VTL ST-85 Valve Power Amplifier

Using the classic workhorse tube that was the foundation of the VTL sound, the EL34 models are studies in compact chassis with short signal paths. These rugged amplifiers are capable of delivering an exceptionally large sweet sound space from any satellite, mini-monitor or small floor-standing system. Listen to what one critic called "the weight of the brass, especially the trombones, is sensational", and "... made me aware of just how good these units are at portraying those subtleties in music reproduction that matter to audiophile music lovers." We accomplished this seemingly contradictory goal through careful engineering... An audiophile: "best buy."

WHY TUBES?

VTL feels tubes are the best technology for audio, for the following primary reasons:

1. Tubes use simple circuits
2. Tubes overload gracefully
3. Tubes are more linear
4. Tubes require less overall negative feedback
5. Tubes are a more viable technology now than in yesteryear, as better components are now available for modern tube amplifiers

Simple Circuitry

In general, passing the signal through a lesser number of components yields a purer, more immediate sound. This is because there are fewer components to change, or distort the sound. Simple circuits also have inherently higher reliability, since there are less of the parts to fail. Tubes are more tolerant of circuit drift and parts deviations, and thus can be used in the simplest circuits.

Dynamic Handling Capability

Music has great dynamic swings, and in tube amplifiers the onset of clip/overload as maximum power is reached is gradual, with a low even-order harmonic.

Even-order harmonic distortion is somewhat benign, and less offensive to the ear than the harsh, odd-order harmonic distortion characteristic displayed by solid-state circuits, even though the distortion figures for tubes are somewhat higher.

Transistor amplifiers generally reach their power limit and clip in a mostly odd-order harmonic, which is more fatiguing to listen to. In such a transistor amplifier the distortion raises very quickly as the maximum power level is reached, with an almost square wave characteristic, and a high DC component, (which can destroy loudspeaker drivers).

As an example of the difference in the distortion characteristics between the two technologies, tube guitar amplifier manufacturers have traditionally designed their equipment to drive the output stages into overload distortion, using the resultant distortion to get the sound they like, also known as "tone". In a tube amplifier this tone contributes to the amplifier's sound, but in a solid-state amplifier this distortion would be intolerable and would destroy speakers.

Linearity and Feedback

Tubes are voltage amplifiers (as opposed to solid-state current amplifiers), with the result that tubes are a more linear amplification technology, requiring less overall negative feedback to make the circuit linear. (Overall negative feedback is a sample of the output of the amplifier re-injected into the input 180 degrees out of phase, and is used to correct non-linearity and distortions.)

Too much negative feedback in general tends to slow the amplifier down and suck the emotion and life out of the music. High feedback designs usually give a sterile and boring, lifeless sound, while low feedback designs give a more immediate sound to the music.

Zero to 20dB of negative feedback is generally considered acceptable, and is usually the maximum amount of feedback needed to make a tube amplifier circuit linear and to keep the output impedance down to an acceptable level. Transistors generally need over 50dB of negative feedback overall.

Power Supply

The higher working voltages present in tube amplifiers generally allows better voltage swing capability and better headroom. This higher working voltage yields higher audible energy storage with a lower value capacitor (audible energy storage is voltage squared divided by 2 multiplied by capacitance) Typically 600 working volts of tube amplifiers vs. 80 working volts of transistor amplifiers. This is most likely why many listeners feel that tubes sound more powerful.

Specifications:
Vacuum Tube Complement: 4 x EL34, 4 x 12AT7
Output Power 20 Hz – 25 kHz ± 0.1dB < 3% THD (Stable to 2W) 80w, 90 w into 8W, 4W
Small signal frequency response (< 0.2% THD @ 1W) 1 - 75 kHz -3dB
Class of output operation: AB1
Inputs / Outputs: Single-ended RCA /1 pair 5 way binding posts
Input Sensitivity / Impedance: 0.7 V / 100KW
Output Impedance: 1.55W
Power Consumption: Idle = 140W Full Power = 500W