The Aleph-X

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#2,242
Taco said:
I used IRFP240 mosfets and bc550c instead of the MPSA18. Also I have changed r12 & r34 in 820 ohm to compensate for the other output devices. Edwin stated that he used 793 ohm to get a current-gain of about 56%. The original 1,2k gave 30-35%.

is with the trimmer VR1 and VR3 at maximum resistance. (used 100k Bournes)

I waited to settle things down, but apparently that was to much for the resistors. (4 watt MOX)
Click to expand...


Hi,

a) I used 930 ohm for 51% on my big beast ... and about 820 ohm on my small beast...

b) The higher the resitance of VR1, the HIGHER the bias. So turn the trimmers way down to 0 ohm... R11 and R33 should provide the base low setting... So lower the resistance! I can trim between about 0.7V and 0.4V ..

c) burned MOX4??? Hmm... 0.7V / 0.22ohm = 3.1A so 3.1A * 0.7V = 2.2W so should be below the 4W threshold...

Nice to hear you are firing up the beasts! 🙂

Edwin
 
#2,243
Thnx Edwin I'll try it asap (read: right now 😀)

P.S. I got 0.85V so it depends a bit on the used components.
0.85V*3.86A=3.3W, but my multimeter isn't that good in the low voltage range.


I turned the trimmer almost to the minimum, and then I got around the 0.6 volt. But when I turned the pot completly to the minium ( I heard a mechnical tick) I blew 2 fuses. But the still warm thermistor could also be the cause :scratch: .
 
#2,245
Set the trimmer midway and use a clip lead to short out R11 and R33. That should put you at the current minimum bias. I think you could also adjust R14 and R31 down in value a little. (Also check that r14 and r31 are 511 ohms to begin with.)
Good luck, Bill
 
#2,250
1. A definitive schematic

2. A Top Secret schematic or portion thereof perhaps smuggled on a paper napkin

3. How to modify an Aleph 3 or 30 for X operation

4. A contest to win a chance to visit Pass Labs to film a two-day reality segment - sort of like "Monster Garage" with transistors.

I'm really serious about that last one!
😎

Mike
 
#2,253
extension topic

Mr. Pass - I would like to propose an idea for a topic in this aleph-x thread. I don't know if people find the topic interesting, but it has been on my mind for some time now.

The topic I would like to adress is the effects of changing the gain of an aleph-x. Since most preamps have a gain of 2 or more (BOSOZ has a gain of 10) it would be very easy to deliver a higher voltage to the balanced input of an aleph-x than the usual 1-2Vrms - for instance +/-6Vpeak. Thereby I could reduce the gain of the aleph-x to 3 (9dB) for an 80W amplifier (change of feedback resistor). Another reason for a higher input voltage to the power amplifier is that the DAC project I am slowly starting, features the new TI PCM1792 DAC has a SNR of 129dB when the output is 4.5Vrms.


The main question is - What is the effect of letting the preamp or DAC do most of the voltage amplification???


As I see it the main things to consider are:

😕 Instability (phase margin to small)

I have tried simulating the circuit and it looks fine (in theory). A possible solution to the instability problem could be increasing the value of the feedback caps, and thereby a lower cutoff frequency and more phase margin.

😕 Hall of mirrors effect. Mr. Pass mentioned that: "the SuperSymmetric circuit is not very forgiving of excess feedback, tending to form an "infinite hall of mirrors" with large open loop gain." (from the x-soz thread)

The method I am proposing here does not change the open loop gain, only the closed loop gain will be altered. But do you guys think that increasing the feedback will lead to the hall of mirrors (have anyone tried to change the feedback)?


😕 Differential pair saturated

What happens when you apply a higher voltage to the input differential. I simulated this and it looks fine (aleph-x running with 20V rails and the input voltage at up to +/-10V).


😕 WHY?

Why change the feedback - Will it work and if it does, will it sound better? There is really no point if it does not lead to any improvement


Any feedback would be appriciated

Regards

Hans
Denmark
 
#2,254
An update of the progress of my Aleph-x, I replaced the bc550 for mpas18 and 1 channel seems te work better. The other channel with the blown sourceresistors isn't working, because the new sourceresistors are blown to. (Luckly it's one half of the channel, the other half appears to be working). Here I also replaced the bc550 with mpsa18 transistors. If I have some time I will measure the fets.

Are there other things to check?
 
#2,256
Netlist said:

I would suspect the output mosfets if your source resistors keep blowing. :att'n:

/Hugo 🙂
Click to expand...

Oke, now another question,

I have bought matched fets, now there are probably 2 dead. Can I simply buy 2 new closly matched fets to repair that half (q2 and q11 with the new closly matched ones?) And on the other spots the 2 fets of the working channel?
 
#2,258
Question to Nelson ( and the other gurus...)

I want to incorporate an ON/STANDBY mode on my Aleph X .

Is it a good way to cut only V+ rail and let V- rail connected to the circuit ? And safe ? Using big relay with capa across the pins to avoid lightening ....?

This because I would like bank capacitor life extended .

Thanks for answers:wiz:
 
#2,259
Re: Question to Nelson ( and the other gurus...)

nar said:
I want to incorporate an ON/STANDBY mode on my Aleph X .
Is it a good way to cut only V+ rail and let V- rail connected to the circuit ?
Click to expand...
To me disconnecting the V+ is pretty much the same as switching the amp off. Also, the risk of DC at the output is too big. There is already half of the negative voltage present at the outputs as absolute DC.
There is a thread about the topic:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=13645
and another one:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=2661

/Hugo 🙂
 
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