OLD WORLD AUDIO U33A VARI-MU STEREO COMPRESSOR INSTALLATION AND OPERATING NOTES INSTALLATION & OVERVIEW The U33A compressor utilizes all vacuum tube technology (no solid state). It is transformer balanced in and out and was built out of the respect for some of the 1960's Abbey Road engineer Geoff Emerick's recordings from the Beatles Revolver era. The unit is not a direct clone of any compressor from the past but we feel the essential features of the EMI RS124 are preserved as well as some newer flexibility. The unit is a cross between an Altec 436, EMI RS124, Universal Audio175B and a Fairchild 670. Running channel A into channel B will yield some very interesting pumping a breathing effects. If High Fidelity is in mind it is Stereo Linkable and will behave itself across a mix beautifully. We hope you enjoy the unit as much as we did designing it and recording with it. The unit was honestly a labor of love and we think it adds a certain magic to the recording experience. It should be installed so as to allow free circulation of air around the unit to ensure good cooling. Avoid the use of close fitting cabinets or enclosures. Hand wound Mu-metal input transformers Large M6 steel transformer in side chain Hand wound Mu-metal output transformers Tube compliment (2) 6386, (2) 12BH7a, (2) 6414 This unit requires 110-130 Volts AC 50/60 Hz for proper operation. Input and Output connections are via standard XLR type connectors Transformer Balanced. The system is non-inverting, Input and Outputs are in phase. Connector pin-out is as follows:
1 Chassis Ground 2 Line 1+ In/Out Pin 2 hot 3 Line 2- In/Out FRONT PANEL CONTROLS INPUT LEVEL Two Input level controls are provided, one for each channel. These controls are used to set the input level to the 6386 variable gain amplifier. Push the input and the unit exhibits a very pleasing tone. OUTPUT LEVEL Two Output level controls are provided, one for each channel. These controls can be used to adjust the output line level to a convenient value chosen by the user. THRESHOLD Two Threshold controls are provided, one for each channel. This control determines the degree of compression relative to the applied input program level. Compression ratio increases with the clockwise Control setting. In STEREO mode both channels use their respective Threshold control for setting compression and these controls should be set as close to each other as possible to achieve best balance. ATTACK Two Attack controls are provided, one for each channel. This control sets the time constant used by the variable gain section to initiate compression. The time constant is increased with clockwise Control setting. In STEREO mode, both channels use the Channel A Attack control for setting attack time and the Channel B Attack control is inactive. Faster attack times tame transients, slower attack times allow transients through before the compression initiates. RELEASE Two Release controls are provided, one for each channel. This control sets the time constant that is used to return the variable gain section to the level prior to compression. The time constant increases with clockwise Control setting. In STEREO mode both channels use the Channel A Release control for setting release time and the Channel B Release control is inactive. Slower release times can control, smooth or add sustain the source material. Faster release times can be reminiscent of the drum and instrument
sounds on the Beatles Revolver album. VU MODE The VU MODE switches permit the switching of the panel meters to monitor any of the three conditions shown below. The data below applies to both channels. BAL. This meter position is used to indicate relative balance of the tube sections is the variable gain stage. Normal operation should show no reading off of the rest stop. A positive/ or negative reading may indicate imbalance of the input amplifier. This may require re-balancing or tube replacement. Please refer to the MAINTAINANCE section of this manual for more information. COMPRESS. This meter position indicates the relative compression as gain reduction of the variable gain amplifier. Under normal no- signal conditions the meter should read 0 DB. Some drift due to tube aging may occur and the meter can be reset by means of the front panel controls marked METER SET. A small screwdriver can be used to adjust these settings. OUTPUT LEVEL. This position allows the user to monitor the output line level. The meter monitors the level at the system output connector after the OUTPUT LEVEL control. A 0 db reading corresponds to a level of +4 dbm. (1.22 volts rms) The VU MODE switch is best left in the COMPRESS position once level checks are made in order to preserve system performance. MONO-STEREO This switch selects the system operating mode. When in the MONO position, the system operates as two separate compressors with individual Threshold, Attack, and Release controls. In the STEREO mode, the Channel A Compression, Attack, and Release controls are used for both channels. This is done to maintain the stereo image and prevent position wander. The Channel B Attack, and Release controls are inactive in STEREO mode. OPERATION Because of the wide degree of operating flexibility afforded by the U33A, it is not possible to provide step-by-step operation details. A basic approach would be as follows assuming a program input level of 0 dbu.: : 1. Turn Input Level to minimum ( set to 0 ). 2. Set Threshold, Attack, and Release controls to 0. 3. Set Output Level to 5. 4. Set VU Mode switch to VU. 5. Increase Input Level to obtain desired output level.
6. Switch VU Mode to Compress position. 7. Depending upon program characteristics, some compression may be seen as evidenced by downward meter deflection. 8. The Threshold control can be advanced to allow more compression and/ or the Input level can be increased to get more compression. The Output level can be adjusted to suit your requirement. The output level can be checked in VU position. 9. The Attack and Release controls can be set to provide the desired compression effect. Typical settings might be 0-3 for Attack and 4-7 for Release. Experimentation is always of value to determine best settings for the program material being used. 10. The above settings apply to both Mono and Stereo operation. In Stereo mode only the Channel A Attack, and Release controls are active. In Mono mode, use the appropriate channel controls. NOTE: Excessive input levels can result in the variable gain stage being overloaded (which you may like). A balance between the Threshold and Input level will be found for all program material and judicious use of the Attack and Release times will result in excellent program control. MAINTAINANCE As the variable gain amplifier tubes (V1A,B) age, their plate current may drop as seen by a reduction below the reference 0 VU mark when in Compression mode. The setting can be restored by adjustment of the front panel Meter Set controls. A drop of more than 5 VU may indicate tube emission is becoming too low for proper operation. In this case the tube should be replaced. BALANCING When replacing V1A or B, or if an unbalance is noted by a change in meter reading when in the BAL position, the following balance operation should be performed. NOTE: At balance, with Tone active, the typical signal output level can be 5 dbu or greater. Use caution when audibly monitoring output level. Excessive monitor gain can result in extremely loud tone, which could damage one s ears, monitor speakers, or both. The following procedure applies to either channel. 1. Set VU MODE switch to BAL position. 2. Adjust rear panel BALANCE control for minimum reading. This constitutes a rough balance and can be refined in step 3 below. 3. Carefully set up to audibly monitor output level. Set Output to 0 position (full counter clockwise). 4. Press the TONE button for the channel to be adjusted. This causes a 60 Hz signal
to be injected into the variable gain stage. Adjust the BALANCE control while pressing TONE for minimum output signal. 5. Inability to audibly balance may be due to circuit malfunction or defective V1 gain or V2 output tubes. Replace V1 orv2 and rebalance. SPECIFICATIONS Frequency Response +0/-1 db 10 Hz- 50 khz. Distortion @1 khz 0.04% +10 Output Level. Noise A Weighted -90 db ref from +10 db Output Level Maximum Gain 30 db Attack Time (variable) 200 microseconds to 47 milliseconds (charging time) Release Time (variable) 0.08-1.5 seconds Compression Ratio Variable 0 to 16:1 typ. Input Impedance 1 K Transformer Balanced. Output Impedance 600 Ohms Transformer Balanced. Output Attenuator range 20 db Maximum Output +26 dbu into 600 Ohm load. Supply voltage 110-130 Volta Ac, 50-60 Hz 60 VA. Fuse 1 Amp, Time Delay, 5x20 mm. Size 19 W x 7 (4U) x 14.5 D Old World Audio reserves the right to change Price and Specifications without notice.