si t amp with altec model 288c driver. - diyAudio
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Old 10th May 2006, 10:00 PM   #1
tade is offline tade  United States
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Default si t amp with altec model 288c driver.

I am wondering if I could use this amp with my altec model 288c compression drivers. The system is bi amped. The drivers have a rated sensitivity of 115db per watt. I worry that because I am using homemade edgar horns there will be an efficiency decrease. These drivers are also 24 ohms. What that means is that they will be consuming fewer watts. So the amp which is rated at about five watts into eight ohms will only be able to deliver a fraction of that into 24 ohms. Will this limit my volume? I suppose my max will be around 117db. Is that a decent volume?

any other comments about this setup are very welcome.
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Thaddeus
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Old 10th May 2006, 10:14 PM   #2
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Thaddeus,

Are you serious? 117dB? That's enough to start a nose bleed. Or do more than ruffle the feathers on a chicken. Heck, 117 dB is enough to make people dance the jitterbug.
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Old 10th May 2006, 10:34 PM   #3
tade is offline tade  United States
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sweet! Thanks for the reassurance! I also have some SLA batteries sitting around and a charger! Time for some experimenting.
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Old 10th May 2006, 10:43 PM   #4
BWRX is offline BWRX  United States
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You will definitely want to change the value of at least the caps in the output filter. With 15ohm speakers you will have a rapidly rising frequency response in the upper frequencies.
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Old 10th May 2006, 10:50 PM   #5
Eva is offline Eva  Spain
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115dB/W is not a valid sensitivity figure. 112dB/W already corresponds to 100% power efficiency, and practical sensitivy figures for the highest performance compression drivers with horns are below 110dB/W.
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Old 10th May 2006, 10:57 PM   #6
tade is offline tade  United States
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Here is where I found the spec:
http://www.voiceofthetheatre.com/images/288C.2.jpg

So lets say that my efficiency is somewhat less. If I really crank up some guitar music or something, will this amp wuss out on really loud transients like picking?

BTW, they are 24 ohm.
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Old 11th May 2006, 03:30 AM   #7
el`Ol is offline el`Ol  Germany
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That will probably be too much, even with 24 Ohm. I had very limited dynamic range with a 4Ohm 96dB driver at low volumes. My experience is that the potentiometer of the T-Amp should be at nine o`clock minimum. A SE 2A3 amp would be a better choice.


PS: There is an ALE midrange compression driver with 120dB and a GOTO with 118.
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Old 11th May 2006, 04:42 AM   #8
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Quote:
Originally posted by Eva
115dB/W is not a valid sensitivity figure. 112dB/W already corresponds to 100% power efficiency, and practical sensitivy figures for the highest performance compression drivers with horns are below 110dB/W.
The answer to this riddle is that the higher sensitivity figure is given for the driver mounted on a horn. Because of the horn's directivity, the on axis power efficiency can appear to be higher than 100%.
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Old 11th May 2006, 06:47 AM   #9
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The Good, the Bad and the Ugly.

Good: You have a great driver that is very sensitive.
Bad: 24 Ohms. You will be hard pressed to get 2 RMS watts out of the SI.

2 watts may be enough for you, or maybe not.

The Ugly:
The Sonic Impact is a low voltage amp. It runs on 13V max. That has advantages, but not in the case of 24 ohm drivers, they need more voltage (but not more power). Tube amps have much more voltage than you'll ever need, so there is a transformer on the output. Easy to make an output tap for the impedance you want - e.g., 4-8-16 ohms. Not so with the Sonic.
Also, as Brian mentioned, the SI output filter is not designed for 24 ohms so it would need to be altered.

So the great detail and speed of the tripath chip would sound great on your Altecs, but the Sonic is ill adapted to the 24 ohm driver.
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Old 11th May 2006, 07:06 AM   #10
Pano is offline Pano  United States
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Quote:
Originally posted by jeff mai
The answer to this riddle is that the higher sensitivity figure is given for the driver mounted on a horn.

Exactly. Sensitivity is related to directionality. Efficiency is not.

One Watt of acoustical power radiated into free space will give 112dB at 1 meter. That's 112dB thru 360 degrees in all directions. If you can restrict that space - direct all the energy into a smaller slice of the space, you could get more than 112dB for 1 Watt. That's what speakers do.

The most efficient speaker I ever heard was almost 50% efficient and was rated at 115dB/watt 1 meter. That's the giant W.E. 15A horn. Very efficient and focused its energy into a limited space. I don't know of any other speaker more efficient.
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