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Old 14th October 2006, 06:34 PM   #1
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Default driver stages

Ok, I've been lurking in the background for a while and I am trying to get the opinions and facts about what I think are the two most popular driver stages for a push pull tube audio amp.
1 the split load "concertina" or
2 the differential stage, i.e. 2 matched triodes with a common constant current source cathode.
I am an objectivist I don't believe in what you imagine you hear, or really hear, unless you can demonstrate it to another person without telling them to hear it. In other words, I believe you can hear something, but I know it has a cause and can be demonstrated to another person. The most effective way I know to prove something is with measurements. If you think measurements are ineffective, invent a new one and submit it to peer review and we will name a new audio effect after you. Everything you really hear has a reason and I've yet to see a cause that wasn't RILL or C based. If it isn't, I will shake your hand in Switzerland.
I see a lot of opinion on the two dominant driver stages, but little fact. What I'd like is a FLAME LESS discussion of the merits of concertina VS differential drivers. The parameters of interest are distortion, affect of feedback on the stage, harmonic structure of the distortion, ability to drive Miller capacitance, slew rate, best tubes for each service, bandwidth and noise. Any other relevant criteria are welcome too. Again, I don't want objectivist calling subjectivist stupid and subjectivist calling objectivist tone deaf. And please no "Magic Pixy Dust"
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Old 14th October 2006, 08:20 PM   #2
anatech is offline anatech  Canada
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Hi TUBESMAN,
Quote:
I don't want objectivist calling subjectivist stupid and subjectivist calling objectivist tone deaf. And please no "Magic Pixy Dust"
Is that even possible?? It is certainly a very tall order.

There have been many discussions about this in and around the forum. I think you might want to define a specific question and allow the answers to roll forth.

-Chris
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Old 14th October 2006, 08:23 PM   #3
SY is offline SY  United States
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In general, the concertina, with a direct coupled common cathode amp driving it will give higher gain and less swing than the constant-current diff amp. How's that in a nutshell?
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Old 14th October 2006, 08:35 PM   #4
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Both the differential and "concertina" phase splitters should be built around high gm types. It's a matter of avoiding slew limiting.

When executed properly, both splitter types perform well. The differential splitter has the advantage of providing gain. We took advantage of the gain a differential splitter provides in "El Cheapo" to create a 2 stage "integrated" amp. The "El Cheapo" saga can be found here.
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Old 15th October 2006, 01:43 AM   #5
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TUBESMAN, you should start by asking a more rational question, then you might receive rational and succinct answers. What you describe as 'drivers' are not drivers at all, they are phase splitters. In fact, the concertina makes a poor driver unless the output tubes are very undemanding, such as EL84s. That is one reason the Dynaco front end has been redesigned countless times.

Quote:
But the dif has more noise.
Say again? The dif has excellent PSRR, a notorious failing of the concertina. In fact, Jim Broskie went to some lengths to tame power supply noise from the concertina splitter with capacitor dividers between B+ and ground.

The dif also gives much greater swing and does not sound so ugly if overdriven on transients. The concertina is unpleasant if overdriven, which is one reason it should be located before a driver with gain, rather than trying to use it directly as a driver.

As to the types of tubes that are most suitable, a low-mu or medium-mu triode (e.g. 6SN7) is generally considered best for a concertina splitter (lower OP impedance), while a high-mu double triode (e.g. 6SL7) works best in a dif splitter (better balance).

PS you ain't gonna get 25 watts or more out of triode-strapped EL34s. Maximum is probably 18 watts, even with fixed bias.
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Old 15th October 2006, 01:45 AM   #6
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I like nutshell answers. After reading many post on the subject of tube types and configurations I decided that I'd like to try a Dif amp with a CCS. I've run Tube Cad simulations and if they can be believed, the dif can be configured for symmetrical output swings easier than a concertina. But the dif has more noise. I figure about 3db more noise power. So I'd take 2 6DJ8's parallel the sections, put 1 in each leg of a dif amp, that should make the noise a wash. Use a CCS in the tail, like the one Morgan Jones described, and call it a day. I want to drive either EL34's, KT77's or something like that , preferably as triodes with fixed bias, to get around 25-35 Watts. A little less power is ok too. Is there an obvious flaw with my idea that I am blind too? Can you use (Vcc)*(Vcc)/2Po to get the a-a load impeadence? If not what other than a graphical method can be used?
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Old 15th October 2006, 03:44 AM   #7
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Hi,

Quote:
Originally posted by TUBESMAN
....the dif can be configured for symmetrical output swings easier than a concertina.
Indeed! And for truly symmetrical output, add a little common-mode local feedback.

Attached is a phase splitter with CMFB I've adapted for a 6V6 amp, based on Fred Nachbaur's (RIP) tube version at: http://dogstar.dantimax.dk/tubestuf/driver02.htm

Cheers!


** edit **
Right attachment this time
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File Type: gif phase_splitter.gif (6.5 KB, 328 views)
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Old 15th October 2006, 03:52 AM   #8
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Tubesman, nothing at all wrong with your idea, but run at a decent current, a single 6DJ8 section is already ridiculously quiet. Paralleling the sections doubles all the capacitances for not much return.

I'm puzzled about the symmetric swing comments. Neither cc diff amp nor concertina have any appreciable asymmetry as long as their loading is symmetric. If the loading is nonsymmetric, neither circuit will have a symmetric output.
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Old 15th October 2006, 06:20 AM   #9
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Power supply noise rejection is not the same as broadband noise from the device itself. A dif amp I've read, right here by one of you guys, has more noise than a single end stage. seems reason able for a bunch of reasons. The comment about the symetrical output is based on what tube cad shows as the maximum positive vs maximum negative output voltage from each type of circuit. With the level of P-P drive voltage I need it's likely its not a factor. I can live with 18 watts, but I'd like a little more, perhaps parallel pairs or triode connected KT88's
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Old 15th October 2006, 06:33 AM   #10
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Geek, the circuit you posted looks like a hybrid version of something I was interested in from Glass Audio. They used a tube to read the balance error of the dif stage and feed it back to correct it. The way it looked, I think it could do two things, provide whatever level of balance you want for the next stage and make sure the two phase signals are exact mirrors of each other.
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