From: "Saved by Internet Explorer 11" Subject: A Dedicated Headphone Amplifier Date: Mon, 14 Mar 2016 15:38:14 -0700 MIME-Version: 1.0 Content-Type: multipart/related; type="text/html"; boundary="----=_NextPart_000_0000_01D17E07.85BD94C0" X-MimeOLE: Produced By Microsoft MimeOLE V6.1.7601.17609 This is a multi-part message in MIME format. ------=_NextPart_000_0000_01D17E07.85BD94C0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.angelfire.com/electronic/funwithtubes/Build_Headphone_Amp.html
A Dedicated Headphone Amplifier.
If you = would rather=20 go small and low power here is a solid=20 state headphone driver you could build an amplifier around.=20
=20 If you like to listen to tube amplifiers you can buy = one on=20 eBay and listen to it through speakers to your hearts content. If = you like=20 listening to headphones you can plug them into a solid state = amplifier and=20 listen to your hearts content. If you would like to listen to tube=20 amplifiers through headphones you are out of luck because the old = tube=20 amplifiers don't have headphone jacks.=20
STOP THE PRESSES!
It seems I was wrong. I received an email informing me = that=20 tube amplifiers with headphone jacks were made by Scott, Fisher, = and=20 others. They didn't find it necessary to fully load the output = when the=20 speakers were switched off. Some designs connected a 25 ohm = resistor to=20 the 16 ohm tap and others used values as high as 100 ohms. This = was=20 apparently sufficient to damp the inductive kick and prevent = oscillation. =20 If you are fortunate enough to own one of these amplifiers you = really=20 don't need to build this project. Of coarse you may want to just = for the=20 fun of it.Potential Problems with Headphone Jacks on Tube = Amplifiers?
That=20 speakers on/off switch could be a problem. Every solid state amp = has one=20 but I have never seen one, although I now know they do exist, on a = tube amp.=20 An S S amp acts more like a regulated power supply than a tube amp. = The=20 amp output supplies what ever amount of current is demanded by the=20 combination of load resistance and output voltage. The voltage = remains=20 uneffected by load resistance. There is no such thing as impedance=20 matching. That's why there are not separate terminals for 4, 8, and = 16 ohm=20 speakers on a solid state amplifier. If the load is an open circuit = there=20 is no current but the voltage remains the same; it does not rise to=20 component damaging values.=20A tube amp on the other hand is matched to the speaker impedance. = If you=20 have 4 ohm speakers you have to connect them to the 4 ohm tap on the = output=20 transformer not the 8 or 16 ohm tap. Moreover tube amps do not like = operating into an open circuit. The design engineers were smart = enough to=20 keep them from going into oscillation but there is a distinct = possibility of=20 damaging the output transformer. If you turned on your tube amp and = left=20 the speaker switch off you might turn the volume up all the way in = an=20 attempt to hear something. The output tubes would be driven between = saturation and cutoff and this would cause the inductance inherent = in the=20 output transformer winding to generate high voltage spikes, possibly = as much=20 as 2000 volts, which could cause the transformer windings to arc = over and=20 ruin the transformer. When a load is on the secondary this effect = is damped=20 and these dangerous voltages are not generated. This load does not = have to=20 be equal to the normal speaker impedance but can be somewhat larger=20 depending on the design. And so I no longer believe that you will = never=20 see a tube amp with a headphone jack.=20
Never say never.
Another way it could be done. The speakers off switch = could=20 totally shut down the power output stage and the headphones could = be=20 driven by a circuit similar to the output stage in this project. = I don't=20 think this was ever done in a commercially available amplifier.=20Requirements.
Modern headphones have an impedance of 100 = ohms. They=20 only require about 50 millivolts to drive them to a normal listening = level. =20 (As opposed to the hearing destroying level some people listen at in = their=20 cars.) That's 2.2 milliwatts; believe it or not. Even if somebody = wanted=20 to listen 20 dB higher than that it's still only 22 milliwatts. It = doesn't=20 take much power to drive headphones. =20The 100 ohm impedance and the small amount of power means that an = output=20 transformer is not necessary. (The headphone amp for the original 4 = ohm=20 Koss phones did need one.) Some engineers might come up with other = ways but=20 my approach was to use a cathode follower. The tube I selected was = a 6CG7. =20 When you look up this tube in the Sylvania tube manual it refers you = to the=20 data for a 6SN7. So it is a 9 pin mini version of that venerable = tube. =20
I set up the circuit on the breadboard and gave it a listening = and=20 distortion test. It sounded really good and measured less than 0.1% = distortion. All without any negative feedback applied except that = inherent=20 in the cathode follower itself. =20
Specifications.
Noise below normal listening level.=20Magnetic Phono Input,=20
58 dB =20
Auxiliary Input,=20
more than 80 dB.=20
Frequency response plus or minus 1 dB,=20
8 to 26000 cycles.=20
Distortion at 10 dB above normal listening level,=20
0.1%=20
If I were to put the whole article on one page it would take a = long time=20 to download over a dialup connection. I have broken it up into = smaller=20 parts. This way you can read what ever you want to and skip the = rest.=20
A heart in love with the beauty
of glowing tubes never grows old.=20
Home= =20
This page last updated July 31, 2004.=20