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Old 17th November 2019, 09:47 AM   #211
cubicincher is offline cubicincher  Germany
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Default to flamethrower #408 and #410

Hello flamethrower,


2mV offset at loudspeakeroutput is nothing to worry about.


If I start up my amps I often go up to 50mV offset (at loudspeaker output)
during adjustment of POT 1 and POT 2.
I think you read the biasing procedure?
Turning POT 1 till your offset at the speakeroutput climbs to somethng between
20mV to 50mV.
Then turn POT 2 till your offset at speakeroutput comes back close to 0 mV offset. You will not get 0 mV! It is fluctuating / drifting to + or minus some mV.
That is normal. Your MOSFETS are starting to work / to be biased.
And they start to heat up - this causes a drift on your loudspeaker-offset-readings.
Turn your TRIMPOTS slow! Give the amp time to warm up (after adjustment
of POT 1 or POT 2).
Then again. POT 1 a little bit more - till your speaker output offset goes up to
circa 20-50mV. Then turn POT 2 to bring the offset (speakeroutput) back close

to 0 - 10 mV.



Check your voltage over the bias resistor of one N-channel MOSFET with one DMM..
Check your voltage over the bias resistor of one P-channel MOSFET with one
DMM.


Only adjust one channel!

This is a time consuming procedure. I often need an hour for one channel.


#410
If you switch off your amp, it is normal that your readings on the DMM at
the loudspeakeroutput are running away.


I hope this helps a little bit.


Greets
Dirk
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Old 17th November 2019, 01:29 PM   #212
Grunnet is offline Grunnet  Denmark
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On trimpot #3. I have seen mentioned, that you can leave it out if you cannot analyze the effect. If you leave it out, what do you do. Replace it with a resistor or leave the mounting holes empty or short circuit the mounting holes or what ??
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Old 17th November 2019, 01:40 PM   #213
flamethrower1 is offline flamethrower1  United States
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Thanks Dirk.
After getting firing up the first board and seeing something other than 0 at the output on one board.
I unhooked that board and hooked up the other one.
That board also read 1.3 mV, that got me thinking "how could this be".
I checked, rechecked everything as I was populating the boards.
Also, I thought at this point, I had better verify all three meters see the same thing so I can be sure to trust them.
One of the meters I had would not read mV even though it was auto ranging.
So, will have to get another meter I guess.
Will follow your suggestions when I get one.

Thanks Greg
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Old 17th November 2019, 01:59 PM   #214
dBel84 is offline dBel84  United States
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Those Sperry meters are great. The low mV reading across the output is not concerning, as Dirk had mentioned..dB
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Old 17th November 2019, 03:23 PM   #215
JeffYoung is offline JeffYoung  Ireland
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Quote:
Originally Posted by Grunnet View Post
On trimpot #3. I have seen mentioned, that you can leave it out if you cannot analyze the effect. If you leave it out, what do you do. Replace it with a resistor or leave the mounting holes empty or short circuit the mounting holes or what ??
Leave the mounting holes empty.
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Old 17th November 2019, 03:30 PM   #216
JeffYoung is offline JeffYoung  Ireland
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Quote:
Originally Posted by flamethrower1 View Post
Thanks Dirk.
After getting firing up the first board and seeing something other than 0 at the output on one board.
I unhooked that board and hooked up the other one.
That board also read 1.3 mV, that got me thinking "how could this be".
...
Hi Greg,

Anything under 50mV is acceptable, although I like to see it stay in single digits (< 10mV). Keep in mind that even if you get it down to 1.3 or 2.0mV it's going to wander around a bit so it won't necessarily stay there.
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Old 17th November 2019, 03:38 PM   #217
ItsAllInMyHead is online now ItsAllInMyHead  United States
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@Flamethrower - I saw a familiar set of steps on your paper...

Is this what is causing you pause from Dirk's post here?

Another F5 Build

"Switch the amp on. Be prepared to switch it off in a case of emergency.
All 3 DMMs should show 0.0 mV - if not - something is wrong.
Now you start to adjust P1 and P2:"

If so, I think even Dirk himself has advised moving on. 1.3mV is nothing to worry about. It doesn't need to be 0.0mV on all three.

Last edited by ItsAllInMyHead; 17th November 2019 at 03:42 PM.
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Old 17th November 2019, 04:30 PM   #218
flamethrower1 is offline flamethrower1  United States
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Yes, that is what made me think that something was not right.
Going to run in town and pick up another DMM and give it another shot this afternoon.
Thanks for all of the help and input you guys.
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Old 17th November 2019, 04:41 PM   #219
ItsAllInMyHead is online now ItsAllInMyHead  United States
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Awesome! Can't wait 'til you get it going. I've been enjoying watching the progress.

Full trust in you, but are you really sure you need another DMM? Depending on the model, even if it's auto-ranging, it may not display really low voltages unless you manually choose the range. You may have a range button to force it to 0.000V resolution or 0000 mV resolution or similar. With mine, I actually like "roughing in" the voltages on a lower resolution setting so it doesn't look like it's drifting so much. Then once I get it really close, I can increase the resolution.

If you're willing... maybe try beginning the process of biasing / nulling offset until you start to get into the 10s or 100s of mV on one meter, then if you swap leads to your suspect meter and get nothing - you may need a new meter. Either way, I fully understand taking the most cautious approach. The thought of burning something up for me was a bit paralyzing during my first few builds.

Last edited by ItsAllInMyHead; 17th November 2019 at 04:55 PM.
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Old 17th November 2019, 04:54 PM   #220
Dennis Hui is online now Dennis Hui  Canada
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Have you tried fresh batteries?

With that meter, do you get zero if you just short the leads together?

What you do want to to make sure the meter can measure a few tens of mV
offset reasonably accurately.

And if you do have a dud meter but two good ones, just use one for bias measurement
and the other on the offset.
__________________
We are fearless amplifier builders. A little heat doesn't scare us at all.
- NP 2002

Last edited by Dennis Hui; 17th November 2019 at 05:02 PM.
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