This is a little project aimed at providing my Quad II power amplifiers with a replacement pre-amplifier. Although the QC22 is a lovely little unit, I wouldn't mind putting together something quite basic that is little more than a buffer stage for my CD player.

Description of circuit

Essentially, this amplifier represents little more than a cathode-follower and a stabilised power supply. A "cathode-follower" is the term given to a valve when you take the audio signal off from the cathode instead of the anode. Thus unlike a conventional voltage amplifier, you do not use a load resistor on the anode, you merely choose a suitable operating point. The advantage of all this is that instead of using the valve as a voltage amplifier - and having a relatively high-impedance driver stage - you use it as a buffer, with no gain but a very, very low output impedance that's ideal for driving long cable runs and other valves. 

I will give the details of two variants of this pre-amplifier. The first will draw its power from a pair of Quad IIs and therefore will be little more than some valves in a box, but the second will have its own power supply - in case you don't have a pair of Quad IIs!

Note: Please do not butcher your existing Quad pre-amplifier for these parts. All are quite easy to get hold of - cheaply - whether you are in America or Europe. Just ask me or someone on the newsgroup rec.audio.tubes if you're having any difficulty. 

Building the basic audio stage

I generally prefer to use valves that have octal bases. There are arguments for and against, with some claiming that octal valves "sound better". I honestly can't say for myself, but octal bases - being larger - are a damn sight easier to wire up than their miniature counterparts. However, the choice of valves is entirely up to you, the constructor. My worked example is for a pair of triode-strapped EF91s (small, 7-pin pentode) since I recently bought some rather cheaply, but this circuit will work just as happily with 6SN7s, 6J5s, or 6SJ7s. American equivalents to the EF91 include the 6AM6, 6024, and 6064.

Going from a big data book, a triode-strapped EF91 may be run linearly between 150V-300V. In order to make the power supply as simple as possible, I propose to choose the 150V option, a suitable bias point (i.e. in the most linear part of the curve) being for 7.5mA at a bias of -1V. A suitable bias resistor would therefore be of 133 Ohms, size at least ¼W. 

The pin-out for the EF91 is: (1) G1, (2) K, (3) F, (4) F, (5) A, (6) EG3, (7) G2. K and G3 (pins 2 and 6) and A and G2 (pins 5 and 7) must be strapped together to turn this pentode into a triode. A 1K resistor should be ideal for this.

Since the Quad II provides 330V of HT at up to 40mA per monobloc, we might as well make the most of this. An 0A2 neon stabiliser tube will require some 10mA for optimum running conditions, and therefore we will need to drop the 180V and dissipate 3.15W of heat for 17.5mA of current using a 10K3 resistor. This is a fair bit of heat, but necessary. For optimum smoothing, I propose to add a sizeable filter capacitor per channel in between two 5K resistors. The resulting circuit, adding some choice coupling capacitors and other items, will look something like this:

This is deliberately a dual-mono design. For optimum matching to your Quad IIs, I would suggest using two volume controls. Something of size 100 KOhms would probably be more than enough, working from a CD source. The Quad connectors should be hooked up so that:

This basic idea may be further refined with a selector dial to choose between different sources, and even for a basic on/off switch to connect to the 2-pin plugs for the Quad IIs.

Variation: An internal power supply