Yet another 12B4 line stage, or is the 12B4 better than the Grounded Grid.....

[Mark A. Gulbrandsen]

no RC stage can compete with inductive loaded stage in that field.

What I'm talking about is not a simple RC stage..... but an active circuit. I have tried chokes but not on this sort of project. Since a simple pass transistor can make a cap look at least 10 times bigger(the multiplication of the caps value is based on the hfe of the device being used) you'd need a whole lot of choke to do what a 100,000 mfd cap could do. Change that simple pass transistor to a mosfet and it looks 10 times larger than the simple pass transistor version. Run that through another pass transistors and all thats left is a few microvolts of noise! Less space, weight, and a whole lot less cost! Take something like the power supply from the ARC PH-3 phono preamp. That power supply elevated to 300 volts would be totally suited to this sort of line stage. Cost to build the active circuit would be less than 25.00. This is why all the expensive iron just doesn't make sense to me....... I can achieve cleaner DC with cheap active circuitry and it would cost hundreds in iron and the results still wouldn't be as good.

Mark

[LuckyLyndy]

All three types of CD's were used in listening tests.

(Mark G.)

Country, Western, and what's the third?

(J.)

I like that J! I think Mark was referring to SACD, Redbook, and DvD-A, but I may be wrong as well.

Or could it be Country, Western, and Classical Country? I mean what else is there?

[Mark A. Gulbrandsen]

Actually all comparisons were done using my old Gene Autry and Cab Calloway 78's that have been transfered to CD..... . The Cabster sounds simply uncanny through the 12B4! The background noise from the 78's sounds extremely detailed and realistic..... Might as well can all this 12B4 stuff and just play the dang 78's!

Mark

[Brett]

Originally posted by Mark A. Gulbrandsen
Then commeth the 12B4!

What can I say when something is so dramatically better.

I can say, I told you so.

Are there any drawbacks? YES! This line stage has just about zero Power Supply Rejection and requires the very cleanest power you can give to it. Does it need to have a CSS or LED's to sound good. NO! This stock circuit is simply amazing running off my regulated bench supplys. The CCS mod might raise the PSR a good bit but good regulated supplies as Dr. Gizmo used to do and as I am doing also works very well. Its either going to take alot of iron or some well designed regulated power supply to get this unit hum free.

Agreed, but it's not vastly different from most CC tube stages. With an excellent PS, they really begin to shine.

[Brian Beck]

I'm late to this thread, but wanted to add that I have been using an almost identical 12B4 circuit as the output stage in a Pioneer DVD/CD player (voltage output DAC). I use 8.2K plate loads, but a CCS in the cathode (well bypassed) to hold current constant. Elaborate power supply filtering (LCRCRCLC). Sounds simply fabulous. No noise, even with headphones. I left the opamps in so I can switch back and forth to **** off solid-state guys. I'm about to do a similar treatment to my Sony SACD player. Screw the warranty!

[wrenchone]

I've had my eye on the 7233 for line amp applications. I was thinking of trying an Akido-type setup, but using a FET-based current source load, as I have some Supertex HV TO-220 depletion-mode MOSFETs looking for something to do. Iwas also looking at using a triode strapped 6AQ5/6005, which would give me a bit more gain. I was looking at the 12B4 as well, but I only have 1-2 used units on hand of unkown condition right now, and I'd like to try out what I've got...

[Mark A. Gulbrandsen]

Agreed, but it's not vastly different from most CC tube stages. With an excellent PS, they really begin to shine.

Well, In another thread about the GG preamp there was a comment from a mamber that the 12B4 is alot less expensive to build. I would completely disagree with that. It's at least as expensive when one takes into acocunt the power supply required for the 12B4.

Brian,
Would you share your CCS ideas with us?

Thanks!

Mark

[Brett]

Originally posted by Mark A. Gulbrandsen
Well, In another thread about the GG preamp there was a comment from a mamber that the 12B4 is alot less expensive to build. I would completely disagree with that. It's at least as expensive when one takes into acocunt the power supply required for the 12B4.

That definitely wasn't me, as I've never posted on the GG preamp, and most common cathode stages have inherently poor PSRR when resistively loaded. A good quiet PS can be expensive of $ (unless you've been collecting iron for years as I have) and space, even if all it does is serve a "simple" circuit like the 12B4A one you posted. If you look at my previous posts on the 12B4 preamp I've been posting on here for at least 3 years, you'll see the lengths I go to to get the PS right, even in the predominantly balanced/differential circuits I prefer.

[Brian Beck]

Brian,
Would you share your CCS ideas with us?

Oh, nothing special at all. I frequently use a CCS in cathode circuits. I just use a good old LM317 and a current setting resistor, well bypassed with a big electrolytic and a small “boutique” film cap. The reason I bother with a CCS in the cathode is to hold the tube’s operating point constant over life and the same between channels. This centers the operating point of the plate voltage and keeps the plate resistance (output resistance) and transconductance close to the same for reasonably well-matched tubes. An LM317 costs maybe $0.49, and I have a bucket of them, so why not use it in many cases? No one should say that they can be heard in such a cathode circuit; the bypass caps completely hide any deleterious effects of a solid-state CCS. All the CCS has to do is to be a good current source at low frequencies, and any high frequency irregularities are hidden by the bypass caps which “short out” the CCS at all audio frequencies. I often use two separate such current sources in push-pull output stages to force primary current balance and ensure DC magnetization cancellation. No adjustments over tube life.

To complete the output filtering for the CD player, I used a passive LRC low-pass between the DAC and 12B4 grid to set a -3dB point at 58KHz, with only -0.026 dB impact at 20KHz.

A CCS as a plate load is a different beast and harder to get right, since you CAN hear the solid-state edge unless great care is taken (a la Gary Pimm’s designs). But great potential exists here, too.

[planet10]

Originally posted by Brian Beck
Oh, nothing special at all. I frequently use a CCS in cathode circuits. I just use a good old LM317 and a current setting resistor

The 317 is limited to a minimum of 7mA as a CCS isn't it?

dave