Pure Learning, Nothing More/Less: Question About Increasing Filter Cap Values On Tascam M30 Mixer

[seanweaverguitar]

I tried posting this over on Gearspace but people here have always been far cooler to me in general, and I like this place a lot more, so wanted to ask with brief context:

I was given a Tascam M30 mixer for free. I'm a guitar player in Nashville with a huge electronics hobby but I'm a pro at guitar only and a total hobbyist/self-trained kindergartener when it comes to actual circuits. Just something I like toying with. Not looking to make this a tone-machine (I have other gear for proper recording). Only to use it as a platform to learn more stuff from.

I found this blog about M30 upgrades where he drastically (as in over 400% if I've calculated correctly) increases the filter caps in the power supply: M-30 PSU upgrade

I have the schematic and service manual but here's my elementary question/concern/what I needed advice about:

Won't increasing the filter caps that much quite significantly increase the inrush current thus potentially straining the power transformer and bridge rectifiers (it uses three W02M on each rail; +6, +15/-15).

I pulled up a data sheet on the bridge rectifier (https://www.rectron.com/data_sheets/w005l-w10l.pdf) but wasn't sure what information to look specifically for on it in order to answer the diode question.

Am I thinking about this properly or is it not the issue that I wonder it might be? Turns out the US units have no fuse board which further made me stop, think, and led to the voice within telling me, "ask smarter people."

Again, I'm under no delusions about this project. If I all I did was remove the board itself and replaced with stock values it'd still be a success. I might even just list it on CraigsList when I'm done having fun but it was a free excuse to continue getting some more experience. My main interest is tube guitar amps but all audio fascinates me.

 

[ruffrecords]

The actual value of inrush current depends on the ESR of the reservoir capacitor and the resistance of the transformer secondary winding. Since at power on the capacitor is discharged it effectively looks like a short circuit. This can lead to a very large inrush current but fortunately this usually only lasts for the first half cycle of the mains frequency. Most components can withstand this overload. The rectifier you mentioned, though only rated at 1.5 amps, can withstand a 50A surge.

Doubling the reservoir capacitor value does not alter the size of this peak, it just makes it last twice as long.

I am surprised the article you linked to seems to imply upgrading the PSU caps reduces hiss. It could reduce hum but is unlikely to improve hiss.

Cheers

Ian

 

[seanweaverguitar]

The actual value of inrush current depends on the ESR of the reservoir capacitor and the resistance of the transformer secondary winding. Since at power on the capacitor is discharged it effectively looks like a short circuit. This can lead to a very large inrush current but fortunately this usually only lasts for the first half cycle of the mains frequency. Most components can withstand this overload. The rectifier you mentioned, though only rated at 1.5 amps, can withstand a 50A surge.

Doubling the reservoir capacitor value does not alter the size of this peak, it just makes it last twice as long.

I am surprised the article you linked to seems to imply upgrading the PSU caps reduces hiss. It could reduce hum but is unlikely to improve hiss.

Cheers

Ian

Thank you sincerely Ian. I post occasionally but mostly just use GroupDIY to read threads and have enjoyed your posts for quite some time. Very helpful to to hear from you on this. That's exactly why I wanted to ask.

Short related follow-up: when the peak lasts twice as long (in the case of doubling the reservoir cap value), are there any calculations or other pieces of education I should pursue to learn/teach myself if that is of any significant strain to the power switch itself? My guess is I'd probably need the data sheet on any given switch for any conclusive answers but if I'm mistaken please let me know.

I'm sort of talking myself out of increasing cap values as I write this but the learning part is all real - it's the only thing I really care about from this so even this discussion alone is a success.

I'll run a web search to not ask too much here, but if you can point me in the direction of a formula/mathematics I'd need to calculate inrush current I'd be fascinated. Would I just measure ESR with an ESR meter and secondary transformer resistance with something like a normal Fluke DMM?

 

[ruffrecords]

For understanding this problem you need little more than ohm's law. You can certainly measure the transformer secondary resistance in situ with a normal DVM. For ESR you really need to take the component out of the circuit. However, most large capacitors have an ESR of less than 0.1 ohm. For the transformer, let's assume it has to provide 24V at 100mA for the mixer. This is equivalent to a load of 240 ohms. A good transformer will have a regulation of better than 10%. Let's be generous and assume it is really good at 5%. This means the internal resistance of the transformer must be no more than 5% of 240 ohms or 12 ohms. The exact numbers do not matter.The point is the transformer resistance is a lot bigger than the capacitor ESR so we can forget the capacitor for the purposes of this exercise.
So now we have a 24V source feeding the capacitor via a 12 ohm resistor. At switch on the capacitor voltage is zero so the peak current through the 12 ohms must be 24/12 = 2 amps which is well within the spec of the bridge rectifier.

The next question is how long does this current last for. The capacitor will charge up to about 60% of the input voltage in CR seconds. So the resistance is 12 ohms and let's say the capacitance is 2200uF CR seconds is 26mS which is a tad longer than one mains cycle. With 50Hz mains, a cycle is 20mS. If you full wave rectify this you get two pules every 20mS so each is 10mS wide and each goes from 0 to 100% in 5mS. So you get a spike of current going into the capacitor every 10mS. The initial spike is not there long enough (5mS) to fully charge the capacitor so the diode becomes reverse biased and the voltage holds until the next half cycle, which will top up the capacitor some more and this repeats until and equilibrium is reached.

I would encourage you to simulate this in LTspice so you can see exactly what happens.

Cheers

Ian

 

[seanweaverguitar]

Just wanted to make sure to say thank you again, Ian. I just learned a TON from your reply above. That all makes perfect sense to me now (thanks to you) and I also learned about LTspice, which I'll definitely check out!