The Aleph-X

[paulb]

Thanks, Grey and Nelson. These are indeed wonderful things that you have shared with us.

My next amp: Aleph-X or Zen 4?

[tschrama]

Hi rtirion,
I think somehow the DC coupled nature of the such desings make them sensitive for proper biasing in some parts of the circuit, the folded-cascode X-version I only simulated so far has that some snack, trim one pot 5% off and a large offset and distortion product appears. This was only simulated and on a 'un-confirmed' circuit only.

I has a few questions about the schenmatic:

Why is the input connected to groud (R19 and R29, 68K). It wastes gain (and therefore feedback) or doesn't it?

Why not AC couple the amp at the input: then DC offset at output doesn't matter since it is a symmetric balanced design.

gr,
Thijs

[GRollins]

I'll try to hit the high points, but will probably miss something...
Harry,
Your point about mounting the input differential on the same heatsink is valid. I have two different PCB layouts going at the moment (remember, I do these things by hand, it'll take a while before I'm satisfied), and that's something I've been trying to factor in, depending on which heatsink I go with.
Regarding open loop compensation, note that the Volksamp 30 & 60 don't have the cap; the 'standard' Alephs did. My experiments here indicate that the Mini-A was quite stable, even under various torture tests. I haven't flogged this circuit quite as hard as the other--for one thing, I ran the Mini-A open loop with no problem (resistive load only, granted)--but I'll get there. My feeling about capping things off is that it should be approached on a case-by-case basis. So far, I've seen no evidence that this circuit has any problems in the HF arena; more in the LF region, meaning DC. I knew that going in, thanks to Nelson's warning. If you want to roll it off, by all means, do so with my blessing. I don't have RF problems here; others do...etc. etc. etc. Follow your own muse on that. Another factor is that the more you roll off the high end, the worse things get. Bear in mind that I've got an unusually complicated system here (quad-amped), and even being down, say, 1 dB at 100kHz is cumulative. By the time you chain a number of components together, all down 1 dB at 100kHz, the rolloff starts encroaching on the 20kHz region.
I'm not saying that your point isn't valid--and with some circuits, it's no doubt imperative--I've just got a different set of priorities.
Out of time...more later.
Jam, still no digital camera or scanner. Besides, the thing's dog-ugly at this point. No one wants to see it until it gets a pretty face on.

Grey

[fcel]

Speaking of pictures .... I like to see "raw" materials before it becomes a product.

[Peter Daniel]

Why not use 2SK389 for input differencial as other X-line amps?

[GRollins]

Thijs,
You e-mailed, but did not attach a schematic. Sorry I didn't reply, but...like I said, there's only one of me, only 24 hours in a day, and I've running on about five hours of sleep a night instead of my usual six. I'd be glad to look at it now, although Nelson's the final arbiter of what's cool and what isn't when it comes to X designs.
rtirion,
Yes, the circuit gets pretty radical, DC wise if you get too far off the nominal values. The reason I put so much play in the adjustment is that---DIY being what it is--someone's going to want to substitute something for the IRFP044s, and they'll need room to negotiate between the front and back ends. There will be a certain amount of variation in Vgs in real world parts, and I wanted to have sufficient lattitude to cover parts changes. Feel free to tighten up the adjustment range for the front end if you wish, but I'd advise building an actual circuit first so that you know what range to shoot for. I started with IRF644s, and had to readjust earlier this week when the IRFP044s came in. Incidentally, I'm using a multi-turn pot there so as to have plenty of control.
Paul,
I intend to use a slightly modified version of this for my tweeters. Lower voltage (+-12V rails), higher current (to handle a 2.5 ohm load), etc. As to whether to build an Aleph or this thing...it's up to you, man...
Thijs,
I played with AC coupling the front end, but the DC came back with a vengeance. The front end needs to be ground referenced, and if you remove the 68.1k resistors, the inputs see only the outputs (via the 100k resistors) and whatever is coming in the inputs (which will vary from nothing hooked up, i.e. infinite resistance, to some unpredictable Zout from whatever you plug into the amp, be it preamp, crossover, electric guitar, etc.) If you want to go that route, then please report back so that anyone else who wants to try it will know what works and what doesn't.
Out of time again...

Grey

[HarryHaller]

The Volksamp 30 has 3 pairs of output devices which is much more capacitance across the 392 ohm resistor where I believe the dominant pole for the amp. Also the 0.001 uF cap for the dynamic current source is missing. This is C7 on the Volksamp schematic. Poor high frequency compensation can cause an amplifier to oscillate under capacitive loads and even when not oscillating can have a great impact on the sound. For those who actually care there is a good article on phase margin at:

http://www.engin.umich.edu/group/ctm/freq/freq.html

H.H.

[HarryHaller]

This is very good idea. The matched pair would give excellent matching for low distortion and good offset figures. Maybe I will give it a shot......

H.H.

[tschrama]

Hi grey,

thanks for your reply, some time ago I have left the X-amp arena to finish my JLH-class A amp and am currently back in headphone-amp arena again.. untill you showed up to mesh-up my whole weekend with your posting .. thanks again for all your efforts..it really get's me thinking about my own futile atempts...maybe an X-headphone-amp anyone ?

greetings,
Thijs

[rtirion]

H.H.

The current source above Q5 and Q7 provides +/- 20 mA. This means +/- 10 mA and 150 to 170 mW dissipation per IRF9610

300 to 350mW for a 2sk389 is a little to much. :-)

Lowering the current in this pair is an option but will wreak havoc on the DC setting. Using a 2sk389 will need a little more work than just a replacement I think.

btw nice article on bode/nyquist stuff. Few years ago I had quite a hard time chewing on this. Still a basic understanding of this is very important if you care about the stabilty of an amplifier.
I for one, do care.

Regards