Early clipping with JX Amp

[JBL4435]

Dear Forum Members and Pass DIY Experts

I did some measurements with my new JX Amp this weekend and see lower power and earlier clipping than I expected.

The JX is designed after my erlier XA Amp which was designed with the AXE1.2 spreadsheet for app. 70W at 7-8 ohms. XA amp had 3 output pairs (12 IRFP240 per channel)

Power supply info
Rail Voltage: 18,3V
99.000uF CLC

Bias "old" XA is 3.3A
AC current gain app 60%

The JX works with the same ps, AC gain (R24=714 ohm) and bias.
It is build with the boards from peter (see picture)
It is build with 2 output pairs (8 IRFP240 per channel)

The JX starts clipping MUCH earlier. It delivers only 49 Watts into 8 ohms at 1kHz with 1% THD whereas the XA delivered 68 Watts into 8 ohms at 1kHz with 1% THD.
The 4 ohm values are 25 Watts into 4 ohms at 1kHz with 1% THD for the JX whereas the XA delivered 51 Watts into 4 ohms at 1kHz with 1% THD.
All THD measurements are made with a HP Audio Analyzer.


My thought was that the JX can roughly be calculated as the XA. What is wrong here?

[JBL4435]

Here are the 4 Ohm graphs

[JBL4435]

I adjust the bias with R27 being a 10k resistor with a 50k pot in series.
See picture (ZenMods idea for adjusting CCS)

[JBL4435]

Correction:
The PS is not 99,000 uF CLC but 198,000 uF CLC

The jfet I use is the 2SJ109BL (app 8mA Idss)

[Zen Mod]

Originally Posted by JBL4435

....
See picture (ZenMods idea for adjusting CCS)

naah ......

that's invented by some much cleverer than Mighty ZM

probably somewhere in Merlin's time



regarding earlier clipping ..... by (previous ) "XA" you mean exactly same output topology as in JX ?

[JBL4435]

Originally Posted by Zen Mod

naah ......

that's invented by some much cleverer than Mighty ZM

Yes, but you brought it into the thread at the right time to the right position

Any idea what's going on with my amp?

[Zen Mod]

I edited previous post , while you were typing yours

[JBL4435]

Originally Posted by Zen Mod

regarding earlier clipping ..... by (previous ) "XA" you mean exactly same output topology as in JX ?

"Old" XA had 3 output pairs (12 IRFP240 per mono block)

"New" JX has 2 output pairs (8 IRFP240 per mono block). I didn't want to use 3 pairs as I'm not sure the 2SJ109 can drive it.

PS (Rail voltage) is the same, Total Bias is the same

[Zen Mod]

well - OLG of amp is this way much smaller ( jfet have lesser xconductance than mosfets in LTP , and now you have 2/3 of output xconductance )

what you need is to recheck AC Bias of Aleph CCS

search on forum , post by Papa .

[JBL4435]

Originally Posted by Zen Mod

what you need is to recheck AC Bias of Aleph CCS

It is checked. 62%
The clipping is unsymetrical if this helps. Negative half clips earlier.

So you mean the JX isn't scaleable to 80W like the XA?
Where do I loose the voltage/current?

Do you think it might help to parallel 2 jfets?