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Old 15th June 2002, 06:52 PM   #1
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Default An odd bridging question

I have two monoblock Leach Amps and I'd like to bridge them.

I know that I will run into trouble if I just connect sources and connect my subwoofer between the live speaker terminals. I think this is because the speaker output current would run in the signal leads, but I'm not sure. But will it help if I put a very thick and very short cable from one Amp's grounded speaker terminal to the other's?

And theoretically, what is the requirement for safe bridging, if you had to pick one or more? Also, which ones are never required?

- Channels have same physical ground point
- Channels are fed from the same mains transformer or power supply
- Channels are inside the same chassis
- Channels have a common ground point (i.e. voltage)
- Channels are placed physically close to each other (how far?)
- Channels have a low-resistance or short connection between their ground points
- Channels clip at the same level, have same gain and have same freq. response

-Kimmo Sundqvist
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Old 15th June 2002, 07:54 PM   #2
paulb is offline paulb  Canada
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www.sound.au.com has some information. See "Miscellaneous Projects" #14 and #20.
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Old 17th June 2002, 12:31 AM   #3
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One of the disadvantages of bridging two channels into a load for higher power is the loss of your ground reference. However, you should be alright in this case because each of your secondaries should be forced to earth / chassis and at about the same potential for the two amplifiers. The load will float with the two outputs. One channel must be and inverted version of the input signal and the speaker is connected between the two main outputs. One amplifier will sink current and the other will source it. There should be no problems running the two leack amps you speak of just follow the above link and it will show you a simple circuit on how to invert and buffer your input signal. Make sure that you have implemented enough heatsinking because each amplifier will see an impedance which is half of the actual speaker load impedance. If you are running an 8 ohm speaker the amp wil see four and you should be ok with audio. However, if you speaker is two ohms or lower the amp may overheat and the current limiting VI slope circuit will activate limiting output power. Good Luck and keep us updated!!!

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Old 17th June 2002, 02:49 PM   #4
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The same problem here as in Finnish usenet newsgroups. I forgot to point out that I know what bridging is. I know the strain it puts on the amplifiers. I know how the signal is inverted and I have propably seen all the possible ways described at least once. I know what one gains and what one loses by bridging.

My plan includes using an Adire Tempest with coils in parallel, and that is 4 Ohms, which means the IV limiter might get active in ordinary Leach Amps with only 2+2 MJ15003/4 at the output. The real problem and the only problem was, how should the earths of the two separate amplifiers be connected to avoid trouble?

My thoughts were that a) the signal leads that have earths that meet at the bridging adapter are not enough, and b) using a thick short cable from one black speaker terminal to another just might do the trick. But if anybody is sure one way or another, will it do the trick, I'd love to hear about it.

-Kimmo Sundqvist
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Old 17th June 2002, 03:50 PM   #5
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Default Asking for Earth Loops

Hello Musher,
In answer to your question, for a bridging amplifier to be electrically correct the earth common of the two amplifier stages needs to be absoloutely common.
A very good connection between the two amplifier negative terminals may work satisfactorily despite parasitic signal ground and parasitic AC power grounds, however this technique may be asking for trouble.
Isolation transformers seperately supplying the inputs would avoid signal level ground looping and may be a soloution.
The proof is in the eating !.

Good luck,
Regards, Eric.
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