Output Buffer Stage
The output stage is again an emitter follower with a 10K emitter resistor, biased from the 4.5V bias source. At the output of the emitter follower is one of the few systemic differences between the members of the TS series. From the emitter of the transistor, there is a low-value resistor in series with a 10uF signal coupling capacitor, then a shunt resistor to ground. The following chart has the values of these resistors from model to model:| Model | 808 | 9 | 9RI | 10 | 5 |
| Series res. | 100 | 470 | 470 | 470 | 470 |
| Shunt res. | 10K | 100K | 100K | 100K | 100K |
So how do these differences affect the sound?
To start with, the series resistance at the emitter limits the amount of drive available to drive an amplifier input (although not by much), and in concert with the series capacitor forms a voltage divider with the output shunt resistor and the input impedance of the amplifier plugged into it. This drops the available signal only a trivial amount, probably inaudibly so. The input resistance of a typical tube amp is most often 1M or over. This is not a serious contributor to the loading of the output follower.
However, the 10K shunt resistor load on the TS808 output does have an effect on the follower's operation. An emitter follower with a resistor load is often assumed to have a very low output impedance, which is true, as long as the signal is going up. However, when the signal is going down, the transistor can only turn off, and the signal is pulled down by the net loading, DC and AC on the emitter. The 10K shunt resistor effectively halves the emitter loading on the output buffer. This does two things; trivially, it reduces the input impedance of the emitter follower; more importantly it reduces the negative going output impedance of the stage by half to about 5K. This means that the output stage can drive the following stage off about twice as hard.
Can this account for the admittedly very subtle audible differences between the TS 9 and 808? It pretty much has to, as the operating current for both stages is the same, the series resistance is only trivially different compared to the forward biased input impedance of a triode grid (10's of K's for the typical duotriode input), and the voltage divider ratio difference between 100/10K and 470/100K is 0.990099 versus 0.995322, an inaudible amplitude difference.
In terms of differences among the models: the TS808, the TS9, and the TS9 reissue are in fact built on the same circuit board pattern. For these three, THE ONLY DIFFERENCES ARE THE OPAMP TYPE AND THE TWO OUTPUT RESISTORS. This is how the various conversions to "TS808 specs" are done - two resistors are changed in value, and the opamp maybe changed to another type. Conversions to "brown sound" or other mystical turbocharging are done by twiddling the values of the resistor and capacitor frequency determining components.
The TS5 is a circuit copy of the TS9, excepting the opamp and a few components in the bias supply section.
The TS10 is the "sleeper". Available on the used market for as low as $20 in working order, it differs from the TS9 In only a few ways:
- extra 220 ohm resistor in series with the (+) input of the clipping amp
- extra emitter follower in series from the input buffer amp to the JFET switch for the bypassed condition
- Different bias voltage for the input buffer transistor.
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