The Aleph-X

[sonnya]

Originally posted by GRollins
Sonny,
--The bias for the front end is about 20mA total, or about 10mA per side.
--Output stage bias is on the order of 2.27A for the circuit as published, i.e. with output bias set for .5V across the Source resistors.
--IRF models? None here. Sorry, I lost all my simulation stuff when the virus hit. I didn't even try to save that; the stories were more important. All I can do is suggest www.irf.com to see if they have it available for download. For accuracy's sake, keep in mind that I'm using IRF MOSFETs and OnSemi MPSA18s here, not Zetex '450s or any of the other likely candidates. Substitution may or may not give useful results...but on the other hand, someone is guaranteed to want to use the Zetex parts with Toshiba MOSFETs (or something...), so I'm sure that any simulation will be of interest to someone, somewhere. Just be sure to specify what models you use.
--Reducing the feedback will increase the measured distortion and reduce damping factor. Whether it will sound better or not I will leave to others. Bear in mind that the closed-loop gain will increase somewhat. This may or may not be a good thing, depending on your circumstances.

I'm still working on a PCB layout. (Have yet to look at the one posted above.) I've started a new pattern that may (if I'm lucky) allow me to post a single-sided board diagram. Maybe. It's slow going and further complicated by having a relative in poor health; my attention is divided at the moment. I'll keep plugging.
I still haven't built even one proper channel. Aside from the medical situation mentioned above, I need to commit to some sort of heatsink. Part of me says to go water-cooled (just build what I need), the other part says to go passive air cooled (in which case I'm at the mercy of what's available on the surplus market--you know that story).
As Pooh says,"Oh, <i>bother!</i>"

Grey

Sorry to hear about your relatives! They are more important than any hobby.

For the models. I think i have all the models i need know... The MPSA18 is from Fairchild though.

What i meant by the feedback network was not to lower the gain, but reducing the size of r18,r28 to 3k3 and r16,r26 to 33k. It will generate led distortion because of your Zenerdiodes capacitance and the mosfets input capacitance.... I know that you have a gate resistor but it will have an effect.. The dominant pole will also move up about 3 times.
The resistor noise will also be lower.

I will make sims for the original circuit and the circuit where i have reduced the values to 1/3.

Harry : Thanks!

Sonny

[Peter Daniel]

Grey,
How important is 10pF cap in a feedback loop? I didn't use it in my Aleph 5 (I couldn't find a good one) and it seems to be OK. I also didn't use 2 other small caps (1nF and 10nF) and no oscillations either.

[GRollins]

The 10pF cap in the feedback loop sets the upper response of the amplifier to about 150kHz. Offhand, I don't remember what the upper limit is without the cap, but the circuit will try (more or less successfully) to do DC to light bandwidth without it. The top end at this point will be governed by the various MOSFET capacitances, the circuit layout, etc. The sum effect will be impossible to predict from where I sit; it'll be up to you to keep tabs on the stuff.
If you go without the cap, you'll be leaving yourself open to RF...and possibly circuit oscillation.
You'll also give Harry a heart attack.
My take is to proceed with caution. If your layout seems stable, and if your load isn't too reactive, and if you don't live too near a radio station, and so on...then you have my permission to experiment.
If in doubt, leave it in.
Note that Mouser and Digikey both sell silver-mica caps that will do just peachy. They're not very expensive, either. Mouser has nice little polystyrene caps in case you want to put the cap across the NPN. They're reasonably priced, also.
Another option is to alter the value somewhat. The smaller the cap, the higher your highs will go. To a first approximation, you can calculate the -3dB point simply by plugging in the 100k value of the feedback resistor. A 5pF cap, for instance, would give you a -3dB point in the region of 300kHz.
Again, proceed with caution, but you have my blessing to alter that value as long as you test thoroughly and take responsibility for your own smoke releases.
I ran both the Mini-A and the Aleph-X without the 10pF caps, and both circuits (at least here, and with the caveat that they were both built on these push boards, which raises the possibility of stray capacitance) were stable. At least in the Mini-A, adding the 10pF did damp a little overshoot on a 10kHz square wave. I can't remember what the square wave on the Aleph-X looked like, with vs. without. If I have time, I'll poke at the circuit tonight when I get home from work and report.
The caps across the NPNs, well...Harry feels they're necessary, Nelson indicates that they <i>may</i> be necessary, and I haven't needed them yet. Your Mileage May Vary.
Thanks for the well-wishes folks, the Grim Reaper has been seen in the neighborhood. Things will simply have to proceed as they will.

Grey

[rljones]

I've read over this thread and looked at the circuit, and have a couple of questions. Essentially this circuit seems to be a bridged Aleph with only one input section. Aside from eliminating a few input mosfets, what advantage does this circuit give you over simply bridging one stereo Aleph, and driving it with a balanced signal?

The main reason I ask this question is that with this circuit, the inputs are also where the feedback is sent. How much interaction might occur between the input and the feedback? With the bridged sterero Aleph, the feedback is somewhat isolated from the input signal as both the + and - input signals would be sent to non-inverting inputs and the feedback would be at the inverting inputs. (Would the DC offset be less of an issue in this case too?)

The other reason I ask this question arises from some listening tests that I've done with bridged amps. I've been playing with various bridged amps (discussed on the solid state forum), using STK4048XI and TDA7293V modules, as compared to a single-ended, non-bridged Aleph (home brew) and a Rowland model 10 amp (bridged using many LM3886 modules). During these comparisons, I also listened to amps with fully balanced inputs (+ and - are both sent to non-inverted inputs) versus those such as the basic Aleph that use a balanced input, but feed the - leg into the inverting input side (where the feedback is located).

The amps sounded much better when fully balanced (and bridged) as opposed to those where the - signal was presented to the inverting input. Therefore, I'm wondering if it's not better simply to bridge a standard Aleph and avoid feedback interactions. Comments?

Regards, Robert

[HarryHaller]

I would check Mr. Pass's article at:
http://www.passlabs.com/pdf/susy.pdf

There are also real avantages in running the inputs on each side as inverting stages in terms of reduction in commom mode signal seen by the input stage. In addition most Power supply noise components will appear as common mode components and will be rejected by a true balance differential amp. The topology also allows smaller supply voltages for a given voltage swing and therefor higher bias currents for a given idle power dissapation. Convinced yet?

[rljones]

HH,

I am convinced; thanks. I had not previously read Mr. Pass's article. It addresses just what I was wondering about.

BTW, very nice looking amp!

Regards, Robert

[13DoW]

Harry,

the bias current for a class-A balanced output stage is not actually higher than that of a single-ended one - you just have two of them. The output transistors are not biased at a higher current relative to the signal current swing than they are in the single-ended case.

There is also a slight efficiency hit as you reduce the voltage rails because the biasing overhead (how close you can swing to the rail) does not decrease so becomes a larger fraction of the DC dissipation. However, anyone subscribing to this board will not be too concerned about that.
And the output resistance doubles compared to single-ended.

That said, I like balanced designs and I'll be interested to read how the Aleph-X compares to a single-ended Aleph. Will even-order cancellation in the Aleph-X enhance or detract from the Aleph sound?

Regards
13th Duke of Wymbourne

[HarryHaller]

The use of lower voltage rails allows one to bias the transistors higher for the same bias dissapation. Power equals voltage times current, less voltage is more current for a given power. The higher the bias current the greater the transconductance and lower the distortion for the amplifier. I plan to bias at about 2.5 amps which is about as far as I dare push the output devices with 16 volt rails.

H.H.

[AudioFreak]

I agree with both HH and 13DoW but on different points.

The lower rails allow higher bias which is good but the bridged design also requires more output current anyway so you do not really gain extra bias current relative to output current.

[HarryHaller]

What!!!!??? Assuming we are talking about the same votage swing into the same load impedance, I am afraid I don't follow your reasoning.

View of Zetex ZTX 450 boards for active current sources. Elna Cerafine elkos, Seimens polystyrenes, and Resista and Corning RN60D resistors.