Subject: Analog tape compression circuit
From: "Saved by Internet Explorer 11"
Date: Thu, 16 Jan 2014 10:38:06 -0800

Analog tape compression circuit updated and revised 6-17-05

There has been much talk about the wonderful sound of magnetic tape. Many
have taken the DSP software approach to tease out what actually happens when
tape is used, with dubious results in my opinion. I have chosen instead to take
the fully analog road and develop a circuit that provides the sonic effect of
actual tape saturation.

This is not just a frequency dependant filter, as some popular product that
claims to capture the sound of tape saturation. This circuit actually duplicates
the effect of what tape does to the transfer curve, and subsequent effects of
pre-emphasis and de-emphasis of tape recording and playback electronics.

I carefully set up several listening tests to determine the correct values
that got closest to that elusive tape sound. There is no mumbo-jumbo here, just
the most elegant implementation.

Be aware of the fact that there is far more to the sonics of magnetic tape
than just some simple little circuit pretending to sound EXACTLY like tape. This
has not yet been accomplished, anywhere! Don't be fooled by cheap promises.

However, if you want a circuit that will give you a very convincing replica
of that elusive effect, this may be for you.

This circuit is a waveform compressor that compresses more at higher
frequencies. It consists of a 70 microsecond (2.25 KHz) pre-emphasis, followed
by a diode type waveform compressor, followed by a 70 microsecond de-emphasis.

This compressor has an unusually good sound considering the lack of time
constants. Distortion and "pumping" effects are both virtually inaudible.

This circuit was designed to simulate the effects of analog tape compression.
It will be useful with digital tape recording equipment if a more "analog" sound
is desired.

Click here for
the schematic.

Notes for this circuit:

1. Power supply is a split supply of +/- 15 volts.

2. Any decent quality op-amps can be used.

3. If equipment feeding or being fed by this circuit has any extraneous DC
voltages, you will need to add blocking capacitors.

4. UPDATES AND REVISIONS:

There have been some changes that were added to the circuit.

C1 was added to prevent any dc from entering the input. R5 was added in
conjunction with the gain stage to provide some make up gain after the diode
block. This also provides a way to get various levels of saturation while having
a constantly adjustable output level. C4 was added as an output blocking
capacitor. It is generally not needed.

Furthermore, you may be inclined to experiment with the value R4 that feeds
the diode block. Previous experiments have shown different waveform compression
charicteristics with different resistor values. I chose the 10k value because it
sounded best to my ear.

PARTS LIST

R1..........100K AUDIO TAPER
R2..........1.5K
R3..........22K
R4..........10K
R5..........100K AUDIO TAPER
R6..........1K
R7..........10K
R8..........22K
R9..........1.5K
R10.........150 OHMS
C1..........0.1UF
C2..........0.003UF
C3..........0.003UF
C4..........10.0UF NOT NEEDED IN MOST APPLICATIONS