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patrick-turner patrick-turner is offline
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Default Boosted cathode follower, super cathode follower etc

Recently we heard about Mr Pogossov's "boosted cathode follower" using a bjt between the cathode and 0V rail where the bjt has base current from an anode R between anode and B+. Current production of bjt is regulated by Vg-k of the follower tube, and all sort of positive claims are made about lower Rout, lower THD etc, but not one measurement of Vac signals possible or distortion have been offered to prove the claims are valid. In the fullness of time, the voltages and THD measurements may follow, we might hope.

Meanwhile, instead of using a bjt as an active current source in the cathode circuit, we can always use the bjt as a passive current source, as seen in my preamp circuits at Fig4 Schematic at page http://www.turneraudio.com.au/Line-preamp-2003.htm

One might be delighted to find THD at a volt of output to an average power amp is so remarkably low because the 12AU7 involved does not have to produce a current change to a load carrying DC, and only has to power the output load which is usually the high input resistance of a following amplifier.
But the amp isn't distortionless, it just has about 1/3 the usual THD merely by making the load "easier" to power, ie, higher ohm value, therefore making the internal triode NFB more effective.

The other form of boosted CF is Allen Wright's Super cathode follower where the anode supply to CF is bootstrapped. It means that insead of a fixed B+ for a CF, you have a second CF triode ( or j-fet, mosfet or darlington pair bjt ) arranged so its cathode output is at say +150Vdc, and its gris is biased with 1M to +143Vsc, and the input Vac is applied to BOTH CFs, so that the lower one's cathode produces the wanted output load signal, with cathode current minimized with a CCS cathode load, while the anode voltage of lower CF has Vac applied at just slightly less amplitude by the top follower, so in reality, the Ea of the bottom follower dealing with wanted signal functions with a constant Ea, as well as a load current that is minimized. Wizards amoung you can work out the resultant lowering of THD by careful load line plotting with curves and a ruler, or even perhaps a simulation program, although it seems hardly anyone here bothers with such BS any more.
One might bootstrap the anode voltage of the follower so MORE Vac is applied to anode than is applied to its grid and arrange the circuit so the anode voltage applied is subject to Vgk, and is in effect, a NFB voltage applied to anode. Nobody seems to have tried that anywhere, yet, and I guess its because its all too hard to do and the average CF is already pure as fresh snow so no need to pay the costs or take the time for more unecessary complexity - ( maybe not if you can go skiing instead of soldering....)

You may recall I mentioned using 2 more additional triodes used to boost the function of what may be a "weak" CF, say 1/2 a 12AX7, by means of using the additionals as a cascaded amp with say EL84 CF output with its cathode connected to 12AX7 cathode, so forcing the internal voltage gain to very close to the µ of the 12AX7, with very much reduced Rout and THD, as could be easily forecast and calculated with nornmal gain&NFB equations. But its all so complex, and easier to just use the EL84 without the 12AX7, and maybe one finds its better and boosted "enough".

The other boosted CF involves using a pentode and forcing it to operate as a pentode while in CF mode. Most CF with say EL84 will have screen tied to B+ with anode, so thay are triode, with open loop gain say 18, so CF gain = 18 / ( 18 + 1 ) = 0.947, and open loop THD of say 1% into RL = 20k at 10Vrms is reduced to 0.053%, so that the 1Vac output level its about 0.005%. Not a high need to boost any darn thing eh?
Well, the EL84 could have its B+ anode voltage kept fixed, but have its screen voltage fed by CCS from B+ higher than anode, and then bypassed with C to cathode. This means OLG is raised from 18 to pentode gain of about 120. The OL THD might be 2% at 10V out, and this is reduced by factor of 1 / ( 1 + 120 )
to become 0.0165%, so at 1Vrms it is 0.00165%, rather low enough for most ppl I suspect.
But any THD in the cathode signal is fed to screen, where it does not act to lower THD. So if the screen was fed from a second triode CF whose grid input comes from the INPUT signal for '84 grid, then I would suggest the effect is closer to boostrapping the anode voltage of a triode, but its easier to do because far less dc current is involved, and screen input resistance is high and easily powered by a small triode. Using a triode-pentode like 6U8A would be a good tube to use as a preamp boosted CF with bootstrappped screen voltage.

I better quit now lest ppl complain about infomation overload, if they ain't already bore ****less.
Patrick Turner.
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Alex Pogossov Alex Pogossov is offline
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Default Boosted cathode follower, super cathode follower etc


"patrick-turner" wrote in message
...
Recently we heard about Mr Pogossov's "boosted cathode follower" using a bjt
between the cathode and 0V rail where the bjt has base current from an anode
R between anode and B+. Current production of bjt is regulated by Vg-k of
the follower tube, and all sort of positive claims are made about lower
Rout, lower THD etc, but not one measurement of Vac signals possible or
distortion have been offered to prove the claims are valid. In the fullness
of time, the voltages and THD measurements may follow, we might hope.

Meanwhile, instead of using a bjt as an active current source in the cathode
circuit, we can always use the bjt as a passive current source, as seen in
my preamp circuits at Fig4 Schematic at page
http://www.turneraudio.com.au/Line-preamp-2003.htm

Alex:
Of course we can use a current source, but it is a more or less standard
solution. You can also use just a 10K...20K resistor from the cathode of
12AU7 to GND and bias the grid to +40...80V.

It is easy to drive a hi-Z load, but what if someone wants to drive a 600ohm
load? Or a line coupling transformer which is nonlinear at low F? Then your
follower on the lousy 12AU7 will have gain of 0.5 instead of 0.95 plus
several % of THD.

Boosted cathode folower with Rout about 5 ohm or lower will just laugh
maintaining almost the same 0.95 gain and low THD.

If you just rewire your venerable MJE340 from a CCS into a booster common
emitter configuration, you will vastly improve performance and NOT increase
parts count. (You will be able to get rid of -125V rail as well.) Something
for nothing? Sounds too good to be true, you might think.

And the last thing -- no one forces you to use this or that circuit. The
website is only for "educational" purpose. Perhaps it might give some reader
some hints.


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patrick-turner patrick-turner is offline
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Default Boosted cathode follower, super cathode follower etc

On Saturday, 2 March 2013 15:20:22 UTC+11, Alex Pogossov wrote:
"patrick-turner" wrote in message ... Recently we heard about Mr Pogossov's "boosted cathode follower" using a bjt between the cathode and 0V rail where the bjt has base current from an anode R between anode and B+.. Current production of bjt is regulated by Vg-k of the follower tube, and all sort of positive claims are made about lower Rout, lower THD etc, but not one measurement of Vac signals possible or distortion have been offered to prove the claims are valid. In the fullness of time, the voltages and THD measurements may follow, we might hope. Meanwhile, instead of using a bjt as an active current source in the cathode circuit, we can always use the bjt as a passive current source, as seen in my preamp circuits at Fig4 Schematic at page http://www.turneraudio.com.au/Line-preamp-2003.htm



Alex: Of course we can use a current source, but it is a more or less standard solution. You can also use just a 10K...20K resistor from the cathode of 12AU7 to GND and bias the grid to +40...80V.

***Indeed, but the CCS not only gives reduction of THD/IMD, but avoids the cut off distortion at highist Vo levels if the load is as low as say 10k, which is OK for 1/2 a 12AU7 to drive.

It is easy to drive a hi-Z load, but what if someone wants to drive a 600ohm load? Or a line coupling transformer which is nonlinear at low F?

***Well I would not expect a 12AU7 to drive such a horribly low load. Standards for professional equipments stipulate all signal handling gear have no less than 10k Rin, and no more than 600 Rout. Anyone wanting to drive 600 ohms with 1/2 12AU7 is a "Pharkahn Idiot".

Then your follower on the lousy 12AU7 will have gain of 0.5 instead of 0.95 plus several % of THD.

*** You are quite correct. But nobody in their right mind will use a 1/2 12AU7 to power 600 ohms. So if one really must power 600ohms, use an OPT, or a mosfet source follower, or maybe a power tube like EL84 as a follower and with 30mA at least for idle Idc so you can get about 11Vrms max, and to keep it linear you may need to boost the gain around the open loop, but it gets complex, so better is the OPT with 10:1 step down ratio, so the BQ5 in triode can work with 6k0 while output load becomes 600 ohms, and Rout at sec is Ra / ZR = 20 ohms so DF with 600 ohms RL = 30. For 12Vrms output, Va = 120V, Vg = 7.5V, so gain overall is 12V / 7.5 = 1.35 approx, and the 6BQ5 acts like a follower but it ain't a follower, because it has slight gain. THD into 6k0 RL is not corrected though.

Boosted cathode folower with Rout about 5 ohm or lower will just laugh maintaining almost the same 0.95 gain and low THD.

***But the bjt is doing the work if you have a 12AX7 follower up to the feeble current limits. At this point, we need to consider SPECIFIC schematics and all items in them to consider driving 600 ohms rather than make general statements to make your booster idea look better than it is. Please try to be scientific at all times.

If you just rewire your venerable MJE340 from a CCS into a booster common emitter configuration, you will vastly improve performance and NOT increase parts count.

***Well, the preamp where those MJE240 exist has been working fine since 2003, and located over 1,000km away, and I don't need to offer the owner an upgrade to something that already sounds so dreamily damn good.

***Please Pardon the human element within scientific discussions.


(You will be able to get rid of -125V rail as well.) Something for nothing? Sounds too good to be true, you might think.


***Next time I find myself farnarkling around with a cathode follower load, I'll try your idea, it shouldn't take more than 30 minutes to install a bjt and anode R etc and do a test for THD.

The last thing -- no one forces you to use this or that circuit. The website is only for "educational" purpose. Perhaps it might give some reader some hints.

*** My website has become +20dB more educational now that I have officially retired from the "Force Of Slaves", that army full of people desperate for munny, and forced to make hand crafted gear for customers who only pay peanuts instead of a socially just wage. Now, when they cry tears to get something nice that I might make, I say just "Make it yourself, its all the easy to follow info is at my website." They never ever offer me a financial deal that's better than the dole or the old age pension, and its quite unthinkable for them to ever offer $32.00 an hour which is now the gross mean wage in australia before tax. Self employed tradesmen usually charge $80 per hour, and the last medical specialist charged $320 for a discussion lasting 35 minutes. So this explains why there will probably not be too many "Pogssov Boosted Followers" built in Australia any time soon, but all that does not take away the interest I have in such matters.

*** Presumably, one could set up a power mosfet with small current sensing R in drain circuit, then cap couple the drain to a bjt base of a darlington pair power BJT in the source to -ve rail circuit, and Id at idle was say 1Amp, then
performance would drive a speaker, or anything not needing more than +/- 1A of peak current. Other ideas can what is dervived from a Ziclai circuit where you have a small signal npn bjt in common emitter gain mode driving a PNP power bjt with its collector output all applied to npn input bjt emitter, and with a CCS or just resistance to a -ve rail. Many SS power amps have used this principle with such ziclai pairs in complementary form. So thus the drive to the power bjts is determined by the current in Q1 collector RL, and the output bjt current forces the Ve of Q1 to chase after the Vb.
But there's no reason why the Q1 of the ziclai pair can't be a vacuum tube and the RLa generates a voltage to drive the base of Q2, a PNP and thus the high NFB involved gives lower THD and lower Rout.

IMHO, Q1 could be say 12AX7 paralleled, and with suitable small value RLa to give a suitable small drive voltage of maybe +/- Vpk to a gate of Q2 NP small power mosfet ( or darlington pair bjt) set up in class A with Idc = 100mA, and with its source going to a +10V rail, and with drain load of 100 ohms (or CCS) to -10V. Drain is also directly connected to AX7 cathode, and the 100r source load ( or CCS )offers the I sink for Ia of tube. The drain voltage output will be at same Vdc as 12AX7 Ek, so cap output coupling is safest, although Eg1 of AX7 may have adjustable Vdc to get Eout to be close to 0Vdc. Then the output will have no trouble driving all loads needed up to about +/- 8Vpk and +/- 0.1A pk, (32 ohm headphones would be fine,) and all action of the mosfet is rigidly controlled by 12AX7.

Another alternative for driving 600 ohms might be to use a pair of EL84 arranged as white follower, with top follower '84 in triode and bottom '84 in pentode, so that bottom unit tends to do most of the work into RL, but controlled with NFB of the top '84 follower. But you still end up with a tube driving a load under say 1k0, not good, and likelihood of oscillations, unless you have an OPT. As load ohms come down, and expected Vout goes higher, and as human expectations increase, then complexity, hardware and expense goes up. So possible solutions need to be assessed with a specific function in mind.

I did once make a headphone amp in a guitar amp for non critical monitoring of levels and using a single 6BM8. The input triode gave some gain and drove the pentode as a triode CF with choke load and about 30mA dc. Even with 32 ohm phones, there was enough Vo. But for better fidelity, there needs to be more voltage gain ( or current gain ) in devices used, which may be a complementary pair of darlington pair bjts. MJE340 and MJE350 with small TO92package bjt drivers will work fine in class A where Ic = 50mA, and with +/- 12V rails. A tube can be used as an input follower if needed, and its cathode load is the high Z load of the solid state arranged as darlingtons. Expect THD 0.01% at a few volts output into say 100 ohms.

Yet another way is to use a pair of input triodes in an LTP with CC tail.
The RLa of V1 is taken to an N channel mosfet source follower,( or to P and N complementary pair ). V1 anode goes to B+ equal to Ea of V1. Input is to V1 grid. Source output from mosfets idling at 0Vdc is connected to grid of V2 for NFB and thus Vout and V1 Vg1 follows after Vin V1 g1. Again, the mosfets don't need to be mosfets and can be pair of NPN and PND darlington pairs.

I'm rather not inclined to ever allow bjts to do the lion's share of the work when an audio signal is involved. Passive CCS are OK because they don't allow the bjt to do anything to the signal level, the tube does it all, and better because the supply of Idc is constant. Mosfets are more forgivable, IMHO, and I like every device used anywhere working in class A and its most linear region whatever the signal level is. I allow the human in me to interfere with the purest scientific rational because the sound of tube circuits justifies their irrational use. Apologies to fragile egos of any readers...
Patrick Turner.
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