MW Pre 01 - Line Pre-amplifier with MM Phono input

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MW Pre 01 - Line Pre-amplifier with MM Phono Input

Uncommon Implementation of a Classical Design

A sophisticated grandson of the SIMPRE

Well, you may be thinking, I'm just another of those all-sound-no-tech pseudo-gurus currently so in fashion! Hopefully you'll see I'm not. I really hope that you can achieve a very precise RIAA curve of very low distortion, but I want you to know that this pre-amp has been designed in order to be built by beginners too, an not only by people crazy for DIY and with a NASA level lab.
This means that if you have a good background and the necessary equipment you can go on modifying the unit to your own needs or requirements. Those who do not have a good lab can just build it up according to design specifications and enjoy listening to it....
Actually, the whole RIAA filter can require a very precise set-up, but very precise instruments are necessary to carry this out. If you have these instruments available, you could well do it; if you haven't, youl can never be sure that the frequency response of your preamp is perfectly correct... but probably you'll never be able to hear it anyhow. that. Hence, don't worry...

Please note: I am not saying that you can not achieve better results with further fine-tuning. I am just saying that a reasonable level of fine-tuning has been performed; especially the phono component's. These values are somewhat different from the initial design, as they have been modified according to frequency response test results. The RIAA accuracy on my prototype is currently better that +- 0.2dB from 20Hz up to 20kHz.
With regards to distortion, it depends also on the tube used. But sound too depends heavily on the tube, and if I must chose to between distortion data or sound perception.... I have no choice.
My prototype measured up to 0.35% harmonic distortion with one set of tubes in the line stage, which is not that low. As it has not been proved that harmonic distortion below 1% is audible (even though I suspect it is...), this is not a major concern. With others sets of tubes it seemed to be lower: hence, again, don't worry...

Design problems

The major problems to cope with in such a design were the following ones.

SE tube stages are definitely unprotected against power supply noise, so that a very good power supply filter must be implemented, in order to reduce the PSU ripple to absolute minimal levels;
At the same time, the power supply output impedance must be as low as possible to achieve good dynamic behaviour;
Both stages (phono mainly, but line too) can be severely subject to microphony problems
Both stages (phono mainly, but line too) can be easily affected by 50-60Hz noise due to ground loops inside or outside the unit
Excess line stage gain must be avoided in order not to increase noise;
Phono input sensitivity must be as high as possible, while Line input should a low sensitivity: the line stage gain required for a correct phono sensitivity is far (about 20dB) higher then the gain required by the input stage
Phono stage could be subject to very high frequency interference (FM radio or similar)

These problems were not all apparent when I started the design: they developed and exposed themselves as far as other problems were solved. The solutions adopted for the previous problems are listed below.
It is clear that one of the major problem in such a pre-amplifier, due to the high impedance and microphony inherent in tube technology, is noise, in all of its forms, while dynamics, due to high power supply voltages, is not an issue.
A major acknowledgment is due to our good friend Thorsten, who had just analysed and solved the same problems for his own pre-amplifier, Toccata, and has been very helpful.
Anyway, I did not want to simply copy all of his solutions, just to design and build my own pre-amplifier, and not simply copying from someone else's design. Actually, a few of the solutions adopted by Thorsten probably take his own pre one step further (surely in quality and probably, but I'm not so sure, in cost) mine. A few details (especially component selection) are basically and philosophically different, so that the two preamps are probably difficult to compare.

The electronic circuit design

Mains connection and power supply switch.
The mains connection wire is currently a TNT Merlino cable, that is a three leads + shielded power wire. On one end is a standard (Italian) three-pin plug, while on the other there is a standard IEC power supply socket. I just have not yet had time to build a Twisted Snake... but it might well be a further improvement. The corresponding panel plug is obviously mounted on the rear of the pre-amplifier.
The three internal leads are connected normally at both ends to mains and earth pins, while the shield is connected to the earth pin at the plug end only, while is completely disconnected at the other end. In this way interference should be earthed at the mains connection socket, and the wire should be completely free from any interference.
The IEC socket really is part of a block containing also an EMI mains filter, a slow blowing mains fuse and the mains switch. The usage of a block containing all these devices is very useful as it helps reduce required volume and, due to the (supposed) lower number of parts interconnections should also hopefully provide a low mains lead resistance as seen from power supply unit.
The internal three pins are directly connected to the transformer mains end and chassis ground.

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