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Old 27th January 2006, 09:31 PM   #1
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Default Q on BOSOZ topology

Hi,

I'm giving some thought to building a multi-channel pre-amp, based on BOSOZ or other balanced single-ended topolgy.

I'm unlikely to use balanced sources (i.e., source devices will connect through singe-ended inputs), but will probably use balanced outputs.

In that case, is there any real advantage for the "X" or super symmetry variations on the basic BOSOZ topology?

Thanks and regards,
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Old 27th January 2006, 09:56 PM   #2
eLarson is offline eLarson  United States
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If you are thinking of using its capability as a unbalanced-to-balanced converter, you might consider replacing the 750 ohm resistors in the Sources of the MOSFETs with constant current sinks.

(Not that it wouldn't work okay as is... I just found it to work a little better with the CCSes.)

I didn't attempt the shunt feedback/X modification so I won't comment on that.
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Old 27th January 2006, 10:43 PM   #3
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Default CCS and Gain

Quote:
Originally posted by eLarson
If you are thinking of using its capability as a unbalanced-to-balanced converter, you might consider replacing the 750 ohm resistors in the Sources of the MOSFETs with constant current sinks.

(Not that it wouldn't work okay as is... I just found it to work a little better with the CCSes.)

I didn't attempt the shunt feedback/X modification so I won't comment on that.
Ok, that's interesting. Just about all of my sources will be line level, meaning I don't need a lot of gain from the circuit. Don't the CCS's bump up the gain quite a bit? If so, would the X feedback be effective in keeping the CL gain sufficiently low (~6dB or so). Or perhaps I could do without. I dunno.

Thanks,

Rob
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Old 28th January 2006, 12:25 AM   #4
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Well, there's surrent sources and then there's current sources...
I think you guys are talking about two different applications.
eLarson, I suspect, is thinking in terms of a CCS under the SOZ, replacing the resistors that bias the differential. This will not increase the gain. Won't decrease it, either. But it will make the amp function better as a phase splitter, which will be useful if you're using unbalanced input signals.
weinstro is thinking of CCSs as loads to the gain devices. Yes, if you use a CCS as a load for a gain device, you'll get more gain. It constitutes an infinite load. The problem you run into there is that the DC won't be quite stable. Note that Nelson published the schematic for the F1 (?) not too long ago. It has a couple of resistors that address this very problem.
Multi-channel implies AV use, which in turn implies at least occasional explosions, volcanoes, earthquakes, and other heartbreaks. The SOZ may not be for you...unless you have very, very efficient speakers indeed.
For what's it's worth, I did a thread on the SOZ with current sources (supplying the differential--not loads) at one point, and offered a schematic with a working circuit for same.

Grey
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Old 28th January 2006, 03:17 AM   #5
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Thanks, Grey

Quote:
Originally posted by GRollins
.....weinstro is thinking of CCSs as loads to the gain devices. Yes, if you use a CCS as a load for a gain device, you'll get more gain. It constitutes an infinite load. The problem you run into there is that the DC won't be quite stable. Note that Nelson published the schematic for the F1 (?) not too long ago. It has a couple of resistors that address this very problem.
Likely. I'll try to dig up the schematic and see what you're talking about.

Quote:
Originally posted by GRollins
Multi-channel implies AV use, which in turn implies at least occasional explosions, volcanoes, earthquakes, and other heartbreaks. The SOZ may not be for you...unless you have very, very efficient speakers indeed.
For what's it's worth, I did a thread on the SOZ with current sources (supplying the differential--not loads) at one point, and offered a schematic with a working circuit for same.
Mostly, I'm interested in high res DVD-Audio, but yes, Dolby 5.1 and DTS movie tracks will be in the mix, too.

That said, this is distinctly for a preamp. I already have a bunch of traditional amplifers (Hafler, Sumo, Marantz) that will be used to drive my inefficient bass-reflex speakers. The idea I'd like to pursue is multi-channel volume and switching control, zero to low feedback, balanced output.

Given the line level outputs, it makes me wonder if I just really need a buffer/follower with a volume knob. But, I'm intrigued by the elegance and simplicity of the balanced, single-ended circuits.
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Old 28th January 2006, 03:45 AM   #6
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My fault...I'm short on sleep and read BOSOZ as SOZ.
Given that the BOSOZ is cap-coupled at the output, you can disregard my previous notes about DC offset unless you feel like guilding the lily.
At the risk of shooting myself in the foot again, I'd recommend at least 6-12dB of gain, perhaps as much as 18dB. Line level outputs are not always enough to drive amps to full power, especially if you end up with a soundtrack/CD/whatever that's mastered lower than usual. You'll need a touch of gain to make up the difference.

Grey
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Old 28th January 2006, 05:33 AM   #7
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As GRollins said the CCS instead of sourceresistors will give a better performance when running this circuit as a singelend/balanced converter, and with Xfeedback you can set the gain to anything you like within the limits of the circuit.
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Old 28th January 2006, 07:16 AM   #8
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and,

X will work with a balanced output regardless of input.

William
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