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Old 16th November 2011, 02:43 PM   #411
Krisfr is offline Krisfr  United States
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This double sided board stuff is TUFF....

Standby

It is like 5 times harder than SS
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Old 16th November 2011, 07:59 PM   #412
AndrewT is offline AndrewT  Scotland
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Ravi,
have you looked at the temperature de-rated SOAR of the 1943/5200 pair when used at the voltages to be found in a 500W amplifier? Almost as bad, have you taken account of the high temperatures likely in a heavy duty 500W amplifier?
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Old 18th November 2011, 10:59 AM   #413
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Glowing LEDs must mean something good right?
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Old 18th November 2011, 11:23 AM   #414
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Glowing good. Mine are glowing too. A question.
With the fuses removed (using 22r resistors in fuse places) and the bias set to maximum resistance my power supply is showing +- 59v which is great. When bias resistance is decreased to set standby current at the correct voltage (20mv between test points) the power supply drops to around 53v and the lamp (series lamp) begins to glow very dimly?
Should I have this voltage drop?

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Simon
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Old 18th November 2011, 11:29 AM   #415
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Quote:
Originally Posted by niss_man View Post
Glowing good. Mine are glowing too. A question.
With the fuses removed (using 22r resistors in fuse places) and the bias set to maximum resistance my power supply is showing +- 59v which is great. When bias resistance is decreased to set standby current at the correct voltage (20mv between test points) the power supply drops to around 53v and the lamp (series lamp) begins to glow very dimly?
Should I have this voltage drop?

Regards

Simon
Hi Simon,

Is the 53V measured after the 22 ohms safety resistors?

6V across those resistors is just about right for about 300mA (100mA per device), however your measured 20mV gives only about ~50mA. I would recommend you also monitor current at your output devices.

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Old 18th November 2011, 11:51 AM   #416
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Actually the P.S. drops to 48v measure before the safety resistors. Way too low I think. At this voltage the power transistors show 10mV across each 0.22r emitter resistor or 45mA Across each.

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Simon
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Old 18th November 2011, 12:02 PM   #417
AndrewT is offline AndrewT  Scotland
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the bulb tester is a tester to help prevent damage when one makes a mistake.

It is not intended for use when setting up the output bias.

Output bias consumes many watts. Many watts is precisely what the bulb tester does not allow to pass.

Similarly the correct fuses must be in place before setting the Vbias above minimum volts.

Get the equipment wired up safely and properly. Check that all devices are appropriately cool. Check circuit voltages and currents. Remove those dangerous test probes. NOW, REMOVE the bulb tester and start up direct on line. If this blows the mains fuse then you need a soft start not a bigger fuse.

Bias up the amplifier.
Test output voltage. Test again. power off, power on, test again.
Finally attach a cheap disposable speaker and listen to noises/hums/buzzes. This should confirm the earlier test output voltage readings.
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Old 18th November 2011, 12:18 PM   #418
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How is the right bias current determined?

My default approach is to set the scope in differential mode, and probe the voltage between the input and output of the output stage. Then you can visually judge linearity when driven into a satisfactory load. The best point changes with loading, and low-ohm loads need a higher bias point. For a 4 ohm load I determined 68mA was best, assuming that Gm-doubling is acceptable compared to switching spikes. For multiple-output amps it is probably different.

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Old 18th November 2011, 12:45 PM   #419
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Ok. Yep it was the bulb tester. Now have it set up as per ostrippers instructions. At 20mv across test points I am getting 10mV across each of the 6 emitter resistors.(50ma per device ). Is this ok?
Also now power supply is rock steady at its supply voltage of 59v. No buzzes hums etc from cheap speaker.

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Simon
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Old 19th November 2011, 12:06 AM   #420
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Thanks keantoken. I will be using 8 ohm speakers so I geuss my 50mA per output device is good then.

Regards

Simon
Quote:
Originally Posted by keantoken View Post
How is the right bias current determined?

My default approach is to set the scope in differential mode, and probe the voltage between the input and output of the output stage. Then you can visually judge linearity when driven into a satisfactory load. The best point changes with loading, and low-ohm loads need a higher bias point. For a 4 ohm load I determined 68mA was best, assuming that Gm-doubling is acceptable compared to switching spikes. For multiple-output amps it is probably different.

- keantoken
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