Basschat 1x12 - Frankenstein project

[Gottastopbuyinggear]

I've been following the 12" Cab Diary Continued thread with interest, but rather than derail that thread I thought I'd start a separate one with my, possibly crazy, thoughts/plans.

I built two of the Mk1 cabs and have been happily gigging with them for well over a year now.  Took a while to get used to the relatively flat response and oodles of low end when used in a pair, but I have no issues at all with the tone.

I've seen discussions about dispersion from cabs quite often, but only really started thinking about it as a result of the thread above.  You could read that as "I now think I have a problem that I've always had but didn't know I had until it was pointed out to me" !

Actually, I don't really have a problem at all, but I'm interested in the promise of an improved experience from the Mk2 cab when standing very close to it. 

It occurred to me that because I've not glued the baffle in on my cabs (they're screwed on, sealed with gasket tape) I could make a new baffle and could squeeze the 127mm diameter manrose pipe based port and the P-Audio tweeter in.  It then occurred to me that I could actually probably squeeze a 5" mid-range driver in, instead of a tweeter, and that seems like quite an attractive idea.

Why a mid-range?  Because I'm not interested in more highs, but I am interested in better dispersion.  I'm assuming that if I crossed over to a mid-range driver at something like 1KHz then more of my sound would be better dispersed, if that makes sense.  This is as opposed to using the tweeter, which I think is crossed over about 2.5KHz, and seeing as most of my playing is with a P bass with flats I think there's quite a lot of the bulk of the sound that's below that point.

Quite early on in the 12" Cab Diary Continued thread someone had mentioned the Faital M5N8-80 mid-range driver, and more recently the new 12PR320 woofer.  From a look at the manufacturer's graphs they look like they would be pretty good match if crossed over at a little over 1Khz.  I know it would not be optimal, but I think I could calculate and build a (for example) 2nd order L-R crossover to achieve this.  I realise it would be in no way tuned to the specific components.  I could actually start with the existing Beyma woofer and attenuate the signal to the mid-range down to match it, just to see what sort of results I get without springing £130 for the 12PR320.

To be clear I'm well aware that this might not end up with a nice flat response, but I think there's a reasonable chance of getting something that sounds pleasing enough.

There's a couple of other things I know I need to consider - for example the mid-range driver will have to be in a chamber which will reduce the volume of the cab, so I'll have to take that into account when calculating the port length, and actually squeezing the chamber sides onto the baffle might also be a bit of a challenge.

I'm interested in people's views on this - is it a mad idea doomed to failure?  I'm well aware that a little bit of knowledge can be a dangerous thing, and a copy of WinISD and a URL for a site with a crossover calculator don't make me a cab designer - but I'm up for a bit of experimentation.
 

[HazBeen]

@Bill Fitzmaurice

Bill will likely have some thoughts and insights on this.

[Gottastopbuyinggear]

44 minutes ago, HazBeen said:

@Bill Fitzmaurice

Bill will likely have some thoughts and insights on this.

...which might be that I should stop messing around and build something from the Simplexx designs I bought from him a couple of years ago!

[Phil Starr]

One of the advantages of a mid range driver is crossing down at a lower frequency, a 12" speaker is already beaming at 1kHz. the larger driver will usually handle more power than most compression drivers too. Years ago I bought a couple of 6" units to try with the Mk1 Basschat 12 but never got around to trying it. I had planned originally to build it into a separate sealed cab and try it with an active crossover to find out the sweet spot for the crossover, I was also going to try it with a 6dB/octave high pass filter built in to see if it could be offered as a design to owners of other cabs to use as a kind of 'bright box'.

The downside is that even a 5" driver starts to beam at quite low frequencies and operates under cone break up above this so you won't get as clean a top end as with a horn tweeter. You kind of have to decide whether mids or the top end are your priority.

There's an alternative to consider , which I wish I had built at the time to get bragging rights on Build something like the mini line source speakers that are described in the recently closed Markbass thread. Using a 2 or 3" driver would be ideal for bass. It'd go high enough to cover all you'd need in the way of top end but would let you have a lower crossover frequency so the mids would have an equally controlled dispersion. The downside is that a single driver that size wouldn't be loud enough. However a line of four would give you the increased power handling and efficiency you'd need and as a bonus a vertical array of drivers would give you a nice wide, flat, fan shaped radiation pattern. Someone beat me to it https://www.genzleramplification.com/shop/bass-array12-3/

I'm still looking to do this as my next project, if you want to go ahead then I'd be really interested in helping, and it may stimulate me to actually get into gear and build the one I've been planning for years.

[Gottastopbuyinggear]

Hi Phil, I was hoping you might come along!

I think that mids are where it's at for me, at the moment at least.  I've taken a look at my raw bass signal on a spectrum analyser plugin and there seems to be precious little going on above 4Khz with my P bass with flats, so whilst the Faital doesn't seem to have much going on above 5Khz I don't think that would bother me.  Most of the time when I gig I've got the tone rolled off and the mids a little boosted anyway.

Apart from the fact that I could only fit a 5" driver on the baffle I'm assuming that I'll get a little better dispersion from a 5" than a 6" mid range.  I'd seen your comments before about trying four smaller drivers and that was quite appealing just for being a little different, but I wouldn't get those on the Basschat v1 baffle and I probably wouldn't be doing this if it meant building a new box specifically to experiment - I'm not that confident in getting something usable!

I actually have a MiniDSP board from a project a couple of years ago and it did occur to me that I could try an active crossover, but I don't have a power amp at the moment, and whilst I've been considering trying power amp and preamp instead of a dedicated bass amp I'm not convinced I want to go there yet.

A couple of questions you might be able to comment on:  First, how rigid does the chamber for the mid-range have to be?  I'd assumed I'd make a box out of some 6mm ply I have in the garage, but it also occurred to me that I could use manrose pipe and cap the end somehow - however that's pretty flexible.  Second, what sort of volume should that chamber be, and is it in any way critical?  I see that Eminence publish ranges with their driver specs, but Faital don't.  I'd guess at somewhere between one and two litres?

I've not come across the Bright Box before.  That's already raised another question which has pushed me way beyond the limit of my understanding - if there's just a high pass filter in that (or something like it that someone might think of building...), and no LPF affecting the woofer, then what do you need to consider regarding the impedance the amp sees - is it not a consideration as the impedance of the woofer is already relatively high at the frequency of the HPF? [EDIT: A slightly clumsy question maybe - I've thought about it a bit more and posted more below]

Such a lot to learn, and I very probably haven't the brain capacity to do it!

Edited March 16, 2018 by Gottastopbuyinggear

[Gottastopbuyinggear]

I was having trouble with the whole business of impedance, and the effect of adding drivers.

I'm happy with the very basics of this - the analogy of a hosepipe where, if I connect two 8 ohm speakers then I'm effectively doubling the size of the hosepipe connected to my tap, so I can get twice as much water down the pipe, or roughly twice as much power to the speakers. 

What I had difficulty getting my head around is that when I play a note on my bass I'm not getting a pure sine wave at one frequency, I'm getting the fundamental plus a load of harmonics.  Thinking about it, is the correct analogy that in that case I have a bunch of taps and a bunch of hoses, and the size of the hosepipe for each higher frequency component is reducing slightly as the frequency goes up (forgetting about the resonant peak)?  

The other aspect of this is that as frequency goes up then power required to excite the speaker goes down - if I recall correctly then for every octave, so every doubling in frequency, I need half as much power to get the same volume.  Hence by the time we're getting up towards 1KHz the power required is already a fraction of what's needed for the fundamental and lower harmonics.  Does this mean that, from the point of view of not overloading the amp, adding another driver in parallel high passed at perhaps 1KHz would not create a problem as the amount of additional power which would be drawn is mimimal up at those frequencies?

That brought me on to another question.  The Trace Elliot Bright Box idea - if the high passed Bright Box is added in parallel with an existing full range cab that is not low pass filtered then will there be interference problems due to both getting the upper range of the signal, and potential phase problems depending on the positioning of the two relative to each other?  Applying this to what I'm considering doing with one of my cabs, could I use a crossed over woofer + mid cabinet on top of a woofer only cabinet with the full range going to that lower woofer?  Would I get better results by balancing the output of the mid-range in the top cabinet with two woofers and low pass filtering the woofer in the lower cab as well?

Finally, who dreamt up such a ridiculous name as "woofer"…
 

[Phil Starr]

I don't think the hosepipe analogy works when you bring frequency into play. A speaker isn't a 'pure' resistor, as the coil moves inside the magnetic field it generates an electric current which is out of phase with the driving current which makes the impedance rise.

So yes the impedance rises with frequency and bringing in a second driver at high frequencies shouldn't stress the amp. There are problems of just introducing a midrange driver with only a simple HPF. both speakers will be producing sound around the crossover point and because they are physically separated you'll get interference between them and response irregularities. That will depend upon the exact drivers, some drivers roll off naturally at fairly low frequencies and this approach will work OK, others will have a big midrange peak (like the Eminence Beta 12)and you'll get potential problems. The Beyma has a fairly flat response in the midrange. The truth is though that both speakers are producing equal volumes at the crossover point and you always get some crossover artefacts with passive crossovers. In the case of a simple HPF they are worse but lot's of commercial designs use them, rather than Stevie's 'proper' crossover.

In a way it's only one step up from using an old school mix of drivers, but these are intended as instrument speakers and some people like a bit of character/response irregularity. I'm not aiming at FRFR with a design like this, just a DIY tweak that people can do cheaply and simply. 

As to the enclosure for a midrange it needs to be pretty much as small as you can make it usually. I go for a 'critically damped' sealed enclosure with a Q of about 0.5. More about that later.

[Phil Starr]

PS a midrange driver is also called a squawker.

[DGBass]

I was thinking of starting a similar 1x12 frankenstien project thread after reading all the 1x12 cab diary threads on Basschat.  But based on modifying a branded cab not originally designed as a 1x12. I don't have WinISD or am technically profficient in cab design. It's more a case of modifying something with the bits to hand and see if it works! I suspect the advice would be 'don't go there' however my latest project seems to have worked out well despite having no technical design. If Could offer one one opinion it is that I agree with your last statement regarding being up for a little bit of experimentation. If anyone is keen, my project was a Musicman RH115 horn loaded cab made from pine and light weight which had a new 3/4 marine ply baffle fitted and converted to a 1x12 cab. The replacement driver was an Eminence Beta 12LF ( 250watts rms) . It sounds pretty darn good and has lows and mids aplenty,  more so than when it was a 15inch speaker cab. Can anyone do the WinISD math on my franken cab to see if it adds up?

[Gottastopbuyinggear]

Normally I’d jump in and say I’d have a go at modelling it, but to be honest I wouldn’t know where to start with anything other than a basic ported cab, and I’m not sure whether WinISD is capable of doing so.  Someone more knowledgeable may come along and comment...

[Phil Starr]

19 hours ago, dangoose said:

I was thinking of starting a similar 1x12 frankenstien project thread after reading all the 1x12 cab diary threads on Basschat.  But based on modifying a branded cab not originally designed as a 1x12. I don't have WinISD or am technically profficient in cab design. It's more a case of modifying something with the bits to hand and see if it works! I suspect the advice would be 'don't go there' however my latest project seems to have worked out well despite having no technical design. If Could offer one one opinion it is that I agree with your last statement regarding being up for a little bit of experimentation. If anyone is keen, my project was a Musicman RH115 horn loaded cab made from pine and light weight which had a new 3/4 marine ply baffle fitted and converted to a 1x12 cab. The replacement driver was an Eminence Beta 12LF ( 250watts rms) . It sounds pretty darn good and has lows and mids aplenty,  more so than when it was a 15inch speaker cab. Can anyone do the WinISD math on my franken cab to see if it adds up?

I designed something similar to that cab back in 1970, before the days when we used Thiele Small and computers had their own rooms and used punched cards. There were many such designs with effectively a conical horn built on the back of the cab with the expectation that the efficiency of the horn would enhance the bass and make the cab 'long throw'. In those days speakers were generally limited to around 50W for a 15" speaker and amps were really expensive so we were mainly looking for high efficiency rather than fidelity. There was a lot of trial and error involved! I suspect there wasn't a lot of calculation involved in the RH115 and Win ISD wouldn't be able to calculate the effect of horn loading on the cab. 

In my 'design' I got lucky. I was working late when someone needed some speakers and the only things to hand were my half finished prototypes. I hadn't finished making the rear horn so I bodged a simple unfolded horn and sent them out, they sounded so good compared to a lot of the **** available at the time that I quit when I was ahead and sold a few to disco's quite successfully. Looking back I don't suppose they probably only worked as bass reflex speakers but people liked the horn look of them.

Just enjoy your cab, it might be worth something someday, people like the old school sound and most of the drivers were long ago replaced with something with a bit more power handling, they could become collectors items.