audio research

Tube stereo power amplifier.



VT100 MKII (Spring 1998)

From the ARC website

The VT100MKII is a new all-tube, balanced stereo power amplifier that draws upon the advanced technology of the flagship Reference Series models. Rated at 100 watts continuous output per channel, the VT100MKII is a Class AB, differential circuit design using two matched pair of 6550 output tubes in each channel, along with four 6922 twin triodes per channel for input and driver stages. A fully regulated, 540-joule power supply is distributed between two separate, isolated output circuit boards for short signal paths and true "dual mono" operation back to the massive main power transformer. Constant-current sources in the input stage help maintain stable drive
characteristics and differential balance under the dynamic conditions of music reproduction.

Ultralinear output transformers and partial cathode coupling to the output terminals give the VT100MKII higher efficiency and lower output impedance than conventional designs for better drive into a wide variety of loudspeaker loads. Inputs include both single ended and balanced. Output terminals are custom-machined, heavy duty, gold-over-brass posts. A spacious chassis and single cooling fan help maintain conservative operating temperatures.

VT100 MKIII (Fall 2001)

From the ARC website

Since the spring of 1996 the VT100 established itself as the benchmark amplifier in its price range. Accolades from the press and music lovers grew even louder with the 1998 introduction of the MKII because of its improved dynamic swing, more refined performance, and ability to drive so many "difficult" loudspeakers which normally could not be used with other vacuum-tube amplifiers. Well, the VT100 in MKIII iteration will shake up the audio world once again and assure that it remains the best-selling vacuum tube amplifier in the world.

The advancements derived from the development of the remarkable VTM200 monoblocks have been so large that we feel compelled to integrate many of them into the VT100 through a MKIII update. This is a major step forward in sonics, reliability, and ease of use.

Externally, the only changes are to the back panel where we have added two welcome features. There is now a Balanced/Single-Ended switch, eliminating the need for shorting links in the XLR balanced connectors when using single-ended RCA cables. We have also incorporated a (dual in/out) 12V power on/off trigger for remote or custom installation purposes. (The soft-start surge-control feature of the VT100MKII power circuits is preserved.) Now you can link the VT100MKIII to the LS25MKII for remote operation!

Internally, the circuitry of the VT100MKIII is derived from the VTM200. Like its more powerful sibling, the MKIII's input stage utilizes direct-coupled JFETS for ultra low noise, and is followed by a 6H30 driver/gain stage, capacitor coupled to a 6H30 follower, driving two matched pairs of 6550C output tubes per channel. The conservative 6H30 driver-follower circuit will help prolong the life of the 6550Cs, and the 6H30 is rated to handle higher currents than the previously used 6922, so it has lots of safety margin for even better reliability. (The 6H30 twin triode is a Russian military tube in current production, with an estimated service life of up to 10,000 hours.) There are now four internal bias adjustments-up from the previous two- to allow more accurate DC balance adjustments for the output tubes. The line fuse has been increased to a 7 amp in 120V units (6.3 amp in high voltage units), allowing the MKIII to handle musical dynamics with greater ease.

Simply put, the VT100MKIII is a major sonic advance. As the original VT100 was greatly improved upon by the MKII, the MKIII eclipses it again by the same margin. Resolution and dynamics are startlingly better: suddenly you can pick out each individual voice in a choir, you can hear the air gently modulated by a hundred diaphragms singing in unison in a great cathedral. Small and large details stand out in bolder relief, tightly focused, wonderfully holographic. Bass is tighter and more dynamic. It sounds as though the power has been increased, with a more effortless quality. The MKIII is easily recognized as having been borne of the VTM200. Lastly, the single-ended RCA input is almost indistinguishable sonically from the balanced-XLR input.

The MKIII update is available for older units and is quite extensive: both left and right main boards are replaced, 6H30s replace the 6922s, and the rear panel is modified (and a sub-board added) for the 12V trigger and XLR/RCA switch. Audio Research will provide one additional year of warranty service to updated units (90 days on new tubes). Please call in advance to schedule an update.


MSRP (US new retail)
$4,995 (VT100 MKII)
$4,995. Increased to $5,995 on Feb 5 2001. Increased to $6,495 on Jan 01 2004. (VT100 MKIII)


Power output 100 watts per channel continuous from 20Hz to 20kHz. Approximate actual power available at "clipping" 110 watts (1kHz)
Power bandwidth (-3dB points) 15Hz to 80kHz
Frequency response (-3dB points at 1 watt) 1Hz to 100 kHz
Input sensitivity 1.9V RMS (Bal or SE) for rated output. (23.5 dB gain into 8 ohms.)
Input impedance 200K ohms Balanced, 100K ohms Single-ended.
Output taps 8 ohms, 4 ohms
Output regulation Approximately 0.8dB 8 ohm load to open circuit
Overall negative feedback 10 dB
Damping factor 11
Distortion 1kHz total harmonic distortion typically 1% at 100 watts, below .05% at 1 watt
Slew rate 15 Volts/µs
Rise time 3 µs
Hum & noise Less than 0.5mV RMS 95dB below rated output (IHF weighted, input shorted)
Power supply energy storage VT100: 270 joules.
  VT100 MKII/III: 540 joules
Power requirements 105-125VAC 60Hz (210-250VAC 50Hz) 585 watts at rated output, 750 watts maximum, 380 watts at "idle"
Tube complement VT100: (4 matched pairs) 6550 power output, (8) 6922
  VT100 MKII: (4 matched pairs) 6550C power output, (4) 6922 driver, (4) 6922 input.
  VT100 MKIII: (4 matched pairs) 6550C power output, (4) 6H30
Dimensions 19" (48.3 cm) W x 8.75" (22.2 cm) H x 19.5" (49.5 cm) D. Handles extend 1.5" (3.8 cm) forward
Weight 65 lbs. (29.5 kg) Net; 83 lbs. (37.7 kg) Shipping.


Issue 111 (VT100)
Vol. 20 No. 3 (VT100), Vol. 21 No. 12 (VT100 MKII)