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Re: Excessive gain in OTL preamp

Posted by Lew (A) on April 3, 2007 at 12:36:32

In Reply to: Re: Excessive gain in OTL preamp posted by Frihed89 on April 3, 2007 at 11:06:32:

I went to the website. There is very little information to help decide what the circuit is like, except to say that it looks like there may be two tubes used for a gain stage, one per channel most likely, and a third tube used as a cathode follower. It's likely that there is a CF output because you would need that to get the stated output impedance as low as it is. To say that this design is "OTL" is not helpful at all. The great majority of tubr preamps are OTL and instead use a capacitor at the output to block DC voltage. I suppose this is applicable to a headphone amp as well, depending upon the nature of the headphones it is intended to drive. So if you really have excessive gain, perhaps you can bypass the gain stage entirely, and just use the unit as a cathode-follower/buffer. Ask the maker whether you can jumper the sockets for the input tubes and just run straight into the CF. If you are a DIYer, you might try modifying the input to accept a low mu triode, instead of medium mu triodes like the ones you named. (However, off the top of my head I cannot think of any dual section low mu triodes, except as a far out choice the 6AS7. Type 27s are great sounding but are single section tubes; you may not have enough sockets to try 27s [mu = 9], and you would have to change the filament voltage to boot).

By the way, I assume you are referring above to 12AU, AV, and AT7, instead of "6au", etc. FYI, 12AU7, 6CG7, 6SN7GT/GTA/GTB, VT231, and 7N7 are all electrically identical and differ only in socket type and pin-outs, tho in my book the 6SN7 types sound best. I personally would not sub some of those other types you list into a circuit designed for the tubes I listed, without making some changes to the current and voltage supplied.

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