DHTs are constructed with the audio signal path directly connected to the heating current of their cathodes. So for high quality amplifiers we must take care that the heating power supply does not interfere with the music signal. It must not add noise and not react to the voltage across the heating filament when the music plays. All dhts develop a signal voltage across the filament when the music plays. This is formed by the difference in slope (gm, mA/V) at the positive and negative sides, in turn caused by the difference in bias at the positive and negative sides of the filament. So for example the WE 300B has a +5V higher voltage at the positive side making the grid to cathode voltage biased 5 volts closer to the grid, giving increased plate current.
When DHT amplifiers were constructed in the 1990s and 2000s many designers used AC-heating because experiments with DC heating sounded so bad. The bad sound arises from voltage regulator chips like the LT1085. These voltage regulators show relatively high noise and an inductive-looking output impedance that reacts with its output capacitance to produce resonance peaks. Worst of all their voltage feedback, which is necessary to set the DC output voltage, reacts to the music signal and across the filament. The voltage regulators see the music signal as an "error" which must be eliminated.
I have been developing DHT heating solutions since 2003. Amplifier constructors found that my heating solutions sounded notably better than AC heating or other DC heating methods, and the use of 'Rod Coleman filament regulators' became widespread.
The improved sound arises from the elimination of intermodulation with 50/60 Hz harmonics (compared to AC heating); and there is no corruption of the music signal (compared to DC voltage regulators). The improvement applies to power amplifiers and to preamplifiers. Preamps also benefit from the super low noise of the regulators, making them quiet at last!
Cooling: The Regulator power transistors are cooled by mounting to a heatsink - or you can use some amplifier-chassis or metalwork for cooling.
Input Voltage: Please add your own rectified DC (rectifiers + capacitors) - these components are not supplied.
Raw DC Kit: If you need a neat solution for the Raw DC, there is now a PCB and some parts are now available, please find it here:Raw DC DIY Kit:
PCB Size and Mounting: The board is 64mm (X) x 40mm (Y). Mounting on 2 Holes, 71-73mm apart (X)
Operation: The Coleman Regulator outputs a stable current, set by a trim pot. The circuit does not sense voltage at the filament terminals. The regulator improves the isolation-performance with circuits to separate the control loop even further.
Adjustment: Most DHTs are specified for voltage across the filament. This must be set exactly, for long life of the filament emissive-surface. The output current flows in the Filament to give a Filament Voltage. The Regulator is fitted with a trimmer potentiometer, which must be adjusted to achieve the rated filament voltage across the Filament Pins. A 25-turn trimmer gives easy and precise adjustment.
Temperature-compensation: The regulator is temperature-compensated to achieve low drift.
Ultra-Low Noise. The regulator does not use noisy bandgap reference diodes to establish the correct heating power. Discrete transistor design allows the lowest possible noise in the output. The result is measured noise level of ca. 7µA rms 20-20kHz (1.2A heating); giving microvolt-region noise across the resistance of the filament.
Wide-Range of voltage and current: The Coleman Regulator uses discrete transistor design to allow voltages up to 32V at the input. High output voltages are possible: allowing Filament-Bias architecture to be used (See AN-Filament-Bias-Supply.pdf for working examples of Filament Bias).
Transmitter and Receiver ranges. Two versions are available, using the same PCB. A high-power Transmitter version for filaments with heating Power 10W and above. And a Receiving-Tube version, for filaments 10W and lower, having all-plastic power transistors, for easy heatsink mounting.
You can buy with PayPal.
Please send me some email: dht (at) lyrima.co.uk and I will issue a PayPal Invoice - it can be paid from your PayPal account, or with Cards. If you need help choosing the right kits, please write, and I will reply quickly.
Cost for 2 (stereo) is USD 45 including tracked delivery
Cost for 4 (2x stereo) USD 78 including tracked delivery
Cost for 6 (3x stereo) USD 105 including tracked delivery
Cost for 2 (stereo) is USD 47 including tracked delivery
Cost for 4 (2x stereo) USD 81 including tracked delivery
Cost for 6 (3x stereo) USD 108 including tracked delivery
Cost for 2 (stereo) is USD 49 including tracked delivery
Cost for 4 (2x stereo) USD 84 including tracked delivery
Cost for 6 (3x stereo) USD 114 including tracked delivery
Documentation for the Regulators. This is a Live Google doc; it is updated every 5 minutes.
DHTs - directly heated triodes - are purpose designed for good sound. The regularity of the triode curves makes this plain to see.
DHTs present a challenge for amplifier design - they are constructed so the audio signal passes directly through the heating element. For high quality amplifiers we must take care that the heating power supply does not interfere with the music signal. It must not add noise and not react to the voltage across the heating filament when the music plays.
All DHTs develop a signal voltage across the filament when the music plays. This is formed (1): by the resistance of the filament, (2): by the difference in slope (gm: mA/V) at the positive and negative sides, in turn caused by the difference in bias at the positive and negative sides of the filament. So for example the WE 300B has a +5V higher voltage at the positive side, making the grid-cathode voltage 5V closer to the grid ('hotter' bias), and driving increased plate current.
The Coleman Regulator is purpose-designed to minimise the effect on this signal, and preserve the integrity