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Old 29th December 2001, 12:33 AM   #1
SteveA is offline SteveA  United States
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Location: St Paul, MN
Question More BOSOZ ?s

I have ordered the PCB's from AudioXpress; I've ordered the parts NP listed in his '97 article (ie, all the board stuff+transformers). Any recommendations for:
1-source selector switch?
2-volume controls?

Also, how does one provide (ie, wire) for both balanced and unbalanced operation? (a paint-by-numbers description, if possible)?

Are there PCB's available for a companion amp? And parts list form same?
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Old 29th December 2001, 02:32 AM   #2
cp642 is offline cp642
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Join Date: Aug 2001
If you're on of those who particularly paranoid about the outcome of the sound of the BOSOZ, you might want to try out Shallco's stepped attenuator and rotary switches....they're way cool.. You might want to check it out yourself...http://www.shallco.com/

As for enclosures, check out http://www.par-metal.com/ or you might just want to carve one out of aluminium slabs as what NP did with his preamps ....
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Old 29th December 2001, 05:32 AM   #3
paulb is offline paulb  Canada
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The companion to the Bride of Son of Zen is the Son of Zen. It is so simple no PCB is needed. But it is very inefficient. See the passdiy.com website.
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Old 29th December 2001, 08:22 AM   #4
grataku is offline grataku  United States
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I used a couple of 10k alps black beauty dual pots to attenuate the output. No input pots and fixed gain (no p5).
I bought a toroidal from plitron 80 VA 60+60V. I had a serious problem with it as with no load the SOB output 73VAC as opposed to 67 that makes 21% regulation as opposed to the declared 13%! with the preamp load the voltage drops to ~71V.
Bottom line, 71*sqrt2 made the initial cap rating of 100VDC a little too close for comfort and having to drop almost 40 V@0.16A meant getting the regulator mosfets red hot.
So I used a couple of 1200uf 200VDC caps and a couple of 150 ohm 25W resistor I had around to chop 25 V dc off and leave the rest for the regulator. Using a pair of headphones with 94 dB sensistivity at full volume and nothing playing I can't hear any trace of hissing.
I made up the rectifiers using 8 IR hexfreds and bypassed all the electrolytics with 1uF MKT caps.
I hetched a little board with 12VDC pcb relays that I mounted directly, with much pain, to the back panel of the rca and XLR connectors.
A little auxiliary board takes care of the power to relay and a simple rotary switch does the commutation between relays.
The logic part could definitely have been handled better.
I used probably 5 times the surface heatsinks to cool the mosfets, I actually put the 750 ohm res and the mosfet on the trace side of the pcb and put the caps and all the other passive components on the other side. The 3w resistors were mounted at about 1/2 inch above the pcb to avoid the stench from burning the g10 and the varnish. I mounted the board upside down with long standoffs so that the caps would be shielded from the heat.
Here are the relevant temp I measured:
PS mosfets 47 deg C (these have the biggest heatsink)
preamp mosfets 57 deg C
resistors 100 deg C (!!!)
I wonder what other people have measured I think I might very well have to supersize the resistors and add cooling to the 4 main mosfets as I don't believe in disposable preamps.

I found that when driving the amp in single ended mode the inverting output is about 25% lower than the non-inverting so I don't know if the unit is really going to work as an unbal. to balanced converter. I am not an expert in balanced systems but it would seem to me that in order to get good CMRR the noise should have equal or close intensity.
good luck
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Old 29th December 2001, 04:16 PM   #5
GRollins is offline GRollins  United States
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Note that I haven't built the preamp, but I think this is one of those circumstances where one might consider a current source under the differential--at least as far as the unbalanced to balanced aspect goes. Shouldn't be a problem when operating bal/bal or unbal/unbal.
You're right, it does sound as though it's running a bit on the warmish side.

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Old 29th December 2001, 04:49 PM   #6
hifi is offline hifi  Sweden
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Location: sweden
By the way..

Is there any benefit in running the signal through the back of the mosfet and throw a screw and the heatsink as on the BoSoZ original layout?....

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Old 29th December 2001, 06:58 PM   #7
grataku is offline grataku  United States
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the balanced problem: maybe one doesn't get full noise and distortion cancellation but still that maybe better than nothing. Nelson must have noticed this and thought it wasn't very important??

in my reverse mounting scheme the signal goes through the standoff which in my case I machined out of brass. In the original scheme one could use brass or cu screws.

IMHO by far the most critical aspect of this project is COOLING!
I definitely will re-engineer the heatsinking and substitute the power resistors. The temp rating for the metal oxide film resistor is 155 C. I am not comfortable with them operating at 100 C.
I wouldn't be so worried if they were wire wound resistors.
I was thinking to either use those or go to to220 style thick film power resistors and heatsink.
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Old 29th December 2001, 08:05 PM   #8
PassFan is offline PassFan  United States
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Location: Central FL
Default BAL / UNBAL

Unless you need balanced right away It will simplify startup to stick with one or the other. I believe the article tells you what to ground for unbalanced operation. You can't just not hook up an input or output for unbal. operation. They have to be grounded in the circuit. Let me know if you need it and I will look over the article and come up with a diagram.

I used alps pots and grayhill selector switches. As for the SOZ amp, it doesn't use a pcb because of low parts counts and the physical size of the power resistors. It is a simple amp to build but it ain't cheap and it ain't light. If you decide to build one keep it small (15 to 20 watts). That really is plenty of power if your speakers are at least 90 db 1watt/1meter. Anything bigger and it gets a little tricky trying to get rid of heat. A 50 watt SOZ will have to dissipitate a 1000 watts of heat and will wind up at around 100lbs or so if built right. Have fun and good luck.
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Old 29th December 2001, 08:30 PM   #9
grataku is offline grataku  United States
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you don't need to ground anything. You may choose to ground the - input and just use the + in and + out in se mode.

I only put in one XLR input and I used a four contacts omron relay from digikey for the xlr and 2 contact relay for the SE inputs. The input on the XLR is normally closed to ground unless it's selected in which case it switches from gnd to the - input of the board.
If you have more than 1 XLR input things get more complicated.
There are a number of ways to do this including logic ICs, flip-flop relays and so on. I went for simple at this stage, I may do something better later.
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Old 29th December 2001, 09:04 PM   #10
ftorres is offline ftorres
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Grataku and Grey,

May the problem with the +/- output levels come from slightly different source resistor values in the differential ?

I built this preamp, but included two CSS, one for each Mosfet. Quite good for CMRR, gives a better isolation from negative supply variations. Brought down the neg. supply to -30 V for a lower dissipation, and used Caddock thick film 15W resistors for drain resistances (TO 126 package), an a BIG heatsink for everybody

But I must mention that in my case, I raised the idle current to 80 mA /mosfet, thus heavy heatsinking is more than mandatory . IMHO, the sound is far better when the MosFets are idling at such high currents. But the preamp now looks more like a power amp
"Learning French is trivial: the word for horse is cheval, and everything else follows in the same way."
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