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Threshold - SA/6e monoblocks class A under review !

This is a great moment for me, maybe the greatest review to date for me... You are about to understand why ...

1-.A bit of history and passion.

During the 80's, when I was (much) younger and already passioned by the true High-end Hi-Fi world, I already knew all the "mythic" amplifiers, such as KRELL, AUDIO-RESEARCH, ROWLAND RESEARCH, CELLO,... and of course the THRESHOLD !
15 years later, I remember them like if I had seen them yesterday for the first time: do you know the Rowland model 8 and 9 ?, the Audio-Research D79C ?, the Krell KSA-50 ?...

But 15 years later, one of them is more present in my memory than the others: the THRESHOLD SA-4e.
Not because it is a monster in terms of weight and power, not because it is an outstanding value in terms of design. But because when I first auditionned it, I had the biggest surprise amongst all the worldwide references I had lessonned to at this time: it was connected to a pair of big PROAC loudspeakers, fed with state of the art D/A THETA and EAD converters, and it gave me the most pleasant listening experience at this period. Its musical qualities were so far beyond I used to ear, its authority in driving loudspeaker was so exceptional, that it was like if the SA-4e amplifier did not know that there was a pair of loudspeaker connected to it !
15 years later, I remember that, at this precise moment, I knew that the THRESHOLD SA-4e had the ability to provide to the listener something that no other amplifier had been able to provide (to my ears), including the aforementionned worldwide references.

It is after having auditionned the SA-4e (whereas I already knew the original SA-3.9e) that I really studied the "like no other"
Nelson PASS designed amplifiers, and particularly the THRESHOLD SA serie for which a detailed documentation was available. Let me just mention a less known point: the external design was styled by Rene BESNE.

And you know what ? I discovered that something even more "monstruous" than the 48kg SA-4e existed: the SA-6e monoblocs... Here we are in the world of the truly exceptionnal products, the "FERRARI like" products that were mainly designed in the 80's...before the era of the "Finance Director governed companies".... These SA-6e class A monoblocs appeared to me "too exclusive" at this time, and I did not imagine that I will be the (happy) owner of them a few years later...

2-.What is in the box...

A FERRARI is a beautiful car, with its big tires showing that it is not the last Honda or Ford with a so common 4 cylinders engine...
By looking at a Ferrari you immediately know that it is not a common car: when you look at the 2 monstruous SA-6e monoblocs, you immediately think the same....

By looking at a Ferrari, you quickly desire to have a look inside: just to see the engine, particularly if you already know that it is a V-12 engine, because you are obviously sure that it is something different... So are you ready to have a tour of these exceptional SA-6e ? ok, so have your seat belt fasten ...

Here is a front view of the "baby", or more exactly a view of one of the 2 babies because the SA-6e is made of 2 monobloc amps:

Threshold SA-6e class A monoblocs, front view.

Sit onto a 15 kg granite base, you immediately see that it is a "beautiful baby": 28 kg per channel, you can bet without any risk that it is not the last "made in Taïwan" amplifier extensively described in many home theater magazines, as being the "star of the summer"...
Do not look for a lot of lighting LEDs, do not expect the latest marketing idea such as the "Turbo bass boost": there is none. There is just a red LED lighting when the amp is on, and an on/off switch, this is all, but this is enough for a (true) power amp, and I do not see anything missing !

For those of you having some knowledge of the THRESHOLD SA serie, I indicate that the thick (1 cm ...) front plate between the (useful) handles is grey, instead of black on the SA-3.9e: this is the only difference at the cabinet level between the SA-3.9e and the SA-6e, each SA-6e monobloc is housed in a SA-3.9e box.

Shall we open this incredible high quality cabinet ?

Threshold SA-6e class A monoblocs, internal top view

The SA-6e monobloc uses 28 power transistors per channel, each one being able to provide 200W. So a total of 56 transistors are used, the famous MOTOROLA MJ-15011 and MJ15012, the same one used in a lot of worldwide references (note that a lot of great amps have used various bipolar transistors). Note that these power transistors are still manufactured by MOTOROLA, in the USA (not in China or Taïwan....), so no problem in case of repairing (a low probability event for sure...).
By looking at these power transistors, you see that they are not directly fixed on the heat dissipators, but they are fixed on a 1 cm massive aluminium plate which is fixed to the heat dissipators.
This is an improvement in the thermal regulation because all the transistors operate at the same temperature, because they are "thermally" linked by the aluminium plate. This mechanical building quality is also exceptional, and you probably have enough fingers on one hand to count the amplifiers manufactured today that would benefit from a so high mechanical standard of quality...

The (transistors) drivers in the front stage belong to the same family and are the MJ-15022 and MJ-15023 (also still manufactured by MOTOROLA).


You can also note the big gauge of the cables in the output section: 2 x 2.5mm2, so the current (Ampere) can easily flow to the speakers. This is not "just in case": as explained later on in this review, the SA-6e are able to deliver a lot of current ! We say  *a lot *...
Each channel of these 28 transistors is powered by a 800VA toroidal transformer, the same one used is the SA-3.9e but there is one in each channel instead of only one for the both channels in the SA-3.9e (there are 2 of these in the SA-4e).
Just behind this transformer, there is a set of 125.000 µF capacitors (computer grade) per channel... so you can easily imagine that the SA-6e push a lot. We say * a lot* !...

The front stage driver transistors are the famous J-FET. For those of you being interested, they are MOTOROLA 2N-5566. You must NOT underestimate the importance of these J-FET:
Nelson PASS has explained in the past that they are far better than the other, and that they are of the highest imortance in the final sound quality (he even wrote an article explaining that there is no comparison, in terms of musicality, between those J-FET and the awful IGBT...).
The J-FET usage was a total new thing in the Hi-Fi world at this period, and a few other brands than Threshold followed this way and also used the J-FET in their later productions.

The other transistors are more common, such as the high voltage MPS-A42 / MPS-A92 and the MPS-U10 / MPS-U60, as well as the PN-4250.

The resistors used in the output stages are rated 2W - 5% (i.e. a good tolerance, but not exceptional) but there is no mention of a specific type of resistor.

The capacitors in the front stages are referenced as being from different types: mica, tantal, film and monolytic.


To get a new proof of the outstanding building quality, just have a look at the plate on which the on/off switch is located: it is a 2mm thick aluminium plate screwed (6 screws...) on the main 1cm thick front plate: today, it would be a piece of plastic, most of the time...

And what about the back ? it is similar to the SA-3.9e or SA-4e. There are 2 handles (very useful to move around the 28 kg of each monobloc), 4 fuses instead of 5 in the stereo SA-3.9e, one IEC plug, 1 speaker connector, and a RCA and a XLR input connectors with a switch in between, to select the one to be used.


This points out an outstanding build quality: these monoblocs, like all the THRESHOLD amps belonging to the SA serie, is built like a tank ! most of these models are still in use, can still be easily repaired, and their owner generally does not sell them... With the exception of the PASS LABS, there is not many amplifiers with a so high level of build quality. Looking inside these monoblocs immediately show you that only the passion can make this kind of amp a reality.

3-.The current (Ampere) output capability.

A very interesting thing is to compare the current output capabilities and the power supplies of the THRESHOLD SA serie amplifiers.
We remind that the power supply is the master piece of a transistors based power amp, and its current capability is of the highest importance, and even more important when it relates to this kind of amps that are class A amps (so the highest musicality is expected from them) and supposed to be able to drive complex loads.

  SA/3.9e SA/4e SA/6e
Output power per channel 60W continuous
120W peak
100W continuous
200W peak
125W continuous
300W peak
Number of power transistors per channel 14 26 28
Current capability (1) per channel 30A permanent
95A peak
35A permanent
120A peak
50A permanent
135A peak
Power supply transformer 800VA 2 x 800VA 2 x 800VA
Power supply storage (total) 125,000 µF 190,000 µF 2x125,000 µF
Weight 28 kg 48,5 kg 2 x 28 kg
(1): the permanent output current capability is limited by the fuses.

One of the most important point of this matrix is probably the output current capapbility and the storage capacity of the power supply.
I have personnally listened the original SA-3.9e, the SA-4e and the upgraded SA-3.9e, and this clearly demonstrates once again that the power supply is of the highest importance in the final music reproduction.
The original SA-3.9e is an excellent power amp with an outstanding musicality, and I know very few amps that can compete with it (when the speakers are not too difficult to drive), particularly because the quality of its medium and highs is really impressive.
The SA-4e is another animal: it has nuch more authority, clearly not playing in the same league: but from a pure musicality viewpoint it is not better in the mids and highs than the 3.9e: remember that Nelson PASS had said that the 3.9e is "maybe the more musical amp"... The SA-4e houses 50% more energy into its capacitors, has 2 transformers (800 VA each) instead of only one in the SA-3.9e, has 28 power transistors per channel instead of 14, and is able to continuously deliver 35A instead of 30A.
The consequence is that the SA-4e gives the illusion to do not even know if a pair of speakers is connected to it....

The SA-6e crosses another gap. There are still the same two 800VA transformers (one in each monoblock) but the capacity storage grows up to 250,000 µF instead of 190,000 µF in the SA-4e, the number of power transistors is 56 instead of 52, and, the most important, the output current capability grows from 35A for the SA-4e up to 50A for the SA-6e. The peak output capability reaches 135A... We are clearly in an exclusive league, and there is not a lot of possible competitors on the planet...

4-.The system setup.

The SA-6e monoblocks have been auditioned on our reference system #1:
  • Digital cable KIMBER Illuminati D-60 BNC / BNC .
  • DAC COUNTERPOINT DA-10 new version 24 bits / 192 kHz (a true reference)
  • Prreamp THRESHOLD T2 upgraded (look at the review on this site)
  • Modulation cable: SIGNAL CABLE Silver Resolution / with EICHMANN Silver Bullet connectors for the RCA version, and NEUTRIK for the XLR version.
  • THRESHOLD SA-6e and THRESHOLD SA-3.9e amps
  • THIEL CS 2.0 loudspeakers
  • Speaker cable: ELEMENT CABLE
  • Custom AC power cords with FURUTECH FI-E11 and WATTGATE connectors

The first thing you have to know is that the SA-6e do require a long time for reaching their optimum temperature: 3 hours is the minimum, that is not a surprise because so powerful class A amps have to be extremely stable from a temperature viewpoint. Getting stability and reliability from a 125W class A amp is not the same story than with a 35W class A or 150W class B... not at all...

The second thing is that the SA-6e, like all the other THRESHOLD SA serie amps, do require a heavy base, preferabily a 15 kg granit plate with cones.

Thirdly, the preamp is connected to the SA-6e monoblocks by using XLR silver cables, whereas the DAC is connected to the preamp with a RCA silver cable (refer to the
DA-10 new version review on this site).

5-.The music.

As you easily imagine, the SA-6e have been deeply auditionned before writing this review: they have been in our system for more than a month...
They are fed by the upgraded THRESHOLD T-2 preamp.
The CD we have used for this review are well known, so we are sure to detect the slightest change in the music reproduction.

So, where do we start ? the simplest is to compare with the original SA-3.9e which is also in use in this system.
When listening the SA-3.9e, we are immediately convinced by the elegance of the sound, its velvet like caracteristic. There are just a few amps able to deliver so many details with so much elegance, particularly in the mid and high frequencies.
The THIEL CS 2.0 used for this review are easy to drive, particularly because they do not deliver enough bass if the amp is not able to deliver a *lot* of current. This is the reason for which I use an active subwoofer, so I get a perfect bass basement in addition to the beautiful mids and highs, and to the space that the 3.9e is able to deliver. The SA-3.9e is marvelous because in this configuration, it delivers both a lot of harmonic details, as well as a velvet like sound which is the sonic signature of the PASS designed, THRESHOLD SA serie.

By switching from the 3.9e to the 6e monoblocks, we are puzzled.

The first most noticeable thing is the bass reproduction. We had never imagined that the THIEL CS 2.0 could go so deep in the bass. The active subwoofer which was absolutely mandatory with the SA-3.9e (despite the fact that it delivers more bass than most of its competitors) is now questionnable. The bass level is so much more present that the active subwoofer becomes quite an option !
The second noticeable thing is the total lack of stress and effort. You can pump up the volume as much as you like, the SA-6e continue to drive the sound up without the slightest sign of effort or stress ! if you continue to pump up the volume, you will afraid yourself, but not the SA-6e... These amps are able to go as far as your tympans accept, with the same stability and total lack of stress...

The sonic signature remains always the same, under any circumstances. They have a lot of power, a lot of Ampere in stock: there is no doubt at all that the 125W class A per channel are here, and that the 50A of permanent curent capability are there !

These THRESHOLD SA-6e are in our opinion the cream of the worldwide reference transistors based amplifiers. Their building quality is outstanding, far beyond the best today munufacturing standards. They are able to drive any speaker load including the ribbon based panels (such as the APOGEE DIVAs) with an absolutely constant (and exceptional) sonic signature: this is the signature of the THRESHOLD class A SA serie, designed by Nelson PASS.

If you are lucky enough to find a pair of them, just buy them: you will not find anything new that could compete with them. Their internal components (transistors) are still available for sale, so there is no pb for repairing them (although I have never seen a Threshold SA amp having a pb...). They are as robust as a tank, capable of authority and sweet but detailed sound. the vast majority of the amps sold at the time they were produced are still in perfect condition and still in use ! They are almost impossible to find in France, extremely difficult to find in Europe (except maybe in Italy) and rare int he USA.

But they are amongst the very best amplifiers on the planet. They win our highest ranking without any doubt !


Article written by Nounours18200

Creation date : 01/06/2007 @ 21:19
Last update : 09/11/2014 @ 20:36
Category : Threshold
Page read 15226 times

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Reactions to this article

Reaction #8 

by Nounours18200 06/01/2011 @ 20:26

 Interesting about the faceplate color differences.  The faceplate on my SA6e are black (not grey) and identical to the faceplate on my SA3.9e.

so there should be different versions of the SA/6e ? maybe due to the date of manufacturing or country of origin ? It is the same situation with the Model number, being sometimes curiously different from a country to another. I will contact Threshold to try to get an answer...

Also, note that rated current capabilities of these amps are not at 8 ohms but rather at fractions of an ohm.  Otherwise, the numbers don't make sense - 35A continuous at 8 ohms would mean 9,800W

right of course, but I will try to get more info from the Service Manual to identify the context of the measurement, because this is the "official" Threshold info...

Reaction #7 

by ey 06/01/2011 @ 19:22

The second to last sentence of my post should be:  If using 35A at 125W, you will get something like 0.102 ohms.

Reaction #6 

by ey 06/01/2011 @ 19:19

Thanks for the great writeup.  I also have the SA3.9e amp as well as the SA6e monoblocks.  I enjoy them both.

Interesting about the faceplate color differences.  The faceplate on my SA6e are black (not grey) and identical to the faceplate on my SA3.9e.

Also, note that rated current capabilities of these amps are not at 8 ohms but rather at fractions of an ohm.  Otherwise, the numbers don't make sense - 35A continuous at 8 ohms would mean 9,800W.  So, if we use 35A at 8 ohms, the impedance is something like 0.102 ohms.
Enjoy listening!

Reaction #5 

by Nounours18200 21/02/2010 @ 18:34

Pour dray177: vous avez tout à fait raison, l'erreur a été corrigée. Et bravo pour votre travail de restauration des ML-2 qui mérite d'être applaudi !

Reaction #4 

by Nounours18200 24/01/2009 @ 12:08

Pour Raymond (dray177): tu as raison ! (article corrigé).

Pour  Borfermy: avec un preamp Kaneda pourquoi pas (il doit tenir les impédances un peu faibles pour alimenter les SA/6e); quant aux Onken, pour excellents qu'ils soient, leur rendement est très haut et je ne suis pas sûr que ce soit ce qui convient le mieux aux SA/6e...

Pour Doctor1: les nouveaux amplis Threshold ? heuh... va écouter les nouveaux PASS avant de prendre une décision....
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