Driver loads and DIY bass cabs

[netcastle]

I just built a set of Bass cabs (4 x 6.5 and 2 x 12). For maximum efficiency I want to wire the drivers in each cab in parallel. This would bring the load of my 4 x 6.5 cab down to 2 ohms and my 2 x 12 cab to 3 ohms. My amp, a Stewart World 600 can push 600W into a 4 ohm load but has no rating 2 or 3 ohms. Am I playing with fire by asking it to push my cabs? At the very least can I get away with 3 ohms? I know its a tough question because nobody would want to make a recomendation that will cause me to blow my amp.

With this in mind, can I wire it to run at a higher impedance and still obtain maximum efficiy from 4 and 2 drivers respectively.

Tim

 

[yan_b]

you should knowthat typical impedance is usually an average impedance.
especially at low frequency's it's tend to be much lower, so your impedance might be lower then you think.

at least in the 4 speakers box you can use series, parallel combination.

 

[netcastle]

I thought about the series parallel thing, but the I lose any sort of electrical advantage that I gain by having that many drivers in the first place.

Really, am I just splitting hairs when talking about the difference between two, three, and four ohms?

Tim

 

[NewYorkDave]

Never run an amp below its minimum rated load impedance. That's asking for trouble.

You're using 8-ohm drivers? Wire your 4X cab in series-parallel and your 2X cab in series. Then run the two cabs in parallel. The total load will be 5.3 ohms. Each driver will receive an equal amount of power.

 

[PRR]

> For maximum efficiency I want to wire the drivers in each cab in parallel.

That does NOT affect efficiency!

You would get the same efficiency wired 2 ohms or 32 ohms.

It seems to be "more efficient" because modern amplifiers make more power in lower impedances (they make roughly constant voltage). Basically, they make more and more current in lower loads until they burn-up (maybe the protection will cut-out before the smoke escapes).

If you found a good old tapped-output amp, with both 2Ω and 32Ω taps, it would give the same loudness either way.

> can push 600W into a 4 ohm load but has no rating 2 or 3 ohms.

If they could say "1,000 watts in 2 ohms", they would. But they only sold you enough heatsinking and transformer to make 600 watts safely. It might burn-up working hard in 2Ω; more likely it will run hot, distort early (like 200 watts), and generally be unhappy or dead.

Also: where do you get 150 Watt 6.5 inch speakers??? There is the old Community mega-driver but that wasn't called 6.5" and wasn't much good for bass. There are HiFi sixers with 150W ratings, but they are low efficiency and can't actually take solo bass waveforms at the 150W level for more than a few moments (they are rated for full-range music well below clipping). There is not a lot of point in carrying a huge amp and driving low-efficiency speakers. The high-efficiency speakers in that size that I can think of have 20W-50W ratings for live instrument use. So even wired series-parallel, 8 ohms, they'd suck 300-400 Watts from that "600W in 4Ω" amp, 75W-100W each, not going to live long.

Same for the 2X12. 3Ω -might- be an allowable load on a "4Ω" amp, as yan_b says, though amp makers know this too. But 300+ Watts each is an awful lot of power for a 12" passing bass-guitar tones. I'd be thinking four 15" drivers or two 18" and 8 or 16 cubic feet to make good use of 600 electrical Watts.

 

[netcastle]

Thank you all for your advice. I have built a few pairs of speakers and am by no means proficient at the craft. I always find new questions to ask because these systems tend to have lots of variables.

The 6.5's are rated at 45 watts continuous. My thinking was to run both the 12's and the 6.5's from one mono amplification source, so I felt that 600 watts would give me plenty of overhead (the 600W 4 ohms is the amplifiers rated output). The problem is that this method created more problems than it solved, including running the amp below its impedence rating.

As I ponder different solutions (quickly eliminating the idea of buying a second amp) I am starting to think that running the amp in stereo and putting 2 6.5's and a 12 on each channel might be the best way to go. That way I run the 6.5's at 4ohms and the 12 at 6ohms. Can I do this?

It's just that I love the amp that I have and I want to get every drop from it before I have to consider buying a new one.

Tim

 

[PRR]

> I love the amp that I have and I want to get every drop from it before I have to consider buying a new one.

Why would you ever want to replace this? I have PA amps from 1978 still in service. Dave probably has older ones around. 600 Watts sure should be enough power. Gosh, I remember when 100 watts was "big" and the 300W SVT was a jeans-creamer. I only ever met one, and only ever knew of one person with two SVTs. Now you can get that power in 10 pounds????

It would be nice to mention what a "Stewart World 600" is; mono? stereo? bass-head? general-purpose PA amp?

Ah: specsheet

FTC Power Rating: 110W x 2 @ 8 Ohms 190W x 2 @ 4 Ohms
FTC Power Rating: (bridged mono): 400W x 1 @ 8 Ohms, 600W x 1 @ 4 Ohms

Frankly, if 2x110W=220W is "not enuff", then 600W is not going to be enough "more" to make a difference.

I also feel that you have Vette power on Yugo tires: a few Twelves isn't going to handle much of a 600 Watt power capacity.

But if you think Twelves and Sixes are "bass cabs", then you must not be a heavy-metal bass-thrasher, more a light-fingered jazz stylist. (Or else you are in trouble.) So maybe this is enough speaker for your needs. I still think this is too much power. I've seen a 2-stack of B*se 901s (16 Fours) sound lovely on a 150W amp, and a few months later the bassist said it sounded wrong and I found ALL the cones SHREDDED. Twelves are bigger, but you just have two (or three counting the quad-Sixes): your total cone area is only 150% of the two-901s, and you are talking four times the power? You better have the LIGHT touch and tough speakers, and pray for zero "accidents".

> putting 2 6.5's and a 12 on each channel

That would not be my first jump. I'd suspect the Sixes need different gain than the Twelves; maybe different EQ. And it does not really solve your impedance dilemma.

I'd wire the four Sixes series-parallel, 8 ohms, and let them have one side of 110 Watts. That's 27W each, on a 45W rating. The alternative, just-parallel, leads to around 100 watts in each 45W speaker. IMHO, for stage-amp, that's just asking for trouble.

The Twelves are tough. How did you end up with 6 ohm speakers? Are these Lead Guitar, MI Bass, or HiFi speakers? Makes a big difference in efficiency, bass cutoff, and ruggedness. 90W in Guitar speakers, ala Fender Twin, can make one heck of a racket. 300W in some 12" home subwoofers will give a dull thud, ample in a living room but not in a club.

The bridged-mono rating seems to imply that each side could drive 2 ohms at 300 watts. But they don't actually say that. In fact it says "600W" only on the front page: on the fine-print page, it only goes up to 400W (8Ω mono bridge). The heatsinking looks just-enough for reliable 2x200W operation. It might survive 600W for days; if you love it, do you want to run it maxed-out and hot?

True, one Twelve (6Ω) and two series Sixes (8Ω+8Ω= 16Ω) is 4.3Ω, on-spec for the 2x190W rating. Each Six gets 24W, 48W total; each Twelve gets 126W. Total electrical power, 350W, which is "same-as" the fine-print 400W rating, and just 2.3dB less than the big-print 600W rating. But will two Sixes, 48W total, "match up with" one Twelve eating 126W?

 

[PRR]

FWIW:

http://www.bgra.net/2004/browse.php?order=DATE&type=poweramp

BGRA: Stewart world 600

"very light, takes up only one rack space, and can prodcue 600 watta beidged into 4 ohms BUT... It overheats and shuts down. i only used this amp once on a gig. I had it in brdged mode, in a to space rack with sans amp rbi above it. The RBI is very shallow, and even though the manual clearly says that it should have a rack space open above it, I thought I could get away with it. Most of the cooling fins were uncovered WRONG! The amp shut down on me at least 4 times during a gig. Granted, I was not following directions--..... It can't really run at its rated watts unless you pamper it. I don't want a piece of gear that's so finicky. ...."

 

[pmroz]

.
was going to post on this yesterday. Thanks PRR for the lookup.
A buddy has the 'World 1200' and has needed service because he disregarded the 'empty rack space above' requirement.
Same thing, amp cuts out etc.
He has also had trouble with adding extension speakers.
A nice clean amp when it works.

 

[Gus]

FWIW I have a home made bass cab, sealed with a 8 OHM 18" F3 at about 60hz Q around 1 about 5 ft^3. The amp is a crown Powerbase 3 wired mono at 500 to 700watts with an SVT pro preamp about 2 foot of 10 gauge wire between amp and speaker cab, 10 gauge inside the speaker soldered connections. The cab is sealed very well and braced 5 way binding posts etc.........

One of my friends, a very good bass player was testing it for me, funny thing the system seems to sound better wired dual mono at 500 to 700watts(from the specs) than wired bridged mono at 1,500 watts 3,000 peak(from the specs)??????

With the amp being a crown this interested me I would have thought the 1,500 would be just louder???

Now I read some of the mac solid state stuff had output transformers with 2 ohm primarys. Did the mac stuff use lower power supply voltages for the power output because of SOA?

 

[netcastle]

I chose the 12" drivers for two uses. I wanted to be able to run them as both the bottom end of a bass rig and as a subwoofer. I did not buy speakers made for use in a bass cab and maybe this was a mistake. The drivers are Eminece 1238r's and have an 8mm excursion with a sens. of 92db/w. This looked good because if I ran two of them I could get 98 db/w and that seemed like a significant advantage. The 6.5's are audax drivers that I got for ten buck a peice. They have a 3.5mm excursion with a sens. of 88db/w. The excursion on both made them look apealling because it is large enough to handle the transients (I think). I know that I am not playing with much power here, but I wanted to make up for that in efficiency. I am gathering that I may have missed the mark here. It's just that on paper it looked so good.

Also, I have used the Stewart for several years and have experienced situations where it cuts out. A small fan blowing on the fins has eliminated this problem in the past and so I am confident that I can use it for this application without overheating it. And yes, it is a great sounding amp in my opinoin so I really want to create a situation in which it will work.

Also, I should clarify that this rig at the moment will only find uses in my buddy's basement where my band practices. We have not yet begun to play out and so maybe that is why I was thinking small (though it didn't seem small when I dreamed it up). I gather that those of you who have used rigs live think that this will be an understated system when I take it on stage. What would your ideal live rig look like for playing in bars if you were going to build one?

Thanks
Tim

 

[netcastle]

Also, PRR, with the 12 and 6 X 6 at 4.3ohms, would mismatching wattage be that big of an issue if I sent the majority of the low end to the 12's where most of the wattage will get used?

 

[Kev]

Don't assume that all the wattaged will end up at the 12's.

that will depend on how you set up a split ... the xover ... series/parrallel of the drivers ... active ...etc
there is lots of ways to do it

the low will have more cone excursion and with filtering this can be directed to the 12 but there can still be wattage demands on the hi drivers. The amount of compression and sustained RMS level can still put much wattage into the smaller drivers.

I see nothing wrong with the general idea and if will be a balance and compromise of what is available.

What about a single 15. You may fine one from eminance that has an eff of close to 98db/1w/1m

 

[PRR]

> The drivers are Eminece 1238r's and have an 8mm excursion with a sens. of 92db/w.

I can't find an Eminence with that number or specs. There is a Goldwing 1238, a rather inexpensive thing that does claim to be useful.

> if I sent the majority of the low end to the 12's where most of the wattage will get used?

If you put the Sixes and Twelves on the same channel, how are you going to steer the bass away from the Sixes? That's why I'd consider Sixes on one channel, Twelves on the other, so you can run separate levels and even EQ.

However, as a basement boomer, just go ahead and try it. It sure should build some bass waves in a home cellar. You can keep a hand on the amp to tell if it seems overloaded.

> What would your ideal live rig look like

After all these years, for non-METAL performance, it is amazing how an Ampeg B-15 will pull the load. One fifteen inch speaker in about 2 cubic feet. Ampeg used various good-price (i.e. low-bid) drivers, but I transplanted a JBL E130 on one. Not a gut-shaker, but a very clear and unstrained acoustic jazz bass sound that could keep up with Freddie Waits.

Whatever happened to Freddie? I see his son Nasheet Waits is active. I found a ballad to the memory of Freddie. And I found this blast from someone's past (maybe my own):

Ted Dunbar, Kenny Barron, Larry Ridley, Monk Montgomery, Frank Foster, Freddie Waits

 

[netcastle]

I got all of the drivers from Madisound. This whole project has been a lot of sweat and tears because it includes a G9 which I can't get the toriods for (Digi-key's lead time has gone from 2 days to 2 months since I ordered the things). Hopefully soon though. After going through these posts and doing some homework, the only way to make this thing go right now is to get a second amp, and so I'll do what I must. I am going to bi-amp these cabs with an active xover (do we have any go plans around here or will Rod Elliots be fine?) and cut out the 6.5's at around 300hz. I have tried the 6.5's and they make a nice practice amp but looking at them I can tell that they will pop right out of their suspensions if I push the low end too much. I have wired this cab to run two speakers at 4ohms from each channel of my stewart. The 12's will get a Carvin DCM600. It seems like a nice amp and gives me the option of adding two more twelves in the future (it can push down to 2 ohms). Hopefully, in the end I will have a system that will be useful.

It's projects like these that keep me humble.

thank you all
Tim