I know this has been gone through many times before. I was told many times here SS amp has lower distortion. I studied Cordell's book and I am reading tube amp book by Morgan Jones and I am looking at which one has lower distortion. It is not clear yet that SS is lower distortion. Let's look at IPS, VAS and OPS. Please tell me if I am wrong as I am still studying. this is my observations.
1) Usually SS use complementary longtail pair for lowest distortion. Also uses degenerate resistors for the longtail pair. This tends to lower the gain. Tube power amp use a triode stage as preamp. Low distortion can be achieved by using Beta-follower which also maintains high gain of 100.
2) VAS is where the most distortion in SS. It is most common to use darlington common emitter stage with either CCS load or complementary CE stage. In tube, most common is long tail PI stage. This stage will have more even harmonics and very little odd harmonics if using triodes. The even harmonics will be cancelled when going through the OT in push pull configuration.
3) OPS in SS particular BJT followers have very sharp output impedance change at crossover. This supposed to be noticeable and cannot be cancelled out by NFB. This is where the push pull power tube with OT shines. The transition of the tubes at crossover is very gentle, no sudden change in output impedance like SS. Yes, the open loop output impedance of tube amps are higher, but this can be lowered by NFB.
4) The last but to me is the most important point. Both BJT and MOSFET has collector/drain curve very similar to pentode plate curve. They all have odd and even harmonics go up to at least 7th order. Triode predominately has 2nd harmonics with very little odd harmonics to start out with. This is a big big advantage over SS devices. There is no SS device equivalent to triode. OPS of tube stage either use triode push pull or Ultra Linear and use the push pull OT to cancel out the even harmonics. SS amp has no provision to cancel out harmonics. Only thing you can do is use degenerate resistor at the emitter or source to linearized it.
I really don't see SS is a clear winner at all. Please correct me.
1) Usually SS use complementary longtail pair for lowest distortion. Also uses degenerate resistors for the longtail pair. This tends to lower the gain. Tube power amp use a triode stage as preamp. Low distortion can be achieved by using Beta-follower which also maintains high gain of 100.
2) VAS is where the most distortion in SS. It is most common to use darlington common emitter stage with either CCS load or complementary CE stage. In tube, most common is long tail PI stage. This stage will have more even harmonics and very little odd harmonics if using triodes. The even harmonics will be cancelled when going through the OT in push pull configuration.
3) OPS in SS particular BJT followers have very sharp output impedance change at crossover. This supposed to be noticeable and cannot be cancelled out by NFB. This is where the push pull power tube with OT shines. The transition of the tubes at crossover is very gentle, no sudden change in output impedance like SS. Yes, the open loop output impedance of tube amps are higher, but this can be lowered by NFB.
4) The last but to me is the most important point. Both BJT and MOSFET has collector/drain curve very similar to pentode plate curve. They all have odd and even harmonics go up to at least 7th order. Triode predominately has 2nd harmonics with very little odd harmonics to start out with. This is a big big advantage over SS devices. There is no SS device equivalent to triode. OPS of tube stage either use triode push pull or Ultra Linear and use the push pull OT to cancel out the even harmonics. SS amp has no provision to cancel out harmonics. Only thing you can do is use degenerate resistor at the emitter or source to linearized it.
I really don't see SS is a clear winner at all. Please correct me.
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No, a push-pull output stage does not cancel even harmonics produced in an earlier stage. It only cancels even order distortion created within the output stage. Sorry, to clarify, a PP output may sometimes cancel even order distortion from an earlier stage but only if presented with this in the right phase. It all depends on how good is the common-mode rejection of the output stage.Alan0354 said:
The reason SS amps have very low distortion is that they have very high global NFB.
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All this thinking!
I have a tube amp, a dynaco ST70 with new electrolytic capacitors, plate coupler capacitors, and metal film resistors over 100 kohm, and output tubes.
I have a solid state amp, a dynaco ST120 with a seven transistor closed loop feedback bias current modification to eliminate the crossover distortion at low volume it was panned for in the 1966 reviews. It does have faster output transistors than RCA 5 digit, as the originals were shorted when I bought it. It has the TIP mod for oscillation kill with faster output transistors and drivers. One channel has original 1970 RCA drivers.
With low distortion speakers (see my signature) the ST120 sounds better. I use piano source material to test speakers, and the ST70 is still a bit fuzzy with hissy sibulants. The attacks are not as pingy as the actual Steinway piano I use as a calibrator.
What tube amps excel at is taking a lightning surge without damage. I had to replace the power switch on the PAS2 preamp, which was arced over, and the turn off pop capacitors which I had added, which was vaporized. Tube amps , when something shorts, do not burn up your speakers that cost six times as much (used) as the amp.
What transistor amps excel at is having parts you can actually buy without being involved in a war of conquest, and not heating the room up. So far the only protection circuit I've owned that actually was tested and worked, was the speaker capacitor in the ST120. The crowbar protection circuit in the PV-1.3k melts the land off the PWB and blows the triac, instead of protecting the speaker or tripping the AC breaker.
BTW the ST120 was so primitive with 4 transistors in the driver stage, that it didn't have any of the features you talk about. With the mod, on the speakers, it sounds exactly the same as a $1000 retail Peavey CS800s that has much more conventional design. Both sound better than the tube amp. Yes, I realize a different driver stage could help the tube amp. Right now, with new output tubes from a European Union source, the ST70 is shorting out after 20 minutes on time, which possibly could be a defective new tube. Waiting room on the electronics table
I have a tube amp, a dynaco ST70 with new electrolytic capacitors, plate coupler capacitors, and metal film resistors over 100 kohm, and output tubes.
I have a solid state amp, a dynaco ST120 with a seven transistor closed loop feedback bias current modification to eliminate the crossover distortion at low volume it was panned for in the 1966 reviews. It does have faster output transistors than RCA 5 digit, as the originals were shorted when I bought it. It has the TIP mod for oscillation kill with faster output transistors and drivers. One channel has original 1970 RCA drivers.
With low distortion speakers (see my signature) the ST120 sounds better. I use piano source material to test speakers, and the ST70 is still a bit fuzzy with hissy sibulants. The attacks are not as pingy as the actual Steinway piano I use as a calibrator.
What tube amps excel at is taking a lightning surge without damage. I had to replace the power switch on the PAS2 preamp, which was arced over, and the turn off pop capacitors which I had added, which was vaporized. Tube amps , when something shorts, do not burn up your speakers that cost six times as much (used) as the amp.
What transistor amps excel at is having parts you can actually buy without being involved in a war of conquest, and not heating the room up. So far the only protection circuit I've owned that actually was tested and worked, was the speaker capacitor in the ST120. The crowbar protection circuit in the PV-1.3k melts the land off the PWB and blows the triac, instead of protecting the speaker or tripping the AC breaker.
BTW the ST120 was so primitive with 4 transistors in the driver stage, that it didn't have any of the features you talk about. With the mod, on the speakers, it sounds exactly the same as a $1000 retail Peavey CS800s that has much more conventional design. Both sound better than the tube amp. Yes, I realize a different driver stage could help the tube amp. Right now, with new output tubes from a European Union source, the ST70 is shorting out after 20 minutes on time, which possibly could be a defective new tube. Waiting room on the electronics table
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What I mean is OT cancel the even harmonics from the longtail differential pair using triode that drive the output tubes. I don't mean the even harmonics from the IPS. The first triode stage is made very low distortion using Beta-follower or mu-follower.
You can have high global NFB with tubes. Loop gain can be just as high for tubes. In my example, I have also 3 stages in the tube. Using mu or Beta follower, the IPS stage has gain of 100. The long tail has gain of about 40. The power tubes with OT is about unity. So it's not that low.
Listening is very subjective, that's the reason I avoid asking about the "sound". Many people here insist low distortion is everything, so my whole post is based on distortion only. This is a measurable quantity, not subjective.
I am basing this discussion using Cordell's book, but there is no equivalent in the books and not even close to the analytical data on tubes as in SS. So I am guessing a lot on the tube side. But tube circuit has improved since 50 years ago also. I believe mu and Beta followers are quite new. The idea is presenting the triode with very high impedance ( over 1M) so the effect of the internal plate resistance becomes insignificant. This eliminate or at least lower the 2nd harmonic.
Sorry, but I would disagree - there's not really anything new, and you can't even buy decent valves any more - nothing like the quality of the old days.
But what are you trying to achieve? - if you want the 'valve' sound, then go for it - it's your choice. But don't try and justify it by specifications, where solidstate wipe the floor with valves on ALL counts, and massively so.
Not exactly true. Many of Nelson Pass' designs use a MOSFET as a single ended gain and output element. With minimal feedback the distortion is mostly 2nd order. In his true push pull designs or single ended amps in bridge mode, the 2nd harmonics "cancel" and the distortion shifts to 3rd order. The key is the circuit configuration and the amount of feedback used, and Emitter or Source degeneration counts as feedback. It shifts the distortion spectrum upward and outward.
Suggested reading: https://www.passdiy.com/project/articles/audio-distortion-and-feedback
SS vs Tubes
It is also useful to separate low power output from higher power output. At lower power output, many Class A/B amps are operating solely in Class A. isn't crossover distortion very low in SS amps? Some amps operate at a fairly high output level in Class A.
It is also useful to talk about SET tube amps which use SETs as driver tubes as the distortion of the input tube is reduced, thus the amp has lower distortion.
Tube amps can be outfitted with high impedance loads (plate chokes or constant current shunt voltage regulators) which reduce their distortion.
It is useful to separate tube amps with output transformers and those without (OTL) to discuss each of their virtues. There is even an OTL amp which uses SETs as output tubes.
Retsel
It is also useful to separate low power output from higher power output. At lower power output, many Class A/B amps are operating solely in Class A. isn't crossover distortion very low in SS amps? Some amps operate at a fairly high output level in Class A.
It is also useful to talk about SET tube amps which use SETs as driver tubes as the distortion of the input tube is reduced, thus the amp has lower distortion.
Tube amps can be outfitted with high impedance loads (plate chokes or constant current shunt voltage regulators) which reduce their distortion.
It is useful to separate tube amps with output transformers and those without (OTL) to discuss each of their virtues. There is even an OTL amp which uses SETs as output tubes.
Retsel
I am not talking about sound, since everyone here said low distortion is everything, that is something that is tangible. That's why I post my impression from studying. You need to explain why SS wipe the floor with valves an ALL counts.
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If you look at the drain curve, it's not like Triode, more the slanted collector curve of BJT. Both BJT and MOSFET has current proportional to (1-e^{-kVa}) where triode is proportional to Va^{3/2} according to the book by Morgan Jones page 90 and 91.
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I mainly refer to tubes with OT.
What I was talking about is using mu-follower or Beta-follow to create an horizontal load line for the triode to get the minimum distortion. The nature of triode is it predominantly have only 2nd harmonic and very little higher harmonics. It is a big big advantage over any SS devices.
My point is I question the often used phrase " SS wipes the floor with tubes". I want to hear analytically why SS is superior.
Sigh How? many times does this need rehashing?
BEST of SS pretty well sounds similar to BEST of Tubes.
Yess there are differences but less than you'd think.
The further down the quality ladder one travels the more the sounds diverge. Not attempting/bothering to say which type is preferable.
Wayyy to many are ready and willing to get up on their Soapboxes on that subject.
But mediocre grade stuff Ain't great... regardless of whether it's SS or tubes.
BEST of SS pretty well sounds similar to BEST of Tubes.
Yess there are differences but less than you'd think.
The further down the quality ladder one travels the more the sounds diverge. Not attempting/bothering to say which type is preferable.
Wayyy to many are ready and willing to get up on their Soapboxes on that subject.
But mediocre grade stuff Ain't great... regardless of whether it's SS or tubes.
Please try to avoid such blanket statements.
In DC coupled stages, relatively easy.
In RC coupled stages ... possible .... if you are careful.
Add an audio transformer to the mix and its inherent narrow band and unavoidable phase shifts throw all high NFB possibilities out of the window.
So much so, that classic amps from the Hi Fi Golden Era of tubes had to resort to local feedback (such as having dedicated windings for cathode feedback; local screen NFB in the Ultralinears, etc.) to be able to increase it somewhat without going end to end ..... where they would have oscillated like crazy above a certain value.
You mean open loop?
No problem, just stack gain stages in series and shield the first ones.
But as soon as you want to close the loop after more than a couple ... beware.
Bringing back NFB signal from some output tap to the proper side of the LTP (which as you say, has around 50X gain) or to the gain stage which drives a cathodyne, where you again have an around 50X stage driving a un ity gain cathodyne, is reasonable and is made all the time.
But if you add ahead of that an extra 100X gain 😱 stage like your mu follower/active load/cascode and bring the same NFB to its cathode ........ I find it very hard to believe it will not oscillate or at least be horribly unstable.
Sorry.
I don't believe 'everyone' here said that at all (and it's completely untrue), but SS amps have massively lower distortion than valve ones.
Much lower distortion, wider frequency response, flatter frequency response, lower noise levels, better damping factor - I don't think there's any measurement you can make where a valve amp outperforms a decent SS one?. Perhaps you can suggest one?.
Shame there is no instruments and units for fluidness, velvet smoothness, sound stage, fluffiness of high range, palpable vividness, warmth of mids and similar “firm” advantages of tubes above semis. Once those categories become established, all of you SS guys will see..... aha...
In my opinion too much bias for ss amp is worse than the optimal due gm doubling (of course there is no question of pure Class A biasing). but when talking about the different types of amplifiers - they are so different - I am enjoying F5 detail, i like hiragas le monstre tube like soft and airy sound ( due low DF and class A). sometimes i prefer leach amps classic, solid sound. But at the same time, I like the sound of my tube amps. Probably the only ones who remain aside - my chip-amps , but they also sound different but not bad at all.Enjoy your amplifiers - it is a passion - for us -fanatics.
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There are some differences that can be important to some people. One of them is clipping - even decent SS amps often perform worse than valves when driven into clipping.
Sorry, but that's hardly a 'measurement' 😀
Both will perform badly when driven in to clipping, but I agree SS amps sound worse when clipped - but are also far less likely to be clipped as they are available with MUCH higher powers.
But if you want to make 'silly' comparisons, transistors withstand blows from a hammer far better than valves 😛
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