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In Reply to: RE: Low-Z SE OPTs? posted by Paul Joppa on August 04, 2014 at 15:19:49
Probably no need for a parafeed cap , suitable chokes will have low-enough DCR to connect the transformer directly across the choke
Al
Follow Ups:
"Probably no need for a parafeed cap , suitable chokes will have low-enough DCR to connect the transformer directly across the choke"
That's an interesting concept. I assume this arrangement might require a source of fixed bias to set the tube up correctly, or maybe a resistor under the choke. Here are my basic goals for this project:
1. 1W output capability
2. No coupling caps from the cathode out
3. Non-boutique OPT with low-Z primary
4. Ultra-low distortion w/o NFB, probably EF86 - 6SN7 driver
It's still a "thought project" at this point, but I've been mulling it over for a couple years. I want to attempt this as soon as my workshop is back together.
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Buy Chinese. Bury freedom.
It's funny to me - some people hate cathode caps, some hate interstage coupling caps, some hate output coupling caps, some hate power supply caps.
To me, they are all in the signal current loop and are all potential sonic issues. In a normal cathode follower, all of the signal current goes through the power supply (most through the last capacitor), so that would be the likeliest culprit from my perspective. But to each their own!
"In a normal cathode follower, all of the signal current goes through the power supply (most through the last capacitor)..."
It's not clear to me that a misbehaving cap that creates minor anomalies at the anode has the same effect on a follower as it would in a common cathode stage. That aside, dealing with the issue of caps in general is somewhat a matter of degree. In this circuit, the last PS cap can be a much smaller value for a given frequency response than the output coupling cap required to drive a low-Z OTL load. That makes the use of a film cap - rather than electrolytic - more practical. Hopefully, that will evidence itself in the form of truer sound. :)
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Buy Chinese. Bury freedom.
You are correct; the power supply is inside the feedback loop. If you stick to this plan, it looks like you will need a custom transformer.
Looking forward to your results!
A resistor will worsen the DC across the transformer so the overall DCR should be kept as low as possible . You could probably achieve a watt with a single 6AS7 with paralleled sections and do away with the output transformer altogether . Feedback required though...
Al
"You could probably achieve a watt with a single 6AS7 with paralleled sections and do away with the output transformer altogether."
That would require a coupling cap. NG for me.
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Buy Chinese. Bury freedom.
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