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USER’S GUIDE 20

ELECTRONAUT COMPANY

M97 COMPRESSOR/LIMITER

UNDERSTANDING THE EFFECTS OF

THE AC AND DC THRESHOLD CONTROLS

Not all variable-mu compressors provide(d) the means to adjust the
DC threshold. The Fairchild 660 and 670 did provide such a control,
although it was relegated to the rear panel with the assumption that a
technician would calibrate the setting, and the audio engineer would
work within the range set by the technician. However, moving the DC
control to the front panel enables the audio engineer to expand the
range of operability to their liking.

To understand how the DC and AC THRESHOLD controls a

ff

ect the

limiting action, consider the waveforms shown in Figure 25.

WAVEFORM A

represents a signal with an average level occurring most

of the time and a peak transient happening occasionally.

WAVEFORM B

represents the e

ff

ect of the AC Threshold, which

controls the overall amplitude of the signal reaching the controlling
amplifier (thus setting the amount of limiting action), and the DC
Threshold, which e

ff

ectively blocks out the average part of the

waveform, leaving only the peaks.

WAVEFORM C

is the result of the settings in B — no signal reaches the

controlling amplifier (and thus no limiting action occurs) except during
the duration of any peaks that exceed the DC Threshold.

The DC and AC threshold controls work interdependently; by varying
the AC and DC Threshold controls it is possible to obtain peak limiting
without a

ff

ecting the basic program material dynamic range; i.e. - it

is possible to calibrate the unit so that basic program material is not
compressed more than 2dB while spikes from snare drums and the
harmonics from di

ffi

cult to control French horns are controlled to

within 6dB of average program; or it is possible to compress in the
ratio of 10:1 or greater by slowing the attack time ratios so there is an
apparent increase in program level of 10 dB or more.

DC THRESHOLD

(CLOCKWISE INCREASE)

AC THRESHOLD

(CLOCKWISE INCREASE)

A

B

C

Average

Peak

Figure 25 —

The DC and AC THRESHOLD controls a

ff

ect the audio that reaches the

Controlling Amplifier, and therefore a

ff

ect the type of limiting behavior.

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Содержание M97

Страница 1: ...M97 COMPRESSOR LIMITER USER s GUIDE Serial Number Built By Build Date Ship Date Line Voltage set to 120 240 ELECTRONAUT COMPANY Revision 0 January 2016 ...

Страница 2: ...UT COMPANY M97 COMPRESSOR LIMITER SERIOUSLY Read this entire manual before plugging in and turning on the M97 Refer servicing to qualified personnel only To prevent electric shock do not operate the M97 near water or moisture ...

Страница 3: ...EC POWER INLET SWITCH 10 INSTALLATION 11 AIRFLOW AND THERMAL CONSIDERATIONS 11 CALIBRATION 11 POWER UP the M97 11 CALIBRATING THE B VOLTAGE 11 CALIBRATING THE BALANCE 12 THE BALANCING SYSTEM 12 BALANCE CALIBRATION PROCEDURE 12 BALANCE CHASSIS 14 AT THIS POINT THE M97 SHOULD BE PROPERLY CALIBRATED AND READY FOR SERVICE REFERENCE 15 INPUT ATTENUATOR 15 THE INPUT ATTENUATOR and the HIGH FREQUENCY RES...

Страница 4: ...SS 27 COMPRESS 27 PASS 27 The FUNCTION SWITCH 28 NORMAL 28 SIDECHAIN 28 INTERLOCK 28 SIDECHAIN INTERLOCK 28 EXTERNAL 28 LINKING TWO M97S FOR STEREO 29 BLOCK DIAGRAM OF SIGNAL AND CONTROL CIRCUITRY 30 POWER SCHEME 31 METER MODE SWITCH 31 IEC POWER INLET 31 IEC POWER CABLE 31 REPLACING THE FUSES 32 MAIN FUSES 32 INTERNAL FUSES 32 REPLACEMENT FUSE PART NUMBERS 33 TUBE COMPLEMENT 33 OPTIONAL REMOTE CU...

Страница 5: ...rmation protected by copyright No part of this manual may be reproduced in any form without prior written consent from Electronaut Company 2002 2016 Electronaut Company M97_UsersManual pdf DECLARATION of CONFORMITY Name of Manufacturer Electronaut Company Address of Maufacturer 333 N Oakley Blvd Ste 100 Chicago IL 60612 USA Product M97 Compressor Limiter Declaration ELECTRONAUT COMPANY is committe...

Страница 6: ...ned with the emergence of internet auction services and a globally interconnected world inspired a frenzy of trading in vintage electronics which in turn inspired a frenzy of manufacturers issuing clones of vintage designs Deeply rooted in Electronaut s philosophy is a belief that audio equipment design should be informed not only by an engineering perspective but also from a musician s perspectiv...

Страница 7: ...that it had a magic combination of flaws what made it great was that it expanded the usability of the variable mu concept to the extent that it could handle nearly any type of program material something previous variable mu compressors arguably could not achieve One of the main advantages of the single stage approach is an incredibly simple signal path an input transformer a tube amplifier and an ...

Страница 8: ...ease notify the carrier immediately and follow the Thorough Inspection Procedure on page 9 before applying power to the M97 OPENING THE CRATE The top cover of the crate is attached by 10 small metal tabs Locate the tabs and use pliars or a screwdriver to bend each tab vertically Once all 10 tabs are vertical lift the cover off the crate and set it aside See Figure 2 CAUTION The metal edges are sha...

Страница 9: ...s of damage Contact Electronaut immediately if any tubes appear to be damaged and do not apply power 5 Take a minute to visually inspect the rest of the internal electronics 6 If everything appears to be okay reinstall the top cover Install all eight screws loosely by hand first then once they are all installed tighten all the screws If the inspection process causes any doubt as to the condition o...

Страница 10: ... is a 10 turn knob to find the center turn the knob in one direction continuously until it stops then turn it back five full rotations You can count the rotation of the brass set screw on the top of the knob See figure Figure 7 Figure 7 The B ADJUST control permits adjustment of the high voltage tube regulator BALANCE CHASSIS KNOB Set the BALANCE CHASSIS knob to its center position This is also a ...

Страница 11: ...shless DC fan is installed on the rear panel The fan runs continuously when the M97 is powered on NOTE The fan has a service life of 80 000 hours equivalent to 18 years of 12 hour per day operation It should be replaced near the end of its service life Install the M97 into the equipment rack making sure to leave adequate space around the unit for airflow Attach the power cable to the IEC power inl...

Страница 12: ... equal for each phase the amplifier is said to be balanced In a perfectly balanced push pull amplifier 2nd order harmonic products created by the tubes are equal and of opposite polarity and are canceled at the output transformer Therefore a reliable way to verify the balance of a push pull amplifier is to detect and measure the presence of 2nd order harmonic products The M97 balancing system is a...

Страница 13: ...mode Set the METER MODE switch to ATTENUATOR and adjust the INPUT ATTENUATOR until the the meter displays a level of 0dB See Figure 15 Figure 15 Set the METER MODE switch to the ATTENUATOR position then adjust the input attenuator until a level of 0 dB is displayed Switch the METER MODE knob to OUTPUT The 2nd harmonic distortion component indicating the amount of amplifier imbalance will be displa...

Страница 14: ...is normal and should not be of concern It should be mentioned that a precise balance is a theoretically correct target For distortion performance other than minimum experiment at will One advantage of having a precise balance control is the ease with which a recalibration for minimum distortion can be achieved after experimenting with other settings BALANCE CHASSIS Most of the time the BALANCE CHA...

Страница 15: ...ridged T network with an impedance of 600 ohms The attenuator is placed between the input XLR and the input transformer and provides 1 dB steps throughout its range with an off mute function at the most counterclockwise position Once the AC and DC Thresholds and the Attack and Release controls have been set up the degree of limiting can be controlled by adjusting the input attenuator See Figure 19...

Страница 16: ...K 5 69 05 9 065 K K 5 69 05 9 065 K K 5 69 05 9 065 K K when ZSOURCE 100 ohms Figure 20 The high frequency response can be extended by setting the INPUT ATTENUATOR to zero and driving the input with a lower impedance source signal as shown here when driven by a 100 ohm signal ...

Страница 17: ...ncy roll off as a problem that should be fixed or as a feature that should be preserved Ultimately the point became moot the M97 can be set to provide either a Fairchild like roll off or a more modern extended frequency response When the INPUT ATTENUATOR is set to any position other than 0dB the Miller capacitance is driven by the attenuator impedance 600 ohms and the high frequency response is ve...

Страница 18: ...ction with the DC THRESHOLD it is set to determine the ratio of compression during limiting Turning the knob clockwise increases the amount of limiter action See Figure 22 Figure 22 The AC THRESHOLD control increases the limiter action as it is rotated clockwise The AC and DC Threshold are explained in more detail in the section titled UNDERSTANDING the effects of the AC and DC Threshold controls ...

Страница 19: ...pically the DC Threshold control is used at the higher end of its range to ensure that only the transient peaks are affected Due to the nature of the timing circuits and the controlling amplifier topology very fast attack times cannot be sustained indefinitely A good analogy to the fast attack limitaiton is the classic camera flash found in old school analog cameras The flash was capable of produc...

Страница 20: ...the amount of limiting action and the DC Threshold which effectively blocks out the average part of the waveform leaving only the peaks WAVEFORM C is the result of the settings in B no signal reaches the controlling amplifier and thus no limiting action occurs except during the duration of any peaks that exceed the DC Threshold The DC and AC threshold controls work interdependently by varying the ...

Страница 21: ... control clockwise causes the limiting action to begin at a lower level When the DC Threshold is at the minimum position the curves have a soft knee resulting in a fairly low compression ratio 2 5 1 in the example shown At this setting only the lower range of the AC Threshold knob is useful since the upper range results in an unacceptably low output When the DC Threshold is at the maximum position...

Страница 22: ...ORKFLOW Once the AC and DC Threshold controls have been set the amount of compression can be controlled by the INPUT ATTENUATOR Since the INPUT ATTENUATOR is adjusting where the signal falls along the compression curve turning the attenuator clockwise moves the signal up right along the curve further into the compression region RULE OF THUMB Driving the audio amplifier harder increases compression...

Страница 23: ...e see POWER SCHEME on page 29 Figure 27 The center of the M97 s interface is the 40 dB peak and persistence meter with the METER MODE knob providing a number of important functions ON When the METER MODE knob is set to ON power is applied to the main power transformer and the amplifier powers up The M97 will be operational within 30 seconds but a warm up time of a few minutes is recommended INPUT ...

Страница 24: ...onjunction with the input attenuator and the meter both of which are 1dB step a lot can be revealed about how the program material is being affected The following are some general tips and suggestions for making good use of the meter and test tone NOTE The OUTPUT is automatically muted whenever TONE or BALANCE is engaged MAKING GOOD USE OF THE TONE AND METER Below are a couple of tips on using the...

Страница 25: ... the input of the controlling amplifier Setting the switch to 120 Hz inserts a 6 dB octave high pass filter with a 3 dB point at 120 Hz into the input of the controlling amplifier The high pass filter can be useful to help keep the limiter from tracking very low frequency program material BYPASS COMPRESS PASS This toggle switch provides three disctinct routing modes See Figure 30 Figure 30 It is p...

Страница 26: ...he INTERLOCK buss connector are affected by the operation of all or one limiter e g it is possible to synchronize one or more limiters for multi channel use where the independent action would provide distortion under separate channel limiting For stereo compression using two M97 Compressor Limiters both units should be set to INTERLOCK mode and a 1 4 mono cable should connect the INTERLOCK jacks o...

Страница 27: ...to the INTERLOCK buss connector on the other unit On both units set the FUNCTION switch to INTERLOCK Set up the limiter controls on each unit Input Attenuator AC DC Thresholds Attack Release The limiter controls should be indentical on both units if an equal amount of influence from each channel is desired over the stereo limiting action It is also possible to use different settings on each unit t...

Страница 28: ...U AMPLIFIER OUTPUT TRANSFORMER ATTENUATOR METER SAMPLE POINT GAIN REDUCTION METER SAMPLE POINT OUTPUT METER SAMPLE POINT INPUT METER SAMPLE POINT OUTPUT TRANSFORMER CONTROLLING AMPLIFIER INPUT TRANSFORMER RECTIFIER AC THRESHOLD PAD HIGH PASS DC THRESHOLD VDC SIDECHAIN INPUT XLR OUTPUT XLR Figure 32 A simplified block diagram of the M97 s signal path and controlling circuitry ...

Страница 29: ...ET The rear panel power inlet is a high quality double fused power connector with a 2 pole power switch and a built in RF filter network Since the RF filtering is already built in the use of external AC Line Voltage filtering equipment is not necessary When the power switch is set to the O position power is completely disconnected from the M97 Setting the switch to the I position activates the sta...

Страница 30: ...ridge and gently pry the cartridge from the IEC power inlet See Figure 35 Figure 35 Removing the fuse cartridge with a small flat screwdriver INTERNAL FUSES To access the internal fuses it is necessary to de install the M97 from the equipment rack and remove the top cover The fuses are located near the center of the rear edge of the main printed circuit board under the small toroidal transformer m...

Страница 31: ...te different tube options the standard configuration is shown in Table 3 Designator Tube Type Requirements V1 6L6GC Matched to V4 V2 12BH7A Both sections matched V3 12AX7A Both sections matched V4 6L6GC Matched to V1 V5 6AS7 6080 V6 12AX7A V7 optional substitute 6386 in place of V9a and V9b Matched quartet V8 optional substitute 6386 in place of V10a and V10b Matched quartet V9a 5749W Matched octe...

Страница 32: ...AUT COMPANY M97 COMPRESSOR LIMITER V9a V7 V8 V6 V3 V2 V5 V1 V4 V11 V12 V9b V14a V14b V10b V10a V13b V13a INTERNAL FUSES Figure 36 The main printed circuit board showing the location of the internal fuses and the tube positions ...

Страница 33: ...experiment with alternate tube types Please contact Electronaut prior to experimentation to ensure compatibility and to reduce the potential hazards The circuit topology of the M97 s audio amplifier is similar to the Fairchild 660 670 in that eight triode sections are required four in parallel per side in a push pull configuration The Fairchild used four 6386 dual triode tubes which contain two tr...

Страница 34: ...e of the 12BA6 NOTE The heater voltage toggle switch only affects the heater voltage for the 7 pin sockets and has no affect the heater voltage for the 9 pin sockets which is fixed at 6 3VDC For a partial list of compatible tube types see TTable 4below 7 pin Tube Types Set Heater Voltage toggle switch to 6BA6 6 3VDC 5749 6 3VDC 12BA6 12 6VDC Table 4 A partial list of compatible tube types There ma...

Страница 35: ...ntact Electronaut to arrange a return of the M97 to the factory where it will be repaired without charge for parts and labour The M97 will then be returned to the customer via prepaid freight and insurance through a carrier of Electronaut s choice All warranties shall become null and void in the event of damage caused by misuse accident neglect unauthorized modification or tampering Electronaut Co...

Страница 36: ...mer balanced Line Output Impedance 120 1 floating transformer balanced internally terminated by 1800 1 Recommended Load Impedance 10 k1 or higher Minimum Recommended Load Impedance 600 1 Signal to Noise Ratio 85 dB Max Output Level for 1 THD no limiting 28 dBu THD Noise at 4 dBu no limiting 0 02 THD Noise at 4 dBu 10 dB limiting 0 7 Recommended Upper Limit of Compression for High Fidelity 12 dB Po...

Страница 37: ...vative audio level metering technology employing a digital processor to produce a visual display of peak and time averaged audio levels in a bar graph style format I am forever grateful to Dorrough Electronics for allowing me to redesign their meter and implement it in new and interesting ways in the M97 Compressor Limiter EARLY BELIEVERS Lastly I owe infinite thanks to the people who helped fund ...

Страница 38: ... the amplifier imbalance 13 Figure 17 The front panel BALANCE knob is used to adjust the symmetry of the audio amplifier in order to minimize distortion 13 Figure 18 The BALANCE CHASSIS control is a secondary balancing control used to fine tune the balance the push pull audio amplifier 14 Figure 19 The INPUT ATTENUATOR is a 3 pole 23 position balanced bridged T stepped attenuator 15 Figure 20 The ...

Страница 39: ... The main fuses are contained inside a removable cartridge that plugs into the IEC power inlet 32 Figure 35 Removing the fuse cartridge with a small flat screwdriver 32 Figure 36 The main printed circuit board showing the location of the internal fuses and the tube positions 34 Figure 37 The audio amplifier tube sockets 35 Figure 38 A detailed view of one of the four tube socket groups that make u...

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