parafeed output amplifier

[tubem]

I would just like to know if anyone can tell me if it is worth building a parafeed output amp,is it awesome or just another novelty?

[Frank Berry]

The parafeed configuration is useful if your amplifier is single-ended and the output transformer is not air-gapped ... not designed to operate with d.c. current flowing through the primary winding.
There is one caution, however. When designing a parafeed amplifier, the combination of capacitance and inductance in the anode circuit can create some interesting low frequency anomalies. The L/C combination becomes a tuned circuit.
If your output transformer is air-gapped, I wouldn't bother with parafeed.

[the_manta]

Furthermore it's a matter of price.
You have to possibilities for a parafeed design cosidering the Anode Load. Either buy a second iron, in this case a choke or use a cheap resistor or a constant current load. The second option may be cheaper but the efficiency decreases to 25% because such a simple load can't store energy.

[CharleyW]

I have a pair (Bottlehead Paramours) - in my opinion, it's worth it.

[ratbagp]

There is an explanation on the Bottlehead website on the advantages of parafeed.

The Bottlehead Technology Story

ray

[twystd]

I have experimented (a lot) with parafeed in a bread boarded line stage, and really like it. There are qualities of parafeed that have some advantages over conventional single ended designs.

Since the transformers don't have the operating current going through the primary, exotic materials like nickle and cobalt can be used. The transformers can also be made small, for the same reason, so they can be wound with less capacitance in the primary, which helps high frequency response.

Parafeed amps use either chokes or CCSs for loads, so they isolate the signal from the power supply to a certain degree. This allows the power supply to have less influence and coloration over the signal.

One of the drawbacks are, like Frank Berry says, the interaction of the parafeed cap, with the tube, load, and output transformer is critical. The size of the parafeed cap has to be chosen carefully, with the aid of a signal generator and oscilloscope. Also the quality of this cap is VERY critical to the quality of the sound.

Another drawback is that parafeed typically requires an extra cap (the parafeed cap). You can get around this by using what is sometimes called the ultrafeed topology, where the parafeed cap goes from the ground leg of the output transformer, to the top of the output tube's cathode, leaving the cathode resistor un-bypassed (no cathode bypass cap).

All in all I really like parafeed. When done right, it seems to have great bass response, and overall, to be a very fast and accurate topology.

twystd

[carlsor]

After researching tube designs for several years in search of the "perfect" design I settled on the parafeed circuit and have built 2 parafeed amps. One is a two stage 76 direct coupled to a 12B4A running at 30ma at 150vdc into a 5K Sowter parafeed OPT via 3.9uf Mundorf Silver/oil caps. The sound is amazing into my Hornshoppe Horn speakers and Grado GS-1000 headphones. The sound is fast, unfatiguing, musical and not at all tubey sounding. It is neutral, not especially warm sounding but is not cold, analytical, or sterile. You get pulled into the music, not the equipment.
My second amp is a 3 stage 2A3 design that I am still tweaking the 2nd stage driver with promising results. With parafeed amps you need super efficient speakers. The circuit is complex and quality parts throughout is expensive. It is worth to me. I hope this was helpful. I would be happy to answer any questions you have about my projects.....Ross

[alexg]

Originally Posted by tubem

I would just like to know if anyone can tell me if it is worth building a parafeed output amp,is it awesome or just another novelty?

I have pretty good luck with parafeed amps, I like the topology on 2a3 and EL84 amps.

I will be experimenting with 12av5 parafeed soon.