On Sunday, 24 February 2013 23:19:59 UTC+11, Alex Pogossov wrote:
On the dusk of my days I decided to follow an example of some folks and launched my website: www.valveradio.net It is supposed to cover some not very common (though not new) circuit ideas related to radio and audio. The site is pretty much empty, with only one relevant article so far: http://www.valveradio.net/en/home/au...-follower.html I will be adding more. Regards, Alex
Keep up the good work Alex. Most of the world quickly tires of having anything explained to them. But I have the time for those few who look deeper, who take longer, and who think every detail matters.
May I make some constructive suggestions?
Where you have schematics, it'd be nice to know what Vac exists at points around the schematic. For example, where you have a "boosted cathode follower" with a bjt in cathode circuit of a triode, just what voltage and current flows in/out of the bjt base? most bjts have very low input Z and less than 1k which would shunt any voltage production at the 33k anode resistance. There may be good reason to use a j-fet instead of the bjt. But then one has to watch out the circuit doesn't become an oscillator.
Your boosted follower looks very much like a White Follower, but the White is using an anode resistor in the top tube follower to create a drive voltage for the bottom tube. as the load on the follower becomes lower and Iac rises, then the bottom tube does more and it works in PP with top tube. Its not a perfect kind of PP follower, and for such an animal, more tubes are needed, and it all gets so complex ya wouldn't wanna do it.
With the REAL boosted follower, you have would have 3 triodes. One might use a 6CG7, and 6BQ5 in triode.
The boosted follower is actually a cascade amp with the 2 halves of 6CG7, and V1 has a lowish value RLa, maybe 10k. Its cathode R goes to a negative rail, maybe -200V, maybe it is 5k0. Signal into V1 generates a voltage at V1a to drive V2 which is normnal common cathode gain stage with gain about 16.. The V2a drives the V3 6BQ5 grid with a signal much higher than Vin at V1. The cathode of V3 is connected to V1 cathode. So what is happening in the circuit is that the internal gain of the 6BQ5 is boosted maybe 30 times by V1 and V2, and the cathode voltage at V1 is then much more determined to follower V1 grid signal, and Rout is many times lower than the Rout of a normal 6BQ5. If the 6BQ5 has gm = 0.005A/V, Rout = RL // ( 1/ Gm ) = 5k0 // ( 1/0.005 ) = 200 ohms approx.
Where RLk is many times 1 / gM, it may be neglected.
Distortion reduction in all followers Dn' = Dn / ( 1 + [ A x ß ] ) where Dn' is the distortion measured with the load value used when tube is in follower mode, Dn is the distortion if the tube where the load used is in a common cathode gain stage, and A = Va-k / Vg-k, and ß = fraction fed back. In the case of a follower, ß = 1.0, because all the output is fed back and is in series with input grid signal. So the follower formula for Dn' becomes more simply, Dn' = Dn / 1 / ( 1 + A ).
The same principles apply to j-fets, mosfets or bjts. A common Garden Variety of signal bjt with Ic = 10mA and emitter load of 2k0 may be found to have voltage gain of its gM x RL = 0.2A/V x 2,000 = 400. ( this is approximate ).
Suppose the bjt generated 5% THD at 5Vrms when 2k0 is between collector and B+ of +40V. AND suppose the emitter was grounded. Then you'd find Vbe needed to generate 10V = 5V / 400 = 0.0125Vac. A tiny voltage. Well, with the 2k0 in emitter circuit, Vbe remains at 0.0125V, and Vb needed is 5.0125Vac, hardly different to Ve output. The THD is reduced from 5% to 5% / 401, = 0.0124 approx.I'm not sure anyone needs to know the formulas for emitter follower distortion that Alex has, but then RDH has formulas as well for tube distortions but hey, why spend all day making mistakes with formulas, or ****ing around with a simulation program when it would be more realistic to build a circuit, try it out, and MEASURE real world performance. I have found the simple formula above holds true for folowers, and I never bother to quantify follower harmonics because followers have such low THD. However, with any bjt, the input resistance to base is low, and non linear, like a diode turn on threshold voltage. The bjt operates over a small voltage range from about 0.45V to 0.75V, only 0.3V range. So when you measure performance of the bjt, one must be fair to your mind and drive the base with extremely low impedance or drive the base with a current source with high Z and because bjts have a fairly linear relationship of base current to collector current, the current drive gives a more linear performance. But any change to collector current affects the base current. From my point of view based on years of experience, I am extremely reluctant to trust any bjt with providing active gain to any circuit anywhere with audio. But my disdain for bjts hasn't stopped me using them as constant current sources where they excel as slaves to the current requirement of the tubes. Not only that, I've just spent 3 weeks of spare time re-building my bench AC volt meter. I did try some CA1430 "fast" op-amps choko-blocko with bjts plus a few input fets, but I found them too slow for over 1MHz of bandwidth so I made the wideband amps and metering amp using split rails, +/-15Vdc, discrete bjts arranged with LTP and CCS, and output bjts with CCS, and all DC coupled so that the devices make a well regulated 0V at their Vo. I MHz at 7Vrms was dead easy, with good gain reduction with NFB from about 2,500 to 10.
Anyway, to conclude, there is a lot more you could tell us about with your boosted cathode follower, but have you asked yourself if anyone knows how a bjts works? They are very tricky things to use, and very prone to oscillations. In my case the self made meter op-amp drives a bridge rectifier to produce 1Madc for the 50 ohm meter for full swing. The rectifier is in series with 90 ohms and join goes to FB connection of amp. My amp wanted to oscillate at 15MHz and it needed 200pF between Q1 collector and VAS amp base to reduce HF gain. Most people posting up circuits for bjt stuff often leave out many of the finer points about the pain and agony it takes to get something working properly.
What I made is better than what it replaced, which was what I made in 1993.
I can now measure 1mV with a nice big meter face, accurately, so I can see 0.1mV, and for all F between 2Hz and 1MHz. Defeating noise was done using an input stage with 2SK369.
It wasn't until I ws 1/2 way done with the meter re-build that I realise there are piles of available fast opamps with rise times of up to 6,000V/us.
I managed to get my op-amps to give 59V/uS, a vast improvement over using normal op-amps such as NE5534, TLO72, etc. Such "audio" op-amps have their place, but in my mind they don't belong in test gear which needs to have extended HF performance above 200kHz.
I recall my first web-page in 2001. Boy, what a dumb page that was! I was told I'd make a lot of money with such a simple page. Maybe I got 2 emails in the first year from guys wanting to pay peanuts. Then I decided that if my website was never to make me rich, I may as well make it useful for those who wish to learn about tube gear worth using with music. I've currently got 3 big new pages underway...... soon be 40MB.....All free, no copywrite. What happens is that some get ideas and build something without copying every single one of maybe 2,000 details in an an average tube power amp. They don't bother to learn basic properties of devices and other stuff. So if they get smoke it ain't my fault. So the free info has a catch.
Smoke is the reward for those who have the confidence of the un-skilled.
Patrick Turner