Why isodyne splitters not popular...
Trying to learn more about phase splitters from this forum and the web (so far I have only tried simple split-load and stock Mullard long-tailed pair). It appears that even though the isodyne topology gets lots of positive remarks (can be tuned to balance very well, etc.), it is seldom used in diy circuits. One of the "cons" from the search is that "tubes (6an8 in the original E.F. Worthen article) need to be closely matched ". Are there other well known problems with this approach that makes it inferior to the others and not competitive? I like the fact that there are adjustable "knob"s for optimization. (Sorry if this topic had been discussed in length in the past, but very little turned out during my last search.)
Actually, I see the feature you mention, that this circuit needs to be "fine tuned", I see as a disadvantage. I perfer to design circuits that don't need fine tuning. Any circuit that needs to be fine tuned will eventually get out of tune as parts and tubes age unevenly. Must of the manufacturing industry perfers circuits that can be assembled and work "out of the box" without adjustments. A circuit that gives high performance no matter what is the best.
I've tried Worthens type phase splitter with other tube types and despite burning much midnight oil, I got worse results (thd) when compared to the simple cathodyne. I never liked the idea of matched tubes in input areas.
There is a raft of triode/pent tubes around which make good cathodyne user potential at decent prices. The beauty with the basic cathodyne is a far more reliable phase splitter solution which functions well with most med mu triode/pent combinations. Setting up the pentode screen grid for lowest thd is another hassle but whatever the design one is faced with something to adjust. If one puts a push-pull voltage driver after the cathodyne (i.e Wiliamson amps) then the impedance mismatch from "upper and lower" outputs really comes to nothing. The trick is to set a stage up with tight layout and look at the common cathode......the 2freq symmetry will show how good the balance as freq is increased upwards and I find it behaves well at a far lower THD than anyother configuration.
Even with LTP I'm not keen using this for HiFi, but for MI is the norm.
The real drawback with the Worthens is a much higher input volts and potty adjustment.
Re: Why isodyne splitters not popular...
An LTP with a good CCS in the tail will work better, isn't so complex, nor does it require the alignment that this thingy does.
I don't see how that got a patent. :bigeyes:
I agree with others' comments. I see the Isodyne as an unnecessarily fussy design for the job it does. It's basically an attempt to make the see-saw splitter behave well despite its inherent drawbacks. I cannot imagine that the end result would be any better than the two most popular and much simpler tube-based splitters the others have mentioned.
Addendum: I think the same criticism probably applies to the Van Scoyoc cross-coupled splitter, which wastes two triodes and also requires tuning.
All of the foregoing ignores transformer splitters, which are a different animal altogether.
Hi all ,
The CATHODYNE or SPLIT-LOAD or CONCERTINA
PHASE SPLITTER , no matter the designation , is the
most simple , the most functional and the most reliable
tube circuit ever made .
The opinions have been divided about it , but the exce-
llent results obtained with it , is unquestionable .
Bruce Rozenblit says : “ With all of its drawbacks , the
simple phase splitter is quite useful , and I have succes-
fully implemented it in several designs. Many applications
can easy tolerate the wide variation in output impedance
between the two signals”
Morgan Jones says : “ Although the Concertina phase
splitter does not provide gain , its output balance is almost
TOTALLY determined by passive components ( anode load
and cathode load resistors ) , and valve characteristics
hardly enter the picture”
It’s very , very good , because no matter if valve ages or is
replaced , the balance ( or equilibrium ) remains the same .
Julio Rueda ( electronic engineer from Argentina and dedi-
cated only to Hi-Fi designs , in the 50’s ) says :
Note : Text was translated from Spanish to English , by me
“ The common cathodine or plate and cathode split-load
phase inverter , is an inverter self-equilibrated and is a good
example of relative theoretical imperfection against actual
practical advantages that overcome very much , anothers
designs that at first sight , seem like most perfects “
And more :
Morgan Jones says : “The phase-splitters based on the diffe-
rential pair were all able to provide overall gain , but this was
obtained at the expense of an output balance that is partially
( ... a lot , if you prefer ) dependent on the matching of “mu”
between the valves “ (... or the two halves of them )
My own experience , I have built all kind of phase-splitters ,
but the one that has the best tonal quality , and wide band-
width , undoubtedly is the cathodine. ( ... in my humble opinion ).
Ray Moth , good to see you on the thread , but I’m sorry
to disagree from you . The J.N. Van Scoyoc phase-splitter does
not need any kind of fine tuning or adjustment , because it is totally self-equilibrated due to the “cross – connections” , and has the great advantage that it cancels the 2nd harmonic , but I agree that it wastes two triodes .
Regards for all ,
The key to getting these results is to overcome that sandophobia and implement the tail CCS with BJTs. Cascoded BJTs give far higher output impedance, and thus LTP balance than anything else. Certainly far better than a simple tail resistor, even if you can make it bigger with a negative rail, and better than a small signal pentode. Besides, that's one less hole you need to make in the chassis. :)
A 6SL7 (high mu triode) in LTP phasesplitter confirms the best balance results for such configuration.
When wanting to use a (split-load)concertina with a full gain pentode preceeding for a particular design, do the stage gain maths. The pentode will heave 44dB front end gain (the split-load concertina gain = 0 ) and a following inter-driving stage will only need a gain of 7-8 to drive output tubes to 50+50V RMS. Using 20dB global NFB one arrives around the 0.5V input for full drive for the output tubes (with plenty of reserve). This was one of many standard setups for 3/4 century. Without NFB from o/p tranny sec the figures look like this .
...-> input 50mV (pent 44dB) split = 8+8V (triode 16dB) = 50+50V RMS. This is enough drive for P-P KT88 class to 100W. The trick is to select a lowish mu second stage tube which won’t increase the pentode noise in preceeding stage. I use ECL82‘s (have plenty of them) config as triodes running at 16mA per sect - despite poor looking curves. . the bandwidth is excellent for driving parallel P-P stages. The noise factor from the 1st stage pentode is the mitigating design problem. It pays to study the 7199 app notes (Neds- triode electronics etc).
The distortion from the pentode driven concertina is quite low (lower than any other configuration and depends on pentode g2 setup. The ECL82 in triode mode will give approx 1% thd at 50V RMS output drive (depending on load), but one can’t throw out everything for nothing. 20dB global NFB will drop all this down to around 0.1%. Simple „ ain’t it.??
With the figures shown without NFB, the A weighted noise figure is around ca-55dB down and with 20dB global feedback this improves to an acceptable –75dB.
Using the LTP, this itself has more gain so the input stage can be strapped as a triode or two cascaded so even more noise reduction results to -80 or -90dB levels.
Some say a noise level of –75db/w down isn’t low enough for HiFi...(WHAT RUBBISH)
With 93dB/w sens LS, there’s barely a whisper.. ..
The Van Scoyoc x-coupled splitter is basically unbalanced. It is normally tuned using a pot between the cathode returns of the two cathode followers, with the slider going to ground.
Another drawback of this splitter is its low signal-level tolerance. The consequence of this is that the splitter must be used early in the amp, usually as the first stage. This forces the designer to use an all-balanced circuit - not necessarily a bad thing in itself, because you get very low PS noise with such a topology.
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