Crack with plate output

[Doc B.]

I'm kind of surprised no one else mentioned this idea yet -

Shawn and I reworked a Crack so that the two stages are cap coupled and the 6080 output stage can now be set up as a standard voltage amp rather than a cathode follower. We used a Speedball PC board set up as a plate load with MJE5731As and 2N2907s rather than the stock TIP50/2N2222A combo. The operating point stays the same, 75V p-k and 30mA (about 20V cathode bias).

The reason for this was to see if the circuit would work as a line stage, possibly a replacement for the Foreplay cathode follower design. Output impedance has of course risen from the stock Crack 120 ohms to about 400 ohms - still plenty low for line driver use and down in the ballpark of the Foreplay output impedance.

First test was as an output stage for the SeDACtion. This had the Crackplate feeding my big single stage 6C45pi control preamp. Results were very encouraging, so the next thing we tried was putting the Crackplate after the single ended output of my Tube Repro on my Nagra T - roughly the equivalent of putting it after a Eros tape head preamp. Again the Crackplate ran into my big preamp. Wow, really wonderful punchy sound with great imaging and monster bass. And the system plays LOUD. Best it has ever sounded!

So we figured we should bypass the big 6C45 preamp and just use the Crackplate to drive the active crossovers of my three way active system.
Curiously we lost some of the punch and the imaging was not as good. Still sounded really nice, it seemed just about as good as using a ForeplayIII, but not as good as when Crackplate used the big 6C45 preamp as a sort of buffer.

The 6C45pi amp has 500:500 ohm input transformers. The crossovers have 25K pots at the input. Somehow the 400 ohm output of the Crack sounds better running into 500 ohms than 25K. Don't have a good explanation for this yet and my system is pretty non-standard so these results may not apply anywhere else, but thought I ought to offer up our research.

Oh last detail - I should note that we did not bypass the 680 ohm cathode resistors on the 6080s. We will do that and try this experiment again next week.

[JC]

It almost sounds as if your test set-up likes the better power transfer out of the 6080.  Maybe the following stage likes some juice!

[Doc B.]

Yeah, I think it's something like that. It's been interesting to discover just how much drive has been missing from the system. I had guessed that the lack of punch was happening at the amps, but it's apparent that that big preamp wants its butt kicked in order to sing. The dynamics are now more like the "big boy" Wilson, Avalon, Magico, etc. systems that a lot of my tape customers have. We have gain to spare and the RAAL ribbon gets very loud very cleanly with that little 2 watt SR45 amp driving it. The stock balanced output stage on the Tube Repro is no slouch with a 6CM7 tube running into a Peerless 15095 15K:500 ohm transformer. But the 6080 seems to have more definition and drive. The next test will be to put an MQ 500:500 ohm transformer at the output of the 6080 to see how it sounds as a balanced output.

[JC]

That sounds like a very interesting experiment.

It might also be fun to try driving that pre with some of your other power amps, like a SEX, to see if you can determine some kind of limit to how much power it likes.

I will have to make a note to keep an eye on this thread...

[Paul Birkeland]

You may as well try the TL431 under the cathodes too while you're at it!  Sounds like some fun experimentation.

[ironbut]

Great stuff Doc!
After hearing how great the Speedballed Crack sounded, I've been toying with the idea of using it for the front end of an amp I've been dreaming up.
It sounds like your experimenting will save me a lot of leg work.
Hearing that the 6080 sounds better into 500 ohms is excellent. Thanks!

[Doc B.]

Added 1000uF bypass caps to the cathode resistors yesterday and this project has now surpassed my big preamp. Took 7 years to find something that would do better. Our next step is to try to figure out a way to shunt regulate this setup.

[ironbut]

Heavy duty Judy!

What kind of voltage is on the cathodes in the Speedplate setup?

[Doc B.]

About 20V. The op point is right off of the published curves. I should have made it clear that the bypass now makes the circuit sound just as good into 25K as into 500 ohms.

[ironbut]

Very cool!
I have a bunch of 1000uF caps but some are as low as 16V. Luckily, the best ones are at least 25V.
This is some exciting stuff Schmalle!

[JC]

Just curios if you have a working theory as to why the bypass helped as it did.  I've never heard of removing local nfb as a factor in improving dynamics, if that's what has happened, but I suppose that a case could be made.  Then again, didn't bypassing the cathode resistor also effect the output impedance of the circuit?

A very interesting experiment!

[Doc B.]

Yeah, I would guess lower output impedance is the difference.