is 60mv too high to drive a tweeter directly?

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hey, on the amplifier i built to replace the amplifier in my powered advents i have between 40-60mv offset on the tweeter amplifier. im worried this is a hair high to be directly coupled to the tweeters.

tho im sure the tweeters will handle 20-30W, 60mv still worries me.

is it safe to have 60mv across a tweeter, or should i use a coupling cap, or try to squeeze a cap in the feedback loop

the tweeters are the late 70s orange advent ones.

amplifier is a typical non-inverting LM3886
 
You didn't mention the impedance.

A 4 Ohm speaker will have a DC resistance between 3 and 4 Ohms, so let us assume 3 Ohms as worst case scenario.

60mV/3Ohm=20mA
60mV x 20mA = 1,2mW

Half that number for an 8 Ohm speaker.

A while ago I stumbled accross a homepage, where a speaker manufacturer had tested the DC power rating of a woofer. It worked up to 1/30 of the nominal AC rating. That would be 0,67 to 1 W for your tweeter.

Looks like enough of a safety margin.
 
i beleive the dc resistance of the tweeter is somewhere between 6 and 8 ohms. the orignal amplifiers said to test them into a 6 ohm load.. yet all advents "non powered" are 8 ohm loads.
i kinda wish i used a servo on this like i did the bridged-parrelled ones, those have 1mv or less offset :)
 
The power dissipated in the voicecoil may be tiny at 60mV, but the voicecoil will be displaced from its normal resting position. This may affect the linearity of reproduction.

Another possible problem with driving tweeters directly (ie. no passive crossover) is that if the power amp fails and the output latches to one of the power supply rails, the tweeter will go up in smoke.

A wise precaution is to add a series film cap that has a largish value. This also has the benefit of implementing a 1st order high pass filter, so that no low frequency components make it to the tweeter. Something above 10uF is in the ball park.
 
A wise precaution is to add a series film cap
that has a largish value.


This is the usual capacitor out coupling.
Has been used for ages in all times in power amplifier
in the junction: amp output -> tweeter, woofer


> For headphones protection we can 100 - 1000 uF, depending on Impedance.
> For single Power supply Amplifiers into 4/8 Ohm speakers,
we can use 2200 - 10000 uF, depending on wanted low freq roll off.
> In speaker crossover X-Over filters,
they use all sorts of values, depending on the actual speaker x-over used and the woofers/tweeters impedances.

To find a really DC Coupled amplifier to speaker woofer connection
we have to look for some Active Loudspeaker Systems.

Where the x-over is done, actively or passively,
BEFORE the power amp INPUT.

Lineup
 
im using active crossovers before both poweramps. thats the whole point of the system.

as far as why the offset is so high, many posts on here suggest it is normal for LM3886's. so i stopped wondering why it was so high

the tweeter amplifier is a single lm3886 with a 2.2uf cap on the input with a 1M resistor to ground , then a 20k pot , a 1k resistor in series to the non inverting input. i have a 20k feedback resistor with a 1k to ground. and a 1000uf 50v cap along with a .1uf cap on each rail right by the chip if i turn the pot all the way down, essentially shorting the input of the opamp right after the 1k resistor, my offset drops to 16mv.

id love to get the offset lower, but im out of ideas?
my feedback resistor matches my resistor to ground on the input "20k"
 
Hi,
change the topology to AC coupled. i.e. both the input DC blocking cap and the NFB DC blocking cap must be fitted.
Then match the resistances seen by the two input pins.
Finally trim the input resistances to remove the last bit of offset.

If you require the offset to stay substantially zero then add a DC servo to correct offset drift as the chip temperature changes.

All the partially DC coupled topologies will generate output offset.
 
thanks for responding so fast andrew,
so is 40-60mv offset on a dc circuit normal? or still on the highside?

i had considered changing my feedback resistor to 50k, and the resistor to ground to keep my 21 gain, but keep the 20k pot on the input, and see how much my offset drops.. and go from there

but if 40-60mv IS normal, its fine by me.
 
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.
 
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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Around 60mV offset is normal with an LM3886 and a circuit like this. :D

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.


[...] 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.
 
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?
 
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!!!
 
thetube0a3 said:
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?
 
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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