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Old 8th January 2010, 03:29 PM   #6571
pedrop is offline pedrop  United States
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Default f5 bias

I successfully biased my newbuilt f5, both channels at 0.59 VDC after 2 hours of warm up.
DC offset in the output of one channel is 90mV and the other is 0.25V. This is way out of NP's recommended value, especially the second value. Is it safe to hook the amp to the speakers at this condition? Any recommendation as to what can be done to bring the offset down to at least 50mv (NP's least acceptable value).
Thanks for any help as I'm really anxious to listen to the amp (like right now!!!)

pedrop
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Old 8th January 2010, 03:33 PM   #6572
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adjust the pots to get the offset down. You might have an imbalance in the bias voltage but you will have low offset. Try it at least, small amount of turns should do it.

Fran
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Old 8th January 2010, 05:02 PM   #6573
sangram is offline sangram  India
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The way I understand it is that each pot controls the relative drive of the half to which it is connected, so increasing the positive side pot increases the offset above zero, and the reverse for the one on the negative side (makes offset negative).

You'll have to readjust the bias. As per the pdf, you have to alternate adjusting the pots. I basically monitor the output with one DMM, and the voltage across one resistor with another. Then keep increasing the bias starting at zero. Once I get about 50mV offset in one direction, then adjust the next pot, and back once the offset swings to the opposite polarity 50mV.

Once I reach around 450mV bias I alternate every 25mV instead of 50mV, and once it hits 570mV, I use a finer adjustment. I strive for as low an offset as possible, not necessarily the matching of the voltage across R11 and R12, there will be small differences due to non-matching of the output devices, I think.

I ended up with 300mV offset the first couple of times before I figured out this method. It is pretty obvious to those who've built discrete amps before, but this was my first and it took me some time to understand the adjustment procedure fully.

Short the input when making the adjustment. I have 0mV on one channel (less than 1mV) and -3.5mV on the other, the second also has a bit of less bias so it needs a little more on the positive side, I'll be doing that a bit later, but 1mV is achievable with some patience.
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Old 8th January 2010, 06:29 PM   #6574
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Has anyone monitored the offset during warming up?
Starting from cold.
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Old 8th January 2010, 06:41 PM   #6575
pedrop is offline pedrop  United States
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Default f5 bias

thanks fran & sangram...
I'll try biasing it again tonight as you indicated. I believe I got to have the patience of a koala.
Thanks...
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Old 8th January 2010, 07:59 PM   #6576
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Quote:
Originally Posted by bobodioulasso View Post
Has anyone monitored the offset during warming up?
Starting from cold.
I watched it every 30 seconds for 10 minutes -- started at 32mV, 53mV then worked it's way down to 26mV, oscillating back and forth about a trend. I ran it at 5W into an 8 ohm load.
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Old 9th January 2010, 06:29 AM   #6577
sangram is offline sangram  India
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My offset starts off around 15mV, gradually reducing over the warm-up period (three hours as of now) and settling at the values listed.
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Old 9th January 2010, 12:32 PM   #6578
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On the other channel the offset starts at around 30mV, and after 10 minutes hovers about zero.

Self considers "within 50mV" to be the upper bound of "acceptable".
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Old 9th January 2010, 02:27 PM   #6579
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Hello,
After following this thread for a while I've started building my F5.
The power supply is finally done, it always takes more time than you think to build something
Specs CLC PS: 500VA - 2x (MUR3020 - 47000uf - 10mH - 2x22000uF)
Next: softstart and drilling holes in the heatsinks.
regards,
Danny
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Old 9th January 2010, 03:32 PM   #6580
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Quote:
Originally Posted by danny_66 View Post
Hello,
After following this thread for a while I've started building my F5.
The power supply is finally done, it always takes more time than you think to build something
Specs CLC PS: 500VA - 2x (MUR3020 - 47000uf - 10mH - 2x22000uF)
Next: softstart and drilling holes in the heatsinks.
regards,
Danny
Hi Danny,
thanx to share your work&experience with us - "behind steppers"...
Hope to here from you very soon about bias setting & sound.
Cheers,
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