Cabinet design for 5" driver - guitar use

[JohnDH]

This thread covers a few things that Im interested in:

My late-night playing is at very low volume, and if you sit close to the speaker, it can still be a full clear sound. But Im playing a 2x12 Vintage Modern. To do this, Im attenuating somewhere between -20 and -30 db, so the amp is still turned up for its best tone.

I think Ive arrived at a good attenuator design to do this, and its taken about a year to figure it out. My current one is referenced 'M', and I started at 'A'.
see March 29 post here:

Simple Attenuators - Design And Testing | Page 15 | MarshallForum.com

It uses an inductive voltage divider at the front end, to let the amp see speaker-like inductance, then switched resistive steps at every -3.5db down to below -30db. And the key part, as Gnobuddy highlights above, is that the output impedance is always controlled and quite high, I target 20 Ohms which matches the small-signal output Z measured from my amp. Its the absolute key to maintaining attenuated tone and this forum is about the only other place it is discussed.

But, a couple if other things:

There's also something about a really small raucous little amp that can be quite fun. My 10W MG has a 6" speaker and its a blast to turn it up and make it sound rude, in a way that the big amps can't be unless they are truely loud.

I also worked up a 1x12 extension cab, working towards the big tone of a 4x12 that I liked but had no space for. Despite plenty of internal volume, it sounded boxy in comparison until I added ports. I tuned these to just below guitar frequencies to lift the low end. It worked and made the change I wanted, though I was told that the theory was all wrong.

 

[Gnobuddy]

I think I've arrived at a good attenuator design to do this, and its taken about a year to figure it out.

Thank you very much for sharing the results of all your hard work! You brought a combination of multiple talents and incredible persistence to your project, and pulled off something very impressive. So many years of snake-oil salesmen selling overpriced, underperforming speaker attenuators - and you've laid all the nonsense to rest, except for the true believers who will not be reasoned with. Amazing!

...plenty of internal volume, it sounded boxy in comparison until I added ports. I tuned these to just below guitar frequencies to lift the low end.

If the speaker resonance frequency (f0) is low enough, using too small a sealed box will create a bass peak (while also raising the frequency of the peak.) A box with "plenty of space" actually produces a lower Q resonance, and depending on the speaker Qts, may have no bass peak at all, or even a drastically rolled-off bass. Good for "tight" bass in HiFi, maybe even bass guitar, but probably not what we want for e-guitar!

You've obviously done so much work for your project that you may know this already, but FWIW: I used LTSpice to simulate the schematic of a Marshall-designed speaker emulation filter that I found on the General Guitar Gadgets website. The simulation has a hefty (roughly 4.5 dB) bass peak at about 100 Hz.

This sort of thing would sound undefined, woofy, and produce "one-note bass" if used for Hi-Fi. But e-guitar is an entirely different animal, and we have had a few decades to get used to the sound of e-guitar through a bass-peaky Marshall 4x12 cab. So if you're going for that specific sound, I can see why a big sloppy bass peak might be required!


-Gnobuddy

 

[JohnDH]

Hi Gnobuddy, Thanks for your comments. The attenuator had bern a lot of fun, and worked out a lot better than Id expected.

Great insight on the cab design. Maybe less volume is more sometimes!

 

[Gnobuddy]

Maybe less volume is more sometimes!

There are anecdotal claims that the original Marshall 4x12 cab was sized just big enough to accommodate the four speakers, with no thought given to internal volume. 🙂

Given that cost (of wood and Tolex), weight, and bulk were all practical considerations, that story might very well be true!

Add in the fact that Thiele/Small didn't even publish their seminal papers until the early 1970s, and it probably took quite a few years after that before speaker manufacturers began to provide T/S parameters for their drivers.

And even if you knew about Thiele/Small's work and had the speaker parameters, you would still have had to go find a computer on which to code and run your simulation - no easy task in those days of giant mainframes that only big labs and universities could afford.

And if you did all that, the high output impedance of the guitar amp probably made your calculations useless anyway.

So I kinda believe the story that Jim Marshall and his tech (or his carpenter!) made the box just big enough to fit the four 12" drivers, and called it good. 🙂

-Gnobuddy