PSU ignorant

[cr0wl3y]

Hi there,

I'm trying to design a PSU for a phono/line preamp I am building. The original schematic calls for a transformer with a 190V secondary, bridge rectified and CLCRC filtered (I have attached the schematic below), to give HT 250V DC.

As it is next to impossible to find transformers with a 190V secondary, unless you make them to order, and conversely, 250V ones are extremely easy to get for next to nothing on ebay, I did just that. I have since then been playing around with PSU Designer II to try and figure out what to tweak.

At this stage, I was wondering if anyone has any idea on how I can figure out what resistive load to add to the simulation. So far, I've used the default 5k load the program starts with.

I have a few other questions, but let's first make sure I get this right.

Thank you,
Nikos

[Bas Horneman]

At this stage, I was wondering if anyone has any idea on how I can figure out what resistive load to add to the simulation.

Easier to do would to actually tell it how big the load is. For instance let's assume 8mA in total.

Go to the last resistor in the simulator (move over it with the mouse until the whole whitespace is highlighted not just the resistor.....)

Then right click and change....to constant current. Click Ok.

Right click again when the constant current thingy is highlighted and enter for value 1 the value 8mA.

You have now added a representative load to your circuit.

[cr0wl3y]

Originally Posted by Bas Horneman

Easier to do would to actually tell it how big the load is. For instance let's assume 8mA in total.

Thanks for this, but why 8mA? How do I find out what current my pre-amp will draw to plug into the program? In theory, the article I have taken this from says that it draws 10mA, but will this not vary according to the power supply itself?

[cr0wl3y]

For example, I just ran a couple of simulations using the same circuit and a 5k resistive load, but using

a. 190V - 0.025A
b. 250V - 0.2A

secondary trafos and got .027A for (a) and a little over .04 for (b)

Would I therefore not be wrong assuming the current draw is fixed and take it from there?

[kevinkr]

Bas' advice is quite good, just set the current source to the article's stated 10mA and design your supply - it is not too critical and once everything is actually built you can then tweak the resistor value(s) slightly to get the voltage spot on. In most cases it is not that critical - a few % one way or the other should not make a significant difference.

[ChrisA]

Originally Posted by cr0wl3y

...
At this stage, I was wondering if anyone has any idea on how I can figure out what resistive load to add to the simulation. So far, I've used the default 5k load the program starts with.

First off you don't need to care a lot about the exact plate voltage. Higher is better up to a point. Your preamp will have better headroom with more voltage. Have to looked at the load line?

The Hammond's 269esx transformer is a 190V and sells for about $40. There is an international version of it too.

With so much extra voltage to kill why not do something more creative? The first thing that comes to mind is to add another RC stage to the filter. You gain another few dB in ripple rejection at a cost of about $4. You could also add a low value 10R resistor between the diodes and first cap to limit in-rush. I notice there is no "bleeder" resistor in the power supply. You _really_ do need one simply to make the amp safe to work on and it could drop a few volts for you. The idea here is to not just burn the voltage up without putting all that the heat to good use. You could do all three: another RC stage, in-rash limiter and bleeder.

If you think about it, a bleeder resistor across the first cap and the inrush resistor toget form a volag devider. A very large in-rush and very small bleeder gives you a voltage drop and a "soft on" feature.
But really, think agin what voltage you need on the tubes the 7AX types can go 300V easy

But still you'd need to pick a value for the last resistor. You best option is to build the amp and measure it. You really can't know in advance because you can't know in advance how the tolerances of all the components will add up.

The nest thing I can think of is regulating the B+ supply. This is more complex but nothing eles you can do would improve it more.

[cr0wl3y]

Thanks for the suggestions.

OK, now onto the next couple questions:

1. Am I right in thinking that the voltage across the load will be the "max" in PSU Designer?
2. Does a value of a little over 210 sound right, given a theoretical "no load" 250v B+ DC supply?
3. Am I right in thinking I should be aiming to tweak values to get the same voltage across the load?
4. What power dissipation resistors would you suggest using?

Thanks!

[ChrisA]

4. What power dissipation resistors would you suggest using?

You could work it out by multipying the voltage drop across the resistor by the current through to get watts but you want these to run cool. Resistors create "hiss" because they are hot and will be quieter if they run cool. It is a minor point but seeing as this is a preamp you want it dead quiet.

Wire wound are the quietest type second is metal oxide. Buy them 4X larger than calculation shows and they will run cool enough. The normal advice is 2X larger than calculation. Never run them at their calculated maximum. or you will find them running at something like 80C or whatever. So do the 4X safety factor then round up to the next size.

Example, 10V drop at 10ma current is 0.1W times 4 is 0.4W so a 1/2W would do. Then I look at my supplier's web site and notice 1/2W cost $0.55 per five and 1W are $0.60 per five. For the extra penny I simply buy all 1W resistors. You can afford to overkill nickel and dime parts

[Wavebourn]

You may use VR-75 and VR-150 (OR SG2S and SG3S) tubes in series instead of a bleeding resistor. Such a way you would get well regulated 225V voltage with very low ripples.

And you are getting cool looking power presence indicators for free...

[boywonder]

Originally Posted by cr0wl3y

Thanks for the suggestions.

OK, now onto the next couple questions:

1. Am I right in thinking that the voltage across the load will be the "max" in PSU Designer?
2. Does a value of a little over 210 sound right, given a theoretical "no load" 250v B+ DC supply?
3. Am I right in thinking I should be aiming to tweak values to get the same voltage across the load?
4. What power dissipation resistors would you suggest using?

Thanks!

The voltage across the load will be max when the tubes are not conducting. With SS diodes, the voltage can rise to sqrt2 X transformer secondary when the tubes are cold.

If you change the load to 0 ma, you should see the max voltage.

Once you've got the load changed to constant current from resistive per Bas' comments above, click on the ma load box (not larger yellow box you clicked on to change from resistive to current, the smaller yellow box of the current load) click the "stepped load" box and put in a second current greater or less than the nominal current to see what voltages you'll end up with. You should do this after 5 seconds or so since you want to change the load when the voltage has settled out (not while it's still rising). Most important for this exercise is to check for ringing/overshoot in the voltage when the current changes. You don't want any overshoot, ie you want the circuit to be overdamped.

Use ohm's law to calculate resistor power ratings, and pick resistors that exceed the power dissipated by a fair bit, like double. They'll run cooler and quiter as already mentioned. You can easily get voltages across the resistors right out of PSUDII.

Since you are using bridge rectification, the diodes need to have a voltage rating at least sqrt2 X transformer secondary as well. It's also a good idea to use fast recovery diodes instead of the plain vanilla ones shown is the schema, ie UF4007 instead of 1N4007. Snubbing each diode with a 10 nf 2kv ceramic cap also helps keep diode switching noise down. Connect the caps in parallel with each diode.