© Don Sachs, 2015

Custom 6SN7 Preamplifier

This is the finest preamp that I have ever heard!

Latest:  All of my costs have gone up about 10% so what I am going to do is to raise the base price of the preamps, but I am going to include the all film capacitor high voltage supply as part of the base price, rather than have it as an $80 upgrade.  So really, the preamp is just going up $20.  I never build the ones with electrolytic capacitors in the high voltage supply anymore anyway.  The only electrolyic capacitors are in the filtering for the regulated 6 volt DC filament supply for the tubes.  The entire signal path now has only quality film capacitors. I am not alone in my opinion of this preamp.  I have gotten two very nice reviews lately from customers who have them.  They are too long to post at the bottom of the page so I have created a customer review page where I post feedback.  There is also a thread going on Audio Circle about the preamp.  I just finished the first preamp with the new dual mono Khozmo remote 64 step attenuators.  Pretty slick (shown at right).  Sounded great too.  I cannot duplicate the dark stained mirror finish cherry case.   That was one of a kind, but I can build preamps with the dual mono Khozmos for $350 extra, vs. the regular stereo remote version that is $200 extra.  The dual mono eliminates the channel level attenuators.  The dual mono Khozmos can be unlinked on the remote and you can raise or lower the level of one channel and then link them again so they go up or down in tandem.  The gives you both a volume and balance control on the remote.  Works like a charm! Update:  I am having 10 preamp cases made out of cherry.  I have the first one and it is shown in the top photo.   I also have some in walnut for a $40 additional cost.  I have priced it out and I can build one in cherry with the new 64 step remote Khozmo attenuator for $1999 with 5 RCA inputs, a tape monitor loop and all tubes.   I can build it with the 48 step manual Khozmo attenuator for $1799.  One pair of XLR inputs with Cinemag transformers for conversion adds $225.  All units include Mundorf Supreme Silver oil caps at 0.33 uF which will easily drive a 100K or higher load.  If you need a bigger cap for a 10-20K solid state amp that will add $50.   The new 64 step version of the fine Khozmo attenuator with remote volume control, featuring a mute button works perfectly and is now available.  It is still a full analog attenuator with only two very high quality resistors in circuit on each channel at any one time.  Once I have experimented with one I can determine pricing, but I expect the remote to be a $200 option.   I have also built a few basic ones in the same simple metal cases that Roy Mottram uses.  I prefer the wooden case with tubes on top, but I can do a basic one for $1295-1395 in a metal case with tubes inside and the excellent 48 step manual Khozmo attenuator.  Price varies depending on how many inputs you want and whether you wish to have a tape monitor loop.  XLR inputs are still extra. I now have a matching tube phono preamp that is equally good!  I have also finished the production version of the matching 70 watt/ch KT88 tube amp and the combination with the line stage is incredibly good! This is the first preamp I have ever heard that I thought was every bit as good a piece of gear as a fully restored Citation II amplifier.  Really, this preamp is so good that it is scary.  You can read my listening impressions here.  The preamp is based on the 6SN7 tube, which has always been one of my favorites.  The bigger octal tubes like the 6SN7 have a much more relaxed, fuller, and richer sound than the smaller tubes like the 12AX7, 12AU7 or 12AT7 tubes used in many other preamps.  You get all of the detail with no brightness.  The preamp is based on the SP14 board from Roy Mottram.  Here is a link to Roy’s SP14 page: http://www.tubes4hifi.com/pre11.htm#SP14 I looked at Roy’s page and decided to build my version because his basic design is absolutely fantastic.  This is essentially a $3000-$4000+ commercial preamp built for $1600-$1900.  It is a cost no object design without all the ridiculous markup and hype of the high end industry.   I have spent numerous hours experimenting with parts choices based on my many years of experience restoring vintage tube gear.  I know what parts sound very good in various positions in the circuit.  This preamp is incredibly good for a number of reasons.  First, the audio circuit itself is a wonderful, but simple design that has a very short signal path and really only one coupling capacitor per channel at the output.  The tubes are direct coupled.  The 6SN7 is a fantastic audio tube as noted above.  If you are used to the sound of small tubes you are in for a treat, and you most likely will never go back.  Small tubes just aren’t going to cut it for you!  Second, the power supply is a fully dual-mono design.  Each channel has its own B+ (high voltage) and filament supplies.  This gives amazing stereo separation because each channel is independent.  The B+ power supplies are tube rectified (the BEST way to go in a tube preamp, none of that solid-state hash) which gives a soft-start and simplifies the design compared to a solid-state design, and it also sounds better! Then the B+ is fed to a separate voltage regulator for each channel that eliminates all variation in the supply, and reduces the noise by at least 20 dB, making for a deep, totally dark background.  The two filament supplies (one for each channel) use ultra-fast schottky diodes, and then the filament voltage is regulated by a low-noise, low-impedance device that is completely isolated from the audio signals by floating the 6 volt supply about 70 volts above ground so there is no hum whatsoever.  The 6 volt DC regulated filament supplies are driven by completely separate transformer windings for each channel.  In fact there are separate filament and high voltage transformers.  The power supply and filament supply design gives stunning channel separation and the blackest background that I have ever heard in ANY preamp, tube or SS. My version uses a wooden case with the tubes on top.  That helps it run very cool and it will be reliable for many years.  Plus you can easily swap tubes.  I use full aluminum front and rear panels to allow easy mounting of controls and jacks, and also to keep the signal path parts well separated from the AC.  All signal cable runs to the jacks are done with Canare Japanese OFC shielded cable.  The front panel controls are wired with mil-spec solid OFC copper Teflon insulated wire.  The B+ and filament runs are done with mil-spec stranded OFC copper Teflon insulated wire.  This preamp is very quiet and the wire will NEVER become brittle with age.  It is the same wire I use in all of the Citation II amps.  All the signal path resistors have been chosen for the best sound.  There are Takman REX carbon film resistors on the grids and cathodes and 1% metal film resistors on the B+ and in the power supply, and a nice pair of Kiwame resistors in a key spot.  It is dead quiet and incredibly dynamic.  The first B+ filtering before the regulators is done using a large film cap, not electrolytic capacitors.  The other electrolytic caps are all very high quality Nichicon or Panasonic 10,000 hour 105 degree rated capacitors.  The volume control is a 48 step Khozmo attenuator.  It is built with very low noise resistors and has hard gold plated contacts that Arek, the manufacturer, says are good for 50,000 cycles!  I have used Gold Point attenuators and they sound very good.  The Khozmo sounds just as good, has 48 steps and a wonderful feel.  I have heard the odd pop or click from a Gold Point attenuator, and none whatsoever from the Khozmo.  It is like a continuous volume pot, but sounds infinitely better.  The individual channel level controls are 21 steps and use the same surface mount resistors found in the Gold Point level controls.  The combination allows for infinite balancing of channels and also lets you dial the level of the whole preamp up or down to match the output of your sources.  Then you use the main volume control.  It works beautifully and is sonically transparent compared to a conventional balance pot.  Lately I have been using really nice old Russian surplus selector switches with big silver contacts and ceramic plates.  I think they actually sound better and I am quite sure they will last forever! I have just completed a build of a preamp using an all film cap supply featuring the excellent Mundorf Tube Caps.  These are very low ESR polypropylene film caps.  They bring the preamp up another notch in clarity.  Bigger, bolder sound with a bit more 3D imaging, which is pretty hard to believe considering how good the basic build is!  This one also had the Duelund CAST-Cu ouput capacitors (photo at right).  I can do the all Mundorf film cap supply for $150 additonal cost.  The preamp has two sets of output jacks, each can be driven by their own pair of output capacitors, or the two can be in parallel and driven by one set of caps.  The basic version of the preamp will come with Mundorf supreme silver oil output capacitors and the jacks in parallel.  I really don’t want to build them with lesser capacitors anymore.  You can use the Mundorfs with your power amp and the second set of jacks to run a subwoofer if you wish.   The base price includes a pair of 0.33 uF silver oil output capacitors which will easily drive any amplifier with an input impedance of > 80K.  Most every tube power amp around is 100K or higher.  If you need to drive a solid state power amp with a lower input impedance I can use larger output capacitors.  For example, the 1.0 uF Mundorf silver oil caps will drive most any amp with > 20K input impedance (any tube amp and almost all SS amps).  These add about $40 to the cost. If you need to drive the standard 10K IHF load then I can use a 1.5 uF silver oil cap for $55 additional cost.  They will drive any amp.  Or if you need a large cap to drive a solid state amp on one set of jacks and you want a smaller, more expensive cap optimized for a tube amp on the other set of jacks that can be done as well.  Usually a .33-1 uF cap driving both outputs in parallel is just fine for most people.  I have recently experimented with the super expensive Duelund CAST PIO-Cu output capacitors.  These are insanely expensive and cost me over 4X what a pair of silver oil caps do, but they are fantastic.  No, they are not 4X as good as the silver oils (which are fantastic), but the background with the Duelunds is even blacker, the separation of instruments and vocals even better, and the soundstage is even deeper.   They are just incredibly musical.  Go online and read about them.  Everything is true.  The parts are very expensive and it will be a $375 upgrade for a pair of 0.47 uF Duelunds.   Also, I have been experimenting with the Jupiter Copper caps.  These are about 1.5X the cost of the silver oils and a bit more detailed.  I quite like them and think they are better than the silver oils.  I think I prefer the Duelunds in my system, but they are MUCH more expensive.   The Jupiters will be a $60 upgrade cost for the standard 0.33 uF size I use, and of course more for larger values if you need them for driving a solid state amp.  With any of these three capacitors this will most likely be the best preamp you have ever heard.  I will not build one with anything less than the Mundorf Silver Oil caps, the Jupiters are better, and the Duelunds are even better, although between the Jupiters and the Duelunds it is a matter of taste and system synergy. I have sent about 30 units out to customers and these are their comments from the first three: Mark from Sacramento: Moment by moment, the sound of this preamp is revealing its magic with absolutely zero detractions.  Among its attributes, I am stunned at the black background on silent passages in music.  I prefer this preamp to the Citation I preamp you modified for me in the past and and the C-I is a fantastic preamp. A SS preamp could not be more quiet. The dynamic extention is breathtaking.  I am eager to have my fellow Bay Area Audio Society members give a listen to this fine preamp as I think the members will not fail to appreciate the accuracy and sonic attributes of this modestly-priced preamp.  No question, this is a stunning preamp as I hear it more.  I think that like fine preamps, it is ruthlessly revealing. The ability to use balanced inputs from a DAC raises the performance of this preamp to a higher level. I think there is some magic in the 6SN7 tubes that almost no other preamp designer uses.  Don has listened to a variety to 6SN7 tubes, the result of which he has optimized the sonic signature of this preamp with the tubes he supplies — no need for expensive tube rolling with this preamp. I have had a dozen tube and SS preamps in my system over the past 45 years and this is the most pleasing preamp I have ever heard.  One could spend 5X-10X the price of this preamp and I suspect would not find their performance exceeding what what Don has created.              Bill from Anaheim: It’s been running 24 / 7 mostly since I got back here on Tuesday.  I turned it off one night.  All other nights I just turned off the amp and let the pre cook.  It continues to improve (or my ability to listen to it appropriately improves).  The improvement is not subtle. Overall, the system is now presenting with extraordinary fidelity … breathtaking dynamics and transparency … combined with a rich and colorful tone. So easy to listen to and so easy to be drawn into whatever performance is playing.  Currently listening to John Williams playing Rodrigo Guitar Concerto … a 70’s recording well done and well transferred to CD … and my rip to a HD and playing back through the Wavelength … Wow! I’m really glad to have this pre-amp! Thank you Don. Jose L. from Oakland: My quest in searching for tube preamp done when I saw this preamp it  was love at first sight without  knowing how it sound.  After contacting Don, very friendly and he knows  his stuff.  He answered all my questions and fears  about  tube amplifier.     This preamp produces the  finest  and incredible  audio   and I cannot imagine getting this quality sound for the price. Thank you  for giving new life for an old hobby Warranty: This preamp should be totally reliable.  So I will guarantee it for two full years.  If anything goes wrong (exept for a tube) I will pay shipping both ways, fix it, and return it for free.  If a tube that I supply fails in 90 days I will replace it.  Now if you have a party and someone spills beer on it then I will fix it for you as cheaply as possible, but the cost is on you.  Realistically I would expect this unit to be trouble-free for many years.  If you get a problem more than two years down the line and you pay the shipping I will fix it for only the cost of parts, forever. Also, if I build one of these for you, I will let you listen to it for 3 days.   If you don’t like it you may return it.  I will sell it and refund your money (-5% restocking fee) provided I receive the preamp and all tubes, and any cord in perfect condition.  It may take a few weeks to sell, but you will receive your refund when it does.  I know that you are going to think this is the best preamp you have ever heard and you will know that before it even warms up! Tubes: The preamp will come with 4 basic 6SN7 tubes and one 6AX5GT rectifier tube.  You are free to experiment with any 6SN7 tube types.  I have a good collection of the “holy grail” 6SN7 types including Tungsol round plates, Kenrad, various Sylvanias, GE, RCA, Russian 6H8C metal base, and my favorite of the batch, the National Union black glass.  All sound good.  However, the modern production Shuguang black treasure series CV181z kills them all.  You can buy them direct from China for about $115 - 130 a pair on the evil auction site.  I buy 5-10 pairs at a time and can sell them for $129 a pair with the preamps.  You will not hear a better tube in this preamp.  Not even close.  If you can afford two pairs it is worth it.  Otherwise use on pair as the main tubes and any good 6SN7 as the buffer pair. Prices and options: Basic SP14 preamp in wooden case with 48 step manual Khzomo attenuator, top quality parts and 5 RCA inputs: $1799 This includes a basic tube set with 4 6SN7 tubes and one 6AX5GT rectifier, one pair of 0.33 uF Mundorf supreme output capacitors with the two outputs in parallel Preamp has an IEC connector, you supply power cord Options: Upgrade from manual Khozmo attenuator to 64 step remote contolled version $200 Change one pair of basic 6SN7 tubes to Shuguang Black Treasure CV181z $129 Upgrade from cherry case to walnut $40 All film cap power supply using Mundorf Tube Caps $150 Change one set of RCA inputs to XLR (balanced) using high quality Cinemag line transformers   $225 Change the output capacitors to one pair of Jupiter Copper Caps $60 Change the output capacitors to one pair of Duelund CAST PIO-Cu (expensive, but even better) $375 10 gauge cryo treated fantastic power cord $109 This cord makes a 10% difference over any other $50-100 cord I have tried.  I am not into snake oil super expensive boutique parts, but this cord has won me over.  Richer, deeper sounding.  Let’s put it this way.  Once you have one of these attached to this preamp you won’t want to swap it out.  Really.  I won’t part with mine! Other options are possible.  I can use any output capacitors you wish.  I could even use two XLR inputs and have a switch between them.  Just contact me to discuss your needs.
Vintage Tube Audio  Restoration and Repair
Front view.  Notice the tape monitor switch and separate level controls for each channel. Rear view with optional XLR inputs. The inside.  Khozmo 64 step remote attenuator and shielded wire.  Mundorf supreme silver oil output capacitors. Tubes are on top to run cool and so you can see them!  Optional Khozmo 64 step remote controlled attenuator.
Line stage and matching power amp All film cap power supply with Mundorf Tube Caps and Duelund CAST-Cu output caps
First preamp with dual mono Khozmo 64 step remote attenuators