trinityamps.com

This is a build log, and also a cautionary tale about GAS*, and what can result from one or two bad assumptions. Don't worry, I think it will have a happy ending!

At a little jam back in July, I had the opportunity to play JHPatton's bass rig, which includes a Traynor YBA-3 bass tube head (circa early '70s). The other guys in attendance that day (LBethune and Coco) are certified tube nuts, but none of us knew much about the YBA-3. It was big, it was mysterious, it had seven tubes glowing in the back! As the windows shook, estimates of its power output grew from unlikely to obscene as the day wore on. After JP had refused all offers to sell it, the rest of us began plotting break-&-enter plans.

Then, just a week later, as The Fates would have it, Coco happened to come into a nearly identical YBA-3, and he offered me first refusal. Well, after a quorum of the "TD Jam Band", including the extended Trinity brain trust had just agreed that the big Traynor was the coolest piece of gear this side of a Hammond B3, how could I pass it up?? Oh, I made a show of inspecting it, of running it through its paces, and of balking at the price, but the truth is: I had to have it.

As I wrote a cheque and dreamed of bringing it home, of plugging into it that weekend, a simple thought struggled to be heard in my lustful mind: "what am I gonna play it through?"

"I'll just have to build something," came the dismissive reply from the other hemisphere. I'd built speakers before, I figured building a cabinet with deep, flat bass would be a simple weekend project. And I was sure I could do it for less than the cost of a name-brand cab from the local music store.

Did you catch that? This project began with two very bad assumptions!

To be continued...
-Bionic

*GAS = Gear Acquisition Syndrome

_________________
Four strings is enough.

(source: yorkville.com)

Here's what we've been able to find out about the Traynor YBA-3:

# made in Toronto by Yorkville Sound (Long & McQuaid)
# 130W "clean" (lots more, dirty!)
# original tube complement: Three 7025As and four 7027As or three 12AX7s and four 6CA7s or (early models) 7027 tubes; also used (late version): Four EL34s
# big heavy Hammond transformers
# dimensions: 8"x28"x10" (HWD); weight: 60 lbs.

Here's mine:


You can see now why I'm calling it the "camo amp." Needless to say, this was not the original covering! I'll be recovering it eventually, in more respectable black tolex or similar.

More info on the Traynor YBA-3:
http://www.informatik.uni-bremen.de/~da ... ecialyba-3

History: the previous owner claims, and Coco has verified, that this very amp can be traced back (via serial number) to it's original purchase by JOHN LENNON (!!) during his stop in Toronto on his solo tour in 1970/71.

Up next: deliberations on a cabinet to match this monster!
-Bionic

_________________
Four strings is enough.

So I laid out my requirements. Then I started researching. I thought I knew a lot about speakers already, but the art-and-science of speaker design is a black hole - the more you learn, the more questions you have! In this log I'll lay out the theory, design, construction, testing and performance of the cab, and I'll try not to get bogged down in the minutiae!

My Requirements:

# Deep Bass - Flat Frequency Response down to Low-E (41Hz). With my current rig (a combo), I have to hit the E-string really hard to play the lowest notes at the same volume as higher notes. This makes it hard to play at a consistent volume.
# Tight Bass – This is not a sound-effects sub for a home theatre system. The transient response of the cabinet must be fast. The consensus seems to be that a group-delay difference of 25ms or more will definitely make the tone sound "muddy". I'll try to keep the group delay below 12ms.
# 130W+ of Power Handling – to match the amp that inspired this project. The YBA-3 is capable of putting out much more than 130W, but that is the threshold of distortion. Because of harmonic generation, the amp won't put out a single tone in excess of 130W power. As they say, "130Wrms, 260W program power."
# Fits In The Car - If the trunk lid has to be bungied, so be it.

Out Of Scope:

# Pretty Finish – It should be durable, and look like it wasn't made by monkeys, but that's all. "Rugged" is an acceptable look for a bassist and his gear.
# Full-Range Frequency Response (20Hz to 20kHz) – Let's keep this first project simple. Deep, tight bass will be enough of a challenge. I can add a 2x10 cabinet for more "tone", later. Note also that "full range sound" from a bass rig tops out around 5000-7000Hz.
# Linearity – As y'all surely know, soft distortion in the amp and in the cabinet is an important part of how a rig gives a guitar it's tonal character. Linearity in a guitar rig is "a disaster" in the words of one commenter. More on this later.

Up next: a brief treatise on speaker design, and what I learned about designing for deep bass!
-Bionic

_________________
Four strings is enough.

(by the way, feel free to interrupt me with questions, comments, advice, or requests for more info!)

Speaker - a box with a driver mounted in it, that makes sound
Driver - the cone-and-magnet thing; part of a speaker

Mr. Theile and Mr. Small
For the fact that it is even remotely possible for hobbyists to dabble in the black art of speaker design, we can thank the two scientists who, in the '50s, first codified it for us: Theile and Small.

Given the "Theile-Small parameters" of a driver, and some simple formulas, it is possible to get a very accurate estimate of how a speaker will sound - or equivalently, to calculate the cabinet size and shape (and porting) that will give the desired sound. Even better, there are now several good freeware packages that will graph it all out for you. I have used WinISD Pro, and it's excellent.

Most speaker-driver manufacturers publish their TS parameters. So, the design phase of speaker building is reduced to simply this:

1. Decide what sort of frequency response you're looking for (i.e. flat to 41Hz),
2. Try out a bunch of different drivers in the design software, and choose the one that will most closely give you the sound that you want,
3. Tweak the speaker design (volume and porting) using that driver, and
4. Buy the driver, the wood, and build it.

Simple, right? That's what I thought!


Sealed vs. Ported vs. Open-backed Cabinets
A sealed box puts a cushion of air behind the cone of the driver, which protects it from over-excursion damage and which gives the driver a quick transient response, resulting in precise reproduction of fast fingerwork, and very realistic tonality. The sound is described as "tight". However, it is difficult to produce the lowest octave of bass from a sealed box. In fact, a sealed box distorts bass signals significantly at high volume levels. Jazz bassists prefer sealed cabs.

A ported (a.k.a. vented, bass-reflex) box is a resonator akin to an organ pipe. The resonant frequency is determined by the size of the box and the dimensions of the port(s). Above its resonant frequency, the cab acts like a sealed box, supporting the driver with a cushion of air. Near the point of resonance, the port acts as a second radiator, adding to the bass tones that the speaker driver is producing. Also, since air suspension of the driver is maximized at the port frequency, there is much less distortion of bass signals at high volume. The result of all this is that vented cabinets produce deeper, louder (+6dB), cleaner bass than their sealed cousins. However, a bass-reflex cabinet has higher signal delay in the range of the port resonance, which can cause transients to sound "muddy" if not taken into account in the design. A badly designed vented cabinet can also sound boomy, if there is a sharp spike in response at the resonant frequency. Rock bassists tend to prefer vented cabs.

(Some of you, by now, are looking at the backs of your Trinity cabs and saying "wait a minute, this thing's open at the back! What gives?" Since bass is not a concern of guitar cabinets, the back is often left open to increase the room reverb and total volume. Guitarists do it with open backs - sounds like a bumper sticker )

There is one other caveat for ported boxes: below the port frequency, the box is essentially open, providing no support to the driver. Loud signals below the port frequency can easily damage the driver.

Modelling the available speaker drivers in various enclosures using Thiele-Small modelling software reveals that the deep bass requirement can only be met in a ported alignment. This project will be ported. I will have to design the box carefully to control group delay and avoid boominess.


Deep, Tight, Small: Pick Any Two
For any given driver (set of T-S parameters), there is an "ideal" enclosure which gives the flattest frequency response and minimizes group delay across the spectrum.

# A bigger box gives deeper bass, with some small increase in group delay.
# The bass response of a small box can be boosted by lower port tuning, but this introduces significant group delay around the port frequency. The resulting speaker would sound very muddy, boomy and indistinct.

In order to get the deep, tight bass that I'm looking for, this cabinet is going to be big. Huge, possibly.


Instrument Drivers ain't Hi-Fi Drivers
Sure, you all knew this, but it was news to me!

Speaker drivers for use in home-stereo loudspeakers and PA's are designed to maximize linearity through the widest possible frequency and dynamic range. This allows the sound of the source material to come through uncoloured and undistorted.

The uncoloured signal from an electric guitar, however, is nothing like the sound we are used to hearing live or in recordings; it is cold and full of harsh transients. It is the job of the speaker (and the tube amp!) to distort softly in a way that produces warm overtones or adds a distinct growl when necessary.

Accordingly, instrument speaker-drivers are designed differently than hi-fi drivers. A hi-fi driver has a narrow voice coil and a deep magnet gap, to keep the coil fully immersed in the magnetic field at all times, for maximum linearity. The surround and spider (the cone's suspension) are designed to operate in this range, and no further. If, through poor design or careless use, the speaker is driven beyond its linear range, damage can easily result.

An instrument driver, on the other hand, has a taller voice coil and shorter magnet gap. In the parlance of the business, the driver is "over-hung". (Hehehehh ) When the speaker is driven beyond its narrow range of linearity, the voice coil begins to leave the magnetic field, resulting in a soft (and safe) compression of its movement. The suspension is designed to operate in this non-linear range, so no damage results. The effect of the soft compression is the creation of warm overtones which are pleasing to the ear, or an attention-grabbing roar at very high levels.

How does this theory enter into the design process?

1. Buy an instrument speaker driver for an instrument cabinet. Avoid the temptation to use a hi-fi or PA driver, no matter how awesome its specs seem to be. The results would be a cold-sounding cab that would be vulnerable to damage.
2.Do-it-yourself-ers will caution new hobbyists to design their speakers such that the cone excursion will never exceed "Xmax" (one of the TS parameters). For hi-fi speakers, they are right. In fact, Xmax is the limit of linear cone movement. For a musical instrument speaker, Xmax is merely the level at which things start to sound interesting... The limit of safe cone excursion without damage to the driver is "Xlim", which is more or less double Xmax for instrument speakers.

Enough theory! Can't we nail down the design, now? See next installment...
-Bionic

_________________
Four strings is enough.

i cant wait, keep it coming bionic.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks, Bionic, I learned quite a bit from your summary of speaker theory.

Having heard your amp, I can't wait for the next update and your description of the sonic properties of your new baby.

JR.

Thanks guys, glad you're enjoying it Wow, that last installment was a bit dry, wasn't it? More pictures from now on, I promise!


Me and the CB15

The Driver - Eminence Legend CB15
I tried out a lot of instrument speaker-drivers in WinISD Pro, including Eminence, Tone Tubby, Electro-Voice, Jensen, Cerwin-Vega, JBL and JL Audio, (Celestion doesn't publish their TS parameters, too bad for them), and the CB15 gave the best deep-bass response that I could find. It is a 15" bass-guitar speaker driver with paper cone, cloth surround, 80oz ferrite magnet, and cast aluminum basket. It's free-air resonant frequency is 34Hz!

Here are its Thiele-Small parameters:


This is where WinISD comes in. You simply enter the TS parameters into the software, and start playing with the "alignment": box volume and port tuning, mainly.

The "optimal alignment" as calculated by WinISD Pro is a 3.75 cu.ft. box with ports tuned to 34Hz. This gives the following frequency response:

CB15, 3.75 cu.ft., 34Hz

Note that the lower -3dB point is at 60Hz, and that the response is -7dB at 41Hz. Most importantly, the difference in response through the lowest octave of interest (41-82Hz) is about 5.3dB. This is pretty good compared with other bass drivers (and already a world better than my combo amp), but is not the "flat to 41Hz" that we're looking for. Fortunately, we still have a few degrees of freedom in which to tweak.

Eminence recommends a vented enclosure of 1.9 to 5.6 cu.ft. volume for this driver. Knowing that a bigger box equals deeper bass extension, let's look at the same driver in a 5.6 cu.ft. box:

CB15, Yellow: 3.75 cu.ft., Orange: 5.6 cu.ft., both tuned to 34Hz.

Wow, look at that, we just picked up about 2.5dB at 41Hz! The difference in response through the lowest octave, where the bassist lives, is now only 3dB! This will make it a LOT easier to play evenly in that range of notes.

Beyond Frequency Response
The software also plots other performance characteristics such as group delay, cone excursion and air velocity in the port (all vs. frequency). These are all important considerations.

Group delay, as discussed, must be kept well below 25ms. In the smaller box, max GD is 9ms; in the larger box, 11ms. Both are quite acceptable, and the difference should be inaudible. Nothing sacrificed here.

Cone excursion (given a 130W signal) is also a little higher in the larger box; in fact, it will exceed Xmax between 43-65Hz (at the amp's maximum output, remember). But we remember that Xmax is only the limit of linear response, and above that the tone will warm up, not unlike that of a tube amp. We are still well below Xlim, the threshold of damage. See chart:



Note also that cone movement gets very small around 34Hz. This is no coincidence. At the port frequency, the air inside the box is moving in exactly the opposite phase of the electrical signal. The cone barely moves at all, and all of the sound is coming from the ports. If this seems counter-intuitive, think about twirling a hoola-hoop around your hips (come on, we've all done it!). Once you've mastered that hoop, found the right groove, and it is orbiting your hips effortlessly, you are barely moving. That's what's happening at the port frequency. This is the magic that explains why ported alignments offer much less distortion in the bass region.

One more set of charts; let's look at the air velocity in the port:

I'm showing a little more of the WinISD Pro window this time. The software calculates the correct port length, given the desired tuning frequency (not shown), and the number and diameter of the port(s).

At the default of one 4" port, both alignments exceed the recommended limit of 17m/s, above 40Hz (the smaller box, not by much). When the airspeed in the ports is too high, a "chuffing" sound can be heard to accompany low notes. This also is a property of my combo amp that I would like to avoid in my new rig. I did a lot of research on the maximum possible air velocity before chuffing sets in, and the answer is... complicated. It depends on port diameter, smoothness, whether the ends of the ports are flared and the radius of that flare, whether one or both ends are flanged, etc. etc. "Use flared port ends and keep it below 17m/s" seems to be a widely accepted rule of thumb.

Increasing the number of ports to two reduces the air speed in each port:


We're now safely below 17m/s. Note also that the required length of each port has nearly tripled! More ports means longer ports - strange but true. This is the esoteric realm of fluid dynamics. It is possible to specify an alignment in which the ports are longer than the box! There are lots of interesting solutions to this problem, such as folded ports, ports that stick out of the box, that wrap around the outside of the box! etc. But no such weirdness should be required here. Two 7.5"-long ports is doable.

4" ports are huge, by the way. Ever plumbed in a new toilet? That drain pipe was either 3" or 4" wide. A sub-woofer I built previously did use a toilet flange and drain-pipe for its port! It looked funny but it worked. This time, I'll order some nice flared port flanges from the speaker supply shop...



So that's a Reader's Digest account of the journey from theory to first design. Not shown was a lot of fiddling with this or that variable to see what the effect was. WinISD Pro was extremely helpful. I remember years ago, designing my first sub-woofer by tracing my finger along the curve on a chart, and reading off the "recommended" box volume and port dimensions, and not knowing the effect if we differed from those recommendations. Speaker design has gotten a lot easier in the past few years!

Up next: from design to blueprints, and the plans hit their first serious snag...
-Bionic

_________________
Four strings is enough.

Wow. Excellent. When this is finished, we'll all be experts on cab design. Looking forward to the design. The suspense is killing me.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Coco liked it so much, he posted twice!

Great commentary, Bionic. I agree with coco's assessment.

JR

So we've gone from design goals to our first high-level design, which is:

# Eminence Legend CB15 15" bass-guitar speaker,
# 5.6 cubic foot enclosure (internal volume of air)
# two 4" ports, 7.5" long, with flared ends

Is it time to hit the woodshop? Well, I checked my tape-measure, and there's no scale in "cubic feet", so there remains a little more work to be done.

Box Shape
Inside the cabinet, sound waves reflect back and forth between the walls. Between parallel walls, standing waves will form at wavelengths that are multiples of 4x the distance between those walls (wavelength is related to frequency). Each frequency at which a standing wave forms represents a box resonance which will create a peak in the speaker's frequency response. Imagine the worst case: a perfectly square cabinet. The standing-wave frequencies between all three pairs of opposite walls will be the same, resulting in huge peaks in response!

So the first rule is to ensure that the box dimensions (length, width, height) are not equal, or integer multiples of each other.

Standing waves can be reduced further by using non-parallel surfaces (e.g. a trapezoidal box) and curved surfaces, but I'm not a woodworking pro. I will go with a rectangular cab.

5.6 Cubic Feet of What?
Air. The air in the box acts as both a spring that supports the cone, and as an oscillating mass. In order for our rectangular box to contain the right amount of air, we need to add the volume of the driver and the ports, and any other internal features of the cabinet! The volume of the ports was easy to calculate, as they are simply cylinders (ignoring the flares): V = 1/4*pi*diameter^2*length. The volume of the back of the driver took a little more work. Some people wrap their driver in plastic and lower it into a calibrated bath of water, but I wasn't feeling that adventurous! I took some measurements with the tape measure, and approximated the driver as a disk (the magnet) and a truncated cone (the driver). The volume of a cone is V = 1/3*pi*radius^2*height.



It's necessary at this point to decide what kind of joinery will be used. I used butt joints with corner braces ("battens"), because they're much easier to make than dovetail or other fancier joints, and quite strong when glue is used. Their drawback is that I must take into account the volume displaced by the battens. Besides the battens, internal braces are strongly recommended for big cabinets, to reduce vibration in the walls. I plan on installing one internal brace which will look something like this:



And one more consideration is the thickness of the panels. I put together a little spreadsheet that takes all these into account and calculates the box's outer dimensions. This allowed me to adjust things like port length without having to re-calculate everything by hand. It also allowed me to play with the shape of the box, by picking the width and depth that I wanted, and letting the spreadsheet calculate the required height. If you'd like a copy of the spreadsheet, just email me.

I set the cabinet width equal to the width of the Traynor amp: 28". The Traynor is only about 10" deep, so I had some freedom in choosing the depth of the cab. I tried a few shapes until I settled on one that would fit in my trunk (just), and that could be cut from one 4'x8' sheet of plywood! The final outer dimensions are:

28" Wide x 15" Deep x 33.5" High

Yeah, that's a big cab. Almost twice the size of your average 1x15 cab you'll see in the music shops. Eminence says this driver should sound good in a box almost one-third this size. A 4'x8' sheet of 3/4" plywood is pretty heavy (~45lbs), and this box is going to use most of one whole sheet. "Deep, Tight, Small: Pick any two." Do I really need the deepest possible bass response? Couldn't I live with a bit more dispersion (group delay) on the lowest notes? Once I add the 17lb driver, the battens and braces, the screws, the glue, the handles, corners and carpet, this thing's gonna weigh...

Aw screw it, let's build 'er anyway! Full steam ahead!

Checks:

# Is there room on the front for one 15" driver and two 4" ports? Yes.
# Will it fit in the trunk of the car?? Yes!

I was all set to start building. I made my shopping list for the hardware store, and waited for the weekend to come. Unfortunately, that gave me too much time to think, and to continue researching...


Snag #1: Source Resistance
Present-day speaker DIYers have it easy, thanks to modern amplifier technology. Solid-state amps closely approximate an "ideal current source" - they maintain a constant output voltage no matter the load. Example: when you halve the load (go from 8-ohm to 4-ohm speakers), you double the output power. In other words, their source resistances are so close to zero that speakers can be designed "in a vacuum", with no consideration for the electronics that will power them.

The night before I went to buy the materials for the cabinet, I learned that life is not so simple when considering a tube amp!

Why is the amp's source resistance important to the speaker design? The answer has to do with damping.

A speaker, simply speaking, is a mass on a spring. The mass is the speaker cone and voice coil, and the spring is the cone's suspension: the surround and spider. A mass on a spring will bounce up and down at a resonant frequency that depends on the stiffness of the spring and the mass of the... mass. In accoustic terms, the speaker is very sensitive (i.e. loud) at frequencies close to its resonant frequency.


Freq. Response of a properly damped speaker (yellow) and an under-damped speaker (orange)

A mass on a spring will bounce indefinitely unless there is some damping force. To draw another example, a car is also a mass on a spring (actually, on four springs). Anyone who has driven a car with worn-out shocks knows the importance of good damping! Similarly, damping is built into a speaker driver, but not with separate components like shocks. A speaker's primary damping mechanism is its motor, and something called counter-current.

Any electrical motor is also a generator, and any electrical generator is also a motor. This holds true for the speaker motor, too: the magnet and voice coil. As the signal from the amp creates the force that moves the coil, so does the motion of the coil generate a current in the speaker wire, but in the opposite direction to the signal current! When the signal from the amp is removed, the speaker cone's momentum continues to generate counter-current. And because any generator is also a motor, this counter-current creates a counter-force which opposes the speaker's own movement. In the absence of a signal, the speaker will quickly come to rest. In other words, the speaker motor damps speaker motion. This "electrical damping" is the speaker's primary damping mechanism.

Because electricity must travel in circuits, the counter-current actually flows backwards along the speaker wires, through the output transformer of the amp, and back. Therefore, the counter-current experiences the amp's source resistance. Since the source resistance of modern solid-state amps is nearly zero, speakers will experience maximum electrical damping no matter which amp they're connected to. Speakers can generally be designed without regard for the peculiarities of the amp which will drive them.

But tube amps have significant source resistance. This reduces the available electrical damping, which makes the speaker much more responsive near its resonant frequency! This is one reason why tube amps are described as having a "warmer" tone: they make the speaker more responsive in the bass region. Unfortunately, the extra resonance also adds to group delay in the region of the peak (there are no free lunches). While cheap automotive subwoofers will take advantage of the extra bass of an underdamped speaker, we can not tollerate such a boomy, muddy sound in a bass-guitar cabinet!

I discovered that the WinISD software has a field where the user can specify the source resistance, and the software will model the effect on the speaker system's performance. But what value should I enter? I spent the next few days researching on the web.

There is not much documentation available on my 30+ year-old amplifier. I found the schematic, but (I'm ashamed to say) don't know how to derive the source resistance from it. It should be the output impedence of the power stage, reflected through the output transformer, and considering the negative feedback (the very purpose of which is to reduce the source resistance). I tried measuring it indirectly, by measuring the voltage and current through two different loads, but the answer was an unbelievable 14.8 ohms! This value created an enormous peak in the calculated mid-bass response, and I was worried. Any changes I made to the alignment to mitigate this peak had other undesirable effects (increased group delay /phase distortion, loss of deep bass response, etc). But given that the recommended speaker load for this amp is 4 to 8 ohms, a source resistance higher than that is unlikely.


Freq. Response of my cabinet with amp source resistance of 0-ohms (yellow), 1.25-ohms (orange), 8-ohms (red) and 14.8-ohms (purple). More bass isn't always better bass: look at the steep slope in the 40-70Hz range. This would make playing with an even tone almost impossible.


Group Delay of my cabinet with amp source resistance of 0-ohms (yellow), 1.25-ohms (orange), 8-ohms (red) and 14.8-ohms (purple). Despite the untidy peaks in the 40Hz+ region, the magnitude of delays remains below 12ms, which is quite acceptable.

I got a good discussion going on www.diyaudio.com's forum. Some experts argued that the most likely SR was 8 ohms (to match the recommended load), but others quoted 1 to 1.5 ohms as typical. The most credible commentor referred to a figure of 1.25 ohms for a "textbook push-pull UL tube amp" (I don't know what the UL stands for; anyone?). 1.25-ohms sweetens the mid-bass response of the speaker by 1-2dB, which will be quite tollerable (perhaps even pleasing). I decided not to modify my cabinet design.

Ironically, some of the speaker drivers which I had rejected earlier in the reseach phase (e.g. the Tone Tubby 15" bass speaker) for having too little bass extension would have matched the tube amp better than the CB15, which by this time I had already purchased!

Next time: Construction begins...
-Bionic

_________________
Four strings is enough.

Dude -get to the important stuff......................................what colour will it be? Great info ,are you a teacher or something?

sorry, double post

Construction is good. More good info bionic.
Hey, planning and design is important right?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I know, this is turning out to be "a tale that grew in the telling"! But I don't want to leave anything out.

Not a teacher, but this is a lot like what I do for a living: I research something new, try it once, write a "Deployment Guideline," then move on to the next new thing... eh Coco?

Lots of messy construction pics coming up in the next post!
-Bionic

_________________
Four strings is enough.

bionic is one of the best. Trust me, the meat is definitely coming.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

The UL should stand for Underwriters Labratories, the organization that certify electrical goods are "OK" for commercial sale in the US. Similar to CE in Canada.

Ok, we're catching up to the present day. This post is based on an email originally sent on Monday 10-Sept '07...

The cab is finished! Well, it's together and working, I had the first full-smoke test at 8pm last night. I built the whole cab with screws only (no glue or caulk), incase after initial testing I decide to change the cabinet dimensions. It's not carpeted or stuffed, and there are no corners or handles yet. The internal wiring was done with old lamp cord! I just wanted to hear it, after a whole weekend of work.


Friday, test & measurement in my "anechoic chamber". The idea is not to trust the manufacturer's published T/S parameters, but to measure them yourself. I tried following this procedure, but found that my solid-state bass-amp wasn't linear enough for the job (in retrospect, I should have used the home-stereo receiver, instead). I decided to go ahead anyway, using the theoretical specs.


The box, with back off, ready for driver and ports. Note the 1x1 braces in all corners, and the weather-stripping around the back. The back panel attaches with nuts and bolts, so it can be removed when adjustments are required.

I mistakenly sized the holes for the ports equal to the outside diameter of the port tubes, assuming that the port flares go inside the tubes. Nope, they go outside. So, the holes are too small, and currently the flares stick out like air horns on a tractor-trailer. It's a provocative look!


The external ports.

It sounds awesome. First I hooked it up to my solid-state amp, and played some music through it (inaugural song: "Crossroads" by Creme, for those keeping track). My friend who lives ten houses down was there before the second chorus. "I hear you got it working, man. Heard you from my livingroom." Hehehehehh. Then I swapped in the Traynor tube amp, hooked up the bass guitar, and belted out some favourite riffs ("War Pigs" was first). The bass extension is... extensive!


The working cabinet.

Enough horsing around. In the days to come, I'll get to the serious testing: impedence measurements, SPL vs frequency, etc. I'll decide if the cab volume is right, tune the ports, then take it completely apart and rebuild it with glue and caulk, and start thinking about accoustic stuffing, carpet, corners and handles.

-Bionic
PS: My neighbours just put their house up for sale. I wonder why?...

_________________
Four strings is enough.

The above email went out on a Monday, to the guys that I usually jam with. By that afternoon, we had plans to get together that Friday eve for a jam. The message was clear: I'd better show up with the camo amp and the new cab, or I'd better not show up at all! Although it was working, there was a lot of work left to do. I still hadn't even glued and caulked the joints!

Why is it so important that a ported box be airtight?
The ports are carefully tuned openings in the box. Besides the ports, the box must be airtight, just like a sealed speaker. Not only can leaks cause audible squeaking, but will actually change the accoustic properties of the box. There can be some "box losses" through the speaker-driver itself (folded-cloth surround, porous dust cap for voice-coil cooling). That's one reason why it is necessary to test-and-tune a new speaker after construction. I'll get to that soon.

Tuesday night: I should have just stayed away from the powertools! First, I put the cab upside down on the workbench, and cut the handle-holes at the wrong end. Ack! No problem, I just removed the baffle (front panel) and flipped it around. Next task was to re-cut the port holes to the right size. Well... somehow I went from too small to too big. One inch too big! There was no saving it this time. I needed a new piece of plywood for a new baffle.

By Friday, I got the new baffle cut and the speaker and ports re-mounted - but not much else. I mounted the handles: big recessed steel handles (thanks Trinity Amps!) which I mounted with more weather-stripping, to ensure an air-tight seal. And I stapled accoustic stuffing around the insides of the box. It still wasn't "finished," but it was ready for the jam.

Stuffing? What's all this talk about stuffing?
As I mentioned, a lot of standing waves can develop inside the cabinet, which result in peaks and troughs in frequency response. Another way to mitigate these is to add accoustic stuffing to the box. Stuffing materials commonly used include long-fibre wool, Dacron pillow-stuffing, fibreglass insulation, etc. Sealed boxes are often 100% full of stuffing. Ported enclosures, however, can not be completely stuffed, unless you want to see stuffing material shooting out the ports! The usual strategy is to staple a few inches of stuffing to the inside of each panel (except the baffle).
Another effect of stuffing is to make the box accoustically larger. Strange but true. Equivalent explanations for this are (a) that the stuffing slows down wavefronts within the box, and (b) the stuffing absorbs some of the sound energy, turning it into heat. The effective size gain is difficult to predict, which is yet another reason that I will do a final tuning at the end of construction. The stuffing also increases total system damping, which is good (remember the discussion on electrical damping?).
In general, stuffing makes a box sound less boxy, tighter and deeper. There are few drawbacks, so nearly everybody does it. I bought 20 sq.ft. of 1" accoustic stuffing from a local speaker shop, cut it to size and stapled it around the insides of my cabinet. It fills probably 40% of the interior volume.

The Innaugural Jam
How does it sound?? AWESOME.



The camo amp and the new cab together are loud enough to be heard over Lbet and jhpatton both playing Trinity Amps' finest hardware turned up to 11. What did they have there Coco, a Trinity IIIs with 2x12, and a Trinity Lightning/Bomb pair?

Volume aside, the tone is SWEET! With the knobs cranked, there's lots of saturation and growl - I don't know if it's tube tone or speaker-motor saturation or both, but it's sweet. This rig howls. The cab seems to be able to take whatever the Traynor can put out.

Bass extension? Waaay better than my old combo! Playing an arpeggio down from mid- to low-E, only the last note or two are noticeably quieter - barely noticeable, that is. A little more finger stiffness brings those notes right up to level with the rest.


Me n' Lbethune.

We played through our usual favourites: ZZ Top, Dire Straits, Neil Young, Black Sabbath, Metallica, Tragically Hip, Santana, Alman Brothers, etc. During one song, Lbethune jumped back and looked at my rig like it had bit him! After that tune, he said "I was wondering why you kept moving closer. Then I saw that thing creeping across the floor towards me, all by itself! It freaked me out, man!" Guess I need some rubber feet on that thing

Special thanks to Coco, lbet, jhpatton, Captain Chris, Security Mark, and Piazza-Manna Rob, for an excellent jam. Let's do it again soon, guys!

Up next: the post-construction, post-jam test-and-tune reveals some surprises! Stay tuned...
-Bionic

_________________
Four strings is enough.

Very nicely done. More surprises to come. Great.

lbet was playing the new sIII Plexi [it then went to Australia] at that time. It's sitting below his original sIII in White tolex. He brought his 212 Trinity cab with a pair of Celestion Greenbacks installed.

jhpatton is playing the TC-15 sitting on top of a Green 212 bomb.

Yes, your amp/cab came through loud & clear!!

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Haha that thing is as big as a 4x12 congrats on a job well done!

it looks like a riot!