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!!
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!!
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).
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).
Output and Load Impedance 100V / 42 Ù, 50V / 10 Ù, (70V / 21 Ù)
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.
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.
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
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
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.
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.
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.
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.
burnedfingers said:
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!
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.
The transformer and amplifier combination will not drive your load.
The solution is what I mentioned ...remove the transformer and drive your speaker direct .
Usually the bigger transformer is the output transformer (the one in the middle with 6" x 6"). To transfer the same power at lower frequencies than i.e. the mains frequency (which would be 50or 60Hz) the core must be much larger (actually the cross-sectional area). To make sure it would be the best to follow the output terminal posts all the way back to the secondary winding of the output transformer - the other winding, the primary, will lead you to the amp output - as 'burnedfingers' already mentioned.
Besides, I won't be so sure if the internal amp of the VP-1240B is a "standard" amplifier driving a step-up transformer for the 100V line. I had once an amplifier to repair which has almost the same power rating and has used a (cheaper) step-down transformer (i.e. from a high-power tube amp). In this case it would be nonsense, if not fatal (for the amp and/or speaker
), to connect a speaker directly to this kind of 'high-voltage' amplifier.
But since I do not know exactly the schematics of the TOA amps I can only guess (and hope for you) that this would work.
In my opinion it would be useless to buy such a heavy amplifier and bypass the output transformer just to use the internal amp. Aren't there any other cheap (PA) amps out there (or in your reach) which you can use instead???
Besides, I won't be so sure if the internal amp of the VP-1240B is a "standard" amplifier driving a step-up transformer for the 100V line. I had once an amplifier to repair which has almost the same power rating and has used a (cheaper) step-down transformer (i.e. from a high-power tube amp). In this case it would be nonsense, if not fatal (for the amp and/or speaker
), to connect a speaker directly to this kind of 'high-voltage' amplifier.But since I do not know exactly the schematics of the TOA amps I can only guess (and hope for you) that this would work.
In my opinion it would be useless to buy such a heavy amplifier and bypass the output transformer just to use the internal amp. Aren't there any other cheap (PA) amps out there (or in your reach) which you can use instead???
i have one too in one of our company's resto............the thing is, its now exhibiting some abnormalities.when you turn it on, the volume of the sound will burst to a high level then gradually decline to the desired level. there is no alarm whatsover in the amp itself...could it be with the volume control module which i checked and found perfectly fine or the speaker network?
I'm just wondering if removing the 100v line step up in a push pull design is a little more complicated than discussed here as I've got the same VP-1240B unit open with feeds to the step up from both banks of transistors one side yellow one side orange with a black tap in the between them on the primary of said step up. Hope this makes sense. I'm a little unsure about how to take a direct speaker feed in this scenario. Here's the schematic. Any help would be very much appreciatedUsually the bigger transformer is the output transformer (the one in the middle with 6" x 6"). To transfer the same power at lower frequencies than i.e. the mains frequency (which would be 50or 60Hz) the core must be much larger (actually the cross-sectional area). To make sure it would be the best to follow the output terminal posts all the way back to the secondary winding of the output transformer - the other winding, the primary, will lead you to the amp output - as 'burnedfingers' already mentioned.
Besides, I won't be so sure if the internal amp of the VP-1240B is a "standard" amplifier driving a step-up transformer for the 100V line. I had once an amplifier to repair which has almost the same power rating and has used a (cheaper) step-down transformer (i.e. from a high-power tube amp). In this case it would be nonsense, if not fatal (for the amp and/or speaker
But since I do not know exactly the schematics of the TOA amps I can only guess (and hope for you) that this would work.
In my opinion it would be useless to buy such a heavy amplifier and bypass the output transformer just to use the internal amp. Aren't there any other cheap (PA) amps out there (or in your reach) which you can use instead???
VP-1240B Schematic
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