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.
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
