Tube Amp and Speaker Impedance


CanyonLands National Park, Utah USA (Photo by Yuichi)

This experiment is to study beheiver of a high output impedance power amplifier when actual speaker system is connected instead of pure resister. The solidstate amplifier usally has very low output impedance. Instead, the tube amp has higher output impedance. The DF or dumping factor is defined as (load impedance) divided by (drive impedance). The DF of the tube amplifiers are 4 or 8 or something like those numbers. However, we have to understand that the basic assumption of this calcuration is that the load speaker impedance is fixed such as 8 or 16 ohms. In reality, speaker impedance value varies very much.

I provided two amplifiers. One is "KT-88" push-pull tube amplifier and the other is very commonly used all-in-one solidstate amp photos in the right.
Also, I provided pure registers and a speaker system as shown. The tube amplifier has three output terminals which are 4, 8 and 16 ohms.
This experiment consists of following process.
1: Use 8 ohms tube amp output terminal.
2: Connect 1 ohm resister load and measure the frequency response at the tube amplifier output terminal.
3: Repeat the same measurements using 2 ohms, then 4 ohms, 8 ohms, 16 ohms, 75 ohms and 150 ohms.
4: Replace the load resister by real spreaker system.
5: Measure the frequency response at the terminal. This is the driver capability (voltage) of the tube amplifier.
6: Calcurate damping factor.
7: Provide microphone to obtain sound output level
8: Measure the sound output level of the speaker system driven by the tube amplifier.
9: Reapeat the same measurement by connection to the other output impedance terminals (ie:other DFs)
9: Repeat the same measurement driven by the solidstate amplifier

Reference data (Output impedance of output terminals on the tube amplifier)
"Output terminal labels 4 ohms" : 2.15 ohms)
"Output terminal labels 8 ohms" : 4.18 ohms)
"Output terminal labels 16 ohms" :7.64 ohms)

Data:1
These curves are amp's driving capabilities at 8 ohms output terminal when a pure resister is loaded. Load resisters used are 150, 75, 16, 8, 4, 2, 1 ohm from upper to bottom curves. This chart shows the drive voltage of the amp veries not negrigible small depending upon the load value.
Data:2
10cm diameter driver is mounted on vented box. The speaker system has above impedance curve. There are two peaks in the resonence frequency domain due to the resonance and in higher frequency domain due to inductance and resistane increase.
Data:3
Connecting the real speaker system replacing pure resistance, driver capability (voltage at the amp output terminal) is measured. There are two curves in between pure resistance property. Upper curve is in case load is connected to 8 ohms and the other to 4 ohms. In both cases, driver voltage vary depending upon inpedance value. This is no surprise considering the Data:1
Data:4
Calucurating the DF from Data:3 I obtained above three curves. Upper one is when the speaker laod is connected to 4 ohms tube amp's output terminal. Middle curve is 8 ohms output and lower curce is 16 ohms out respectively.
We now understand how DF vary. So, commonly used word "DF of this amp equals 2 or 4 or 8" is just a reference number but not realistic number.
Data:5
Previous measurements are all in electronics world. I measured power ams's capability. Now, we have to know how it affects to sound pressure level. Above are SPL-Frequency reponse curve when the speaker system is connected to 4,8,16 ohms terminals in the tube amp. We can find out deferent properties. Those are quite resonable but surprize.
Data:6
Now, I did the same measurement by the solidstate amp. Comarison between the tube amp's property is quite defferent. This is, I guess, one of the major contribution factor of tube amp and the solid state amp beheives deferentry depending upon speaker system impedance property.
As a conclusion of this experiment, higher sound pressure can be obtained in the area of higher load impedance

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