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Minipre: DIY kit phono and line tube preamp

by Selected Audio Components Selection Components

An introduction to tubes

[Italian version]

Product: Minipre - DIY kit phono and line tube preamp
Manufacturer: Selected Audio Components - via Busoni 12, 20137 Milano - Italy
Phone: ++39 2 55 18 70 73 - Fax: ++39 2 55 18 90 95
Approx. price: 300 Euro/US $ (as a kit)

The kit I am going to write about is an old friend of mine. It has been living with me for nearly three years, but in the meanwhile has been modified to such an extent that I estimated necessary a new test of another unit supplied directly by the manufacturer to be sure to describe you the "original" sound. It is a small tube pre-amplifier with a reduced price, which does not happen so frequently with tube devices...

The look

The pre-amp look is nice, compact and solid.
It is quite small, 26cm wide, 21 cm deep and 14cm high. The cabinet is black, in extruded aluminium and iron sheet, 5cm high, powder painted, with a matt finish, very thick and tough. The four tubes, in their metal shield (there are different types in aluminium or other metal) are aligned at the left end of the top plate, while the power supply transformer is placed at the rear right corner.

The fascia is a 8mm thick aluminium panel, black, perfectly matching the cabinet colour. The holes for the four screws connecting the front panel to the cabinet sides and a large square depression, where the two control knobs are placed, are milled into the panel.
The labels are in italics, in Italian.

On the rear side there are the twelve RCA connectors, gilded and insulated from the cabinet. There also is the power switch (real good quality, never had any problem), the connector for the detachable power supply cord and the ground terminal for the ground wire coming from the turntable, originally very protected but uncomfortable, now comfortable but unprotected.

The "foots" are little aluminium spikes, supplied with surface protection discs. As these cones are rather low, when you have to place the pre-amp somewhere you must try to match the hole in each disc with the point of the spikes moving around delicately the pre-amp... as a pass-time is really exciting :-)
As a whole perhaps it is not a real hi-end look, but it shows a care in the design and an attention to details which is definitely very uncommon in a low-cost kit.

Technical analysis

The circuit is classic, without peculiarities. There is a single power supply transformer, with one secondary for filaments and one for the anode voltage. The filaments are supplied with DC, obtaining by rectification of the AC voltage coming from the transformer by four solid state diodes and two electrolytic capacitors.

Anode voltage instead has a RC pi filter. The capacitors value is for sure not too high, but this helps in reducing rectifying diodes commutation noise. Such capacitors values are possible, however, even because the polarisation current through the tubes is extremely low.

Any active element of phono stage has a further local RC filter, with rather high value resistors, which grants for a good de-coupling of these elements.
The phono stage is built around a passive RIAA network. Input stage uses an EF86 signal pentode, famous for its good sound, for each channel. The tube is polarised as a pentode, in order to obtain the maximum possible gain; the cathode is coupled, through a film capacitor, to the passive RIAA network, this too made up of film capacitors.

Then there is the second phono stage tube, which is a half E88CC/6922 (a double triode, professional version of ECC88, much appreciated for its sound) for each channel. The 6922 cathode is coupled through another film capacitor to the input selector. In my version the pull down resistor was missing, so that the selector side of the capacitor is floating when not selected, causing the classical bump (actually really reduced) when selecting the phono input.
Phono stage gain is rather high, hence no problem should be expected with high output level devices.

There are three line inputs, connected directly to the input selector. There is also a tape output, connected to the selector output through a resistor.

The selector output is connected to the volume potentiometer, originally a carbon, sealed Philips device, now substituted with a low-cost carbon one, because the original component is no longer available and the film components tested were excluded because of their bad sound.

The potentiometer output goes through a further coupling capacitor, to the line stage, which is carried out with another half E88CC/6922 for each channel in "cathode follower" configuration, that is with unit gain. The cathodic output is de-coupled with another capacitor.

All tubes have a grid-stopper resistor (a resistor mounted by the tube on the grid circuit, which makes with the grid capacitance a low pass filter extremely useful in reducing high frequency noise or instability) and polarisation is automatic everywhere, the best for a beginner kit.
The quality of the components in this new version is slightly different from the original one. Resistors are currently all metal 1%, while originally were Holco's. The sound difference should anyway be really reduced. Apart this I noticed the potentiometer, chosen as said for its musical sound.

The care in external design has not spoiled the internal component layout, which is really extremely accurate. For example input selector and volume potentiometer are placed in the rear, as near as possible to the input pins, so that a shaft extension has been necessary.

Summing up, coming from phono stage signal goes through 4 capacitors. For sure they are not few, but what matters in the end is their effect on sound, if any.
Another discussion topic can be the usage of a common anode stage. There are two different points of view. The first, the obvious one, is that the common anode stage has intrinsically a very high feedback. Hence with all feedback problems, someone says. But it is local and intrinsic feedback, someone else says, without any problem of the overall feedback. I do not enter the match, anyone can use its own ears to make up his mind.

The second point is less pleasant and subtler. The tube cathode is placed at a relatively high voltage, about 80 volts, and it is good practise to place the filament at the same voltage as the cathode; actually even though, as in our case, no cathode-filament voltage maximum limit is disregarded, bad auto-oscillations can take place, normally limiting their effect to appearing as subtle whistles from the loudspeakers, but potentially very dangerous for tweeters.
This - by the way - makes it really advisable to avoid non professional tubes (just to be clear ECC88 should not be used). A note: I do not remember any low-cost kit with cathode follower or SRPP configured tubes in which this problem was not present... in these cases there is no choice, the small budget hits hard. In the Minipre originally there was a circuit especially designed for taking the filament circuit to a voltage half way between the two cathodes' voltage, but they told me it was abandoned as it made things worse.

A very relative fault is the fact that the cabinet is nearly full: if you want to upgrade any component you must carefully check its size.
Another problem to be taken into account is that the tubes are outside and are rigidly connected both to the cabinet and to the printed board, which for sure does not help with microphony.
By the way, 6922's are rather microphonic, hence you must select them carefully. If you really want to exaggerate, and stay on the safe side with tubes absolutely safe from the microphony and noise point of view, you could select the top quality Golden Dragon, which are rather expensive and have a not too much seducing sound.

At the other extreme there are the Philips ECG, which are rather microphonic, but have a really marvellous sound; half way, rather noisy but with a rather pleasant sound, the Mullard. On the version supplied for the review I found Colomor. Currently I have been using for at least one year on my tweaked version United National and AEG, which are scarcely microphonic but have a not too much exciting sound (I am sparing the Mullard and ECG stock for future usage...).

Further care in selecting the tubes is required because of the design, specifically because of the absence of a balance control; while for the line stage no problem arises, (cathode follower as unit gain), a matched gain is required for the two EF86 and the two halves of the 6922 in phono stage.
In general I never experienced problems neither with EF86 nor with ECC88 and similar types, but once I found a set of Philips ECG 6922 probably faulty, so that the gain of one channel result notably (2-3dB) lower then the other; by the way even testing the same tubes on a tube tester there appeared only a gain difference of only 0.5dB, probably for working point used by Minipre is peculiar and far different from the standard one checked in the tester.
I must repeat that in ANY other E88CC/6922/ECC88/6DJ8 I put my hands on the gain of the two halves was quite similar, anyway if you are experiencing a similar problem and after all possible checks you cannot get rid of the problem, ask for the supplier for tube checking: there is a 3 month guarantee.

An enormous advantage is instead the fact that there is a single side printed circuit board, which makes things simple for beginners, but the components, apart the tubes, are mounted on the copper side, which remains later accessible opening the bottom of the cabinet. This make it possible any component substitution or upgrade.

From the point of view of measures, line stage has an exceptionally low distortion, clearly because of the heavy feedback and the rather low level signal. Even noise is rather low in this stage.
Very low distortion even in the phono stage, in this case because of the very low level signals; noise here is definitely higher even though not as high as to become really annoying in normal usage.

The sound

What follows is a synthesis from several listening tests, with different systems.

Sound is neutral, even though there is a roll-off at high frequencies that can be easily detected. As a matter of fact mid and mid-bass bands are prominent above the rest.
Treble is there when it is necessary, but it always remains a little "behind", so that you can listen to it without any problem for hours. I cannot detect any other peculiar coloration. Low frequencies are just a little slow. It is not always easy to separate the single instruments and localisation is not perfect, especially the depth of the image, even with very good recordings, is compressed, but the sonic mixture you get is really pleasant. Dynamics OK.
The image is veiled, but only by a thin veil difficult to detect. Actually probably it is not so much transparent, which could make everything far better. Speed is not excessive, but the impression of a very pleasant sound remains.
After a while, anyway, you stop looking for problems and just enjoy the music. It is a real Music machine, it seems really a pre-amp designed for listening to music and not to the recording noise or the throat of the singer.
Actually, even though it shows a few problems if compared with the reference unit, if you listen to it for a long time you appreciate its musicality and you don't miss so much the reference unit, as Minipre remains easier to listen to, less fatiguing than the reference.

With regards to the phono stage, the Minipre has a rather good impact, even though not perfectly involving. Just to make it clear: there is no "air guitar" effect, but after a while you find your foot tapping & following the rhythm, so I would say that it is really a very good result.

Minipre's phono input shows a typical "tubular" character: low frequencies are full, rounded and even neat, but the speed is not so good, so that impact is reduced and you are not perfectly carried away by music. At both upper and lower end anyway there is some "trouble".
Specifically, there is a little mess as usual at the top end. There is not so much depth and detail, even though voices are accurately carved. No bad at all.
By the way, the version under test has rubbery foots, which do not help in getting a precise and detailed image. Mounting the spikes normally supplied the sound gets far better, cleaning up and getting much more detailed.

You must anyway be careful to avoid unbalancing the sound increasing treble too much: the sound is perfect for pleasant and easy listening just as is: perfectly adequate for anyone who wants to enjoy the music and not reproduction details.

Comparing the line stage with the TNT MW-Pre 01, whose components cost is about double, you realize that the major difference is in sound refinement. The MW-Pre sound is more detailed, agile, dynamic, dancing. The Minipre sound is slightly fuller, bass is more rounded, but less refined at high frequencies, while very high frequencies are missing. Anyway the sound is quite natural and the differences are not huge at all. It is not as transparent as the reference system, but probably this is even too much transparent. A strange thing: sometimes stereophony is exaggerated with an hint of "tennis table" effect between the two 'speakers.

Even with the phono stage the differences are rather limited and probably the TNT MW-Pre can be better defined as more musical in a general sense. The Minipre dynamics anyway are good, but as we said before it is not able to carry you away, at least not as much as some solid state phono stage does.
No grain can be detected.

Troubles

Sometimes you can hear a subtle whistle, probably indicating a stability problems. I have never been able to understand if the cause of the oscillation is just acoustic feedback, which for sure helps in sustaining it, or it is triggered by other problems. Anyway selecting accurately the tubes you can get rid of the problem.

Was a balance or gain control for phono stage available, it might solve any unmatched tube gain problem without special tube selections. In the unit under test there was no need for it, anyway.
A technical discussion of the circuit could be very interesting for the beginner and it might be worth adding it to the supplied documentation (by the way, I have always seen it in Italian, better check if it is available in other languages).

Notes

The Kit assembly is clearly described into the supplied documentation, which gives a detailed description of any operation to be completed step by step. Hence anyone can assembly it, even though not so much experienced: you must have a minimal experience and a few tools (soldering iron, multimeter, screwdrivers, scissors...), must be able to read the value of a resistor from the colour code and to use the soldering iron: soldering on the silver plated copper solid core wire used for any ground connection is not soooo easy.

Once completed, it is clearly an handy pre-amp to be used with not too high level systems: it can beat some well known competitors, but you must not overdo it.
Can be a very good solution in an edgy system.

According to me, moreover, it offers a great advantage, as any other kit: you can modify it with no risk of voiding any warranty or of not being supported by the supplier's official assistance centres: it is really hard to imagine a kit supplier refusing to help a customer who has got into troubles, even though he has done a mess. And now, read on

Tweaking

Just to make it clear: all following modifications and tweakings are my own personal interpratation of the kit, not tweakings proposed by the supplier. As they can give a notably better sound, and the teask is still not so dangerous and difficult, according to me they are worth testing.

First of all, you should better try to dampen the upper panel as much as possible BEFORE mounting anything on it: later it will become impossible. Personally, I would try with lead foil or damping foils used by coachbuilders.
There should not be any heat dissipation problem, as all tubes are outside, but check carefully the thickness of the foils, as there is not so much room available inside. I have not done this, I realized the problem only after building it up and then it was too late.
I am pretty sure anyway that it is worth the expense; if you are able to make the connection between tubes sockets and panel a bit elastic, while granting a good electric contact, it would be even better; but sincerely it doesn't appear to me any easy at all.

Then you can substitute the coupling capacitors: you should avoid changing them all at once: you would better substitute a couple of them (I would begin from the one placed between the potentiometer and V4 grid, which still have an acceptable value), then listen to the pre-amp for at least one month to get used to the new sound: only after this time you should decide if to substitute another pair or stop.
The sound could get too thin, cold and transparent for your taste. For sure the sound gets far mode transparent and detailed, but if the system is not correctly balanced you risk to exaggerate.
A very important note: do not try to save money by reducing capacitors value: if you have not enough money for Jensen paper in oil capacitors, look for another type, but do not reduce the values, if you do not want to loose a good deal of bass, as they are already near to the limit.
It seems not possible, but you can hear the difference in terms of impact just moving a cutoff frequency from 3 to 6 Hz even with loudspeakers definitely lacking at the low end.

What happens with paper in oil capacitors? The sound is more detailed, precise, fast; the soundstage gets a little deeper and the presentation is more lively.
To end with, you can also transform the output stage into a common cathod stage. It is a possible modification, but it is not very simple as components are crowded up. I remember I had to struggle a little. As a matter of fact I

Be careful to avoid unexpected and undue contacts between components.

Looking back to the notes I wrote when I made the modifications, I read that I had to substitute the line stage tube, as it was too much microphonic, which is probably connected with the high increase in gain. Anyway I did not found any other problem. The resulting sound is more lively, notes arrive more naturally, instruments have a lot of air around, the depth is there.
Sound is brigther, but low frequencies are still at their place. You cannot say it is a very refined sound, but it is for sure better then the original one. A note: please take into account that the common cathod output should not be used with power amps with low input impedance (under 30Kohm, let's say).

Conclusions

With this kit anyone with a little experience (I would exclude the beginner only for for the high voltages: the assembly is not difficult at all) can enjoy building a tube pre-amplifier with phono input, with a look and a sound which for sure are not poor at all.

If you have not yet noted it, the price is less than 300$: I do not know any commercial preamp at this price, left alone a tube pre-amp, and there are really few kits.
Perhaps it is not able to take the place of a hi-end preamplifier, but for sure you cannot consider the price as an excuse not to buy it.
Be careful, anyway: once you have listened to it, you could be not able to do without tubes; yes, tubes are addictive, they are just a little worse than drugs...

Not only price does matter (and less than this...), but even the seller attitude and available services, and anything that can reduce the risk of a disappointing expense is for sure to be highly appreciated.

© Copyright 1999 Giorgio Pozzoli - http://www.tnt-audio.com

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