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I am in the process of designing a split power aupply with a discrete regulator to power an integrated circuit amplifier (LM1875.) I am trying to determine the proper resistor value for the base resistor (R1) in my regulator.

The LM1875 produces max output at +/-30V and works as low as +/-8V. I'm shooting for around a +/-25V power supply for good power output without exceeding the maximum rated supply of +/-30V.

I'm starting the design based on a 44VAC dual secondary transformer with a current rating of 1.82A per.

Each secondary gives 22VAC RMS. After multiplying the RMS voltage by 1.414 to get peak and then subtracting the 0.7V drop due to the rectifier diode, I get about 30V peak of pulsed DC on each rail.

I chose a filter capacitor of 4700uF for a large value that's not too costly. I calculated the ripple left after the capacitor with the formula V=I/(2fC); where I is my max current of 1.82A, f is 60Hz for US mains power, and C is the 4700uF capacitor, though since the capacitor is 20% tollerance, I used 3760uF as a worst case scenario. This gives me 4 volts of ripple that still needs smoothing.

Rather than an IC voltage regultor, I decided on a discrete design. I'm hoping to incorporate the idea of a capacitance multipler that I saw in a pretty fabulous EEV Blog video [https://www.youtube.com/watch?v=wopmEyZKnYo]. My understanding is the IC regulators incorporate negative feedback that deminishes this capacitance multiplying effect.

I've created a schematic for what I'm thinking.

enter image description here

What's troubling me is how to appropriately size R1. I've tentatively come up with a value of 220 Ohms, based on this line of reasoning:

  • The maximum collector current is 1.82A. That's what the transformer can output continuously. (It could surge, I suppose because of the reserve charge in the filter caps, but I had to start somewhere.) 1.82A is what I used in my ripple calculation.
  • The Beta of the TIP120 is listed at 1000 minimum. To get a collector current of 1.82A, I need at least 1.82mA at the base, but 10x that provides some margin, so I chose 18.2mA.
  • The maximum voltage across R1 is equal to my ripple voltage of 4V.
  • Ohm's law says a 220 Ohm resistor will give me the 22.8mA of base current I'm looking for.
  • A 100uF capacitor at the base seemed like a reasonable starting point, though perhaps on the large side of things.

The -3dB point for an RC filter with a 220 Ohm resistor (R1) and a 100uF capacitor (C4) works out to 7Hz, which is well below the 60Hz line noise I'm trying to filter. This might even result in a soft turn on for the amp. So far, so good.

The effective capacitance in terms of ripple suppresion should be hfe x 100uF or 100000uF. That's like a 10x stack of 10000uF caps which should be enough to make any audiophile wet their pants.

My question is, have I calculated R1 correctly? Was my assumption to use the maximum ripple voltage the right thing or should it be RMS or something else entirely? Is the transistor getting enough current at the base? Too much? These are things I could not find in my research so far.

On a side note, the LM1875 data sheet claims a 94dB power supply ripple rejection. That seems pretty good, so perhaps my regulation efforts are overkill? This is a guitar amp, not hi-fi, and a little 60Hz hum might add to the mystique, but I don't want it to be obnoxious. 4V of supply ripple remaining after the 4700uF capacitor seems like a lot. Unless it's not a problem.

Any insight is appreciated.

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    \$\begingroup\$ You are going fine .Now what is your max DC volts under no load at high mains ? Double check power disipation in the zener and the 220 ohm you chose .increase R1 if needed .Now check SOA worse case of Q1 . \$\endgroup\$
    – Autistic
    Jun 5, 2021 at 22:56
  • \$\begingroup\$ As far as the max voltage under no load question, I do not have the part yet, so I can't measure it. But the data sheet gives a % voltage regulation of 10.8%. Should I take that to mean there is a maximum of 10.8% variance between unloaded and fully loaded? If that's the case it would be 1.108 times the fully loaded voltage, I think, or about 33V. That's a 7V drop over R1 and about 32mA current. I would think that's still acceptable. Should be about 800mW dissipated by the zener. There are 1W and higher parts available for that voltage. \$\endgroup\$
    – Dave H.
    Jun 5, 2021 at 23:28
  • \$\begingroup\$ I missed the edit window on my comment, but I wanted to add the max. base current for the transistor is 120mA, so the 32mA is well within range. \$\endgroup\$
    – Dave H.
    Jun 5, 2021 at 23:36
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    \$\begingroup\$ You might be interested in this LM1875 design: github.com/nradulovic/hw_minima and the discussion about the power supply. The schematics are in the PDF file minima.pdf \$\endgroup\$
    – ErikR
    Jun 6, 2021 at 4:09
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    \$\begingroup\$ @DaveH. You may also want to read this, as it directly discusses a capacitor multiplier method for higher power situations, as well. \$\endgroup\$
    – jonk
    Oct 19, 2021 at 19:34

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