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Old 13th September 2011, 07:17 PM   #1
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Default Output caps - or no output caps?

I would be happy to hear your opinion on this subject.

I am building a Headphone amp with (almost) no compromise on price as it is simple as it could be, similar to the Wire with bare metal resistors and so on...

I had hard time to think but can not decide on DC coupling or using output caps.

I know, output caps cause distortion but prevents DC on output.
1) Those who are using DC coupled amplifiers, how do You make sure Your headphones are safe all the time?
2) Those who are on the capacitor side, what output (or input) caps do You recommend in an audiophile amplifier?
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Old 13th September 2011, 07:37 PM   #2
Atilla is offline Atilla  Norway
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Well, this all depends on the design, really.

For no coupling, you have Relay+DC sensing protection, or servos integrated into the design.

For output DC coupling you need fairly big electrolytics. Simply put, the bigger, the better. First - because of the high-pass filter that'll it'll form with the load impedance. That's something you can calculate. Second, according to Douglas Self's experiments, low-freq distortion falls with bigger values of the electrolytic coupling cap.

Just what exactly are you planning on making?
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Old 14th September 2011, 02:14 PM   #3
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My main system is fully DC coupled, however if I were to build something new it would probably have cap coupled output AND be single supply.

Electrolytics tend to perform better with with some DC bias across it, half of the supply voltage does just that and don't forget your bleeder resistors to ground.
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Old 14th September 2011, 08:03 PM   #4
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Quote:
Originally Posted by Atilla View Post
Just what exactly are you planning on making?
Simple opamp headphone amp with premium components mainly built around LME49990 and LME49600. Power supply is dual (+-) 15V, volume control is my own LDR attenuator with separate supply.

OPC had great success with these opamps in the "Wire". I don't want to decrease that quality, but afraid of damage in headphones I could afford only once in a lifetime (almost).

I thought of a relay based protection but many said that the relay will cause loss especially after years...

The other solution would be an idea from a member: he said when sensing DC just short the output with a cheap relay. This way the headphone is protected... the LME49600 is short circuit protected so it will survive, but this sounds quite drastic for me. Altrough I can admit this way no other series components are needed in line neither a relay.

What do You think?
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Old 14th September 2011, 08:22 PM   #5
Atilla is offline Atilla  Norway
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So .. why would a crowbar protection with a relay be better than a disconnecting relay?

I wouldn't crowbar my output, protections or not.

Quote:
I thought of a relay based protection but many said that the relay will cause loss especially after years...
What loss? And how may years? Come on... pick a nice, quality relay and be done with it. In 15 years, if you need to replace 1 relay, big deal.

Otherwise, you can just DC-couple the output, use good, severely oversized caps and you'll never have to worry.

In both cases, if you can hear the difference, you're a lucky man.
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Old 14th September 2011, 08:23 PM   #6
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I would just use high quality film capacitors in the input. That is quite transparent. You don't need output caps as you have dual power supply.
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Old 14th September 2011, 08:32 PM   #7
Atilla is offline Atilla  Norway
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Well, there's the potential of losing one of the rails and having the output hugging the other one. Low chance, but still...

I personally feel the safest with output relays. Just stay away from the 50-pence onces and it should be fine. You don't have to go the other extreme though - spending 50 quid on one is probably not worth it.
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Old 14th September 2011, 09:04 PM   #8
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Quote:
Originally Posted by Atilla View Post
So .. why would a crowbar protection with a relay be better than a disconnecting relay?
I think, because if You are "DC less" this protection should never activate. If You use the series relay that should always be there regardless of DC free operation.
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Old 14th September 2011, 09:09 PM   #9
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If this were a cost-no-object crazy headphone amp I guess I'd stick huge film caps in parallel on the outputs, but then the caps would probably cost more than your headphones. A more practical DC coupled amp can use a servo, however that would not protect anything if you lose a power rail.

--
Personally I tend to prefer a output mute switch separate from the power switch, and always turn on/off my amp muted.

A DPDT switch can use a couple resistors + LEDs each channel on the muted side to ground so that if you ever lose a power rail they would fully light up and tell you something's wrong... plus it should be cool to look at while something's playing when muted.

Not the most foolproof but this sort of compromise I can live with in DIY.
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Old 15th September 2011, 09:43 AM   #10
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It shouldn't be too hard to include a proper protection circuit in a "cost-no-object" amp, right? Speaker amps have had these for ages, maybe one can find a corresponding IC.

I would use the kind of variety that runs more or less independently and controls the amp power supply rails via relays (or one relay for both). Checking output DC offset and +/- voltage levels should normally do.

Like this, the relays are operated with sufficient wetting current, and you don't need ridiculously big output coupling caps.
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