Bill Fitzmaurice

[quote name='Mog' timestamp='1385633808' post='2290385'] These "Smart Power" cabs have intrigued me for a while. The principle is sound but something in the back of my mind keeps saying gimmick.[/quote]The principle has been known for 40 odd years. Take two drivers, configure them so that only one of the cones is exposed to the air and one to the cabinet, and you get the same response curve with half the net box size as is required with one driver. That's a plus. You also get the same net output as with one driver. That's not a plus. This concept was useful said 40 years ago in home stereo applications, because the driver technology of that time often meant it took a box in excess of 12 cubic feet to get strong output at low frequencies. Using a second driver to get the box size down had its advantages. But even in stereo applications advancements in driver technology rendered the isobaric concept obsolete by the 1990s. By the same token isobaric cabs would have made sense in 1965, but not in 1995, let alone today.

[quote name='Iheartreverb' timestamp='1385637197' post='2290443'] The issue is, I can't get the tone I want at a decent volume. What are my options? [/quote]Either they turn down, or you turn up. In your case that means adding another 810. In their case they'll invariably whine "But I can't get my tone if I turn down". That's true, because getting their tone means pushing the drivers to break-up. That brings us right back to the same point: to get the same break-up with four twelves versus one means playing 12dB louder. If you're regularly playing at Wembley or the Royal Albert then 412s are justified. Using a 412 in an average club, let alone a rehearsal space, is like using a sledgehammer to swat at a fly.

[quote name='bassmachine2112' timestamp='1385541554' post='2289374'] Guitarists in the know use 50 watt or less versions so they can get the sound without destroying buildings [/quote]They also use 112s that can be pushed to break up with 1/4 the power that it takes to do so with a 412. 112s also have twice the dispersion angle of a 212 or 412, with no comb filtering of the highs. The trouble is that the vast majority of guitarists aren't in the know, and buy amps based on what they see their heros using, who use what they're paid to use: the biggest, ie., the most expensive, rigs. This guy, however, is in the know:

[quote name='discreet' timestamp='1385502976' post='2289142'] There's your problem right there. [/quote]+1, an 810 can't compete with two 412s. Haul in three more 810s and tell them that's your new rig, unless they get sensible 112 combos.

[quote name='1970' timestamp='1385399670' post='2287766'] Even if this amp is solid state? The wattage doubles @ 4ohms... so I was thinking that headroom/volume would be a distinct bonus of running a 4ohm cab instead of 8? [/quote]Wattage is moot. Volume is limited by voltage swing, and whatever you gain by increasing the voltage sensitivity into a 4 ohm cab is offset by the increased current draw. If you can't get the volume you need from an 8 ohm cab you don't need more power, you need a second identical cab. That will give you a 6dB increase, which equals quadrupling your power/doubling the voltage into one cab, assuming it could handle it, which it probably can not.

[quote name='chris_b' timestamp='1385299906' post='2286515'] The -15db is just a fixed volume control. If your amp has volume controls there is no point in a -nn db pad. [/quote]Amps place the volume control after the first stage of amplification, if not even further back in the signal chain. If the signal chain is overloaded prior to the volume control, causing clipping, the volume control will not eliminate either the overload or the clipping. A pad may reduce the sensitivity of the first input stage, it may attenuate the output of the first stage, it can even be a total bypass of one stage of amplification. But whatever it does it always occurs before the volume control.

The point of the active input is to attenuate the input signal if it's too much for the passive input to handle without clipping. That assumes the output of the active bass in question is some 10 to 15dB hotter than a passive. But not all are, so if it works into the passive input better that's what you should use.

[quote name='bridge' timestamp='1384840206' post='2281290'] has anyone had the experience of using something different with a compact, such as the Shroeder 12L, for sale on here? does it mix ok? [/quote]The two together will probably sound better than either alone, that's usually the case. However, the BF uses a long excursion driver, the Schro does not, so the Schro would be a weak link.

If it was me I'd stop using guitar twelves and get a real bass cabinet. You're giving away an entire octave or more of bass response with the G12-65s, and G12-30s are no better. The only thing 'bass' about the JCM was what they chose to call it.

[quote name='tall_martin' timestamp='1384287893' post='2274937'] Or, is it possible to turn the pair of 8 ohm cabs into 2ohm cabs with the right speakers? Under £200 for new speakers. [/quote]If they're loaded with 4 ohm drivers series wired you just need to wire them parallel to get 2 ohms.

[quote name='subrob' timestamp='1384155395' post='2273210'] Damping factor is also halved. [/quote]Damping factor is moot. http://www.diyspeakers.net/Articles/Richard%20Pierce%20DAMPING%20FACTOR.pdf [quote]What effect does increased current draw have?[/quote]Potential for clipping, and heat, which shortens the life of amp components. Think of an amp as an engine. You wouldn't want to drive a car with the RPMs always red-lined.

[quote name='gsgbass' timestamp='1384132742' post='2273177'] I believe if the 4 ohms is the amps max, it will have an edge over the amp @ 8 ohms. More power normally pushes more sound from it. [/quote]Rule # 1 of loudspeaker design: There's no such thing as a free lunch. What you gain with respect to voltage sensitivity with a lower impedance load comes at the expense of increased current draw. To preserve a balance between voltage and current headroom you should avoid running an amp into its minimum rated impedance load.

[quote name='xgsjx' timestamp='1383745411' post='2268442'] Does running an amp at it's maximum ohms affect the sound that it produces? [/quote]No. Voltage sensitivity is increased by 3dB, but that's at the cost of doubled current draw. Seldom is maximum output affected, as that's determined by the driver displacement limit, which on average is only half the thermal limit.

[quote name='LukeFRC' timestamp='1383350230' post='2263614'] so florescent tube bulbs give a wide frequency light in the greeny bit of the spectrum, and incandescent a narrower frequency of light in the yellowy spectrum.[/quote]Flourescents waste less energy as heat. For proof, take a CFL and incandescent of the same lumens output, put your hands on each. By the same token low sensitivity speakers convert less energy into sound, more into heat than high sensitivity speakers. Equal power applied, totally different results, by as much as a 20:1 ratio best case to worst. So you can have a 5 watt amp and speaker combination that has the same decibel output as another that's a 100 watt amp and speaker combination. Watts? Not totally irrelivent, but almost.

[quote name='RandomBass' timestamp='1383346284' post='2263563'] You are missing the point. Comparing fluorescent to tungsten is irrelevant. As you increase the wattage of either, the brightness increases. Nothing to do with loudspeakers at all.[/quote]Au contraire. A higher sensitivity speaker will be louder than a lower sensitivity speaker with the same power applied. A speaker with lower frequency response will be louder at low frequencies than one with less low frequency response. If one only looks at watts one has no true inkling as to the actual result.

[quote name='RandomBass' timestamp='1383332605' post='2263342'] Even RMS Watts is a marketing ploy. It makes no sense. [/quote]It makes as much sense as using the wattage rating of a lightbulb to gauge its brightness, which is to say, none. But the powers that be decided long ago not to provide the consumer with the specs that really matter.

[quote name='pby' timestamp='1383323290' post='2263222'] I think RandomBass was confused about specs from ampeg manual, the units used are confusing. "2 x 900 watts rms @ 2 ohms (600 watts continuous), 3% THD" I don't know how they got the 900 watts rms value but pretty sure it's electrically irrelevant as the real/apparent power of the amp is way less than that. I'm guessing the continuous power IS the average power. [/quote]Looks like specsmanship to me. Traditionally RMS was assumed to mean continous, anything other than continuous was called peak, or music power, or any of a half dozen other marketing euphemisms created for the sole purpose of inflating the actual power output.

[quote name='sebastian' timestamp='1383310250' post='2262948'] Does it mean that when we're talking about the cab of 600 Watts - the amp of 900 Watts is appropriate or the other way around - the amp of around 300 Watts? [/quote]Both of the above and anywhere in between. [quote]As a slight aside, I have never fully understood the term of RMS watts.[/quote]Measure the amp's maximum output RMS voltage swing into the rated load at the rated THD and that's 'RMS' watts. The watts aren't RMS, the voltage swing is.

[quote name='sebastian' timestamp='1383303923' post='2262811'] Taking the example of SVT 4 Pro, which has 350W rms @ 8 ohms on the power amp A and B. Does that mean that I will be able to use it with the Carvin cab and will I only get 350W of power or - is this going to be too much for the cab? [/quote]The cabs power rating is how much it can take before the voice coils melt. In most cases that's well above how much the cab can take before the drivers distort (displacement limited power). Cab companies should tell you what the displacement limited power is, but they won't, since the much lower figure would impact sales. In practice you'll never over-power a cab if you don't push it beyond the point where you hear distortion. That means an amp rated anywhere between one-half and twice the power rating of your cabs is usually appropriate. Less than half probably won't drive your cabs to their displacement limit, more than twice is power you can't make use of anyway.

[quote name='6v6' timestamp='1382629258' post='2254694'] - the component can be attached to the board with tie-wraps or silicone [/quote]As they can, and for that matter should be, with wood. OTOH I've seen many a phenolic board that cracked, and the traces along with it, but I've never seen a wood base crack. Much ado about nothing you are making.

[quote name='6v6' timestamp='1382528950' post='2253198'] Well I never, google says it's true! [/quote]Google is wrong. Bulbs aren't employed in crossovers as fuses, although they will in effect act as fuses if the cab is over powered or fed with very highly distorted signals, which can blow them out. Bulbs are used as simple compressors. As the filament heats with higher power input the resistance of the filament goes up, reducing the voltage delivered to the tweeter. [quote]Same goes for the crossover, which should be constructed on a bit of fiberglass or phenolic board with turrets/tags, not a bit of ply which can absorb moisture and become conductive.[/quote]There is no reason to use fiberglass or phenolic other than that they can be made with conductive traces, which speeds the construction process in high volume operations. Making a board with conductive traces slows the construction process in typical DIY production runs of one or two cabs, even assuming the DIYer has the capability to make traced boards at all. Wood as a material for crossover boards has been used since the first crossovers were constructed circa 1920, with no ill effect.

[quote name='Dev70' timestamp='1382099437' post='2247923'] bought 2 sovereign pro 500s 12" my plan is to run them in 2 separate old but solid H/H cabs [/quote]There are dozens of threads about the dangers of randomly tossing together drivers and enclosures.

[quote name='RPaul' timestamp='1381907708' post='2245120'] apart from taking it to the nearest specialist to take a look at and opening up the wallet, is there anything I could do? [/quote]Even a specialist won't be able to do anything, A poor signal to noise ratio is a common charateristic of an inexpensive design. The cure is a better/more expensive amp.

[quote name='Billy Apple' timestamp='1381150371' post='2234953'] My point is not whether it can or can't be done, my point is that there are differences in opinion. In this case, one from a very knowledgeable chap such as yourself, and another from the manufacturer. For my money, I think the OP should be speaking to Eminence. [/quote]That depends on who at Eminence. For the most part when you email Eminence for advice you deal with a customer service rep, who gives advice based on a drop-down menu that they have of stock answers to what amount to FAQs. These people aren't engineers. They will have a far higher degree of product expertise than a retailer, but they don't make their livings designing either transducers or enclosures. Not that there aren't engineers at Eminence, but said engineers work in the engineering department, not customer service. You will not be put in contact with them, or even know their names, unless you are an industry professional, like myself or Alex. That doesn't mean that one needs the advice of an industry professional. There are certainly many amateurs quite capable of using speaker modeling software able to give definitive advise on how a certain driver will function in a specific cabinet. That advise could include specific details of the need to high pass at a specific frequency and to limit amplifier output to a specific voltage swing to insure that the driver cannot be harmed. You won't get that from Eminence's customer service department. The closest you'll get to that is if you look at their published plans for their drivers, which do come from their engineering department. But even that information can be duplicated by anyone with modeling software, and the knowledge of how to use it.

[quote name='Billy Apple' timestamp='1380979328' post='2233053'] Fair enough Bill, but below is a copy of the email Eminence sent me, and after all, they manufacture them. Anyway, my point is, everyone has an opinion, but for my money I'd listen to the maker. Hi Sean, Yes, you can remove the front gasket pretty easily without harming the speaker. Take a utility knife and cut it off, being careful not to cut any portion of the cone’s surround. Once you get the gasket started, you may be able to pull it off. Best Regards, A****** L**** Technical Support Eminence Speaker, LLC [/quote]I'm well aware that it can be done. My point is that it shouldn't be, there's nothing to be gained, and potentially a driver to be lost.