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Old 23rd July 2004, 02:25 AM   #1
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Default Question about direct coupling a anode follower into a cathode follower.

As I was shown and taught by Sy (haven't heard from him in a while) a simple cathode follower with a single sided power supply has some limitations because of the grid being referenced to ground and sitting essentially at zero volts. In a direct coupled two stage circuit where an anode follower is direct coupled to a cathode follower does the cathode follower still suffer from those limitations? It would seem to me that it does not as the grid is lifted quite a ways above ground by the plate voltage of the anode follower. Am I looking at this correctly?
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Old 23rd July 2004, 02:32 AM   #2
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Hi,

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Am I looking at this correctly?
You do...Not that direct coupling all of a sudden turns the humble CF into a super CF but, yeah...it does rid it of that particular problem.

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Old 23rd July 2004, 02:52 AM   #3
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So I could direct couple a 5687 CF to the anode follower of my 5687 preamp and lower the output z significantly. How do you think that would sound sonically Frank?
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Old 23rd July 2004, 03:16 AM   #4
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Hi,

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So I could direct couple a 5687 CF to the anode follower of my 5687 preamp and lower the output z significantly.
You mean the other way around, right ?
Does that lower the Zout of the CF ? I don't recall it doing that, can't see how it would really.

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How do you think that would sound sonically Frank?
This is the kind of circuit we've been trying to avoid for the past ten years, maybe for the wrong reasons...

Anyway, from previous posts I gather you plan on building a preamp with good sonics and low Zout.
Why not try something with either the 12B4A or EL84/6BQ5 trioded with either choke or resistive load?
While certainly not the last word in Zout it's undoubtedly hard to beat on sound.

If you want lower Zout with that type of tube and still need some gain then you can also try a WOT with this.

Or, go for one like this one if you listen to headphones as well:

Cheers,
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Old 23rd July 2004, 06:06 AM   #5
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a simple cathode follower with a single sided power supply has some limitations because of the grid being referenced to ground and sitting essentially at zero volts.
Yes, a CF connected this way is not very linear and can not handle large voltage swing.

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In a direct coupled two stage circuit where an anode follower is direct coupled to a cathode follower does the cathode follower still suffer from those limitations?
No it doesn't if you allow enough voltage over the cathode resistor, (this has to be larger than in an ordinary CF of course). the output impedance of the CF will be the same but will be shunted by a higher resistance.

Another way of increasing grid voltage and allowable voltage swing is to use 2 resistors in series between cathode and ground and then connect the grid leak to the connection point between those resistors. With this connection you can use capacitor coupling.

In any case if you allow 100V or more between cathode and ground of the CF using one of the above methods the linearity will in practise be the same as when using a CCS in the cathode, the only thing you gain by using a CCS is that you can use somewhat lower supply voltage but you dont really gain anything in linearity.

BTW, please don't call it anode follower, a follower is an amplifier with gain of ~1, call it cathode grounded amplifier that is the usual name.

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Old 23rd July 2004, 08:28 AM   #6
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Or, it's more stable with grid leak taken as a voltage divider from B+ to GND. Which is the same grid-leak-DC-operationage-ism as direct coupling to a plate.

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Old 23rd July 2004, 09:02 AM   #7
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Which is the same grid-leak-DC-operationage-ism as direct coupling to a plate.
Except for the lower input impedance, if you want to maintain the very high input impedance of the CF the solution with 2 series connected cathode resistors works very well, stability for any alternative is not of any serious concern except if you want to DC couple the output.

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Old 23rd July 2004, 12:43 PM   #8
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You can bootstrap the divider too.

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Old 23rd July 2004, 12:58 PM   #9
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You can bootstrap the divider too
I think it is what I have already described... the input impedance of a CF with dual cathode resistors is RG/(1-A*RL/*RL+Rk()where RL is the resistor closest to ground and Rk is the resistor closest to cathode, quite a difference to your variant using a potential divider between B+ and ground... I try never to waste gain or power in resistors....

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Old 23rd July 2004, 01:40 PM   #10
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Not like a few hundred K is much lost impedance. No worse than a light grid leak on any other grounded cathode stage.

I'd set up a voltage divider (from +300V to bias at 150Vk, two 1M resistors would do it), cap couple to cathode and run a resistor to grid. Impedance should be something like Rg/(1-gain) or however you'd express it. Which is electrically identical to the cathode biased method, except the voltage divider loads the output slightly. (It'll notice a gnat flying around the envelope before it notices 500k load, so, yeah.)

I don't know how a triode would tend to behave when self-biased as you advocate, but it doesn't feel like good, stable practice. Certainly wouldn't cut it in direct-coupled circuits like my active preamp design.

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