Sunday, August 2, 2015

Re-imagining the 10LS/1LB Preamp, Part 3: The Lama Kazu LK5H

I built the 10LS/1LB preamp in 2008, and modified it a few times over the years. It has mostly been serving as a direct injection (DI) box for guitars. I recently undertook a major rework. See Part 1  and  Part 2 for more background.

The Final Circuit

The final circuit
For the source follower stage, I selected The STQ2HNK60ZR-AP MOSFET. It is rated at 500V, is zener protected, and comes in a TO-92 package. I essentially kept with my previous plan for the rest of the circuit, except for a couple very minor changes: I added a 1 MΩ grid leak resistor on the pentode and 10 kΩ stopper resistors at the grid of the pentode and gate of the MOSFET; I reduced the source resistor value to 47 kΩ on the source follower section.

Although not depicted in the schematic, I added a textbook regulated DC heater supply, with a couple capacitors and a 7806 linear voltage regulator.  I decided to build this on a miniature terminal strip (0.25" spacing between lugs) that I had on hand.

The Chassis

Since the circuit is so much simpler than its original version, and so much more compact due to the change to a solid-state follower section, it became possible (with a little fussing) to take all the components inside the chassis box:

First steps in chassis modifications
First, I removed the tube socket turretboard and filter capacitor from the "top side" and plugged the holes with electrical box knockout plugs.

Then, I moved the input jack and neon power light off the "front" of the chassis and put them at the "back", i.e. the side that already had the power input, fuse, output connector and phase/lift switches. (I actually put in a different neon— I had bought a few smaller ones from the closing sale at my local Radio Shack.)

The pentode socket turretboard
Once those spaces were clear, I had room to move the filter choke into the area that had mostly been taken up with the old neon and wiring.

I modified the power supply board by removing the old "virtual center tap" heater resistors, adding a turret, and soldering on a monolithic bridge rectifier.  (The rectifier is over-rated for modest needs of the 150 mA, 6V heater, but I also had it on hand from Radio Shack).

The finished "gutshot"
I placed some adhesive bases for later mounting of the filter capacitor, moved the handle to the side, and plugged all the larger holes.  Not visible at the "front" side, I put some copper foil tape (with conductive adhesive) on the inside where all the jacks & potentiometers had been.

Outside back
To get the pentode inside the box, I made a little baby turretboard (about 1.8" × 3.4") mounted on a small section of aluminum angle.

The filter capacitor (with a few resistors directly attached) was attached to the adhesive bases with nylon cable ties.

Front view
The entire source follower circuit, including the 1μF coupling capacitor was assembled onto a miniature terminal strip similar to the one I used for the heater regulated supply.

It all fit, though the last stages of assembly were a bit cramped.  To finish it off, I designed a label and printed it on cardstock.  I put some adhesive aluminum tape on the back, and laminated it to the now blank "front" side of the chassis.  I decided to resurrect the "Lama Kazu" brand that I used for my first guitar amp project. I call this the LK5H, with the "5" referring to the pentode and the "H" for hybrid.


The tone is essentially the same as it was in its earlier incarnations, but it's much cleaner.  Obviously hum has been reduced, down to about -60 dBu with input grounded, which is well below the level of hum picked up by any of my guitar pickups.  Most likely this hum is coupled in from the power transformers to the output transformer.  There's no way to get any mu-metal shielding inside this box, so this is how it will stay.

The noise floor looks to be on the order of -75 dBu, not exactly "audiophile", but good enough for an electric guitar.

Monday, July 20, 2015

Re-imagining the 10LS/1LB Preamp, Part 2

I built the 10LS/1LB preamp in 2008, and modified it a few times over the years. It has mostly been serving as a direct injection (DI) box for guitars. I recently undertook a major rework. See Part 1 for more background.

In-situ Modifications

Previous configuration.
For the first round of modifications, I kept everything in its original enclosure and on the turretboard.  For the first experiment, I removed the 12BZ7 dual triode and associated circuits, as well as the level control.

In-situ modifications
To replace the cathode follower stage, I put in a source follower with a LND150 MOSFET.  This might not be an ideal component for source follower service due to its relatively high RDS /low transconductance, but I had some on hand and it has a sufficient voltage rating (500V) for the application. I used the solder lugs from the 12BZ7 socket as a terminal strip for the MOSFET. The bias network on the 5879 was adjusted somewhat to allow the source follower to be center biased with DC coupling to the pentode stage.  Power supply and heater elevation circuits are omitted in the schematics for clarity.


There was a decrease in hum when compared to the old circuit (-48dBu) and the tone seemed at least similar to the old circuit, but there were some new problems:
  • The gain was not quite enough. E.g.: Playing my Danelectro '63 baritone through it, I was peaking at about -15dBFS on my DAW.
  • Removing the 12BZ7's heater load sent the heater voltage excessively high-- about 8.4V AC! Remember that the heater transformer has a voltage regulation of 30% and was rated for 115V (i.e. Canadian) input, so a very high open circuit voltage happens with 120V on the primary.

Next steps

Next round of planned modifications.
The gain can be raised by increasing the anode resistor, and adjust the operating point to center bias it again. Of course there's a compromise: Increasing the anode resistor means reducing the idle current, which means lower transconductance and lower gain.  A 330k resistor seems to be about right.  The circuit will be something like the one here.

For the purposes of reducing hum, I was already contemplating a DC heater supply.  After seeing the overvoltage condition, it's obvious I need to do it regardless of the hum issue.  (Incidentally, I never measured the heater circuit voltage when the 12BZ7 was still in.  It was probably somewhat high, but not quite this high.)

I will probably do a by-the-book supply with a capacitor filter and linear regulator.  No reason to get fancy. A low-dropout regulator is not even going to be necessary with input voltage this high.

Since I'll need to add a heater power supply PCB, it's probably time to remove the turretboard and rewire the amplifier circuit.  The circuit is so simple, with so many components connected to the pentode socket, it makes sense to go point-to-point, on old fashioned terminal strips.  I might even be able to move everything (pentode, reservoir capacitor, and choke) inside the enclosure.

Thursday, July 16, 2015

Discrete solid-state microphone preamp with low fixed gain

Proposed schematic
When I removed the microphone input from the 10LS/1LB Preamp, I was left with a spare Edcor MX8cs transformer.

When Radio Shack closed its retail stores and got out of the DIY component buisiness, I picked up a bunch of assorted "jellybean" transistors, as well as some 1N4733 5.1-volt zener diodes (along with a ton of other things).

All these solutions were in need of a problem, and I found it with my occasional need to record a drum kit on an audio interface with only two microphone preamps (The now-discontinued Echo AudioFire 4).

Even with a low-sensitivity microphone (like a SM57), a loud source (like a snare) will do just fine with the 18 dB of voltage gain inherent in the transformer.  All that's needed is to bring the impedance down (after it is increased by a factor of (8.2)² in the transformer) and drive a balanced line.

Here's the thought:

  • Drive the line from a two emitter followers using Sziklai pairs.  Biasing is through dc coupling to the transformer.
  • Use the 5.1V zener to derive a virtual ground point that's one VBE drop away from half of a 9V battery.
  • Input impedance can be selected with resistor R10.
More to come when I get around to building it...