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The Millett Hybrid MiniMAX - Tweaks

Millett MiniMAX Boutique (revised - 12/24/2008)

Can't Miss MiniMAX Boutique Parts

CA2: Nichicon Muse ES, 1000uf 16V, Black Gate NX 1000uf 25V
CA7: Nichicon Muse ES, 470uf 35V, Black Gate NX 680uf 35V
CA9: Wima MKP10 or Vishay-Roederstein MKPT 0.22uf 250V
*** Note: Try leaving CA9 blank if Black Gates are used. ***
CA8: VitaminQ 0.18uf 100V, Russian K42 0.1uf 100V, or
the Wima or Vishay Roederstein listed for CA9

Output Transistors:

Note: Mixing and matching is OK, but keep transistor complements together at all costs. Keep the Vitamin Q's or other boutique film caps away from the CA9 position. The Wima or Roederstein is best there, or nothing at all.

Note that the 0.22uf Vitamin Q at will not fit the MiniMAX board in the Lansing case. Use the 0.18uf VitQ, only.

Also - while these haven't been personally tested, consider the use of Nichicon Muse Fine Gold (FG) caps. There is a 1000uf, 35V FG capacitor (25mm tall) available that may work well at CA7 for low-impedance phones and IEM's. Corresponding versions are available for CA2, as well.

The Elna Cerfaine 470uf 35V is only 25mm tall and is a top-notch selection for CA7 (perhaps as good as Black Gates). However, the Elna Cerafines were discontinued and may be pricy where found.

Warning! The following subject matter has been known to illicit strong and aggressive disputes! It is offered only within the context that experiences differ from build to build and from person to person.

Boutique Summary
The question of boutique parts often arises when considering amplifier tweaks, especially on tube amps and tube hybrid amps. The MiniMAX is no different. Two things are extremely important in considering any boutique parts for the MiniMAX, however:
1. The single, greatest tweak that you can provide for the MiniMAX is to use the custom-machined, Beezar-designed, Lansing case. Your MiniMAX will run cooler, support higher buffer biases, and in general, promote longer-lasting part quality than any other "boutique tweak" that you can accomplish. 2. If the Lansing custom-machined enclosure is used, no part must be taller than 1" (25mm) above the surface of the PCB.
The MiniMAX has five board locations where boutique quality is useful:

  • CA2 Cathode Bypass (electrolytic) - must be 1000uf minimum, largest effect of bass on the board.
  • CA9 Cathode Bypass (film bypass) - highly sensitive, use a Wima/Vishay-Roederstein or leave empty if in doubt, a typical "blooming mids" (roll off in bass and highs) can destroy the MAX response.
  • CA7 Output Coupling (electrolytic) - classic coupling cap, all signal passes through this cap - must be sized large enough to prevent becoming a bass filter.
  • CA8 Output Coupling (film) - bypass for CA7, helps to remove typical harshness that may exist in an electrolytic. Less effective when CA7 is a boutique cap, but the best CA8 will smooth and extend the response from CA7, regardless. The wrong cap here will still damage the bass, but not as badly as in CA9.
  • RB14 Output Resistors - Resistors in the MAX offer the only other means of tempering gain than the tubes. The tubes actually change gain as in a solid-state circuit, but the lowest gain tubes (12FK6) may still be too high in some instances. The RB14 resistors will only attenuate in every case. So, care must be used to prevent harming the dynamics of the amp. Jumpering the positions is best for dynamics. However, if use of the RB14 resistors is needed, then a boutique resistor may minimize the effects of attentuation.

  • Boutique Background
    Forum user "dsavitsk" (Doug Savitsky) has an excellent review article posted on boutique coupling caps and bypassing:
    Notes on Output Coupling Capacitors
    This link can give you some great ideas on the caps to use, although not all are mentioned.
    Boutique Electrolytics
    Excellent boutique electrolytics include Black Gate NX's, Elna Cerafine and RFS Silmic II, and Nichicon Muse KZ, FG, and ES. Boutique electrolytics don't necessarily have better ratings than good quality power caps (FM's, FC's, UPW's, UHE's) - many don't, as a matter of fact. However, they exhibit less harshness when placed directly in the signal path. In the case of the Black Gate NX's, they may perform just as well as a film cap.
    Boutique Film Caps (also includes non-film)
    Film caps are as varied as any DIY part, and the suggested mfrs are almost too numerous to mention: Mundorf, Auricap, Reliacap, Solen, Sonicap, Orange Drops, Hovland, Audiocap, Multicap, Clarity, etc., etc. Then there are the older NOS-style tube caps such as Vitamin Q's, Russian PIO's of various types, Teflons, and motor run oil caps. There are also the slightly different type caps such as polystyrene or silver mica. The differences in all of these caps and their applications are far beyond the ability to explain on this website.

    One thing to note: the WIMA MKP10's specified in the MAX BOM under the Mouser listing are very economical and sound very good. Unfortunately, they are not included in dsavitsk's reviews, which might enable a comparison (hint, hint). Nevertheless, be confident that you will lose nothing in frequency response or dynamics by using these caps in the MAX. The Vishay Roedersteins give every indication of being an outstanding equivalent of the Wima, perhaps superior in some aspects. It may depend on the type of sound you desire. The Wima's are very neutral, but forward-sounding in the mids. The Vishay-Roedersteins are dark, but detailed and very quiet.

    Bypassing Principles with Capacitors
    Note that the film caps in the Millett MAX follow the Millett heritage by "bypassing" the electrolytic in each case. This is a common practice for many amp designs, although hotly debated in some circles. The idea is to combine the film cap with the electrolytic so as to pass the higher frequencies through the film cap, whose sound quality is generally much better than the electrolytic. Most electrolytics, even boutique ones, suffer from some harshness in the higher frequencies. The philosophy of bypassing can have such good results that boutique electrolytics may not be needed - common power electrolytics may suffice.

    You are encouraged to try the commonly used excellent power caps that are available at Mouser, DigiKey and elsewhere. In combination with the excellent film bypasses, use of Nichicon UPW, UHE, or Panasonic FC, FM at CA2 and CA7 may sound perfectly fine, if not better than fine..

    Conversely, it has been documented in several places that using bypass film caps with Black Gate NX's may actually degrade the quality of sound. Some of us who've had a chance to test different combinations with the Black Gates have also found that many film cap bypasses have absolutely no effect at all. Typical practice is to delete the film bypass caps when investing in Black Gates. However, this may not work with the other boutiques and film bypasses may still offer better results than none at all. For instance, the Nichicon Muse ES is one that works well with a good film bypass, but not as good without.

    Output Coupling Caps
    After deciding on a boutique electrolytic, one must determine the size needed. In the case of the output coupling caps, C7 combines with the impedance of your headphones to form an RC circuit. This means that unless you are careful to size the cap, it will filter some of the frequencies we want to hear. To determine this, the following equation applies for calculating the corner frequency:

    F = 1/(2 * pi * C * R)
    (It's called a "corner frequency" because a curve of the frequency response drops so much at this point it looks like a "corner.")

    2Hz is a good corner frequency to use, because phase distortions can occur up to 10 times the corner frequency. So if we interested in hearing 20Hz, we should calculate for a corner frequency of 2Hz or less. The following chart displays the cap size vs. the resulting corner frequency for commonly available headphone impedances:

    For special details on how to use and mount Russian K42 caps, refer to Tweaks->K42 PIO Caps.

    For special details on how to use and mount Vitamin Q caps, refer to Tweaks->Vitamin Q PIO Caps.

    Cathode Bypass Caps
    Traditionally, a similar philosophy was followed for sizing the cathode bypass caps, CA2. However, in the case of CA2, the tube plate resistance is the resistance that combines with the capacitor to form the RC circuit. However, a good CCS can make the plate resistance seem very high. As result, the cap size does not have to be very large. The revMH Millett often used electrolytic cathode bypass caps sized at 220uF. This was for two reasons: 1) the space was limited on the board since C2 was directly adjacent to the DB board, and 2) the calculation outlined above usually resulted in cap sizes of 100uF or less for this position. It was thought that 220uF was plenty to allow for all bass frequencies desired from the tube.

    Experience with the MAX contradicts this. The actual source for the reason is not known at this time, but 220uF caps are not sufficient for the CA2 positions. Team Millett MiniMAX recommends a 1000uF cap in this position for the best in bass. Several builders of earlier Millett MAXes have confirmed that this makes a noticeable difference. The necessary voltage rating is still very low - 16V or less, perhaps. Accordingly, the Millett MiniMAX BOM shows a 1000uF, 16V cap for the CA2 positions.

    NOTE: Further testing has revealed some important facts about the CA9 film cap bypasses at the cathode bypass position.
    Please be very careful what film cap you use - if any - to bypass the larger electrolytic in this position. The film cap positions are CA9, Left and Right. It has been found that some "boutique" film caps will have very bad effects in this position and may destroy the bass response. This is because most "boutique" film caps emphasize fluid and silky mids in return for some rolloff in the bass (some in the highs, too). In classic output coupling positions (such as CA8 in the MAX) this may go unnoticed if noticed at all - often improving the sound - the MiniMAX is no different. However, in the cathode bypass positions the caps are acting as sort of a "shunt" to "catch" all the of the stronger music frequencies when they go out of bounds of the differential swing setup by the tube bias point. This most often affects the bass. If the film cap exerts any influence at all, it seems to destroy many of the wanted lower frequencies right at the most critical point in the amp - at the tube.

    Another way to look at this is that the cathode bypass caps provide extra room for the tube to expand the music signal as necessary - without cutting anything off. So, the greater that space and the room available for the signal, the less the signal is attenuated or clipped. A small boutique film cap chokes off many of these signals.

    The effects are unpredictable. Interestingly, the default Wima film caps have been found to have little effect on frequency response in the CA2 position. In some cases, such as pairing with the Nichicon Muse ES, they even seem to actually enhance the bass. So, they work well in the CA2 positions and out of the circuit bypassing the CA4 and CA5 caps. They will exhibit a bit of grain in the high-mids and highs on the output, however - but it may take you hours of listening on very detailed headphones to notice this. A MiniMAX constructed entirely of Wima film caps bypassing the specified sizes in electrolytics will result in amp that will best many designs. As stated previously, the Vishay-Roederstein gives every indication of being an acceptable alternative to the Wima's.

    NOTE: After repeated testing with many caps, the Wima film caps may be all that's necessary in CA8. While the wrong film cap in CA8 may have disastrous effects on the MAX's frequency response, it seems that higher quality caps - even the Vitamin Q's - will reduce bass response.

    A good economical compromise (translated: no Black Gates) for outstanding performance with the MAX is to use the Nichicon Muse ES caps at the CA2 and CA7 positions, bypass CA2 with the Wima's (at CA9), then use a Vitamin Q (0.18uf) to bypass the ES's on the output at CA7. The result is outstandingly deep and slamming bass with fantastic highs. This combination approaches the quality of a Black Gate NX in detail and neutrality.

    Output Resistors
    Finally, the output resistors on the MAX, RB14, are another opportunity for boutique parts. Traditionally, these resistors were used to protect from shorts in the output stage. In the case of the Intersil buffer option in the revMH Millett, these were absolutely necessary. In the case of the MAX, however, either BJT or MOSFET output stage is robust enough to tolerate the intermittent shorting that may occur from plugging/ unplugging headphones.
    For those wishing the ultimate in power performance and uncolored sound output, the best thing to do is simply jumper the RB14 positions. On the other hand, the RB14 resistors offer two opportunities: 1)to give an additional sort of "gain reduction" where the lower gain 12FK6 tube is not low enough, and 2) use of a boutique resistor can tame some high-end harshness or a slight bit of tube noise.
    Several Millett builders have had good expience with Kiwame and Stackpole output resistors. If you are wishing to tame some high-end harshness or tube noise, however, do not use Vishay-Dales. They won't help ... at all.
    Resistance values from 10 to 150 ohms may be used. Stick with the low resistances if you are interested in sweetening the high end slightly. Use the higher resistances for gain reduction. You may have to use as much as 100 ohms to start detecting a difference with gain. On the other hand, as low as 10 ohms may remove some harshness, if present.

    file last changed:Monday, December 29, 2008 6:00:00 AM
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