100W Isobaric Compound Practice Amp

[roycruse]

bassed on alex's advice im going to scrap the isobaric chamber and put all these bits in a new cab configured the more traditional way nwith both 8 inch drivers visible and facing forward.

With some playing around in bass box pro software i have finalised on a 600mm x 400mm x 300mm box with a 68 x 220 tuning port which gives the attached yellow plot line (orange plot is the cab as it is now in isobaric config).

Ive also decided to adda tweeter to the box - dont really think it needs it for the bass guitar but when im practising and I am using the drum loops in the Zoom b2.1u they sound very dull.

so on to my next question.

Ive decided to add a very cheap piezo tweeter and have ordered one of these [url="http://www.skytronic.co.uk/product/index.php?s=902.475"]http://www.skytronic.co.uk/product/index.php?s=902.475[/url]

I liked the idea of it being high impedance and not needing a crossover. but it does say it needs to be connected with a "small series resistance" but I cant find any information about how small that needs to be.

Ideally I would like to add a variable resistance and give my self a seperate control on the back of the unit to adjust the level of the tweeter but havent got a clue where to start... ohms, kohms Mohms ??? and also dont know if I need a linear or log type pot.

Any ideas appreciated.

Edited February 24, 2009 by roycruse

[alexclaber]

The series resistance with the piezo is just to ensure your amp doesn't see too low an impedance in the very highest frequencies and therefore shut down. You can't adjust the level of a piezo with a series resistance or an L-pad unless you also add a crossover and a dummy load. Best way with piezos is to use multiple ones in a vertical line and add a switch to shut some off to make them quieter.

Be interested to hear how the midrange sounds with the pair direct-radiating compared to the isobaric design.

Alex

[wizbat]

[quote name='Hamster' post='414917' date='Feb 20 2009, 12:32 AM']I'm intrigued by what this would sound like as a small home practice 'combo' - and on stage with 2x15" speakers and a 1000w amp! [/quote]


I say build the beast and they will come...... along with most of the women in the room if you hit the right note!!

[Mr. Foxen]

Was the Acoustic 408 bass cab isobaric then, or is it some other thing entirely? [url="http://acoustic.homeunix.net/twiki/bin/view/Acoustic/BassCabinet408"]link[/url]

[alexclaber]

[quote name='Mr. Foxen' post='418725' date='Feb 24 2009, 08:38 PM']Was the Acoustic 408 bass cab isobaric then, or is it some other thing entirely? [url="http://acoustic.homeunix.net/twiki/bin/view/Acoustic/BassCabinet408"]link[/url][/quote]

No, it was a ported direct radiator with the central fifteens acoustically lowpassed by the chamber they're firing into - like Schroeder cabs. With the much greater Xmax of modern fifteens you can get even more bottom from a pair than the Acoustic could do with four, hence them not being around any more.

Alex

[Hamster]

I'm getting lost now Did anyone see the footy earlier?