ART MPA Gold two channel microphone preamp - JJ tubes installed

ARTThis tube mic amp sounds great and can be "tweaked" by swapping tubes.  The impedance knob seems to do quite a bit, making sources sound richer or thinner, to taste.  All functions seem to be working normally.  It had a little case of scratchy pots when we first got it in on a trade, but that seems to be going away.  It has a lot of cosmetic flaws, scratches on the top that were touched up, rack rash, rack ears that were bent and bent back and dings that are visible on the face panel.  It currently has JJ 12AX7 tubes in it.  Are these the correct tubes for this unit?  We're not sure, it sounds great, but looking at forum discussions, a lot of folks seem to invest in NOS Telefunken tubes.  

A professional tube microphone preamplifier is defined by warmth, presence and clarity - core elements which make a performance truly come alive. Sister to the ART Digital MPA, the MPA Gold performs to incredibly high standards, and that it's tone, functionality and performance will satisfy even the most discerning user.

Like the Digital MPA, the MPA Gold features Variable Input Impedance. Variable Input Impedance is intended to optimize the MPA Gold for the widest range of microphones. Variation of the input impedance of a microphone preamplifier changes the load against which the microphone has to push. This variation dramatically alters the performance of a microphone, and creates numerous sonic possibilities for microphone performance. The variable impedance control of the MPA Gold is infinitely variable, as opposed to steps. This allows users much finer control over the tuning of the impedance.

Features:
Variable Input Impedance For Flexible Microphone Voicing (150 Ohms to 3000 Ohms)

Selectable Tube Plate Voltage

Large VU Meters with Range Switch

Tube Warmth LED Meter

Improved Discrete Class A Input Microphone Preamplifier

Lower Noise At Low Gains, Wider Frequency Response and Lower THD Than our Pro MPA

Front Accessible Instrument Input Jack With Very High Input Impedance

Automatically Switches To Instrument Input When You Plug In

The selectable Tube Plate Voltage function delivers a wide variety of tones. The tube circuit is differential, and provides the equivalent of a 300V power supply on the high voltage setting. Not to mention it has very low distortion and noise. Our normal setting maintains ART's classic tube sound (warm with smooth overload characteristics).

The MPA Gold's unique tube circuitry utilizes two 12AX7A vacuum tubes and renders unsurpassed sonic detail and clarity with the broadest range of microphones. The 12AX7A tube preamplification section provides gain while maintaining the incredible dynamic range of the previous stage and controlled overload characteristics.

Condenser microphones, ribbon microphones, dynamic microphones - new and vintage - all become more versatile and deliver their maximum performance when they interact with the MPA Gold's Variable Input Impedance. You'll discover that your microphones will sound better than ever.

In addition to the VU meters, the MPA Gold has Tube Warmth LED Metering which measures the left and right audio channel levels. The meters act like a normal histogram for the average signal level and the display holds the highest LED on alone for one second. While the average level function attack and release are measured in milliseconds, the peak function has a 20 microsecond attack time. Additionally, the analog meter range is switchable between -10dB and +4dBU. The MPA Gold's High Pass Filter is continuously variable from 15Hz to 150Hz. Many other preamps have a fixed switch. This makes our MPA Gold more flexible audio processor.

The MPA Gold is truly a modern, world-class tube microphone preamplifier, and its audio performance will surely exceed your expectations.

CMRR: >90dB (typical @ 1kHz)

Controls: Input Gain, Impedance, Output level, and High Pass Filter SWITCHES: +20dB gain, Phase Reversal, Plate Voltage, Meter, Sensitivity, Phantom Power with LED switches

Dimensions: 6.5in D x 19in W x 3.5in

Dynamic Range: >100dB (20Hz to 20kHz)

Equivalent Input Noise: -133dBu (XLR to XLR 'A' weighted)

Frequency Response: 15Hz to 120kHz (+0, -1dB)

Input Impedance: 150-3K ohms (XLR), 840k ohms (1/4 in.)

Analog Connections: Bal. XLR (pin2 hot) inputs/outputs and bal. 1/4 in. outputs (TRS), unbal. inst. inputs

Maximum Gain: XLR to XLR 75dB

Maximum Input Level: +20dBu (XLR), +17dBu (1/4 in.)

Maximum Output Level: +28dBu (XLR)

Output Impedance: 47 ohms (XLR)

Power Requirements: 100-125VAC, 25W Export units configured for country of destination

Total Harmonic Distortion: >0.005% (typical)

Weight: 12 lbs.