John Swenson's (In my eyes revolutionary) preamp!

[John Swenson]

Hi Bas, wow someone is actually trying this out! A few hints on this design: the maximum input voltage without causing distortion varies with the current through the tube (ie the volume setting) , so the lower the volume setting the lower the maximum input signal you can have. Thus to maximise the input level you want the current flow at max level setting to be as high as possible. You have to ballance that with the plate dissipation and the maximum through the tube. If I remember correctly the tube is rated for 20mA cathode current, try to get close to that.

Of course the greater the current the greater the voltage drop across the anode resistors, you have to take that into account when designing the PS and specing the resistors.

The resuslt is that I found that when I had the volume very low (say for listening late at night) a high output CD player would start distorting, at higher volume levels it was fine. The DDDAC with its somewhat lower levels worked fine at all settings.

On my tweaked version of this design I put transformers on the output rather than the resistors. An IT with tapped primary is what is needed here. I tried the cheap Hammond 124B here and it worked VERY well. The only problem is you have to make sure you get the plates balanced so there is no DC voltage on the transformer, I did this with a pot on the non driven deflector, wiper to the defelctor and +-3V on the ends. With this I could tweak the balance. Of course you could go parafeed if you wanted but that starts getting pretty expensive.

The other main problem is that the tubes are highly microphonic so tube dampers etc can be very useful. But they do get HOT when run at close to full current so be careful to get dampers that can withstand that.

The other thing to remember is that the mu is low, about 6, so you will not be able to get much gain out of it, but that is not usually that big a deal with a line stage these days. In practical use a gain of 4 was about it.

Its very easy to alter the overall gain, just change the anode resistors, what you use is going to take a little playing around so you get enough gain for a soft source but don't have to turn the volume down too far for loud sources.

BTW at 20mA on the cathode it will handle a 90V signal on the input without significant distorion, its only when the volume is very low and the current is in the microamp range that you have problems.

Have fun!

John S.

[Bas Horneman]

Have fun!

Thanks!!! Will do...

[Bas Horneman]

[mourip]

Hi John,

I am thinking of adding a tubed linestage to my class D amped system...

After this much time has passed are you still enthusiastic about the circuit? What are you using as your regular preamp now? Was this a "keeper" for you?

Best,

Paul

[John Swenson]

Hi Paul,
actually I'm not using it anymore. After I built a single ended parafeed amp with 0.03% distortion (yep thats right) the residual distortions and primarily the microphonics were too much. I started realizing the preamp was the limiting factor. Its not that it sounds BAD, but the rest of the system had improved so much (including my DIY USB DAC) I was starting to hear the little problems with this. It was the best bang for the buck I had ever come up with but it wasn't cutting it with the rest of the system.

What I wound up with was doesn't even use tubes (oh horror!) its a modification of Gary Pimm's CCS circuit into an amplifier and buffer, the result is really incredible. Gary has been playing with these concepts as well and likes to call them the solid state pentode, the circuits look and behave vary similar to tube circuits but aren't. They even run at high voltages and use tranformers and such just like equivalent tube circuits.


One of the things I REALLY like about the BDT preamp is the volume control does not cary signal, thus you don't need expensive controlls or switched attenuators etc. Also it makes remote controls really easy. On the one mentioned above I had to spend 4 times as much money on the volume control as the whole BDT preamp cost in order to get to the same quality level.

John S.

[mourip]

Thanks for the update John. Is there a link online for some of Gary's work? I had heard of his CCS but not this new idea.

I am using a Sowter TVC and like it very much but am always on the look for something new and better

Best,

Paul

[gurevise]

To John Swenson,

What do you use for volume control with solïd state pentode preamp?

Thanks
Sergey

[MIKET]

In order to not dilute this post with a different project which I'm sure most of us are also very interested in, I started a new post for the Solid State Triode http://www.diyaudio.com/forums/showt...866#post887866

[MIKET]

I should have said Solid State Pentode, that is what happens when you post a 6AM, brain cells still not stimulated by the morning coffee yet

[mourip]

Originally posted by MIKET
In order to not dilute this post with a different project which I'm sure most of us are also very interested in, I started a new post for the Solid State Triode http://www.diyaudio.com/forums/showt...866#post887866

Good call. Thanks Mike.