AX100 100W Aleph-X Monoblocks

[macka]

Hi Graeme.

Thanks for your report.

I have been glued to the Australian Open so nothing done.

But my matched JFets are on their way.

My relative offset (with 9610) also shifts a bit after full warm up but not much.

I also have to capacitively couple the inputs on both sides.

Even with my X2.5 connected it seems to throw out the relative offset.

A question. Are you J Fets touching each other on each side?

As long as its consistant I dont thing its a real problem.

As William say a 10 ohm pot maybe a good thing on the souce of one side.

Macka

[gl]

Hi Ian,

Thank you for your thoughts. Are you waiting for your new JFET's before proceeding? I had thought you had the parts you needed for this particular project already on hand.

It has occurred to me that drift in the relative offset is being caused by current hogging in the JFETs. So your suggestion about Williams pot is a good one. I don't like using Source resistors in JFET circuits. I decided years ago I don't like the way they make a circuit sound. A highly personal and arbitrary statement I realize. In a SuSy circuit I would prefer to see the common mode feedback resistors connect directly to the Sources and not to a node that puts any Source resistors in series. I could use four Source resistors, one for each JFET, and then four MacMillan resistors of twice the current value, again one to each Source. But that's a lot of parts. What do you think?

Tonight I am going to fire up the amp, let it heat up to operating temp, then measure the temperature of each JFET package to see if they are heating up unevenly. And no the J109s are not touching each other.

Cheers,
Graeme

[macka]

Hi Graeme,

The parts I have on had are not of suitable match.

I think finding out the relative offset is happening before any modification is a good idea.

A thought is to swap sides for the Jfets.

The differecne is coming from somewhere and the output stage gain and rising temperayure is magnifying it.



Macka

[gl]

Hi Ian,

Yes I agree. The issue regarding the rising relative offset needs to be solved first. I've been thinking that since it is only 3 to 4 mv at turn on that the existing component matching is fine. The problem is the result of component parameters changing during warm up. The first thing I think I'll try is 10 ohms in series with each JFET source and see if that changes the relative offset performance.

Cheers,
Graeme

[wuffwaff]

Hi,

just to make it clear. I´ve used a 10R trimpot. Connected the current source to the wiper and the fets to both ends,

In the middle position the fets have a 5R source resistor. Turning the pot makes one bigger and the other smaller and compensates for not perfect matching.

It starts at around 0mV and stays at around 0mV.....

In my amp both fets touch eachother.

William

[macka]

William.

Very clever..reminds me of the UGS adjustment.

Macka

[daly41k]

Erno Borbely in his balanced line differential tube line amp did the same thing in two different places. He put a pot between the plate resistors and between the current sources at the cathodes of the input tubes to make sure the voltage gain on both tubes was the same for +/- signals-works well. dave

[wuffwaff]

Yes,

I also read the Borbely articles

William

[daly41k]

William, is there a sonic penalty to putting in a trimpot or trim resistor in series with the fet's. I don't know myself, just asking.
It seems to me to be very logical and can save a considerable amount of time and money. I think I trimmed one side of my A75 diff input pair but I can't remember becasuse I built it in 1993.

I didn't do that in my AX120 but I struggled to get a good match.

dave

[wuffwaff]

Hi,

I don´t know because I never tried without. It should be small as the source resistors are quite small as such.
Not using them would probably be better but I found a list of my 2sj74. It were 34 pieces and I measured Idss and Vp but they varied quite a lot.

I´m in the proces of building a CCS-CS-XBOSOZ for a friend (who was very happy with his Aleph 3´s). After that maybe I´ll have some time to try a few things on my Aleph X´s.

BTW changed speakers from Thiel CS6 to Jordan "the Wall". Quite a different concept but not sounding bad at all.
Must only find a suitable sub.

Anybody interested in a pair of Thiels ?

William