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VLC Microphone Preamplifier/EQ/DI          

HAZELRIGG INDUSTRIES

 

 

18 

THEORY OF OPERATION  

Input section  

The first stage is a selected 6072 tube configured as a Class A voltage amplifier with a gain of 

approximately  30.  Negative  feedback  from  the  plate  of  the  second  stage  reduces  distortion, 

flattens  the  frequency  response,  and  makes  the  gain  of  the  first  two  stages  less  dependent  on 

individual vacuum tube characteristics.  

 

Second stage  

The  output  of  the  first  stage  is  coupled  to  the  grid  of  the  second  stage  through  a  polystyrene 

capacitor. This stage operates as a Class A voltage amplifier with a gain of approximately 30. The 

plate  is  coupled  through  a  polypropylene  capacitor  to  the  top  of  a  conductive  plastic  rotary 

potentiometer (Gain).  

 

Third Stage  

The arm of the Attenuation potentiometer feeds the grid of the third stage (a 6072A), which also 

operates Class A with a gain of approximately 30. This stage is capacitively-coupled to the grid 

of the output stage through a polystyrene capacitor.  

 

Output Stage  

The output stage operates as a cathode follower, presenting a comparatively low output 
impedance (approximately 800 ohms).  

 

Equalization Section  

The passive LC (inductor/capacitor) equalization is inserted at the mic preamp output.  

 

 

Содержание LP-1

Страница 1: ...HAZELRIGG INDUSTRIES VLC Vacuum Tube Preamplifier EQ DI Operating Instructions ...

Страница 2: ...ne Preamplifier EQ DI HAZELRIGG INDUSTRIES 2 How to Contact us Telephone 567 DW FEARN 567 393 3276 Shipping Address 124 Tartan Terrace Chalfont PA 18914 U S A e mail support hazelriggindustries com www dwfearn com ...

Страница 3: ...s committed to manufacturing products that are fully compliant with the EU RoHS Directive The following products are compliant VT 1 VT 2 VT 24 VLC 1 VT 3 VT I F VDI VT 4 VT 5 VT 7 VT 15 LP 1 PDB This declaration is based on our understanding of the current RoHS Directive and from information provided by the supplier material declarations with regard to materials contained in the component that mak...

Страница 4: ... Tube Microphone Preamplifier Final Test Report Serial Number Mains Voltage VAC Date Tested by THD Noise 20 cps 200 cps 2 kc s 20 kc s Operational Tests 48V 20 pad Phase Reverse EQ In Line Input Instrument Input Equalizer LF Cut LF Boost HF Boost HF Cut Listening Test ...

Страница 5: ... EQ DI HAZELRIGG INDUSTRIES Table of Contents Warranty 6 Specifications 7 Description 8 Installation 9 Operation 12 Theory 18 Maintenance 20 List of Illustrations Rear Panel Connections 11 Front Panel Controls and Connections 13 ...

Страница 6: ...ty but are warranted for 90 days from date of purchase Except as expressly set forth in this Warranty D W Fearn Hazelrigg Industries makes no other warranties express or implied including any implied warranty of merchantability and fit ness for a particular purpose D W Fearn Hazelrigg Industries shall not be liable for technical or editorial errors or omissions in this manual nor for incidental or...

Страница 7: ...m Line Input Load Impedance 40k ohms Gain 55 dB minimum Frequency Response 0 5 dB 20 cps to 20 kc THD Noise 0 35 20 cps to 20 kc Intermodulation Distortion SMPTE 0 80 Signal to Noise Ratio 72 dB typical operation Equivalent Input Noise 124 dbm maximum Output low Z transformer balanced Maximum Output Level 22 dBm unterminated Power Requirements 120 or 220 VAC 50 60Hz 25 W Dimensions 19 48 26cm W 3 ...

Страница 8: ... warmth that solid state preamps lack Its modern design and construction allows the VLC to exceed the performance of vintage vacuum tube preamps It is designed for use in the professional recording environment It accepts all low impedance balanced microphones It features a regulated 48 volt supply for phantom powering condenser microphones a switchable 20 dB input pad and phase polarity reversal s...

Страница 9: ...efit all your equipment The VLC can also be used without a rack placed on a table counter or even on the floor Optional rubber feet are available when requested at the time of the order Moderate electrical and magnetic fields in the vicinity of the VLC should not cause any degradation in noise performance due to the well shielded construction but proximity to devices with motors or large power tra...

Страница 10: ...h and pin 3 or low The VLC is optimized for feeding balanced bridging inputs Virtually all modern audio equipment has bridging inputs The output is transformer balanced The GND terminal is for use when an external grounding scheme is utilized The Fuse is a 5mm x 20mm 1 amp for 115 VAC operation and 0 5 amp for 220 240 volts The AC input connector is used with the mating line cord supplied For 115 ...

Страница 11: ...Shields A full discussion of proper studio wiring schemes is beyond the scope of this manual but in general the Input mating XLR connector must have the cable shield connected to pin 1 With most microphones this shield must also be connected to pin 1 at the microphone end of the cable Whether the shield is connected to pin 1 of the output connector depends on the standard in your studio The shield...

Страница 12: ... then fed back to the control room Avoid locating the VLC where it will be subjected to high sound levels or excessive vibration such as on a drum riser Output The output of the VLC is line level transformer balanced Note that vacuum tube equipment is more sensitive to load impedance than solid state units The VLC design is optimized for feeding a balanced bridging input 20k ohms or greater When f...

Страница 13: ...experiment with the microphone pad if it has one the VLC 20 switch position or both Let your ears be the judge Slight to moderate overdriving of the VLC often adds edge power or excitement to the sound If it is necessary to operate the Gain control near the top of its range the input level is too low and any padding at the mic or on the VLC should be reduced or removed Input switch 2 The input swi...

Страница 14: ...ics Although leaving the 48 on will not damage any properly wired balanced mic some ribbon mics have reportedly been damaged when connected or disconnected from phantom powering Also some engineers feel that the performance of some dynamic mics may be subtly degraded with the phantom power on The phantom power circuit used in the VLC is suitable for use with all Neumann micro phones AKG 12 and 48 ...

Страница 15: ...t built before standardization may reverse the polarity in the recording monitoring chain The effect of reversed absolute polarity is usually subtle but can be significant with some sounds If the reverse position sounds better use it With more than one microphone on the same sound source or picking up leakage from another sound source the POL switch may have a profound effect on the audio quality ...

Страница 16: ...rking but it is suggested that you turn on the power at least five minutes prior to use The tubes are often noisy until all the internal elements reach a stable operating temperature Initial Set Up The VLC should be installed as detailed in the Installation section With the outputs connected to an appropriate destination typically to audio recorder inputs configure the studio to monitor the VLC ou...

Страница 17: ... located in the studio or in the control room but use the shortest possible cable between the mic and the VLC 2 Use the best quality mic cable you can We don t believe you have to use esoteric wire but do use a good cable designed for low impedance microphones A quality cable with gold contact connectors is best 3 There should be no additional cables connectors junction boxes patch jacks etc betwe...

Страница 18: ...lystyrene capacitor This stage operates as a Class A voltage amplifier with a gain of approximately 30 The plate is coupled through a polypropylene capacitor to the top of a conductive plastic rotary potentiometer Gain Third Stage The arm of the Attenuation potentiometer feeds the grid of the third stage a 6072A which also operates Class A with a gain of approximately 30 This stage is capacitively...

Страница 19: ...imary taps for 115 and 220 240 volt operation A switch on the rear makes it quick and easy to go between voltages Be sure to also change the fuse to the appropriate value Filament supply The power transformer output is rectified by a bridge rectifier and filtered before being regulated to 12 0 volts by a three terminal regulator The negative output of this supply is grounded Although the tube fila...

Страница 20: ...problem since measurements in a quiet room will indicate perfect performance Gently tapping the tube shields while listening to the output at a normal monitor level should reveal nothing more than a slight clank On a peak reading meter connected to the VLC output with 50 dB gain any microphonic response above 55 dBm is excessive Replacement is indicated unless the VLC always operates in a quiet an...

Страница 21: ...cps noise the filter capacitors should be suspected They should be replaced with new capacitors of equivalent capacitance and voltage rating and the replacements should be specified for a minimum ten year service life Electrolytic capacitors are also used as plate and cathode decouplers In choosing a replacement the same considerations as with the filter capacitors should be followed TROUBLESHOOTI...

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