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Author Topic: Borbely tape preamp  (Read 3381 times)

Offline bobschneider

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Borbely tape preamp
« on: August 13, 2010, 04:47:49 PM »
After a long time on hold, I've finally finished my project to build a high quality solid state tape preamp.  The basis of my design is a phono preamp kit sold by the well known designer Erno Borbely.  www.borbelyaudio.com.  I used his EB804/419 All JFET MC/MM phono preamp as my starting point.  This is the simplest and least expensive of the RIAA preamps he offers.  I used it because it could be set up for either active or passive EQ, and because I already had one I was using as a phono preamp.  The articles on his website (originally in AudioXpress in 2005) give a lot of explanation of how the preamp works, including the information needed to design either an active or passive EQ version.

Dr. Borbely was extremely helpful in helping me work though what I needed to do.  I bought a pair of surplus silver/ceramic 4P5T rotary switches (genuine Red Army surplus!) off fleaBay, since I wanted to be able to switch between tape and phono.  One switch switches between the 3 pairs of RCA jack inputs on the back panel: MC, MM and Tape Head. One pair of switch poles changes the RCA jacks (with associated input loading), the other pair changes the feedback resistor which changes the gain of the input stage (I ended up with the gain for the head on my Otari MX-5050 close to the MC gain).  The other switch changes the passive EQ network between the two stages of the preamp (a series R and a shunt RC network, except for RIAA which needs a second C shunt in parallel with the RC) - I can switch between IEC 15ips, NAB 15ips, IEC/NAB 7.5ips (those curves are identical), and RIAA.  I could have set it up for old LP curves as well, but all my LPs use the RIAA curve.

I am very pleased with the sound of this, using my Otari MX-5050 with the Tape Project transport mods.  The sound is much clearer and more vivid than the stock playback electronics, which I have extensively upgraded (Burr Brown op amps, modern capacitors including Blackgate bipolars in the coupling positions, etc).  Cymbals sound like metal (I'm listening to Waltz for Debbie as I type).  This is the best sound I've ever had in my system!

Unfortunately, Dr. Borbely is retiring, and shutting down his kit business later this year, so if you want to build one of these you should act soon.  If anyone's interested in building one of these, I'm happy to let you know the values of the parts you need to modify Dr. Borbely's phono preamp kit.  His "starter" phono preamp with the power supply is Euro 380 plus shipping (about $500), the additional caps and resistors might run $50 more, I paid something like $20 for the switches, and the case and RCAs are whatever you want to spend.
Bob Schneider