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Threshold - Fet-10 HE 'competition upgrade' Level 1
THRESHOLD Fet-10 He PREAMPLIFIER
The THRESHOLD Fet-10 He « competition upgrade » - Level 1
Well... We have promised that you will get a review of a highly upgraded THRESHOLD Fet-10 HE preamplifier: here we are... This upgrade took a few months because it does not simply consist to change one or two parts, but it is far more sophisticated. Here are the changes that we have made:
As the title may suggest, " Step One " means that a "Step Two " will probably follow in the coming months: more details are given at the end of the article. But let us back to this already very serious upgrade...
As you know if you have read the THRESHOLD Fet-10HE preamp original review, this preamplifier has been largely acclaimed and provided us with an unexpected level of sonic performance. It easily surpassed the Upgraded THRESHOLD T2 preamp, and is one of the rare preamp to compete with the upgraded JADIS JP80... When it comes to sonic performance, this « David » can compete with more or less any commercial « Goliath » preamp, providing more than a lot of surprises to the listeners...
The following photographies help you to understand to differences and improvement between the original power supply and the upgraded power supply.
The original power supply on the left, and the upgraded power supply on the right:
Nevertheless, we thought that we can hire it up to a higher level by working on some « strategic areas ». In this preamplifier, we identified the 4 areas pointed out with the bullets at the beginning of this article.
1-.Working on the external Power Supply.
This study has been performed with one of our friend, jfg67, and the schematic is available on request. Although this is a very serious upgraded power supply, the step 2 will go even further in the P.S. sophistication. The upgraded power supply features:
1-a-.The two bridges (rectifiers).
To improve rectification and power handling, the upgraded power supply uses 2 bridges instead of one. These bridges are not « ready to use » bridges, but they are made by the assembly of 4 diodes. These diodes are 4 Fast " Soft recovery " diodes, more known under the name F.R.E.D. diodes (F.R.E.D. stands for Fast Recovery Epitaxial Diode) with a very important "Soft Recovery" characteristic. Each diode is able to handle at least 4A - 600V manufactured by Fairchild ®; of course 4 diodes are required to build each bridge.
This approach allows to use the best available diodes to build the best bridge.
According to the Nelson PASS experiments, we do not recommend using the fast diodes for the power amplifiers P.S.
1-b-.Capacitor before each bridge.
Before each bridge we use a small capacitor: these capacitors are the excellent MIT (made by Reliable Capacitors ®) belonging to their best RTX serie. They are matched per pair (<1%) and their outer foil is identified for an optimum orientation.
1-c-.Decoupling capacitor in // with the main capacitors.
They are also MIT RTX. The pair is matched.
1-d-.Upgraded main capacitors.
The upgraded main capacitors are 2 NICHICON © KG Gold Tune, 22,000µF / 63V each.
1-e-.Internal on air cabling.
The internal cabling is « on air » , using an OFC, 1.5mm2 Teflon insulated cable for the capacitors to the small board connections, whereas we used a pure Silver Teflon insulated cable for the connections on the small board (i.e the Bridges / MIT RTX / resistors / diodes interconnects). The solder is the Wonder Solder Signature made by TRT ® and the AudioNote ® silver solder.
The transformer is a custom ordered model, rated 300VA. When not connected to any load, it delivers 20V at the secondaries (meaning 1 or 2 volts less than the original one), but of its bigger size the power supply delivers exactly the same voltage when the preamp is on. Of course, a so bigger transformer has a huge impact on the final sonic performance of the power supply.
Herebelow there is a closeup of the entire new power supply:
On the left of the photography, you can see the external power supply Tajimi connector, that is exactly the same than the original one in order to allow a direct A/B comparison between the two power supplies.
So, is there any improvement after having installed this new Power Supply ?
Well... yes Sir, and not a small one ! The improvement can be described as follow:
A conclusion comes with evidence: the Power Supply is definitely in the signal path....
2-.Working on the potentiometers.
The THRESHOLD Fet-10 He is equipped with 2 potentiometers manufactured by NOBLE: they are the black 25kohms models, one is a stereo version used for the volume control, the other one is used for the balance control.
Although the black NOBLE potentiometers are known to be very good ones, we wanted to go a step further and decided to:
Given the limited available space inside the preamplifier cabinet, it has been impossible to find a stereo 25kohms stepped attenuator small enough to fit in...we were obliged to go for a mono version, but this was not a problem because we had also decided to remove the balance control that we never use (and that is also a supplementary device in the signal path).
Consequently, we have used the location made available by the removal of the balance potentiometer to install the mono left channel stepped attenuator, and we installed the right channel mono stepped attenuator in the original volume potentiometer location.
The 25kohms impedance also limits the choice of the stepped attenuator (only a few manufacturers propose this impedance, for example DACT does not propose this value) and we decided to choose the GOLDPOINT Mini-V mono stepped attenuators (one for each channel). They are available in a mono version for $79 each and have an excellent reputation in the DIY world.
The main problem for installing these GOLDPOINT stepped attenuators is that they firstly require a larger hole in the main front plate than the NOBLE, and secondly their axis (shafts) has a 6mm (0.237") diameter, meaning slightly smaller than the original NOBLE ones. Consequently, it has been necessary to drill the Fet-10 He main front plate, a 10mm thick aluminium plate (we nevertheless thank Mr PASS for this beautiful look...). This implies a cautious work that we ask POWER ELECTRONICS to deal with: because they have a lot of experience with this kind of work... POWER ELECTRONICS really did a perfect work, as explained below.
Here is a view of the GOLDPOINT stepped attenuators, you can refer to the original Fet-10 He preamp review to have a look at the NOBLE potentiometers that they replace:
Before answering to the question « how do the GOLDPOINT Mini-V stepped attenuator sound ? », I have to explain that we first performed the Power Supply upgrade, and only later, when we had enough listened to the preamp to be sure of the exact Power Supply upgrade sonic impact, we performed the GOLDPOINT upgrade. By doing this, we cannot make a confusion regarding the respective sonic benefits of the Power Supply upgrade and of the GOLDPOINT stepped attenuators upgrade.
Answering to the question « how do the GOLDPOINT Mini-V stepped attenuator sound ? » is very easy: if you still have a potentiometersin your preamp, what are you waiting for to upgrade to a stepped attenuator ???
As you have understood, the improvement is terrific ! obviously very different than the P.S. upgrade but terrific in another manner. The improvement can be described as follow:
3-.Working on the capacitors.
The THRESHOLD Fet-10 He uses 8 white capacitors, 0.1µF / 400V, named « Wonder Cap ® Ultima with Danor TM », installed on the main board in the preamp cabinet.
After having discussed on the DIY Audio forum about what capacitors I should use if I was looking for an improvement, I have been pushed to use the MIT RTX capacitors.
So we ordered 8 of them, 1% matched pair, with outer foil identification for the best possible orientation.
Here is a view of the Fet-10 He after the capacitors upgrade, you can refer to the original Fet-10 He preamp review to have a look at the capacitors they replace:
Did this upgrade bring some improvement ?
For the time being we do not answer to this question because it is not evident to say yes or no: contrarily to the section 1 (power supply) and section 2 (stepped attenuator), we cannot listen to an improvement or a degradation in the sound: we prefer to wait for a few weeks before giving an advice on this modification.
It is highly probable that these capacitors need a burn-in period, so we will update this article in a couple of weeks.
A dedicated topic is opened on our Forum to discuss about this upgrade: do not hesitate to use it.
Article written by Nounours18200.
Creation date : 04/01/2011 @ 21:37
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