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Old 19th October 2010, 12:55 AM   #21
jrenkin is offline jrenkin  United States
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Quote:
Originally Posted by SY View Post
Cascode LTP. Use high gm FETs on the bottom.
I am just trying to find a straightforward schematic example of a cascoded LTP. I am having trouble wrapping my mental imagination around it. Thx.
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Old 19th October 2010, 01:51 AM   #22
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Mile's Vixen. It even feeds to 807's (in AB1) so is the answer to all the OP's questions...
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Old 19th October 2010, 01:56 AM   #23
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oops - wrong Miles design, it should actually be this one...
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Old 19th October 2010, 03:08 AM   #24
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a quicker link to Mile's cascoded LTP diagram:
http://i40.tinypic.com/2jaghdw.jpg

------------------------------

"This Kulish driver is not well thought out. It looks like a hack someone tossed together carelessly. ........ It should be stricken from the records and forgotten as so much tube folly."

Hehehe.....Thats why I like the circuit, tubies look at it and immediately analyze it as a Darlington and conclude it's Cuckoo. Sorry about the IGBT symbol, I just like it better since it is more readily understandable for analysis. (I did label it as a Mosfet)

You do need to read the earlier linked Kulish Circuit thread to fully understand its operation. It uses feedforward in an elegant way, and achieves around 100X better linearity than a conventional gain stage.

To briefly recap the modus operandi, (ignoring for the moment the effect of R2) R1 current is (Vgate-Vgs)/R1. Then putting R2 back in, R2 current is Vgs/R2. Since R2 is very nearly equal to R1, the combined current in R1 will now be (Vgate-Vgs+Vgs)/R1 or just Vgate/R1. This same current will appear in the load RL (currents from V1 and Q1 summed). So Vout is proportional to Vgate*RL/R1. The current thru R2 is "nearly" constant, since it is the Vthreshold voltage of Q1 plus a small variation for gm activation of Q1. Assuming a constant current thru V1 would just result in a constant voltage across V1's grid to cathode. We are almost there. If Vin is just a constant off from Vgate, then Voutput would be directly proportional to Vinput (AC).

The small variation of Vgs affecting V1 current is the remaining detail. This is where R2 "nearly" = to R1 comes in to play. By misadjusting R2 ever so slightly, we can arrange so that its earlier feedforward correction current to R1 can take into account the slight current variation thru V1 (this assumes a near constant gm of V1 over this limited range). This requires a tweek adjustment of R2 (using a dist. analyzer) until the distortion nulls. Practical results achieved with this circuit using all SS devices have achieved 100X linearity improvements over a simple one device stage. (by the way, there is no Darlington like product of gm's here due to R2's near constant current grossly swamping out V1's gm, it's not a Darlington)

Several topological variations of the circuit exist, including cascode. Device implementation flexibility exists, but factors like bipolar gate currents and finite Beta lead to some adjustments. This circuit uses feedforward correction in a remarkable way, rarely seen implemented in audio. These circuits were originally published in a Russian magazine by one of our members Mikhail Kulish. Last time I checked there were around a half dozen threads in the SS forum referencing this design (well, not the one with a tube it).

Don
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Last edited by smoking-amp; 19th October 2010 at 03:36 AM.
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Old 19th October 2010, 11:03 AM   #25
SY is offline SY  United States
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Quote:
Originally Posted by jrenkin View Post
I am just trying to find a straightforward schematic example of a cascoded LTP. I am having trouble wrapping my mental imagination around it. Thx.
Take a look at Joe Curcio's ST70 mod published in Glass Audio. Might also be on his website.
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Old 19th October 2010, 11:56 AM   #26
AJT is offline AJT  Philippines
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@smoking-amp, can you post links please? thanks....
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Old 19th October 2010, 12:43 PM   #27
jrenkin is offline jrenkin  United States
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Thanks Smoking-amp and Sy. Now I see it better.
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Old 19th October 2010, 01:21 PM   #28
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Default 807 - status update

Today I used the pentodes in long tailed pair.
I first tried the EF184, with 22k anode resistors etc like yvesm suggested.
It worked and has plenty of gain (a little too much even) But I was a little short on voltage to run the EF184 optimally (preamp B++ being around 300V)
So I replaced it with EF80 tubes and used 170V on screens and anode. Works like a charm now! (altough I might still raise the B++ voltage a little)
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Old 19th October 2010, 01:28 PM   #29
waltube is offline waltube  Italy
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One question:
If you have a balanced input, why you want to use a LTP?

Bye

Walter
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Old 19th October 2010, 02:05 PM   #30
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"@smoking-amp, can you post links please? thanks...."

Kulish Circuit

Single darlington line preamp.

The more general circuit uses an R3 with an R1 not equal to R2. This extension allows further flexibility to match the correction of the first device for not only its average gm slope but for the avg. curvature variation of its gm also, I believe. The RL load impedance would also affect the range of variation of Q1's gm versus V1's gm variation, so is another factor affecting the tracking/correction of V1.

For tube circuitry, the simplified and easier to use version presented above may be adequate for many cases since it allows cancellation of the Mosfet distortion, leaving some tube signature likely. (although tube and Mosfet devices both reasonably approx. square law curvature already due to the additional "Island effect" from tube grids, pushing the order from 3/2 to 4/2, so correction/ tracking of V1 could be decent too. Use of an RL that is more tube like (as opposed to 8 Ohms say) will likely improve the correction tracking for V1. A try it and see design approach.)


I can't read the Russian article, so would greatly appreciate any feedback on additional details or variations. I'm assuming the above simplified R1 "nearly" = R2 approach was used for the cascode design in the article. Further tube variant discussion of the "Kulish Circuit" would maybe best be continued in the "Kulish Circuit" thread, since we appear to be heading off topic here.

Don
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Last edited by smoking-amp; 19th October 2010 at 02:34 PM.
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