High Pass filter

[musgit]

Hi,

I have a bit of a technical question.

The background

I am going to build a small  1 x 10" cab for rehearsals etc using a Beyma sm110/N.   This will be sealed. I also have a sealed 1 x 12" cab with the Beyma SM212, decided from the build in this forum, which I use for medium size gigs  (Jazz, Big band) and  a 1 x15" ( ported) with a Precision Devices PD156,  which I use for outside gigs.

I'm thinking that I could have the option of using the 1 x 10" with either of the bigger cabs for an easier going, nicely defined, two-cab sound, but I'll need to high pass the 1 x 10" at about 120Hz to keep the excursion within limits. I won't need the bass end of the cab,  especially with the PD156). Will this work with a capacitor as a first order filter?

Any thoughts would be welcome,

 

Thanks  

[musgit]

Should read "filter" of course!

 

[Phil Starr]

It will filter as you say as a first order filter, 6dB/octave and as such will offer a small amount of protection from over excursion for your 10. Usually you'd want a much sharper cut off as 6dB down at 60Hz isn't going to give you complete peace of mind, though you could model it in WinISD easily enough. However you will still get interference between the two speakers from  phase differences and you will hear the combined frequency responses of both speakers at most frequencies. In a way you are doing the opposite of what Barefaced are doing where they allow the speakers to double their output below the crossover point though in this case it isn't really a crossover of course. Your combination is going to sound bass light with a bit of a bass shelf below 120Hz. 3dB down at  80-100Hz is going to be quite noticeable. The advantage is that it is a really cheap fix and you can try varying the value of your capacitor to see what sounds best. I personally tend to roll of below 100Hz in most venues so it could be a sound you like but the only way you'll find out is to try it.

You might be better off just building a smaller cab for a second SM112,  In a really small cab power handling wouldn't be much of an issue and you'd have the same frequency response above 200Hz so you wouldn't need to re eq when adding the second speaker. It depends upon just how much you want to experiment though. I quite like the SM110 on paper and I can understand the appeal of constant experimentation, it's a sort of intellectual GAS

[musgit]

Hi Phil, thanks for you reply.

In hindsight, buying another SM212 and putting in a smaller cab would have been more strategic, but I have had the SM110 sitting on the shelf for ages  and I've modelled a tiny cab (21 litres or so) that I'm hoping  should give me a similar response to my Sm212 ( in 44 litres ), albeit a lot quieter, for rehearsals and stuff.   I'm hoping  that I can use the 10" with the other two for maximum flexibilty.  It'll probably be most useful with the PD156, which has oodles of bottom end but lacks a bit from mid-bass upwards.

I've fiddled around in WinISD quite a lot with the cut off and phase differences, but I guess I was really concerned about whether using just a capacitor will introduce any problems for the amp.  There's info on why it's not a good idea for low pass at bass frequencies but not the other way. Any thoughts?

[Phil Starr]

You shouldn't have a problem as a load with the cab in parallel with another presumably 8ohm cab at all. The only situation I can think of where I'd worry would be using the cab on it's own with a valve amp and your cab presenting a high impedance within the amps pass band. Even then I wouldn't expect a problem but I'd want to investigate, it's early 50 years since I've used valve amps so it's beyond my pay grade to offer advice about that.

 

[Bill Fitzmaurice]

When you add a second cab to your rig to provide more power handling and output where that power handling and output is required isn't above 120Hz, it's below 120Hz, so I question the value of what you propose.  I wouldn't use the SM110 in a sealed cab, partly because its specs aren't well suited for sealed, partly because it won't have enough low end even as a rehearsal cab. I'd put it in a 40 L (net) VB with 50Hz tuning to get out of that driver what it has to give. It's still not like it will be a huge cab by any means. Then if it adds worthwhile output used along with one of your other cabs all well and good. If not at least you have a decent rehearsal cab. As for high passing at 120Hz, to have a useful result you'd need at least second order filtering, which is impractical and expensive with a passive.

[musgit]

Phil and Bill, thanks for your responses.

I already have a 44 litre sealed cab ( with the 12") which is really good for big band, more than enough oomph, great tone and nice playing response. I think I like a fairly skinny bass end. The 22 litre cab ( for the 10") is for  putting in a rucksack and doing jazz duos and rehearsals, with acoustic instruments, where I don't need a car . I have also thought about a 22 litre ported cab, which has a sealed type roll - off but keeps cone excursion down and give a bit more at the bass end. 

 I agree with both of you that  the high pass filter bodge is going to sound really bass light -  and having looked at capacitors it's not going to be that cheap to experiment, even with a first order filter. 

I can try to play both cabs together anyway, but ultimately, for a bigger sound for funk, I think I'll have to build another sealed 12"......

 

 

 

 

[Phil Starr]

The capacitors shouldn't cost much https://www.ebay.co.uk/itm/Qty-10-Bipolar-Non-Polar-Electrolytic-Capacitors-Reversible-NP-Cap-Various-/120859793251

 

[musgit]

Thanks Phil - the 100v ones should be fine I would think. Safety first!

I'll give it  a whirl and report back.

[musgit]

It's been a while, but I have been busy. I did build the titchy 1 x 10" with the Beyma SM110 . Photographic evidence included!  I've also put a photo of my sealed SM212 cab up for size comparison.

It ended up being 17 Litres with a port - not sealed. I took Biil's advice, sort of, and thought about the advantage of porting a very small cab , like fdeck's one from talkbass.

I used WinISD to model 100w signal and juggled things around so it would stay within cone excursion limit using the variable HPF on my Acoustic Image Amp .The calculated port length for 48 hz  ( about 23cm)  tuned it very low - and I ended up using 19cm ( 68mm  downpipe ) port  which tuned it at  roughly 44hz ( using the salt- on -cone method). 

I haven't tried high passing it to use with the  SM212 , 44l sealed cab but I have tried them together and it doesn't  really  work, either tonally or volume wise - as suspected.

It sounds very clear and responds nicely to EQ. It's loud enough for acoustic duo/trio ( piano , sax, git) and usable for big band rehearsal, although does struggle for enough bass end with a loud drummer.   It now makes my other cab sound muddy, especially when I'm next to it - so I'm thinking of rebuilding my sealed cab with a mid and crossover...........

 

 

[Chienmortbb]

Downward firing port. I wonder what the issues are, if any,  with it  that close to the ground

[ahpook]

On 05/01/2019 at 19:56, musgit said:

It's been a while, but I have been busy....

Nice ! Looks the biz !

[musgit]

Thanks!  I've made a few cabs now, all with natural ply finish.  In the photos you can see our new, ply kitchen ( minus drawer fronts) which i made using techniques learned almost entirely from making bass cabs.

The port  had to be downfiring to fit it  in , the original calculated length anyway. I measured the port resonance with it lying down, port up against the wall , otherwise the salt didn't stay!  

I did think that a bottom port could be a way to get more, deep, bass and it does sound that way. it's a bit 'bloomy' though and sounds more balanced on it's side, which has little feet too.

 

Next up, now that I have a light cab, is finding a mid to go with the SM212 so I don't have stand 10 feet away from it.  Any ideas on how to choose one? Is it mainly matching sensitivities near the cross over? It looks like a can of worms.