Elliott Sound Products PO Box 233 Thornleigh NSW 2120 Australia

The ESP VP103 Hi-Fi Valve Preamplifier is a premium vacuum tube preamplifier, designed for discerning hi-fi enthusiasts who will appreciate the unique sound of valves.  (See photo of prototype.)

Price - AU$1,800 (+ 10% GST for Australian residents)

The design is no compromise, with the following features:

• Massive overload margin - input signals up to 10V rms will not clip the input amplifier


• A timer changes the front panel power indicator as the unit warms up
• Red - Should not use, not temperature stable
• Orange - May use, but not completely stable yet
• Green - Unit is ready for use


• The power supply uses massive capacitance (for a valve circuit), completely eliminating hum on the DC supply, and providing excellent rejection of short-term mains variations as well as providing an extremely low impedance voltage source
• Fully regulated 12.6 Volt DC heater supply to eliminate induced hum and noise and provide optimum heater operating voltage at all mains voltages
• Current limited heater supply, to reduce inrush current (typically a major cause of valve heater failures)


• Standard valves are used at the lowest practicable voltage and current, ensuring maximum valve life · Local feedback is used on each amplifying stage to reduce the effects of valve ageing, and reduce output impedance and distortion


• Very low distortion. Even at an output level of 5 Volts rms and a load impedance of 10k, distortion remains below 0.2%


• Frequency response from 10 Hz to 150 kHz (-1dB)


• Channel balance better than 0.5 dB


• Auxiliary mains output for VR102 Phono preamp or other low current device (this must not be used for power amplifiers). NOTE: Aux mains output is switched but not fused


• Rear panel level controls for CD and Tuner inputs, to allow their level to be matched to other components in the system


• All signal paths use traditional point-to point wiring, and not a printed circuit board


• All signal paths are shielded by a ground plane, made from un-etched fibreglass PCB to ensure minimal hum and noise pickup


• Valves are matched to ensure optimum channel balance is maintained


• High quality 1% metal film resistors are used exclusively (except for one 1 watt resistor in the power supply), to provide low noise and long term stability


• All electrolytic capacitors in the signal path are low leakage types, and are bypassed with polyester caps to ensure optimum high frequency performance

Specifications (all measurements relative to 2V rms output unless otherwise stated)
 

*   I have found it necessary to provide two gain settings (switch selectable), since a gain of 18dB is too high for many power amplifiers.
** Channel balance is expected to be better in the production version, but it is not possible to predict by what margin at this stage.

The unit follows the minimalist approach, with the only front panel controls being the power switch and indicator, a 6 position rotary input selector switch and a volume control.

Inputs are provided for:

• CD Player
• Tuner
• Phono (external preamp required)
• DVD Player
• Tape
• Auxiliary

• Tape (to Cassette, MiniDisk, DAT, etc)
• Main (to amplifier)

Rear panel level controls are provided for the CD and Tuner inputs, since these devices rarely have output level adjustment. This allows the gain for each device to be set, so there are no radical differences in sound level when changing signal sources.

In anticipation of the VR102 Phono Equaliser, there is a switched rear panel mains output. This is not to be used for the power amplifier (however tempted one may be), as it is low current only (<2 Amps max.).

All input and output RCA connectors are gold plated.

The case design for production is not complete, but will be all aluminium, powder coated in dark grey, with ventilation slots on the top and bottom. The front panel is black anodised aluminium, with all markings engraved.

Dimensions are fairly standard for hi-fi equipment, being:

• 44mm (1 3/4") High (1RU)
• 420mm (16 1/2") Wide
• 300mm (11 3/4") Deep

Photo of the interior of the prototype VP-103

The 3 dual triodes can be seen on the right, and are mounted on a piece of PCB for shielding only.  The power supply occupies the left hand side, and the heatsink is for the 12.6V DC heater regulator.

Input switching is all performed at the rear (hence the long extension shaft), close to the input connectors.  The two pots at the rear are for the tuner and CD inputs, to allow them to be adjusted to match the level of other sound sources.

As this is the prototype, the case is not a final design, but an off the shelf 1 Unit rack cabinet (with the ends cut off).  The final design is completed (but not yet available), and is far more robust and looks better, too.

There is something of a resurgence of valve (or tube) amplifiers of late, and it is worth while taking a look at the reasons. There are those who will simply buy whatever is in fashion, but they are a small minority, so what is about valves that has experienced hi-fi buffs waxing lyrical about technology that is (or was thought to be) well past its use-by date?

I don't have the answer (unless 42 is acceptable), but during the design of the VP103, some very interesting facts came to light (or to be more precise, I was reminded of a few things which had faded somewhat).

Everyone knows that valves have greater distortion than transistors or integrated circuits, but is this a bad thing? Apparently not, provided the levels are kept low and are predominantly second harmonic. This is a characteristic of valves - if properly designed - and it seems to be accepted as a part of that 'valve sound'.

Where it gets interesting is in overload. All transistor and IC amplifiers are limited by relatively low supply voltages (typically +/- 15 Volts), and at some point just below the power supply voltage they clip - not gently or progressively, but hard and fast. So fast in fact, that harmonics are immediately generated well beyond the limits of our hearing.

"Ah, but a preamplifier should never clip - there should be plenty of headroom for all practical input sources", I hear. I agree with this, and in my own testing have never seen a properly designed preamp clipping internally (unless you do something silly, like turn the power amp gain right down and then drive the preamp into distortion).

However, it stands to reason that 20dB of headroom might just help with transients, and perhaps this is one of the deciding factors?

Then of course there is 'slew-rate', which is the speed that the output can change. This is typically measured in volts per microsecond (V/us), and most modern IC devices have a slew rate which is greater than 10 V/us. This is far greater than is needed for audio, where a measly 0.5 V/us is actually sufficient even for a low power amp - except it sounds awful.

Valves have no slew-rate limit as such, since there is no compensation capacitor used (although the internal grid to plate capacitance does have a similar effect, but is much less savage).  Instead, the signal simply rolls off smoothly once the upper limit has been reached. At no frequency or amplitude will a valve convert a sine wave into a triangular wave, as will many opamps and power amplifiers.

When a preamp is built using IC or discrete opamps, there are actually tens to several hundred active devices in the signal path. Each of these contributes it own little bit (gain, noise, distortion, etc), and the overall signal is tidied up using negative feedback. This works so well that with the latest opamps, distortion cannot be measured with a simple noise and distortion measuring set. Indeed, the signal source is likely to be an order of magnitude (or several) worse than the device under test.

Perhaps it is this clinical treatment of our sounds that is somehow to blame - can an amplifier be too good, too clean? I don't know, but there is a marked difference between a 'solid state' and a good valve preamp - with the latter having more 'air', and a sense of openness that IC and transistor units don't.  Even here, it is difficult to be objective, since I used an opamp preamp for quite a few years, and it is quite possible that the difference is imagined.

Most valve circuits use little or no negative feedback in the preamp stages. The VP103 is an exception, but even here, the level of feedback is very low. Valves have a relatively low gain (especially compared to opamps, which have a low frequency gain of over 100,000), so massive feedback is not an option, since there is not enough gain to spare.

A valve preamp is pure Class A - it has to be, since it is too difficult (and a completely useless exercise into the bargain) to build a Class B valve preamp. Virtually all IC opamps have a push-pull output stage which spends some of its time in Class B. Perhaps this generates some artefact which I have been completely unable to measure (along with many others), but which is audible. The various laws of acoustics would indicate that any such artefacts would be inaudible (due to masking, among other things), but there seem to be many things in audio which don't seem to obey the law! (At least according to some reviewers and the like.)

This is one area where the valve is measurably vastly superior to opamps. The gain of an ordinary opamp from DC to (about) 100 Hz is typically 100,000 - but after the compensation circuit is added, this rolls off at 6dB/octave after the 100 Hz point is reached. This means that at 200 Hz, the gain is down to 50,000 and by the time you get to 25 kHz, the gain will be down to a little under 400 (or less). Premium opamps are better, but the same thing still happens - its just not as severe.

What this means is that the input signal gets less and less feedback compensation as the frequency increases, so the distortion and output impedance will both increase in proportion.

With a valve preamp circuit, the gain is (or should be) pretty constant from the lowest operating frequency (determined by coupling capacitors, which cannot be avoided), up to 50 kHz or so, and will roll off very gently after this. Even if feedback is used, it will be constant over the audio range and beyond, which may account for some of the perceived difference.

I don't know! What I do know is that my valve preamp really does sound better than (or maybe just different from) a transistor or IC preamp. As little as 10 years ago I would have probably taken the opposite view, but upon reading reviews and then deciding to build one just to hear it has changed my thinking.