Negative Feedback!?


More insights into the mastermind behind Trinity's ultra-high-end audio products. This time Dietmar Bräuer talks about negative feedback. For general understanding, the negative feedback is a control loop that compares the input signal with the output signal and adjusts the output signal, if it deviates from the input signal. 


Such a control loop has a few sound-relevant advantages.

1. Decreasing Distortions
  • In the TRINITY PA, the applied negative feedback decreases the distortions down to 0.00005%
2. Decreasing Output Impedance
  • In the Trinity PA, the applied negative feedback decreases the output impedance down to a few µOhms. This effect can be seen very well in the frequency response measurements via the load impedances. This effect can be seen very well in the fr frequency response measurements over the load impedances, because there the measurement curves lie exactly on top of each other, as in the ideal case.
3. Stabilization of Voltage Gain         
  • In the Trinity PA, the applied negative feedback stabilizes the voltage gain over temperature, frequency, and so on.
Let me start with a 500years old quote: “All things are poison and nothing is without poison; only the dose makes a thing, not a poison.”- Paracelsus

Even if the statement refers more to a medical reason, one can also derive an analog to electronics. 1977 wrote Otala in his paper “The Theory of Transient Intermodulation Distortion”

“The application of strong negative practice feedback in audio amplifier has become standard practice over the last decade because transformerless transistor amplifier circuits have enabled its easy use."

At that time, a distinction was already made between strong and weak negative feedback.
 
The benefit of negative feedback was never generally questioned only the dose. The advantages of negative feedback are real and not fantasy.
 
By the way, I have also designed concepts without negative feedback, e.g. current conveyors used as integrators in high-frequency channel equalizers around 1992.
 
However, a lot has happened in electronics since 1977, and statements that were maybe justified at that time are completely outdated today.
 
For a deeper explanation have a look at these two articles published at EDN  
 

 
How strong feedback can depend on the achieved open loop gain of the amplifier.
 
Since the open loop gain drops vs frequency with 20dB for a decade a much higher open loop gain always increases the loop gain over all frequencies.

Here is the open-loop gain of the TRINITY Power Amp. Even at 100kHz, we have almost 60dB open loop gain.

 
The small signal bandwidth of the current gain stage of the TRINITY power amplifier is around 6MHz and the large signal bandwidth under full load is 500kHz.
 
I don't think you need a wider bandwidth, because the best microphones can only convert music up to 50kHz into an electrical signal, if we then consider the recording side with a 192kHz AD converter then we see that when we talk about music we are talking about a bandwidth limited signal.
 
All these discussions about distortion caused by a “slew rate limited amplifier” have never considered that the rise time of an audio signal is limited and if we speak about 50kHz audio bandwidth we speak about a rise time of 7us (for a first order design).
 
The rise time of the TRINITY amp driving 10A is 800ns. That is almost a factor of 10 higher than the rise time which can be created by musical content.

 
If I see the matter correctly, such amplifier concepts without negative feedback are mainly found in amplifiers with huge output power, which requires a large number of big transistors whose wiring and the input capacitance represent quite a complex load that can cause the amplifier to start oscillating when you close a negative feedback loop.

However, when an amplifier oscillates, the intended negative feedback becomes positive feedback, which is essential for oscillators. 

Or as Murphy would say “Oscillators never oscillate, while amplifiers always do.” Enclosed the Murphy's Laws If people talk exclusively about the “sonic advantages” (there are no measuring advantages) of an open loop, then perhaps it is only because they cannot realize a stable closed loop in their concept.
 
From my point of view, amplifier concepts for music reproduction without feedback are no longer up to date and never were, because there are no sound-relevant parameters that are worsened by feedback, on the contrary, all sound-relevant parameters are significantly improved by feedback and the TRINITY PA is the best example of this.
 
Side note about kW audio amplifier: Similar to the weight and external dimensions, the maximum amplifier power is not a sound-relevant parameter.