What are the implications of turning off DC filtering on the inputs?

[KSTR]

I was planning on having an RME FS BE handling my ADC duties and having it set off.

You also have to remove (short) the physical DC blocking capacitors at the ADC input...

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As for DC-coupled DAC outputs, amplifier inputs etc, my stance is that idiot-proof consumer devices should block DC to prevent mishap.

Professional devices should never block DC as it is assumed the user knows what he/she's doing and the device must not make any assumptions how it is going to be used (sometimes they are used in research labs etc, not even handling audio signals). The same goes for those dreaded auto-standby features and such. They may offer those features but there must be options to turn them off.

 

[KSTR]

Can you explain why is that?
Is there a good reason for any source device to have any DC output?

- audiophiles generally hate coupling caps
- they cost money and occupy board space
- digital implementation is seldom straightforward when using common DAC chips as is, which means extra cost and complexity to implement DC protection
- some users might want to have DC coupled option, for measurements or non-audio duty or whatever
- last not least: "it's not our problem (says the DAC designer) when you use a dangerous source signal" -- the same would apply to other dangerous signals like 20kHz full scale, btw...

 

[restorer-john]

As for DC-coupled DAC outputs, amplifier inputs etc, my stance is that idiot-proof consumer devices should block DC to prevent mishap.

Professional devices should never block DC as it is assumed the user knows what he/she's doing and the device must not make any assumptions how it is going to be used (sometimes they are used in research labs etc, not even handling audio signals). The same goes for those dreaded auto-standby features and such. They may offer those features but there must be options to turn them off.

I agree 100% with this.

Auto standby/power off (APO) is bad enough on my DMMs. Each has a different power on keypress to disable it and the Flukes don't even have an APO indicator so it's anyone's guess until you turn around and it's gone to sleep.

 

[Sokel]

Wasn't there a thread about that couple of weeks ago which we made measurements and tests to output DC?

 

[Sokel]

I agree 100% with this.

Auto standby/power off (APO) is bad enough on my DMMs.

I don't mind about APO too much but I get furious about the light of my extech which lasts 2-3 seconds and then you have to press again or hold it to disengage auto shut off.

 

[Cars-N-Cans]

Most amplifiers are AC coupled anyway and the better ones that are full DC usually have either DC servo (which, within limits) removes DC at the output or have DC protection that disconnects the speaker/headphone.

There may well be designs that do not have any protection which would be a severe design fault... owners might not be aware.
Usually a loud 'pop' when switching inputs/devices can be a dead give away.
After such a 'pop' you really won't know until the woofer/headphone starts to smoke or stop working. Maybe, when the DC is not very high you may never know it is there.

The more inexpensive headphone amps (and some of the more expensive ones, probably) from Schiit don't, that's for sure. Bought one of their Magni 3 Heresy amps, and despite the whole show put on by the relay wasting 10 seconds of my life each time I turn it on, there's no DC offset protection at all. Almost cooked a set of headphones with it. Just wanted to try it out with a laptop real quick when I first got it, and fortunately I saw though the vents in the top that the circuitry looked suspiciously simple despite the reassuring sounds of the relay. Sure enough there was several volts of offset with it set to high gain. To me any sort of audio amplifier that is straight DC coupled should not exist... But they do, and seldom mention it, and most people don't even know to check for it before hooking up headphones or speakers.

Even better was a video I saw at a high-end audio show where they had a DC coupled pre-amp for the turn-table. Spent the whole video watching the woofer cones neatly go in and out while it tracked the surface warpage of the LP they were playing. I wish I could find it again. Its the best example I have ever seen of why a rumble-filter exists.

 

[restorer-john]

I don't mind about APO too much but I get furious about the light of my extech which lasts 2-3 seconds and then you have to press again or hold it to disengage auto shut off.

What do they say- first world problems? LOL.

But yeah, I built an auto power off for my bedroom audio system back in the early 90s so I could play music and not have to worry about shutting down the system after the music (CD) had finished. The RC was set for around 10 minutes and worked like a charm.

 

[restorer-john]

Even better was a video I saw at a high-end audio show where they had a DC coupled pre-amp for the turn-table. Spent the whole video watching the woofer cone neatly go in and out while it tracked the surface warpage of the LP they were playing. I wish I could find it again. Its the best example I have ever seen of why a rumble-filter exists.

Wait until @Frank Dernie reads that comment.

I know he doesn't love DC-daylight phono stages, or the lack of a decent shelving infrasonic filter.

 

[solderdude]

The more inexpensive headphone amps (and some of the more expensive ones, probably) from Schiit don't, that's for sure. Bought one of their Magni 3 Heresy amps, and despite the whole show put on by the relay wasting 10 seconds of my life each time I turn it on, there's no DC offset protection at all. Almost cooked a set of headphones with it. Just wanted to try it out with a laptop real quick when I first got it, and fortunately I saw though the vents in the top that the circuitry looked suspiciously simple despite the reassuring sounds of the relay. Sure enough there was several volts of offset with it set to high gain. To me any sort of audio amplifier that is straight DC coupled should not exist... But they do, and seldom mention it, and most people don't even know to check for it before hooking up headphones or speakers.

Even better was a video I saw at a high-end audio show where they had a DC coupled pre-amp for the turn-table. Spent the whole video watching the woofer cones neatly go in and out while it tracked the surface warpage of the LP they were playing. I wish I could find it again. Its the best example I have ever seen of why a rumble-filter exists.

Yes, especially cheaper amps do not have DC protection. I thought recently Schiit starting adding it but not in the cheapest models.
Whenever an amp has a softstart relay (and/or quick-off) to prevent 'thumps' it only requires very few added components to add DC protection. There are even IC's for it.
Many headphones have been ruined by this.
And even IF there is DC protection there may be circumstances it fails. The L30 debacle (first runs) are testimony of that.
It should be noted that this is an amp problem though and not an ADC or DAC issue. These simply 'record' and 'reproduce' what is handed to it.

Failing amp(s) is another matter. Unfortunately a lot of manufacturers do not add such a headphone/speaker saving feature. Either because of ignorance or to save a few $.

 

[Ken Tajalli]

- audiophiles generally hate coupling caps

I must be one!

- they cost money and occupy board space

tuppence . . .

- digital implementation is seldom straightforward when using common DAC chips as is, which means extra cost and complexity to implement DC protection

I was under the impression it was a needed thing in I to V transfer.

- some users might want to have DC coupled option, for measurements or non-audio duty or whatever

Well we are in Audio science review, non-audio duties should be beyond our discussion.

- last not least: "it's not our problem (says the DAC designer) when you use a dangerous source signal" --

That is in line with restorer-john's assertion of irresponsibility, cheap ....

 

[Mnyb]

Is the pass the buck situation it's always someone elses problem and then you end up with no DC protection

So it's wrong of amp mfgs to assume DC free sources and also wrong of DAC mfgs to assume DC blocked amplifiers and very sloppy of a recording studio to have DC levels in their releases ( it does happen ) My amateur POW is the ADC should DC block who knows what kind of offsetts would creep in from crusty vintage consoles and mic preamps or guitar amps ?