mA meter tester/questions

[mkstewartesq]

OK, so I have a question about pedal draw power in milliamps. The reason for my question is that I bought a 1Spot tester because I have a shitload of pedal now and wanted to make sure I wasn’t starving any of them. (I was going to build the tester that Robert is going to make available, but I found I needed the tester sooner). ANYWAY….

• Some of the results I’m getting are exactly what I would expect – for example, my Gigrig 10-loop switcher states that it needs to be plugged into a 500 mA supply and I see that with all loops activated it draws about 320 mA. Some other pedals I have tested are about within the range I would expect as well.
• But I built the Clandestine Preamp, which is based on the Chase Tone Secret Preamp and it only shows as drawing 8 mA, even though the Chase Tone website says the real pedal draws 150 mA.
• My two—in- one compressor build shows as drawing 0 mA. Zero. Same for my stripboard build of the Pendulum tremolo.
• Just for kicks, I decided to see how much my home built seven loop switcher drew. It’s basically just 7 3PDTs but it has 7 blue LEDs. With all seven LEDs activated it only draws a total of 4 mA where as I expected it to be a fair amount more because, you know, SEVEN LEDs.

What am I missing here?

Thanks,
Mike

 

[mkstewartesq]

OK well, Dipshit figured out some of the problems were of his own making.

As background, if you don’t own one, these testers work by unplugging the power from the pedal, plugging it into the tester to power the tester, and then running a second test cable to the pedal.

For a lot of the pedals that were reading lower than I expected, I realized it was because I was powering the tester by using the tap from the daisy chain power cable (i.e., 5 pedals daisychained power together - when I tested a pedal I unplugged it‘s power supply on the daisychain and put it into the tester and then ran the test cable to the pedal.) Obviously this mucked with the readings. When I used a dedicated wall wart to run the tester, I got more expected (and frightening) results (my two in one Spirit Box/ Unison Double Tracker sucks up over 150 mA, I assume largely due to the FV-1?).

Not sure if the weird readings from the daisy chain tap are just because daisy chains are weird or because that unexpectedly low reading itself was demonstrating that the other pedals on the chain were exerting such a draw that whatever pedal I was testing wasn’t even able to draw its normal amount. That’s why I wanted the tester – to inventory my pedal draw versus how I had them set up on my isolated power supply to make sure everything was getting the power it needed.

• In any event, even using the wall wart, I am still getting the same same 8 mA reading for the Clandestine, which I don’t understand (because the Chase tone website said it drew 150 mA, this has been on its own dedicated 200 mA tap at all times).
• Also, my Tearjerker wah shows as drawing 0 mA but maybe that’s the nature of a wah?

M

 

[darwin999]

I'm looking at the PPCB schematic, and it's really hard to see how anything close to 150mA could flow thru the Chase SP circuit (unless the PPCB version has left something out, which seems unlikely). The charge pump itself and LED each use little current (<2mA each), so then it's all about the 2N5457. Only if the 50kohm Trim pot was dialed down to practically zero could the current possibly get that high. If that Trim pot has even 1Kohm left in it, the current thru the transistor will be <20mA. Also, per the spec sheet, the 1044 charge pump's output voltage is seriously affected for current draws of even 50mA.

So either my evening glass of wine really fried my brain, or the Chase SP 150mA "requirement" is assuming multiple daisy-chained pedals + the SP. Bottom line: I think that your current measurement for the SP is quite reasonable. The only pedal I have that uses such high currents is my TC Electronic Hall of Fame 2 reverb, and I strongly suspect that the current draw is mostly for the digital control elements rather than the signal channel.

 

[Erik S]

Them belton bricks are pretty power hungry.

 

[mkstewartesq]

I'm looking at the PPCB schematic, and it's really hard to see how anything close to 150mA could flow thru the Chase SP circuit (unless the PPCB version has left something out, which seems unlikely). The charge pump itself and LED each use little current (<2mA each), so then it's all about the 2N5457. Only if the 50kohm Trim pot was dialed down to practically zero could the current possibly get that high. If that Trim pot has even 1Kohm left in it, the current thru the transistor will be <20mA. Also, per the spec sheet, the 1044 charge pump's output voltage is seriously affected for current draws of even 50mA.

So either my evening glass of wine really fried my brain, or the Chase SP 150mA "requirement" is assuming multiple daisy-chained pedals + the SP.

Thanks for the detailed analysis. Here is the specific language from the Chase Tone page. As you can see, it’s a bit ambiguous – as you noted, they do in fact references daisychained power supply – but then go on to specifically say that you need 150 miA to 250 mA for the preamp. Like much of that page, it’s somewhat poorly written, so it’s not clear where the reference to the daisychain is just to make sure that it’s negative polarity or if it in someway is factored into the current draw for the SP itself.

”150mA 9VDC Power Requirement. If you are using a daisy-chained power supply, please make sure you are only using 9VDC standard polarity and there’s at least 150mA to 200mA current available for the SP™.”

Again, thanks for the detailed look at this issue.

M

 

[mkstewartesq]

Them belton bricks are pretty power hungry.

View attachment 69319

Wow – well that certainly explains why combining this with the FV-1 unison double tracker is drawing enough power to take down the grid on the eastern seaboard.

M

 

[darwin999]

”150mA 9VDC Power Requirement. If you are using a daisy-chained power supply, please make sure you are only using 9VDC standard polarity and there’s at least 150mA to 200mA current available for the SP™.”

In terms of average power requirements, I'm just not seeing anything like that number from the PPCB schematic.

So perhaps they are thinking about the initial transient of charging up the capacitors, when the device is initially powered (and the audibility of the associated "pop").

Of course, your power supply also has capacitors on its output side, so if they are of sufficient size they can store enough charge to handle the initial current transient (i.e., surge). The stored charge Q on a capacitor plate is equal to C•V, the capacitance times the voltage.

 

[Robert]

I'm looking at the PPCB schematic, and it's really hard to see how anything close to 150mA could flow thru the Chase SP circuit (unless the PPCB version has left something out, which seems unlikely).

R101 isn't present in the original. Current is dumped directly to ground from the charge pump through D105, and as much as I don't like to change things, I can't stress how bad of a design choice that is.

You can jumper R101 if you want to give it a shot, but expect D105 to get warm for no good reason... the voltage will still be the same.

 

[darwin999]

R101 isn't present in the original. Current is dumped directly to ground from the charge pump through D105, and as much as I don't like to change things, I can't stress how bad of a design choice that is.

Incredible!
Maybe that's the real secret of this preamp...