Yes, they exist and in large numbers from a wide variety of manufacturers.
14 responses Add your response
Yu11375, thanks for the comment on the S-30.
Onhwy61 is correct- most preamps have no output transformer. But with preamps the usual limitation at the output of the preamp is often the coupling cap, rather than an output transformer. Removing this part can result in a huge increase in detail/transparency and apparent speed without edginess or brightness.
Doing this is a but tricky; there are only a handful of ways to do it. Reliability is a factor- you don't want the preamp making DC voltages that could damage a power amplifier or speakers. We patented an elegant (IMO) method of doing this some years ago. Seems to me there are a few other tube preamps that also have direct-coupled outputs but this is quite rare! I think, other than the ones we make, you can count the others on one hand with fingers left over.
I believe you need a cap to block DC at either the ouput of the preamp, or the input of the amp. You don't need both, so it may be safe to remove one or the other as long as you keep this in mind when changing to other components. I'm no tech, just a tinkerer, so please, someone correct me if I am wrong. This cap will change the sound, but I suppose it's possible to prefer it with, rather than without. I removed them from my vintage amps and it did improve the sound. I now have to be careful though that any pre I use will have output caps.
08-30-12: HifiharvThere are some tube preamp designs that will put out DC voltages on the order of 150 volts or more if the output coupling capacitor is eliminated. The voltage would be sourced through an impedance that is most likely high enough to limit current flow to non-lethal levels in the event of accidental bodily contact. But I would not make any such change to a tube preamp without careful study of the schematic, or input from a qualified person who has knowledge of the specific design.
The input circuit of the amp, and especially the voltage rating of its input coupling capacitor, should also be looked at to assure it can withstand that voltage, and with considerable margin so that long-term reliability is not degraded.
Another concern that occurs to me in that situation, where the preamp design is such that removal of its output coupling capacitor results in a large DC voltage, is the possibility of a significant increase in the severity of the turn-on transient that can result if the preamp is inadvertently turned on after the amp has already been powered up.