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Exploring tube tones sans tubes

15 Jul 2015  | Paul Pickering

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The original TS808 designs used the JRC4558D dual bipolar opamp from New Japan Radio; later models substituted the RC4558P, the JRC 2043DD or the Toshiba TA75558, but these are generally regarded as sonically inferior.

Figure 2 shows the schematic of a TS808 Tube Screamer. The main sections are as follows:

Input buffer stage: a DC blocking capacitor at the input, then an emitter follower into a low pass filter.

Frequency-dependent variable gain & clipping stage. For a given setting of the distortion pot P1, the non-inverting circuit boosts higher frequency components of the signal due to the 0.0047uF capacitor C3. The back-to-back diodes symmetrically clip the output to VF , which is around ±0.9V for this diode. The 51pF capacitor C4 acts to "soften" the corners of the clipped waveform by passing higher frequencies.

Tone & volume circuit, beginning with a first-order RC low pass filter formed by R7/C5 with a 3dB point of 723Hz and attenuation of 20dB/decade.

The tone pot P2 changes the characteristics of the active filter: when the pot is at the full "bass" setting, series combination C6/R8 is in parallel with capacitor C5, giving a low-pass filter in conjunction with R7.

At the other extreme, the C6/R8 combination shunts feedback frequencies above 3.2kHz to ground. In combination with the R7/C5 low-pass stage, this results in a bandpass filter.

Output buffer: emitter follower stage.

Latching bypass circuit consisting of Q2, Q4, Q5 & Q6 and associated components. Q5 and Q6 form a discrete flip-flop circuit controlled by the momentary-action footswitch. The flip-flop outputs control JFETs Q2 and Q4, routing either the unmodified (buffered) input or the processed output as required.

If you want to dive into the details of this classic circuit, a detailed discussion of the TS808 circuit operation can be found here.

The Tube Screamer has many sites devoted to it and there is a thriving market in modifying existing pedals. Here's a site from Analog Man that discusses the differences between the various models in great detail.

Initially only bipolar opamps were available for overdrive pedals, but later Tube Screamer competitors adopted opamps with a JFET input stage such as the TL072, again due to the perceived improvement in sound quality.

How much of this sound difference is real? Check out this comparison of a dozen or so different opamps – the tester apparently has a fleet-fingered assistant who swaps devices between two identical pedals while he plays. Not sure about this "Brown-Burr" company he mentions, though.


Le roi est mort, vive le roi.....
Numerous other pedals start with the basic Tube Screamer circuit and tweak it slightly to claim some degree of differentiation. Periodically, a new design comes along which is hailed as the ultimate answer, only to be supplanted later by a new contender. Who knew that vast improvements can still be made using the same $5 worth of parts?

Being a highly trained professional, I am, of course, immune to such irrationality. I do have several such pedals, though, purely for the purpose of benchmarking—my current favourite is the Ethos TLE. Here's a demo by Tim Lerch, a wonderful guitar player from Seattle, comparing the TLE and the Hermida Zen Drive, another highly-rated pedal. Check out his new CD!


DSP modelling of amplifiers
The designs so far have been all analogue, but once DSP technology became sufficiently powerful, designers began using it to model tube amplifiers. These are now available both as stand-alone pedals and bundled with a power amplifier and speaker as a "modelling amplifier".

Line 6 is the most prominent maker of modelling amplifiers today but even Fender, manufacturers of some of the most storied tube amplifiers, has got in on the act.


Figure 3: Line 6 algorithm overview. (source: dlacm.org)


How do they work? Not being a DSP guy, I'm indebted to Rob Coker of Pythagorean Development, who did his master's thesis on the topic, for guidance on this section.

The hardware is pretty straightforward. Any reasonably powered 32bit microprocessor or DSP will work, provided it has DAC and ADCs of sufficient power; preferably 20+ bits of resolution and at least 48kHz sampling rate.

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