | 430.8 | 2707 |
As you can see the inductive reactance of the driver, XL, dominates the input impedance above about 200Hz.
Below is the mini line, the nearer of the two springs is missing.
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At the far end is the plate with V's where the suspension taut-wires solder. Just visible is the hair-fine suspension taut-wire itself which passes through the magnetic driver where it passes through a small (tiny) cylindrical magnet, then forms a hook to catch the end of the spring itself. This magnetic driver is as basic as they come, a coil on a bobbin slipped on a U-shaped bit of steel. In fact the only difference between the ends are the coils on the bobbins. This unit has only one spring in each run, and this passes through an anti-slosh loop to stop it twanging around too much. This is only to prevent damage. If you slosh any line enough to make it hit its protection stops the impact will launch a massive crash onto the lines, and out of the amplifier. |
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ANZamps Date: Mon, 30 May 2005 From: Anthony Ralph Subject: Reverb tank transducer question? Group, trying to work out how the transducers in the standard reverb tanks actually articulate with the springs. |
“Actually articulate” - a well-framed question Tony.
There are three different modes a springline can be driven in:
- longitudinally (push-pull)
- radially (side-to-side)
- axially (twist)
In longitudinal mode the spring is driven along its axis producing compression waves that change the spacing of the spring turns along the spring axis.
In radial mode the springline is driven side-to-side or up-and-down producing lateral displacement waves at right-angles across the spring axis.
In axial mode the spring is driven with a rotary motion around the axis producing a change of the spring diameter.
To simply produce reverb there is little to chose between these modes, and I have built a reverb line using army surplus moving-armature headphones as the transducers in push-pull mode and it worked just fine, and you can have great fun experimenting with different springs.
That may be okay in a studio, but real reverb lines hang out in the bottom of combo speaker cabinets and the risk of audio feedback between the speakers and the springline is very real, even inside a padded bag.
If we think about it a bit we realise that the sound from the speaker acts on the springs by moving them about, just like radial modulation, so that doesn't look good.
Longitudinal push-pull looks a better prospect, but the vertical component of radial modulation by the speaker changes the weight of the spring, and so part of incident energy is translated into a push-pull. Better, but still not great.
While the spring may be moved in bulk by speaker sound there is little energy across its width to generate a torque or rotary twisting motion between the sides, meaning that an axial or twisting modulation can pass down the noisy spring in a similar way to a balanced line with common-mode noise. There is poor coupling between the modes so we have better noise immunity.
This axial, twisting or torque drive is obtained from a tiny cylindrical magnet mounted around the taut-wire suspension. This magnet is not what you expect from looking at it - it is magnetised across its diameter, not along its length. Moreover it is mounted with its poles side-to-side, not up-and-down facing the driver or pickup poles.
Looking along the suspension
When the driver poles are magnetised by driving current the attractive force acts on each side of the magnet causing it to rotate in place, thus driving the spring around its axis.
Because the Laws of Induction are bi-lateral the pickup does the same in reverse, at rest the flux from each magnet pole cancels-out in the pickup poles. When it is twisted one or other magnet pole will dominate on each side and the resulting flux change picked up by the coil.
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These photos show a magnet recovered from a broken line on a compass, the only difference is that the magnet has been rolled over half a turn between shots. In one shot North is attracted, in the other repelled, showing the poles of the magnet are on the sides, not the ends. A sewing needle will only hang off parts of the sides, not the parts between, nor the ends.
Some magnets are marked with a dot of paint, but be warned that this normally means “this way up”, not the North pole as is conventional, this marking being exactly half-way between the poles. Unlike guitar pickups the magnets appear to be aligned in the same orientation, not opposing.
These normally fail either electrically with a lead-in wire dropped off the back of the socket, easily fixed, or the suspension wire breaking, generally right at the back of the magnet.
I've done a lot of moving-coil meter repairs, but if anyone has had any success in repairing these without a new suspension, magnet and hook, say by gluing, I'd like to hear about it.
Given these parts repair would be easy enough. Knock the solder and broken suspension out of the anchor tube. Align and solder the new one.
Experimenters who don't have ex-army headphones might think in terms of small hobby-motors, the smaller the better, with the shaft locked in something like rubber so it can twitch. Ideally connection should be made direct to the armature windings using very fine and flexable wire to avoid commutator contact problems.
See also Gibson repair.
