For a verbal description click=20 here.
A gain block is a circuit consisting of one or two triode=20 amplifiers that can be replicated as many times as needed in an = integrated=20 amplifier or preamplifier to makeup for losses in tone, volume, and = balance,=20 controls. These gain blocks are most often found outside of a = global=20 feedback loop so they need to have inherently low distortion. A = single=20 stage with an unbypassed cathode resistor is usually not good = enough unless=20 it operates at a very low level. Gain blocks usually consist of two = triodes=20 with feedback from output plate to input triode cathode to diminish=20 distortion to a very low level and tightly control the value of the = gain. =20 Here is a circuit I developed that will give a fairly wide range of = gain at=20 very low levels of distortion.=20Here are a couple of solid=20 state versions of the same circuit.=20
For a verbal description click=20 here.
This circuit has a lot = of open=20 loop gain which is traded off for low distortion. Somewhere else on = this=20 site I know I said that this is often done because gain is cheap and = easy to=20 come by. The gain may be changed by changing the cathode resistor, = Rk, in=20 the first triode. Changing this resistor has a very small effect on = the=20 operating point of either triode. The numbers are given in the = table below.=20
A very good approximation for the gain is,=20
Cathode Resistor, Rk. Gain. Distortion. Output Voltage. 36 k ohms. 6.67 0.035%. 10 v. 27 k ohms. 8.47 0.04%. 10 v. 22 k ohms. 10.1 0.045%. 10 v. 9.1 k ohms. 21.7 0.045%. 10 v. 5.1 k ohms. 33.6 0.060%. 10 v.
Av =3D 1 + Rf / (Rk + 1100)=20 Where Rk and Rf are as labeled in the schematic and = Av is=20 the gain. =20You see that the feedback is DC coupled, not through a capacitor, = from=20 the plate of the second triode to the cathode of the first. An = earlier=20 design used AC coupled feedback. This version was unstable at low=20 frequencies. Bode analysis showed that a 3 microfarad coupling = capacitor=20 would have been needed to make it stable. That circuit was posted = here in=20 the early days of March 2005. If you saw it and decided to use it = please=20 update your schematics.=20
The 220 k ohm resistor makes up most of the load for the second = triode=20 which is where most of the distortion occurs. I don't recommend = changing=20 the value of Rf because it effects the operating point of the second = triode.=20 If this resistor is changed the cathode resistor in the second = triode will=20 have to be readjusted for minimum distortion.=20
Someone emailed me with the information that when this circuit = was run on=20 a spice simulator there was a 1 dB peak in the frequency response = below 10=20 cycles. This really isn't very important but it still might bother = some=20 people. It was caused by the fact that the feedback which goes to = the=20 cathode of the first triode and is negative also is fed in a small = amount to=20 the grid which is positive feedback. I downloaded a spice simulator = and=20 checked it out and confirmed the peak. The trick is to adjust the = relative=20 sizes of the inter-stage coupling capacitor and the input coupling = capacitor=20 so the positive part of the feedback is rolled off before it can = effect the=20 gain. I also tested it with the volume control set at half = resistance which=20 is the maximum impedance condition. The roll-off is now smooth at = both=20 ends. I trust everyone, perfectionists included, can now sleep. Am = I a=20 perfectionists? Well, grudgingly, yes.=20
You may be wondering why I have gone for such low distortion = while on=20 other pages of this section I didn't indicate it was that important. = This=20 amplifier is NOT meant to be inside the global feedback loop. As = such it=20 needs to have an inherently low distortion level because nothing is = going to=20 improve it later. This circuit may be used where ever a stage with = low=20 distortion, relatively low output impedance, and closely controlled = gain is=20 required.=20
Happy building and good luck. Keep watching, there will be more = in the=20 future for the builder or the curious experimenter.=20