1. Basic considerations
This design could be used with various kinds of tube preamps, etc. RIAA, microphone, guitar. Throughout the years, I have built many well known and popular DIY preamps, EQ, but quite often when powered on for the first time, they didn't work as expected. It seems that this problem is common for all DIY project builders.
Usually, DIY schematic providers emphasize the preamp action. Despite that, the characteristics of power supply used with also have a considerable influence on the performance of the device, this aspect of the system is one which is often being ignored. Most of the native power supplies provided with schematics were far from acceptable performance (noisy, poor long-time stability of output voltage). In my opinion, if you are going to invest the time and expenses to build project, why not build a project of exemplary performance in one breath?
To avoid wasting time on power supply adjustments building new schematics for each project, I implemented simple, easy to build PSU (Power Supply Unit) design. It is based on my own experience and designs which could be found in readings that are enumerated in „Recommended Further Reading“ at bottom of this page.
2. Schematics
Usually a tube preamp needs many power supplies: DC High Voltage (anode) supply, AC or DC Low Voltage heater 6.3 V and/or 12.6 V supply, supply for relays, indicators, DC Phantom Power +48 V supply, sometimes DC +/- 15 V supply if there is a need for additional modules.
Offered PSU contains complete HV and two independent heater supplies (which could be connected consequently) on one 100x160 mm one-sided PCB. HV supply output voltage could be trimmed up to +250 V, heater supplies also have wide trimming range, up to +35V, the value of output voltage depends on your demands.
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HV supply created on integrated stabilizing circuits TL783KC (Texas Instruments) IC101 + IC201. It contains two identical stacked supplies with output voltage trimming possibility up to +125 V each. HV supply provides low-noise and low-hum very stable output voltage, what allows it to be suitable for RIAA, Microphone and Guitar preamps.
The trimming voltage of one stack is selected by means of a resistive dividers Vout1 = Vref(1 + ((R107||R108||R109) + R110)/R106) and Vout2 = Vref(1 + ((R207||R208||R209) + R210)/R206) (see schematics). Total HV value Vout = Vout1 + Vout2. Values of adequate resistors on both stacks should be equal, etc. R10(6,7,8,9) = R20(6,7,8,9), resistors tolerance could be 5%. Divider parameters are provided in Table 1.
3. Technical details
Each stack should be trimmed separately. Dissipation power of parallel-connected resistors R107||R108||R109 and R207||R208||R209 is 6 W, this seems overpowered, but ensures excellent thermal stability, which is important when the load is switched on for long time.
Dissipation on stabilizing circuits (IC101, IC201) depends on input-to-output differential voltage and output current P = V(in-out)*Iout. For example, if IC101 Vin = 120 V, Vout = 100 V, V(in-out) = 120V – 100 V = 20 V and Iout = 100 mA, dissipated power on IC101 will be 2W; with used thermal sinks IC temperature rises 20 - 25 °C over ambient temperature, such warming isn‘t critical for stability.
LV heater supply realized on circuit LM317. They are designed to supply up to 1,5 A of load current with an output voltage adjustable over a 1,2 to 37 V range. The nominal output voltage is selected using resistive divider. Schematics description could be found on LM317 datasheet.
LV output could be calculated by equation Vout = Vref(1 + (R303 + R304)/R302). Vref = 1,25 V. With selected values R302 = 240 Ohm, R303 = 560 Ohm and trimmer pot 2,2 K, Vout could be trimmed from 4,2 V to 14,8 V, using adequate transformer. Minimal input-to-output differential voltage should be 3 V, if Vout = 6,3 V is required, the secondary voltage should be 9 V.
Is recommended to use two separated transformers for HV and LV sections. HV supply works well with Triad Magnetics toroidal transformers (for example 25 VA VPT230-110 was used), LV - with Triad Magnetics 20 VA (FS20-1000-C2) laminated transformer. Both are available at www.digikey.com.
4. Components list (Farnell order)
NOTE: Values of resistors, capacitors and other components could be changed to meet personal needs. Ask for advice by email (see CONTACT section).
5. Recommended Further Reading
Douglas Self Small Signal Audio Design
Morgan Jones Valve Amplifiers
Burkhard Vogel Balanced Phono-Amps An Extension to the ‘The Sound of Silence’ Editions
Merlin Blencowe Designing Tube Preamps for Guitar and Bass, 2nd Edition
John Linsley Hood Valve and Transistor Audio Amplifiers
G. Randy Slone The Audiophile's Project Sourcebook