Once again, a small valued resistor is placed in series with the load and ground. A sampling of the voltage across this resistor reveals the current flowing into the load. So only a single feedback loop is required.
   The third model is the current-to-current amplifier. Like the voltage-to-current amplifier, this amplifier has an infinitely high output impedance, but differs in that its input functions as a dead short, as the input is current not voltage.

    For homework, I recommend finding a fascinating article from early 80's in the Journal of the Audio Engineering Society, JAES, written by Malcolm J Hawksford. Unfortunately, I have lost my copy. This article explains the many advantages that derive from driving loudspeaker with voltage-to-current amplifiers.

Subject: OTL output impedance
I have built the driver circuit similar to your differential circuit on
www.tubecad.com/april_may 2001. I used this to driver a totem pole of 2 x 6C33C (total just 2 tubes). I found that the output impedance is about 23 ohm as follow:

Load 8.7 ohms
No load output 40v p-p
Loaded output 11v p-p
Zo = (40-11)/(11/8.7)=29/1.26=22.9 ohm

The expected impedance is that of 2 cathode follower in parallel:
(Rp/1+u)/2 = (100/1+3)/2 = 12.5 ohm.
    Well this is what I have measured, do you think this accurate?
    Also, the other single tube driver on the same page is actually a Futterman variation, too claimed to have lower output impedance of (Rp/(2+2u) i.e. about 13 ohms with 2 x 6C33C, I have built this circuit before, but I unfortunately I didn't (forget to) measure the output impedance.
    So, would appreciate if you can please explain the difference between the two circuits, as to which one has lower output impedance. I have read this article beside your web magazine:
http://members.aol.com/aria3/otlpaper/otlhist.htm

p. k.
Malaysia

Two reasons for the higher output impedance present themselves. The first that the rp of a triode is not a constant. It like the

   The fourth model is the current-to-voltage amplifier, AKA the I-to-V converter. This circuit appears in all single unit CD players, as the output of the DAC is in current. (Those DACs that yield a voltage output have internal to their package an I-to-V converter.)

    Of all these models, the voltage-to-current amplifier is probably the easiest to achieve with vacuum tubes. Feedback is taken from the cathode (or cathodes) of the output tubes, which radically increases the effective rp of the tube.
    One possible benefit to be derived from such a topology is the accompanying decrease in power supply noise from the output stage's, as the high rp makes a poor voltage divider with the reflected impedance on the primary of the output transformer. But do expect transformer distortion to increase simultaneously.

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