F5 power amplifier - Page 658

pedrop · 8th January 2010, 03:29 PM

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

woodturner-fran · 8th January 2010, 03:33 PM

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

sangram · 8th January 2010, 05:02 PM

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.

bobodioulasso · 8th January 2010, 06:29 PM

Has anyone monitored the offset during warming up?
Starting from cold.

pedrop · 8th January 2010, 06:41 PM

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...

jackinnj · 8th January 2010, 07:59 PM

Quote:

Originally Posted by bobodioulasso

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.

sangram · 9th January 2010, 06:29 AM

My offset starts off around 15mV, gradually reducing over the warm-up period (three hours as of now) and settling at the values listed.

jackinnj · 9th January 2010, 12:32 PM

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".

danny_66 · 9th January 2010, 02:27 PM

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

ma_coule · 9th January 2010, 03:32 PM

Quote:

Originally Posted by danny_66

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,

8th January 2010, 03:29 PM	#6571
pedrop diyAudio Member Join Date: Mar 2008	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

8th January 2010, 06:41 PM	#6575
pedrop diyAudio Member Join Date: Mar 2008	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...

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Rockford Fosgate Power 20001 bd amplifier power rating??	pachoorion	Car Audio	8	8th May 2011 10:49 PM
Power transformers versus amplifier output power..what is your option?	destroyer X	Solid State	38	9th May 2009 05:23 PM
McIntosh Power Amplifier Power Guard	johnnyrt	Solid State	2	23rd August 2007 10:22 PM
Output power for a power amplifier	Progg70	Solid State	33	10th September 2006 08:44 AM
Amplifier 3000 Wats Rms Power + Smps Higcht Power Bestiality	MARAVILLASAUDIO	Class D	1	5th November 2004 04:06 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

8th January 2010, 03:33 PM	#6572
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	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

8th January 2010, 05:02 PM	#6573
sangram diyAudio Member Join Date: Sep 2002 Location: India	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.

8th January 2010, 06:29 PM	#6574
bobodioulasso diyAudio Member Join Date: May 2008	Has anyone monitored the offset during warming up? Starting from cold.

9th January 2010, 06:29 AM	#6577
sangram diyAudio Member Join Date: Sep 2002 Location: India	My offset starts off around 15mV, gradually reducing over the warm-up period (three hours as of now) and settling at the values listed.

9th January 2010, 12:32 PM	#6578
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	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".