Alice PYE 4061S Stereo Feed Forward Compression Amplifier

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.

Compression operation
Notably, the 4061S lacks a dedicated 'ratio' control; instead, the compression ratio adjusts dynamically with the audio level.
Below - 20dB, the ratio is nearly 1:1, representing minimal compression. As the signal approaches 0dB, the ratio rapidly increases, approaching 20:1, indicating true limiting.

Release
The release control sets the time that the compressor takes to recover after it has applied compression. Short release times generally add 'urgency' to the sound, but the creation of good compression effect
is always a careful combination of attack and release settings.

Width and Balance
The width control sets the stereo width of the output, from mono
to 150%. When set vertically upwards, the stereo image is the same as the program material entering the unit.
The balance control allows variation in gain between left and right, and allows for fine left/right balance adjustment.
The make-up gain controls the level of signal from the main outputs. This recovers volume lost in the compression process as well as setting the output level.

More about Transformers
The input transformers are unconventional in that they operate in current mode: The audio is transferred as a small AC current with effectively no voltage generated.
This mode means that there can be no power loss in the transformer and therefore no ‘overload’ condition, audio distortion is eliminated.
Output transformers are special high power low distortion types manufactured in the UK by Stephens and Billington especially for the 4061S.

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.

Set the amount of compression by increasing the input level during level tests, but use caution as it is extremely easy to increase the level too far without realising, this will not produce distortion but will cause the 4061S to work as a limiter.
Because of automatic compression ratio it is almost impossible to overload the compressor.
Release the ‘compressor’ button to hear a comparison to the original audio, this bypasses the compressor.