Originally Posted by theearbone
I'm looking at investing in a ss amp kit of some sort and would like to get something that drives my he400s to their best, but also has spare juice to power more demanding planars that I don't yet own. I've built amps before and can do that safely, I just need some guidance on how to choose/reject the design from the umpteen options out there.
1. How to calculate the output capacity of a solid state amp?
a For current
Transistor spec sheets feature current gain for a couple of pairs of voltages and currents, I have no idea how to get a figure for the sensible sustainable out for different voltages and currents.
b For voltage swing
I'd not thought that this was an issue and then saw someone claiming that the ksa5 didn't have big voltage swing for some highend difficult to drive planars that wanted 12v. If I invest in another amp I'd like to have all bases covered
2 How to know what headphones need?
This seems to require a knowldege of target spl, headphone( + cable) impedance and headphone sensitivity. Which is fine if I had a rule of thumb figure for a spl to aim for, I don't want to get into louder than should ever be used territory, but would assume that some sensible headroom for peaks is required.
1.a. It's usually specified as maximum voltage and/or maximum power into a specified load. You could calculate current from that, but you don't need to. The key is power into a load. The amp has been tested at some point and has specifications, it's all there. No need to calculate anything.
1. b. If a headphone required 12V rms to play at a reasonable level they should be rejected as a poor design. Consider the actual power dissipated by a headphone with 12 V applied to a 30 ohm load: 4.8 W. Power is "work done", and very little power is converted into actual sound in any transducer. Most of it is converted to heat, and heating is the "work done". Where is that 4.8 W going? Cooking your ears? Of course, your ears would also be damaged because in even the worst case I could find (HE1000 90dB/2.6mW) 12V would result in SPL at around 122dB. That's just the worst case I could find quickly, but as I looked, I didn't find a lot of headphones even close to that, typically they are 0.25mW/90dB max, and most way below that.
Your HE400 would not likely survive 12 V for long, but while they were still working your ears would be hit with between 127 and 132dB SPL (sorry, I don't know which version you have) . The KSA-5 could do more than 12V, but the corresponding SPL from anything would be damaging to hearing and headphones. You can always buy new headphones, but....
2. Headphones require a certain amount of power to achieve a certain sound pressure level, SPL. The parameter is sensitivity. Their sensitivity is usually specified as power to reach a reference SPL, usually 90dB. Since power is a function of voltage applied to a load, you can calculate the required voltage to deliver that power if you also know the headphone impedance.
If you can't find specifications for your particular headphones, you could try looking them up on innerfidelity.com to see if they've been tested.
As far as you original question of what amp that drives your HE400 to their best, it wouldn't take anything exotic at all. They are a reasonable load, and fairly sensitive. All an amp would do for them is buffer a source device with wimpy drive capabilities, or provide more gain. Look at the specs of the amps you're considering, and look for output impedance. It should be 1/10 of your load or lower. Beyond that, not knowing what your source device is, it's pretty hard to say what the benefits would be, if any.