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Old 3rd January 2009, 01:09 AM   #571
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Hi Nelson,

I was referring to the current of the output stage (with JFet front end like the Aleph J).

I sense having heard a diy Aleph J that less is more (better) when adjusting the A/c current share..as I recall the resister is like 1200R.

Despite the low transconductance of single Jfet pair it works beautifully in that design!!

I am taking an educated guess here here but with the improved linearity of the Jfet over the 9610 the 50/50 share and cancellation of distortion of the output current share may not work to best advantage.

Then again perhaps it just arbitrary.

Anyways I have bought some 10 k linear dual gang pots and propose to strap them across 1300R with 2K in series with the pots.

This will give me an adjustment range of about 800- 1200 ohms for the current share resister on the fly. The linear pots track quite closely.

You can tell I like to tinker.

Ian
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Old 4th January 2009, 02:30 PM   #572
daly41k is offline daly41k  United States
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Default aleph1.2 to AX100 jfet mod and initial check out

GL and others, my second full day of checkout after converting my ax100 to jfets went as follows:

One full day of running the amp hooked up to a 5ohm 125watt resistor on the outputs with the inputs shorted.

Input:

330mv dc on the gates of the input fets and this remained steady so I will use the input coupling caps as GL recommends. I am attributing this to the different resistor values used as the Mosfet version did not have any DC.

80mv dc relative offset voltage on the outputs. I am leaving this adjustment alone. The amp starts up at 40mv and after 8hours is ramps up to 80mv.

The absolute dc on the output starts out at 13 volts and slowly drops to about 3.5 volts dc after several hours. This poses no safety hazard so for now I am not going to try playing with resistor values to get it lower. I would be interested in what others are getting? I think this number is temp dependent. My mosfet verstion the absolue dc was about 800mv after a long warm up. The rail voltage is a little higher than GL's at 32 volts.

In checking my output devices (they are going on 9 years old as they were part of my original Aleph 1.2 I built in 2000); I am getting 600mv across the 1 ohm source resistors. This is an average as some outputs are 700mv and some are 450mv. I guess you can say I didn't match them but I am getting lazy. I am using 24 total per side or 6 per quad.

I did run some square waves and they look ok but I have to use the generator as a single ended input and I don't have a shielded cable to connect it to the amp (I am using an ordinary wire). Therefore I think the 10k square wave on the + input looks really clean but the - input which is connected to ground looks a bit rounded. I will listen to some music this week and see if I hear any distortion but my test equipment is ancient.

I also cleaned up some wires and drilled some additional holesin the amps top cover. More to come in later. dave
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Old 5th January 2009, 02:03 AM   #573
daly41k is offline daly41k  United States
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Default aleph1.2 to AX100 jfet mod and initial check out

GL after running the amp on the test bench all day today and bolting it back up I noticed the output absolute offset went down below a volt but the relative offset climbed up to 300mv. I have modified the amps cover to allow more cooling (I drilled 1 inch holes along the top sides (like boulder) and covered them from the inside with black screen and I think this has affected the bias as the amps run cooler.

I think I will install the source resistors but I may use a 10 ohm pot and attach the wiper to the fet current source and each leg to the +/- signal input jfets. I can then adjust this to see if I can zero the relative offset. I read up on the threads and it seems patience is the best solution. dave
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Old 5th January 2009, 11:35 PM   #574
gl is offline gl  United States
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Hi Dave,

Minimum relative offset IMO is governed by close matching. It also plays a role in the absolute offset although other factors play a large potential role there too.

Like you say, your output transistors don't appear to be matched as well as they could be. I matched mine to within .01 - .02 volts on the Vgs. The voltage drop across the Source resistors differed by only a few millivolts as I recall. The 10 gain (output) transistors in each channel were matched to each other and the 10 CCS transistors were matched to each other.

Definitely use the coupling caps.

Graeme
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Old 6th January 2009, 01:08 PM   #575
daly41k is offline daly41k  United States
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Default aleph1.2 to AX100 jfet mod and initial check out

GL, one other question is that you did not use gate resistors for the jfets. In the mosfet version the traditional 220 ohm resistors at the gates were used. I do not see any oscillations on my scope so for now I have left this alone. dave
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Old 6th January 2009, 03:45 PM   #576
gl is offline gl  United States
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Dave,

You need the gate resistors for the mosfets but not for the jfets.

Graeme
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Old 17th January 2009, 04:34 PM   #577
ashaw is offline ashaw  Australia
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Nelson, What are you doing with the pendeg disscontinuation of the 2SK170/ 2SJ74?
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Old 17th January 2009, 07:06 PM   #578
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It's not pending, it's done.

I'm planning on living off my inventory until better replacements
for the P channel parts appear.

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Old 18th January 2009, 02:59 PM   #579
daly41k is offline daly41k  United States
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Default aleph1.2 to AX100 jfet mod and initial check out

GL, I have been fighting some heat gremlins. After successfully converting to the all JFet front end and checking things out with the cover off --perfection--70mv dc offset and all electrical dc voltages spot on. The absolute offset took a bit longer to reach about 1 volt than the previous mosfet version.

However after I put the cover on the relative dc on the outputs shot up to 300mv after about an hour. So what to do.

My plan now is to reduce the bias on the 24 outputs (6 per quad) to .450. from about .600. This will give me 5.4 amps per leg or from the PS perspective a total of 10.8 amps at idle. This should reduce the heat load considerably.

I have drilled new cooling holes in the top but this did not help.
Adjusting U2 did not get it lower.

I measured the voltage across the R23 and R25 392 drain resistors and got the following:

+side=4.91v
-side=4.68 v This is a difference of .23v with a current of 12.6ma (4.91/392=12.6). So I thought I should put in a 14ohm source resistor on the +side (230mv/12.6ma=14ohms) but that caused the relative dc to jump to 1.68volts on the outputs with no other changes.

So I am trying the reduced bias first as in the cold condition everything is ok (the square waves looked good). I have 10uf ppcaps on the inputs in balanced mode.

I am not sure what exactly is causing this as the voltage across R23 and R25 did not change much with the addition of the source resistor. I may try to put in a 30ohm pot with the wiper connected to the current source and each leg to one of the pot connections and adjust this after the amp gets hot.

The way I had this bias set I would be getting 14.4 amps at idle and estimated 200watts of class A which I do not need.

I am replacing the 100k R11/23 with a 100k pot set for 75k today and adjust from there. I will keep in touch-dave

I will keep in touch-dave
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Old 11th March 2009, 09:57 PM   #580
gl is offline gl  United States
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Hi Dave,

How are things going with your heat issues and the relative offset issue too?

How are your amps sounding these days? I'm presuming that you're doing a lot of listening.

Cheers,
Graeme
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