Can I add a tweeter without impedance issues?

[Moos3h]

I have an old 12” cab (8 ohm speaker) and would like to add a tweeter to it. I don’t want to bring the overall impedance lower than 8 ohms - is there a tweeter that can be wired inline that won’t affect what the amp ‘sees’?

 

Thanks in advance

[Bill Fitzmaurice]

You must use a crossover to send the lows to the woofer and the highs to the tweeter. This keeps the impedance correct and prevents the tweeter from being blown. Even piezo tweeters, which are advertised as being able to be run without a crossover, really need one. One reason why they get a bad rap is that they're almost never used with a crossover.

[Killed_by_Death]

 

The crossover in my MarkBass cabinet did not split the lows out to the woofer, but it did restrict only highs for the tweeter.

Your tweeter won't be inline (series), it will be in parallel.

However, the Capacitor to discriminate frequencies will be in Series:

 

[Bill Fitzmaurice]

That's not a crossover, it's a high pass filter, and a very poor one at that. It only attenuates below its corner frequency at a 6dB/octave rate. Worse, as the power density of a signal increases by 3dB with each octave drop in frequency the net attenuation is only 3dB/octave, which is barely enough to provide any protection for the tweeter. With no low pass filter on the woofer it will still produce enough high frequency content to destructively interfere with the tweeter output. A good crossover will at the very least consist of a 12dB/octave low pass filter on the woofer and an 18dB/octave high pass filter on the tweeter. This is commonly referred to as a 2nd order low pass/3rd order high pass crossover.

[Killed_by_Death]

 

3 minutes ago, Bill Fitzmaurice said:

That's not a crossover, it's a high pass filter,

That's exactly what I commented about the MarkBass 'crossover' years ago, but a design engineer told me even if the full spectrum of frequencies were going to the woofer, it would be considered a crossover.

MarkBass claim they are allowing all frequencies to the woofer.

 

[Moos3h]

Thanks all - this is really interesting reading.

 

So:

 

- can one buy crossovers for this purpose or are they a case of buying the components?

- if one is used, does the amp ‘see’ the crossover (impedance wise) or the speakers connected to it?

Thanks,

James

[stevie]

Very good question. The answer is that you're quite likely to have impedance issues unless you know what you're doing. Here's an example of a commercial cab with serious problems in this area. It's not the first one I've seen either.

 

[stevie]

You can indeed buy ready-made crossovers but I'd compare them to having your hair cut with a pudding bowl. Yes, it's a haircut and some people might even like it -  but nobody's should believe it's a proper haircut.

A 'proper' crossover needs to be designed for the drivers (and cabinet) being used. Unfortunately, just about every bass guitar cab I've seen has a minimalist crossover like the one @Killed_by_Death showed earlier - because it's cheap and nobody seems to care anyway.

The impedance the amp sees is a combination of the crossover and the driver. If you'd like a solution that's been tested and documented on this very forum, check out the Lockdown cab build thread. There, you'll find an inexpensive but very good compression driver/horn combination along with a cheap-to-build crossover that was originally designed for use with a Celestion 10 but won't be a million miles out if used with another bass driver. 

[beans-on-toast]

Ask yourself, do you really want a tweeter. It can reveal hiss in your signal and that can be annoying. Try the tweeter and crossover with the 12” cab before cutting a hole and mounting it.

As part of the design, I would include a LPad to control the level, or at the very least an [off]-[pad]-[fully on] switch.

 

Edited May 19, 2021 by beans-on-toast

[Bill Fitzmaurice]

9 hours ago, Killed_by_Death said:

 

That's exactly what I commented about the MarkBass 'crossover' years ago, but a design engineer told me even if the full spectrum of frequencies were going to the woofer, it would be considered a crossover.

No acoustical engineer would say that. Maybe an EE, but they tend not to be familiar with speakers.

Quote

MarkBass claim they are allowing all frequencies to the woofer.

They are, and that's not a good thing. There's only one reason to use a 1st order highpass with no low pass, that being it's the least expensive option.

Quote

As part of the design, I would include a rheostat

You cannot use a rheostat, as it does not present a constant impedance load. One must use an LPad.

[stevie]

There are plenty of examples of commercial speakers that use a driver's natural rolloff as part of a crossover. I'd mention the Spendor BC1 as just one example. That used the natural rolloff of the Celestion HF1300 to *cross it over* to a Coles supertweeter. It wasn't very elegant, but it worked.

As far as bass cabs are concerned, using a properly designed two-way crossover is a much better solution, but nobody does it because it costs money and doesn't add to the perceived value of the product for the average punter. 

[Bill Fitzmaurice]

7 hours ago, Moos3h said:

 can one buy crossovers for this purpose or are they a case of buying the components?

You may, but they're designed to operate into a fixed driver impedance. Drivers don't have a fixed impedance, for instance a typical 8 ohm woofer may have an actual impedance of 20 ohms or more at the crossover frequency, so the results will be far less than optimal. Designing a crossover is just as involved as designing the rest of the speaker.

 

Quote

- if one is used, does the amp ‘see’ the crossover (impedance wise) or the speakers connected to it?

Both. That's part of why you can't build a speaker like ordering Chinese food, with a woofer from Column A, a tweeter from Column B and a crossover from Column C. But you'd be surprised how many manufacturers do just that. A few years back I designed an award winning 112/6 speaker for a manufacturer. The driver/box design process took a matter of weeks. Getting the crossover exactly right, which included seemingly endless Beta testing by at least a dozen users, took months.

[Phil Starr]

I think you might be getting some technically correct answers to a practical question. 

If you want to bodge some sort of tweeter to see if you like the sound there are a number of ways you could successfully do that, but it will be a bodge. The Mark Bass is a bodge after all and it works in the sense that lots of people quite like it and it doesn't blow amps. Would a better crossover with a few weeks design work and a few pounds worth of components wok better, yes it probably would.

I think you have a few options which would 'work' in the sense of giving you a bit of extra tops wthout blowing anything.

use a piezo tweeter, cheap and easy but they will always sound like a piezo.

use the MB single capacitor circuit as used in hundreds of commercial designs. There's not a lot of protection for the tweeter and all sorts of problms around the crossover point but half the bassists in the world are probably living with this sort of sub optimal 'design'.

use @stevie design. It'll look after the tweeter and if you are lucky work ok with your speaker.

buy a commercial 2way crossover. it won't exactly match your speakers but you avoid some of the problems of interference around the crossover area. You'll probably need to pad down the signal to the horn so this is only an option if you can work out how to do that.

I'd go for using Stevie's easy build design