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| A Single-Ended OTL Amplifier for Dynamic Headphones | |||||
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by Aren van Waarde
For many years, I have used a solid-state headphone amplifier in my shack. It is an op-amp type of circuit which is built with discrete components (BC559 and BC560 transistors, with a BD139/BD140 output pair). It runs in class A and has no output coupling capacitor. The measured specifications are quite good and it sounds decent after it has warmed up for about 20 minutes. However, I started wondering how a tube circuit might sound. In the stereo of my living room, I now use a tube preamp (Curcio Daniel) which sounds excellent, both on CD and MC phono. Rudy van Stratum has published a schematic for a tube headphone amplifier in the April and September 1995 issues of the Dutch magazine Audio & Techniek. It was just a circuit idea, without any guarantee that it would actually work. The schematic caught my attention for the following reasons:
I decided to give the circuit a try, and after prolonged listening tests (more than 3 months, with both CD and analog tape as signal sources) I can report that it works very well. THE AMPLIFIER
My own (slightly modified) schematic of the headphone amp is shown in Fig. 1. The first stage uses one-half of a E88CC (6922/6DJ8/ECC88) in a common cathode configuration. This is directly coupled to a cathode follower which employs one-half of a 6AS7G. I have added a volume potentiometer and a grid stopper resistor to the original schematic. The size of the output coupling capacitor was also increased (from 100 to 220 uF), simply because I had this value in stock and also because I intended to use 32 and 60 Ohm headphones. With 60 Ohm headphones, the calculated -3 dB cutoff point is now at 12 Hz, whereas with 32 Ohm phones, it is at 22 Hz. A prototype which I made on a piece of plywood worked immediately and I really liked it. With good recordings there is a life-like quality to the sound. Voices and instruments are pinpointed on the stage, with lots of musical detail and "air". Cathode followers have the reputation of "muffled" and "boring" sound, but Rudy's circuit renders dynamic contrasts very well and it grips your attention. Minor details in recordings become audible. One can hear, for example, the difference between different violoncellos, and the fact that different tracks of a CD are recorded in a slightly different recording venue.The solid-state amp sounds "hard", somewhat "glassy" and "mechanical" in comparison, with less detail and less precise imaging. This surprised me greatly since the tube amp has an output coupling capacitor which the solid-state amp lacks. Apparently, the absence of global feedback and the simplicity of the tube circuit works wonders for the sound. The single-ended topology of the tube circuit may also result in a spectrum of harmonics which is different from that of the solid-state amp which has a push-pull topology. Since I am quite happy with the sound, I have now built a final version of the circuit on a matt black aluminum chassis (size 4 x 8 x 1 inches). The amp is hard-wired, without any PCB. For my listening tests, I used Sennheiser HD 465 (60 Ohms) and Panasonic EAH-S30 headphones (32 Ohms). However, I suspect that the tube amp would sound even better when 600 Ohm headphones were used such as the Sennheiser HD 580 or the AKG K240. THE POWER SUPPLY
Modifications of the power supply have a marked effect on the sound quality of this simple amplifier. For the initial listening tests, the amp was fed from the high-voltage power supply on my workbench. Then I tried it with a regulated solid-state power supply (schematic with 2 x BF459 from a book by R. zur Linde). The regulated power supply was not an improvement (as I had expected) but it caused a sonic degradation! The "magical quality" was gone, the amp began to sound like its solid-state counterpart... Subsequently I wanted to try a power supply with a vacuum tube rectifier, but the EZ81 which I intended to use proved hard to obtain. Finally I settled for an inductor-smoothed power supply with solid-state rectification (Fig.2). This simple circuit sounds very well. The tube heaters are fed from a DC supply with a LT1084CP regulator (see Fig. 2). Since this IC dissipates about 10 W of power, it is bolted to the aluminum chassis. The rectifier diodes get hot too, and they are mounted at some distance of the chassis, with sufficient ventilation. The 1k variable resistor (P2) is used to adjust the output voltage (6.3 Volts, loaded). I have not included details for the primary circuit of the transformers in Fig. 2. Please use the appropriate fuses for your transformers and mains voltage. My power supply has a mains switch (which activates the heater power supply) and a standby switch (which brings +150V to the plates after about 30 seconds of heating). The power supply is hard-wired and built on a separate chassis (12 x 6 x 2 inches). MEASUREMENTS Since I am a hobbyist, I have limited possibilities for amplifier measurements. Here is the only data I was able to collect:
< 10 Hz.... > 100 kHz (0.775 V out in both 60 and 600W) (I therefore suspect that the output capacitors have a larger value than the specified 220 mF. NB My sinewave generator runs only from 10 Hz...100 kHz) Output voltage:
ca. 28 Vtt in 600 Ohms (onset of clipping) = 10 Veff Max. power output:
170 mW in 600 W Voltage gain: 8 x (i.e. 100 mV at the input produces 800 mV output at a 600 Ohm load, volume potentiometer at maximum) Squarewaves (1 kHz, 10 kHz, 20 kHz) look perfect, at low and very high frequencies (smaller than 100 Hz or greater than 50 kHz) one sees the influence of the output coupling capacitor. I think these specifications are good, but the best measuring instruments are the human ears. PARTS LIST (AMPLIFIER)
R1 - 1M ohms, 1 Watt carbon resistor R2 - 33 ohms, 0.5 Watt metal film resistor R3 - 47K ohms, 1 Watt carbon resistor R4 - 820 ohms, 1 Watt carbon resistor R5 - 4k7 ohms, 5 Watt wire-wound resistor R6 - 3k3 ohms, 10 Watt wire-wound resistor R7 - 10k ohms, 0.5 Watt carbon resistor
C1,C2 - 220 uF, 400 V electrolytic capacitor (Nichicon)
V1 - E88CC (Brimar)
1 - noval chassis-type tube socket (ceramic, gold-plated contacts) Please note:
The shield between the two halves of the E88CC is grounded. The heater power supply doesn't float, but is grounded to avoid the pickup of hum. With shorted inputs, or a low impedance signal source, the amplifier is completely free of hum and noise, even at full volume. In practice, the volume control is never increased more than half-way. PARTS LIST (POWER SUPPLY):
R8,R9 - 6.8 Ohm, 1 Watt carbon resistor R10,R11 - 180 Ohm, 0.25 Watt metal film resistor
C5,C6 - 22 nF, 1 kV MKT (DDR stock)
IC1 - LT1084CP (Linear Technology)
T1 - 220:2 x 115 V, 30 VA isolation transformer
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© Chu Moy, 2001