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Understanding THD in tube amplifiers

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JohnJ    411

Simply does the warmth or timbre that I recognize to be from a tube amp correlate to the higher measured level of thd in the specifications of that amp somehow?

Conversely the old Yamaha "Natural Sound" amps with .0007 or any measured beyond a thousandth did that contribute to the cold clinical sound that I can get turning the variable loudness off.

Thinking not all tube amplifiers have specifications of thd measured in tenths. If the details of the source are also revealed to a much greater extent that goes against what I might have incorrectly believed about SS specs for a long time.

 

** http://kenrockwell.com/audio/why-tubes-sound-better.htm

Plausible ideas, I did find it on the www though.

Edited by JohnJ

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Ski Bum    308

The Rockwell article has lots of kernels of truth.

 

Tube amps can add both linear and non-linear distortion.  

 

The linear distortion is due to the higher output impedance of tube amps, so the response will reflect the impedance curves, although this  is extremely subtle IME, not anywhere close to the extent of a tone control like the Yammie's variable loudness.  The output impedances of the myriad tube amps (as well as some ss designs, i.e. some of Pass' single ended First Watt amps) are all over the map.  High output impedance amp will add some plump at woofer resonance, and some sparkle up high as the tweet's impedance rises.  Again, it's pretty subtle until you veer off into transconductance amp territory, but small changes to the bass can result in big changes in subjective impressions.  

 

The non-linear distortion aspect is where it gets a bit confusing.  Most tube amps inherently have higher distortion, often corresponding inversely with the complexity of the circuit (compare the simplicity and off the reservation measurements of a SET to the complexity of something like your reasonably well sorted Yamaha).  Different topologies exhibit different distortion characteristics, which can and will dramatically change the presentation if/when clipping occurs.  Single ended amps generally have low order, monotonic distortion spectra, and as a bonus are completely devoid of crossover distortion (manifested as high, odd order spectra).  PP topologies cancel even order harmonics, so they tend to produce predominantly odd order spectra.  They inherently have crossover distortion, although it is mitigated in good designs via proper biasing and the correct application of nfb to beat it down to negligible levels (ditto for conventional ss a/b amps).  Solid state amps are typically squeaky clean up to the point of clipping, where they exhibit extended, high, odd order spectra. 

 

Reality:  We clip our amps more frequently than we might realize, even using sensitive speakers.  I would posit that this accounts for the vast bulk of reported audible differences between amps, regardless of class or amplifying device.  Lower power tube amps will clip sooner than more powerful amps, and thus excite their respective spectra of harmonics, dynamically, in relation to signal strength.  In overdrive conditions, the monotonic pattern of single ended amps acts almost like the ideal compressor, exposing details in the mix while simultaneously not shredding your ears with true to input peaks, and they can be run well into the red before the sound becomes objectionable.  As a result they're both highly resolving and easy on the ears, which is a great combo with Klipsh.  PP can be over-driven as well, but not as far into distortion as a single ended amp due to it's odd order spectra, which is more brassy or edgy sounding.  The more well engineered pp tube amps, which are necessarily complex, expensive affairs, when operated within their limits are very hard to distinguish from boring ol' ss.  Excessive, gross clipping sounds bad on either (unless you're a musician).  SS clipping is just audible ugliness however you slice it, and should be avoided if at all possible.

 

Second reality: low distortion at full rated power is irrelevant if you spend the bulk of you time at micro-watt levels, so how the amps perform at the low end of their envelope is just as, if not more important than how they do at rated power.  From this perspective, I'm convinced there are merits to certain tube amp approaches, particularly the single ended jobs that avoid crossover distortion.  Phase splitters always seem to ruin the magic.  

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tube fanatic    557

The reality is that it is impossible to correlate a single parameter with a particular sound.  There is simply too much going on.  Rod Elliott, with whom I agree about 99% of the time, did a nice overview of this issue:

 

http://sound.whsites.net/amp-sound.htm

 

37 minutes ago, Ski Bum said:

 From this perspective, I'm convinced there are merits to certain tube amp approaches, particularly the single ended jobs that avoid crossover distortion.  Phase splitters always seem to ruin the magic.  

 

No question that Ski Bum's statement is true based on my years of experience.  Push-pull is a necessity when high power is desired from tubes at reasonable cost, but overall it has many disadvantages.  The only way push-pull can deliver on its lower distortion promise is to maintain perfect balance in the phase splitter and output stage.  Unfortunately, tubes never age identically so the balance is always in a state of flux. And further, there's the issue of so-called matched tubes.  Regrettably, most vendors match at a single operating point which is vastly different from the actual operating conditions.  A curve tracer, which few have, is the only way to match tubes to perform similarly across their entire power curve.  Single ended, using cathode bias, is truly the best way to go as it is a "plug and play" arrangement which self adjusts as the tubes age  (in my opinion it also sounds vastly superior to push-pull amps as well).

 

Maynard

 

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Ski Bum    308

You won't find a more reality based take on amps to learn from than Rod Elliot (well, aside from his SET hatred).  Maynard is our resident sweep tube ninja.  Build one of his amps and you won't be disappointed.

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JohnJ    411

@Ski Bum Good to know, my "angle" isn't going that far. I've got a new solid state system set up but I'm just not satisfied with what I'm getting out of my records. New cartridge helped the frequency response and overall sound and depth, but I've got "it" bad and want more out of the lps. Found some deals on both tube and ss pre amps, the tubes are more. The signal going from the lp to my speakers would have to go through my integrated amp. It is a decent amp, but I wasn't knowledgeable of the hows and whys of tubes.

I comprehend more now, thinking that even tubes on the output side of the phono amp might get the sound that I'm seeking. Need to go hear a comparison, there's a place near here that stocks what I'm looking at, maybe I could take a listen and reach a conclusion. 

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Ski Bum    308

I'm cynical, so I see kit with tube line stages as having them for the sake of marketing purposes.  All that stuff about the thd in tube amplifiers upthead assumes a tube amp coupled to a speaker, not merely a line stage, where the task at hand is much less complex than driving a reactive load. Maybe for the phono pre, since that too is a transducer, but otherwise I tend to see it as just bling.  Those items do indeed tend to cost a lot, and be relatively poor values.  Approach with caution and skepticism, don't let the consumerist impulses win without full deliberation.

 

 

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Schu    2430

Perhaps it's more effectual to compare the thd quotes, and it's effects on changes in sound, in a similar topology comparison instead of across completely different platforms.

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JohnJ    411
15 hours ago, Ski Bum said:

Approach with caution and skepticism, don't let the consumerist impulses win without full deliberation.

 

 

Along with everything else you've imparted I appreciate this too. Narrowed it down to two (I think).

https://ifi-audio.com/portfolio-view/micro-itube2/

http://www.pro-jectusa.com/en-us/products/pro-ject-box-designs/ds-line/phono-boxes/tube-box-ds

 

 

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Ski Bum    308

Hmm.  Well, I personally don't find either of those products to be remotely compelling.  I may be the worst enabler ever for you, as I'm feeling very 'Rod Elliot' in sentiment after looking at those products.  Those strike me as perfect examples of the sort of gear you'll find along the dubious audiophile primrose path of consumerism.  Ok, maybe the phono pre makes sense, but I'm still not too impressed.

 

 

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JohnJ    411

Hey @Ski Bum I've seen your posts besides this thread for the six or more months I've been here. Not looking for an agreeable voice, just opinions! Thanks for your time spent with me here. My new rig is Cambridge CX80, CXN, CXC Audio Technica AT-LP5 with AT440MLb cart. The sound is nice with the Forte IIIs and good cabling. Built in preamp in the tt is lacking, and I can switch it completely out of the signal path. With your knowledge should I just stick with Cambridge for the phono preamp? Would the benefit of spending almost double to fold tubes into the record sound be nil? The signal does still have to travel through the ss CX80 to get to the F3s.

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Ski Bum    308

I'm no expert on vinyl rigs, so hopefully others will chip in.  Your Cambridge integrated is a fine piece of kit, so I doubt they would have dropped the ball with the phono stage, so it may boil down to your cart or setup.  A tubed phono pre will likely sound different, though, and possibly better to you.  So go for the low hanging fruit first, and only if you're sure your rig is optimized as it stands, then consider an external phono pre.

 

Your cambridge has pre-outs, and would make an excellent front end to feed a tube amplifier.  If you're going to dabble in tubes, I think you should go that route.  And don't pussyfoot around with a nearly linear pp amp, go all the way to the furthest depths of depravity and get a SET.  

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JohnJ    411

I use my CXN as the preamp so that I can control the stereo with my phone if I want. That also makes the CXN's per channel DACs (connected with balanced XLRs to the amp) what everything else flows through. The phono could go USB2 to connect to the cxn but I've got it connected directly to the CX80 with some very quiet RCAs. The new and broken in cartridge is good enough it has improved the lp play a lot, but not so high end that it can't play high milage lps that are 35 years old well. As far as I can go under four bills!

Thanks again, and I don't think you're a bum at all! 

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Ski Bum    308

Well, if you have soldering skills and enough awareness to not electrocute yourself, one of Maynard's designs would set you back less than that.  Lil Sweetie parts cost is probably three bills or so.  Just under 2 watts of single ended goodness that would have you scuba diving through the layers in the mix.  You've got the speakers for it.    

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JohnJ    411

I remember ohms law but haven't been prolific soldering since the late 70s, found the schematics.....

For the price of admission and to perceive the difference I was hoping for by putting tubes in one small part of the lp signal I could not justify it. It is a need vs want thing. If I have to revisit the turntable after trying the simple preamp I've ordered, that will be next year. Maybe I'll try to squeeze that iTube2 in then?

I figure that I can't go too far with this since half of my lps have a lot of miles and 4/5 ths of them are decades old.

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Marvel    2281
On 9/14/2017 at 3:07 PM, Ski Bum said:

...go all the way to the furthest depths of depravity and get a SET.  

 

Now you're talkin' ;)

 

Great explanation of the different amplifier types as well.

 

Bruce

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JohnJ    411

@Ski Bum Tried a simple ss preamp that did not work out. Picked up the iFi iTube2 and the sounds my record player makes now are incredible. Using it as a buffer after my tt internal preamp, set to +9db, switched from "classic tube" to "SET" to "push/pull" settings on it and wow the texture in the sound has come alive! If a SET amp sounds rich and kind of a flat sound I get that with the setting. The classic is more rich and midrange sounds are just very forward into the room...vocals, sax, flutes, horns, guitar. Push/pull sounds like the classic setting tempered a little with what the SET sounds like.

Wish I'd been more aware of tube gear before I started my 30 year stereo rebuild last year, but now my lps really shine with this little trick after my tt.

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Ski Bum    308

Cool.  Glad you're liking it.

 

That's an interesting device.  It's different settings apparently (based on their cryptic verbiage) manipulate the amount of feedback around the tube stage to get the different sound characteristics.  That's a far cry from a tube amp coupled to a transducer, but if mere audibly different profiles are the goal it should do that.  Since you're still using your big amp, you also don't suffer from the main drawback of those fine sounding single ended tube power amps: low power.  

 

.  

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JohnJ    411
1 hour ago, Ski Bum said:

Cool.  Glad you're liking it.

 

.  

Was. But then..... I was changing records last night enjoying that realistic rich sound. Didn't turn the volume down to 1 or 0 like I have every time for forty years (it was only at about 8:30 on the pot) and i hear this humming through the speakers. This thing was advertised as zero ie dead sound when not in use, even on the product page. One review said put it in line turn it on and listen..... listen to absolutely nothing is what will happen. Switched out the cables, plugged it into a differently grouped socket on my Furman pfi15 no change. Aye yaa yaaa....... Loved the sound but if that's going through the speakers at higher volumes I don't love that. Just sitting back and closing my eyes enjoying it I did not discern that hum, but on to plan C now!

The box was not sealed when I got it, the folded cardboard ends didn't look like they had been opened and closed once or repeatedly... but I wonder... it was from amazon & they've pulled shenanigans with me before!

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