The Aleph-X

Jump to Latest
#2,104
nania said:
The issue of optimum voltage rails was mentioned earlier (before Peter's post) in this thread and as far as I know went without a defintive answer.
Click to expand...
There is not much in the way of a definitive answer. Like tubes, Mosfets prefer the higher voltages. When you look at the variability of the input capacitance you see that below 5 volts Drain-Source it's not very linear. You should plan for linear operation within an envelope which is at least 5 volts inside the rail voltages, and more is preferred.

Not that you can't run the output right up to the rails; it's not hard to get within a volt, but if you look at the distortion characteristic or listen to it, it's not very pretty.
 
#2,105
Nelson Pass

Earlier, you mentioned your preference for 25V rails for the Aleph-X but I didn't see a reply to the comparison of an Aleph-X built with 16V rails and 20A of bias vs. one built with 24V rails and 12A of bias, which is more likely to sound better given an impedence load of 3.2 ohms? What about the mosfet used, will that factor into the voltage rail decsion? In other words, will certain mosfets tolerate a lower voltage rail and still sound as good as they can? I've noticed that the 150V has rather large input capacitance numbers but its transconductance is huge! Does the ratio of transconductance to capacitance matter or is the absolute value of the input capacitance the determining factor in making the best sound?
 
#2,110
Nelson Pass said:
Since Fets, Mosfet, and tubes do not have current gain as such, transconductance is the term you want.


On more than one occasion I have found it helpful to measure the transconductance of Mosfets as part of the matching procedure, and in the case of many Mosfets it is equally important to measure transconductance versus frequency, as this varies for a lot of the chips we use. The IRF610 and its relatives, for example, go through about a 6 dB transition in transconductance through the audio midband, and choosing your Mosfets based on variations in this figure can make a performance difference.

(Bet that transition doesn't show up in your Spice model, either.)

I have a nice little circuit for this, and I'll dig it up and post it.


:wiz:
Click to expand...


P L E A S E
🙂 🙂 🙂
 
#2,114
Peter Daniel said:
Which feedback caps are you talking about?

I'm not using 10pF caps and I think amp sounds better without them.
Click to expand...

Right, the square wave really looks like crap without them. On the other hand the caps seem to have a deleterious effect on other aspects of the waveform.
I know it's silly but it is still not clear to me what's the best way to look at the floating output with the scope. Right now I am looking at each side to gnd, which is not optimal.
 
#2,120
Differential scope measurements

originally posted by grataku
I know it's silly but it is still not clear to me what's the best way to look at the floating output with the scope. Right now I am looking at each side to gnd, which is not optimal.
Click to expand...

originally posted by wuffwaff
I think the simplest way to measure the X is to use two probes and push the "add" button on your scope (invert one channel).
This way no earth link is made.
You can even push the "invert" button again and see if both halves are equal
Click to expand...

Two single probes are maybe the simplest way but as the difference between the probes (and the scopes vertical amplifiers if not so well matched) could falsify the measurements (particular towards higher frequencies) a real differential probe would be the best for this task.

For audio frequency range this could be a DIY-project as well but I guess for the money You can get for instance a Tektronix P6046 Active Differential Probe Set at ebay, IMO this is not worth the efforts in time and the costs for parts. Clean & complete Probe Sets with attenuators, case, power supply, probe-adaptors, manual etc. usually go at ebay for around $60-$120. I wouldn`t buy anything else than a complete set (maybe unless the manual which is offered separately quite frequently) as missing parts of a set are harder to get.


http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2521837793&category=4677

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2521307408&category=25415

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2520657769&category=25415

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2521205780&category=1504

etc. etc.
 
You must log in or register to reply here.
Top Bottom