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Old 17th September 2008, 01:14 PM   #11
AndrewT is offline AndrewT  Scotland
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Tube,
post a schematic or link to the schematic you are using.
list the component values you have adopted.
Then we can advise on what is normal.

I have discrete SS amps (without any DC servo) that have output offsets that never stray outside +-10mVdc for all temperatures and operating conditions.
I have one amp (symasym and again no DC servo) that stays very close to +-2mVdc for all conditions.
Careful design and implementation makes these offsets possible.
40mV to 60mVdc offset is way beyond what is possible and can easily be designed out.
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Old 17th September 2008, 03:54 PM   #12
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ok heres a quick copy i hand drew of the schematic,sorry for the quality

all passives are 1206 surface mount, tho i have some extra pads in series and parallel to ground i can use on the input trace

the feedback look will be very hard to modify tho unless its just values.. and i didnt draw the caps on the power rails, which i said are 1000uf and .1uf per rail

thanks btw for the help, i cant thank you guys enough for your time
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Old 17th September 2008, 03:56 PM   #13
SY is offline SY  United States
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Put a 100-220uF nonpolar electrolytic in series with the 1k feedback sunt resistor (bypass it with a small film cap if that will make you feel better) and your offset will only be a memory.
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Old 17th September 2008, 04:32 PM   #14
sek is offline sek  Germany
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Around 60mV offset is normal with an LM3886 and a circuit like this.

I'd recommend to either do as SY advised, or match the input impedance levels (according to the LM3886's datasheet or any opamp textbook) i.e. using a trimpot temporarily in place of the series 1k resistor at the noninverting input.

With the volume pot at the input as a given, you would then have to find a tradeoff between offset, noise and drive level.


Quote:
[...] the voicecoil will be displaced from its normal resting position.
Why would an offset voltage steadily displace the voice coil in the air gap?
If it's truly an unmodulated (i.e. non-motorboating) DC voltage, it only causes a force during turn-on and turn-off, but not while steadily constant.

Cheers,
Sebastian.
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Old 17th September 2008, 04:55 PM   #15
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it would slightly hold the voice coil either more in, or more out depending on polarity as the tweeter is direct coupled to the amplifier.

if there is no easy solution to this, and the offset is safe ill probably just leave it as right now i have NO noticable noise. infact, i love these amps! ive never used any chipamps before now and im totally amazed. this amplifier did 55W rms into an 8 ohm resistive load with extreamly low distortion from near DC to almost 100KHz and noise seems nonexsistant . atleast to me

the only thing that bothers me is the offset. and unfortunatly my boards are already made, and all surface mount, so installing a huge film cap would look terrible, so im hoping i wont have too

btw, if i decide to install a capacitor inline with the tweeter, how large would i want? the crossover point is 1500Hz, so i want as little of effect as possible to atleast 1500Hz, id prefer far lower so the active crossover is the only thing filtering it im assuming 50-100uf? or do i still need much larger?
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Old 17th September 2008, 05:07 PM   #16
AndrewT is offline AndrewT  Scotland
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Hi,
convert that topology to AC coupled.
At the present you have 1k0//20k on the inverting input pin, ~952r.
The non-inverting input pin has a variable source resistance of 1k0 to 21k0.
That guarantees a variable output offset voltage.
Will the offset get worse with changes in operating temperature?
Will it get worse over time?
FIX it.

Tweeters have a very low Xmax, often <<1mm.
How much DC offset is necessary to use up all your Xmax?
What if half your Xmax is used just for the 60mV of offset. How well will the tweeter play with only half the remaining Xmax in one direction but three times as much in the other direction?
FIX it.

Will the tweeter run warm? How much will the warm/hot VC affect performance?
FIX it.

BTW,
13uF will give a 1.5kHz single pole High Pass filter if the tweeter has an 8ohm impedance around that frequency.

1500Hz is quite low for a good tweeter!!!
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Old 17th September 2008, 05:11 PM   #17
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is there anyway to fix it without going ac coupled?

what doesnt make sense to me is id assume i should have low offset with 20k on the input which i dont, i have 60mv.
with the pot all the way to ground i have 16mv.

seems backwords , no?
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Old 17th September 2008, 05:17 PM   #18
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by thetube0a3
is there anyway to fix it without going ac coupled?

what doesnt make sense to me is id assume i should have low offset with 20k on the input which i dont, i have 60mv.
with the pot all the way to ground i have 16mv.

seems backwords , no?
when the pot is at maximum the resistance seen by the non-inverting input is 1k0+20k=21k.
When the pot is at minimum (wiper close to ground) the resistance is 1k0. Very close to that 952r value on the inverting input.

Since the tweeter amp only needs to operate flat to 1500Hz the input filter to the amp can have F-3dB set to about 150Hz.
This allows the NFB filter to be set to about 100Hz. Using that 1k0 NFB value you can use a high quality plastic film/foil or metallised plastic film cap of about 1.6uF (use 1.8uF or 1uF//1uF or 2.2uF).

Finally, why on earth are you using 2u2F as the input DC blocking cap for the tweeter amplifier. What are you thinking about?
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Old 17th September 2008, 05:29 PM   #19
Nuuk is offline Nuuk  United Kingdom
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Probably much good as you have already built your 3886 amps, but something like the OPA541 or OPA549 should guarantee you almost 0v offset. It's what I would use for an active speaker.
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Old 17th September 2008, 07:12 PM   #20
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ooooh i was looking at it wrong, so if i switch my input resistor to say, 10K and my feedback resistors to 200K and 10K id have really low offset. or 5K input, and 5K and 100K feedback

is there any negative effects to raising the values on the feedback resistors? possibility of oscillation, or noise?

i was considering throwing a capacitor in series after the pot and a resistor to ground, so the pot wouldnt effect offset anymore.

as far as the 2.2uf cap, its because i made the amplifiers to be universal, so i could use them with or without the active crossover daughterboards that plug in seperately. the crossover has subsonic filters on it.

and by universal, i mean so i could build a 50W amplifier to drive speakers, or a 200W amplifier, or a 200W + 50W amplifier.
the other section uses the BPA-200 with servos.
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