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Mimì two way compact speakers

Many hears ago, in a Hi-Fi show, I listened the ProAc Response Two speakers and I was really impressed by their performance. These speakers just simply disappeared, leaving behind an ambient soundfield that changed very little with head movement. Driven by an excellent Audio Reserch tube pre/power amplifier, they had put the singer right here in the room with me!! Also all acoustic instruments like the guitars, violins, percussions were played with a breathtaking sense of 3-D realism. In last September I saw among ads sale two 6.5″ Scanspeak 18W8543 polypropylene bass/midrange drivers and two ¾” dome tweeters D2010  Scanspeak and remembering the previous ProAc listen experience ( Response two speakers were equipped with these drivers ) I thought….mmh…..why not?


To build the cabinet I choose 19 mm thick marine type plywood. To compensate the time delay tweeter-woofer emission I adopt a sloping baffle. Furthermore, I put  a 30x30mm battens in the internal rear perimeter to allow the removable rear panel ( to adjust crossover, damping and so on ). A 19x80x200 mm bracing is put between the two sides to minimize the box vibrations and silicone sealant on all internal edges to seal any micro cracks. The single front-mounted tuning port is a slot 12 mm height and width as a baffle ( 200 mm ). The total length is 140,2 mm. As you can see in the photos, the internal sides ( excluding the front panel and the bottom ) are coated with 30mm dacron fibre mat. In the tweeter cell crossover I suggest to use 5 W non inductive resistors  and MKP high quality capacitors as Mundorf, AuriCap, Multicap or similar. The coil for woofer cell MUST BE air wrapped, 1.4 mm Cu to minimize series resistance. For the other coil is sufficient 0,5-0,6 mm Cu diameter. The red soft carpet put in the baffle smooth out the 6 mm speaker edges.






Attila 6C33cb SE Power Amplifier


The 6C33C-b is an attractive tube. This indirectly heated, high transconductance, high current capability and low internal resistance, inexpensive triode, from Russia, with a large 60W anode dissipation, originally designed as a voltage regulator for the avionics in Russian military aircraft, it is the ideal “high current” triode for audio applications.

The downside? The 6C33C-B requires a relatively high anode current (typically 180-220mA at 180-210V) a high heater current ( 3,6 A with filaments connect in series or 7,2 A with parallel connection). For this reason the tube runs very hot ( 250-300°C ) and a ceramic socket and adequate ventilation are required. My choice in this project is to use this tube in SE ( Single Ended ) Class A configuration without negative feedback. Due to relatively 6C33C-b low plate resistance, with a 600 Ohm primary impedance output transformer we can obtain 15 W rms at 200 V anode voltage and 200mA of plate current ( 40 W anode dissipation is a good safer choice for a 60 W max tube ). To adopt a 600 ohm output transformer with a very low turns ratio and, as a consequence, extended frequency decade factor (as compared even to the most advanced modern vacuum tube amplifiers), as well as dramatically reduced leakage inductance.

The 6C33C-b needs a very large voltage swing at the grid input to produce 15 Wrms at output side. It’s very important the choice of input/driver tube type and configuration for best sound. I spent two months for testing various front-end tubes and topologies:

At last my choice was for the circuit below:


It’s very simple with a very short signal path but with outstanding performance. The Lundahl LL9201 input transformers, offers a galvanic isolation from the upstream stage, a signal gain ( due to 1:2 configuration ) and lastly but not last  it allows to adopt a fixed bias per the 6SN7GT. I love this old double triode due her exceptional linearity and sound. Each triode has a raw amplification factor of 20 at a moderately low plate resistance of about 7,7 KOhms.

I remember this valve in the famous Williamson Amplifier  the first true hi-fi design. I try this tube with both triode in parallel. In this configuration the plate resistance halves ( 3,85 KOhm ), a must for transformer load to obtain the best low frequency response. As interstage transformer I use a custom made 1:1 from AE Europe, 5 Kohm on both sides, 50H (nominal ) of primary inductance  ( ). 6SN7GT( paralleled ) plus AE transformer, driven under a 15mA current ( 7,5 mA per triode ) at 380 V anode voltage, provide an impressive set of performance: up to 50Vrms ( 140Vp-p ) of output voltage with a very low harmonic distorsion, wide flat bandwith. This stage is perfect to drive a tank-tube as 6C33C-b.

The output transformer is the beautiful, heavy ( 5,2Kg ) James JS-6130 HF. It is rated for 30 W power output with a primary DC current max of 250mA. Primary impedance is 600 Ohm and 3,26 H of inductance. Bandwith at -3dB is extended from 14,3 Hz to 148Khz.


If made in a single chassis this amplifier will be very heavy due to many transformers and choke. This raises two choices: two mono amplifiers or separate power supply from the amplifier chassis. I adopt this last layout because I only have one power transformer for input stage and one for 6C33C-b output stage. Power supply and amplifier it joined with a big military type connector. Please note the last big MKP 330uF reservoir capacitor in the 6C33cb power supply is put in the amp chassis very close to output transformer. Also 33uF MKP capacitor is put very close to interstage transformer and 6SN7.

In the power supply, I adopt a vacuum tube rectifier ( 5V4G ) for 6SN7GT input stage. The first capacitor after 5V4G rectifier MUST be paper in oil type. In the schematic you can see a LCLC type filter for best ripple reduction followed by two other cells LC to separate right, left channel power supply line. Due to high current ( and voltage drop ) of 6C33C-b output stage, vacuum tube rectifier are not recommended. Here a solid state bridge MUR860 600V, 8A ultrafast rectifiers is for me the best choice. After the first 33 uF MKP capacitor, the power supply is splitted in two sections for Left and Right channel. Here I use a LCLC filter with two double 3H chokes per channel, and MKP capacitors to obtain a very low residual ripple. As you can see in the photos, the power supply chassis is made in two levels: in the downside is put the 6C33C-b part ( HT plus filament transformer ) while the upper level houses the 6SN7 power supply and timer. Please note in the 6C33C-b power supply the double choke wound in same core to improve inductance up to 12H and ripple rejection  ( note the phase point in the positive and negative leg ).

Bypass MUR 860 diodes with 1,5-2,2 nF-1000V Silver Mica or Ceramic capacitors. In general I like fixed bias for tubes in each stage but here in the 6C33C-b stage is indispensable to avoid matched tubes, high dissipation in the cathode resistors, and a higher anode voltage for compensate the voltage drop in these resistors. The 6C33C-b has huge inconsistency of parameters when it is new, so it is very difficult to measure the parameters. The reason that these tubes show varying characteristics is not because of bad quality manufacturing but is that as the 6C33C-b has very high transconductance the internal spacing between the electrodes is very small and therefore even a tiny difference in the spacing will have a big effect on parameters. It’s useful to make bias supply available from minus 50V to minus 120V. Biasing resistor that goes to the 6C33C-b grid should be the best possible quality and has value less than 100K. For grid stopper resistors use carbon composite type Allen-Bradley or similar and Blackgate or Elna SilmicII capacitors in the negative side of bias. Although the 6C33C-b marked for 60W plate dissipation, I recommended a plate dissipation 40 W for a full tube ( both halves paralled ). To protect the ceramic tube sockets and maximize air exchange, the sockets are mounted floating on stand-offs, and a forced ventilation with two fan at downside of chassis put in axis of power tube help to dissipate heat. These two 12 V fans are powered at 6V DC to reduce speed and noise.

6C33C-b burn in: when you take as brand new tube, heat its filaments up for 2 minutes and start the amp with 20mA on plate. In 10-15 minutes the plate current will rise to 150mA – let it be this way.  Burn the tune for another 3-6 hours with 100mA-150mA and then consider it ready to be use at full power.
This amplifier employs a relay-controlled timer to allow the tubes time to warm up and stabilize. These circuits provide a 3-minute delay to pre-warm the output tubes. The capacitor Ct set the delay time ( for 3 min,  100uF is the correct value ).

Except for the negative bias circuit and the time delay put in a common board, I adopt the point to point (PTP) wiring technique. All circuit connections are made directly between components, using the leads of those components themselves and very little wire. In the PTP amp, the signal path is usually as short as possible—which, when done right, can help minimize interference and noise in the circuit and the circuit flows logically from input to output. The circuit itself, therefore, tends to look very much like the schematic diagram from which it is built.


When we “measure the bias” we actually measure the voltage drop across the cathode resistor of that tube. In the output stage, check and adjust the bias voltage every 10 hours during the next 50 hours break-in with a millivolmeter across a 1ohm 1% resistor ( TP2 ). I recommend keeping bias voltage at 200mV DC ( 200mA in the 6C33C-b ) for best accordance tube life/output power. In the input/driver stage 150mV across 10 ohm resistor is the correct bias value ( 15 mA in the 6SN7GT group ). Both bias measurement resistors are put in short by push button normally closed P1 and P2: these resistors are insert in the circuit only when we test bias.

About listening experience I found this amplifier is a quite spectacular full bandwidth ‘muscle’ triode amps. Dynamic attacks are in the first class amplifiers but also delicate nuances like female voices or violin strings are played as the best 2A3 tubes. If midrange is true direct-heated triode without the excess bloom, bass has the speed, grip, tremendous impact and shocking growl that lives somewhere between a good transistor amp for transients and control and the saturation of a high-power p/p valve machine. Iron and velvet: This amp sings freely and let’s go with a flourish.




New Crossover for K2 Klipschorn clone

During the last year of listening my K2 Klipschorn super clone speakers (info and specs here) I put the attention on some point to upgrade. I focused my attention on the crossover frequency on both low-mid and mid-high cells. After many weeks of listening session I found some benefits to lowered both crossover frequency.

The inductor in series to K33e woofer is now 8,2 mH ( old value 3,6mH ) put in a separate plastic case very near the woofer, and the capacitor in series with Gauss HF 4000 driver is increased now to 68 uF ( two paper in oil capacitors 66uF + 2 uF in parallel ).


The mid-bass region is now clearer and the dirty, muddy of K33e break up cone less unpleasant. The 0,25 mH inductor in the mid cell lowers the cut of frequency of mid-high range and the 3636 autotransformer is set to -6dB of attenuation for the Gauss HF 4000. In the high section, I increased to 1 uF the capacitor and I put a 8 ohm L-pad for the tweeter Coral 104 to optimize the audio level.

I found some benefit to adopt the dielectric polarization technique in this section, instead of 1 uF capacitor I put two 2 uF capacitor in series with a +9V bias pole battery in the common point ( see JBL, Infinity…, on their best crossover a DC bias is used on capacitors to reduce the effects of dielectric absorption ).

The Red Priest ( Vivaldi )

title3My line reference SRPP ( Series Regulated Push Pull ) fitted with NOS Philips ECC186SQ as voltage amplifier and 5687 for low impedance high current output buffer. Power supply is a fine all tube Shunt Regulator. ECC186SQ is the best ECC82 family tube ever made. The crucial mid range ( male/female voices ) is astouning but unfortunately …this tube now It’s very expensive !! All capacitors are MKP or Paper in oil, carbon composite resistors in the signal path, double C-Core power transformer and choke PSU.
Schematics…..coming soon.

Madama Butterfly

Clipboard02Another 2A3 SE amp !!!! oh no !!!!

What in the added value in my implementation?
What I present in this blog is the result of many, many….. hours of listening sessions to various configurations and components about a SE class A 2A3 Amplifier. My target was the best design to reproduce the body, sonic texture and fine gradation characteristic of natural sound. This is what, in my opinion, is the best result in terms of listening pleasure, that I obtained. First of all, I think is useful adopt the monoblock concept for optimize power supply and internal wiring ( layout), furthermore ensures short cable between amp and speakers. The starting point for any DHT-SE tube amp in the couple audio output transformer- DHT power tube. In this critical stage I choose the well reputed excellent Tamura F-2007 SE output transformer: 3Kohms primary impedance ( 100mA ), 4-8-16 ohms in the secondary side and Shuguang Treasure 2A3-Z as power tube.
About the topologies I test various type starting with the classic Loftin-White direct coupled, RC coupled with various tube front end. I try out pentode ( 6SJ7 ) or triode ( 6J5 and various other ) in the first stage. In the RC capacitor coupled schema I test several type of capacitors : paper in oil NOS or new production, polystyrene, polypropylene and so on. At last I choose the IT interstage transformer coupled topology with fixed bias for DHT power tube.
 As input/driver I found that the old pentode EF6 in triode mode strapped goes very well with Lundahl LL 1660 10mA interstage transformer. The high primary inductance of 130 H ensure good low frequency response. To get out the best performance from couple EF6/Lundahl 1660 I thought to supply it with a fine tube CCS ( Constant Current Source)/voltage regulator.
Being very simple in circuit design, every part is important when building quality DHT SE amp. You should know the tonal characteristic of the part you are using. For my ears carbon composite resistors ( Allen Bradley 2W ) and paper in oil ( or polypropylene in oil ) in PS give the best audio natural performance. In power supply, I want to try TV damper vacuum diode as rectifier for their excellent specs about voltage drop, huge peak currents, and slow warm-ups (about 30 seconds). Furthermore TV damper diodes are the quietest from the viewpoint of switching noise. Power supply for HT is a fine CLCLC schema. The fist capacitor ( 4 uF) MUST be paper in oil the other polypropylene in oil or MKP. For wiring in the signal path use thin solid core copper ( max 0.4mm in diameter ) teflon or cotton insulated. Only for filament supply thicker stranded wiring can be used. In the EF 6 cathode R-C group I think the best choice is Black-Gate or Elna Silmic II and a Allen Bradley Carbon composite 2 W resistor ( or Mills wirewound ). About the schema layout It’s good thing also providing separated ground returns for both low-level ( SG ) and high-level ( PG ) as in the schema below. To reduce hum I adopted a LM317 based regulated slow turn on power supply for EF6 filament. The N ch mosfet souce follower in the negative bias circuit keep constant the 2A3 grid voltage even if a little grid current flow.

DUAL 1019 Naked

The Dual 1019 without doubt is one of few finest turntable ever built. It first began production in 1965 and is a fully-auto, idler driven with integral tonearm and pitch control and heavy massive platter ( 3.4 kg ).
The mechanics are outstanding…
This electromechanical masterpiece justify the adoption of a marble throne…
On the bottom base of 430×375 mm and 20 mm thickness yellow egyptian marble are screwed three 80 mm height solid wood turrets plus 3 mm thickness aluminum turnable supports.
This turnable can hold two gold plated RCA connections like this.
I finally replaced the old Dual wire with more flexible tonearm pure litz copper cable 45×0.005 mm cotton sheath and add a green ground wire screwed to the metal chassis. On the other side I have removed the spring contacts on the shell and soldered the four litz wires at gold plated cartidge clips.


Clipboard01nnnnI often want to listen my favorite music at nighttime without disturbing the neighbors….In the modern apartment with thin walls it’s impossible…….Yes I think I need a good headphone and a suitable amplifier.
But common low cost op-amps found in most consumer audio equipment just don’t have the grunt force and the finesse to actually let the headphones do what they do, which is get out of the way and deliver great music. Well, this was a first for me, building an headphone amp, so I decided I’d prototype an amp and see how it goes. The goal was as cheap as possible, with the minimum of hassle with the focus on the circuit and sound, not a final product so to speak. Here’s how it went. Vacuum tube circuit, have a simplicity unbeatable from any transistor amplifier, however, there are not many projects that use a single active component ( minimalism philosophy ).
This is a single ended transformer coupled amplifier that employs a single pentode tube D3A as active device. There are other excellent candidates : E55L, EC8020, 3A167M, 437A but all are more expensive. Normally a pentode in triode strapped reveal unlinear curve. Not so with D3A. This tube in triode mode give excellent data: amplification factor=70, trasconductance=46mA/V, plate resistance=1.5k; plate dissipation 4,5W and very linear anode curve. The perfect choice for my cheap transformer load, compact, easy-to-building, SE headphone amp.
Take a look at schema: I chose the fixed bias for D3A. Put a DC millivoltmeter across a 1 ohm resistor ( TP ) and turn VR bias trimmer until you read 16 mV ( 16 mA in the tube ) with 170 V in the anode. I noted that this is the operating point for the “right sound” with astounding results in terms of transparency and musicality ( low THD at measurement ). If you want, you can easily use the VR pot for little change and find your optimal “sweet spot” . In this application I chose Lundahl LL1689/18mA as output transformer in ALT/R ( 18:1 ) configuration ( ). A primary inductance of 90 H ensure good low frequency response. Use alternative configuration ( 18:4, 18:2 ) for best match your headphone impedance. Put a GS ( grid stopper ) carbon composite 470-1K ohms resistor on tube grid pin to prevent nasty hf-influences. In power supply I suggest to use tube rectifier like 5Y3GT followed by LCLC type regulator. For the lowest hum and best L/R separation use a Mosfet capacitance multiplier stage. The 0,5 uF capacitors in the schema must be the highest possible quality ( polystyrene or polypropylene type ). About the power transformer the secondary windings must have: 240+240 V at 100mA for HT, 6,3 V at 1 A for 6C45Pi filament, 5 V at 2 A for 5Y3GT and 5 V at 100mA for negative bias.
Sound quality
This really was a big surprise to me! The amplifier has a most detailed, warm and very good balanced sound. Voices are very fine lifelike and excellent dynamics attacks. You can listen to this amp for days and days without your ears getting tired – music comes through fast and accurate without any buttery soupiness, but still open and airy without a hard edge. So for little money one can listen to expensive tube sound!
4053-026542             noctu4053-026542
As mentioned above there are other excellent tube candidates : E55L, EC8020, 3A167M, 437A and so on.. but all are more expensive. I found a small triode with excellent data: amplification factor=52, trasconductance=45mA/V, plate resistance=1k; plate dissipation 7,8W the 6C45Pi. In this case the audio output transformer must have a 5Kohms primary impedance and a secondary that well match the impedance of your headphone. A good choice can be Sowter 9351 single ended output transformer ( ): it has a 5K ohms primary and 4 separate secondary windings configurable for 40-150-300-600 ohms. Adjust the bias trimmer for 35 mA in the tube with 170 V in the anode. I noted that this is the best operating point for 6C45Pi. If you want, you can easily use the VR pot for little change and find your optimal “sweet spot” . 6C45Pi has 2 grid pin connection ( 2, 8 ). Use a GS ( grid stopper ) carbon composite 470-1K ohms resistor on each pin to prevent nasty hf-influences. The power supply may be the same of D3A version or adopt a more cheaper type as tube rectifier 5Y3GT followed by CLC type configuration regulator. First capacitor MUST BE paper in oil type 4 uF/ 400-600V.

K2 Klipschorn (super) klone…

This is my reference speakers……
I heard hundreds of speaker systems but at last….I always return at my Klipschorn super clone. Why super ? OK bass bin is almost identical to original Klipschorn, also fitted with original K33E Klipsch woofer but mid-high section is very different.


I use high-end expensive component like Gauss HF4000 as midrange assembled with JBL 2350 aluminum horn. Gauss HF4000 2”compression drivers are very impressive components: heavy (13 Kg ), alnico magnet, large aluminum diaphragm, wide frequency response ( 500 to 15.000 Hz ) excellent matched with JBL 2350 40×90 radial horn with 220 Hz taper cutoff frequency. This excellent heavy ( 16.3 Kg ) aluminum horn measure 80,3 x 50,8 x 20,3 cm and so match very well with size of bass cab Klipschorn. For high range I chosen the Coral H-104 horn tweeters.
This big ( 3.5 Kg ) alnico tweeter is mounted in the same vertical plane of the Gauss driver diaphragms for best time alignment mid-high response. The crossover filter is an variant of Klipsch type-A. I choose first order slope crossover for his best sound. I tried various crossover type also electronic ( tube and solid state ) but I returned back to the simplest 6dB/oct in the schema. Be carefuI on quality components: for mid-range I use paper in oil capacitors, in the high section copper MKP I found has more detailed sound. 10 Ohms resistor must be metal/oxide 1%, 10W. The 3,6mH inductor for low pass filter must be high quality, low RDC, ferrite ( or best transformer ) core with almost 1,4 mm copper section.
Mid range level , due very efficient Gauss driver, must be attenuate with an autotransformer ( like model 3636 from B&K Sound ). In my system I use pin 0-5 as input and 0-3 as output. These speakers have a first rate SPL factor. 3 W of 2A3 SE tube amplifier, are sufficient to produce a powerfull, open, deep sounstage. It is very difficult for any speaker to play music as a live session….. these speakers are among the very few that succeed in this uneasy task.

Die zauberflote‭!!‬…

Zauberfline2uuuuuuuuDie zauberflote‭!!‬…it‭’‬s‭ ‬a magic flute now I thought when I listened for first time this DAC.
‭ ‬Yes,‭ ‬the original Musiland monitor‭ ‬02Us was a honest good cheap USB DAC but nothing more. ‭ ‬All low cost DAC or CD player use a modest analog op amp based output stage.‭ ‬But a quick look in the schematic and specs DAC I saw a very good digital part‭ ‬:‭ ‬high speed USB interface,‭ ‬SNR and dynamic range of‭ ‬113dB,‭ ‬good PCM1793‭ ‬DAC,‭ ‬excellent‭ ‬Windows driver and so on.‭ ‬Well,‭ ‬this is a good,‭ ‬true‭ ‬24‭ ‬bit/192kHz digital device.‭ ‬So it‭’‬s worthwhile a serious heavy upgrade:‭ ‬a tube push-pull output stage.‭ ‬In this role a good candidate is the Russian tube‭ ‬6H30:‭ ‬very linear,‭ ‬low plate resistance,‭ ‬adequate gain‭…‬a good choice for transformer load stage. ‭ ‬The tube new circuit must replace the op-amp output stage on the Musiland board taking the input signals as in the schema below.‭ ‬CCS‭ ( ‬constant current source‭) ‬on the cathodes‭ ‬6H30‭ ‬is based on DN2540‭ ‬N channel depletion mode vertical mosfet.‭ ‬With the source resistor in the schema the current is set to‭ ‬40mA‭ ( ‬20mA per tube‭)‬.
‭ ‬GS is a carbon composite gate stopper resistor:‭ ‬any value from‭ ‬220‭ ‬and‭ ‬1k ohms 0,5W is OK, for the other resistors use 1% precision metal or carbon type 0,5W.‭ ‬50‭ ‬ohms multiturn trimmer allows to balance current in primay push-pull transformer‭ ( ‬e.g.‭ ‬set the equal‭ ‬6h30‭ ‬anode voltage‭)‬.‭ A poor DC current balance might saturate the core and reduce the inductance of the windings ( worse low frequency response ). ‬Power supply‭ ‬it‭’‬s a very fine tube rectifier,‭ ‬choke input circuit ( choke-fed supply are much quieter due to the choke slowing down the rate-of-charge of the main 47uF cap ) and‭ ‬need a transformer with‭ ‬3‭ ‬windings:‭ ‬260+260V at‭ ‬100mA for HT,‭ ‬5‭ ‬V at‭ ‬2A‭ (‬min‭) ‬for‭ ‬5Y3GT filament,‭ ‬6,3‭ ‬V at‭ ‬2A (min‭) ‬for‭ ‬6H30‭ ‬filament. ‭ ‬Note in Power supply schema,‭ ‬after first‭ ‬47uF cap,‭ ‬the anode voltage are splitted for best R-L separation performance.‭ ‬All capacitors used are MKP type‭ ‬bypassed with‭ ‬0,1uF polystyrene or silver mica.‭ ‬Don‭’‬t forget to ground one leg of‭ ‬6H30‭ ‬filament to reduce hum.

Sheherazade dsd-pcm Sabre32 DAC….

Sheherazade dsd-pcm Sabre32 DAC….

This is the ”reference” DAC in my audio system. It’s based on Twisted Pear Audio ( ) Buffalo III SE board and consist in my implementation for best use this module.
The heart of Buffalo III board is the ES 9018 ESS Sabre32 Reference DAC chip. Whith its amazing performance is probably the best DAC actually available. It can handle 32 bit PCM data via I2S input as well as DSD or SPDIF data with DNR of 135dB and THD+N of -120dB. To obtain a USB port and connect a personal computer as player for high-resolution files I use the Amanero Combo 384 module ( Amanero Combo 384 module support a sample rate of 44.1Khz 48Khz 88.2Khz 96Khz 176.4Khz 192Khz 352.8Khz 384Khz with output format I2S 32bit or DSD64, DSD128, on Windows DSD256,DSD512 too. For Mac OSX 10.6+ and Linux with UAC2 compliant kernel no drivers needed otherwise you can download KS/Wasapi/WDM/ASIO Drivers for MS OS XP to Win8 32-64bit. The Buffalo III SE board need a power supply of 5,25 V at 440 mA. I tested various type of power supply and in particular it’s “sound characteristics” and at last I adopted a mosfet shunt regulated type equipped with Schottky diode as rectifiers. The most important feature of this DAC is the adopted two audio transformers to match the output signals. The Buffalo III need an I/V stage buffer to match the differential current output from ES 9018 DAC. I tried various circuit and device ( tube, opamp, fet ) but the best performance I obtained with Triad HS-56V audio transformers, in 600:600 ohms configuration, connected directly in the +, – pin out on the Buffalo board. No DC current flow in primary side due the equal DC voltage on the +, – DAC sides. Usually a I/V low value resistor is put on transformer primary side but after various listening test I prefer to omit I/V resistor. About the sound, I love this DAC. Very dynamic, open, spacious, wide and deep sound stage, realistic voices even with DSD, PCM high resolution files and also 16/44 khz music from CD. All kind of music is played at best quality very, very close to the best vinyl set up.