TOA VP-1240B Power Amp - diyAudio
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Old 28th May 2008, 01:54 PM   #1
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Question TOA VP-1240B Power Amp

Hi everyone,

I came across this power amp for sale at a very attractive price. However i'm a bit confused as to how to use it in my home system.

Was planning to use two of this amp to drive a DVC rockford RFR3115 subwoofer, 2ohm per coil. However, the amp appears to be only rated for 10ohm load minimum, is this correct?

Attached are the specs for both amp and sub. Appreciate any comments and tips!

Thanks!!
Attached Files
File Type: pdf vp-1240b.pdf (90.5 KB, 204 views)
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Old 28th May 2008, 01:59 PM   #2
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Sorry for double posting, this is the spec for the sub.
Attached Files
File Type: pdf rfr3115.pdf (67.8 KB, 116 views)
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Old 28th May 2008, 02:15 PM   #3
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Hi,

I've got a TOA VP1240B.

Yes, minimum speaker impedance is 10Ohm.
8Ohm would be OK, but 2Ohm/4Ohm is too low.
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Old 28th May 2008, 02:17 PM   #4
Corax is offline Corax  Germany
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Hi rhapsodee,

at a weight of 20.8kg I assume this is a mainly a 100V distribution amplifier which has a (heavy) transformer at the output (like in tube amps). Therefore you're able to select either 100V/42 ohms or 50V/10 ohms.

With the mentioned chassis I've my doubts that this is what you really want. Only in case you buy two of 'em and connect all (four) voice coils in series on one output channel of the TOA VP-1240B, resulting in a total load impedance of 8 ohms, it will work - even if you turn the volume knob all the way up. In all other cases I'm pretty sure the overcurrent protection will trigger quite soon when you turn up the volume knob, unless you will rewind the output transformer (if there's such a 'beast' in the VP-1240B).

The final result is:
The Rockford subwoofer chassis is definately no good match for such an amplifier. With 2 ohm voice coils they're designed for car audio (amplifier) use (only).
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Old 28th May 2008, 02:18 PM   #5
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Output and Load Impedance 100V / 42 , 50V / 10 , (70V / 21 )

What this means is if the amplifier is driving a 100 volt line the load cannot be lower in impedance than 42 ohm. Driving a 50 volt line the load cannot be lower than 10 ohms. Driving a 70.7 volt line the load cannot be lower than 21 ohms.

This is a commercial amplifier and it is not designed to drive a low impedance load. Its not suited to driving your subwoofer. Find something else.
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Old 28th May 2008, 02:18 PM   #6
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I will totally disagree with trying to drive a straight speaker without a transformer with this amplifier.

If your going to try something like that it would be far better to remove the step up transformer and drive your 8 ohm load direct off the amplifier. This amplifier isn't that much different than an A924 II. It will drive a low impedance load provided the transformer is bypassed
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Old 28th May 2008, 02:35 PM   #7
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Wow,

That was quick, thanks for all the replies!

Looking from the top, there was a massive (approx) 6in x 6in sized transformer in the middle, and a smaller 4in x 4in transformer by the side.

Andy : What do you use the amp for? How does it sound?

burnedfingers: how would i remove the step up transformer from the output? Is it the larger or smaller of the transformers? If i understand how transformers work, the power rating is still 240W with the ouput transformer bypassed right?

I guess the advantage of the high voltage high impedance loads is lower resistive losses in the speaker cables?

Sorry, i'm a mechanical engineer by training, so not very strong fundamentals in electrics and electronics.
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Old 28th May 2008, 02:58 PM   #8
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I guess the advantage of the high voltage high impedance loads is lower resistive losses in the speaker cables?

The idea behind the 100 volt and 70 volt lines is the ability to run long lengths of speaker line without having to use large gauge wire. By stepping up the voltage at the amplifier this is possible. At the speaker a step down transformer is used which accomplishes several things. The speaker can have its tap selected and therefore allow you to tap one speaker at 4 watts for example and other speakers at lower or higher taps as needed. The output at each speaker will be limited to the tap selected.

Sorry but I go nuts when I hear anyone mention the idea of using a speaker without a transformer. Its not an industry practice to do so. It also makes my life a living hell when I get to a site and have to remove a bunch of 8 ohm speakers some idiot hooked up to a system and then wondered why the amplifier was going into protection or why the amplifier fried. Another pet peave is installing a 8 ohm speaker directly to the output of a 70 volt volume control. The amplifier still sees the 8 ohm speaker as a nasty load when the volume control is set to maximum. Electricians are famous for that one.

Unfortunately the 1240 doesn't have a link to allow you to use the transformer or bypass it. The transformer is easy to spot. Just follow the leads from the speaker terminal strip and you will find the output of the amplifier feeding the transformer primary.
By taking the transformer out of the circuit the amplifier will drive a 4 ohm load without a problem.

When you look at the impedance rating it will work but the results would't be as good as driving the speaker without the step up transformer.
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Old 28th May 2008, 03:10 PM   #9
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Quote:
Originally posted by burnedfingers

When you look at the impedance rating it will work but the results would't be as good as driving the speaker without the step up transformer.
How is it not as good without the transformer? I can vaguely remember some high school electronics about using a transformer (at the speaker) to improve load matching, ie reduce the phase angle between the voltage and current, hence improving the power transfer efficiency. Am i making sense?

How does this affect sound quality in the case of a speaker? A transformer adds a resistive and inductive load, to even out a capacitive load, but is a speaker a capacitive load?

From your post, you must be involved in PA setups. I digress, but have you heard of a metal band called Manowar? They run very high power and high sound quality concerts, using tens of thousands of watts to achieve crystal clear sound. Think they still hold the world record for loudest concert.

Cheers!
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Old 28th May 2008, 03:27 PM   #10
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Quote:
How is it not as good without the transformer?
Quote:
When you look at the impedance rating it will work but the results would't be as good as driving the speaker without the step up transformer.
I believe I said the results won't be as good as driving the speaker without the transformer.

The frequency response is going to suffer somewhat with the transformer. The transformer is designed for general PA usage not for Hi Fi. The rating might say for example the frequency response is 20- 20k when the response may very well be 9 or 12db down at 20 hz.

Yes, I generally repair very large commercail systems like football stadiums and the like. I generally play with 50K watts on up.

Quote:
How is it not as good without the transformer? I can vaguely remember some high school electronics about using a transformer (at the speaker) to improve load matching, ie reduce the phase angle between the voltage and current, hence improving the power transfer efficiency. Am i making sense?
The transformer and amplifier combination will not drive your load.
The solution is what I mentioned ...remove the transformer and drive your speaker direct .

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