I am trying to use some parts I have on hand, both transformers and tubes and build something relatively simple. I am looking to build a headphone amp to use a line out on a "prosumer" sound card. (This is all assuming my design is adequate.)
I've read conflicting and/or confusing info on this. If I understand correctly -10dB line level is more or less going to be the same as headphone level, assuming the headphones had a line level impedance? Does that mean, in essence, a headphone amp is more or less an impedance matcher? In my case, (and my other thread on PSU design is related) I want to use a conventional triode stage with a conventional triode strapped pentode transformer coupled output.
Assuming my math is correct in the design, I would use an 8:1 transformer to match the tube to 32ohm headphones. If that calculation is correct, then if I had enough gain on the first stage, to account for the 8:1 on the output, I would more or less have ~.32v on my output after the transformer., and that would be sufficient? What am I missing here?
I've read conflicting and/or confusing info on this. If I understand correctly -10dB line level is more or less going to be the same as headphone level, assuming the headphones had a line level impedance? Does that mean, in essence, a headphone amp is more or less an impedance matcher? In my case, (and my other thread on PSU design is related) I want to use a conventional triode stage with a conventional triode strapped pentode transformer coupled output.
Assuming my math is correct in the design, I would use an 8:1 transformer to match the tube to 32ohm headphones. If that calculation is correct, then if I had enough gain on the first stage, to account for the 8:1 on the output, I would more or less have ~.32v on my output after the transformer., and that would be sufficient? What am I missing here?
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Headphone sensitivity is usually rated in db/mW and it's usually around 100 db/mW. Let's say you need 120 db max, you're going to need 100mW or so. With 32 ohm headphones that would be about 3.2v and 0.1 amp peak. Line level is usually 2v peak to peak (1v peak) with no specified impedance, but it's usually fairly high. You're going to need a gain of 3 total, so calculate your gain and transformer stages that way.
pro audio uses 600 Ohms as the "standard load" - which is never actually seen, just used for level calculations
dBu (1 mW into 600 Ohms "untermenated") is ~ 0.77 Vrms
often pro audio equipment can push +20-24 dBu output
many newer headphones are designed with 3.6 LiIon cell DAP in mind - 1 Vrms is now sometimes used as the headphone sensitivity reference level rather than 1 mW into whatever the Z of the headphone in question
dBu (1 mW into 600 Ohms "untermenated") is ~ 0.77 Vrms
often pro audio equipment can push +20-24 dBu output
many newer headphones are designed with 3.6 LiIon cell DAP in mind - 1 Vrms is now sometimes used as the headphone sensitivity reference level rather than 1 mW into whatever the Z of the headphone in question
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Headphones come in most anything from 16 to 600 ohms and sensitivities of little over 80 to more than 130 dB SPL / 1 Vrms (though most of the time you are looking at about 95-125 dB/V). Some are very picky with output impedance and ideally want well under 1 ohm, while others show little coloration even around 100 ohms. With full-sized models you're generally fine if things are well under 10 ohms. A few full-sized models play best with 33-100 ohms, otherwise low Zout is desired.
You generally want a maximum output of at least 100, preferably 110 dB SPL, while keeping noise well under 30 dB SPL (<20 dB SPL preferred). Over the sensitivity range given, that generally means some form of volume and/or gain control is required in order to avoid bumping into either end of the dynamic range. (Output series impedance can be used to address some of the sensitivity difference, but as stated it's not exactly recommended in hi-fi applications.)
A nominal -10 dBV output usually means a 2 Vrms full-scale (0 dBFS) output voltage. This should be correlated to the maximum level requirement above. Normally you'll be listening far quieter though. Maybe 0 dBFS = 80 dB SPL or so.
Since both PCs and tube amps tend to be safety earthed and have their grounds connected to this as well, consider going for a balanced connection. (Does your "prosumer" card have balanced outputs? It's not too hard to add ones if it doesn't, you just need to determine output impedance and put something equal in the "cold" line.) That typically means an input xfmr in case of hollow state. These are quite handy, as they'll accept both balanced and unbalanced signals without too much fuss.
You generally want a maximum output of at least 100, preferably 110 dB SPL, while keeping noise well under 30 dB SPL (<20 dB SPL preferred). Over the sensitivity range given, that generally means some form of volume and/or gain control is required in order to avoid bumping into either end of the dynamic range. (Output series impedance can be used to address some of the sensitivity difference, but as stated it's not exactly recommended in hi-fi applications.)
A nominal -10 dBV output usually means a 2 Vrms full-scale (0 dBFS) output voltage. This should be correlated to the maximum level requirement above. Normally you'll be listening far quieter though. Maybe 0 dBFS = 80 dB SPL or so.
Since both PCs and tube amps tend to be safety earthed and have their grounds connected to this as well, consider going for a balanced connection. (Does your "prosumer" card have balanced outputs? It's not too hard to add ones if it doesn't, you just need to determine output impedance and put something equal in the "cold" line.) That typically means an input xfmr in case of hollow state. These are quite handy, as they'll accept both balanced and unbalanced signals without too much fuss.
Personally, in my experiments I've found that an HPA that can supply 10Vrms and 330mA can drive *any* headphone (ie. +/-18VDC rails). Here's a chart that plots manufacturer supplied max power solved for impedance on a fair sampling of headphones I found published.
Take the chart with a large dose of salt as 2watts into a Denon AH-2000 (the phones I own) would cause hearing loss even though they are rated to take it.
Slightly off-topic, but 100 watts into my JBL 2" compression driver mounted on an EV HP9040 full sized horn would be a couple orders of magnitude above deafening while following manufacturer's ratings. Which is why the poweramp I use is 3 watts and still has way more headroom than I need
Take the chart with a large dose of salt as 2watts into a Denon AH-2000 (the phones I own) would cause hearing loss even though they are rated to take it.
Slightly off-topic, but 100 watts into my JBL 2" compression driver mounted on an EV HP9040 full sized horn would be a couple orders of magnitude above deafening while following manufacturer's ratings. Which is why the poweramp I use is 3 watts and still has way more headroom than I need
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That's an interesting idea. IMO it would make more sense to mark the points of a given amount of SPL output though (like 100 or 110 dB), since as you say e.g. AH-D2000s would make you go deaf before reaching their maximum power rating. I know of very few cans that reach their rated power at "sensible" volumes (Beyer DT231s are a bit tight at 95 dB/mW and 50 mW max, but even that still means 112 dB max; Hifiman HE-6s are kinda similar except at much higher power).
10 Vrms and 330 mA seems like a good bet. That's enough voltage for a K240M/DF or K340 and still gives 2 W into a HE-6 (still voltage limited, actually). Now you "only" need to accomodate sensitive in-ears as well, without annoying hiss levels and a volume pot way into bad unbalance territory...
10 Vrms and 330 mA seems like a good bet. That's enough voltage for a K240M/DF or K340 and still gives 2 W into a HE-6 (still voltage limited, actually). Now you "only" need to accomodate sensitive in-ears as well, without annoying hiss levels and a volume pot way into bad unbalance territory...
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