MiniMax tube rolling

Since I own this little monster, this post is pretty much for my own record keeping purposes.

Source: extract from AudioAsylum Amp/preamp section Review by Hin H. on December 31, 2004

Minimax Preamp Performance Review

Results of Tube Rolling

The Minimax preamp was designed by audiophile Alex Yeung in Hong Kong and manufactured in China. Since its introduction in 2002, the preamp has already received a number of rave reviews in the U.S. This write up is concerned with the results of tube rolling that I have done recently.

The design philosophy of this line stage preamp follows a purist’s approach that calls for a minimization of the length of signal paths and the use of point-to-point wiring to enhance signal integrity. Secondary functions that may affect the signal quality, such as remote control, gain attenuation, etc., are eliminated. The design layout achieved a small size that measures only 12.5” wide, 4” deep, and 2.5” tall and the unit weigh approximately 9 lbs. The face plate is made of solid aluminum with variable thickness so as to make it less susceptible to resonant excitation by acoustics and vibrations in the surrounding environment. The chassis and internal hardware are very well built. The dark gray glossy color scheme together with the silver face plate creates a simple yet elegant appearance.

Power supply of the unit is done by means of choke filtered voltage rectification scheme using a 6X4 type valve (tube). The same kind of approach can be found on high end preamps. One 12AU7/ECC82 triode tube is used for the buffer stage and another 12AU7 for the amplification stage. Since only three tubes are used on the preamp, it is particularly cost effective for tube rolling in comparison with other preamps such as BAT and Audio Research in my possession.

Audio Setup:

The components of the audio system are listed below for reference. The listening room features a 18 ft ceiling and 760 square ft of carpeted floor space. The rear wall is not parallel to the front wall, and because of this geometry, stationary waves are more difficult to build up in this room than in a rectangular shaped room. Considering the Martin Logan speakers that are operated in a dipole mode, they emit signals in both forward and backward directions. Some reflection is needed from the front wall to make the music alive. Accordingly, the front wall is lightly treated to control the amount of wall reflection. No echo has been found in a sound sweep test and reverberation in the room is moderate.

The source and amplification equipment are placed in a room adjacent to the listening room separated by a wall. This arrangement would provide isolation of the equipment from the sound field and vibrations transmission from the speakers.

A few tweaks were used to enhance the performance of the audio system. These include the use of: Bybee quantum purifiers and Auri Caps installed at the output terminals of the power conditioner, Bybee purifiers installed at the drivers of the speakers, the use of risers to raise the speaker cables and the power cables for the speakers from the floor, and to allow the spikes at the bottom of the speakers to penetrate the carpet cut-through and land onto the concrete floor (note: don’t do this tweak at home unless you have the consent of your spouse). Furthermore, the Minimax’s ground wire was lifted.

The system has been tweaked to a point that the back ground hiss and noises emitting from the speakers have been virtually eliminated.

The Music:

To test the fidelity of an audio system, I like to use the distinct sonic characteristics of a violin, a piano, a soprano, an ensemble of multiple instruments, and a symphony orchestra. Five pieces of music are selected for the review, they are described as follows:

1. Paganini, Caprice No. 24 for violin, by J. Ehnes, Telac 80398.

Paganini’s music has been regarded as the most challenging for the violinists. This caprice demands both lyrics and skill presentation. In the musical passage, the theme changes pace with great transient variations. A century later, Rachmaniov’s famous rhapsody based on this theme had captured many music lovers’ imagination.

This playback allows the listener to examine the timbre of the violin sound and the musicality of the lyrics. In my opinion, the violin presentation should be natural and have an inherent inner strength and texture such that it can reveal the finesse of the instrument as well as the feeling or passion of the performer. A hint of sweetness would be nice, but excessive coloration may lead to an unnatural euphoric sound. As the audition turned out, this song proves to be a difficult piece for several tubes to achieve a full score.

2. Chopin, Prelude No. 20 for Piano, Reference Gold/Inter Sound RDW 3622.

The stunning effect of this master piece was demonstrated in the first few bars of the prelude. With 10 key strokes being played simultaneously, the richness and complexity of the sound are impressive.

This playback allows the listener to focus on the harmonics from the resonant chamber of the piano and examine the superb inner details, transparency, and richness. The crisp wave front due to each key stroke and its subsequence decay would provide a good example to check the performance of the equipment being used in the playback.

3. Verdi, Rigoletto Act 1, By Sumi Jo, Erato 450 97239-2.

This is perhaps one of the highest frequency songs a human can sing and the skill to sing this song is beyond imagination. Yet the presentation was so beautiful and graceful that the listener may have a temptation to repeat it for enjoyment.

For the listener, this is a nice song for testing the reproduction of frequency extreme of the equipment. For tube gears, excessive coloration would lead to an un-natural sound. For SS gears, the lack of second harmonic may render the vocal music lean and analytical. The tonal accurately, transparency, lean vs. full body voice, inner texture of the soprano voice, articulation of singing skill, and the overall musicality and liveliness, all come into play in this playback.

4. Inca Son, Vol. 1, El Condor Pasa, VMMT21, DID017042, 1993.

This popular South American song was performed by street musicians from Peru. It is an exceptional performance with four pieces of instruments, namely, a panpipe, a flute, a guitar, and an Indian drum. Using the Condor as a metaphor, the musicians express their deep feeling toward freedom and their home land. Especially in the second passage of the song, the flute brings out the emotion of the music with plenty of air. The drum beats were punchy and with a rhythm that made the music full of energy.

This song allows the equipment to demonstrate the three dimensional sound stage and pin point images separation. For this song, I would give up a little bit for the tonal accuracy in favor of the emotion and liveliness of the music.

5. St.Saens, Symphony No.3, “Organ”, Track 4, Boston symphony, RCA Victor,092026-61500-2.

This song offers a ground shaking effect due to the low frequency sound generated by the organ. This historical symphonic performance under the baton of Munce was perhaps the best among all recordings. However, the re-mastering process for the CD was less than perfect.

With over 60 pieces of instruments and a grand organ, the grandeur of a symphony orchestra is demonstrated in this song. The resolution of the audio system in general, and the performance of the preamp in particular, would be the focal point in the audition. The musicality of the playback would be demonstrated through the coherence of the whole musical passage, the foreground vs. background music, the micro vs. macro dynamics, and the very rich and powerful bass reproduction of the organ.

The Pre-Screening Process:

Of the vintage tubes that are in my possession, six are rectifier tubes (Genalex Gold Lion U707, Marconi EZ90/U78, GEC CV4005-1982 version, Tungsol 6X4W, Tungsol 6X4WA, and Haltron CV 493), one Westinghouse branded E80CC tube, and ten ECC82/12AU7 tubes (Mullard 10 M, Tungsram UK, Tungsol dark glass, Mullard 12AU7WA/CV 4003 ribbed plate, Mullard 8136 CV 4003 box plate, Siemens silver plate, TEN- from Japan, ITT Lorenz-CV491, TFK ribbed plate, Raytheon JAN 5814A red label, and Sylvania USN 5963 gold label).

Due to the large number of permutations for combining these tubes for the rectifier (Left), Buffer (Center), and Amplification (Right) positions, a pre-screening process was done in order to sort out the combinations that provide better performance before serious listening was carried out. This pre-screening process was done through casual listening.

As a result of the prescreening process, the tube combinations chosen for review are the ones that have better performance, despite the rankings between them are to be revealed later in a direct comparison. Technical measures such as acoustic imaging and sound stage are satisfactory for the tube combinations being used in the final comparison, and therefore these features will not be mentioned regularly in the following write up.

Some of the tubes were new, i.e. the Gold Lion U707, the Haltron CV493, the E80CC, and the TEN, their burn-in process was time consuming. I assumed the U707 had gone through some burn-in time during the factory selection process; it was burned only approximately 30 hrs in my system before the evaluation started. On the other hand, the other new tubes have gone through up to 100 hrs of burn-in time.

Of the six rectifier tubes, the performance of the Gold Lion U707, Marconi U78, and Tungsol 6X4W clearly stood out in comparison with the rest. To limit the scope of the write up, my review is focused on these three rectifying tubes.

Evaluation of three Rectifying Tubes:

Three pairs of ECC82 (namely, Mullard CV4003 box plate, Tungsram UK 12AU7, and Raytheon 5814A) were chosen to match with each of the three rectifying tubes (i.e. Gold Lion U707, Marconi U78, and Tungsol 6X4W). Hence there was a 3X3 matrix for evaluation.

The winning combinations were the Marconi U78 and the Tungsram 12AU7 pair, and the Marconi U78 and the Raytheon 5814A pair. The second best of the rectifier tubes were the Tungsol 6X4W and the Gold Lion U707, both of them tied for the second place; and their best matches remain the Tungsram and Raytheon. I believe the Gold Lion U707 can be improved in due course after a couple hundred hours of further burn-in. Regardless the rectifier tube was used, the combinations with the box plate Mullard CV4003 pair had less impressive performance. The following is a summary of the test matrix:

Marconi U78 and pair of Tungsram UK 12AU7:

Pros: Best overall, extended high, sweet mid, and accurate bass. Excellent inner rersoulation. Articulate and very musical presentation. More neutral presentation than others. Cons: None.

Marconi U78 and pair of Raytheon 5814A:

Pros: Very rich, articulate, realistic texture, excel in harmonics, and full of life. Magnificent in symphony orchestra music reproduction. Cons: None.

Marconi U78 and pair of Mullard CV 4003 Box Plate:

Pros: Play back was slightly better than with the U707, but overall in the same style. Cons: a little lean on violin sound.

Gold Lion U707 and pair of Tungsram UK 12AU7:

Pros: Exquisite sound, well balance for the high, mid range, and bass. Cons: Somewhat less relax probably due to insufficient burn-in.

Gold Lion U707 and pair of Raytheon 5814A:

Pros: Rich and full body sound, musical and articulate, vibrant at the bottom end. Cons: same as with Tungsram.

Gold Lion U707 and pair of Mullard CV4003 Box Plate:

Pros: Very good for reproduction of piano, soprano, ensemble, and symphony orchestra. Cons: slight coloration on violin sound.

Tungsol 6X4W and pair of Tungsram UK 12AU7:

Pros: Good tonal accuracy, rich in harmonics, relax, good resolution, articulate and very musical. Cons: None.

Tungsol 6X4W and pair of Raytheon 5814A:

Pros: Energetic, lots of guts, rich in harmonics, good resolution, full body. Achieved theatrical effect for the Organ music. Cons: none.

Tungsol 6X4W and pair of Mullard CV4003 Box Plate:

Pros: More musical than the combo with the U707. Cons: lack of emotion.

Entering the E80CC:

The E80CC is a sub ECC82 which draws a stronger current in operation. Based on the comments of many audiophiles, an E80CC is an excellent substitute for the ECC82 tube. For the buffer stage, it provides a linear response over a range exceeding that of an ECC82. By using this tube at the buffer stage, both the micro and macro dynamics are better reproduced while a full body sound can be maintained.

The E80CC tube being used in this reviewed is a Westinghouse branded tube with tinned pins. Its manufacturer is unknown. It seems to me the E80CC tubes made by Phillips and Valvo were featured with gold pins.

After a comparison with the performance with other ECC82s for the buffer stage, I reached the conclusion that the E80CC was the proper choice for this position. This choice led to a great simplification of the tube rolling because the only variable left was the tube for the amplification stage.

To evaluate the tubes for the amplification stage, I used the best tubes found earlier for the rectification and buffer stage to match, i.e. the Marconi U78 and the Westinghouse E80CC were chosen for this purpose. The ECC82/12AU7 tubes were inserted one at a time to the amplification stage to reproduce the five test songs. The scoring of each tube does not have absolute meaning; it is merely a way to show the relative overall average scale of performance. The results are summarized as follows:

A Comparison of the Amplification Tubes (ECC82/12AU7):

1. Mullard 10M: This tube is by far the most impressive among the 10 tubes being evaluated, with the combination of the U78 and the E80CC. The music reproduced by the Minimax was exquisite, articulate, full body, transparent, energetic, and have plenty of life. Regardless it was a reproduction of solo or ensemble, vocal or instrument, this tube performed well over the entire bandwidth of the sound spectrum with very good inner details and fine texture.

2. Tungsram UK: The construction of this tube is identical to the Mullard 10M and its sonic characteristics are also similar. In my mind, this tube could be a Mullard with a Tungsram brand. If a score for the previous combination is 10, I would give a relative scale of 9.9 for this combination.

3. Mullard CV4003 ribbed plate: This is a Mullard ECC82 with military spec. For military application, an amplification tube should be low in noise and less pronounced on second harmonic in the output. The sonic characteristics of this tube are similar to the Mullard 10M, with the exception that its output has less inner details and the violin sound has noticeable coloration. Overall, it offers the familiar Mullard sound. I would give a 9.8 for this combination.

4. TEN- Japan: The construction of this tube is identical to the Mullard CV4003 ribbed plate. The sonic characteristics of this tube are the same as the Mullard, but the coloration of the violin sound is less pronounced. It is a very good tube with all around performance. I suspect this tube was made in Japan using the same manufacturing tools from Mullard. It deserves a 9.8 for this review.

5. Raytheon 5814A: The sound of this tube is the richest among the 10 tubes being evaluated. I would use the words: vibrant, exciting, energetic, and full body to describe the output. Yet the music preserved the details and fineness. This is my tube of choice to reproduce the magnificent orchestra music. In terms of scoring, it deserves a relative scale of 9.9.

6. Tungsol 12AU7 black glass: This is another great tube in the line up. It is smooth, graceful, transparent, and articulate. Its treble is as good as the Mullard 10M but presented in a different style. Instead of using the word “exquisite” to describe the mid and high frequency sound, I would use the words: relax, open, and excellent tonal accuracy, for description. The bass is deep and very well defined. Overall speaking, this is a tube of great musicality and the fidelity of timbre is among the best. A solid 9.9 was marked on my notes.

7. Siemens ECC82 silver plate: This tube is very special due to its unique sonic characteristics. The second harmonic was not so pronounced in the output. However, the music can be described as bright, accurate, very good in clarity, and energetic. If the sonic characteristic of your audio system is on the dark side, this tube can surely cure the problem. Its overall performance is 9.8.

8. ITT Lorenz C491: The sonic characteristic of this tube is somewhat close to the Raytheon 5814A, but not as musical. It produced a heavy sound in a forward and full body style. In my opinion, this tube can perform much better at the buffer position than at the amplification position. Its overall performance is 9.7.

9. TFK ECC82: This tube is very good for the mid range and bass reproduction. Especially for the song No. 2, it produced a vibrant piano sound which is unheard of from the other 9 tubes. Apparently, this tube “generates” a bloom sound at the lower mid and upper bass frequencies. In many occasions, this extra energy made the music stunning in presentation. It has good transient characteristic as well as a full body sound reproduction. For the treble, however, it is inferior to most of the tubes being evaluated due to the early roll-off at the frequency extreme. The relative score is 9.7.

10. Sylvania USN 5963: This mil spec tube produces a clean output as expected. I would use the word “accurate” to describe the tone. However, if you are looking for a rich and emotional presentation, this tube is not your choice. The relative score is 9.6.