The Phono Stage

The Vinyl Groove

A record spins at 33 1/3 RPM, at this speed it takes two seconds of music to travel approximately 36 inches around the outside of the record and 14.9 inches around the inside.

As the groove of a vinyl record spirals inward from the outside of the disc towards the label, the length of the groove per revolution on the inside of the disc is shorter than on the outside of the disc.

Because of this, the reduced space on the disc near to the label restricts the spacing between the groove walls, the depth of the groove and subsequently affects the stylus ability to track the groove, consequently, the record suffers some hi frequency loss plus distortion halfway through the LP.

The Cutting Engineer 

The cutting engineer compensates for this hi frequency loss plus distortion by cutting louder tracks on the outside of the disc and quieter tracks on the inside of the disc.

Louder tracks require more space for a deeper, wider groove depth and cut for volume and clarity.

The shorter in time the tracks are on an LP, the better, resulting in improved dynamics and volume.

The longer or more tracks that are cut,  the quieter and less dynamic the record will be, plus there will be increased surface noise. 

Study the grooves on a 12’’ remix record running at 45rpm and you will see that the groove is cut deeper and wider this is because there is only one track per side and the cutting engineer can use all the space available to cut a deeper and wider groove resulting in a louder and clearer vinyl cut.

This is why 12’’ 45rpm discs with one or two tracks always sound better than 12’’ 33 1/3 rpm with multiple tracks.

12’’ Vinyl Disc at 33 1/3 rpm offers an optimum of 12 to 14 mins and maximum of 22 to 24 mins cutting time

12’’ Vinyl Disc at 45rpm offers an optimum of 9 mins and maximum of 12 to 15 minutes cutting time.

The Cutting Engineer and the RIAA Pre-Emphasis Equalisation Curve(Recording Industry Association of America)

During the process of cutting the vinyl record the cutting engineer uses what’s called an RIAA Pre-Emphasis Equalisation Curve

this equalisation reduces frequencies below 1Khz by 6 dB per octave reaching -20dB at 20Hz

and boosts frequencies above 1Khz by 6 dB per octave reaching +20dB at 20Khz.

The reason for using this equalisation curve is to increase the cutting area on the disc.

By reducing the lower frequencies, the width of the groove is reduced, permitting longer track times. 

The louder the bass, the wider the groove.

Boosting the high frequencies greatly reduces high-frequency surface noise during the cutting process.

the-listening-suite-hifiology-riaa-pre-emphasis.jpg

The Phono Stage Equaliser Section and the RIAA De-Emphasis Playback Equalisation Curve 

During playback of the vinyl record the Phono Stage acts as both an equaliser and line amplifier plus uses a RIAA De-Emphasis Equalisation Curve, reversing the RIAA Pre-Emphasis Equalisation Curve used by the cutting engineer. 

The RIAA De-Emphasis Equalisation Curve boosts the Low frequencies and reduces the Hi frequencies canceling the effect of the Pre-Emphasis curve applied during the cutting process.

the-listening-suite-hifiology-riaa-de-emphasis.jpg

The Phono Stage - Line Amplifier Section and Electrical Specifications

As the cantilever / magnet or cantilever / coil assembly vibrates between the cartridges main magnets or coils

It creates an electrical signal which varies in voltage depending upon the type of cartridge.

There are two inputs relating to the Line Amplifier of the Phono stage, Moving Magnet & Moving Coil.

A basic Phono Stage will have a Moving Magnet input set at 47 KOhms and the Moving Coil Input at 100 Ohms with a set Gain level and set MM Capacitance / MC Resistance settings.

More elaborate Phono Stages allow the selection or variation in the Gain level and MM Capacitance / MC Resistance settings.

Phono Stage Input Settings for a Moving Magnet cartridge

Cartridge Type: Moving Magnet

Cartridge Output Voltage: The output voltage of the moving magnet assembly measured in milli-volts (mV) can range between 2.5 mV and 7 mV

Phono Stage Load Impedance / Resistance: For Moving Magnet set to 47 kOhm, 47,000 Ohms

Phono Stage Load Capacitance: The Cartridge manufactures recommended Capacitance setting measured in picofarads (pF) 

Capacitance Range: 

Example Settings:

0 - 50pF - 100pF - 150pF - 200pF - 250pF - 300pF - 350pF - 400pF - 450pF - 500pF - 550pF - 600pF - 650pF - 700pF (Pico farads)

Phono Stage Gain Settings

Gain: 10dB - 20dB - 30dB - 40dB - 50dB - 60dB - 70dB

Recommended settings are 40dB for Moving Magnet cartridges and settings between 50dB and 70dB for Moving Coil cartridges.

Adjusting The Settings

Impedance, Capacitance and Gain settings within the Phono Stage can be set in the following ways:

Using resistors and capacitors of varying values to obtain the optimum settings, this method requires the knowledge and skill of soldering.

Operating tiny On/Off switches at the rear or underneath the chassis of the phono stage, or simply dialling in the values required at the front of the phono stage.

Most Phono Stages usually have preset settings and may not offer an extended range of capacitance values.

Experiment with the settings provided by your phono stage and listen carefully to hear how they alter the sound.

Increasing the capacitance value reduces the upper hi frequency response creating a slightly dull sound. 

Decreasing the capacitance value increases the hi frequency response creating a slightly brighter sound.

Follow the cartridge manufacturers recommended setting and then experiment to hear the differences.

Try to find the setting which presents and overall balanced sound.

Remember your cartridge and tonearm settings also play a major part in the performance and settings of your phono stage.

Note: The capacitance value of the tonearms internal wiring and cable need to be taken into account when matching a MM cartridge to the phono stage.

Room Acoustics

Whether you adjust the phono stage settings by ear or use computer software, your room acoustics will dictate how your overall system sounds.

Remember you are tuning the system based upon the acoustics or sound of your listening room, your room acoustics are unique, no other room can sound like yours.

For example your friend may have exactly the same hifi system as you but when you listen in their room everything sounds different.

Note the list of variables which effect the acoustics or sound of a room: 

Room Size, High or Low Ceiling, Carpet or Hard floor, How many Windows and Doors, Curtains or Blinds, Mirrors, Wall Hangings, Paintings, Fabric, Leather or Wood Furniture, etc. etc.

Phono Stage Input Settings for a Moving Coil Cartridge

There are three types of Moving Coil Cartridges 

  1. Low Output Moving Coil 
  2. Medium Output Moving Coil
  3. High Output Moving Coil 

 The output voltage of the moving coil assembly measured in milli-volts (mV) can range between 0.3 mV and 2.8 mV

Low Output Moving Coil  

Small size coil assembly = Low Output Voltage

Output Voltage: 0.3millvolts to 0.7millivolts

Cartridge Impedance: 5 ohms - 32 ohms

Low output moving coil cartridges are by far the fastest and most responsive to the constantly varying width and depth of the vinyl groove.

They produce a signal with less harmonic distortion and a more open and dynamic sound presentation. 

This is due to the miniature and extremely light weight coil attached to the cantilever.

The down side is that with the decrease in weight there is also a decrease in the output voltage of the cartridge.

Medium Output Moving Coil

Medium size coil assembly = Medium Output Voltage

Output Voltage: 0.7millvolts to 1.0millivolts

Cartridge Impedance: 5 ohms - 32 ohms

Medium output moving coil cartridges have a slightly slower response time due to an increase in size and weight of the coil attached to the cantilever.        

But do provide an increase in their output voltage.

Low and Medium Output Moving Coil Cartridges & Step-Up Transformers

The benefits associated with a Low Output Moving Coil cartridge are 

it has a much faster response time to the constantly varying width and depth of the vinyl groove.

This is due to the light weight and finer coil windings attached the cantilever, 

the cantilever has less weight to carry, hence its faster response time. 

The down side is that with the decrease in weight there is also a decrease in the output voltage of the cartridge.

Due to this decrease in voltage it is recommended to use what’s known as a Step Up Transformer.

Why use a Step-Up Transformer

Step-Up transformers offer an added improvement to the low level signal coming from the cartridge.

Step-Up Transformers have a variable frequency-dependent input impedance, unlike Phono Stages which have selectable impedance load settings.

The results of this variable frequency-dependent input impedance is less harmonic distortion with a more open and dynamic sound presentation. 

The Step Up transformer amplifies the low level voltage from the cartridge passively, that is not requiring mains power,

which in turn reduces electrical interference to the low output signal, producing a cleaner and amplified signal 

suitable for the phono stages higher level Moving Magnet input.

A phono stage is still required as the Step Up transformer does not have a RIAA De-Emphasis Equalisation Curve within its circuit.

High Output Moving Coil 

Large size coil assembly = High Output Voltage

Output Voltage: 2.8 millivolts 

Cartridge Impedance: 1000ohm

High Output Moving Coil cartridges offer the benefits of a moving coil assembly but also offer a higher output voltage 

which can be connected directly to the Moving Magnet input of your integrated amplifier.

This can be seen as an alternative to purchasing a dedicated Moving Magnet / Moving Coil, Phono Stage and can be a less costly method.

Moving Coil Phono Stage Input Settings

Phono Stage Load Impedance: 100Ω ohms (Recommended)

Phono Stage Resistive Load Range: 

Example Settings: 22/47/100/150/330/1Kohm 

-010-020-030-040-050-060-070-080-090-100

-110-120-130-140-150-160-170-180-190-200

-210-220-230-240-250-260-270-280-290-300

-310-320-330-340-350-360-370-380-390-400

-410-420-430-440-450-460-470-480-490-500                     

-510-520-530-540-550-560-570-580-590-600

-610-620-630-640-650-660-670-680-690-700

-710-720-730-740-750-760-770-780-790-800

-810-820-830-840-850-860-870-880-890-900

-910-920-930-940-950-960-970-980-990-1000Ω

Phono Stage Gain Settings

Gain: 10dB - 20dB - 30dB - 40dB - 50dB - 60dB - 70dB

Adjusting The Settings

Impedance, Capacitance and Gain settings within the Phono Stage can be set in the following ways:

Using resistors and capacitors of varying values to obtain the optimum settings, this method requires the knowledge and skill of soldering.

Operating tiny On/Off switches at the rear or underneath the chassis of the phono stage,

Simply dialling in the values required at the front of the phono stage.

Most Phono Stages usually have preset settings and may not offer an extended range of resistance values.

Experiment with the settings provided by your phono stage and listen carefully to hear how they alter the sound.

Follow the cartridge manufacturers recommended setting and then experiment to hear the differences.

Try to find the setting which presents and overall balanced sound.

Remember your cartridge and tonearm settings also play a major part in the performance and settings of your phono stage.

This graph indicates the variation in the cartridges frequency response by varying the impedance setting.

impedance-settings-the-listening-suite.jpg 

Impedance Setting / Frequency Response

Increasing the impedance value boosts the upper hi frequencies approx 18Khz and slightly increases the middle & upper treble frequencies from 8Khz to 10Khz which may result in a slightly bright and forward sound.

A setting of 100K equals roughly a +12dB increase at around 18Khz

A setting of 150Ω equals roughly a +4dB increase at around 18Khz

The recommended setting of 100Ω is the closest to a level just above 0dB at 18Khz

A setting of 68Ω equals roughly a -2dB reduction at around 18Khz

A setting of 33Ω equals roughly a -4dB reduction at around 18Khz 

Room Acoustics

Whether you adjust the phono stage settings by ear or use computer software, your room acoustics will dictate how your overall system sounds.

Remember you are tuning the system based upon the acoustics or sound of your listening room, your room acoustics are unique, no other room can sound like yours.

For example your friend may have exactly the same hifi system as you but when you listen in their room everything sounds different.

Note the list of variables which effect the acoustics or sound of a room: 

Room Size, High or Low Ceiling, Carpet or Hard floor, How many Windows and Doors, Curtains or Blinds, Mirrors, Wall Hangings, Paintings, Fabric, Leather or Wood Furniture, etc. etc.

Note:

Impedance 

Is the reduction of current flow in an Alternating Current circuit which is specified in Ohms.

Capacitance:  

Capacitance is the ability of a component to collect and store energy in the form of an electrical charge.

The number of electrons it can hold under a set voltage is called its capacitance, expressed in pF (pico farads)