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Double Bass Installing new tweeter in Markbass combo

PaulKing

PaulKing

Apr 17, 2004
354
154
58
London, UK
www.myspace.com
#1
I've read a couple of posts concerning this, but have a specific question that isn't clearly answered elsewhere.

I have a MB CMD121P, with passive internal crossover splitting signal to 8ohm speaker and piezo tweeter.

The piezo is blown, and I have an IMG Stageline HT-30 to replace it. Perfect fit into the hole...

The new tweeter is a dynamic (ferro-fluid) speaker, 100W max, 8ohm. I have read several posts from people who have used this tweeter in MB combos. So that sounds encouraging.
(btw, I've also been using an external CTS Powerline piezo tweeter ... works very well, but annoying to have it clipped on top of cab... so I'd prefer to use the new IMG Stageline if I can)

BUT

Question 1)
I don't fully understand how this new tweeter may affect the load on my amp:
- Does the new 8ohm tweeter, combined with the main 8ohm speaker via the crossover, still only present 8ohms to the amp?
- Can I still safely run an 8ohm extension cab and keep overall load at 4ohms?

Question 2)
I have also read several articles saying that piezo and dynamic speakers may require different capacitors / resistors in the crossover to work properly / avoid damage. Equally I've read that the crossover in the CMD121P is 'identical' (at least visually) to the one in the CMD121H with its dynamic compression horn.
Does anyone know if the existing crossover in my amp is safe to drive the new tweeter?

Thanks for any help ...
 
#2
If the impedance of the new tweeter nominally matches that of the old one, then the load presented will not change substantially. The much bigger issue is whether the crossover frequency is workable for the new tweeter, whether its level will be a close enough match to the old, and whether the phase response with the new tweeter will be close enough so as not to produce any unwanted cancellation or addition.

Of those considerations, a potential level mismatch is most likely to be an issue. You can try to tame that with tone-shaping.

It would likely be best to replace the tweeter with another of the original one. Then again, you can give it a shot.
 
#3
Thanks drurb ... that's the rub though. Piezo tweeters seem to have unquantified, very high impedance, so don't really count in a way that is comparable to dynamic speakers. So the 8ohm replacement certainly ISN'T comparable to the original piezo.
I've also read that most piezo tweeters don't actually NEED a crossover at all.

That's why i worry that connecting my new 8ohm tweeter to the crossover in place of the HiZ piezo tweeter might not be wise. The crossover might be configured for piezo only. QAlternatively it might be a one-size fits all for MB amps, whether they have piezo or dynamic tweeter as factory setting.

If the chief effect is going to be a sound quality one, I'm quite prepared to try it and make a call ... I just don't want to risk damaging a $$$$ amp for the sake of a $15 tweeter!!

BTW, I'm not keen on replacing the original LeSon tweeter with an identical one ... not very nice sound.
 
#4
Hi Paul,

you are right. It is not worth the risk and I think the risk is rather large.
You can try to ask the company, but I bet they advise you to send your cab in for repair (which might become a bit expensive).
An alternative would be to install a new crossover into the cab that works with 8 Ohms speakers for both, woofer and tweeter. This should not harm your amp if you do it right, but might change the sound of the cab. (Often the tweeter is damped because of the directionality of tweeter horns.)
So, it's your decision, but your worries are reasonable. The cab might even sound better with a dynamic tweeter than with a piezo, but needs a better crossover and may loose some sensitivity. (Piezos tweeters are not popular because of a bit harsh sound.)

Good luck with your repair.
 
#5
Cheers, all wise words.
I've asked Markbass, hope they get back to me, but yes I expect they'd recommend their very expensive tech support!

Interestingly, the manufacturer of the new tweeter said this:

"Yes, you can use the HT-30 as a direct replacement.
You won’t have any issues with the crossover and the overall ohms of the cabinet.
So in effect you can simply switch the terminals.
I hope this helps
Kind regards

Technical Sales Engineer
Monacor UK Ltd"

I'll proceed with caution !
 
#6
That is an interesting question, as crossovers are not normally used with piezo tweeters, and they have a frequency dependent impedance that starts high and gets lower as the frequency goes up.

I would
1. Replace the piezo with another similar piezo, or
2. Replace the crossover and tweeter with a proper new 8 Ohm set, however, where to set the crossover point may be hard to determine. Be sure it can handle the huge power of the amp! .
 
#7
Bassamatic said:
...and they have a frequency dependent impedance that starts high and gets lower as the frequency goes up.
Click to expand...

Well, of course, impedance, by definition, is frequency dependent but I get your point about the particular impedance curves of piezos.
 
#11
I wouldn't trust Monacor here. Sometimes Piezo tweeters have a capacitor in series as a high bass filter, but a piezo acts like a capacitor on its own.
For use of a dynamic tweeter you need to keep the low frequencies off (or they may damage the tweeter) and the high frequencies off the woofer (otherwise the impedance will drop below 8 Ohms for some frequency range and this might damage the amp).

Bassamatic is completly right, these are your options (if you don't want the expensive professional repair).
You might ask Markbass how much a replacement tweeter costs (only the part without mounting). Maybe this won't get too expensive then.
 
#13
Interestingly, I took alook inside the companion extension cab, which is an 8ohm cab.
It has 8ohm woofer, 8ohm dynamic tweeter, and yes, a crossover that looks exactly the same ... two identical components on it, labelled identically, same model number printed on the crossover.

Certainly seems possible the same crossover is used, regardless of whether they install piezo or dynamic tweeter in the cab.

I'll still get confirmation from MB firts...
 
#15
PaulKing said:
Interestingly, I took alook inside the companion extension cab, which is an 8ohm cab.
It has 8ohm woofer, 8ohm dynamic tweeter, and yes, a crossover that looks exactly the same ... two identical components on it, labelled identically, same model number printed on the crossover.

Certainly seems possible the same crossover is used, regardless of whether they install piezo or dynamic tweeter in the cab.

I'll still get confirmation from MB firts...
Click to expand...

Did you ever get an answer from the Italians ?
 
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