solderdude
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Post by solderdude on Aug 30, 2013 20:17:10 GMT
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dicky
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Post by dicky on Aug 31, 2013 7:16:24 GMT
Thanks for this, Frans. This will probably be one of my projects this winter - I have a Naim NAC22/NAP120 that I would like to try to use to drive my AKG340 and/or T50.
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solderdude
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Post by solderdude on Aug 31, 2013 8:03:10 GMT
Fortunately, because of specifications of both headphones and the right output power both headphones can be driven from the same voltage divider even though they differ considerably in impedance 50 vs 300 Ohm.
R1 = 27 Ohm, R2= 39 Ohm.... all 3W-5W rated.
This will protect your K340 from over voltage and drive T50RP to high levels. You will be having about 16 Ohm output R and about 10V on K340 at max setting which is what it can handle. The T50RP will be limited to 1.3W (slightly over what Ember will reach)
a perfect match for both headphones.
You could also try to listen to K340 on Ember set to 120 Ohm and see if you like it better at higher R's (or 35 Ohm) Perhaps the K340 likes higher output R better.
In this case the solution is very simple.
Make a box with 2 output sockets and make 2 different adapters in that box. They can be paralleled without any problems, this is NO problem for the amplifier as the load it sees is very high compared to what it is supposed to drive.
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juke
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Post by juke on Aug 31, 2013 11:22:31 GMT
Great to see the final bit of this article Frans, much appreciated.
My amp manual states 23W into 8 ohms and my HE-6 are 50W impedance per their website. Power handling isn't quoted but I think it's safe to assume your 3-5W table is fine.
So do I go for R1 as 15 or 18 and R2 as open or use the next value 100?
Thanks
Syd
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solderdude
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Post by solderdude on Aug 31, 2013 12:12:44 GMT
Yes... R1 = 15 Ohm, R2 = 100 Ohm will give you 1.9W at max power. The HE-6 can easily handle 5W.
At full power without any resistors max power will be 4W (3dB more headroom)
R1 = 15 Ohm and R2 = open will do 2.3W that is 0.8dB louder than with the 100 Ohm
In your case you could connect it directly but is safer to operate with a 15 Ohm (or 10 Ohm) in series.
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Rabbit
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Post by Rabbit on Aug 31, 2013 12:32:12 GMT
Frans, how do you calculate the output impedance of the amp?
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solderdude
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Post by solderdude on Aug 31, 2013 12:47:08 GMT
The output impedance of a speaker amp cannot be calculated ... well.. it could but involves soooo many equations. It can be easily measured though. Output resistance also can differ depending on the frequency at which it is measured.
It is safe to say speaker amps (unless they are tube amps) all will be between 0.01 and 0.1 Ohm.
Headphone amps is another matter and is determined by the circuit itself and-or the resistor that is connected in series.
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Rabbit
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Post by Rabbit on Aug 31, 2013 13:24:44 GMT
OK, so it's very low in any case!
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solderdude
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Post by solderdude on Aug 31, 2013 15:13:45 GMT
Very low, I would say negligible low. If the damping factor is given of an amp you can calculate the value. If an amp is rated at 8 Ohms and having a damping factor of 50, the output resistance is 8/50 = 0.16 Ohm. The lower the resistance, the higher the damping factor the less influence a load (speaker) will have on the frequency response of that speaker (not the amp). Going below a damping factor of 100 is rather pointless. In the past some amplifiers were even having damping factors of over 10000 and used special cables with 4 wires to achieve that. There are even amplifiers designs that have negative damping factors. These numbers were considered important in those days and were a great selling point. You had to have a high damping factor, forget about the other numbers/figures was the advice in those days. It still lingers in everyone's mind and also made it's way to headphone audio, mostly by NwAvGuy. The reason high damping factors are completely pointless is because in the end it is the impedance of the speaker itself that determines the damping current. A resistor of 0.1 Ohm or 0.00001 Ohm in series with an 8 Ohm resistance of a speaker will have NO effect although many will claim better this and that, such cannot be shown to exist... nothing new there though. Another reason why an output resistance of 0.00001 Ohm and 0.1 Ohm do not make that much difference is that the wiring and the connectors used are in series with the amplifier and as there are 2 wires this doubles !. Also (and this is the main contributor) one has to realise that all woofers have a crossover filter coil in series (unless one has active speakers ) and the resistance of the used inductors are also in the 0.1 Ohm numbers or even higher. Some tube amps (depending on the circuit) can even have output resistances above 1 Ohm which will most likely result in sonic differences with most speakers due to voltage division caused by wildly varying impedances due to crossover filters.
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Rabbit
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Post by Rabbit on Aug 31, 2013 16:48:13 GMT
So not the same as many old amplifiers did for the headphone socket? I thought they just took a feed from speaker out with resistors in line to give you a headphone connection in the 70's and 80's. They were so awful, I got into headphone amps as a result.
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juke
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Post by juke on Aug 31, 2013 18:16:40 GMT
Thanks Frans, just wanted to be sure nothing went 'pop'! I've opened my Hifi-Man protector box and I can adapt it to this improved use tomorrow, I hope Syd
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solderdude
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Post by solderdude on Aug 31, 2013 18:52:47 GMT
Hi Syd,
It seems they use a power resistor over the amplifier as a 'load' and have a 25 Ohm resistor as 'R1' and 'open' as R2. That extra load resistor is really only needed for some tube amplifiers (those with output transformers).
So in this box you can replace the 25 Ohm for a 15 Ohm resistor or simply keep it as it is.
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solderdude
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Post by solderdude on Aug 31, 2013 18:59:46 GMT
So not the same as many old amplifiers did for the headphone socket? I thought they just took a feed from speaker out with resistors in line to give you a headphone connection in the 70's and 80's. They were so awful, I got into headphone amps as a result. Yes they did it this way. That resistor, however, was often higher in value than 120 Ohm or also was a voltage divider, sometimes they even used 3 resistors. Not all headphones liked that arrangement though. The power-amp adapter as described basically does the same but with a lower output resistance.
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juke
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Post by juke on Aug 31, 2013 22:08:29 GMT
Hi Syd, It seems they use a power resistor over the amplifier as a 'load' and have a 25 Ohm resistor as 'R1' and 'open' as R2. That extra load resistor is really only needed for some tube amplifiers (those with output transformers). So in this box you can replace the 25 Ohm for a 15 Ohm resistor or simply keep it as it is. Hi Frans I was assuming I would desolder the 10 Ohm from the - tag and solder it into the outlet wire, removing the 25W resistors. The problem at present is that all the useable volume spreads over a very small arc of knob. That wouldn't help then? Syd
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solderdude
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Post by solderdude on Sept 1, 2013 6:17:47 GMT
If you want more travel on the volpot what may be best is to make a voltage divider. I assume it is playing MORE than loud enough with the adapter as it is now.
If you make R1 and R2 33 Ohm the whole volume will come down 5dB (max SPL will be 108dB)
When you want even less power make R1 = 47 Ohm and R2 = 22 Ohm (as if you are driving 500mW headphones) this will lower the volume by 10dB (that is half as loud). Max output power will be shrunk to 0.4W though. You will still get a very respectable 103dB SPL with that though.
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