Headphones from power amp questions
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Category: DIY AUDIO
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Topic: Headphones from power amp questions
Posted By: tg [RIP]
Subject: Headphones from power amp questions
Date Posted: 07 May 2012 at 4:45am
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I have previously seen discussion WRT driving headphones from power amp outputs. My interest has been sparked by recent acquisition of a rather cute, 2 stage, single valve SET amp with a power output of approximately 2 watts into 8 ohms and I thought to add a headphone out to this. Which naturally raises a few questions for me I found a general description at http://sound.westhost.com/project100.htm - http://sound.westhost.com/project100.htm however, the table of resistor values given does not cover my requirement. Question 1 - is there a formula readily usable that might yield appropriate values ? Question 2 - setting appropriate output impedance, particularly for K701, how to determine ? Question 3 - what effect does varying the output impedance have on heaphone performance in general terms ? |
Replies:
Posted By: Graham Slee
Date Posted: 07 May 2012 at 9:44am
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DIN standard (IEC1938): 120 Ohms series resistor! Except most HP manufacturers never heard of standards! They mainly developed their HP's to sound good with their own gear and that's a truth I've learned the hard way the last 13 - 35 years... ------------- That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps |
Posted By: mrarroyo
Date Posted: 07 May 2012 at 11:06am
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I would only use the speaker tabs of a power amp for very special and demanding cans. For now I can only think of: 1. AKG K1000 earspeakers using a pigtail terminated into a 4-Pin balance plug. 2. Stax using one of the many adapter boxes. 3. Something like the HiFiMAN HE6 planar headphone. ------------- Miguel |
Posted By: tg [RIP]
Date Posted: 07 May 2012 at 2:41pm
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Just to clarify, the intent is not to drive headphones from speaker terminals. I headed the topic that way, because in electrical wiring terms that is what is required, but that was not the point of my questions. The point is to install a headphone jack in the amp by tapping the transformer outputs and reducing them to a suitable level. The questions were aimed at establishing suitable values for that reduction. So far, lions 2, heathens nil. Plan B is SI&S |
Posted By: Graham Slee
Date Posted: 07 May 2012 at 3:32pm
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Tony, the rms output is going to be 4 volts (check sum: 4 squared divided by 8 Ohms = 2 watts). 4 volts is not uncommon for a HPA output before the usual (and advisable) series resistor. I would expect there not to be any DC here (check with meter). Now, if that were the only OPT tap it really should need to switch to a 8 Ohm dummy load (or the amp could "motor-boat"), but if there is a 16 Ohm tap a 16 or 15 Ohm (preferable) dummy load resistor would not have to be rated so high - perhaps a 10 watt wirewound would not get so hot. So, once suitably loaded via a switch to select between speaker and dummy load, then take the HP feed series resistor from there. I suspect a 15 Ohm 1 watt resistor would bring good results with Grado's and K701's. If this is the wrong answer then please break it to me gently.  An aside: a solid state amp should not need a dummy load resistor normally... ------------- That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps |
Posted By: tg [RIP]
Date Posted: 07 May 2012 at 5:08pm
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Thank you Graham, that is ample guidance for me to proceed with a degree of confidence that I will neither blow up the amp or the cans. I have let the "magic smoke" out of one device already this week, damn hard to get it back in too. Heatsinks can be wonderful things. BTW SI&S = Suck It & See - which can result in the above type of unhappiness. |
