RETRO 176, User Manual

Page 1: ...Retro Instruments Inc User s Manual 176 Tube Limiting Amplifier ...

Page 2: ...Retro Instruments Inc 176 Tube Limiting Amplifier User s Manual 2008 Retro Instruments Inc P O Box 5066 Modesto CA 95352 5066 USA 209 810 3344 ...

Page 3: ...n any form by any means without the written authorization of Retro Instruments Inc Printed in U S A User Manual Part Number 176001 01 XLR RCA Xcelite UA are trademarks of their respective companies Retro attempts to provide information that is accurate complete and useful If you require further information or find discrepancies in the text within please contact Retro Instruments Retro Instruments ...

Page 4: ... Interlock 3 2 Amplifier Active Bypass Switch 3 3 Input Level Control 3 3 Output Level Control 3 4 Compression Ratio Control 3 5 Attack and Release Controls 3 6 Sidechain High Pass Filter 3 7 Asymmetry Switch 3 8 Meter Select Switch 3 9 Stereo Linked Operation Section 4 Principles of Operation 4 1 Amplifier Circuit Board 4 1 1 Input Transformer Stage 4 1 2 6BC8 Variable Gain Stage 4 1 3 Interstage...

Page 5: ...n Reduction Meter Zero 5 2 Internal Balance Test Switch 5 3 Balancing the 6BC8 Gain Reduction Stage 5 4 Balancing the Amplifier Stage 5 5 Noise and Distortion Tests 5 6 Frequency Response Tests 5 7 Hum and Microphonics Section 6 Schematics and Drawings 6 1 Board Layouts and Schematics 6 2 Recall Sheet Section 7 Support and Service 7 1 Service 7 2 Support 7 3 Replacement Parts 7 4 Warranty 7 5 Fact...

Page 6: ... tube compression handles dynamics the 176 enhances dimension depth and detail Independent tracks such as vocal bass acoustic guitar piano fill out nicely without harsh restraint The 176 is great for tracking mixing and multibuss compression applications Possibly the best application for the 176 is use on the stereo buss buffing it to a glossy finish The Retro 176 adjusts tone in a way that equali...

Page 7: ...itch that interrupts the AC mains power when the door is opened This switch should not be defeated Only trained technicians should open the front door to gain access to the internal adjustments Tubes should not be removed or inserted with the power on An accidental breakage of the glass envelope can expose high voltage on the plate of the tube This product is grounded through the AC Power Cord For...

Page 8: ...ply transformers If you experience 60 Hz hum in any of your gear try unplugging adjacent gear first If you purchased a matched pair with the link panel mount the panel between the two units and connect the RCA cables to the Couple jack on each 176 2 2 Power Connections 2 21 AC Line Voltage Selection AC Mains Voltage is applied to the IEC standard AC receptacle on the rear of the unit Set the Red A...

Page 9: ...r It is transformer balanced and floating meaning there is no path to ground from pin 2 and 3 Unbalanced sources connect the center conductor to pin 2 and the shield to pin 3 2 4 Output Connection Audio Output is a standard Male XLR connector It is transformer balanced and floating meaning there is no path to ground from pin 2 and 3 Unbalanced loads connect the center conductor to pin 2 and the sh...

Page 10: ...he 176 is primarily grounded through the AC Mains For safety do not disable the AC ground The chassis is also grounded through the rack ears to the rack The XLR connectors are grounded on pin 1 Pin 1 is only used to drain the shield of the audio cable on one end of the cable The exception to the rule is a microphone cable where pin 1 is used to carry ground to the microphone case ...

Page 11: ...des a hard wired bypass of the 176 for instant A B comparisons 3 3 Input Level Control The Retro 176 has a fixed threshold The gain reduction is adjusted by the Input Level control The input is optimized for standard 4 dBm line levels to provide 0 20 dB gain reduction 3 3 Output Level Control The Output Level control matches the Output Level of the 176 to the Input Level or provides less or more a...

Page 12: ...gram controlled time constant with the knob IN makes the Retro 176 gain reduction more natural better controlling dynamics with less distortion 3 6 Sidechain High Pass Filter The Sidechain HPF is a very effective tool to eliminate pumping on low frequency material It allows the compression to work more effectively controlling dynamics in the midrange frequencies Bass frequencies will pass through ...

Page 13: ...mplifier Bypass switch Normally the meter is kept in the GR position 3 9 Stereo Linked Operation Stereo linked operation is generally only desired when the stereo image must not wander For example when dialogue is slightly panned and you are using heavy amounts of compression If the 176 s are unlinked the panning would tend to move to the center For contemporary music recording we prefer the 176 s...

Page 14: ...rovides a distinctive load for the variable gain stage to work into Alternatively the interstage transformer can be bypassed which changes the tone and character of the compression 4 1 4 12AX7 and 12BH7 Amplifier Stage The amplifier stage is a fully push pull circuit The first stage a 12AX7 and the second stage a 12BH7 are closely tied through a dual balanced negative feedback loop This provides g...

Page 15: ...nt to observe balance during gain reduction It allows the technician a way to balance the amplifier without test equipment See the Aignment and Maintenance section for procedures 4 3 Power Supply Circuit Board 4 3 1 Mains Input The AC Power enters the 176 through the IEC power receptacle It travels to the fuse the door interlock switch and the front panel User Power switch Finally it reaches the V...

Page 16: ... 3 or the light dances in the tube causing a rumble or squeal in the audio signal With normal gain reduction this tube will dim slightly This is normal 4 3 5 Bleeder Resistor There is a 270K Ohm bleeder resistor across the first power supply capacitor In the event of a choke failure the input filtering capacitor will discharge Under normal conditions the power supply will not carry residual voltag...

Page 17: ...etry switch should have no effect on the meter zero If it does the 6AL5 tube may be at fault 5 2 Internal Balance Test Switch The Internal Balance Test Switch has three settings For normal operation it must be in the center position The Top position static test is used to set VR2 and VR4 The Bottom position dynamic test is used to set VR3 The Meter Zero VR1 may have to be trimmed after balancing 5...

Page 18: ... 0 Read the residual noise on the analyzer It should be less than 76 dB If it is not remove the 6BC8 and see if the noise remains If the noise vanishes suspect the 6BC8 tube or 0B2 tube If the noise remains suspect the 12AX7 or 12BH7 Distortion can be measured by feeding 4 dBm signal into the 176 Adjust the Input Level for the desired amount of gain reduction then measure the distortion at the out...

Page 19: ...ke a tink or bonk sound or nothing at all An excessively microphonic tube will make spastic sounds when tapped which can help isolate an intermittently noisy tube The 6BC8 will always exhibit some microphonics because it is the first stage of amplification and has a relatively tiny signal passing through it ...

Page 20: ...Section 6 Schematics and Drawings 6 1 Board Layouts and Schematics ...

Page 21: ...ent by telephone at 209 810 3344 or contact us through our service page at http retroinstruments com We will give you instructions how to return your product for service Please fill out the Factory Service Form and include it with the product return 7 2 Support If you have a question or concern about your product you can contact our technical support at 209 810 3344 Monday through Friday between 8...

Page 22: ...uments within 45 days of the date of purchase Retro Instruments neither assumes nor authorizes any representatives or other persons to assume for the company any other liability in connection with the sale or any shipment of Retro Instruments products There are no other warranties expressed or implied given by Retro Instruments extending beyond or different from the express warranty stated herein ...

Page 23: ..._______________________ Company __________________________________ Shipping Address __________________________ City State Zip _____________________________ Phone Number ______________________ e mail ________________________ Model _____________________ Serial Number _________________________ Describe Problem Special Instructions ...