SE vs. PP

[dice45]

Originally posted by planet10

Good to hear. I have least a couple PP amps (EL95/EL84/EL34) i want to give Allen's treatment to.

Hello Dave,



I would like to limit my recommendation to the output stage and the fact all stages are differential.



Allen's experimental thing had an ECC88 as input tube.

Q: how many swing can an ECC88 / 6DJ8 handle at its grid?

A: 1.4 V p-p which is 0.5 V RMS

The gorgeous listening comparison was done with a >103dB/W/m horn speaker.

Under those circumstances 0.5V RMS applied to the amp's input will make your ears bleed.



However, I would prefer my amp to have a little bit more headroom at the input. And if I intend to use the amp at a medium-efficient speaker, possibe headroom is used up already. I would feel comfortable with 10V p--p before the input tube is drawing grid current.

[dice45]

Originally posted by dice45











Allen's experimental thing had an ECC88 as input tube.






Q: how many swing can an ECC88 / 6DJ8 handle at its grid?






A: 1.4 V p-p which is 0.5 V RMS






The gorgeous listening comparison was done with a >103dB/W/m horn speaker. Under those circumstances 0.5V RMS applied to the amp's input will make your ears bleed.













However, I would prefer my amp to have a little bit more headroom at the input. And if I intend to use the amp at a medium-efficient speaker, possibe headroom is used up already. I would feel comfortable with 10V p--p before the input tube is drawing grid current.

Hello Dave, Allen and all,









I have to correct some BS from my last post. When I wrote it, I had no schematic in mind. I apologize, Allen.




Allen informed me he had unbypassed cathode resistors in his input stage providing enough bias in differential mode, acting as current feedback and as a side effect, linearizing the stage a lot . He says he has 9V p-p input swing limit.









Let me see, as the stage is a differential pair of cascodes with a current source isolating the stage's virtual ground from a, say, -25V supply and so the input grids will float to signal ground if the preamp is connected and as the tube is running atleast at 10mA, the operating point probably is not located in the knee of the transfer characteristics.




Okay, admitted, the stage not only sounds good but also seems to work properly an to swallow the claimed 9V p-p at the input.









I stated already the amp is sounding wonderful. However, what I still do not understand:




why using a tube so badly fitting for the job and then taming it? Why not using a tube where taming is not necessary? (Allen, I am citing Manfred Huber here and I share his opinion) . Maybe there is sonic potential left in the amp? Dunno, don't have Allen's experience.








I decided for my own amp to go the way using a tube with enough bias and no linearizing current feedback. 1st stage is a differential pair of E80CC with anode chokes (although I later may settle on something like the 5687/6900), 2nd stage is a pair of cathode followers, each with a CCS in the cathode, the CFs facing an eeeezy load. This amp will sound different to Allen's amp however I presume it will suit my taste.









It does me another favour, I have a 3 stage topology and I can tuck 2 XO networks between the 3 stages: no separated speaker XO, be it active or passive. I don't have to; a common cathode stage can be DC-coupled to a following CF and then the coupling cap between CF and output stage is the only one in the whole preamp/poweramp/speaker signal path, providing the LF-rolloff necessary for my "fullrange"-speaker to cooperate with the subwoofer.

[Denis]

[QUOTE]Originally posted by P.Lacombe
[B]The sound of SE is more natural, but the design of the output transformer is difficult, specially for low frequencies, because of core saturation. In multi-amplification systems, PP is used for woofers, and SE for medium and tweeters.


Doubtfully. The properly designed PP amp(and with pentodes!) will leave a SE one no chance.

The common opinion on the superiority of the SE amps may be based on the far from perfect implementation of PP circuits.

For instance, a 300B SE amp can be considered a good amplifier below 1W (roughly -10 dB in respect to its full output). This will be indeed enoug for the loudspeakers of about 100dB sensitivity.
It is true, the linearity at thes level and below will be remarkable, and there will be enough headroom in the output current.
But nothing prevents one from making a PP amplifier with the same or even better distortion behaviour, but with far greater dynamic range and load tolerance. It is a pity, the designers of amplifiers often make the same errors, and do not look beyound old textbooks. I'm not saying these books always lie, but they rathe tell not the whole truth.

[Brett]

Hi Bernhard,

An oasis of sense at last. I agree with everything you've written in your posts above basically. Well implemented PP is superior to SE, ALWAYS IME. Statements like Mohans above "Single Ended amps are for those who are pure in heart and mind" strike me as a lot of crap. Note to Mohan: I'm not intending to be insulting towards you personally, but I hear this sort of statement all the time and I find it has no relationship at all to my personal experience. Or to reality. Quite the contrary, I have heard maybe two dozen different SET designs in my system, and that many again in another I was familiar with, and they just don't gel. I found a post on the Asylum recently that I'll quote as it describes what I hear in varying degrees with all SETs
<i>"The harmonics were of the "Haagen-Dazs Double-Fudge Chocolate Chunk with extra Nutella & half-bottle of liquid Nestle's Quik, topped with sugar cubes & honey" school of delightful excess, just listening to the thing too long would give you a toothache! The image, correspondingly, was HUUUUUUUGE"</i>

But hey, if you have a SET and it floats your boat, more power to you. In the end, only an individuals' chioces count, and if their chosen pathway to musical ecstacy takes them there, I bow deeply.

Even the famed low level linearity of a SE at sub watt powers is easily matched by a well implemented PP, as well as better SNR and greatly reduced susceptability to PSU generated sonic characteristics. But add in a complex load like all real world speakers present, and the performance of SETs is varying all over the place.

<i>Denis</i>
But nothing prevents one from making a PP amplifier with the same or even better distortion behaviour, but with far greater dynamic range and load tolerance. It is a pity, the designers of amplifiers often make the same errors, and do not look beyound old textbooks. I'm not saying these books always lie, but they rathe tell not the whole truth.

Amen.

FWIW, I'm building a set of Allen's PP1C's for a friend, and an RTP pre for me. My PP EL84 will be very similar. I also have most of the components (tubes and iron) for a pair of Lynn Olson's Amity amps, as this design's elegance and Lynn's articles were a large part of the impetus for me to start building amps again a couple of years ago. I just don't need 20W so I didn't build the Amity, and I have the highest repect for Allen as a designer.

<i> Bernhard</i>
Allen's experimental thing had an ECC88 as input tube.
Q: how many swing can an ECC88 / 6DJ8 handle at its grid?
A: 1.4 V p-p which is 0.5 V RMS
The gorgeous listening comparison was done with a >103dB/W/m horn speaker. Under those circumstances 0.5V RMS applied to the amp's input will make your ears bleed.

However, I would prefer my amp to have a little bit more headroom at the input. And if I intend to use the amp at a medium-efficient speaker, possibe headroom is used up already. I would feel comfortable with 10V p--p before the input tube is drawing grid current.

I asked Allen a similar question recently, and got a similar answer. He also added that the noise and distirtion at my expected power levels using 105dB horns would not be easily heard or measured. My question was based on the published performance of the PP-1C. BTW the input stage is a cascode.

SETs hold one great advantage at the moment. There are so many individuals and companies out there working on designs for SETs, and variations like the Ultrapath and parafeed, that there is bound to be an increase in performance generally for this topology. Put the same resources and effort into PP (esp DHT) and I don't think there would be any question of the superiority of PP. I think this will begin to change in 10 years or so as people begin to realise the limitations inherent in SE, and look to eliminate them by developing innovative, and refining existing PP topologies. And SE has almost become a cult.

Ciao

[PassFan]

I guess I may as well check in on this one as well. While new to tubes, I recently built an older Magnavox paraphase p-p design that blew my SE 6L6 right out of my set up and into the garage. Now I don't proffess to know why, but I trust my measurment system to no end (my ears). I believe a crappy design is just that, and it does'nt matter if it's SE or PP.

[planet10]

A good SE amp is much much easier to build than a good PP amp, which is why we see so many good ones. I think that PP in the end will prove out to be the better topology.

dave

[PassFan]

Hey stig:
I had to revise my auction. I sent you an e-mail to your berkeley address. Hope this doesn't change your mind but I won't hold it against you if it does. Good luck on your amp.

[yves]

Would anyone care to shed some light on the
connection of the 2x1000uF caps and the 68 ohm
resistors?
The caps create a virtual ground at the
cathodes, at ac, they are shorted together.
The 470 ohm resistors bias the EL34's.
But the 68 ohm?

On another note, imagine a
constant current source biasing the output
tubes with their cathodes connected together
without a resistor. The ccs goes to ground
or some -ve bias. If I were to connect a oil cap
(say 100uF) from B+ to the virtual ground
at the cathode junction, would that make it
a p-p ultrapath with one cap?

TIA,
Yv