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Old 13th February 2004, 04:03 PM   #1
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Default 2-ohm stable amplifier PCBs or kits available?

I've searched around the forums, but so far haven't come across what I'm looking for. I'm getting ready to build an MTM speaker where the mids are 6 ohms nominal. If I put them in series, I don't get the increased sensitivity I want, but if I put them in parallel I get a load that looks like 2.5-3 ohms through the midrange.

SO, has anyone put together an amplifier PCB or kit that will deal with a 2-ohm load? I'd be looking for something that could generate 300WPC into 2 ohms, roughly.

Thanks in advance for any advice you might have.

Paul Pickard
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Old 13th February 2004, 04:10 PM   #2
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It sounds like what you should really be focusing on is addressing impedance compensation in your crossover design and not a 2 ohm capable amp.

However, if you are set in your ways, you can search on "BPA200" and "bridgeclone" to see what you like in the 2 ohm world.
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Old 13th February 2004, 04:19 PM   #3
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Default impedance compensation?

I'm fairly new at this with only a couple of basic designs under my belt, but what can you do in the passband besides add series resistance? Certainly that raises the impedance, but it also lowers the sensitivity.

Paul
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Old 13th February 2004, 04:27 PM   #4
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Default Resistance

Yes, put the speakers in parallel and add a 1 ohm 25 or 50 watt resistor in series. This will bring them up to 4 ohms. You do lose about 1 db in sensitivity but you can't have everything.
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Old 13th February 2004, 04:31 PM   #5
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How about 4 or 6 LM3886 in parallel?
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Old 13th February 2004, 04:49 PM   #6
tiroth is offline tiroth  United States
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If you have a 3 ohm load and you put 1 ohm in series with it, you are into tube range damping factor. Depending on the crossover you could have significant anomolies from the predicted response. I believe Rod Elliot's article on damping addresses this.
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Old 13th February 2004, 05:01 PM   #7
jcx is offline jcx  United States
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why not "tri-amp"?

separate amps for each driver; the M amps could share power supply, the output devices/heatsinking will cost nearly the same with 2 amp approach, the only "extra" cost over a lo-Z amp is small signal/dirver stage stuf and a superior performing low power T amp and the active crossover
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Old 13th February 2004, 08:18 PM   #8
sreten is offline sreten  United Kingdom
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The advice about series resistance is completely incorrect.

Its as lot easier in terms of output devices to build a 300W/8ohm
amplifier than a 300W/2 ohm amplifier, personally I can't see the
point of your concern with voltage sensitivity, efficiency of the
series or parallel connected drivers is identical.

(Any difference in speaker voltage sensistivity in a multi channel
set up can be addressed by adjusting the amplifier gains.)

sreten.
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Old 13th February 2004, 08:38 PM   #9
mlloyd1 is offline mlloyd1  United States
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I'm hoping my famously bad memory doesn't get me into trouble yet again

I think ribbon speakers are low impedance (right?) so this Borbely Audio amp may be worth investigating

On the other hand, "ribbon" is probably just a name for the amp model.

mlloyd1
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Old 14th February 2004, 02:28 PM   #10
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Default sensitivity not a concern?

Quote:
Originally posted by sreten
The advice about series resistance is completely incorrect.

Its as lot easier in terms of output devices to build a 300W/8ohm
amplifier than a 300W/2 ohm amplifier, personally I can't see the
point of your concern with voltage sensitivity, efficiency of the
series or parallel connected drivers is identical.

(Any difference in speaker voltage sensistivity in a multi channel
set up can be addressed by adjusting the amplifier gains.)

sreten.
OK, I'm a gearhead so I will admit that this stuff isn't easy for me, but let me explain my thinking. When using two midranges, folks typically assume a 6db increase in sensitivity (at 2.83 V/1m), which as I understand it is because you've doubled the radiating area and doubled the current from the amp. So, if you wire these two drivers in series you double the radiating area but halve the current through the drivers, which then gives the same sensitivity as a single driver. Is this correct? If not, please explain where my I'm goofing this up.

In my MTM I have 2 mids that are 92db/2.83V/1m, and a tweeter that is 96db/2.83V/1m. Ignoring baffle step (for the moment), if I want a 96db/2.83V/1m MTM, I need to put these 6 ohm mids in parallel. Before anybody says it, I'm already planning an active system (which is why I can ignore the baffle step for the MTM): the MTM from 300hz to 20kHz, Lambda TD-12S from 300Hz to about 60 Hz, and a dedicated subwoofer from 60 Hz down. My wife is tolerant of this hobby, but I don't know how tolerant she would be of having to turn on the CD player, preamp, crossover, and 4 amplifiers to listen to music. I'm already getting comments about how difficult it is to work my bi-amped setup today.
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