From: "Saved by Internet Explorer 11" Subject: DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project Date: Thu, 23 Jan 2014 14:08:18 -0800 MIME-Version: 1.0 Content-Type: multipart/related; type="text/html"; boundary="----=_NextPart_000_003D_01CF1844.90CEABE0" X-MimeOLE: Produced By Microsoft MimeOLE V6.1.7601.17609 This is a multi-part message in MIME format. ------=_NextPart_000_003D_01CF1844.90CEABE0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: http://diyaudioprojects.com/Tubes/6DJ8-Tube-Headphone-Amp/
Bruce Heran |
=20 | =20 |
This is a project that has roots to back about 2 years ago. I wanted = a=20 quality headphone amplifier to be an addition to my Hi-Fi stereo system. = It was=20 to be something to use when I didn't want to disturb everyone else in = the house.=20 I tried several different tube types in the amplifier design process. = I used=20 6J6, 12AU7, 6SN7, 6CG7/6FQ7 and 12AT7. Generally all worked, but none = seemed to=20 have either the needed output level or didn't match well (likely to the = choice=20 of transformers) and sounded on the dull side. The 6DJ8 (ECC88) dual triodes worked = like a=20 charm. They had the combination of sufficient current flow, modest = voltages on=20 the anodes (important for the matching transformers) and enough gain to = make the=20 amplifier sing. The next best tube type was the 6CG7/6FQ7. Almost as = good, but=20 they are getting a bit scarce and the new production JJ E88CC are now = quite common=20 and reasonably priced. I am sure other triodes could be used and there = is no=20 reason why a high impedance ( on the primary side) small push-pull = output=20 transformer would not work. I doubt that you can find one much cheaper = than the=20 matching transformers I used. Certainly not with similar response and=20 distortion characteristics.
This low power tube headphone amplifier is designed along the lines = of the Oddwa= tt power=20 amplifiers, except that this headamp does not require a driver = stage. All=20 the circuit gain is in the single output stage. Once again I used a constan= t=20 current source (CCS) IC in the cathodes but since only low power = levels were=20 needed there is no provision for balancing the load between the two = triode=20 sections. None was needed either as the output is quite symmetrical to = levels=20 way above anything you could use with headphones. I wanted simplicity = in the=20 circuit. It has neither coupling capacitors nor any feedback loop. In = fact=20 there are a total of only 7 components including the tube in each = channel. The=20 power supply, alas, has many more components. This is common in many of = my=20 designs. I find that a very clean power supply is essential to achieve = high=20 quality audio performance.
Approximately 100 millivolts (mV) of signal input will result in = about 50=20 milliwatts (mW) of output power. This is much more output than is = needed for=20 really excessive sound levels with most headphones. The audio output=20 transformers used are Edcor=20 XSM10K/150. These transformers are designed for line level matching = and are=20 rated at 2.5W with a center tapped primary impedance of 10k and a center = tapped=20 secondary of 150 ohms. These impedance values lend themselves for use = with many=20 Hi-Fi headphones. While there is some loss of efficiency in using = headphones=20 below 32 ohms on the 75 ohm tap there was minimal effect on the quality = of the=20 sound. For use with higher impedance headphones you can use the Edcor XSM10K/600. The primary impedance of the = Edcor=20 XSM10K/600 is 10k with a 600 ohm center tapped secondary that is ideal = for use=20 with 300 ohm and 600 ohm headphones.
The power supply provides direct current (DC) for the tube heaters = and a well=20 filtered B+. I believe that using alternating current (AC) on the tube = heaters=20 would also be just fine, but since I have the PCBs and components to get = the DC,=20 that is what I used.
Aside from the Edcor transformers, most of the required parts for the =
tube=20
headphone amplifier project can be sourced from the likes of Parts Express, MCM Electronics,
and Tube =
Depot among=20
others. To help you along with the construction of the tube headamp =
here are=20
some suggestions for design and construction tips for =
vacuum tube=20
amplifiers and the tube amplifier wiring color code I use.
Warning: This tube headphone amplifier project uses high = voltage in=20 the 250 to 270 Volt (V) range. Contact with voltages of this magnitude = can=20 cause serious injury or possibly be fatal. If you do not know how to = build=20 projects that have these potentials or you are not comfortable in with = projects=20 that use these voltage levels, I strongly recommend you do not built = this=20 headphone amplifier. As always, please follow your governing electrical = code=20 for all wiring and connections.
Construction of the headphone amplifier is straight forward and=20 uncomplicated. The layout of the circuit is not critical as the = amplifier gain=20 is relatively low. The PCB and chassis you see in the photographs are = left over=20 parts from Forewatt=20 12AU7 Tube line stage preamplifier prototypes. Being somewhat = frugal, I=20 tend to use parts that are available. The 1 uF and 0.1 uF capacitors in = the=20 circuit should be "poly" types. The 0.1 uF capacitors across the tube = heaters=20 can be low voltage ceramic types. For best performance the 1 uF = capacitor shown=20 on the amplifier section near the audio output transformer should be = physically=20 located close to the actual B+ connection on the audio output = transformer. You=20 can use the standard size of LM317=20 voltage regulator in place of the LM317L which comes in a TO-92 transistor=20 case if you wish. The power actual dissipation is rather small at = about 75=20 mW. The 7806 voltage regulator used for the heater circuit should have = a small=20 heat sink as it will be dissipating a little over one Watt if you use = the Edcor=20 transformer shown in the schematic. I recommend against using standard = type=20 rectifiers in the HT (B+) power supply. The ultra-fast UF400X series = rectifiers=20 work far better and generate considerably less noise than the 1N400X = types do=20 and are nearly as cheap. The bridge rectifier in the heater circuit can = however=20 be a generic one. As always, be careful not to create ground loops. = For more=20 informations about grounding, see my article about groun= ding=20 and shielding for your DIY audio projects.
The amplifier will deliver over 100 mW of output into pretty much = anything=20 from about 32 ohms to 300 ohms. With the Edcor XSM10K/150 audio output=20 transformers the optimum match is at 75 and 150 ohms. Using headphones = with an=20 impedance below 32 ohms will result in decreased performance as the load = on the=20 tubes will be below the best range. Above 300 ohms it will also be less = than=20 the best match, but many phones in that range seem to be more sensitive = than=20 those with lower impedance and it may not matter. The frequency = response at the=20 50 mW level is from 20 Hz to 28 kHz within 0.5 dB and the wide band=20 signal-to-noise is at -84 dB. I didn't run a full distortion series on = the=20 amplifier as the output levels are rather low (voltage wise) and the = ambient EMI=20 in my shop would tend to confuse the results. On a spectrum analyzer I = was able=20 to get as much as 2 volts output before there were obvious harmonics = appearing=20 on the display. At more typical listening levels (about 0.1 to 0.2 = volts into=20 100 ohms) there was no sign of any harmonics above the noise floor. I = would=20 conclude from this that the actual distortion levels at such levels are = well=20 below 1%. Power consumption of the headphone amplifier is approximately = 20=20 Watts (W).
The testing equipment setup used to measure the amplifier performance = consisted of:
So to the important question - How does it sound? The sound in = short, is=20 excellent. With my heaphones - Koss Pro4AAT (250 ohms) and Sennheiser = HD-280=20 PRO (64 ohms) there was no audible noise or hum and the response was = rich and=20 clean. I won't give up my electrostatic speakers and other gear to swap = for=20 headphones and the headphone amp but it sure is nice for those occasions = when=20 private listening is needed. The quality of the sound will depend a lot = on your=20 choice of headphones. The amplifier can not make cruddy headphones = sound great=20 but will sound great on quality phones. I found the sound particularity = nice on=20 my Sennheiser HD-280 PRO headphones. They have a rather neutral overall = sound=20 and a lot of detail. I have used them on other sources, most notably = with my=20 OPPO BDP83SE Blu-ray player. For those of you unfamiliar with the OPPO = player,=20 it earned a class A rating in Stereophile for audio performance. When I = feed=20 the output of the OPPO player into the headphone amp I can not really = tell that=20 the sound has changed in any obvious way. My conclusion is that the amp = is=20 pretty linear and clean. There is no hum and noise with the Sennheiser=20 headphones which are rather sensitive. Solid bass, clean mids and = highs, nice=20 detail and obviously wide sound stage. I actually considered adding a = blend=20 control as I prefer some channel mixing similar to what you would have = with=20 speaker use. There are plenty of tube options as the tube headphone = amplifier=20 should work well with most the 6DJ8 / ECC88 and 6922 / E88CC = tubes.
I hope you enjoy this headphone amp project as much as I have. =
Comments and=20
questions about this project are welcome in the 6DJ8=20
(ECC88) Tube Headphone Amp Project thread.
Good listening, =
Bruce =20
Bruce Heran is the VP for Design and Support of Oddwatt Audio. He has been involved in the field = of=20 electronics in one form or another for nearly 50 years. Bruce is a = strong=20 supporter of DIY audio projects and generally specializes in vacuum tube = designs=20 for Hi-Fi. One of his main goals is to educate and encourage new comers = in the=20 building of affordable high performance audio equipment.
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