House Jam Micro Cab

[Chienmortbb]

2 hours ago, cbd said:

Is there any benefit to putting a capacitor in series with the speaker as a crude high pass filter? Just had a go on a crossover calculator website which reckons a 220uf bipolar would give you a 6db/octave cutoff at around 90Hz - that would cost about £2.50. Or would it have to be steeper than that to avoid taking too much off the higher frequencies?

To make a decent HPF at the driver end  mean big heavy and expensive components . You need IMHO something line 18-24dB. That entails a 3rd or 4th order filter. A first or even 2nd order filter ( 6-12dB) would putter cutoff frequency too high. 
 

As @Woodwind says it is easier to achieve in ghe signal path rather after the power stages. I think the Thumpinator sets the -3dB point too low although there are lots of them about. 
 

If you have WinISD, you can model the driver and then add various filters to see the effect on the low increase by adding higher order filters. 

[Phil Starr]

On 16/12/2024 at 21:10, cbd said:

Is there any benefit to putting a capacitor in series with the speaker as a crude high pass filter? Just had a go on a crossover calculator website which reckons a 220uf bipolar would give you a 6db/octave cutoff at around 90Hz - that would cost about £2.50. Or would it have to be steeper than that to avoid taking too much off the higher frequencies?

 

You almost certainly have a tone control which will roll off the bass by 12db or more. It would be easier to just use that as a starting point, not least because it costs you nothing and you can restore the system back to normal when that has finished. I don't want to speculate too much but anyone using the cab for the first time will sense the loss of the bottom frequencies and probably boost the bass on their amp or bass if it is active. We've all played with guitarists who swear blind that they haven't turned up even when you can see it on the meters. Bassists can be just as naughty

 

The thing is that it is perfectly acceptable to use a big amp with a small speaker if you are aware of all the parameters. As the owner of everything you know the risks and will be careful accordingly. For a bassist just jamming with their mates and no knowledge of the gear involved you'll just go for it. Just swapping from a bridge pup to a neck pup could be enbough to put you in the red zone.  At open mic nights I reckon I'd match my cabs to double the amp power for 'safety' as you have no idea what fx or playing style you will come across and some people are brutal with their equipment.

Edited yesterday at 09:46 by Phil Starr

[Phil Starr]

Another factor with a capacitor is that the whole design of this cab with the very high tuning of 87Hz was to get the flattest response possible down to below 80 Hz. As you can see with your own WINIsd plot it's hard to get a flat response low down with other tunings. All cab design is a matter of compromise and this one was done by compromising on power handling and the speaker being almost unloaded below 70Hz in part mitigted by it's intended use with a Gnome/Elf/BAM style amp which has a bit of HPF going on.

 

I'm a bit prejudiced against a simple in line capacitor. In the olden days when I built amps I found a few that really didn't lke a capacitative load. In addition the capacitor produces a phase shift. @stevie is more expert than I but I instinctively wouldn't want that happening at the port tuning frequency. Being lazy I haven't looked at any calculations though

[pete.young]

7 minutes ago, Phil Starr said:

 

 

I'm a bit prejudiced against a simple in line capacitor.

A capacitor is how Tecamp (and presumably Eich) high-pass the feed to the mid/upper driver in their combos and cabs. Greeeted with widespread derision in my repair thread about my Puma combo!

[Chienmortbb]

7 minutes ago, Phil Starr said:

I'm a bit prejudiced against a simple in line capacitor. In the olden days when I built amps I found a few that really didn't like a capacitative load.

Yes you need to be careful here. Amplifiers have limits to the size of capacitive load and you could cause your amp to shut down. This is true of Class D amps but may class A/B amps have trouble with capacitive loads.

[Chienmortbb]

2 minutes ago, pete.young said:

A capacitor is how Tecamp (and presumably Eich) high-pass the feed to the mid/upper driver in their combos and cabs

It has been a thing at the cheaper end of the market, paying lip service to the higher frequencies. Shame on Eich for saving money this way although they are not alone. It is one of the reasons that boutique makers like Greenboy, LFSys, and others exist, because the usual suspects play fast and loose with the top end. SEarch on here or on T*lkbass for Markbass combos and the  issues with their top ends. If the best you can do is a tweeter and capacitor, do not bother.

[cbd]

On 16/12/2024 at 23:41, Chienmortbb said:

If you have WinISD, you can model the driver and then add various filters to see the effect on the low increase by adding higher order filters. 

I didn't know about that feature, great idea. If I add an 80Hz 1st order high pass filter to my winISD model of the 10L house jam cab it looks like it takes out lots more around from ~120Hz than I expected.

 

5 hours ago, Phil Starr said:

As you can see with your own WINIsd plot it's hard to get a flat response low down with other tunings.

Yep - it's super interesting with that speaker actually how tuning the box lower suddenly makes all the bass disappear. Quite counter-intuitive, I'd never have expected tuning higher to increase the low end but that's what's happening here as far as I've understood WinISD!

[Phil Starr]

19 hours ago, cbd said:

Yep - it's super interesting with that speaker actually how tuning the box lower suddenly makes all the bass disappear. Quite counter-intuitive, I'd never have expected tuning higher to increase the low end but that's what's happening here as far as I've understood WinISD!

Yes it's all about tuning a whle set of frequency dependent factors, some of them characteristic of the driver and others of the box itself. Obviously you cant really change the characteristics of the driver other than by going for a different one.

 

Basically you have control of the volume of the box and in a ported enclosure the resonance of the cab. Essentially the volume of the cab affects the bass response, a large volume giving more bass with a lower roll off point but with a drooping response, a small cab raises the roll off point and can give a significant boost in output around the cut off point which is at a higher frequency. That boost can become unpleasant and you can have a cab which is too big or too small. There's a Goldilocks size for each speaker and an area within which you can fiddle around a bit and optimise the output to your design spec or just your taste.

 

With a ported cab you aim to extend the flat area of the output by getting the port tuned just below the point where the speaker's output us falling and the cone is starting to flap around outside it's safe excursion limits. At the tuning point the cab resonates on it's own (sort of) and 180deg out of phase with the cone which started the resonance. That means the air in the cab is moving the 'wrong' way and creates enough pressure on the speaker cone to stop it moving, What you hear is all coming out of the port.

 

The Goldilocks point for the cab is where it is 'maximally flat' the flattest response possible. WINIsd goes for those first (They are called things like QB3, BB4, SBB4 and C4) and they can be calculated simply just knowing two parameters Qts and Vas. The clever bit of programs like WINIsd is that you can go off piste, They can calculate exactly how far the cone can move at every frequency and work out the frequency response, power handling, excursion and so on for pretty much any design changes you make. That's brilliant because it's so quick if like me you've ever had to do the calculations by hand.

 

If you are interested I can go through the process that helped me come up with the original House Jam cab to Illustrate one approach to designing a cab and choosing the right driver to go in it.