First Foray Into Amp Building - Please Help

[nipperiodic]

Well, I'm now waiting for some parts to arrive so I figured I might as well asks some questions about "features" for the speaker cabinet.

Should I install dual, parallel input jacks for daisy chaining? Not sure I would ever use this, but since I have the option to do it, why not?

Again since I have the option to do it now, should I consider using those fancy speakon type jacks or should I just stick with 1/4" jacks? Always been a little perplexed by the use of 1/4" jacks, since they don't seem very robust.

Edit: Also, do must people roll their own speaker cable for connecting amp heads to cabinets? I can't seem to find inexpensive short lengths of cable for this use.

Thanks, and a belated Happy New Year to everyone.

[galaxiex]

I have built a few, (3 to be exact) speaker cabinets, and always installed dual parallel input jacks for daisy chaining.
I didn't know if I would ever use the daisy chain feature, but thought better to have it and not need it...
So far I have not needed it...

I used 1/4 inch jacks, considered the Speakon's and figured they are more for pro audio gear PA stuff rather than guitar cabs.

I make my own short cable for connecting the cab to amp.

To be honest... the cabs I built are more like extension cabs for a combo amp that has an extension speaker output, rather than a dedicated cab for a amp head.
(although I'm working on one of those right now)

For my extension cabs I followed the guidelines in this book. >>> http://www.londonpower.com/Speaker-Book

Fascinating read, even if you don't build your cab according to his guidelines, it's still well worth buying/reading that book!  <3)

VERY different approach to speaker cabs for guitar! I highly recommend it!  (if only for the excellent info it contains, it's still worth it)

Cheers!
Dale

[Roly]

I use XLR's because the are robust, lock in, have very good contact area for carrying current, and the cable clamps work well with light/med weight standard round mains cable.  "Oxygen-free" is a total scam; what you really need is circular mills, cross-sectional area of the conductor (figure 5 amps minimum), and good road-tough covering (which mains cable is).

1/4-inch/6.5mm aren't really up to the duty (fragile, lack cable clamp), and Speakons are expensive overkill.

[g1]

  Dual bananas are also fairly expensive.
Agree with Roly, XLR are a good option for carrying high current.
There are also 1/4" jacks with 2 tip contacts which helps, but they are not very common.

[Roly]

The problem with 1/4-inch hardware is that it was designed for the telephone system with a power level of milliwatts at 600 ohms impedance.

Double tip contacts are better than single, but in a speaker cab with a 50 watt amplifier the peak current is of the order of 5 amps!  If you have a close look at a jack plug in its socket you soon realise that the total contact area for both the ground and hot is none too impressive, and why 1/4-inch/6.5mm/TRS hardware really doesn't cut it for speaker connections.

The body sleeve effectively contacts at only two points, and with the common single hot spring, at only one point on the hot side, and this is a sphere against a plate, the contact area is a pinprick, yet we are expecting it to carry up to five amps.  If you look at such a speaker lead plugs after they have been in service for a while you may see a ring of pitting around the back of the tip ball where this point contact has got hot enough to melt the metal.  {and the 1/4-inch situation is not helped by manufacturers who can't stick to the size standards, so you get plugs that are very tight or won't insert, which get their tips trapped and can't be extracted, etc.}

In the XLR the trick is in the socket where the contact is a sleeve with a pair of helical cuts.  The ribbon thus created wraps intimately around the pin giving a large number of points of contact.

We are even more stuck with 1/4-inch at the input end, but really, they aren't up to this duty either.

[nipperiodic]

The body sleeve effectively contacts at only two points, and with the common single hot spring, at only one point on the hot side, and this is a sphere against a plate, the contact area is a pinprick, yet we are expecting it to carry up to five amps.

This is what I've always found perplexing. The 1/4" jack just never seemed like a good way to transfer power. Although, it has worked for how long now?

I'm certainly not going to be touring with the cab or the amp anytime soon, but I was thinking I might as well attempt to do it right from the outset. Never thought about using XLR jacks. I'll look into them and see what I come up with.

Thanks.

[Roly]

The 1/4-inch/"TRS"/6.5mm/Jack has never been a great connector for anything much, but for most musicians their shortcomings are below the irritation/action threshold, so they go with the flow.

Good quality hardware seems able enough up to a few amps peak (e.g. 50W into 8r), but starts to show signs of pitting as you get up to 10 amps peak.

A lot of the time I've also run a PA as well as a guitar rig and keyboard pit and amp rig, and with so many connectors to deal with jacks where they weren't needed were a pain in the bum needing constant attention to keep them working properly.

There was also the matter of having an extensive low level signal system and a high level signal system with interchangeable connectors, so I adopted XLR's for the speaker system.  The mikes also use XLR's so a mistake is still not impossible, just less likely, but the XLR's are so much more robust and roadworthy than jack plugs; they are hard to break, and they are less prone to picking up dirt on the contacts.

But for low power at home?  meh - 1/4-inch works well enough.

[nipperiodic]

Just wanted to thank everyone again for the help and to add a bit of an update to this. I have a box of components but haven't had time to actually do anything with them yet. Hopefully I'll get some time in the near future to do some finger burning.

However, I did get a chance to get/make parts for a speaker cabinet. My living space just doesn't allow me to do any cutting or sawing of anything anymore, so I decided to go the laser cutting service route. I haven't had time to glue and screw the parts up yet, but I think it will turn into a cool little cabinet. At the very least, it will be unique.

Unfortunately, the service I used doesn't cut 1/2" ply any longer (no service seems to) so I had to go with 1/4" bamboo and lots of bracing. I know it's not standard, but I'm really having a lot of fun working this project out and seeing what comes of it. At least I hope something comes of it.

I designed the cab to sort of kind of look like an 8" tweed champ. There were a lot of ways I could have gone with the design, but I just wanted something simple and relatively cost effective. Not sure about finish yet, but the bamboo looks pretty cool, so I'm contemplating doing some type of wipe on finish. This cab is never going to see heavy duty use on the road, so it shouldn't be a problem. I'll have to get some pictures put up of the pieces when I get a chance.

So the above is a long way of bringing me to my next question, which I think is a fairly important one. Any suggestions for 8" speakers or are there any specs I should pay particular attention to? Speaker specs are a bit dizzying and as expected, every hobbyist seems to have their own idea about which speakers are great. One speaker that looked kind of interesting (although it looks like it's a not a true guitar speaker) is the Eminence Alpha 8A. I even found some clips of some guy using it as a guitar speaker. Anyways, I'm digressing. Any helpful hints?

[Roly]

Power efficiency - dB/W - the more the merrier.   :dbtu:

[nipperiodic]

So, just to make sure I'm understanding you exactly, I want both dB and W to be as big as possible when selecting a speaker?

I'll use an example. So that Eminence speaker I saw comes in a couple of flavors. They are (with specs in the order sensitivity, power rating, usable frequency):

Eminence Alpha 8A: 94dB, 125W, 58Hz - 5kHz
Eminence Alpha 8MRA: 100.9dB, 125W, 400Hz - 4.8kHz
Eminence Beta 8A: 95.1dB, 225W, 78Hz - 4.5kHz

Again these are only examples, but your selection would be the Alpha 8MRA or Beta 8A due to the higher sensitivity and power rating?

Thanks for clarifying.

[Roly]

No, the wattage rating only has to be "enough". It's the sensitivity/efficiency in dB/W that is more important.

The dB rating is dB/W - dB per watt at one metre, generally measured at the 1 watt level.  This is the conversion efficiency of watts into Sound Pressure Level, SPL, sound.

While 94dB, 95.1dB and 100.9dB all appear to be pretty similar they are actually on a logarithmic scale, so the 100.9dB/W unit is actually quite a bit more sensitive than the other two.


As a sidebar; the ultimate SPL, that each driver is capable of has to be multiplied by the ratio of the power limit to the reference level, 1 watt.

The ratio of 1 watt to 125 watts is

dB = 10 log₁₀(P1/P2)

125/1 = 125

log₁₀(125) = 2.09691001

x10 = 20.969dB

So if it makes 100.9dB at 1 metre with 1 watt it will make 100.9+20.969 = 121.9dB at 125 watts.

Similarly the Alpha 8A will make 94 + 20.969 = 114.969dB at 125 watts.

The Beta 8A will make;

log₁₀(225) = 2.35218252 * 10 = 23.52182520 dB

95.1 + 23.522 = 118.6 dB at 225 watts.


The bottom line with a low power amp is that you need all the speaker efficiency, dB/W, you can get, so the Alpha 8MRA.


(ed: speelinge)

[g1]

  Also note the frequency response.  The 8MRA is designed as a mid-range driver with freq. response dropping off below 400hz. 
  This may or may not be an issue, depending on how much low end response you need.
  But, keep in mind, A440 is the high E string at the 5th fret, open low E string is 82.41Hz.   

[nipperiodic]

Apologies Roly, I misunderstood. I thought you meant to take the sensitivity and power ratings and come up with some magic ratio. At least I was correct in assuming that sensitivity was important.

There aren't many 8" speakers with (relatively?) high sensitivity. Hence why I was looking out of the box at the 8MRA? I imagine it still produces at 82.41Hz, just not as loudly. I don't know if that's a good thing, a bad thing, or just something outside the norm. I wouldn't step on a bassists toes if I played in a band. Is the frequency rating really that important?

If I stick strictly with "high" sensitivity "guitar" speakers, really the most readily available speakers are the Eminence 820H, WGS G8C and Celestion Eight 15.

Anyways, I realize I'm doing lots of typing with very little building. Time to take some pictures of the cabinet pieces.

[Roly]

mmmm....yermmm... g1 makes a very good point.

The bandwidth is usually quoted at the "half power point" or 3dB down on the midband response.  With a CR network we expect the rolloff outside those points to be 6dB/octave, but a speaker is a complex resonant circuit and it's important what the resonant frequency f₀ is because below this the response drops, (IIRC) at 24dB/octave - right down a mine shaft.  So this driver might not have much response at all for musically important midband frequencies like A440 or Middle-C256, and could be pretty well inaudible at your open low-E string.

[J M Fahey]

The Eminence BMRA is closed back for simplicity of installation and so can only be used as a midrange driver.
Even without a cabinet ... because the blind/closed frame works like a very very small one.

Now sometimes the exact same speaker comes in a standard open back frame, like a conventional speaker.

Those are very good efficient speakers for instruments with little bass content.

That speaker (even smaller 6"version, open back frame) is used in the diminute ZR lunchbox to get a loud , stage usable amp, in a mini size cabinet.

Amazing:
https://www.youtube.com/watch?v=w5ave1vU4ds

that said, they cheat a lot:

1) the amp actually "has bass" ... up to a point.
The circuit is secret but I found that it's reasonably flat at low volume, but at high power (say above 20/30W ), bass is limited below 200 Hz ... which by the way is a sensible choice.

2) 200W RMS?  :duh :loco :lmao:
Not in their wildest dreams.

It actually is around 60W RMS ... respectable enough.

That said, I like the little beast, very clever design.

See that they test it with a trebly Tele ... don't even dream of plugging your drop tuned 7 string there.

But the idea is very good, perfect for a busy Pro playing weddings and such.

Back to the BMRA, if you find same with open back ... may work for guitar.

FWIW`Celestion also has some similar "midrange"speakers, open backed.

They are just not in the guitar catalog but on the PA one.

Also check Jensen (both standard and MOD series), they might have something interesting.

You search the datasheets and link them here