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Showing results for tags 'swamping resistor'.
Hello, my name is Daniel and this is my first post here. Couple of weeks ago, I bought a pair of black 1988 LaScala for a very good price. All drivers are original, cabinet in fair condition that I plan to re-veneer in the next few months. CrossOver is the infamous AL type. I read a lot of very good information here, and quickly get interested in the crossover. I have a Tube Amp based on the RH-807 design. It is basically a class A pentode based single ended output stage. The Amp have very little feedback, around 6db I figured. One very important characteristic is the output impedance that I measured at a huge 20 Ohms. In fact, any pentode based amplifier, including push-pull without a lot of feedback will exhibit a high output impedance. In theory, the output impedance is the tube plate resistance reflected on the secondary of the output transformer. A typical plate resistance value for a 6L6 based tube is around 30 kOhms. With a 5k to 8 ohms transfo, the plate resistance (output impedance) is 30000/625= 48 Ohms. Considering the 6db feedback, it fall to 24 Ohms, not very far from the value I measured at 20 Ohms. Now, why bother??? This 20 Ohms resistance is in series between the load (Speaker) and the source (The Amp) Consider the lower frequency where the speaker presents itself as a 6 Ohms load. For this case, the voltage at the speaker input terminal will be: Vin = Vsource (6/26) = .23Vsource Consider the mid frequency where the speaker presents itself as a 30 Ohms load. For this case, the voltage at the speaker input terminal will be: Vin = Vsource (30/50) = .6Vsource It can be concluded that the midrange will be 2.6 time more driven than the bass (.6/.23). a 8.3db bump. At first, I was skeptical about this. Then I measure the input signal to the La Scala from the tube Amp and effectively proven right. It also explains very well why anything midrange was screaming at me :-) I then used a more modern transistor Amp with a <1 Ohms output impedance, and everything was normal (speaker Input Voltage and sound). The 1 Ohms output impedance does not absorb any significant voltage from the source, whatever the speaker impedance. Conclusion is, output impedance matters a lot when using speakers presenting a non constant impedance with frequency. Any pentode based design without feedback needs to pay attention. Triode based design are much less affected due to a lower plate resistance, but will still exhibit the bump if no feedback is used. Solid state get a free pass :-) The next step was to fix the problem, which will be detailed in a next post