PYE 4061S Stereo Feed Forward Compression Amplifier

The Pye Compressor 4061S The 4061S is a dual channel analogue limiter/compressor designed for stereo effects compression, limiting and
mastering level control.

The early PYE compressors, the BBC type 4060, 4061 and others,were mono units using the principle of pulse width modulation. It was a ground breaking design setting a standard for the technologyof the time. That technology worked well but had limitations in its noise performance. The new PYE compressor 4061S is also a ground
breaking design, it is a re-application of Ted Fletcher’s earlier work on exciting opto compressors, combined with advantages provided by feed forward technology.

The PYE 4061S in detail
A stereo pair of audio inputs feed via XLR or balanced jack connectors via output routing relays to precision balancing transformers working in Ted Fletcher’s ‘current mode’design which produces extremely linear output with minimal distortion.

An illuminated ‘compressor’ switch operates the input relays switching the output connectors from direct (or by-pass) to compressed (compressor output). The stereo audio then enters a ‘sum and difference’ matrix where a width control affects the ‘difference’ signal and sets the stereo image width.

A feed from the ‘sum’ signal feeds filter networks and is converted to a carefully regulated DC current with controls for the speed of attack and decay.

The ‘sum and difference’ (M and S) audio feeds direct to a pair of current controlled (transconductance) amplifiers which are gain controlled by the DC control current and operate in feedback around a pair of precision low distortion operational amplifiers.

Buffer amplifiers convert the compressed M and S audio back to ‘left and right’, they are affected by the ‘gain make up’ control and drive specialist high quality output transformers.
Balanced outputs are available on both XLR and jack connectors. Audio volume level is shown on a pair of illuminated peak programme meters developed specifically for the PYE 4061S project.

An illuminated switch selects either a normal mode of left and right outputs, or the left meter showing a mono (sum) signal from a point before the output ‘gain make up’ control, and a measure of gain reduction on the right meter.

Limiter operation

The operation of the limiter/compressor section is not a simple one. The sidechain (the audio signal that feeds forward to the control elements) has a by-pass filter that gives immediate passage to short transient bursts of audio, this allows the control amplifier (a transconductance amplifier) to react extremely quickly, in the order of microseconds.
Further filters set the normal attack time which can be controlled by the attack switch setting the time it takes for the compressor to operate. A loud signal comes along and if it contains a transient ‘burst’ of sound, then the transient filter controls it. It then takes a small fraction of a second for the compressor to ‘catch’ the main sound level, ‘attack’ sets how small the fraction is.

The feed forward system allows for extremely fast and accurate attack times with zero overshoot; so when a transient ‘spike’ appears, this is controlled (limited) separately whichever attack time is selected.The character of the sound of the compressor is affected significantly by the attack control. An engineer-friendly feature is the ‘comp off’ position, deactivating the compression amplifiers while retaining system gain.

Loudness

The 4061S is uniquely suited for controlling perceived loudness, essential for transferring program material to streaming services like Spotify or YouTube.
It excels in ensuring clear audibility without distortion. By increasing the input control, the audio level approaches a limiter peak, concurrently elevating the compression ratio and perceived loudness.

Using the PYE 4061S

It’s worth pointing out at the beginning, the audio output level can be very high and it’s easy to overload any following equipment.

The input gain control is marked with approximate system gains from 0dB to +20dB, this is for convenience of engineers to recognise that a high-volume gain might be in the system when it is not necessarily audible at the output because of the 4061S limiting the output.

Keep the ‘make up gain’ control turned down at first. When starting it’s advisable to have the meter switch engaged, this gives instant indication of what’s happening to the audio just before the make up gain control and at the same time gives an indication of volume compression. When the meter switch is pressed in(illuminated), the LH meter reads the sum of the L and R audio. Both the attack and release controls have a very wide range. Take the settings in the table on the next page as a starting basis.

INPUTS 20Kohms transformer balanced, XLR and TRS jack
• OUTPUTS 56 ohms transformer balanced, XLR and TRS jack
• AMPLITUDE FREQUENCY RESPONSE +0 -0.2dB 20Hz to 20KHz
(40Hz to 10KHz within 0.06dB)
• HARMONIC DISTORTION at 1KHz at normal operating level 0.01%
2nd order 0.03% 3rd order. LF distortion dependent on release
settings
• NOISE approx. 110dB below peak operating level
• SYSTEM GAIN max 30dB.
• MAX LEVELS, INPUT +30dBu, OUTPUT +28dBu
• NORMAL OPERATING LEVEL limited to +8dB peak
• WEIGHT 2.8Kgs
• POWER 95 to 245VAC 5 watts max