Updated: 14-Jan-09 17:08
Doede's Audio Tweaks...

Tweak of the week: A simple GENERAL PURPOSE PCB solution for Voltage and Current Sources

Most of us are using DC to heat the small signal tubes and the Penthodes in the MU stage. For the real DHT lovers, a current source is an absolute MUST !! A real good current source is also fed by a regulated voltage already. So for example, for a Stereo Line amp with DHT's  you already need 2 Voltage sources and 2 current sources. Most of us does it in hard wiring or on a simple piece of general purpose PCB with a raster of holes and Soldering islands. No problem, can be done and it works just fine...............


BUT: My opinion was that it does not LOOK neat and it is a lot of work, specially with a RIAA / Line amp and MU stages.... Would it not be easy, if you could just grab in the drawer and take out a simple Module, just like you take a component??? Ohhhh yeeaaaa, that would be easy, eh ??? Solution was on hand of course; I made a simple general purpose PCB solution. No ROCKETS SCIENCE !!!! just an industrial quality PCB with 2 Voltage sources and 2 Current sources:

You can use it in one piece for Stereo; just connect the Voltage Output to the Current Source Input on the PCB Or cut the whole thing in 2 or 4 pieces, Just what YOU need for your application !!!

The next 2 Images are showing in more detail how the PCB's can be used. There is enough room (and drilled holes !!!) for several different types resistors and Capacitors, making it really general purpose. The cooling element is the very standard SK104. You can get these in 3 sizes, 25mm 38mm and 50mm. All with different K/W ratio. The interesting thing is, that you can SOLDER this element on the PCB !!! very easy and solid construction is the result.......

lm317pcb4.jpg (9522 bytes)


Techy Portion.......:

I used the following basic circuit for the Voltage and Current  source:

For the voltage source, with the given values for R2 you can reach aprox 15 Volt. With R2 = 110 Ohm, you can get 25 Volt out. Of course your input voltage needs to be (after the C minus ripple pk-pk !!) aprox 2 Volt more than the desired Output Voltage

Do not forget that this can consume very nicely some power !!! Example: 15 Volt DC after the C, 6.3 Volt output at 600mA... Pd of the LM317 is now aprox (15-6.3)Volt x 0.6A = 5.22 Watt. If you use the 50mm SK104 (10k/W) the heatsink will become almost 70 degrees !!! In this particular case I would suggest to lower the input DC a bit...... My recommendation is, not to heat up above 50 degrees !!!
So limit the Pd to 3Watt with the 50mm Heatsink !!!

For the current sources the formula is very simple:  Iout = 1.25 / Rl   or     RI= 1.25 / Iout
Example: for a Ba Tube I need 500mA, this will result in 3.5 Volt across the filament. For Ri the value is now: 1.25 / .5 = 2.5 Ohm. Use a 2.7 Ohm resistor and trimmer of 100 Ohm in parallel for proper adjustment as the trimmer would read aprox 35 Ohm.... The input DC should be at MINIMUM:  2 Volt (for the regulator) + 1.25 Volt (the current source resistor ref voltage !!) + 3.5V (Voltage across the Filament). thus:  aprox: 7 Volt.... more is ok, but you will have more power dissipation and more heat........

OK, last technical help than:  How large does the input C need to be??
Very simple, with the above you define the MINIMUM DC voltage you need and current drawn by the load.

The ripple voltage on the C (pk to pk) will be now   Urtt[Volt] = 1.5 x Iload[mA] / C[uF]
Add this to the minimum DC, add 2 Volt for the Diode Bridge and divide by 1.4 and   you have the Transformer AC Voltage...
Example: Need 6.3 Volt at 600mA and use a C of 1000 uF.  Urtt  = 1.5 x 600   / 1000 = 0.9 Volt ------>.
Ut = [6.3V (DC out) + 2V(LM317) + 1 V (ripple) + 2V (Bridge) ]/ 1.4 = 8Volt,
Best is now to use an industry standard 9 Volt AC output transformer.......

Just imagine you came out on 9.5 Volt for a certain application.... mmmhhhh what to do??? simple increase the C to 2.200uF, you will reduce the ripple enough to come below the 9 Volt now.... personally use the  following rule of thumb as Capacitors are cheap anyway ----->

I load



1.000 uF

100mA - 500mA

2.200 uF

500mA- 1.5 A

4.700 uF


For easy building up of the PCB, below is the lay out seen from the component side...

Do you need / want it from the soldering side??? Just click the Image .........


Well, isn't this just one of those nice & easy things, which makes our hobby life a bit more enjoyable ????


Can I get these PCB's somewhere??
Yes, Just look here


Happy Listening and Building !!!!

Doede Douma


IMPORTANT: The information provided on this page is intended as guide for DIY activities and therefore free to copy and or publish. If any one wishes to use any of the information from my WEB site, please make sure to refer and footnote to my URL Link as source! Doede Douma