transformers/output impedance

To clarify a few points made immediately above:

Negative feedback has only a minor effect on high frequency bandwidth. This is because HF bandwidth is a function of the risetime (or slew rate) of the circuit, which will not be affected by bandwidth. However, there is a *perception* that it has this effect, because feedback will add trace amounts of odd ordered harmonic distortion, which is perceived as brightness by the human ear.

Transformers do indeed transform impedance, and in *both* directions. So if for example a 16 ohm tap is loaded with a 4 ohm load, the power tubes will see a load that is considerably lower than they will be able to drive properly. Distortion will be higher and output power will be lower. This is why its important to select the right tap.

The use of feedback has little to do with the transformer, and everything to do with the linearity of the amplifier circuit that precedes it. Usually the driver sections are fairly linear, but output sections may not be if tetrodes or pentodes are used. It is this reason that feedback is used rather than the choice of output transformer.

Almost any transformer currently in use in a hifi amp is capable of operating in a design without feedback. Again, feedback will not have a large effect on improving bandwidth of the transformer due to the demands that would be made on the preceding circuitry.

IOW, the reason you see zero feedback tube designs is not because a transformer is particularly suited for it, more so it has everything to do with the linearity of the circuitry preceding. In Charles1dad's case, his amps have triode power tubes which are inherently linear- no need for feedback.

To be clear, the use of a transformer can indeed reduce the output impedance, but there is a myth that adding loop negative feedback will too. It doesn't. I'll explain that further if anyone asks.

Hi Bifwynne, first you are correct that the cathode cross-coupling thing you see in ARC amplifiers is a form of negative feedback. I've used this a lot when modifying Dyna ST-70s. I think the oldest amp I have seen with that was employed in a Leslie organ speaker made in the 1950s.

To your second point- the highish output impedance of a Power Paradigm amplifier (SETs being a good example) may or may not be a problem, based entirely on what speaker you are using. You may recall in that article I wrote on the subject that I mentioned that the two design approaches can't be mixed or else you wind up with tonal anomalies (due to the voltage response of the amp on the speaker).

This is why horns had such a bad reputation for so long- their crossovers were designed for amps with a higher output impedance, and so the older ones in particular can sound 'honky' as drivers in the system can be trying to operate outside of the passband the designer had in mind.

Conversely, a choke in series with a woofer is supposed to cause the woofer response to fall off at a certain frequency, but if the amplifier output impedance is high, the amplifier power will not drop off in the way that it is supposed to, causing the woofer to play higher frequencies than the designer had in mind (this would be an example of a Power Paradigm amp being used with a Voltage Paradigm speaker).

Regarding the output impedance (for those that did not follow the link Al dropped), its pretty simple. Amps with a lower output impedance can drive lower impedance loads with greater ease than amps with a higher output impedance. I think we all understand that simple fact.

So if we add feedback, will that cause the amplifier to make more power into lower impedances? The answer is 'no'. You get more power into lower impedances by having larger heatsinks, larger power transformers, larger output transformers, more tubes, etc. You can't get more power out of an amp by adding feedback- that would violate a fundamental rule of electronics known as Kirchoff's Law.

IOW, the term 'output impedance' as we generally understand it in audio is a 'charged term' in that it has a meaning that only works inside the Voltage Paradigm in audio. Anywhere else the term refers to the internal impedance of the output circuit, whatever that might be.

I think the point that Ralph was trying to make is that using a meter to measure an amp's impedance/resistance at the output taps is **not** what the term "output impedance" is speaking to. Instead, it's referring to the amp's operational behavior in delivering current/power to a speaker load **as though** it's output impedance was a specified number.

Bingo!

I'm sure Ralph has maximum power in mind.

Correct. The maximum output power will be a function of the internal impedance of the output stage. Its Ohm's Law after all :)