The Aleph-X

[Joe Berry]

Grey,

I took a fresh look at your schematic this AM and may have found a minor glitch: I believe that R21 should also lift R19 and R29 from ground along with D2-D5. Looks great otherwise. Congratulations and keep us up to date!

[grataku]

Grey,
I am not kiddin about building small stuff like the mini-AX I mean come on! How much power is enough power? More to the point how much weigh in aluminum can your floor support? I just got done _sort of_ building 4 hiraga monsters, and 2 Hiraga le crappe'...er le classe A for my active MTM system. Anyways to make the story short, which is related to the crappy vendors we discussed above, and maybe come to think of it, to the 'real' reason why Nelson is building single ended amps instead of push-pull (damn genius if you ask me), I want to go to single ended specifically, to mini-A....where the hell am I going with this?(sorry I had 0 sleep last night up all night working and not at stereo-building) oh yeah... I think that with 36 W total x channel I have pleeeenty of power. Maybe I am getting old but my ears seem not to be able to put up with high volumes anymore. No that's not listening fatigue or the fact that deep down I am not satisfied about my system...well...I am not but for other reasons.
Is anyone there that just turned thirty-something having the same hearing affliction?

'odd damn I m so *******' tired

[Joe Berry]

Grey, I think I misunderstood the role of this resistor earlier, based on a flawed memory of the Aleph 30/60 grounding scheme. After re-checking the A30/60 schematics, I now believe that R21 is there only to float any RFI that may try to couple between the amp's ground and the input via the parasitic capacitance of D2-D5. If this is true, then your initial connection is correct, of course. My oops, sorry!

[GRollins]

Joe,
Just to mess with your head, notice that Nelson uses it (on 30 & 60) under the + input resistors, but not the - input, which goes directly to ground. At any rate, 3 ohms is far, far less than the part to part variation you'd get from using even a 1% resistor.
To anyone who wants to really shave this circuit down, you can remove R21 entirely. Also D2-5. You'll be exposing yourself to the possibility of ground loops (not really a common problem) and opening the door to static discharge blowing a hole in your front end MOSFETs. But if you're careful when hooking and unhooking the signal connections, you don't really need them. Note that the A-75 doesn't have them, and there are a number of those cruising along happily even as we speak.
The caps I omitted from the back end, the ones across Q3 & Q8 that Harry was mentioning and the one that the Alephs have (but the Volksamps don't) between the feedback line from the output and the output MOSFET's Gate are my way of streamlining things. If it makes you more comfortable to have them in there, by all means put them in, but I've found the circuit to be quite well behaved so far. Who knows, I may go back and put them in later--I just haven't seen a need to do so. The circuit just doesn't seem to be twitchy in that manner.
Now, DC...that's another matter, entirely...
grataku,
Volume itself doesn't offend me--it's the content that I find bothersome sometimes.
Just as I intend to run my copy of the amp at 12 volts, there's no reason you couldn't drop the rail a little further. But once you get below about 10V, you need to reevaluate some parts of the circuit. The current source for the front end will have to be redone, for instance, since once you get below 9V, D1 won't turn on. This isn't a fatal problem. You could use a 6V Zener, for instance, and drop the value of R24 accordingly. Or you could just abandon the Zener strategy entirely and use another MPSA18, playing its Vbe against R24 to set the current. Looks like about 33 ohms for R24 would do the trick. Using a transistor to watch the voltage drop across R24 would not only give you a lower voltage drop across the current source, but would also be a better performing current source because the feedback loop would tend to reduce current variations that the Zener strategy doesn't even notice.
Thirty-something? Hmmm, I'm 44 and confess that I got really bored with what passes for rock these days about five or ten years ago. After a period of aimless dissatisfaction, I shifted sideways into jazz rather abruptly and have been quite content with that. It's improved my entire outlook on life. Classical remains constant in interest level.
To those who might feel the need to defend their favorite group a quick story (which I may have related before, don't remember): Just for fun, pulled out In-A-Gadda-Da-Vida by Iron Butterfly to play for one of the sprouts a few months ago. Now, the title track is pretty much unique, but when I turned the record over and started playing the songs on the other side, she started giving me this,"Why, that sounds like XXX, and the other cut sounds like YYY, and...oh, listen...that sounds like ZZZ," nonsense. My reply was simply to show her the copyright (1967 or '68, something like that) and ask her who sounded like whom...after a few moments of embarrassed silence, she conceeded that perhaps I do have a point when I say that all the stuff that you hear now has been done before--and frequently done better the first time around. The phrase 'been there, done that' was invented to cover situations like this. Just as the early rock players listened to blues and jazz (and in some cases classical [Yes, for example]), the current crop are listening to early rock. Unfortunately, the relationship becomes a little too incestuous at that point and creativity suffers.
******************************
Come to think of it--and now you've got me waxing philosophical--perhaps that's why I'm not all that interested in the standard three-stage solid state amp. Self, Slone, and others have pretty much flogged that topology to death. I'd rather mess with something newer and less picked-over.
I'll think about this some more. It's an interesting point.
******************************

Grey

[Joe Berry]

Originally posted by GRollins
. . . notice that Nelson uses it (on 30 & 60) under the + input resistors, but not the - input, which goes directly to ground. At any rate, 3 ohms is far, far less than the part to part variation you'd get from using even a 1% resistor.

Yep, exactly that observation led me to rethink its role in the circuit.

To anyone who wants to really shave this circuit down, you can remove R21 entirely. Also D2-5.

Maybe we should ask NP's advice on this one. He's normally pretty ruthless in weeding out extraneous parts. On that basis, I looked for a reason for having an "R21" in the A30/60, and (think I) found it in D2-D5 and the AC noise suppression caps which shunt power-line RFI to the chassis. If these caps OR D2-D5 are present, R21 also may be advisable; I'm not sure I'd trust the circuit's inherent common-mode rejection at these frequencies. But I will of course defer to the Designer's opinion.

I intend to run my copy of the amp at 12 volts . . .

Are you implying the use of a rechargeable battery supply? Now THIS I think could be the really trick setup, giving a decent 25W or so output swing (assuming +/- 12V), and of course, making both supply regulation AND RFI management less of an issue.

[HarryHaller]

And making battery life one HELL of an issue........ I have got to give up comedy in my post, you guys are so much better at it.

H.H. (cHarge Hourly)

[Joe Berry]

HairyHowler,

Once you regain your composure --

What would you say to a hybrid approach? The amp could have a conventional internal power supply, which could also serve as a charger for optional external batteries. A switch on the back could determine the operating mode -- AC, Battery, or Recharge.

Or do you reject the idea of battery power altogether as not offering a worthwhile improvement, based on your own experience with same?

[HarryHaller]

I don't know, it never occured to me put batteries on an amp that draws several amps. I would rather put to money in a a good regulated supply which can sound excellent with attention to details in design and construction ie. soft recovery diodes and good caps.

H.H.

[Joe Berry]

HH, I really don't know if battery power is worth the trouble either. I only want to point out that it's a feasible option in the context of this project. This is even more true if you are already planning to do a regulated supply, because the raw DC requirements for a regulator and a charging circuit are about the same (+/-17 or 18V in this case).

A quick google search on "lead acid charging circuit" turned up several schematics, all of which are about as complex as a regulator. The major cost for battery operation is going to be the batteries themselves, but I think you could add a battery option to this project for about (US) $200 that would give you 3-4 hours of operation per charge. At that price, I for one would be tempted to experiment with it. If nothing else, it would be a (relatively) cheap education.

[GRollins]

Joe,
While I did mention battery operation as an option for the Mini-A, it would be pretty rough with this amp, considering the bias requirements. The +-12V rails popped out of my calculations simply because I plugged in 25W/8 ohms as my starting point for output. Given that the ribbons have such a low resistance (purely resistive, thank goodness), I've got to bias heavily, and a lower rail brings down the device dissipation to a 'reasonable' 35W or so. Actually, given that the power into 2.5 ohms will be far more than I anticipate needing (I only run the tweeters from 5kHz on up), I could drop even further, but I'm kinda hung on this silly ML-2 image I've got in my mind. No, I've never owned a pair--just kinda like the conspicuous overkill approach that they represented. I suppose I could just as easily get by with 10W/8 ohms and let that double, but then grataku would show up at my door, wanting to borrow the thing all the time.
One option related to battery power would be to use higher impedance speakers; at least 8 ohms and maybe 16...does anyone even make 32 ohm speakers anymore? Anyway, you could reduce the bias considerably if you happened to own high Z speakers. Maybe you could series several drivers. If you happen to have even 8 ohm speakers that have a relatively flat impedance curve (say, minimum at 6 ohms or so), you could back off on the bias quite easily, thus making the battery option more attractive.
I've always regarded battery power as being more appropriate for preamps and such, although I've seen that there are battery powered amps out there. No, I've never tried it...have no opinion as to whether it's better or worse.
Oh, R21 is to break ground loops.
Harry,
I had intended to abuse the amp some more, trying to force it into instability, but have run out of time today. Perhaps tomorrow...maybe...I've already got a lengthy list of things to do before I go to work. If I can make it misbehave, I'll stick in some caps. No electrostats on hand; I'll have to make some kind of nasty load. The Mini-A was pretty much bullet proof, at least here, so I don't anticipate this being easy. About the only thing I can see is RF, which is kinda a non-issue in central SC. Most of these people live in the Stone Age, and distrust newfangled things like light bulbs. They won't get to radio until somewhere around the year 2135. Perhaps someone in NYC or LA could build one and comment on RF stability.

Grey