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Old 20th February 2003, 03:55 PM   #1
2Bak is offline 2Bak  Denmark
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Default Bridged Gainclone?

I'm doing a multichannel gainclone to be used as surround L+R and center amplifier.

I use this inverted GC circuit, without input pots.

Is it possible to use this Elliott way to wire two channels into one?
Or should I do it differently since CGs are inverted? Something like in this thread on the first drawing?

Please recommend. I would like to bridge two of my four channels, to drive my center. So I will have three channels total. Something like 2*30 watts for surround channels and 1*60 watt (this will be 4 ohm in Elliots way) for center channel?

What must I take into consideration about the toroid used for bridged channels? The toroid for the two L+R surround channels is a 300VA, 25+25. I use LM3875T's.

I plan to bridge cause I guess a center channel speaker need more power than surround speaker, but maybe I'm wrong...If this is not the case I will build identical channels...

Thanks in advance!
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Old 20th February 2003, 04:05 PM   #2
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Hi!

The topic of bridging LM 2875 has been discussed widely on this board, with National's recommendations on bridgin being probably the best way to do it (with buffering OPs), look at the application notes "BPA200".

But I don't think that the center speaker requires more power than the satellites... In fact, there are some DIY surround sets out there (at least I know of two Visaton sets that are build this way), which use for all five main channels (LF, RF, Center, LR, RR) the same speaker, a d'Appolito construction.

If properly configured (high-pass), the output power of a LM 3875 should be more than enough, if maybe not for getting a THX certificate, but who knows?

When testing my GCs, I never got to their limits (using 8 ohms speakers), if I didn't want to bring my neighbours wrath down on me...

Bye,

Arndt
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Old 20th February 2003, 09:34 PM   #3
2Bak is offline 2Bak  Denmark
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Thanks Arndt, you convinced me. I'll leave it as it is with four identical channels.

Since my Thule pre/processor does in fact have two (identical) center line outputs, four CG channels gives me the chance to experiment with bi-amping of a single center speaker, or using two center speakers. Or even try to power up a passive sub with good efficiency from the preamps sub line out.

"If properly configured (high-pass), the output power of a LM 3875 should be more than enough, if maybe not for getting a THX certificate, but who knows?"

- what exactly do you mean by high-pass?

/Jan
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Old 21st February 2003, 06:30 AM   #4
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Hi!

Quote:
Originally posted by 2Bak
"If properly configured (high-pass), the output power of a LM 3875 should be more than enough, if maybe not for getting a THX certificate, but who knows?"

- what exactly do you mean by high-pass?

/Jan
If the Thule pre-amp you mention is a proper Dolby Surround preamp, high pass should already be enabled for satellites and center speakers. High pass means that the pre amplifier uses an active (active means before the signal has been through the main ampflifier) crossover, letting only the high frequencies (over a certain threshold, most often configurable) pass. The smaller a speaker chassis is, the less power it can handle in lower frequencies. But if you just leave out those frequencies, those smaller chassis play more efficiently, effectivly producing louder output with less needed power and less distortions.

Of course this is also the same for the subwoofer, but the other way around (low pass).

I for example drive my satellites actively cut off at about 62 Hz, with a slope (?) of about 24 db (this indicates the steepness, with which the frequency curve drops off at the cut off point, 24 db is pretty steep). But I do not use a pre amp, I use a DSP in my computer, but your pre-amp will do about the same. There is no real recommendation about how to properly configure an active crossover, you will have to do listening tests (maybe even frequency measurements) and decide for yourself...

Bye,

Arndt
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Old 22nd February 2003, 06:36 AM   #5
2Bak is offline 2Bak  Denmark
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...yes, my Thule pre/processor has those high-pass facilities.

Additional question: Can a 500VA 28+28 v be used for the LM3875 in this circuit?

/Jan
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