A Low-Mu Triode Preamp - pmillett.com

1 Just about every stereo systemuses a Preamp or line amplifier,if you prefer. This article de-scribes a Preamp (Photo 1) that sa little different from most, using Low-Mu power triodes to get good perfor-mance with only one amplifier stage,and no negative DO WE NEED A Preamp ?The most visible functions of a preampare to provide source signal selection( , CD player, phono Preamp , and soforth) and volume control for youraudio system. A Preamp doesn t reallyneed very much voltage gain; in fact,some designs, called passive pre-amps, don t have an amplifier stage atall. There is much to be said for thesedesigns after all, with no gain stageand only switches and passive attenua-tors in the signal path, the noise anddistortion performance is hard to why would we need an amplifier?

2 The biggest problem (for me) with apassive Preamp is that its outputs havea relatively high source impedance,and one that tends to vary substantiallydepending on the position of the vol-ume control and the output impedanceof whatever is driving it. This works OKas long as the load (the amplifier) iswell behaved, with a high load imped-ance, and there isn t too much capaci-tance in the amp or interconnect my system, this isn t usually thecase. I typically have several poweramps connected, with only one pair inuse at a time. Often I also have a head-phone amp or two plugged in. Each ofthe loads is usually pretty benign, butwhen you parallel several along withtheir interconnect cables you wind upwith a load that re-ally needs to be dri-ven with a lowsource impedanceto sound amplifiersalso aren t all thatwell behaved.

3 Peo-ple assume that aclass A1 load (suchas the input of theirpower amp) is al-ways a high imped-ance. At DC, this is usually the case; butthe effective input capacitance can bequite high, especially in designs thatuse high-gain triodes in the Preamp DESIGNSSo, you want a Preamp , and you regoing to build it with tubes. You canchoose from several common designtopologies used in tube preamps , aswell as some unconventional oldest Preamp designs are essen-tially small single-ended power ampli-fiers. They use a gain stage with an out-put transformer as the plate load of atube, with the turns ratio set to provide ahigh load impedance to the plate, and alow output impedance.

4 Many people be-lieve that this is the best type of circuit,partly because it doesn t use any cou-pling capacitors in the signal path. Thedownside is that it s expensive to imple-ment a good line output transformercan cost several hundred dollars, andthe cheap ones don t perform very classic design that s typical of1960s hi-fi equipment uses a high gaintriode such as a 12AX7 as a groundedcathode amplifier, coupled to anothertriode acting as a cathode follower. With-out negative feedback, this type of cir-cuit has very high gain (around 80) anddistortion, and high output of overall negative feedback is usedto get the gain down to close to 10 or 20,which reduces distortion and output im-pedance.

5 Though it measures well, Idon t like the way this type of circuitsounds. I suspect that I m hearing thelarge amount of feedback that increaseshigher-order harmonic similar design uses a medium-mutriode such as the 6SN7, and no feed-back. This type of amplifier, if properlydesigned and not overloaded, soundspretty good to me. It has low output im-pedance and distortion, but cannot de-liver lots of current to the load (currentdelivery is limited by the amount of cur-rent flowing through the tube, regard-less of output impedance). Cathode followers are not withouttheir problems (and critics), though. I vehad problems with the heater-cathodeinterface inducing noise, and even insu-lation breakdown, since the cathode isoperated at an elevated common Preamp stage usestwo triodes in an SRPP circuit, with thetriodes connected in series and theThis author describes how to achieve low distortionfrom power triodes.

6 By Pete MillettA Low-Mu Triode Preamp4audioXpress 2 1: Top view of Low-Mu Triode tube acting as both a cathode fol-lower and a plate load for the lowertube. These circuits can achieve verylow distortion and output impedance aswell, but suffer from the same issues asa cathode follower output design takes a different path asingle gain stage using a Low-Mu triodewith very low plate resistance (Photo 2).This achieves the modest gain requiredalong with very low output POWER TRIODESThe output impedance of a grounded-cathode amplifier is roughly equal tothe plate resistance of the tube in paral-lel with the plate load resistor. Youneed to make the plate load resistorlarge relative to the plate resistance toget low distortion.

7 So, to get low outputimpedanceandlow distortion, youneed a tube with a low plate number of large power triodeshave low plate resistance (Rp), such asthe 6AS7 and 6080, 5998, 7236,WE421A, 6528A, and 6336A. Most weredesigned for voltage regulator service;the 7236 (I believe) was designed todrive motors for vacuum-tube computertape drives!Unfortunately, at first glance, thesetubes don t look linear. In a normal re-sistor-loaded, grounded-cathode ampli-fier, they have high distortion, mostlyeven harmonics. This distortion de-creases with increasing plate load resis-tance, but a larger resistor means thatyou need a higher supply voltage.

8 Luck-ily, there is a way to present a very highload to the plate of the tube withoutusing a big resistor and high voltagepower supply a constant-currentsource (CCS) PLATE LOADSA constant-current source (CCS) platePHOTO 2: Quarter view.+C6330uF 100VJ13R4150 2 WLLRR51 KLSW13 POLE ROTARY SWR810 KPPJ7X+C5330uF 100VV1EL343527184R91K 1 WRWWRC110UF 400 VCCJ9J11 INPUT 47236 / 6AS7213 OUTPUT 2 RYWWJ6 MFEL*XR101K 2W7236 / 6AS754678LC210UF 400V+320V_LWWXR31KJ1 VOLUMEINPUT 1 CCC310UF 400 VOUTPUT 3 RPP+320V_RJ4 INPUTPPMFOUTPUT 1V2EL343527184LJ2J14R13100K STEREO POT132654 WWJ8MF = METAL FILM RESISTORCCEL*LR710 KINPUT 3 YINPUT 2PP = POLYPROPYLENE CAP, SOLEN OR EQJ3J10R12330 5 WWWYPPR6150 2WJ12LJ5EL* = ELECTROLYTIC CAP, AUDIO GRADEV3 ACC = CARBON COMP\ RESISTORWW = WIREWOUND RESISTORCCWWR11330 5WC410UF 400 VRFIGURE 1 2 uses an active device (usually atube or FET)

9 That s connected so that italways wants to supply a constant cur-rent to its load, which in this case is theplate of a Triode . This provides a veryhigh resistance load to the plate essentially, a near-horizontal load a CCS load, the load line that thetube sees is dominated by the currentthat s delivered to the are many possible designs forCCS plate loads some very simple (justa depletion-mode FET and a resistor),and some very complex, using bothtubes and solid-state devices. If you re in-terested in CCS plate load designs, thereis a gold mine of info about them on theInternet (see References at the end ofthe article for some places to look).

10 I experimented with simple designsusing an FET and a pentode. They bothperformed well. I opted for a pentode,partly because I had them they weresimple and performed well in this appli-cation and partly because they don tneed a heatsink. Using a tube lets youdissipate the power above the chassis!One of the problems with using aCCS especially a tube CCS is thatyou need to have substantial voltageacross them, especially if the outputvoltage swing is substantial. With a Low-Mu power Triode Preamp , this isn t toomuch of an issue; the output swing isonly a few volts, and the Triode can behappily run at low plate voltage, leavingmore of the available power supply volt-age to drop across the Preamp DESIGNThe Preamp design is straightforward,using EL34 pentodes wired as CCSs asthe plate load for the Low-Mu Triode .