is 60mv too high to drive a tweeter directly? - diyAudio
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Old 14th September 2008, 03:23 PM   #1
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Default is 60mv too high to drive a tweeter directly?

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
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Old 15th September 2008, 07:57 AM   #2
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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.
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Old 15th September 2008, 11:42 PM   #3
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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
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Old 17th September 2008, 08:50 AM   #4
glennb is offline glennb  Australia
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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.
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Old 17th September 2008, 09:06 AM   #5
lineup is offline lineup  Sweden
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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
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Old 17th September 2008, 09:32 AM   #6
frank1 is offline frank1  United Kingdom
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Default Re: is 60mv too high to drive a tweeter directly?

Quote:
Originally posted by thetube0a3
hey, on the amplifier i built to replace the amplifier in my powered advents i have between 40-60mv offset on the tweeter amplifier.
Its safe but such a high offset is not normal.
You really need to find the problem that is causing this and fix it.

Frank
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Old 17th September 2008, 11:36 AM   #7
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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"
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Old 17th September 2008, 12:05 PM   #8
AndrewT is offline AndrewT  Scotland
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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.
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Old 17th September 2008, 12:40 PM   #9
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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.
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Old 17th September 2008, 12:45 PM   #10
glennb is offline glennb  Australia
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Quote:
Originally posted by lineup
To find a really DC Coupled amplifier to speaker woofer connection we have to look for some Active Loudspeaker Systems.
In conventional passive crossovers all woofers are DC coupled from the amplifier, ie. there are no capacitors connected in series to block DC, they only use inductors, which conduct DC.
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