Aleph-X builder's thread

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#722
Another question from a friend who is building an Aleph-X. This amp can be run at lower power (or put on standby) by lowering the idle current. Now, if you have a power transformer with multiple primary taps, can you do the same thing by switching taps? That would drop the rail voltage, but I would suspect would not affect the idle current (or would it?). Nonetheless, it does represent a lowering of dissipation.

Is this feasible or is there something stupid that I'm not seeing?
 
#723
Hi,

lowering the voltage without changing anything else will raise the bias (as experienced when powering up the X through a variac).

I would just turn it off if not needed. Sound quality is very good from cold and putting it on standby won´t change the warmup behaviour very much (abs. dc offset).

William
 
#724
The issue is not so much standby mode, but using less than maximum power depending on the speakers. So the idea is switching both voltage and speakers, so that it's not necessary to run the same dissipation for (say) speakers that work OK with 50W as one would for speakers that require 150 W. But the amp would be capable of returning to 150W easily when higher power requirement speakers are in place.
 
#725
ok,

I still think this would be not that simple (but possible). You would have to setup two modes (high power/low power) and have settings for:

1. bias (two pots)
2. ac-current-gain (two pots)
3. diff pair bias (one pot) to set abs. dc offset

This could be done by setting one mode, measuring the resistor values, setting up the other mode, measuring the resistor values and then switching 5 resistors per channel plus the supply voltage

The changes to the supply voltage can´t be too big otherwise other resistors will have to be changed as well.

I would keep it a bit simpler and only change the 5 resistors without changing the supply voltage. If changing speakers only happens two times a year I would fit these resistors in little plug-in modules and change those when needed.

William
 
#727
I think that's one of those places where you start using op
amps to actively control the absolute DC offset. As you change
the bias, the absolute DC needs adjusting. The "typical DC
servo" you see in AudioXpress and elsewhere could be adapted
for this job.
 
#731
Hi Guys,

I have just aquire a pair of the HiFiZen boards and am looking forward to building this little amp. All of the threads about this amp are so long now that I'm having trouble getting a grasp on what's what. By the time I read through 20 pages or so, I feel like I know the posters better than I do the circuit.

I guess I need to decide on the power supply first so I will know the rail voltage to help define what other parts I need. I am currently listening to music on my newly finished Krell KSA clone. I like the power level of the Krell and so would like to be somewhere in that neighborhood with the AX. Can one of you help out this newb and give me an idea what voltage range I should be looking at for a transformer/s? I would like to go mono-block on this one.

Thanks, Terry

EDIT:

If 19v rails will suffice, would a transformer like this work for a dual PSU? Does it have to be two seperate transformers?
 
#732
Hi,

I don´t think this transformer will fit very well. You have either 12 or 24V secondaries wich will give around 14-16 or 28-31V supply rails. The first is a bit low, the second is as in the AX200 and will give very high dissipation (and power output).

Have a look at the wiki, here all the information is concentrated and will give you something to start with.

William
 
#733
Hi William

OK, that leads back to my original question. What voltage should I be looking for in a transformer to get the output that I'm hoping for. My Krell is biased for 50W Class A. It seemed that I rad that the first amps built were using 15V rails. Did that not work out well? This transformer that I linked to should put out 16V rails. I see that you have a working model. May I ask what rail voltage your amp is running?
What kind of power is it putting out?

Got a link for the wiki?

Thanks, Terry
 
#734
Hi,

my rail voltage is around 22V from an 18V transformer and a C-L-C power supply. Power into 8 Ohms is around 100 watts, 90watts into 4 ohms at 6,5A bias and 51% of ac-current-gain.

The 12V transformer will be giving a bit less than 16V when you use a C-R-C or C-L-C power supply.
You can calculate the expected power with the AXE-1 sheet that´s mentioned in the WIKI http://www.diyaudio.com/wiki/index.php?page=Aleph-X

William
 
#735
I'm just ironing the final kinks out of my AX monoblocks, but I have designed and built mine in the same direction as what Terry (still4given) seems to be planning. I used one Plitron 225VA 2x12V trafo for each monoblock, IXYS fast/soft recovery diodes for the rectifier bridges, followed by a CLC of 94,000uF - 1.5mH - 47,000uF on each rail. I built the amplifier circuit using IRFP044 output transistors with 5.1A bais (2.55A on each side of the balanced circuit). This resulted in +/-15.5V rails at the amplifier circuit, and gives me roughly 50W output into 8 or 4 ohms.

I've ironed out the stability issues I was having, got my relative offsets under 50mV, and all I have left to do is change the resistance value on the diff. pair CCS, because I didn't give myself enough adjustment range to turn the diff. pair bais down low enough to optomize the absolute offset.

Cheers, from another Terry
 
#739
Hi guys,

Well my wheels are still turning. I am trying to get a grasp on the AlephX but man, there is a lot to read. I am going to have a couple of 25-0-25 400vA trafos available. Is there a way to use these for an X? If I don't have to buy any other trafos that would be great. I would like to be able to run at 4ohms. I need to get these things settled before I can start buying parts.
I have extra output board so I can add quite a few outputs if necessary.



Thanks, Terry
 
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