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Original Message

RE: Balance Compensation

Posted by cellai@space.it on June 30, 2008 at 10:20:52:
I have nothing against Futterman....rather the contrary is true!

In the present context I would just prefer to compensate for the intrinsic imbalance of the output stage (I remind that the OTL amplifier is in use since a month ago and that I am quite satisfied of its current performance) by adding a simple resistor (rather than by adding a new 450-500 V rail and by introducing some strong modification of the current OTL schematic).

An Italian proverb (a sort of Murphy's law) states that when something is almost good enough, you should not modify it, otherwise it fails.

The following table shows the calculated variation of the gain of the upper and lower triodes of the output stage as a function of the speaker impedance (from 1 Ohm to 17 Ohm, assuming a fixed compensation resistor of 5.5 Ohm, that is, four 22 Ohm resistors in parallel).

The imbalance with no compensating resistor at all is 19.8%.

With a 5.5 Ohm compensating resistor (that is best for 8 Ohm speakers) the imbalance is less than one half (i.e. approximately 10%) when the speaker impedance varies from 4 to 14 Ohm. A good result for a simple resistor, don't you agree?

Tube Type 6080 (4X) mu=2, rp=75
Speaker Up Lower Up-Low/Up
1 0.0216 0.0256 -0.157
2 0.0428 0.0494 -0.134
3 0.0635 0.0714 -0.111
4 0.0838 0.0920 -0.089
5 0.1036 0.1111 -0.067
6 0.1231 0.1290 -0.046
7 0.1421 0.1458 -0.025
8 0.1608 0.1616 -0.005
9 0.1791 0.1765 0.015
10 0.1970 0.1905 0.034
11 0.2146 0.2037 0.054
12 0.2319 0.2162 0.072
13 0.2488 0.2281 0.091
14 0.2654 0.2393 0.109
15 0.2817 0.2500 0.127
16 0.2977 0.2602 0.144
17 0.3134 0.2698 0.161

Last but not least I have also found that the sensitivity of the % of residual imbalance does not vary significantly with mu and rp: this is really intriguing.