This amplifier was found to be so extraordinarily sensitive to the various kinds of external electrical noise and micro vibration reducing tweak modifications we have already explored in this column, that the mods developed for the interfacing and support of it merited a separate category in the System Tweak pages. This sensitivity and improvability appears to be mainly because this design is unusually good in the time domain (that is, time coherent) in addition to having a flat relative phase characteristic over the audio band, plus the other qualities identified in the various reviews. Prior to adding the various external tweak modifications detailed here performance with the NuForce Ref. 8.02 amps (with all updates up to the present) was excellent in various areas of performance but still not completely satisfying ­ especially due to a little electronic hashiness and edginess in the high frequencies. After all of the external tweaks described here were implemented with NuForces, every aspect of the system's sound was unbelievably improved, to a sort of "supernatural naturalism" (someone else coined this phrase). The system now has previously inexperienced (and unimagined) levels of see-through clarity, resolution, dynamics, musicality and holographic imaging. The previous favorite amps, a pair of superb 30w push-pull 2A3 tube monoblocks, have been relegated for use only for occasional experiments. The speakers are 96db sensitive hybrid ribbon designs ­ the Ambience Reference 1600s (380 Hz crossover), and TBI VIP subwoofers.

It should be pointed out that most of these techniques were also tried out on the tube amps with great results, but coming nowhere near the performance of the NuForces with the same tweaks.

Of course, we have not tried all available kinds of conditioners, filters, equipment supports, vibrational damping, etc., but this particular combination of external mods with the NuForce Ref. 8s has produced dramatic sonic improvements that should be to some extent translatable to other systems.

Power conditioning:
Dedicated AC conditioning/EMI filtering seems to be essential for the NuForce amps, presumably due to the EMI generated by the 500KHZ switching power supply. Another separate power conditioner also needs to be used for the front end components, to further isolate and reduce interference from the NuForce switching supply EMI. We found the combination of two AC power conditioners in series to be an optimal arrangement for the NuForces. The PS Audio UPC 200 balun-type conditioner works very well ­ an immediate increase in clarity, lucidity and reduction of a slight "edginess". An even greater improvement resulted from adding a separate Foundation Research LC-2 power conditioner/power cable for each NuForce monoblock. Since the LC-2 is by design floating (no earth ground connection), an earth ground connection for each amp had to be improvised. Although a supplementary earth ground connection needs to be provided, the Foundation Research LC-2 is the best single power conditioner found yet for use with the NuForces (two required, one for each monoblock). The Brick Wall also works well with the NuForces, but is not as refined, smooth and resolving, and also is incompatible with the Foundation Research units.

EMI filter at amplifier speaker outputs:
Considerable improvement was obtained from use of Empirical Audio's ferrite-bead Image Clarifiers, one for the + output and one for the ­ output. These units have a spade at one end and a banana receptacle at the other.

EMI absorbing shielding material:
EMI is apparently radiated especially through the ventilation openings in the chassis. A significant improvement is made by taping a couple of two-layer patches of the ERS EMI shielding cloth just over these vents. The amplifier runs so cool there was no concern about sealing these vents off.

Special Supporting Feet:
With these amplifiers the differences between rigid cone type supports and flexible or spring-like devices were much greater than usual. We found that rigid support feet of all kinds resulted in various degrees of hard, edgy coloration. The best results were with the use of three Hyperion Magnetic Floaters - superb clarity, transparency and smoothness. These devices also had the advantage of relatively low cost, $90.00 for a set of four. They are made by the manufacturer of the highly regarded Hyperion HPS938 speaker. The Tightrope devices were also excellent with these amplifiers, but they were somewhat too large for the small chassis. The small physical size and weight of these amplifiers evidently makes the transmission of vibrational energy from the supporting rack the dominant mechanism, not direct acoustic pickup. Therefore a premium is put upon using feet that absorb as much of that energy as possible. The feet should be pointing up.

Damping:
   The largest single improvement of all of these tweaks resulted from greatly reducing micro vibration through mechanical damping using special elastomeric materials (along with the special supporting feet). Be warned that damping accomplished this way has a poor WAF rating because it must be done by placing ugly rectangular pieces of damping material all over the amplifier chassis.
   This amplifier is so sensitive to vibration that we found that a more sophisticated approach was need for damping. This requires two layers of different materials, where the underlayer is stiffer with a high mechanical impedance to absorb vibrational energy from the chassis surface without reflecting much back into the chassis, and the upper layer is softer but still dense and reduces reflection of vibrational energy from the top surface back through the material to the chassis.
   The damping material placed on the chassis should be rectangular patches consisting of 4 layers of EAR Isodamp SD40 placed on the metal surface, followed by 3 layers of Parts Express automotive sound deadening sheet, part no. 268-020. These materials are adhesive backed. There may be better materials for this purpose out there, but for now they are the best I have found. The damping patches I have used in my experimentation have been about 1" x 2", but presumably larger pieces would be even more effective. The damping material should cover most of the surface area of the top, bottom, sides and front except the ventilation holes and the places where the chassis rests on the 3 upward pointing magnetic feet. The EAR materials are available from Michael Percy Audio, www.percyaudio.com.

Weights:
The optimal weight to place on each NuForce amp seems to about 5 pounds. I used a. bar of lead babbit (McMaster-Carr part no. 8899K11), which has a slightly better sound than regular lead, and is easily available in bars of the right weight and shape. Each bar needs to be damped with two layers of EAR SD125 on all four sides - this is necessary because even lead has a slight metallic ring. Addition of the lead weights drastically clarifies, deepens and widens the sonic image by further reducing micro vibration.

Install Bybee quantum purifiers:
The Bybee filters produce wonderful improvements in see-through clarity, intertransient silence and musicality, with the effects especially pronounced if implemented after controlling microvibration. They should be installed at the interconnect inputs, speaker outputs, and most importantly at the IEC AC power input. The least expensive approach is to get the bare purifiers and solder them into adaptors. For the interconnect inputs and speaker outputs, there should be filter units on both + and -. The small Purifiers are fine for these applications. For the AC input, the large Purifiers should be used, one on AC (line) and one on neutral. Surprisingly, the Purifiers' sound improves if they are each damped by a 3 or 4 layer piece of EAR SD40.

Improve quality of input load impedance:
This last touch is quite amazing in the degree of improvement obtained, after all the previous techniques. This only goes to show how really subtle and complex are the underlying phenomena of high end audio. The standard input impedance of the NuForce is 50,000 ohms (50K). This resistance is certainly of good quality, but improvement of it still makes a big difference. For several reasons this needs to be done externally with an adaptor with a very high quality resistor connected between signal and ground. The RCA to RCA or XLR to XLR adaptor is inserted at the interconnect input.

Certain very high quality resistors have such superbly low noise and high time coherence characteristics that there is an immediate, large sonic improvement of the same type as with the Bybee filters, though considerably less dramatic. The very high quality resistor constitutes a parallel path from input signal to ground, and the noise level and coherence of the signal input to the amplifier (at the junction point of the very high quality resistor and the amp's internal load resistor) is significantly improved. The ear-brain system immediately recognizes this, because of having the ability to detect and fixate upon small improvements in time coherence and noise level. Two ultra-quality resistor types have presently been identified for this purpose: Caddock TF-020 (metal-oxide film), and Mills 1w non-inductive wire wound. These are available from Michael Percy. A value of about 100K ohms has been found to work the best so far, with the best results a parallel combination of one 200K TF-020 and one 200K Mills. The net input impedance of the amplifier is changed a little (reduced from 50K to 33K ohms), but this is unimportant in most systems.