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How to calculate amp current output capacity and what a planar headphone would like?

post #1 of 8
Thread Starter 

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.

post #2 of 8
Quote:
Originally Posted by theearbone View Post
 

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. 

post #3 of 8
Thread Starter 

The 12V was something I think I saw on superaudiofriends about some older harder to drive high end planars HE6, LCD2 or some such.  I had some recollection about the ksa5 not being best for those super hard to drive models, checked on a thread by Gilmore and he then contrasted running them at higher voltage for oomph, I have to take his word for the calculations as a reliable source.

 

I measured the the voltage going into my he400S as 1.4ish, they have a spec impedance of 22ohm so I think thats 6ma for less than a 10th of a watt. 

 

Currently going from mojo in lineout mode to a  lme49710 buffered by two lme49600 per channel in dual mono, a jimsaudio pcb headphone amp.  You'd think at that low power it would be better going straight from the mojo but its not, similar goes for hd600 but not T90, which obviously had some detail removed from the sound - shame I didn't like the T90s.

 

1a I want to be able to calculate it if building or modding a kit.  Most bag of bits off ebay won't come with detailed measurements, and even if they did it would probably have been done by someone else under variable circumstances.

post #4 of 8
Thread Starter 

That should be 60ma and 1 watt

post #5 of 8
Quote:
Originally Posted by theearbone View Post
 

The 12V was something I think I saw on superaudiofriends about some older harder to drive high end planars HE6, LCD2 or some such.  I had some recollection about the ksa5 not being best for those super hard to drive models, checked on a thread by Gilmore and he then contrasted running them at higher voltage for oomph, I have to take his word for the calculations as a reliable source.

The HE6 are horribly inefficient, you're throwing almost 3W at them to get 110dB SPL.

Quote:
Originally Posted by theearbone View Post

I measured the the voltage going into my he400S as 1.4ish, they have a spec impedance of 22ohm so I think thats 6ma for less than a 10th of a watt. 

That's a pretty hot signal your'e measuring. Probably using just a voltmeter for that?

Quote:
Originally Posted by theearbone View Post

1a I want to be able to calculate it if building or modding a kit.  Most bag of bits off ebay won't come with detailed measurements, and even if they did it would probably have been done by someone else under variable circumstances.

Think of a power amp as a modulated power supply.  The primary limitations are all in the power supply. Its maximum voltage swing will be lightly below the supply rails, and the maximum current will be that of the power supply with whatever protection there may be in the circuit to prevent output device damage.  The maximum voltage swing will depend on the specific type of output device and circuit topology, so bipolar devices have more drop across them than MOSFETS.  Protection may take the form of a built-out resistor at the output (dumb, and simple, and increases output Z), or something more elaborate and active, but it will limit current through the devices by throttling back their available power.  You also have to include the load the rest of the circuit presents to the supply in the total load. For example, if it's running from a single-ended supply there's possibly a virtual ground in there somewhere which shouldn't add much, but is included.

 

Personally, I wouldn't bother with eBay kits unless they are a parts pack for something really well designed.  I'd also avoid the headphones with exotic drive requirements. They are few and there are so many excellent headphones with no special drive requirements, I wouldn't worry about them.

post #6 of 8
Thread Starter 

The Gilmore Krell clone is probably about as well designed as it gets - a guy that knows what he is doing cloning a super expensive amp by a guy that knows what he is doing and had senior engineers to answer to.  I think the ksa5 guy is now their chief designer.

 

That 1.4V was just a multi meter reading playing but not on my head fairly loud but not very dynamic music, thats 62ma, about a tenth of a watt.  Innerfidelity talks of 0.086Vrms for 90db spl, I have to assume that these are not like for like measurements. 

post #7 of 8
You can't get any meaningful readings measuring music with a multimeter. You need a tone in the range the the meter is calibrated for. Just use the test data from innerfidelity.
post #8 of 8
Thread Starter 

Thanks, always good to learn something.

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