capacitor coupled vs. direct coupled ouput

repairing an old amp with just a single transfo, and caps at the output to prevent DC at the speaker connectors, I considered to use such capacitors at the outputs of my amps (between output and centre tap of symmetric psu).

I don't like relais anyway, neither complex DC protection circuits. Those caps are easy to use and reliable and will protect my speakers if necessary.

Do you think there are any disadvantages involved with this use?
I know the caps must be large to pass the lower frequencies. Why isn't this method used anymore (costs, distortion)?




On Hiatus
2002-11-05 6:14 pm
There was a time, I remember, when the big thing about an new amplifier was the words "Direct DC coupled".
Now most amps use it. It is no big thing.

But soundwise it is not so bad to use Caps, as one might think, especially since
there has been some development in quality for Big Caps.
Most people would not notice any change in sound by using caps.

I ask:
Why the fear of using a cap in Amp output,
if you use them in Loudspeaker input
filter anyway?
Capacitot output!

hei Hugo!

For the capacitors in the output do not introduce significant distortion they need to be correctely a amplifier with dual rails (+-) this condition os not meet.

You coud use two back to back but their ESR will add:eek:

Resuming : the ouput with a capacitor only make sense in a single rail power amplifier...of corse in my humble opinion:D


Jorge Santos
Re: Capacitot output!

Tube_Dude said:

Resuming : the ouput with a capacitor only make sense in a single rail power amplifier...of corse in my humble opinion:D

yes I know, if everything runs smoothly. I was thinking to add them to prevent damage to speakers when something goes wrong inside the amp ;) I'm just too lazy to make some DC protection, and caps are so easy to add :D

Well, you figure you need a cap of at least 4700uF/50V for the average amp, but then consider the ripple current requirements and it becomes more difficult.

Of course, large electrolytic in the signal path is considered heresy. ^_^ Seriously, though, there is ample evidence of the electrolytics causing problems as coupling caps. This isn't like a hovland vs wima difference.

I am working on some tweeter amps which will be capacitor coupled. Setting the pole 2 octaves below the rated bandwidth still allows for reasonably sized polypropylene cap.
Re: Capacitot output!

Tube_Dude said:
You coud use two back to back but their ESR will add:eek:

In this case in it's totally unneccessary since you HAVE DC all the time across the cap.

Also to remove the capacitor is not an easy task. The bias point at the output stage is not at the signal ground, nor half the voltage. Forget it (for now)!.
back to back capacitors

Hei Paranders

I only talk about the back to back electrolytics because in the case of the first post in the trhead the power supply is symetric...Hugobross said:( I considered to use such capacitors at the outputs of my amps between output and centre tap of symmetric psu).

Of course only in such case the back to back make sense!


Jorge Santos
the speakers are directly connected to the middle point of the amplifier

Any problem during amplification of the stability point, one rail getter higher or lower V. Or one side of transistors getter hotter biased or colder cause DC to appear on the speakers which can cause serious sound issues.

The feedback is directly taken from the varying Z and temperature dependent reactance and impedance of the voice coils, mitigating high frequency response under lower amounts of feedback.

Small inductors are needed to the outputs of each transistors. etc etc.

There are very scientific problems to the designs.

if the amplifiers constantly fight the perceived motion of the cone through Z and Reactances and capacitance as well, the speakers will never act as an instrument resolving and impulse, sustain and decay , instead it is in a perpertual fighting motion.

this could be without feecback as well with DC coupled designs

In sound, there is always one impulse, then the sound is let reconate then it is stopped, like in a piano, you hit the cord, then let it resonate, then release it and dampers cut the sound.

A good amplifier is like this, it will not use a fake low Z, it will send the proper signal then let the speaker have some distortion from the mechanics and limitation of the matter.

IOW the solid state coupled high feedback are too perfect for the actual device and poison the rest. So a modest capacitor coupled SS amp can sound excellent for this reason among others
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This thread is 8 years old!
Back to back electrolytics sound funny on high freq 1/4 watt source tracks. I tried 2x10000 uf series output on a Peavey CS800s split supply amp. The CS800s has reliable Dc detection anyway, Enzo said. DC detected lifts a serious DC rated speaker relay.
The direct coupled CS800s & my AX6/ST120 (djoffe mod) with both speaker & input cap sound exactly the same on my SP2-XT speakers at 1/4 watt base level with 70 w peaks.
Don't like designing protection: repair a PA amp that has it. The CS800s uses 8 times the electricity of the AX6/ST120 at music room volumes. 4 pairs output transistors/channel instead of one. I repaired a PV-4c with 2 pairs for my HDTV room with $10 speakers instead of the $600 SP2-XT. DC detection on the PV-4c is a *****y speaker crowbar.
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doesn’t the ngf loop work to keep dc offset under control in most amplifiers when they are properly working ?

Interestingly, a split rail amp, single channel only, with no connection between power trafo centre tap and ground is virtually equivalent to a capacitor coupled output single rail amp. In my single channel TGM7 amplifier I put a resistor between the amplifier power ground and the centre tap of the power supply trafo as a crude form of speaker protection. A dc fault resulting in dc current through the speaker must also flow back to the transformer centre tap through ground so the resistor would limit the current.

The rest of my amps use SS relays on their outputs to stop turn on and turn off noises. Adding dc detect and protection is then a small ask. Single rail amps with output capacitors sometimes suffer from turn on thumps because they are not usually split rail so you have to charge the cap with a charge up pulse. For me, this is why split rails are handy, not because of capacitor sound.
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2004-12-01 6:46 pm
Next door
Not yet quoted : Cyril Bateman proposed to connect two bipolar capacitors in series to lower the distorsion of capacitive connections. I think he once told me he designed a special cap of high value (10000 µF) for an amps manufacturer.
Michael Renardson designed amplifiers with an output capacitor included in the feedback loop because he met problems with the output relay contacts of a DC coupled output amplifier. I also met this problem, I think electrolytics are more reliable. By the way, Renardson's Mosfet amplifiers give extremly low distortion results.
MJR7-Mk5 Mosfet Power Amplifier
Not yet quoted : Cyril Bateman proposed to connect two bipolar capacitors in series to lower the distorsion of capacitive connections....

Thanks for mentioning those things. It was a very significant discovery by Cyril. It was first mentioned in EW Jan 2003 'Capacitor Sounds' series p17,18 available at Linear Audio here

It is again mentioned for improving Douglas Self's amp in the EW 'Capacitor & Amplifier Distortion' series sometime Oct-Dec 2004 where he measured a 20 fold reduction in distortion to -120dB using a double 220uF 63V bipolar instead of the best polar 47uF (C4 across the Vbe multiplier). Reference is given to Patent application GB 0227606.