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?
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?
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?
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?
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.
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.
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?
So .. why would a crowbar protection with a relay be better than a disconnecting relay?
I wouldn't crowbar my output, protections or not.
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.
I wouldn't crowbar my output, protections or not.
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.
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.
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.
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.
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.
--
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.
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.
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.
You may misunderstood me. My goal is the highest quality possible with no question on how much those parts cost. I do not think that stuffing money in there will do much for me but the simplest design with the least compromises. Yes I can do microcontrolled magic stuff in there it is no question, the question was:
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?
There are many commercial DC amplifiers, (Firestone Spitfire, etc) so there are a lot of chance if You buy a headphone amp it will be DC coupled.
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?
There are many commercial DC amplifiers, (Firestone Spitfire, etc) so there are a lot of chance if You buy a headphone amp it will be DC coupled.
Last edited:
I agree with sgrossklass. Get a DC-sensing protection circuit, there are many ready-made ones. Make it not cut off the output, but the power rails instead. That way, the relay contacts aren't in your signal path, so you've nothing to be concerned with. Also, on any error you shut everything down and have less chance of any further component failure.
This is for DC-coupled.
For input caps - good quality film cap. There are many good brands, but beware of snake-oil advice. Don't go overboard, any good quality cap will do. Sizing will depend on the input impedance of the amp, but basically you can't go too big. I would recommend you Obbligato caps. Great quality for normal prices. I've got Film-in oil, they're totally transparent, but large. There are other film types that are smaller. For output caps - literally as big as you can go. You'll probably need an electrolytic. Get a good brand cap and as large as possible. You won't need a big voltage rating, so you can get some tremendous values in uF.
This is for DC-coupled.
For input caps - good quality film cap. There are many good brands, but beware of snake-oil advice. Don't go overboard, any good quality cap will do. Sizing will depend on the input impedance of the amp, but basically you can't go too big. I would recommend you Obbligato caps. Great quality for normal prices. I've got Film-in oil, they're totally transparent, but large. There are other film types that are smaller. For output caps - literally as big as you can go. You'll probably need an electrolytic. Get a good brand cap and as large as possible. You won't need a big voltage rating, so you can get some tremendous values in uF.
Last edited:
use low rdson fets as a switch instead of relays if you must. dc protect circuits are often not that quick, or transparent. personally i never use them, dc coupled all the way for me. relays add too much by way of output impedance, so having them at the output in the signal with a headphone amp just doesnt work.
personally my every day portable is balanced and actually dc coupled input and output with the sabre dac having 1.65v dc at its output. i dont forsee any problems, failure would have to be pretty unusual to cause issues as i;m using fully differential opamps so the chances of one phase failing are unlikely, i have low charge detect and auto-shutdown on the regulated bipolar psu and the mcu has a faulty detect. if theres a problem with the cable and one phase becomes disconnected there simply isnt a signal
personally my every day portable is balanced and actually dc coupled input and output with the sabre dac having 1.65v dc at its output. i dont forsee any problems, failure would have to be pretty unusual to cause issues as i;m using fully differential opamps so the chances of one phase failing are unlikely, i have low charge detect and auto-shutdown on the regulated bipolar psu and the mcu has a faulty detect. if theres a problem with the cable and one phase becomes disconnected there simply isnt a signal
Last edited:
I have a balanced power amp design I've been working on periodically that sort of self protects against a shorted output by way of the DC servo reaction. It esentially places the other phase to the same potential as the shorted output so the speaker becomes a 'bird on a wire'.
A LP filter, some simple logic, and a duel comparator for bi-polar control to transistors that switch the rails on or off. Since you only have mA inside a headphone amp small signal devices will suffice. And you can use a momentary switch and a flip-flop circuit for standby/active.
As far as input cap, polypropelyne film is good. Polystyrene is also a very good low absorbtion dielectric but unfourtunately is hard to find in values larger than 10nf.
Paper in oil is very good, but as pointed out they are physically large. I'm not a fan of output caps because the values need be so large and film caps of that uf's would take too much space and $. Electros generally have issues with specific types of distortion. This is mostly why I go for DC coupled output.
Last edited:
As qusp mentions, maybe go for solid state relays. Read it all... they work well.
http://www.diyaudio.com/forums/solid-state/191449-output-relays.html
I would never go back to mechanical relays now.
As to AC coupling... well it offers the simplest approach in many ways although for perfection the output should still be muted (shorted with a relay ?) for a few seconds at power up to prevent thumps etc.
I used AC coupling here,
http://www.diyaudio.com/forums/head...rmanium-single-ended-class-headphone-amp.html
just a bit of fun that one... surprisingly good though.
http://www.diyaudio.com/forums/solid-state/191449-output-relays.html
I would never go back to mechanical relays now.
As to AC coupling... well it offers the simplest approach in many ways although for perfection the output should still be muted (shorted with a relay ?) for a few seconds at power up to prevent thumps etc.
I used AC coupling here,
http://www.diyaudio.com/forums/head...rmanium-single-ended-class-headphone-amp.html
just a bit of fun that one... surprisingly good though.
My issue with Dc coupled/servoed headphone outputs is that the dc servo opamp typically has a huge output (compared to the signal level) in order to null the DC ouput. The expert on DC servos is on this forum, his name is Mr Gootee. He has a website and a very eye opening post which using his spicemodels. I have seen that most class A headphone output stages have the DC servo up to -25 dB well into the midrange even when filtered below 1hz. The effect with a deep class A type diamond buffer is a very noticeable straight downward slope from 20khz to 1khz on a 1khz FFT.
Not saying they don't sound good, but most dc servo's can't be called as transperant as a DC blocking cap (at least as far as measurements.)
That said I don't like the sound of most big electrolytic DC blocking caps on the outputs. What has worked well with 300 to 600 ohms headphones are a couple of the ASC 50uf motor run caps, with 100uf the bass loss is minimal, but subjectively (I can't prove it) they just seem to sound better than a typical 470uf Black Gate.
In the end I think even for SS a good output tansformer is the best bet for headphones.
Last edited:
years ago, it was a push-pull mofset output buffer with output tansformer, but he had to go thru a lot of mofsets to get a match that didn't saturate the primary with dc, he used trimmers and had both mofsets on the same heatsink.
I am sure there are better ways to do it but there are transformer coupled designs out there, I think the k&k lundhal forum has some info.
I am sure there are better ways to do it but there are transformer coupled designs out there, I think the k&k lundhal forum has some info.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.