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Old 2nd February 2002, 07:14 PM   #1
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Default New P-P output topology? the Elliptron

For those who have been wanting to try a distributed loading design using partial cathode follower action, but couldn't find a transformer available or affordable: the Elliptron! Just use a commonly available Ultra-linear configured transformer but wired up like a Circlotron. Except! - connect the output tube cathodes to the 40% taps. The primary center tap gets grounded like in the Circlotron topology and the plates still connect to the usual primary endpoints through separate floating power supplies. (Cathodes for a given tube connect to the 40% tap on the OPPOSITE side to its plate/pwr. connection) For a typical 40% tapped transformer, the result becomes 28.5% cathode follower action which is much easier to drive. ( .4/(1+.4) ) Suggest using two cheap dual bobbin isolation transformers with 117x2 secondaries in series for plate supplies to minimize common mode capacitance to ground. The Hammond 1650T output transformer with its low 1900 Ohm CT primary and UL taps is a good fit for this idea. Can make as a Modified UltraLinear (per N. Crowhurst) (see Glass Audio 3/96 page 20 ) if desired by just using a fixed screen voltage to ground.
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Old 2nd February 2002, 07:48 PM   #2
dice45 is offline dice45  Germany
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Sorry,

got lost at the floating power supplies, what wil force the current flow through the primary part?
Please post schematic.
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Old 2nd February 2002, 08:11 PM   #3
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Default verbal schematic

For one tube, the connections are as follows: Cathode connects to 40% tap (Left side primary say), Plate connects to positive terminal of its own floating power supply. Negative terminal of floating supply connects to end of primary (but on Right side of primary center tap).

Second tube: (just mirror image but I'll spell it out) Cathode connects to 40% tap (Right side primary), Plate connects to positive terminal of it own (2nd) floating power supply. Negative terminal of 2nd floating supply connects to other end of primary (on Left side of primary center tap).

Center tap of primary is connected to ground reference. Output tube grids are driven with respect to ground reference.

If you have a schematic for a Circlotron available, just move the tube cathodes inward to the 40% taps. Works the same way as Circlotron but has 28.5% cathode follower action instead of usual 50% cathode follower action.

Note: Using this connection of course alters the turns ratio and hence impedace rating from the usual transformer specification. As an example: the Hammond 1650T xfmer has a specified rating of 1900 Ohm plate to plate (ie. across the entire primary end to end) With Elliptron connection this becomes .7 of total primary turns from end to opposite 40% tap, so impedance seen by one tube is .7x.7x1900= 931 Ohms. This would be equivalent to a 3724 Ohm Plate-Plate rated transformer in usual center tapped push pull output stage. (4x impedance with 2x turns)
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Old 3rd February 2002, 05:16 PM   #4
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Default precision schematic

********************************U.L. output transformer
************I---------- +PWR- ------------------>||***connected
************I************************>||******as
************I************************>||****Ellipt ron
***********plate**********************>||***output
*************************************>||
******o-------grid******************40% >||
**********************I---------------------->||<------o
**********cathode*****I**************>||<*****xfmr .
************I*********I*******C.T.------>||<*****out
************I*********I**********o gnd***>||<------o
************I-------------------------------------->||
**********************I********40%**>||
**********************I**************>||
Note: screen*********cathode***********>||
*******grids**************************>||
*******not******o--------grid************>||
******shown************************I-->||
*********************plate**********I
***********************I----- +PWR- ---I
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Old 4th February 2002, 12:22 AM   #5
dice45 is offline dice45  Germany
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Hello Don,



your precision schematic made me lost in the smoke again but your verbal schematic was very helpful and totally clear. I took a parer and drew it from your words. Thanxalot!



I got lost where to connect the pwer supply, AFA circlotron is concerned I am a total newbie.



What is the benefit of this circuit compared to an orthodox PP output stage? I get the hunch it could yield a lower output Z without having to apply negative loop feedback.
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Old 4th February 2002, 02:59 AM   #6
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Hi Bernard,
Sorry about the "precision" schematic. It all moved around after posting, now it's aMAZEing! Regarding the Elliptron output, its main advantage, as you surmised, is its low output impedance due to partial cathode follower output. It also should be low distortion due to the local feedback effect. The McIntosh unity gain output and Circlotron are similar in this respect, but difficult to drive because of their 50% cathode follower action (large voltage swing). The 28.5 % cath. fol. action in the Elliptron requires significantly less drive voltage, so is easier to design the driver stage. May still want to use bootstrapped driver plate voltages, but not absolutely necessary as with McIntosh and Circ. The main disadvantage, of course, is the need for TWO FLOATING power supplies in the Elliptron (and also the Circ.). These supplies swing around at the audio output levels, so is critical to provide low common mode capacitance to ground in the power transformer(s). Also need low common mode capacitance between the two power supplies since they swing in opposite phase. (important consideration if both High Voltage windings are on the same transformer.) Easiest way to get two highly isolated HV supplies is to use two DUAL BOBBIN (line volt. primary(s) and secondary(s) are on separate bobbins) isolation transformers (usually have two 120V primaries and two 120V secondaries). These are commonly available and inexpensive (Stancor, Hammond, Magnetek Triad, ....)

The partial feedback distributed loading topologies have always been rated among the best, but are difficult to find transformers for. Plitron offers some in their Specialist Transformer section, but are pricey. The beauty of the Elliptron scheme is that cheap and available Ultralinear transformers can be used. Also, the Circ. and Elliptron use low impedance primary transformers since each tube uses more than half the primary winding. These generally have better frequency response than high impedance transformers.

Some good information on distributed loading can be found at these sites:
http://homepages.enterprise.net/iced...w/distribu.htm
and several articles on McIntosh and Circlotron:
www.tubecad.com
Don
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Old 4th February 2002, 05:20 AM   #7
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A few, no a lot, more thoughts.
The question naturally comes up in Circlotron (and Elliptron) circuits as to where to get plate voltages for the input tube stages without adding a third power supply. Usually this is done by connecting two high value resistors to the output tube plates and using the common point between for B+ (low current available of course, make sure to bypass to ground with a good cap. or will feed back output signal to input stages)

The driver tubes which generate the large drive signals to the output tube grids usually use bootstrapping plate resistors from the opposite (than the one driven) output tube plate. (cross coupled) This way they have a high voltage available when needed at signal peaks. This technique makes the bootstrap plate resistors look like higher value resistors if the output plate voltage variation on the resistor is equal too (or a little less than) the driver tube plate voltage variation. (If exactly equal variation on both ends, the driver plate resistor draws constant current just like a constant current source!) This technique works well in the circlotron and McIntosh since output plates and cathodes have the same amount of variation on them with 50% cathode follower action. With the Elliptron however, the lower 28% cathode follower action will cause the plate bootstrap to get too much voltage variation from the output plate, so a little resistive divider (to some fixed voltage, maybe to the screen supply) attenuation is in order or one will end up with positive feedback thru the bootstraps causing it to oscillate. Best to look at some existing McIntosh schematics for ideas on using bootstrapping. Unfortunately, not much published on Unity gain/bootstrapping technology, everyone is afraid to touch it in current publications. Too complex. Some possible sources are in the original publications: Audio Engineering, Dec 1949 pg 9 Mcintosh/Gow and Audio, Nov 1954 Tomcik and Wiggens (Circlotron) I haven't looked at these yet, wish someone would put them on the Web or re-publish. (They are not even in the Audio Anthology series) McIntosh patent is available on the Web though: #2477074 (1949) www.uspto.gov

Since bootstrapping is a little tricky to get right without making a power oscillator, I would recommend just using separate additional power supplies for a first go at this type of design. One can economize later. One will need some driver tubes with good voltage rating for this design if more than say 60 watts output is planned. Some cheap NOS (and decent) tubes are 6CM7, 6CS7, 6FJ7, 6EA7, 6FM7 (500V or more rated).

As for screen voltage for the output tubes, several approaches are possible. The McIntosh series used bootstrapping from cross coupled output plates for this too! They lived dangerously and used the full output plate voltages for screen voltages. I would suggest reducing the bootstrap voltages with zeners (paralleled by a cap.) referenced to the respective cathodes. Alternatively, one could use a fixed screen voltage power supply for both output tubes (referenced to ground) as used in Modified Ultralinear stages described in the N. Crowhurst article. This same supply could then be used for plate voltages for the input stages too. Modified Ultralinear is highly regarded sound wise and would help to soften clipping in an output stage using local feedback such as Elliptron, Circ., & McIntosh. Time to sign off!
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Old 5th February 2002, 05:42 AM   #8
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Default KISS: 3 independent power supplies.

Hello Don,





the bootstrap technique used for providing enough supply voltage fot the driver stage is tricky, prone to oscillation and interstage crosstalk.
Hurts the KISS maxime,IMO.




I had the valuable opportunity to listen to a normal PP amp before and after PS upgrade which was providing regulated independent PSs for every stage. The step forward to more clarity, open-ness, detail resolution and relaxed-ness was HUGE. I don't think it was due to the fact th PS were regulated, it was the interstage crosstalk barrier of the independent PSs. Several preamp upgrades in my environment seem to indicate the same.





I admit several PSs are costly, but i do believe that is is easier tounderstand, track and nail down what (mess) happens in the amp if there is no IS crosstalk via the power supply. Electrically as sonically it follows the KISS principle. In my book, there is just one more PS added and i may have to use one or two different PS trannies. Not much added complexity compared to the benefits





AFA the screen grids are concerned. why not just pull them to the own plate via 100Ohm? Triode operation! It always is the first watt of an amp that counts most!
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Old 5th February 2002, 09:12 PM   #9
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Hi Bernhard,

You are of course on the best track to use separate power supplies, I agree. And bootstrapping is shaky business, but is one of those mountains I would like to climb just once to say I have been there! Fortunately, the lower 28% cathode feedback (compared to Circ. and McIntosh) allows one to go either way, so this is an experimenters paradise.

The issue that arises in bootstrapping (besides stability if not done right!) is whether the increased plate load impedance possible for the driver tubes is outweighed by the small positive feedback of some output distortion components. I believe I read in some N. Crowhurst article that bootstrapping did actually win out distortion-wise (but no info was given on distribution of harmonics generated, something I will want to measure) over using the USUAL resistive plate loads from a fixed supply. But, a set of current source loads for the driver tubes with a separate power supply was BETTER YET. An interesting approach might be to use the bootstrap voltages for power supplies to operate some mosfet plate current sources (triode driver tubes). Especially since the bootstrap voltages available (in the Elliptron case) are too big to begin with. But these complexities should be reserved for later tweaks, after building a working amp the simplest way first.

Triode outputs of course should be fine, but will generally produce less output power per available plate dissipation and power input, as in any design. The original thinking behind distributed loading designs, I believe, was that cathode feedback would be a great way to get the benefit of local feedback, inherent in triodes, but by using more efficient pentodes. Cathode feedback has the advantage of local feedback giving low output impedance (potentially much lower than triodes even) and low distortion without lowering the power handling capability of the pentode, unlike Ultra-linear mode. To get the most benefit from cathode feedback, one would want to use a high Gm (transconductance) tube. This, I realize, is in direct conflict with the usual thinking for low distortion gain with low Gm triodes. But one should keep in mind here that the output tube is providing little voltage gain (about 3 in the Elliptron case). The higher its Gm, the less error in output voltage the tube will tolerate in cathode follower mode, due to the local feedback effect.

The one consideration for Ultra-linearizing the output tube screens with distributed output topologies would be to soften the clipping effects when driven to the rails. By using a fixed screen voltage supply one automatically gets so called Modified Ultra-linear mode since the cathode follower voltage swing on the cathodes effectiviely subtracts from the fixed screen voltage. The 28% cathode follower action here (versus usual 40% in U-L or 50% in Circ. or McIntosh) is helpful when using pentodes with screen voltages less than the plate voltage since the amount of ultra-linear effect is really based on the percentage variation of the screen to cathode voltage. So will be greater than 28% U-L with lower screen voltages.

The driver tubes are the critical voltage gain tubes in Circ., Elliptron, and McIntosh designs. (I should qualify this to "voltage gain driver" tubes since some designs follow these with a cathode follower to actually drive the output grids.) This is where I would be selective in choosing a quality sounding triode. For the unlimited budget one might even consider a set of 300Bs or competitive tubes. (more practical tubes like 7119 or 5687 etc. don't have the plate voltage ratings needed unless for a low power amp.) I have been doing a little research on constant Mu triodes and many NOS triode vertical amp. (TV) tubes are fairly decent and quite cheap and plentiful. Most have robust plate voltage ratings too. Their power ratings are sufficiently high (with low plate resistance) so that no cathode follower grid driver stage should be necessary. For the first shot at designing this topology I will be keeping to cheap components.

Regards,
Don
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Old 14th February 2002, 12:17 AM   #10
dice45 is offline dice45  Germany
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Hello Don,







may i tell you about my unease, reading your post? Technical unease, of course.



1st,



this is not intended as downtalk, i am deeply impressed how deep you are into that compley topology and i have to admit, i do not yet know whare i got lost and where i still keep track of you thoughts. So don't take my remarks personal and DON'T get discouraged, ok?







Reading your descripition, i somehow get the feeling you are more interested in having your amp crossing a technical hurdle than: how does this amp contribute to bringing you nearer to your music. And even more important: how to influence, manipulate the amp to approach more to your personal sonic heaven.







Crossing the hurdle (in a technical way) can be disgress to Selbstzweck (a German word, my dictionaire translates this as "end in itself" but misses the target, IMO, i would translate it as "an action or concept targeting to justify its own existence/completion and besides that, leading to nothing").







IF you are so deep in your amp concept that you do not loose track of which change triggers which sonic consequence and technical side effects having sonic consequences themselves, then the amp is simple for you.







Simple amps have the advantage that a given change has noticable sonic consequences and causes for this can be easily understood.



So many simple amps sound just wonderful be3cause the amp designer had an easy job in getting the amp sonically right. No matter whether the amp has the targeted output Z or THD.







Primary target for an amp IMO still is:



* terrific sonics, it has to SING!



* bench results ensuring that i do not compare apples and and oranges.







Quote:
Originally posted by smoking-amp



You are of course on the best track to use separate power supplies, I agree. And bootstrapping is shaky business, but is one of those mountains I would like to climb just once to say I have been there!








see what i mean?







Quote:
Fortunately, the lower 28% cathode feedback (compared to Circ. and McIntosh) allows one to go either way, so this is an experimenters paradise.






Always good to have options







Quote:
The issue that arises in bootstrapping (besides stability if not done right!) is whether the increased plate load impedance possible for the driver tubes is outweighed by the small positive feedback of some output distortion components. I believe I read in some N. Crowhurst article that bootstrapping did actually win out distortion-wise (but no info was given on distribution of harmonics generated, something I will want to measure) over using the USUAL resistive plate loads from a fixed supply.






Thats interesting. THe DIY gang i am proud to be member of tends to prefer inductive loads whereever possible and why inductors sound better is TMK not yet fully understood. Maybe this could be a hint!



With your topology, you in fact have inductive loads both in the cathode and anode. Combined with the fact the cathode load induces local neg.feedback this shoud lead to reduced distortion.







Quote:
But, a set of current source loads for the driver tubes with a separate power supply was BETTER YET. An interesting approach might be to use the bootstrap voltages for power supplies to operate some mosfet plate current sources (triode driver tubes). Especially since the bootstrap voltages available (in the Elliptron case) are too big to begin with. But these complexities should be reserved for later tweaks, after building a working amp the simplest way first.






Agreed. Simplest way first.



BTW, a properly designed CCS has an almost infinite Z to the PS thus yielding superior PSRR. Maybe as good as a well-balanced differential stage, maybe even better. And the tube runs with an infinite plate load, having gain= which leads to very low distortion. A plate choke however may sound better and may also measure better on the bench.



Sonically, a question of taste, yes, and of money, good plate chokes rip your wallet







Quote:
Triode outputs of course should be fine, but will generally produce less output power per available plate dissipation and power input, as in any design. The original thinking behind distributed loading designs, I believe, was that cathode feedback would be a great way to get the benefit of local feedback, inherent in triodes, but by using more efficient pentodes. Cathode feedback has the advantage of local feedback giving low output impedance (potentially much lower than triodes even) and low distortion without lowering the power handling capability of the pentode, unlike Ultra-linear mode.






Sounds promising. But how does it sound? I wait on yor results







Quote:
To get the most benefit from cathode feedback, one would want to use a high Gm (transconductance) tube. This, I realize, is in direct conflict with the usual thinking for low distortion gain with low Gm triodes. But one should keep in mind here that the output tube is providing little voltage gain (about 3 in the Elliptron case). The higher its Gm, the less error in output voltage the tube will tolerate in cathode follower mode, due to the local feedback effect.






For a cathode follower indeed a high Gm tube is preferable.



Sonds logical to me, the CF stage has unity gain, how then shall it amplify distortion? Agreeing with your conclusions.







Quote:
... The driver tubes are the critical voltage gain tubes in Circ., Elliptron, and McIntosh designs. (I should qualify this to "voltage gain driver" tubes since some designs follow these with a cathode follower to actually drive the output grids.) This is where I would be selective in choosing a quality sounding triode. For the unlimited budget one might even consider a set of 300Bs or competitive tubes. (more practical tubes like 7119 or 5687






if you hunt the Bendix 6900, you'll have a 5687 capable of 600 V at the plate .... and another hole in your wallet



The 6900 sonically are reputed to outperform even the fanciest 6SN7



.... and if worn out you still can use them as bullets







But 300B? I would sell them and buy some serious tubes for the money. Can you live with such small ? Consider you have to handle floating filament supply for each driver tube then, could add to complexity.



Dunno what driving power you need but for a driver the 71A and the #45 (depending on your sonic preferences) are very recommended small power DHTs and still affordable. They vastly outperform any 300B concerning transparency, detail resolution, speed. Some folks on the SET asylum consider the 71A to be the best small power DHT.



If you want to go cheapochaepo, you could choose a triode-wired #46, should not exceed $10 apiece and is sonic gold.







Jeremy, if you are reading this, your comments please.







Quote:
...etc. don't have the plate voltage ratings needed unless for a low power amp.) I have been doing a little research on constant Mu triodes and many NOS triode vertical amp. (TV) tubes are fairly decent and quite cheap and plentiful. Most have robust plate voltage ratings too. Their power ratings are sufficiently high (with low plate resistance) so that no cathode follower grid driver stage should be necessary. For the first shot at designing this topology I will be keeping to cheap components.






Wise. BTW, it pays to search the TV tube field. Tube rectifiers are a widely discussed sound voodoo issue (which recitfier tube sounds best in amp XYZ, yapyap..). Iron man Thomas Mayer started to use TV damper diodes instead of fancy famous $$$ tube rectifiers. Never heard a better system .



He also built a 2 tube integrated amp i.e. 2 tubes from MC phono cartridge to loudspeaker, okok, with the help of some transformers.



Sounded fine. He used ultra-high Gm TV "beam triodes" with =115 for that and only pedants complain about the tube's plate requiring 1200V to get alive.
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