Hello, I'm changing a bias Rk resistor to minimize distortion and odd Harmonics and i measured the following 3 results to choose (always at 1 Watt output):
1) Min Overall THD: THD=0,28%, 2ndH=-65dB, 3rdH=-55dB, 5thH=-80db
2) Min 3rdH: THD=1%, 2ndH=-45dB, 3rdH=-65dB, 5thH=-80db
3) Min 5thH: THD=2%, 2ndH=-40dB, 3rdH=-55dB, , 5thH=-100db
Which is better to minimise: overall distortion but with predominant 3rdH, or minimise the 5th, that is the most unpleasant, or minimise the 3rd which is the predominant odd H?
1) Min Overall THD: THD=0,28%, 2ndH=-65dB, 3rdH=-55dB, 5thH=-80db
2) Min 3rdH: THD=1%, 2ndH=-45dB, 3rdH=-65dB, 5thH=-80db
3) Min 5thH: THD=2%, 2ndH=-40dB, 3rdH=-55dB, , 5thH=-100db
Which is better to minimise: overall distortion but with predominant 3rdH, or minimise the 5th, that is the most unpleasant, or minimise the 3rd which is the predominant odd H?
Hi
I can't answer which one sounds best, but I am curious about your test setup.
Is it SE or PP? Are you using only the Rk resistor to bias the output valves? Does the valve have the same dissipation in all cases?
There was a recent thread here about "shifting" distortion by changing the tail resistor in a PP stage. Also Morgan Jones applied this technique in an amplifier, shown in the 4rd edition of Valve amplifiers. I still have to do some tests for myself 🙂
Best regards, Erik
I can't answer which one sounds best, but I am curious about your test setup.
Is it SE or PP? Are you using only the Rk resistor to bias the output valves? Does the valve have the same dissipation in all cases?
There was a recent thread here about "shifting" distortion by changing the tail resistor in a PP stage. Also Morgan Jones applied this technique in an amplifier, shown in the 4rd edition of Valve amplifiers. I still have to do some tests for myself 🙂
Best regards, Erik
With no listen seems to me this would be better:
3) Min 5thH: THD=2%, 2ndH=-40dB, 3rdH=-55dB, , 5thH=-100db
With one listening session the situation also similar due many variables in the play, as equips, cables, room, media format etc
What would clear the doubts would be a long time evaluation to a full year.
IMO, the best sounding HD distribution is the so called waterfall of 2nd > 3rd > 4th > 5th ... As for THD level, personal preference definitely plays a part. However, it (IMO) pays to keep in mind that implicit in the term HIFI is minimal alteration of the source signal. FWIW, I prefer stuff whose THD is >= 0.3%, at full power O/P. Yes, I am a H/K Cit. 2 "lover".
Again IMO, any discussion about distortion needs to address the IM issue. IMD, being unrelated to the music signal, is much more objectionable than HD. A design whose HD level is low, but whose IMD level is substantial, rates to sound bad. IMD is not as a clear target as HD is. To begin with, there are at least 2 reasonable measurement methods, IHF and SMPTE.
Again IMO, any discussion about distortion needs to address the IM issue. IMD, being unrelated to the music signal, is much more objectionable than HD. A design whose HD level is low, but whose IMD level is substantial, rates to sound bad. IMD is not as a clear target as HD is. To begin with, there are at least 2 reasonable measurement methods, IHF and SMPTE.
test
My circuit is a SET and I'm varying just the Rk (cap-bypassed) of the driver tubes, which are two triodes in parallel.
Listening test showed immediately that the best was N°2, that is the one who minimised just the 3rd harmonic, for which 2nd harmonic component is predominant (i.e the best sounding HD distribution was REALLY the so called waterfall of 2nd > 3rd > 4th > 5th ...) : sound was the most "open" and "natural" that confirms all studies which say that even harmonics give a warmer, organic, and natural sound while odd harmonics impart a more harsh and metallic sound.
Pay attention that even if I’m using zero tolerance Rk (i.e. selected with a multimeter) Left and Right channel show a different distortion distribution, probably due to the intrinsic tolerance of each triode. So the three Rk were optimised just by measuring one channel, and by applying the result to the other channel too.
Tube is 6n2P (=ECC83) with paralleled triodes
Rp=75K which means 150K per tube
Rk (bypassed) that minimised 3rdH=840 ohm, which means 1680 ohm per tube
I suspect also that the result is a combination of the power section+transformer and not just effect of the driver alone: in fact i was not able to simulate the 3rd harmonic minimisation with any program and only measurements (and only for one single channel) showed me the right Rk.
My circuit is a SET and I'm varying just the Rk (cap-bypassed) of the driver tubes, which are two triodes in parallel.
Listening test showed immediately that the best was N°2, that is the one who minimised just the 3rd harmonic, for which 2nd harmonic component is predominant (i.e the best sounding HD distribution was REALLY the so called waterfall of 2nd > 3rd > 4th > 5th ...) : sound was the most "open" and "natural" that confirms all studies which say that even harmonics give a warmer, organic, and natural sound while odd harmonics impart a more harsh and metallic sound.
Pay attention that even if I’m using zero tolerance Rk (i.e. selected with a multimeter) Left and Right channel show a different distortion distribution, probably due to the intrinsic tolerance of each triode. So the three Rk were optimised just by measuring one channel, and by applying the result to the other channel too.
Tube is 6n2P (=ECC83) with paralleled triodes
Rp=75K which means 150K per tube
Rk (bypassed) that minimised 3rdH=840 ohm, which means 1680 ohm per tube
I suspect also that the result is a combination of the power section+transformer and not just effect of the driver alone: in fact i was not able to simulate the 3rd harmonic minimisation with any program and only measurements (and only for one single channel) showed me the right Rk.
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Clearly the minimum 3rd H sounds best, as expected. There is another important factor that solid-state amplifiers not always follow: The harmonics should monotonously decrease with decreasing power. It is frequent case that they have a valley at say 1W and they are higher at 0.1W and below. That is unpleasant to the ear. This is not an issue with SE tube amplifiers, though.
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