Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request From: brian@america.Telebit.COM (Brian Lloyd) Newsgroups: rec.audio.high-end Subject: Re: Tube pre-amps <$1000 Message-ID: <12721@uwm.edu> Date: 3 Jun 91 12:46:30 GMT Sender: news@uwm.edu Lines: 76 Approved: tjk@csd4.csd.uwm.edu Originator: tjk@csd4.csd.uwm.edu I have received NUMEROUS responses to my posting. It seems that there is a great deal of interest in my old technology. What I thought that I would do is to describe how I came to create the different preamp architectures that I did (and that I am learing that others have been copying) and how to build your own INEXPENSIVE vacuum tube and solid- state preamps. Heck, I designed a solid-state preamp for Dennison Electrostatics that was an ARC SP6A killer in its day. Parts cost was about $20 (more if you put it in a box). It was low tech but it sounded quite good (musical -- I personally don't care too much about "accuracy" because who knows how the instrument(s) were played or recorded, I just want to be able to listen to my recordings again and again and again and not get tired of the sound). Brian Lloyd's Rules of Amplifier Design Actually, building a preamp (or any amplifier for that matter) that sounds good is not hard to do IF you think about what you want and don't get too carried away with technology. Here are some of my guidelines: * Every component degrades any signal that passes through it so keep the parts count low. * The human ear seems to be more sensitive to transient phenomena so avoid topologies that can aggravate transient problems. You can live with .5% THD and .1% IMD IF the transient distortions don't rise above those levels. * Match the input range of a stage to the expected output range of a preceeding stage so that you avoid overload of intermediate stages (this is critical if you are working with feedback stabilized amplifiers) * Provide clean, stable, low impedance, well decoupled power to each stage. Here is my basic topology for two types of preamps, one with no feedback loops and one with a feedback loop in one stage: eq eq |\ -------- |\ ---------------------- |\ input >---| >--| 75us |--| >--| 318 us and 3180 us |--| >---> output |/ -------- |/ ---------------------- |/ 1st gain 2nd gain buffer stage stage eq |\ -------- + input >---| >--| 75us |-----|\ |/ -------- | >---------------------------> output 1st gain --|/ | stage | - | | --------------------- | -----| 318us and 3180 us |-- --------------------- Notice that the first stage is never in a feedback loop. This is because the signal there is at its fastest and most prone to transient distortion. You just have to make sure that this stage is wide bandwidth, unconditionally stable, quite, and has very wide dynamic range. After the signal passes through the first part of the RIAA EQ (the 75 microsecond deemphasis) the risetimes are much slower so that most well-designed feedback stabilized amplifiers can deal with the signal now. Anyway, that is all I have time for now. I will write more later and get into actual schematics for individual stages. -- Brian Lloyd, WB6RQN Telebit Corporation Network Systems Architect 1315 Chesapeake Terrace brian@napa.telebit.com Sunnyvale, CA 94089-1100 voice (408) 745-3103 FAX (408) 734-3333