|Elliott Sound Products||VP103 Hi-Fi Valve Preamplifier|
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.)
VP103 Hi-Fi Valve Preamplifier
Price - Sorry, this unit is not available. This article is preserved for historical reasons only.
The design is no compromise, with the following features:
The VP103 is not a production item, and while it can be built to order this is not usually practical. The unit will not go into regular production, and this has been determined by the responses I received to various market probes (this being one of them). A photograph of the interior of the prototype is shown at the end of this section. The final version would typically use a high quality custom made case with CNC engraved front and rear panels.
This preamp sounds superb - open, with a stable stereo image, and detailed reproduction of all frequencies. The specifications are as follows:
Specifications (all measurements relative to 2V RMS output unless otherwise stated)
|Distortion||< 0.2% (10k load)|
|Signal to Noise Ratio||> 70dB|
|Maximum input level||10V RMS|
|Maximum gain||18dB / 10dB ¹|
|Tape Output Gain||6dB|
|Crosstalk (at 1kHz)||- 65 dB|
|Crosstalk (at 10kHz)||-50 dB|
|Frequency Response||10 Hz - 150 kHz (-1dB)|
|Channel Balance||Within 0.5dB ²|
|Operating voltage||115 / 230 / 240 Volts AC|
1 I have found it necessary to provide two gain settings (switch selectable), since a gain of 18dB is too high for many power amplifiers.
2 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:
And the outputs are ...
Note that the phono input is a simple line level input, since the VP103 has no inbuilt RIAA equalisation amplifier. The VR102 Phono Equaliser was intended to become available shortly after the VP103 is in production, or the user may choose any other RIAA equaliser which has an output voltage of around 300mV to 1V RMS. Project 06 is ESP's premium phono input stage.
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:
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. 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 low order. 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? I think it's probably irrelevant if the gain structure is right.
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/µs), and most modern IC devices have a slew rate which is greater than 10 V/µs. 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 often sounds awful (depending on the opamp of course).
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. However, it must be remembered that all amplifying devices are non-linear, as they can conduct in one direction only. Any capacitance (including stray) can be charged or discharged by the active device, but not both.
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 claimed to have 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 use one again now), and it is quite
possible probable 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 (supposedly) 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 laws of physics (at least according to some reviewers).
In reality, many opamps have distortion (from any source within the IC) that's virtually immeasurable with average/ typical test equipment. It's difficult with the most sophisticated equipment available, so distortion should never be an issue.
Open-Loop Frequency Response
This is one area where the valve is measurably superior to opamps. The gain of an ordinary opamp from DC to (about) 100 Hz is typically 100,000 or more - 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. 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 (or imagined) differences. I consider this to be clutching at straws, but ... ?
I don't know! What I do know is that my valve preamp appeared to 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 changed my thinking to a degree (at least until I cam to my senses ).
However, my current system (now in use for over 10 years) is based on the Project 88 hi-fi preamp, and includes the Project 06 phono preamp and Project 09 electronic crossover (configured for stereo 3-way). I have no intention of reverting to the valve system. I suspect that the 'differences' heard were completely imaginary, because I always knew what I was listening to, and that's a sure-fire way to get a result that doesn't stand up to scrutiny!
Not that the technical results are lacking in any way - the preamp measured very well, and could be improved even further. However, there really is no point, partly because the valves available today are not as good as those made during the final days of valve equipment. In addition, many opamps are so good that it's almost impossible to measure their distortion, noise is much lower than you can get from any valve, and there's more than enough headroom to ensure that the signal never clips.
Would I ever go back to using a valve preamp? In a word, "no".