A Heretical Unity Gain Line Stage part III

[fdegrove]

Hi,

Given the achieved performance in this configuration, it's hard for me to see the benefit of cascoding.

Cascoding the follower?
You'd need approximately twice the B+ for that to work with not much to gain IMO... If at all.

Nice work by those bottles of wine, SY....How many left?

Just to be my annoying self, servos can degrade sound quality just as much or more than any cap can....
You knew I'd say that, didn't ya?

Cheers,

[SY]

Happily, I have about 20 more cases of various vintages running from '88 to '02. That should be enough to get me through the next article.

[Brian Beck]

servos can degrade sound quality

Quite so, but not always. I typically add a second pole (and sometimes more than one pole) after the servo opamp, spaced far enough from the main pole to avoid making the loop unstable or even the least bit bouncy, just to filter away the opamp's feeble audio output. At least SY adds an enormous 10MEG resistor which offers some isolation by essentially making the correction signal behave as a current source that is shunted out by the pot at higher frequencies. The input coupling cap and pot will add some phase shift (a pole and a zero) to the servo loop, but apparently he’s not noticed any bounce. One can also use a solid-state low-offset transistor pair to sample the offset, but use other circuits besides opamp integrators to provide gain and filtering. Or try a high gain opamp with resistive feedback followed by a passive RC that kills the opamp output. You may not get “infinite” gain, but the loop will null plenty well enough. As a side note, I’ve also built servo amps with a fast acquisition time constant for quick settling, and then after a 555 timer delay, slowed the loop way down. Idea borrowed from phase lock loop design work. Not easy to implement, but, hey, anything for DIY. Again, there’s more than one way to skin a cat, and I think SY has offered a reasonable circuit for others to build.

[SY]

Actually, I mentioned the option of the second pole (I'll usually put a passive RC with a 50Hz f3 after the opamp output), but I eliminated it here. I tried it both ways and didn't hear or measure a difference.

The 10M is actually shunted off by a parallel combination of the pot, the 15K resistor, and (depending on the setting) the source impedance of whatever is driving the preamp.

[Brian Beck]

SY,

I look forward to seeing your servo-modulated B+ implementation. I don’t think I’ve ever seen that idea before, but it should work fine. Getting rid of the output coupling cap is a noble cause, but your present design still requires an input coupling cap. Since it’s small, you have options for decent dielectric material and construction techniques. Still, it seems that eliminating all coupling caps would/could be a good thing.

The servo-modulated current source approach is probably equally appealing to the modulated B+ approach, because both are shifting the tube’s operating point slightly to find an output null. In fact, when you promised a servo design in your earlier postings, I imagined it was going to be the modulated current source approach. Looking at your schematic, it occurs to me that you could easily implement the modulated current source with mostly the same parts and only a minor rewiring of your present design. The same may be true of your B+ idea – we’ll see what you come up with. The 200 ohm output resistor (or something in that value range) would go in series between cathode and current source collector. The resistor would be sized to drop the same voltage as Vgk with the desired operating current through it. Output would be taken from the “south” end of that resistor, which would sit nominally at zero volts DC. Remove the input coupling cap. Use a dual opamp to achieve a needed polarity inversion (inverting integrator followed by unity gain inverter). Drive the lower transistor base, disconnecting the LED and the 560 ohm resistor from this base but leaving those two components otherwise connected to bias the cascode upper base. A few resistor tweaks and a cap re-evaluation (for servo time constant) should be all that is needed, unless I’m missing something. Oh, maybe a resistor-diode stack at the lower base, or something along those lines, to limit start-up current until the servo settles.

So then you’d have three servo options for builders to choose from. It will be interesting to hear if folks build them and compare them. Certainly your excellent measurements suggest a promising result no matter which option is chosen. Thanks for offering the design and the tutorial.

[SY]

I'm not at the computer with the full schematic, but here's the outline drawing.

[SY]

Your modulated CS concept is obvious in retrospect (always the sign of a good idea!). I'll have to do some drawing to see if it has the same (or better) rejection of servo anomalies as the "standard" servo and the modulated B+ concept. If so, it's a better way to go, I think. You raise the source impedance a bit, but get rid of all caps. It's possible that you don't even raise the source Z, since the biasing resistor could doube as a cathode-stopper.

[Brian Beck]

SY,

I think your modulated B+ approach will work just as well (better actually) without the input coupling cap, zener diode, 220uF cap and +12V supply connection at the grid. Just reference the grid to ground via the pot (I always add a 1MEG safety resistor from grid to ground in case the wiper goes open). Now add that same 200 ohm (roughly) “cathode-stopper” resistor in series between cathode and the CS/output node to return the output to roughly zero volts. The servo will take care of the rest of the DC nulling, and the 200 ohm resistor will bias your Vgk. Works the same whether B+ is modulated or the current source is modulated. Either one is just gently readjusting the tube operating point by a small amount to force the output to zero. The output Z will still be <300 ohms depending on the bias current setting. You could replace the 200 ohm with a battery, an LED or several diodes to drop the output Z, but I wouldn’t risk the introduction of new distortion mechanisms, especially when 300 ohms is decently low anyway (same Zout as your original servo design). Either the CS modulation or B+ modulation will have good inherent rejection of opamp output garbage since the cathode wants to follow the grid, and not the plate voltage or the plate current (or at least not by much). When you injected the servo error signal into the grid, any residual garbage at the south end of your 10MEG would come right through to the output, although, as you pointed out, there was a lot of signal attenuation there.

[moamps]

Originally posted by SY
I'm not at the computer with the full schematic, but here's the outline drawing.

Vgk=+Vled

[SY]

Mistake on the sketch. The LED is biased by the negative rail to give -1.7V on the grid.