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JBL4435
diyAudio Member

Join Date: Oct 2006
Quote:
 Originally Posted by EUVL It is a voltage clip. The circuit has a gain of 6. So output is at 13.8V with 16V rails. As expected not enough headroom left for the MOSFETs. Since you are using 2 output pairs, you can increase rail voltage and reduce bias current per FET while keeping dissipation per FET below 32W. I would try 20V 1.6A per FET as a start. Patrick
Thanks Patrick
I thought about that but I use 22V Rails and 1.3A per FET.

Can you explain how you get the 6dB gain? With 6dB I get 4.4V out with 2.2V in. Wasn't it 15dB?

Last edited by JBL4435; 20th October 2011 at 05:55 AM.

JBL4435
diyAudio Member

Join Date: Oct 2006
Quote:
 Originally Posted by NYCOne This is how I built one: Building a World-Class Cascoded Balanced XF5 on a Budget I used 1K and 100K as NP did in the original and have no problems.
Thanks for the help but this was solved a few posts before. 1k and 33.2k

 20th October 2011, 06:12 AM #23 EUVL   diyAudio Member   Join Date: Oct 2003 Gain of 6, not 6dB (gain of 2). The closed loop gain of a feedback amp is given by (Rfeedback + Rshunt) / Rfeedback. In this case, Rfeedback = 50R, Rshunt = 10R. So gain = (50+10)/10 = 6. Patrick
JBL4435
diyAudio Member

Join Date: Oct 2006
Quote:
 Originally Posted by EUVL Gain of 6, not 6dB (gain of 2). The closed loop gain of a feedback amp is given by (Rfeedback + Rshunt) / Rfeedback. In this case, Rfeedback = 50R, Rshunt = 10R. So gain = (50+10)/10 = 6. Patrick
I've got this wrong. Thank you for clearification.

Any idea why I get this clipping? With +/- 22V Rail I should be safe.

 20th October 2011, 06:35 AM #25 EUVL   diyAudio Member   Join Date: Oct 2003 Your output voltage should not come closer than say 4V to the rail voltage if you want to avoid clipping. The MOSFETs do not have sufficient Vds to operate properly. Patrick
JBL4435
diyAudio Member

Join Date: Oct 2006
Quote:
 Originally Posted by EUVL Your output voltage should not come closer than say 4V to the rail voltage if you want to avoid clipping. The MOSFETs do not have sufficient Vds to operate properly. Patrick
I understand that, but I'm missing a few Volts

Output with 2,3V in and gain of 6 (15,5dB) is 13,8V. With 22V Volt rail....

My PS is ultra stable. 800VA Transformer 2x18V, CLC with 2x99.000 uF, 2x 2,2mH/0,12Ohm.

 20th October 2011, 07:16 AM #27 EUVL   diyAudio Member   Join Date: Oct 2003 You need to show an output waveform for the clipping for us to understand your problem. Patrick
JBL4435
diyAudio Member

Join Date: Oct 2006
Quote:
 Originally Posted by EUVL You need to show an output waveform for the clipping for us to understand your problem. Patrick
OK, I'll make several pics.

JBL4435
diyAudio Member

Join Date: Oct 2006
Measurements and Waveform 1kHz 8Ohm

Amp info:
Vrail +/- 21.8V
Ibias 0.6V across 0.47 Ohm 1.3A 2 parallel output Fets Total Bias 2.6A
I out of PS measured 5.4A

NOT X-ed yet.

1. 8 Ohm 2.5Vin 0.5%THD right before clipping
2. 8 Ohm 2.6Vin 1.5%THD start of clipping
3. 8 Ohm 2.8Vin 4.5%THD clipping
Attached Images
 8Ohm_2,5Vin_0,5THD.JPG (52.3 KB, 397 views) 8Ohm_2,6Vin_1,5THD.JPG (54.7 KB, 388 views) 8Ohm_2,8Vin_4,5THD.JPG (53.6 KB, 387 views)

JBL4435
diyAudio Member

Join Date: Oct 2006
Measurements and Waveform 1kHz 4Ohm

1. 4 Ohm 2.3Vin 0.6%THD right before clipping
2. 4 Ohm 2.5Vin 3.5%THD start of clipping
3. 4 Ohm 2.8Vin 7.5%THD clipping
Attached Images
 4Ohm_2,3Vin_0,6THD.JPG (50.6 KB, 283 views) 4Ohm_2,5Vin_3,0THD.JPG (48.2 KB, 273 views) 4Ohm_2,8Vin_7,5THD.JPG (51.3 KB, 23 views)

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