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Old 12th May 2009, 07:01 PM   #1
bluegti is offline bluegti  United States
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Default PSUD - How do interpret the results?

I have just recently begun to understand how an amp works (as opposed to just following directions and putting them together). I want to design my own amp as an exercise - I learn better by doing than just reading. I have gotten to the point where I have a (very) basic understanding of operating points and load lines. Now that I have enough information to be dangerous (but still safety conscious) I'm looking at Power Supplies.

I often read suggestions to use Duncan PSUD to design/model power supplies. However, I haven't found a good site explaining how to interpret the results.

Is there a "tutorial" or guide for beginners anywhere?

If not, is there someone willing to act as a mentor to help me learn?

Finally, if there is a better forum for me to ask this question, please point it out to me. I have been unable to find one.
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Old 12th May 2009, 07:07 PM   #2
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1. No tutorial that I am aware of, other than the "Help" section of the software. There used to be a forum on the Duncan web site, but I don't know how active it is (I don't use it).

2. I would be more than willing to help you out, as I am able. I love the software; it works amazingly well.

3. I believe this to be your best forum. Are you interesting in building solid state or tubed amplifiers? I suggest tube
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Old 12th May 2009, 07:21 PM   #3
DougL is offline DougL  United States
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There were a couple of articles in Bas's DIY Magazine.
That should get you started.
I will look for a link after work.

I am sure many of us would be willing to help.

Doug
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Old 13th May 2009, 04:50 AM   #4
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here we go.

http://www.basaudio.net/pubs/DIY_2005_5.pdf
http://www.basaudio.net/pubs/DIY_2006_1.pdf
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Old 13th May 2009, 01:48 PM   #5
bluegti is offline bluegti  United States
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Thanks for those links. It looks like there is some great information in them. Although I think I have some even more basic questions.

Starting with the transformer properties. Since I will ultimately be building a 6V6-based tube amp, I will be focusing on a power supply for it. I will use a couple of readily available transformers for my examples, but I'm not really sure if they are appropriate yet. Hopefully, others will be able to learn from this as well.

The default design that comes up in PSUD shows T1 is rated at 333V (31Ohms):

Click the image to open in full size.

You can edit the properties by right-mouse clicking on the transformer and choosing Edit. This will bring up the Edit transformer properties window.

Click the image to open in full size.

Based on the PSUD Help, I assume I should enter 260 for the 370DAX and 330 for the PA774, but perhaps not because it says I should enter the Off-load voltage. How do I determine this value? I don't see it in any of the specifications. Please point out where I can find this information.

In addition, I don't know what to enter for the Source impedence (Source res). According to the help, I can calculate the value by clicking on the ... button. This brings up the Source Impedance Calculator:

Click the image to open in full size.

How do I determine the values to enter? Again, I do not see this information listed in any of the specifications.

Thank you for your help. I hope this will be useful to others as well.
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Old 13th May 2009, 02:00 PM   #6
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Good question, and this is probably the most important part of the simulator in order to obtain accurate results. I have found the best way to get a good transformer model is to measure your transformer (if you have one in your hands). Fortunately, the small transformers we deal with in audio have most of their impedance due to DC resistance, not leakage inductance. So by measuring the DCR of each winding, you have addressed over 90% of the actual impedance of the transformer.

So measure the DCR of the primary and secondary windings with your multimeter. Then, energize the primary of the transformer with 120VAC, or whatever you have at your wall outlet. Measure the secondary voltage, which is the no-load voltage. Plug these into PSUD as it requests it.

If you don't have the transformer in your hands, I have found reasonable accuracy by using the top "Value" option, and entering a percent impedance. This can be inferred from the datasheet.

Do you have the transformer(s) ?
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Old 13th May 2009, 03:43 PM   #7
bluegti is offline bluegti  United States
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I don't actually have these transformers... I'm trying to decide what to buy once I get to the next step in my design. Not even sure if these are appropriate, but I figured a lot of people probably use hammond transformers.

Just to be clear, I measure DCR by measuring the Ohms (with the power off) by connecting my probes to the different leads/terminals for the primary and secondaries. The value I get is the DCR. That's it? That's easy.

To measure the off load voltage I set my multimeter to VAC, clip the leads to the secondaries (I prefer clips when measuring high voltages), then turn on power to primaries. The value I get is the off load voltage. Pretty easy too!

Hey... I just noticed the Off-Load Voltage Calculator. This must be what you meant by using the top "Value" option. Click on the ... button next to the Value RMS V field in the Edit Transformer Properties form.

Click the image to open in full size.

How do I infer the percent impedance from the datasheet? I don't see how to do it given the information provided on the websites linked above.
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Old 13th May 2009, 03:54 PM   #8
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Since your transformers have center taps, you may also want to model full-wave rectification (instead of bridge). To get the target voltages that you are looking for with bridge rectification, you will only be using half of the secondary winding.

To change from bridge to full wave, highlight the first block (the entire block, not just the transformer) and select full wave.

I input the DCR of the secondaries into the transformer properties box, but IIRC, this may be a slight simplification. I also input the loaded (rated) transformer voltage, and have found the program to be quite accurate.

One more tip is that if you use a CRC or CLC design, you can adjust the value of the first C to dial in the desired B+. Lower 1st C acts more like a choke input, and higher first C acts more like a cap input.

Yes, you can measure the transformer as you have described, it's that simple. Use clip leads and keep your hands off when testing live secondaries.

The offload regulation refers to how high the secondary voltage will go without a load; larger transformers usually better regulation.

You may also want to change the load from resistive to a current load, which is the sum of the idle currents for all of the tubes, and influenced most by the output tubes. Right-click the load section, and (not the load R) and select current load. Current as a load is way more intuitive for me, anyway.
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Old 13th May 2009, 05:00 PM   #9
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Those are good rec's by BoyWonder. Yes, it is that easy, IF you have the transformer in your hands.

If we assume you are using the Hammond 370DAX, we can assume you have a 54VA rated winding (76 - 6.3*3.5). So, our rated secondary winding current is close to 54 / 520 = 104 mA.

In your dialogue box, you would enter nominal output of 260V (let's assume for now you will be going full wave center tap) at 104mA. The regulation is the tricky part, and is dependent on the transformer. Regulation between 10-12% would be a reasonable estimate. Filament transformers for LT supplies tend closer to 15-18%. If you had the transformer, it would approximately be the percentage voltage drop from no load to full load.

Let the software provide the values for you, and you're ready to start playing. Solid state or tube rectification? Cap or choke input?
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