I am trying to find out if it is common practice to use AC coupling to filter out DC in modern pro audio amplifiers. I can't seem to find any info on the matter on the web. Specifically I am curious if it IS the standard to use DC blocking caps... if they are just used on the input and not the output and if they ARE used on the output, what some common values might be. I found the formula: 1/(2*3.14*(Cap value)*(Resistance) for the cutoff frequency of the RC highpass filter that the DC cap plus the impedance of the driver would create. So I guess the value of the output cap would have to be huge and costly for decent low frequency response of the amplifier into say a 4 ohm driver as the load?
I believe amplifier INPUT coupling caps are required for speaker safety. Some will claim added distortions but these are so small that to me it's not worth the risk of a pure DC path.
But output caps are indeed obsolete and not required or desired in a ground referenced amplifier. At higher power levels coupling caps do introduce a lot of distortions.
The only exception would be on a tweeter amp in a bi-ampled system where you don't want tens or hundreds of watts directly available to the tweeter. The problems with an output capacitor in a tweeter circuit are much more manageable. Also conside that any passive speaker crossover already has a cap in series with the tweeter anyway.
But output caps are indeed obsolete and not required or desired in a ground referenced amplifier. At higher power levels coupling caps do introduce a lot of distortions.
The only exception would be on a tweeter amp in a bi-ampled system where you don't want tens or hundreds of watts directly available to the tweeter. The problems with an output capacitor in a tweeter circuit are much more manageable. Also conside that any passive speaker crossover already has a cap in series with the tweeter anyway.
Line level filters are most common, rare to put a DC block on the output of an amplifier although it used to be common due to early transistor OP topologies. and it still can be useful as a speaker fail safe tool.
Values are typically 2000 to 10,000 uF for full range
PAs typically require extra high pass filter stages (DC block) for maximum SPL output > look at the speaker installation documentation.
Speakers hate DC, music needs no DC , most commercial amps have DC blocks inside up to 2 or 3 stages.
not many amplifiers are rated at DC other than some specialized laboratory uses.
not sure what THX says about it, but might be useful as a spec for commercial installs.
Correct the Xc should be low compared to the speaker Re at a frequency lower ( 1/2 octave or less ) than the mounted speakers Fb
Values are typically 2000 to 10,000 uF for full range
PAs typically require extra high pass filter stages (DC block) for maximum SPL output > look at the speaker installation documentation.
Speakers hate DC, music needs no DC , most commercial amps have DC blocks inside up to 2 or 3 stages.
not many amplifiers are rated at DC other than some specialized laboratory uses.
not sure what THX says about it, but might be useful as a spec for commercial installs.
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Large scale PA stuff typically has a DC block on the power amp input, and has additional (and rather higher frequency poles) high pass filtering in the drive signal processing electronics, no point in driving a horn loaded sub below resonance, the driver just tears itself to bits.
Often the amps have various filtering options built in as well, either in DSP or on a bank of dip swiches.
Generally casual input highpass poles will be below a couple of Hz, while the driverack DSP might have a third order (or sometimes even higher) at somewhere in the few tens to a few hundreds of Hz region.
Almost all of these amps have DC sensing on the output which either opens a relay or trips the power supply depending on the technology (With switchmode power becoming more or less ubiquitous in the touring scene (My back loves this) tripping the power supply is an increasingly common way to handle catastrophic failures).
Often the amps have various filtering options built in as well, either in DSP or on a bank of dip swiches.
Generally casual input highpass poles will be below a couple of Hz, while the driverack DSP might have a third order (or sometimes even higher) at somewhere in the few tens to a few hundreds of Hz region.
Almost all of these amps have DC sensing on the output which either opens a relay or trips the power supply depending on the technology (With switchmode power becoming more or less ubiquitous in the touring scene (My back loves this) tripping the power supply is an increasingly common way to handle catastrophic failures).
D.C. blocking capacitors at the input of power amps are quite common. Even those power amps which do not have an input capacitor will often then have a D.C. servo circuit, which will serve much the same purpose, at least, within reasonable offset limits. However, the primary function of a D.C. servo is to null any D.C. output offset produced by the amps own circuit.
Any amp specified as, "D.C. coupled" should be used with an input signal that is free of D.C., else it will amplify the D.C. and force direct current through your woofers, which you do not want to occur.
Any amp specified as, "D.C. coupled" should be used with an input signal that is free of D.C., else it will amplify the D.C. and force direct current through your woofers, which you do not want to occur.
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You will find some of the few amps that are not DC coupled at the output in PA gear - that's those funny "upside down" concepts which are driving their current into ground and connect the speaker between the big filter caps. Those are there anyway and have sizable capacitance. Essentially free DC protection - well, you do need separate secondaries for each channel, and the caps may contribute some distortion (but then in PA, who really cares?). It's very effective though, even if the output stage blows up bigtime.
There's an urban legend story in Hollywood that years ago some big mixing console had a power supply failure in the middle of the night. The engineer hooked up two 9volt batteries to a Crown DC300 and adjusted the volume controls for the needed +/- DC voltage to power the console from the DC300 speaker outputs.
I guess it could be true.
Don't forget the McIntosh autotransformer. They designed the early output stages at two ohms. The autotransformer then matched that to the speaker load. A side advantage is that the transformer impedance was fractions of an ohm at DC so failed outputs would simply blow the rail fuses and the speaker never saw the DC.
I have a Heathkit AR-15 receiver that uses a single supply with 4000 uf capacitors to couple to the speakers. Those capacitors have to charge to half the rail voltage and the path is through the speaker. On power up, there is a loud "WHOOOMP" and a lesser "whooomp" on power down. Woofer cones can be seen violently moving. I haven't tried to measure the physical unit but a simulation shows over 5 volts on the output at turn on, followed by very slow decaying oscillation over 20-30 seconds. Additionally, applying or removing a large low frequency signal results in DC offset that never completely clears when the signal is on.
I investigated a redesign but the power transformer has no center tap, so this is not possible.
I investigated a redesign but the power transformer has no center tap, so this is not possible.
some pretty crazy early transistor designs, for sure a lot of magnetics where involved, interstage and OPT. heck even 9V pocket radios had two audio transformers each.
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