100W Isobaric Compound Practice Amp

[roycruse]

100W RMS Velleman Amp
2 X 100W 8" Skytronic Drivers
100mm diameter X 195mm Reflex Tube

Hoping to run it off of the Line output from my zoom effects unit (when it arrives tomorrow)

[steve]

Looks interesting, did you design it yourself or buy some plans?

What's the dimensions?

[budget bassist]

Forgive me for maybe being a bit dense, but what's the point in putting one driver behind the other? and are there no controls on the actual amp?

[roycruse]

Its the result of several evenings of playing around with the softwatre "Bass Box Pro"

I wanted to build something really cheap - that i could just use at home that wasnt huge but could still hold its own down in the 40Hz regions

The Skytronic 8" Woofers were only £10 each - the velleman amp kit was £15

The problem I had was to suck the full 100W RMS out of the amp I needed 4ohms - which meant either a more expensive speaker or 2 cheap 8 ohm ones.

Thowing the parameters of these cheap speakers into the software and asking for good performance down to 40Hz I was getting suggestions of huge 180 litre enclosures and after 2 days of varying port lengths and widths etc - i gave up and started looking at other options.

I noticed the Isobaric Compound method of mounting the speakers in one of the pages of the software and Eureka I suddenly had a box that was only 400mm X 400mm X 400mm and held its own right down to below 40Hz. (see the attached plot from Bass Box Pro)

The only control on the amp is a simple gain on the back - the amp has simple line level input - and my zoom B2.1u has line out so i figured why complicate things (and ramp the price up). I will use the amp modeling, eq and effects of the Zoom unit to give me control over the overall sound.

Its the first time Ive ever done anything like this - so please pile in with the critisism as I like to learn from my own and other peoples mistakes.

Only reason I posted it here was because its an unusual configuration and wondered if anyone else had seen anything like it.

[steve]

fascinating, I'll be interested to see how this works out

[Prosebass]

Superb

good to see you trying something different , would love to hear the results.
Its much more satisfying than just going and buying something.

Paul

[Hamster]

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!

[JPJ]

Hmmm this looks interesting. I thought Isobaric was when the two drivers faced each other and were out of phase, but I'm probably be wrong. Didnt Ampeg make an Isobaric 2x15 where the speakers faced each other at about 45 degrees? I'm sure Alex or Bill will be along shortly to add the physics lesson

I will be very interested to here how this works out.

[alexclaber]

Excellent idea for getting deep response out of a tiny combo. The only caveat with isobaric cabs is the increased moving mass lowers the sensitivity by 3dB vs a single driver and 6dB vs two drivers, but if it's for practising then that isn't a problem.

Isobaric means constant pressure, which is referring to the air in between the two drivers. They run in phase (you reverse the polarity of one if it's a clamshell design) and this halves the Vas but doubles the Mms, hence you get the same transfer function in half the cab volume but with a cost to sensitivity.

Alex

[Happy Jack]

TBH I'm a bit disappointed. Your third photo shows a whole set of really cool stainless steel components in the background, and I was waiting to see how you were going to incorporate them into your design ...

[peted]

What a very interesting little combo you've built there. So, how does it sound? Can you compare it to anything we might be familiar with?

[alexclaber]

One thing I will add is that the rise in response you have around the tuning frequency combined with the low F3 you may find the combo sounds extremely bassy once room gain in a house is added. If you lower the tuning frequency by extending the port you should be able to solve this.

Alex

[roycruse]

Im afraid I will not be able to comment on how it sounds in comparison to anything else for a while. My Zoom B2.1u arrives today so will be able to plug it in with the correct line level its expecting later on. - I built this thing to keep me going while whilst I put some cash away for something decent.

Ive really fell in love with the TC Electronics RH450. Im a complete gadget freak (i love the digital knobs on the front of that amp and the car stereo style memory buttons) but the price tag on that head and a couple of their cabs is way out of my price range for quite some time to come. (comments on whether this is a good thing to be saving towards also gratefully recieved)

I used to play bass when I was in my teens (well kind of played - was never very good back then) but my bass got stolen from my flat and my life went in a different direction. I picked up another bass only a few weeks ago so am really enjoying picking up where I left off - So Im a fair way off of needing anything other than a practice amp anyway.

One thing I would like to know from the boffins - will I have reduced the high frequency response of the drivers putting them in isobaric config. they were supposed to be good up to 8kHz - which I figured should be enough - but this is where my little bit of knowledge comes to an end. I was wondering if it would be worth putting in a small mid range or tweeter. I left it all out so far as simple and cheap was the main motto for this.

[roycruse]

[quote name='alexclaber' post='415037' date='Feb 20 2009, 09:48 AM']One thing I will add is that the rise in response you have around the tuning frequency combined with the low F3 you may find the combo sounds extremely bassy once room gain in a house is added. If you lower the tuning frequency by extending the port you should be able to solve this.

Alex[/quote]You are right - the first couple of fretts on the E string do boom out noticably louder than the rest of the fretboard. I was going to just E.Q this out but ill have a play with making the port longer - Thanks for the tip.

[alexclaber]

[quote name='roycruse' post='415061' date='Feb 20 2009, 10:14 AM']One thing I would like to know from the boffins - will I have reduced the high frequency response of the drivers putting them in isobaric config.[/quote]

I imagine you have because you've increased the moving mass that much more because the cones are moving an additional air load. You might like to try removing the back speaker and seeing if you like the reduced bottom and increased mids and highs - it should also get louder because although you'll increase the load impedance the gain in sensitivity will more than compensate.

Alex

[roycruse]

So in other words the reason you dont see alot of speakers set up like this as it doesnt really give you alot of benefits

Can you suggest a configuration that might work better - with the hardware I got (using both speakers) - i dont mind spending another £20 on wood and starting again with the box.

I just want it to stay as small as possible. What I did wrong here is pile into this knowing a little bit about speaker cab design - but nothing about what parameters make a good bass guitar speaker. But that is kind of the way i do things - i like to find out how everything works myself the hard way

Details on the drivers are :-

Re 7.1
Fs 53
Qms 2.4
Qes 1.13
Qts 0.75
Sd(CM2) 216
Vas(L) 20
Cms(uM/N) 300
Mms(g) 30
B x L 7.9
SPL 1W/1m2 86
Prms 100
Pmax 200
Xmax 5

Edited February 20, 2009 by roycruse

[Hamster]

[quote name='roycruse' post='415148' date='Feb 20 2009, 11:48 AM']So in other words the reason you dont see alot of speakers set up like this as it doesnt really give you alot of benefits

Can you suggest a configuration that might work better - with the hardware I got (using both speakers) - i dont mind spending another £20 on wood and starting again with the box.

I just want it to stay as small as possible. What I did wrong here is pile into this knowing a little bit about speaker cab design - but nothing about what parameters make a good bass guitar speaker. But that is kind of the way i do things - i like to find out how everything works myself the hard way

Details on the drivers are :-

Re 7.1
Fs 53
Qms 2.4
Qes 1.13
Qts 0.75
Sd(CM2) 216
Vas(L) 20
Cms(uM/N) 300
Mms(g) 30
B x L 7.9
SPL 1W/1m2 86
Prms 100
Pmax 200
Xmax 5[/quote]

You'll find a wealth of ideas and information on Bill's website and forum - [url="http://billfitzmaurice.net/"]http://billfitzmaurice.net/[/url]

[alexclaber]

Ah, as I suspected it's high Qts. It I were you I'd take the back woofer out, plug the port and then stuff the whole box with polyfill. That should get rid of the hump yet still get you a lower F3 than most bass cabs.

Alex

[Mr. Foxen]

There are isobaric bass cabs out there, usually clamsheel type with speakers facing eachother. Bass cabs got a bit boring after that golden age of drivers facing in all directions and folded horns and such.

[roycruse]

[quote name='alexclaber' post='415163' date='Feb 20 2009, 11:59 AM']Ah, as I suspected it's high Qts. It I were you I'd take the back woofer out, plug the port and then stuff the whole box with polyfill. That should get rid of the hump yet still get you a lower F3 than most bass cabs.

Alex[/quote]Alex - Whats the average F3 for a low-medium cost bass cabinet and what would be considered good...

[alexclaber]

[quote name='roycruse' post='415756' date='Feb 20 2009, 08:34 PM']Alex - Whats the average F3 for a low-medium cost bass cabinet and what would be considered good...[/quote]

About 60-70Hz. Good isn't so much managing to lower the F3 alone as also managing to increase power handling and thus maximum SPL as you drop further below 100Hz.

[url="http://billfitzmaurice.net/phpBB3/viewtopic.php?f=12&t=2357"]http://billfitzmaurice.net/phpBB3/viewtopi...f=12&t=2357[/url]

Bill has done plots of typical twelves and fifteens as well, which are all worth checking out.

Alex

[gary mac]

I'll have to revisit this thread when I'm not so tired, it's making my head all fuzzy

[roycruse]

[quote name='alexclaber' post='415837' date='Feb 20 2009, 10:27 PM']About 60-70Hz. Good isn't so much managing to lower the F3 alone as also managing to increase power handling and thus maximum SPL as you drop further below 100Hz.[/quote]

Im aware that i am one of those dangerous people where a little knowledge is worse than knowing nothing at all

So please bear with me if my logic is floored.

If avarage bass guitar cabs have an F3 of 60-70Hz then this means that any signal fed to them below this will be a minimum of half as loud as notes higher up where the SPL is nominal - (is my logic ok so far) This means most bass cabs have a much lower SPL on any notes from about C2 downwards.

This to me raises the following questions

1) is this actually the desired sound for the bass guitar where the lowest notes are less loud.

or 2) bass guitarists buy rigs far bigger and more powerfull than they need and use EQ to make up for the quieter low end

or 3) i dont know what the hell im talking about (in which case please help )

[alexclaber]

Basically there's a lot more to a note than its fundamental frequency - it's the balance of overtones that make it sound musical and separate the sound of one instrument from another. And although the F3 is the half power point it's the F10 that is the half volume point. The half loudness point is in between the two and varies with SPL according to Fletcher-Munson.

Alex

[Guest subaudio]

Fascinating stuff, nice one for having a go, hope it works out