Damping factor

[Unda Maris]

What Kenwood called Sigma Drive is nothing else than a sense circuit which extends the feedback loop to the ends of the speaker cable.

But this approach has a huge drawback:

If you don´t use a special low inductance cable, the amplifier starts to develop an tendency for oscillations, because the cable itself introduces servere phaseshifts at higher frequencies (well above the audio spectrum but still quite low frequency ).

Since every amplifier has a certain phase margin, which allows a certain open loop gain and a certain open loop bandwidth, the additional phase shift of the four wire feedback will force the designer to artificially reduce the open loop bandwidth, in order stabilize the amplifier.

All amplifiers I know, which allow such four wire cables of several feet length have an open loop bandwidth, which is well below 20kHz, sometimes even lower than 1kHz.

This means that the DF of these amplifiers at least doubles every octave above this corner frequency. Lets say the amplifier has e.g. a DF of 1000 and a 400Hz open loop bandwidth. This would mean that the DF decreases down to 100 at 4 kHz and 20 at 20kHz. These are realistic figures for such amplifiers !

Amplifiers with lots of NFB and very low open loop bandwidth also tend to be unable to control crossover- and switching distortions of the output stage and produce all these bad TIM, IIM and Differential tone distortions.

I don´t think, that Kenwood was able to deliver a DF of 1000 at the end of a ten feet cable at 20kHz ! That sounds impossible for me !

 

[Deang]

http://www.transcendentsound.com/amplifier_output_impedance.htm

 

[Paul C]

"The open-loop perfomance of a circuit design is the most important factor.What's done after with neg feedback is less important than what happened before neg feedback was applied."

If you open loop 99% of the amps on the market, an input slightly over 0 will result in the amp swinging the output to nearly full voltage, less drop through the output transistor and driver. An input slightly below 0 will swing output the other way, nearly full voltage of the other rail (whether positive or negative depending on if the amp inverts or not).

Negative feedback controls gain and distortion... it makes the output follow the input. There is nothing wrong with negative feedback, and it is necessary to make the amp work.

 

[tcdriver]

High feedback?

If you open loop 99% of the amps on the market, an input slightly over 0 will result in the amp swinging the output to nearly full voltage, less drop through the output transistor and driver. An input slightly below 0 will swing output the other way, nearly full voltage of the other rail (whether positive or negative depending on if the amp inverts or not).

Negative feedback controls gain and distortion... it makes the output follow the input. There is nothing wrong with negative feedback, and it is necessary to make the amp work.

The other 1% of the amplifiers on the market are designed to operate without closed loop negative feedback. It is unnecessary to design an amplifier with closed loop negative feedback. The people who design such amplifiers often claim that they sound better than the more conventional high feedback amplifiers. In the early 80’s, there was a lot of ink spilled describing the excessive use of high feedback amplifiers and the purported negative effects it had on the sound. Does anyone remember TIM and SID distortions?

 

[Unda Maris]

Did you know that really no (artificial) NFB-less amplifier design is able to operate without coupling capacitors and transformers in the signal chain in order to get rid of these huge dc offset problems, which plague all these designs ?
Often there are even multiple capacitors or transformers neccessary in order to realize a simple power amplifier ...

(Don´t forget a DC servo loop is NFB !) :yes:

So what is worse ? Intelligent use of NFB or low frequency distortion of an audio transformer and a coupling capacitor ?

So who said, that NFB is bad ? Perhaps manufacturers of audio transformers and silver foil capacitors ...

 

[tcdriver]

So what is worse ? Intelligent use of NFB or low frequency distortion of an audio transformer and a coupling capacitor ?

Or to put it another way, what is worse, intelligent use of capacitors and transformers or SID and TIM caused by the use of excessive NFB?

 

[Unda Maris]

tcdriver,

thats true, but in between there is a region, where neither lots of capacitors nor audio transformers or huge amounts of overall NFB *) are used.

That´s the region where the best amplifiers come from ...

Such amplifiers can also have high damping factors at high frequencies without all these bad TIM,SID,DIM, IIM and lots of other measureable distortions with "I".

One example:

My actual amplifier design has a BW of 25 MHz (!!), a gain of 1 and a DF of 10.000 at 20kHz and negligible distortions of any kind (<130dB). Even at 1 MHz the damping factor is still above 100. That´s possible, because all available open loop gain is used for output impedance and not for closed loop voltage amplification. The voltage amplification is made by a dedicated voltage amplification stage without any overall NFB between both.

You can imagine, that such an amplifier can only be realized when you design it like a RF transmitter, making all traces very short ...

How does it sound ? Dry, tight control but also very rich in a unique way !
Put a resitor in series with the output and you also have the advantages a low damping factor may have with certain speakers.

*) "Huge amounts of negative feedback" means for me very bad open loop linearity, too many (>2) amplification stages, very high open loop gain and low open loop bandwidth within the audio range (The classical operational amplifiers were made in such a way e.g. the LM741, but also nearly all solid state amplifiers to date, too)

 

[tcdriver]

Fast Amp

One example:My actual amplifier design has a BW of 25 MHz (!!), a gain of 1 and a DF of 10.000 at 20kHz and negligible distortions of any kind (<130dB). Even at 1 MHz the damping factor is still above 100. That´s possible, because all available open loop gain is used for output impedance and not for closed loop voltage amplification. The voltage amplification is made by a dedicated voltage amplification stage without any overall NFB between both.

Your amplifier sounds interesting. If I understand what you are saying, you have an amplifier that has gain followed by a non-amplifier that has no gain. The non-amplifier has a high damping factor and is not included in the overall feedback loop with the amplifier that has gain. What kind of power do you get from your combination amplifier? Are the two amplifiers on one chassis or two?

 

[Unda Maris]

You got it !

The new "no gain" amplifier output stage is able to deliver "classical" 100W/8Ohms and 200W/4 Ohms. It runs in Class AB (with a quite high bias current of 500mA) and includes a special feed forward crossover distortion canceler, which I am developing now since a few years ...
It was originally intended as a V-Fet emulator, but emulating V-Fets means obviously building a crossover distortion canceler ...
The output devices are high power complementary switching MOSFETs (300W types, the same power capability like five SONY V-Fets ...).

The voltage gain will be realized by a separate module with separate supplies, which is integrated also in the same housing. I am thinking about a special transimpedance amplifier, which has no inherent NFB and high output impedance. But this will need a coupling capacitor ...

This is still on work, but I have an equivalently performing older design running in my main system for about seven years which has a BW of even 40MHz (world record ?) and 20W/8Ohms pure Class A, which doesn´t have a voltage gain stage integrated at all. I drive it directly with my Wega Lab Zero preamplifier (=SONY TA-E88) which has enough internal gain and output voltage swing (+- 20Volts) to drive it sufficiently high with line signals. For MC/MM unfortunately there is a little bit too low overall gain, but it´s sounding nevertheless phenomenal.

The advantage is clear: Every kind of distortion is about 20..30dB lower than with any other amplifier (these have voltage gain in the same range) and this amplifier is really independent from load and can be considered to be ground free.

I also tried to use a tube amplifier (Dynaco ST 70 clone) as an "external voltage amplifier stage" and it sounded great, just like a tube amplifier with a damping factor of 10.000 at 20kHz ... The tube amplifier doesn´t see any (or only a defined small dummy-) load in this setup, which reduces load dependency to almost zero.