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?
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?
Attachments
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Power supply, JFet
Correction:
The PS is not 99,000 uF CLC but 198,000 uF CLC
The jfet I use is the 2SJ109BL (app 8mA Idss)
Correction:
The PS is not 99,000 uF CLC but 198,000 uF CLC
The jfet I use is the 2SJ109BL (app 8mA Idss)
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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 ?
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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?
"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
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 .
what you need is to recheck AC Bias of Aleph CCS
search on forum , post by Papa .
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?
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Using an Audio Analyzer
Raising input voltage in 0.1V steps, reading distortion, put everything into excel and let excel make a graph.
Schematics
With values / changes
Absolute and relative DC offset is low and stable BTW
Voltage R8 is 8.4V
With values / changes
Absolute and relative DC offset is low and stable BTW
Voltage R8 is 8.4V
Attachments
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I have a drastic THD increase when I switch from 6.6ohm to 8.8ohm load.
I'll try the second amp with 3 output pairs as I think I need more current.
I'll try the second amp with 3 output pairs as I think I need more current.
Maybe your negative current limiters are kicking in. Try reducing R14's to 221
ohms and see what you get.
😎
ohms and see what you get.
😎
Thank you Nelson for chiming in here! 🙂
THAT sounds good. I'll try this later. Is there a reason why you set R14 so high? I see it is 100 ohms in the published AX schematics and I built my AX's like that.
I just see Aleph 3 was 100 ohms, Aleph 30 was 392 ohms
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😉
good to have Real McCay coying around
problem solved probably ......
what is value of source resistors you put , and what's Iq per mosfet vertical ( now you have 2+2 of them ) ?
good to have Real McCay coying around
problem solved probably ......
what is value of source resistors you put , and what's Iq per mosfet vertical ( now you have 2+2 of them ) ?
We will see....
R16,17,18,19 are 0.5ohms/3watt Dale RS-2B
Bias current for each is 0.83A
Total bias is 3.3A with 62% AC gain (R24 being 714ohms)
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