More BOSOZ ?s

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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?
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...

As for enclosures, check out or you might just want to carve one out of aluminium slabs as what NP did with his preamps ....:D
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
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.

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.

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.
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.
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 :(
Nelson is in a funny position on this stuff. He wants it to perform well, but doesn't want people calling him at 3AM asking why their durawowsler (I'll explain some other time...) doesn't work. So he keeps it simple (besides simple, all things being equal, will sound better anyway--those who want nice measurements may leave through the door at the rear of the room). In the case of the preamp, resistors are simpler than current sources. Now, a lot of folks on this site can handle the construction (and theory of) a current source, but he ain't doing these things for them who can, he's doing them as an exercise for them who can't.
(Note that I'm playing Devil's Advocate here, as I like using resistors in place of current sources whenever possible--while noting that current sources do have their place in the overall scheme of things.)
Note the number of posts requesting help on small points here. If we get that many folks are out there in the wilderness who don't know we're here and can't figure out what's wrong with their circuit?
Caddock TO-220 resistors w/heatsinks might be a good option. No, I haven't used them myself, but they have a good reputation.
PassFan, grataku,
Actually, you ground the unused input if you want to use it as an unbalanced to balanced converter. Otherwise the unused gate tends to flop around and you get a weaker output on the backside. Yes, you'll get something, but it won't be quite what it should be.
The operating principle of a differential used as a phase splitter (i.e. unbalanced to balanced converter) says that the more resistance you've got underneath the source/emitter/cathode, the better balanced the output will be. The ideal, of course, being infinite resistance. Enter the current source, which acts as an 'infinite resistance' and gives pretty near perfect balance between the two outputs.
This is not to say that a large enough resistor won't do a good job. The BOSOZ uses 750?/1500 ohms? (I don't have the schematic handy.) Whether that's enough to give good balance, I don't know, as I haven't built the circuit.
It's nothing to see 10-20k under the tail end of a tube phase splitter, but then you've got these really high rails to play with--not the case here, unfortunately.
Oh, one other thing, going from resistor to current source will play havoc with your gain, depending on what you're doing with it. I'll leave it as an exercise for the reader to figure out why. (I always wanted to say that...)
Besides, I don't see anything wrong with a muscular-looking preamp.

Grey and ftorres,
in my case, a 6dB gain, grounding or not grounding the - in while operating in SE mode makes no difference whatsoever in the output levels. I am planning to put in p5 and see what happens at higher gains.

I like the passive current sources, in some circles they are thought to sound better than the active ones and I tend to agree.
I don't know where you got that I have a problem with Nelson philosophy.
I think that he was too conservative with the cooling. If nothing else, toasted components may increase the rate of angry phone calls at 3 am.
It's not that grounding the unused input will increase the input on the 'front' side--it should increase the 'back' side (this for a balanced output, don't worry about it if you're just going unbalanced). If I'm recalling correctly (into sleep deprivation mode again--may slip a cog here), it will lower noise, hum, etc. by some slight amount if that other gate is at ground potential.
The reason I said the stuff about Nelson wasn't that I thought that you thought he was goofy (you still with me?), but that you said something to the effect above that 'Nelson must have noticed, but thought it wasn't important?...' and in that context he might simplify a circuit in a manner such that it's easier to build, even at the expense of degrading performance a hair.
(I think I said what I meant. The question is did you understand what I thought I I'm confusing myself...)
Heat...dunno, man...if I get a spare minute, I'll scrounge up the schematic and calculate a few power dissipations, here and there.

Grey and Passfan,
I have the -input to ground as well just in case, although it does not appear to be of any consequence in unbalanced mode.
I have used a 99 db sens. headphone and the thing is DEAD quiet either way. I bumped up the gain to 10dB, just as well.
What I found is a constant 0.35-0.4 V difference between the two output levels this is the pure circuit with no pots and it doesn't matter whether I ground the -in or not.
I also fed two the same signal into both -in and +in thinking that it should get nulled out except for random noise and device mismatch, and once again I got my 0.3-something volts out.
I guess that tells me my devices are completely out of whack.

no grounding of unused outputs. Nelson has already replyed to that extent in someother thread.

Thanks for the info on the outputs. For myself, all I have read or been told about differential circuits is that the unused device still contributes to the circuit and must be given a reference to voltage. I freely admit my ignorance to the mathmatical element of design and find myself at the mercy of those who know. While I have learned alot in the last 4 years I will probably never understand 95 % of "The Art of Electronics". Never the less, I am grateful to gleen and share any help I receive from my fellow hobbyists on this and other forumns. I generally like to use the tips before I advise someone else. It's easy enough to burn our own projects to the ground without getting bad advice from someone else. Anyway, may every circuit you build start the first time. Thanks:cool:
I've got a bad feeling about this...
Han Solo

You're right, there's something odd going on here (and I'm still concerned about the heat). If you're getting different gains on the two sides and the CMRR is that far off...
I know I'm being silly, but you did use matched devices, didn't you?

The one and only
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I don't recall off hand whether the 25% difference in
balanced output with a single input is the right figure, but
it sounds a bit high. There will certainly be some difference.

This is mostly a function of R15 versus R3-6. If you want
to improve the performance in this regard, You can increase
the values of R3-6, and correspondingly increase the
negative supply voltage. Of course the best way to get
what you want here is to use constant current sources
at about 40 mA in place of R3,4 and R5,6.

You can also decrease the value of R15, which of course will
increase the gain. At that point, you might want to
consider an input potentiometer to attenuate the input.
Thank you all, especially Nelson and Grey
for your kind support. I didn't intend for this thread to turn into my personal odessey with the circuit.
I have put the BOSOZ on ice for the moment, the pre amp works in SE mode and actually sounds wonderful. The best pre amp I have ever owned. No listenitng fatigue, the ear drilling hi-freq sensation is gone and yet the amp is amplifying up to 20kHz. Ain't that a thing?
However, I need to make extensive mods to address the heating issue before the preamp can be put back online. The chassis is completely sealed and has extruded Al sides. So what I am trying to do is to use these sides to achieve proper cooling and thereby avoid fires.
I am expecting some new resistors and I will then go back and re-check all the components including matching (which I thought I did).
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