– with tubes ánd transistors –
Part 1 – Circuit design
If you don’t like to spend too much money on sound quality, you should buy a good headphone. The next thing you need is a good headphone amplifier. Perhaps one based on the design presented here. Including tubes, of course.
The interesting thing of this circuit is – besides the absence of a output transformer (which is apparently not needed) – the use of transistors at some points. Transistors and tubes are used in such a way that their strengths are combined
Schema
This schema was drawn for a design contest where a maximum of 30(?) components was required. That’s why this design is quite lean and mean. And that’s why you see complete rectifiers in stead of single diodes. After counting the components it was clear that the power trafo should not be on the parts list… but be sure, it is needed absolutely!
Nevertheless the schema can be reduced even more: only for very critical applications DC current sources are needed for the filament; in most cases AC power can be used without hearing any hum.
Safety
The mains transformer is not drawn in the schema. But you really need it! Do not connect the 220V AC power terminals directly to the mains because te amplifier and headphone will result in a kind of electric chair… Also when using a transformer a tube circuit like this is a little bit hazardous because of the high voltages (> 300V) at several points.
Input stage
Prominent in the input stage is the anode load of the ECC82 triode. It’s a current source build with T1 and T2 having an output impedance of about 2M. This has the following advantages:
- The anode is effectively shielded from noise and hum of the power supply.
- The gain of the tube will be near its maximimum amplification factor. That means equal to µ (= e.g. 17 for a ECC82). So the amplification is defined by only one tube parameter, even the most constant one, just enhancing the linearity.
Although the cathode voltage is kept constant by the constant anode current, two capacitors C1 and C2 are added. They eliminate the thermal noise of R2 and they decrease the output impedance of this input stage from 120K to only 30K. Connecting both triodes of one ECC82 in parallel also lowers – although not so much – the noise. And it decreases the output impedance which increases the bandwidth.
Output stage
The output stage is biassed to supply enough voltage and current to drive a 600 ohm headphone giving earshattering performance. The value of C5 makes a LF cut-off at 2Hz when a 32 ohm headphone is connected. Together with the 1Hz cut-off resulting from coupling capacitor C3 the phase shift in the audio frequencies is neglectably low.
The screen grid of the pentode has its own filter because the anode current of a pentode depends more on the screen voltage than on the anode voltage. Without this filter the anode voltage had to be filtered more consciously to make the output state free of power supply hum.
Filament current
Every tube has its own current source. Thanks to these current sources no huge transient current can flow through the cold filaments, increasing the lifetime of the tubes. The potmeters have to be set to a value giving 6.3V filament voltage.
Construction
(NB: the print lay-outs are not on this site. You can ask for them at ben@benefactus.nl)
The main print is double sided. The filament current flows at the component side coming from the power supply print via a band cable. The LM337s can be mounted without isolation. They can also be mounted on one piece of metal for cooling purposes.
If a potmeter is connected to the input stage as a volume control, R1 can be removed. Even better is to choose a value of 1M for R1 to ensure the input is grounded when the gliding point of the potmeter accidently looses contact. No cinch- or jackplug-connectors are provided on the print. The constructor is free to place them on a convenient place in the housing.
Most of the print surface is used by the FKP capacitors which are sublime for transients and pulses. Use parts of very good quality: metal film resistors and gold plated tube sockets.
This design can handle several other tubes than ECC82: for instance ECC81, ECC83 or ECC90. The amplification factor will vary with the µ of the tube. A ECC82 has a sufficient and useful gain. When using other tubes be careful that the anode voltage is between 50V and 200V. Sometimes R1 and/or R3 have/has to be changed.
Modifications / enhancements
You can change this design concept in many ways. For instance: the output voltage swing can be increased by stepping up the voltage on the input gate of the pentode. Or the cathode resistor of the pentode can be replaced by a transistorized current source, resulting in a slightly higher output current but an equivalent higher output impedance.
If you like to have overwhelming output power, the EL84 can be replaced by a more tough output tube. That can be very useful to monitor audio recordings where you can’t sit in a separate room. Most standard recording equipment is not able to deliver enough output power to let you hear the recorded audio apart from the live sound. This amplifier can do that without problems. But be cautious, you only have one pair of ears…
This amplifier can also be used as a line-driver.