Nearly 10 years ego I've build OTL studio preamp with 12AX7 and 12AU7 anode output just using caps and output level pot for load instead of a trafo. It' been a great preamp but lately I've been thinking of adding output transformers. Apart from being safer, I'm not sure if there would be any advantage, maybe different tone, don't know. But I'm at the loss for what to get and not finding anything either. It'll need to be 20hz to 20khz range as it is for recording. I've got bunch of old radio trafos I could try to adapt but the Hz range would be unknown so that's probably waste of time.
I'd welcome any suggestions or even criticism
I'd welcome any suggestions or even criticism
Forget about it. With an Rp as high as these of the 12Axx you will get MASSIVE frequency response problems with an output transformer. You will need an ungodly inductance for an acceptable LF rolloff and that many windings will give you a hell lot of interwinding capacitance screwing your HF response.
Seems you guys think I should leave it OTL than I might just leave it as it is and not even bother trying using trafos than. Sometimes I do feel little conscious about having high voltage separated from output by simple cap but I guess that's the price for having OTL. I also have another preamp where I made the output as a cathode follower so that's safe but I just don't like the way it sounds. I've tried several cathode follower designs but didn't like the sound of any of them so sticking with anode out.
But what about commercial tube preamps, what do they use for output? Transistor stage, cathode, trafo?
But what about commercial tube preamps, what do they use for output? Transistor stage, cathode, trafo?
Ok, so what would be a output stage most would use? Just maybe a transistor buffer or simple preamp IC on the cheap units? Somehow I can't imagine commercial preamp with only a cap at the end to separate from high Voltage. When I look at some schemas, often there is an OT like on this one for example.
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Start imagining them because that's what you'll find everywhere.
Even the example you show has a cap separating high voltage at the plate from the next element.
As of the transformer, it's needed if you want to drive, say, a 600 ohms input impedance balanced load.
Which is fine if your Audio system components are separated by tens of yards (up to, say, 100-200 yards); otherwise, not needed at regular component to component distances.
A modern plastic cap is extremely unlikely to go short. Even if it does, the output would be a shock hazard but very unlikely to be life threatening since the current woudl still be limited to a few milliamps at most.
Nevertheless, a cap by itself is a bit dodgy because you can still get high voltage transients that may blow the solid-state amp at the other end of the cable. This is something I address in my book.
If you want to avoid high voltage of common cathode ground stage you would redesign the preamp around common anode stage and also easier to install additional neon or zeners protection and reduce transience. Maybe neon protection @ the input also.
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