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Old 17th October 2011, 02:24 PM   #1
cs is offline cs  United Kingdom
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Default All-tube vs tube/mosfet hybrid ?

How does the sound quality compare between all-tube and tube/mosfet hybrid amplifiers (of a similar output power) ?

I'm interested in building a HiFi tube (or hybrid) amp, after being very impressed by a friend's EL34 based PP amp. A hybrid would potentially be more compact, but would it sacrifice sound quality ?
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Old 17th October 2011, 04:00 PM   #2
Defiant is offline Defiant  United States
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Originally Posted by cs View Post
How does the sound quality compare between all-tube and tube/mosfet hybrid amplifiers (of a similar output power) ?
That depends on the quality of each amp and the listener. Audio and music is a very subjective thing. Also, it's possible to make good or bad versions of either topology.

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I'm interested in building a HiFi tube (or hybrid) amp, after being very impressed by a friend's EL34 based PP amp. A hybrid would potentially be more compact, but would it sacrifice sound quality ?
i prefer all tubes in my signal path. Most hybrids use SS output sections, but IMO the output tubes and transformer are what give tube amps most of their distinctive qualities. SS guys call it "euphonic distortion", but I don't buy that explanation since I have built and listened to tube amps that measure as good as a decent solid state amp, but they still sound like a tube amp. Since you're buddy's amp was presumably all tube and you liked it, you should probably build an all tube amp.

Also, FWIW even though mosfets are the "in" thing these days, I still prefer good bipolar transistors over mosfets in SS amps. But I will still take tooooobs over SS any day

Last edited by Defiant; 17th October 2011 at 04:03 PM.
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Old 17th October 2011, 04:05 PM   #3
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Going hybrid would be no problem in terms of SQ. Going hybrid and doing a poor job would. Just as a poor quality tuber will sound like a potato (bad). Defiant has the goat in the pen on this one. Unless everything in your music library was mastered with all tubes, a grain of sand here and there won't hurt at all.

Just cross your fingers, and try not to think of all those op amps that silicon-free sound went through on the mixing board!
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Old 17th October 2011, 04:27 PM   #4
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All tube mixing boards are still being made! Also some recordists are opting for purist techniques using only two mics and a tube pre-amps. Also, I see more use of Ampex and Studer tube recorders. Now when we get to CD recording and reproduction, we are opening another can of silicon entirely.Regards
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Old 17th October 2011, 04:42 PM   #5
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Thanks very much - all useful comments.
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Old 17th October 2011, 11:44 PM   #6
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Another point of view, I prefer triode connected output tubes over UL and over hybrid and over SS. My madness is quite simple. The impedance of speakers varies over frequency so the coupled power output of a SS amp having very low impedance really does not care. It is designed to go to a specific output voltage level and the resultant power is V^2/R. So as the impedance of the speakers goes up, output power goes down. Hybrid outputs are basically the same as SS in this regard, leaving UL and triode connected tubes. UL connected tubes have localized feedback from the UL tap in the transformer that help to control the gain and linearity of the output section desensitizing the tube to output impedance changes. Triode connected tubes are on their own. The speaker impedance is reflected back to the plate so as the speakers impedance increases, so does the gain of the triode, acting as self compensation in correcting, increasing output voltage as speaker impedance increases. I believe it has a lot to do with the individuals' speakers, I can tell a difference in mine. In a lot of tube amps, the choice is a simple flip of a switch to chose output mode, I prefer to hardwire triode mode.......
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Old 18th October 2011, 12:08 AM   #7
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by cs View Post
How does the sound quality compare between all-tube and tube/mosfet hybrid amplifiers (of a similar output power) ?

I'm interested in building a HiFi tube (or hybrid) amp, after being very impressed by a friend's EL34 based PP amp. A hybrid would potentially be more compact, but would it sacrifice sound quality ?
These amps are really common for bass guitars. Maybe the most common type. They will use a tube preamp and SS power section. Typical power is 200 to 1,000 watts You just can't get there with tubes for a sane price, size and weight.

For home HIFi use I'm sure it would work but mostly you will have a mosfet amp. The one little tube in the input secton will not next to nothing to the sound. All that "tube sound" in a HiFi amp is in the power section and that includes the output transformer ( Again, we are talking HiFi here - very different answer if the subject is live music)

I think if you'd build a mosfet power section you get mosfet sound. Has to do with how you'd handle feedback in the amp, I think. It could work, and work well but it will not have a tube sound.


The bass player can use a tube preamp and SS power because he uses the preamp to introduce intentional distortion (maybe 10 to 20% THD) This adds some overtones very much like a wooden sound board does on an acoustic instrument. But this use case is absolutely contrary to what you want. Your preamp tube would be running at very low gain and low distortion. I think hybrids are worth the effort only for live sound, not HiFi.

Last edited by ChrisA; 18th October 2011 at 12:18 AM.
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Old 18th October 2011, 03:09 AM   #8
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Guys,

The Devil is very much in the details. One amp in my collection is an older AVA FET-Valve hybrid that sounds damned good. You really can't talk amps, without mentioning the speakers they are mated with. A big part of getting good performance is amp to speaker matching. For instance, the AVA is not good with horns, as things are badly overdamped. OTOH, paired with AR3s or AR9s is "hog heaven".
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Old 18th October 2011, 05:08 AM   #9
Defiant is offline Defiant  United States
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Originally Posted by Marine Boat View Post
Just cross your fingers, and try not to think of all those op amps that silicon-free sound went through on the mixing board!
Fortunately, as long as the op-amps are kept well away from saturation and have good bandwidth, they won't damage the sound too bad. Unless the mixing board is a low quality POS the result depends a lot more on the skill of the recording engineer than the active devices used in the mixing board.

I don't do the thing of putting together a super high-end system to listen to "audiophile" recordings of crap I don't like. I build gear to listen to stuff I do like, and most of it isn't the best when it comes to the engineering and skill in mastering. IME, tubes do a much better job of making such recordings listenable and enjoyable. Many SS amps make you want to stick icepicks in your ears after listening to these same recordings for an hour. By the same token, a good tube amp will do an absolutely stunning job of rendering a good recording, provided you have good matched speakers of course (goes for any system regardless of technology).

Costwise, a decent tube amp will always cost more than a decent SS amp, but truly high end gear is comparable in cost either way. Cheaper SS gear is built with regular parts purchased in tape and reel form and stuffed by automated machines. The circuits are designed to be to be tolerant of wide variations in the active devices so that no culling or matching of parts are needed. High end SS isn't this way - go read some of Nelson Pass's posts where he talks about culling and hand matching transistors. You may have to go through dozens of devices just to get a good matched pair, let alone the larger quantities used in output stages. SS amps also typically use more discrete components, which drives the cost up further. The point is, if you want to build a decent amp a hybrid may be cheaper, but building a GOOD hybrid (or purely SS) amp isn't going to be.

Last edited by Defiant; 18th October 2011 at 05:29 AM.
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Old 18th October 2011, 01:16 PM   #10
cs is offline cs  United Kingdom
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I've noticed that some commercial hybrid amps (such as those from Croft Acoustics), use the output mosfets completely open-loop. But in simulations at least, I've found that the mosfet distortion massively exceeds the triode distortion once you get beyond the 3rd harmonic, producing a raft of higher order products which are absent from the tube.

I guess this means that the mosfet stage would have to be linearised with feedback, but then it might just sound like any other SS amp, as mentioned in the posts above.

Maybe there isn't any substitute for power tubes and transformers, in order to achieve the tube magic.
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