Speakers and cabinets

[Morten Stenbaek]

Anyone have any experiences with speakers and cabinets they want to shear ?
Speakers : 8” – 10” –12” different sizes in combination etc.

Cabinet:
I’m thinking about size (debt and with) is oversize a way to go for bigger sound ?
Bafflebord thickness ?
Closed / open back ?
How should speakers be placed if an oversized cabinet is used (placed evenly or close together ?) ?

Looking forward to the debate : )

[Earl Norton]

For Harp?????

[Morten Stenbaek]

Well this section of the forum is dealing with harp amps right ?

[Bruce / Mission Amps]

Originally Posted by Morten Stenbaek

Well this section of the forum is dealing with harp amps right ?

HA ha ha ... yes.

Make the carcass deeper then the standard 9"-10" thing and the baffle thinner, like no more then 3/8" plywood or 5/16" MDO and use light weight lumber for the cabinet carcass.
IMHO, 3/4" Baltic birch is a little bit stiff for a harp amp.
I think I prefer a little thinner cabinet made out of 5/8" or softer 3/4" pine that has been finished down to around 11/16".

[MWJB]

8" - limited choices in 8" especially in 8ohm, typically lower efficiency than 10"s , often have to wire in series parallel (not good) to get impedances right.

10" - loads of choices (relatively), I'd recommend Eminence 102, Kendrick Blackframe & Fane AX10 (only use 2 in a 4x10"). Better efficiency than 8"s, better feedback rejection than 12"s.

12" - Not that many good choices for a harp amp, worse at rejecting feedback than 10"s (you will therefore struggle to get the same volume from the amp), cones are slower off the mark and can be dull sounding compared to 10"s, better sound dispersal than 8" or 10". That's not to say that they can never sound good (never say never) but this will not be the easiest route to take (I have a dozen 12"s just gathering dust).

[MWJB]

Also...

The final choice of which size of speaker and how many you intend to use will depend to some degree on the type of circuit you use (fixed/cathode bias, for instance).

I also think that there is an advantage in building multi speaker cabs tall & narrow, where possible) rather than low & wide, in respect to both feeback rejection and on stage monitoring. E.g. a 2x10" may be arranged with one speaker above the other vertically, rather than side by side on the horizontal plane (though obviously a 4x10" built like this would be impractical).

Speaker configuration plays a very important part in limiting feedback, which in turn, is the main limiting factor in getting good power from a harp amp.

[clyde1]

8's are the bomb, minimum 4, try 6 even. They just have that tight sound and with more, they're thicker.

[MWJB]

Which 8"s? A lot of 8"s are 4ohms, how do you wire them in parallel when using 4 or 6 of them (even at 8ohms)?

[dl5mhk]

Originally Posted by MWJB

Which 8"s? A lot of 8"s are 4ohms, how do you wire them in parallel when using 4 or 6 of them (even at 8ohms)?

4-4-4 parallel with
4-4-4 makes 6 Ohms.

8-8
8-8
8-8 makes 5,3 Ohms.

6 speakers with 16 Ohms makes 2,66 Ohms when wired parallel.

The 1st two choices work with 4 or 8 Ohm OT's, the last one with 4 Ohm,
don't you agree?

cu,
Ralf

[MWJB]

Agreed. The last 2 choices will "work" with 4 ohm OT. 6x8" at 16ohm,
in parallel (2.66ohms) will also work with a 2ohm OT.

My point is that connecting speakers in series sacrifices efficiency (all things being equal 8"s are not as efficient as 10"s) and lowers the ceiling of feedback with harp.

[dl5mhk]

Originally Posted by MWJB

My point is that connecting speakers in series sacrifices efficiency (all things being equal 8"s are not as efficient as 10"s) and lowers the ceiling of feedback with harp.

Oh...my Alligator amp makes people wonder about his feedback resistance, and it uses 6x8. Eventually 6x8" is not as efficient as 4x10", but what i like with them so much is that they are "faster" than those. For me, they sound more "crisp". But as always: Your mileage may vary.

Btw: Would you rather use a selfbalancing paraphase PI or would a fixed one work better for harp? I changed the longtail PI to paraphase selfbalancing style (as in the 5D4 Super Amp), it's not bad, but the most of the sound comes from the preamp in the moment, that means: soundwise is not so much difference.

cu,
Ralf

[clyde1]

The best harp amp I've heard extensively is the Gibson Maestro,
4 8's wired series parallel for 8 ohms. I've never heard the Sonny Jr.'s or the Gibson GA-90 with 6 8's, reportedly one of Kim Wilson's favourites.

[MWJB]

Sure it's "swings and roundabouts" and personal preference plays a big part.

I've not investigated parphase PI's to any great extent, only built & modded long tail PIs. The theory is that you always get some grind because the signal going to the 2nd half has already been amplified by the first. Harp amps have been built using both styles.

[MWJB]

Indeed, I've heard great things about the Maestro 4x8" and the GA40LP that it resembles (apart from speaker compliment). But would it sound better & be more efficient with a 2ohm OT and speakers in parallel? I am sure it would.

The amp's suitability for harp (I'm not suggesting that it doesn't sound great, but that doesn't mean it couldn't be improved) wouldn't have been the first thing on the designer's mind, how to utilise the 4 speakers with the same 8ohm OT as the GA40 would have been.

A friend of mine auditioned a GA90, cool sound, a monster for feedback but still only a recording/small venue amp. Kim was rumoured to have used one on Tiger Man...though he is supposed to have told an aquaintance that he "always uses the same thing in the studio...a champ". I take it all with a pinch of salt.

[Bruce / Mission Amps]

Originally Posted by MWJB

...
My point is that connecting speakers in series sacrifices efficiency (all things being equal 8"s are not as efficient as 10"s) and lowers the ceiling of feedback with harp.

Huh? I'm confused by this "series sacrifices efficiency" statement... do you mean you believe the speakers are not as efficient because they are wired one way or another with respect to series or parallel, in order to acheive a correct impedance match?
That doesn't make any sense to me so I must not understand what you are meaning here.
The efficiency of a speaker or speakers is totally independent of the signal phase so what am I missing?

[dl5mhk]

Originally Posted by MWJB

I've not investigated parphase PI's to any great extent, only built & modded long tail PIs. The theory is that you always get some grind because the signal going to the 2nd half has already been amplified by the first. Harp amps have been built using both styles.

Well, i'll see tonight - i'm playing on a jam session, there can i crank it a bit. Here @ home is no chance to let it really sing.
At first, i had some switches built in, for a different cathode bypass cap, with or without NFB, and some more. Meanwhile i eliminated all aof them except one: for switching the plate voltage of the 1st preamp stage, 150 volts or 40 volts. Nice headroom with 150, and lots of sweet distortion with 40 - then the amp has that "Rod Piazza-Sound" on some of his CD's, especially when played with a chromatic...sounds kinda broken. Love it!

cu, Ralf

[MWJB]

That is exactly what I am saying, but only in the context of harp, where acoustic feedback imposes a limit on the max power output (a guitarist would simply turn up his guitar and the differences in power available would pretty well even out, though the tone would change slightly).

The point you seem to be picking up on is that the individual speaker's efficiency remains unchanged (agreed), but when arranged in a series array it's a harder signal for harp to push, compared to parallel. Volume drops off, feedback starts earlier plus you sometimes get unpleasant harmonics and a "stodgy" tone. It's not a subtle difference. Similar listening tests by a trusted 3rd party came to the same conclusion.

My basis for this conclusion was A/B testing the same amp with 2x8 ohms in series (16ohm tap) and 2x16ohms in parallel (8ohm tap).

This is a good trick for harp playing Blues DeVille owners, the OT already has a 2ohm tap, rewiring the speakers in parallel will help (you can junk the stock 4/8ohm switching jack assy), as will increasing the series resistance on the fixed resistor at the top of the volume pot (430K-570K total).

This is why I am surprised at the number of "harp" amps that are on the scene with series wired speakers. Builders tend to make the numbers work out with the parts on hand, assuming that either scenario will work the same.

[Bruce / Mission Amps]

Originally Posted by MWJB

The point you seem to be picking up on is that the individual speaker's efficiency remains unchanged (agreed), but when arranged in a series array it's a harder signal for harp to push, compared to parallel. Volume drops off, feedback starts earlier plus you sometimes get unpleasant harmonics and a "stodgy" tone.

Wow...
OK, I'll have to experiment with that too because electronically (in a technical sense) and intellectually, this should make zero defference in anything you've encountered or just said!
However, I think I can hear a slight improvement in overall tone when running a guitar amps OT at its highest speaker output load...
ie.,
a multi tap OT running at 16 ohms with a 16 ohm load if the OT is built with 4, 8 and 16 ohm taps.
A NFB issue could pop up by running different loads on an amp where the 8 ohm tap or 2 ohm tap changes the amount of NFB to a previous stage.

[MWJB]

Hi Bruce,

Your skepticism is understandable. However, from a logical viewpoint does it not make sense that, if nothing else, the lower load of all speakers in parallel makes it easier for the signal to push, compared to 4x the load running 4 speakers in series-parallel? Plus, any current only has to push half the diaphragm mass?

[clyde1]

If you're pushing 4 speakers, doesn't diaphragm mass stay the same no matter what impedance wiring configuration you use? If the OPT is made for 8 ohms impedance, running a 2 ohm speaker setup will not yield optimum power from your amp and is this the tone difference you're hearing?

[MWJB]

I'm talking about running matched loads in all scenarios. I do know people who like twice the rated speaker load, but as you say there is an audible drop in efficiency. For this reason I prefer matched loads.

Thinking about it further, the theory does back up what I'm saying...

Lets take 2 otherwise identical amps (or one amp with 2ohm & 8ohm taps) running 4x8ohm speakers. Let's say we're getting 30W at the speaker in both scenarios (you won't, but just for arguments sake...).

With a parallel 2 ohm load you get 3.87A with 4 paths to ground, all signal only has to push a speaker cone once.

8 ohm load load gives 1.94A with 2 paths to ground, signal must go through one speaker cone, then immediately push a second cone before going to ground (like pushing a speaker with twice the diaphragm, mass)...twice the work.

[clyde1]

I think you're confused, same # of speakers, same mass pushed in both configurations. I don't see any difference electrically. You can argue there is a tone difference, but no difference in the math I'd say.

[dl5mhk]

Originally Posted by MWJB

8 ohm load load gives 1.94A with 2 paths to ground, signal must go through one speaker cone, then immediately push a second cone before going to ground (like pushing a speaker with twice the diaphragm, mass)...twice the work.

That doesn't matter, since the current is the same in both speakers of a series. It's not that the current would become weaker after driving the 1st one, and then would be left less to drive the 2nd one.
The reason for choosing a 2 Ohms OT winding instead for i.e. an 8 ohms configuration with 2x2 speakers in the Fender Bassman and other 4x10" amps was rather to save copper in the OT -> saving money.

cu,
Ralf

[MWJB]

I'm not confused. The math is clearly as I explained it. As I have said (more than once), in the context of electric guitar you would simply adjust the volume control, if necessary, to achieve the same result volume-wise. However. with the limitations imposed by acoustic feedback when cupping a harmonica to a mic you will not achieve the same result in the 2 scenarios I describe.

It takes less time to rewire 4x speakers than it does to read this thread, perhaps you should try doing that, and investigate the practicalities, rather than "suggesting" that there "may" not be a difference, because you don't "see" it.

[MWJB]

But the current is less in the series scenario.

It only doesn't matter if it doesn't sound any different, or doesn't work any better or worse. 8 ohms sounds different and doesn't work as well. Rather than arguing perhaps you should try it?

The reason for going 2 ohms was the fact that they switched to 4x10" to stop the bass farting out on the low E, compared to the original 1x15" format of the bassman & Fender used parallel wiring & 8ohm Jensens. The reason the 4x10" tweed bassman had 4x10" RCA jacks for the speaker wiring was to stop a potential current bottleneck in the speaker harness...for the same reasons I stated earlier.

I would have thought that there was MORE copper in an OT with a 44:1 turns ratio (approx for 2ohm) than in an OT with a turns ratio of around 22:1 (8ohm)?

[clyde1]

I'm sorry MJ, I didn't mean to suggest there is no difference and I'm sure you've heard it. I just don't understand the mathematical calculations you're
using to support your theory. I would indeed like to try rewiring the speakers in my Maestro, however then I would have to change the OPT to a 2-ohm
piece, which I am loath to do.

[MWJB]

Hi Clyde1,

Indeed, I wouldn't change the OT in the Maestro either (unless you could find one that would fit exactly without drilling the chassis) especially if it sounds good as is...however, if I was scratch building a stage amp, specifically for harp, I would aim to keep multiple speakers in parallel.

The power calculations I was referring to are I=E/R:

30W into 2 ohms:
I= 7.75v/2ohms
I= 3.87A

30W into 8 ohms:
I= 15.49v/8ohms
I= 1.94A

The voltages in the equations can either be measured at the speaker (as in the case of the 2ohm example) or arrived at by the square root of (30W*8ohms) divided by 8ohms = 15.49v...if we assume that both amps would hit a realistic 30W with a mic & speakers NOT a signal generator (no acoustic component to cause feedback) & a dummy load (which can't feedback either).

Cheers, Mark.

[dl5mhk]

Originally Posted by MWJB

But the current is less in the series scenario.

Yep. But the voltage is higher...

I would have thought that there was MORE copper in an OT with a 44:1 turns ratio (approx for 2ohm) than in an OT with a turns ratio of around 22:1 (8ohm)?

I would calculate the primary side of the OT first...and then the secondary, because the inductance of the primary (and the core size) has a big influence on the frequency range of the whole thing. After that i would do the math for the secondary - and that means less copper...

cu,
Ralf

[MWJB]

The current is the important number in this instance.

Yes, I see what you are saying about the OTs.