Converting from single ended parafeed to differential

[dsavitsk]

My current preamp is a basic CCS loaded parafeed design that uses a 12J5 per side and a Magnequest B7 output transformer.

I was thinking of switching it to a differential design, something like this



It seems that this will require an additional CCS per side, an input transformer, and swapping from 12J5's to 12SN7's.

So, the question is, Can I still use the same OPTs. They don't have a center tap like the ones shown in the schematic. However, it seems like I could hook them up like this instead



Also, the OPTs are 15K:500. My understanding is that this will effectively cut the loading in half, which I think should be fine here, though I don't really know.

Thoughts?

[analog_sa]

I don't see any importance of having the centre tap. Btw, what is the coupling cap for?

[dsavitsk]

Originally posted by analog_sa
Btw, what is the coupling cap for?

Well, my assumption is that if the tubes don't bias up exactly the same, then there will be offset across the transformer and thus current flow. But, I emailed Electra-Print about winding some transformers for a similar circuit, and Jack's comment was the same as yours. So, I don't know the answer. maybe someone with more technical knowledge can weigh in.

[analog_sa]

Very possibly the tubes won't bias the same. It is still trivial to adjust the ccs for zero voltage.

Could you tell us what is the source of the circuit?

[dsavitsk]

http://www.raleighaudio.com/chapter_4.htm

and the one at the bottom: http://www.nutshellhifi.com/library/ETF.html

[David Davenport]

Since the transformer that I was using had a split primary (Lundahl LL1674) I put the capacitor there for esthetic reasons. There is no reason that it couldn't be put on one side or the other.

One advantage of using parafeed in a preamplifier circuit is that you can use smaller transformers that are generally less expensive than an equivalent quality large transformer. Many small transformers, like the LL1674, cannot tolerate more than a few micro amps of direct current before the bass suffers. Even if the two sides of the circuit are perfectly balanced when new, it may drift off as it ages. Therefore a capacitor is used to ensure no DC will flow.

Since you are now using the B7 in a single-ended circuit, it should be able to tolerate the current from an imbalance between the two sides. Therefore you may not need the parafeed capacitor. Try it and see.

Dave

[Bas Horneman]

Since you are now using the B7 in a single-ended circuit,

The Magnequest B7 is a parafeed transformer so the cap will have to be kept in the circuit.

[dsavitsk]

On the input transformer, there is a centertap that is grounded. This seems to be the case on all similar designs I have seen. Is this a necessary connection here, or will an input transformer w/o a center tap (like the Jensen JT-11P-1) work here.

Originally posted by David Davenport
Since you are now using the B7 in a single-ended circuit, it should be able to tolerate the current from an imbalance between the two sides. Therefore you may not need the parafeed capacitor. Try it and see.

That raises an interesting question, if I were to buy a transformer for the task, what does more damage to the sound, an air gap, or a cap (which, I realize is just another sort of air gap.) But, would I be better off getting a transformer that can tolerate a little DC and forgo the cap altogether? A sort of capless parafeed?

[David Davenport]

You can use a non-center-tapped transformer as shown in figure 3 in this article: http://www.raleighaudio.com/active_output.htm

There is no magic to parafeed – you can get excellent results with either series feed or parafeed. If I were considering a larger (and more expensive) transformer, I would go with the “Differential (forced balance)” configuration in Lynn’s presentation. I would not choose a single-ended gapped transformer.

Assuming equivalent high quality transformers, and an excellent capacitor like the Cardas, you will get different but equivalent excellent performance from either the Differential (forced balance) and Differential Parallel Feed configurations. Your trade-off is a smaller less expensive transformer plus an expensive capacitor versus a larger more expensive transformer and no capacitor.

Since you already have a pair of B7s, why not start with that? Bas is correct; you will need to use a parafeed capacitor. If a Cardas capacitor is a little rich for your budget, start with a Kimber Kap that has very good price-performance.

Dave

[dsavitsk]

Thanks Dave, I appreciate the help.