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Old 23rd November 2011, 09:32 PM   #21
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Originally Posted by chris661 View Post
Assuming bridged operation, 48v peak output = 34v RMS = 144w (using V^2/R) into 8ohm.
You need to check your sums!.

48v p-p is only 17V RMS, giving you only 36W to a 8 ohm speaker.

Using a 4 ohm (if the amp would take it) would give you 72W or so.

As you're building a mains amp, it's pretty silly to use such a low supply rail, just because you have one to hand.
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Old 24th November 2011, 07:48 AM   #22
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Originally Posted by Nigel Goodwin View Post
You need to check your sums!.

48v p-p is only 17V RMS, giving you only 36W to a 8 ohm speaker.

Using a 4 ohm (if the amp would take it) would give you 72W or so.

As you're building a mains amp, it's pretty silly to use such a low supply rail, just because you have one to hand.
Ah yes, you don't double the volts twice.

I was intending the 24v supply as a test to make sure the amplifier and speaker works, with the idea of moving up to a 36v 10A PSU at a later date.
This would give me about 80w before clipping, which is more like it.

The alternative to a cheap switching amplifier would be to find a class AB one (or higher voltage switching) and use the pair of 68v split (linear) supplies I have here.
If I put them in series, that'd be +/-68v rails, giving... 288w into 8ohm, though I don't think the transformers would survive running 4ohm - they're about 250VA each.

Decisions...

Chris
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Old 24th November 2011, 01:03 PM   #23
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Hi all! Just a couple thoughts: amplifying guitar is way different than amplifying most other stuff. That means: 1.Limited bass response is a target. Depending on your style, you don't need much below a couple hundred Hz. Okay, quite vague... Let's put it this way: you only need to get close to 100Hz if you play strong low Heavy Metal riffs. So, there's not so much air to move - again: depending on your style - so there's good chance a 6" can do. Note: limited treble is desired too, specially for using distortion pedals. 2.You don't get linear or uncompressed. If you do, you're not going to like the sound. That means you can't go for the "musical power" of your speakers to spec your amp. The dynamic range is way lower than acoustic stuff (disregarding post-90 CDs...), and you get more harmonics than normal (again and again: depending on your style), so respect the RMS power limit. 3.Open rear cabinets help on cooling the speaker. But you have to limit the bass output not to blow the speaker on overexcursion. Watch out: the desired nonlinearities create sub-harmonics and so on, so put your limit after overdriven stages! Important question (excuse me if you pointed so already): which is your style? Best regards, Emerson
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Old 24th November 2011, 04:08 PM   #24
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Originally Posted by Emerson Prado View Post
So, there's not so much air to move - again: depending on your style - so there's good chance a 6" can do.
But volume is still all about moving air, and a 6 inch driver doesn't move that much air.

To get the same volume from a 6" driver as a 12" would require a LOT more power, and a 6" driver capable of much greater cone movement.
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Old 24th November 2011, 05:24 PM   #25
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I think I have a plan.

Going to get hold of one of these Eminence speakers, and a 2nd hand graphic eq pedal (to dial the exact sound I want). Using a sufficiently large power amplifier (I have something big enough here that'll do for a test) and the limiter/mixing desk to limit the bass going to the speaker.
I was thinking of going for a ported box to improve the LF power handling of the speaker, and provide some form of ventilation. Anyone got any thoughts on that?

Chris
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Old 24th November 2011, 06:14 PM   #26
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Originally Posted by Nigel Goodwin View Post
But volume is still all about moving air, and a 6 inch driver doesn't move that much air
Sure. What I wanted to emphasize is the need of air movement is frequency-dependant - the lower the frequency, more air should be moved. Since a guitar amp is not supposed to deliver much bass, the need for air movement is drastically smaller than, say, a bass guitar speaker. So Chris might be able to get high SPL from a small cone if he doesn't abuse on the low end - what, again, depends on his style.
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Originally Posted by Nigel Goodwin View Post
To get the same volume from a 6" driver as a 12" would require a LOT more power, and a 6" driver capable of much greater cone movement.
Cone excursion might be a limitation, indeed. I guess wider speakers have greater excursion limits in general (though I never checked this). But, again, higher frequencies demand less excursion, and the above apply. IMHO, Chris has a chance, just will have to be happy without lots of bass.
Other things to consider: our ears are way more sensible to mids, which are predominant in most guitar styles. So the absolute SPL for the guitar can be lower than the bass and drums. Also: hardly ever the other musicians want to hear the guitar as loud as the guitarist wants - get the amp close and pointed towards you.
Another thing I forgot to mention: smaller cones tend to make guitars harsh. Have an eye on the tone character for the amp.
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Emerson
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Old 24th November 2011, 09:48 PM   #27
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I was thinking of going for a ported box to improve the LF power handling of the speaker, and provide some form of ventilation. Anyone got any thoughts on that?
Porting DECREASES the power handling of the speaker, a sealed box (infinite baffle) is the best for power handling, the limited space restrains movement of the cone.
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Old 24th November 2011, 09:54 PM   #28
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Originally Posted by Emerson Prado View Post
Sure. What I wanted to emphasize is the need of air movement is frequency-dependant - the lower the frequency, more air should be moved. Since a guitar amp is not supposed to deliver much bass, the need for air movement is drastically smaller than, say, a bass guitar speaker. So Chris might be able to get high SPL from a small cone if he doesn't abuse on the low end - what, again, depends on his style.
Certainly the lower the frequencies, the more excursion of the cone - but guitar isn't bass heavy anyway, so it's not really that much of a concern.

But trying to get similar volume levels from a smaller speaker require MUCH greater cone excursions (simply pumping air remember), so a 6" speaker will require far greater excursion capability, and much higher powers top provide the same volume.

Quote:

Cone excursion might be a limitation, indeed. I guess wider speakers have greater excursion limits in general (though I never checked this).
Not particularly, larger speakers don't need as large an excursion as smaller ones, as they pump more air by moving less.

Large excursion speakers are often smallish HiFi types, aiming to give good bass from a small speaker/cabinet, by using high powers.
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Old 25th November 2011, 06:54 AM   #29
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Originally Posted by Nigel Goodwin View Post
Porting DECREASES the power handling of the speaker, a sealed box (infinite baffle) is the best for power handling, the limited space restrains movement of the cone.
I'd beg to differ there.

Assuming a steep low cut below port tuning, the port provides more excursion damping than a small sealed box - it almost stops the cone altogether at port tuning.

The maxSPL chart in WinISD (you'll have to enter the Xmax data for the driver) shows what I mean quite nicely.
Taking an 8L cabinet for both (ported tuned to 90Hz), at 100Hz the ported cabinet will run out of thermal power handling at 111dB, the sealed box runs out of excursion at 101dB.
At 80Hz, the ported does 109dB running at full power, the sealed just below 98dB (excursion limited).
Now, the ported cabinet is much smaller than the optimal ~15L, but still takes the sealed box to town.
Combine that with the fact that, using two ports, you can get convection cooling for all the internals, and I can't see why anyone would use a sealed cabinet.

Chris
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Old 25th November 2011, 12:38 PM   #30
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Chris, which is your style? Most good guitar cabs are open back, while some are sealed, and I can't recall of a ported one (I don't know as many, though). A sealed can add some bottom, and a ported can add some more, but do you really want more bottom? That's why I'm asking your style. Depending on what you play, more bottom will harm more than help. OTOH, good cabs use 10" to 15" speakers. With 6", design path might have to change anyway... Best regards, Emerson
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