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Old 29th December 2001, 10:03 PM   #11
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grataku,
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...how 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.
ftorres,
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
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Old 29th December 2001, 11:44 PM   #12
grataku is offline grataku  United States
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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.

grey
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.
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Old 30th December 2001, 02:37 AM   #13
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grataku,
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 said...waitaminnit...now 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
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Old 31st December 2001, 04:23 AM   #14
PassFan is offline PassFan  United States
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Default grounded input

Grey:
I was aware of the - input to ground. Thats how I have mine set up. I believe Nelson mentions that in the SOZ article for unbal input. I wasn't sure about the output but it seems to have been answered here already. Thanks
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Old 31st December 2001, 06:56 AM   #15
grataku is offline grataku  United States
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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.

passfan
no grounding of unused outputs. Nelson has already replyed to that extent in someother thread.
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Old 31st December 2001, 02:21 PM   #16
PassFan is offline PassFan  United States
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Default outputs

Grataku:
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
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Old 31st December 2001, 03:53 PM   #17
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I've got a bad feeling about this...
Han Solo

grataku,
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?

Grey
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Old 31st December 2001, 04:40 PM   #18
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Default BOZ Preamp

I used Clarostat mylar film pots from Digikey, and housed it in an aluminum Sescom case. It looks and sounds beautiful.
For the balanced version, you could poaaibly double up on the input and output rca jacks, and use a dpst switch to switch 2 inputs or outputs to ground when going single ended.
mg16
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Old 31st December 2001, 07:57 PM   #19
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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.
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Old 2nd January 2002, 03:07 PM   #20
grataku is offline grataku  United States
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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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