Can someone explain parafeed & direct coupled & a.c. coupled?

[mwmitchell]

I'm wanting to build an ouput stage to my preamp. I have an 2SK170 driving a 12au7. I want to go into one more stage to drive a transformer.

I'm very interested in trying out a cheap power transformer for the line output. How is this done? I have been reading about parafeed, by why is it called parafeed? Isn't that just a.c. coupled to the transformer? I don't mind if it doesn't have perfect specs. I want to do it with a cheap power tranny for the fun.

How does push pull parafeed work?

Thanks a lot for any info!@

- matt

 

[Brett]

I'm wanting to build an ouput stage to my preamp. I have an 2SK170 driving a 12au7. I want to go into one more stage to drive a transformer.

I'm very interested in trying out a cheap power transformer for the line output. How is this done? I have been reading about parafeed, by why is it called parafeed? Isn't that just a.c. coupled to the transformer? I don't mind if it doesn't have perfect specs. I want to do it with a cheap power tranny for the fun.

Matt,
In a standard transformer coupled output, the DC bias for the tube also runs through the primary of the transformer. In parallel-feed, the B+ is fed therough a choke or active load, and the transformer is capacitively coupled between the plate and ground. Often the bottom of the primary is returned to the cathode.

The advantage to this arrangement is, the core of the transformer can be made smaller, an optimised for AC signals only, since the core doesn't have to deal with the flux generated by a DC current passing through it. The core can then be made out of other materials than the standard iron cores used, like permalloy and nickel, often with significantly increased performance. Optimised parafeed transformers or autoformers usually have significantly higher primary inductance then series fed for improved bass performance, all else being equal.

The disadvantage is requiring an extra choke or active load per tube, which costs more, and often the parafeed trannies cost more too (few suppliers and low volume). However for a line output, the LL1660 Lundahl is very good, as are the Magnequests and undoubtedly others. Remember the plate load and the transformer primary are in parallel with each other, so make sure you have enough net load at all frequencies for your pre tube to be happy. Active loads can help a lot here as they're almost always cheper then a good signal plate choke by a goodly amount.

If you have the gain to spare, small mains torroids have been used as step down outputs to good effect by several punters, such as <a href="http://home.t-online.de/home/MHuber/pre-5687.htm">Manfred Huber</a>. The turns ratio on the transformer will allow you to run a higher effective load as the Z of the next stage will be increased (from the preamp tube's POV).

Bottlehead are big proponents of <a href="http://www.bottlehead.com/et/adobespc/parafeed.pdf">parafeed</a> and this link is for Doc B's explanation of it.

How does push pull parafeed work?

Basically the same way as the SE does, except the primary floats between the two plates of the tubes, rather than plate-> ground/cathode. You need a balanced signal for the two tubes, and unless you can control very stiffly the DC conditions of the circuit, it will still need to be cap coupled. However, if you have a transformer with dual primaries, and can take the plate voltages, a single cap can be series wired between the primaries. Even better, use active loads and control the Vp, and no cap at all might be neccessary.

Cheers
Brett

PS. Why do you need or want 3 stages in a preamp? Even if the 2SK170/12AU7 is a cascode, you'll have way, way too much gain even with most step down OPTs.

 

[mwmitchell]

Brett,

Thanks for your great explanation. That really helps. The reason I was going into another 12au7 is becuase, well for one it is available. And I thought for sure I'd need to gain to drive the step-down transformer at the ouput. But do you think I can drive a suitable transformer with just the cascode? I mean, I need to just try it out.

One more thing I'm wondering about is, what kind of load impedance does the cascode stage want to see? How do you know the output Z? As a very basic test (don't laugh!) I tie a cap on to the plate, then touch the other side of the cap while touching ground. I use an oscilliscope to watch the output and if the level of the signal sinks too much, then I know the output Z is pretty hi. After a while, I can usually get good results by testing it out, and changing things around.

Although I know that I don't need a really low output Z from the cascode (nor can I expect one) , what should it be? Thanks for all of your help! And sorry for the ranting!

- Matt