Preamplifiers

Lamm LL1 Signature Stereo Tube Preamplifier - An Audiophile's Dream

ARTICLE INDEX

The Design

First, let's talk about "rectifiers". Incoming AC to your wall socket cannot be used directly to create the music signal. Initially, it passes through a rectifier, which converts the AC (60 Hz in US, 50 Hz in Europe) to DC. The rectifier can be either half-wave or full-wave. A half-wave rectifier (first photo, below) chops off the negative portion of the AC's 60 Hz sine wave. The current is now flowing in only one direction, but half the power is lost.

half-wave-rectifier-tube-diode

With full-wave rectification (photo below), the negative portion of the AC is converted to positive. All of the power from the wall AC is conserved this way.

full-wave-rectifier-tube-diode

There are two 6X4 rectifier tubes in the LL1 power supply, and the circuit is a full-wave rectifier. There are two anodes (plates) in each 6X4 tube instead of just one. Here is a pin-out schematic of the 6X4:

6X4-schematic-pin-out

Following rectification, however, the voltage is pulsating at 120 Hz, going from 0 volts to 120 volts (or whatever voltage the secondary on the transformer is producing) and back to 0 again. So, the power still needs some work to make it usable. This is where power supply capacitors and chokes (called LC-filters) come in. The output of the rectifier is connected to the LC-filters, and power is stored there. The voltage and current needed for preamplification is drawn from the stored power, and it is very smooth DC. It's not perfect - as exemplified by the presence of "ripple" peaks in the output - but it is as close to being perfectly smooth as it can get.

There is also a custom wound transformer, and capacitors and resistors so esoteric, I have never heard of the brands. You can see two analog voltage regulators in the middle. These are for keeping the filament heater voltage steady. They are not in the signal path. The reason I am describing the LL1 power supply in such detail, is that single-ended designs tend to suffer from AC hum, a problem that is attenuated in balanced designs. So, Lamm has gone to great length to build a power supply that will not allow hum to contaminate the music that you are listening to.

lamm-ll1-preamplifier-inside-power-supply-chassis

Inside the main control chassis, shown below, you can see four 6H30's (sometimes called the "Supertube") on the right. Each one is a low impedance (800 ohms) dual triode, so basically here, you have eight triodes driving the signal. Because of so much power, the preamplifier has only one gain stage (that's all it needs), and therefore, the output signal is inverted. So, you will probably need to reverse the connections on your speaker binding posts if your power amplifier is non-inverting.

lamm-ll1-preamplifier-inside-control-chassis

The photo below shows a close-up of the four 6H30 tubes in the control chassis. They have silicone rings around them to limit microphonics (vibrations that cause electrical noise). Notice also that the tube sockets are mounted on small posts rather than being flat against the circuit board (see inset for detail).

lamm-ll1-preamplifier-closeup-of-tubes

Here is a schematic of the 6H30 dual triode. Cathodes 1 and 2 are indirectly heated by a heater filament. The two grids are situated between the two plates (anodes). There is a screen that separates the two triodes from each other.

6H30-schematic

The front of the LL1 has plenty of toggle switches, and that is only on one of the channels ! ! !

The main chassis is on top and the power supply is on the bottom. In use, you would want to separate the two chassis by at least 4", to keep power supply electromagnetic radiation from interfering with the control circuits in the main chassis, but also for ventilation.

On the top, from left to right, are toggles for Output 1/Output 2, Mute/Operate, Normal/-12 dB (use this when you want more movement in the volume control), Line/Direct (Direct takes the signal from the corresponding input straight to the volume control, thus bypassing the rest of the switches), and Line1/Line 2 (input selection). On the bottom (power supply) are toggles for Power On/Off, Accessory Outlets On/Off (you can plug a CD player or other low wattage component here), and Remote 1/Remote 2 (remote control triggers for turning on other Lamm components, such as their power amplifiers).

lamm-ll1-preamplifier-front-600-pixels

On the rear panel are the RCA and XLR inputs and outputs. There are two sets of outputs, switchable from the front panel. There is also a tape loop. You can see the six-pin connector sockets that connect the power supply to the main control chassis via the supplied cable. On the bottom (power supply) you can see the accessory outlets, remote control trigger sockets 1 and 2, a grounding terminal, and the detachable grounded AC cord socket.

lamm-ll1-preamplifier-rear-600-pixels

To give you an idea of the size of this product - keeping in mind it is a stereo preamplifier - look at this photo of the shipping boxes as they arrived on my deck. Each chassis is housed in a wooden crate, with dozens of long screws. It took me half an hour just to get the screws out of the top of the four boxes.

lamm-ll1-preamplifier-in-shipping-boxes

I had to place them on the floor in front of the rack that has my sources and one of the power amplifiers. To move ahead on the story a bit, I loved the sound of the LL1 and have purchased the review units as my new preamplifier reference. An AnthroCart, custom built just for the LL1 to sit on four shelves, and an SACD/CD player to sit on the middle shelf, is on the way.

lamm-ll1-preamplifier-chassis-on-floor-no-connections