JE Labs phono and line preamp

!!!WARNING!!! 

The voltages found in this circuit can be lethal, build at your own risk!!!

Phono stage

For severaI years I have been hacking the Dyna PAS chassis with various cascoded and cascaded phono circuits all employing negative feedback RIAA EQ in search for a circuit that will satisfy my musical requirements. My research ended when I discovered this unglamorous 2-stage cascaded passive RIAA EQ phono preamp circuit at the back page of an RCA tube manual. 

The most important lesson I learned in this excercise is negative feedback chokes the sound! I did not alter any component values except for using Ni-mH batteries for cathode bias which I thought improved detail and definition. I strongly advice against the use of a cathode follower and make sure the circuit drives a minimum load of 220K as originally specified otherwise you will lose bandwidth at both frequency extremes. I use an Alps 250K but have also tried the cheap Alpha 250K dual log taper carbon pot sold by AES. It does not have the tracking precision of Alps but at normal volume settings the channels are balanced enough.

I have been using this phono circuit since the early 90s starting with a PAS chassis with Telefunken ECC83/12AX7. Through the years I developed fondness for octals and nowadays I almost exclusively use 6SL7s/5691s which further improved the bandwidth and depth due to its slightly greater transconductance, vis-a-vis, the 12AX7. 

JE Labs phono stage adapted from the RCA circuit

There are people who swear by using high transconductance tubes like WE417A or 6DJ8. To hear what this was all about, I bread boarded a famous circuit using these tubes but did not find the sound as appealing, must have been just a matter of taste...I find the superior linearity of true audio tubes like a 12AX7 or even better yet 6SL7 octals play a lot more LPs.

MC transformer

For MC cartridge use a transformer step up device like this Tamura TKS-83 or mic transformer  to boost the signal to MM level. I am not a fan of active MC stages and rather not go there...

76>6SN7 cathode follower line stage

Another very simple topology with a single gain stage direct coupled to a cathode follower. Originally I was just using a 6SN7 dual triode for both stages but was inspired by the Berman article in SP 13 to try a type 76 as the gain stage. I use a lower B+ supply and run the tube at lower plate dissipation and also prefer the sound with the 2.2K cathode bias bypassed with a 100uf/16V cap [in this case I can justify using Black Gate]. 

At this signal level the direct coupled 6SN7 GTA or GTB cathode follower is sonically beneficial due to its low output impedance. Thus, it can drive longer interconnect cables without any detriment to the frequency extremes. [Cable geeks take note: many times the "difference in cable sound" you hear is more a function of load/capacitance especially if your preamp does not have low output Z].

Transformer coupled line stage using Tango NP216N

I briefly flirted with this design as a linestage and in spite of the fact that all my SETUP friends liked the sound [whose ears I respect], I eventually went back to the above linestage. The Tango NP216N is a wonderful piece of iron although I have to admit that I needed greater bandwidth than it has to offer. However this is a circuit that offers a solid 600 ohm [as used in studios] output impedance and can even be wired in balanced mode.

At some point I may try this topology again at the output of a tube microphone preamp project. Please do not take my seeming lack of enthusiasm as a dismissal of this topology. I included this schematic because it may well be appealing to other ears and/or applications.

Joel's preamp

12SL7 phono + 12SN7/Tango NP216N line stage + external PS

Nonong's preamp

5691 phono + 76>Tango NP216N line + external PS

Power Supply

During my Dynaco hacking days, I experimented with regulated HV using solid state devices which was 'in tune' to the high end sensibilities of the time. I also chanced upon a Heath tube regulated [6L6GC] Lab PS and hooked this to my preamp, there was a slight improvement towards my goal but my ears longed for a more natural sound.

When I acquired my first copy of the Radiotron 3rd edition, I regressed to 1930s technology. This is the PS circuit I've been using for the past couple of years for deluxe versions of my preamp. A full choke input followed by another choke/capacitor stage in order to get a very well filtered B+ supply.

I use a bridge rectified and highly filtered DC filament supply for the heaters without having to use silicon regulators. If you pay close attention to the schematic you will notice that there is a voltage divider network [120K series connected to a 47K and 68uf/160V combo shunted to ground] at the output of the B+. This network is mounted on the preamp chassis and keeps the filament floated above ground to further minimize hum and noise and also to keep the heater to cathode voltage on the 6SN7 cathode follower below maximum limit.

Note: the numbers in the schematic pertain to the terminals of a 4-prong connector/umbilical cord to connect the preamp to the power supply.

If your power transformer has a 5V, 3A rectifier winding, you can experiment with rectifiers. My favorites are the 5V4G and 5Y3, but have also used GZ34/5AR4 and 5U4. If you know how to use a DVM, you will note that different rectifiers will produce higher or lower B+ voltage which alter the operating point of the circuit. In general, the different voltage is what changes the sound not the rectifier itself.

Prototype

Built on a PAS chassis
12AX7 phono + 6SN7 line + external PS

Octal prototype
6SL7 phono + 6SN7 line + external PS

5691 phono + 76/6SN7GTB line
Gen Rad PS

under the hood

Variations on a Theme

5691 phono + 6SN7GTB line
External PS

6SL7 phono + 6SN7 line
External PS

Simple version 

6SL7 phono + 6SN7 (single gain) line stage
no cathode follower +
simple CLC PS

6SL7 phono + 6SN7 (single gain) line stage
no cathode follower +
simple CLC PS

76>6SN7 line stage + CLC PS, ca. 2006

6SL7 phono + 6SN7 (single gain) line stage +
PS in one chassis

My last project in the USA - a single chassis preamp in 2007 for my brother's system. With careful attention to wiring to avoid ground loops, it is just as quiet as the 2-chassis version.

76>6SN7 line stage preamp
with remote
volume control

JE Labs goes high tech - built by Joel Villanueva for this system and featured in the November 2011 Hi-Fi Show. It is a minimal line stage preamp with remote volume control on one chassis from Harana Audio.

Have fun and happy listening!