AX100 100W Aleph-X Monoblocks

[wuffwaff]

Steenoe,

maybe you should lower the resistors that are in series with VR1 and VR3. I changed them to 27k.

19V seems very low! Normally transformers are specified at rated power. With big torroids this means 3-5% more voltage at no load. Do you use a C-R-C supply?

Normally the voltage over the source resistors goes down with temperature not up!

William

[steenoe]

Hi William. Thanks for the reply. I am using a C-L-C supply consisting of 68.000uF-2mH-68.000uF on each rail. I had the suspicion that too much current is running through the amp.
It just came to mind that the voltage drop over the output to GND resistor is around 12V, that seems like a lot to me also. They are at 124r as far as I remember.
I did lower the resistors in series with the pot's but that helped only a little (I have around 27K there too), and the voltage keep rising. I have triple checked all connections, so I dont excpect any faults, but I better check again.

Steen

[wuffwaff]

Hi,

the 12V over the output resistors is the absolute dc offset. Try to lower it with VR2 and keep it near zero until the amp has warmed (or blown...)up. See if anything changes.

William

[gl]

Hi Dave,

The only advice I can give you about info on the MacMillan resistors is to read the first 40 or 50 pages of the big Aleph-X thread. It's all there in bits and pieces.


Hi Steen,

I agree with William. On my amps the Source resistor voltage drops 25mv from turn-on to full operating temperature on the heat sinks. Your absolute DC offset should not be staying at 11V. You should initially adjust it to as close to zero as possible over several hours (or days). From then on it should settle down within an hour after turn-on to something you can live with - for me that's less than a volt. Check the voltage across the BJT control transistor in the Aleph CS. It should be 4 to 5 volts. I suspect there is a wiring error in your amp or a part with the wrong value. It's just a guess. (BTW with the first channel I turned on I discovered I had all the main rectifiers installed backwards. I also had a serious error in the Base circuit of the CCS BJT. And I had checked all the connections three times!)


Hi Cheff,

Yes a BJT would make the best cascode and for all the reasons you state. I just suggested the mosfets because if you already have a working amp with 9610's as the diff pair it would be easy and cheap to try the mod you described.

I'm not sure that the ZVP33xx is going to make a big enough difference to affect the sound of the amp. My preferred solution would be to use a 2SJ109 with BJT cascodes. I know that this has been discussed many times and I am aware of the current drive/power dissipation limitations, but I am currently building a new version of this amp based on this idea.

I look forward to hearing about the results of your work with your new board.

Cheers,
Graeme

[steenoe]

William and Graeme, thanks for your advice on this matter. I will triple check everything again, before going any further. I didnt see the blue smoke yet, and I would hate to let it out now

Steen.

[Algar_emi]

Thanks QL for the information. I have a pair of 750VA, 2 X 30V transfo. I guest they would be perfect for a pair of X100 monoblocks isn't?

[wuffwaff]

Hi Graeme,

I replaced the 3k3 McMillan (R46/47) resistors with 10k ones yesterday.

Absolute dc behaviour changed from 6V at startup to 16V while it takes a lot longer to come below 2V. I´m not shure I like this and will try some other values too.

I did listen a bit yesterday but since I´ve changed quite a bit the last few months (cascoded x-bosoz, cd-transport, room) it is not so easy to tell the difference.

First impressions:
-no change in the lower frequencies
-the mid/highs did change (and this sounds a bit funny) sounding more free and powerfull and at the same time more "rounded". On some CD´s this was almost too much (the power I mean)

I will have to listen a bit more and go back to 6k8 or 4k7 to be shure I don´t imagine things.......

William

[wuffwaff]

Hi,

10k probably is a bit too high for my amps. Voltage keeps creeping down even after 3 or 4 hours of playing. Startup would be something like 18-20V wich is almost rail voltage if I would compensate for this.

So now I put 6k8 in one channel and will see what happens.

If I remember correctly when I first build the amps with zeners as a voltage reference one amp went from plus to minus and the other one was the other way round. Changing the zeners for precision voltage references made both amps behave the same way (going from plus to minus).
Wouldn´t it be possible to look for a zener that sits between the two I had in my amps and lets the amp stay at 0V from startup? Or some other temperature compensating part?

William

[gl]

Hi Algar_emi,

Your transformer will yield 30 * 1.414 = 42V (approximately) for power rails. This is too high, even if you used R-C or L-C sections to drop the voltage. Besides you would waste too much power. A transformer with 18V to 24V secondaries would be in the ballpark.

Hi William,

Generally speaking you seem to be getting similar results to mine as far as the quality of the sound goes. I understand your situation about not wanting to change too many things at once. At this point I am getting used to turning the amps on 30 to 60 minutes before starting to play music. The PassLabs manuals recommend doing this. Maybe this is part of the reason. I think that Nelson Pass is much more tolerant of absolute offset than most of the DIYers here. I am teaching myself to be braver about this because I believe that there are sonic benefits.

I'm afraid I don't remember any details about your Aleph-X and I didn't understand the details of your voltage reference remarks. Is there a thread reference you could give me? Both of my amps go from plus to minus.

Cheers,
Graeme

[wuffwaff]

Hi Graeme,

here´s the thread:

One Aleph-X working, One to go

I would like to measure a "standard" AX-160 to see how it behaves. I don´t think it´ll start up at 16-20V though.

William

P.S. the 6k8 seems to be OK. Startup is 12V and it stays around 0V after a few hours. I listened to it while the other channel still had the 10k resistors. Didn´t hear any unbalance........
I will put the 6k8 in the other channel today and listen a bit more.

William