LS 300 Preamp Design
In the last post, I discussed the important elements of a good preamp and what I feel is important, as well as the design elements that I consider key to good sound and performance.
Enter the LS 300, our Reference Balanced tube preamplifier.
As stated previously, I prefer a preamp to have lean functions and a clean signal path. The LS 300 does not have balance control, headphone jack or other functions. It has mute, volume, input select, phase control, and home theater bypass. It includes two pairs of fully balanced XLR inputs and outputs, three pairs of RCA inputs and two pairs of RCA outputs.
It has a simple remote control for volume, mute, phase and power.
The tube complement is (2) 6922’s and (2) 5AR4/GZ34 or 5R4GY and all equivalents, including but not limited to, 5U4G, 5U4GB, 5V4, 5V4GB, 274B. It is a fully dual mono design with external supply and tube rectification.
Like all of our tube preamp designs, it has a single gain stage. In fact, it has a single stage, period! Our phono stages are the only products that differ from this approach. This single gain stage must also offer very low output impedance! Some designs use Mu stage or similar, but the LS 300 doesn’t.
Starting from the inputs and working our way forward:
All inputs are logic-selected via sealed signal relays. This keeps signal paths short and limits noise entering the signal before it is amplified.
The signal is then transformer-coupled, via differential volume control, which is implemented in a quasi-shunt configuration. This reduces sonic impact of the volume pot on the signal.
From here, the signal feeds the grid of the tubes, where it is amplified by inductive load and transformer coupled out, with no capacitors in the signal path. Phase switching and mute are accomplished at this point with sealed relays and the differential signal is fed to the input stage where it is routed to RCA and XLR outputs and Home Theater bypass switching.
The key to the elegance of this design is found in a few parts:
First of all, all inductive and tube components in the signal path. No solid-state plate loads, buffers or other complex circuitry. The differential circuits allow for common mode noise cancellation and transformer coupled outputs allow for very low output impedance, determined by the square of the transformers stepdown turns ratio. Gain and low Zout are achieved cleanly and elegantly.
The power supply is where most of the work is really done, as it is in most products. The LS 300 is a dual mono design with two power tansformers in the outboard supply. To keep noise levels as low as possible, the power transformers are kept as far as possible from the signal transformers to avoid electromagnetic interference induced noise. Dual umbilicals carry the various raw AC voltages to the internal preamp power supplies then where the voltages are tube rectified and choke regulated. The dual mono DC supply voltages are then fed to solid state finishing voltage regulators to ensure rock solid DC supply that is clean, quiet and immune to AC mains fluctuations.
So, we keep the signal path simple.
We provide adequate gain.
Volume control with the least impact on the signal.
Source select via sealed relays.
Provide very low output impedance via inductive step-down coupling.
Provide adequate drive to overcome single ended and balanced cable losses and impedances.
And most importantly, we do this with a low noise floor, low distortion and wide frequency response.
The use of vacuum tubes in this and other designs, allow for this simple design approach. The result resolving and detailed sound with excellent tonality and 3-D soundstage.
It is simple and it works! Yes, the details matter.
ModWright LS 300 Refernce Balanced Tube Preamp https://www.modwright.com/products/ls300-balanced-tube-preamplifier/