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Rolls Preamps us tip negative power - Why?
Rolls Preamps us tip negative power - Why?
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Old 15th April 2019, 08:54 AM   #11
JensH is offline JensH  Denmark
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Rolls Preamps us tip negative power - Why?
Quote:
Originally Posted by nigelwright7557 View Post
I have seen equipment with a diode to ground to short out wrongly connected power supply ! Why ? Its just as easy to put it in series then it protects with out blowing up the power supply.
Probably to eliminate the voltage drop across the diode (and the power consumption associated with it).
I have seen - and used - both, depending on customer preference. And I have also seen both polarities used.
If the supply is short circuit protected, the diode across the input will not be an issue. If it's not short circuit protected, it will be an issue
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Old 16th April 2019, 06:32 AM   #12
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by amplitode View Post
I have a Rolls Preamp - the MP13. I am curious to understand why the power supply input is tip negative while everything else I own and everything I can think of has tip positive power connection. The inversion is inconvenient when setting up battery powered systems with common power. Someday I will use the wrong plug. Thanks in advance!
Please DO NOT REWIRE IT.

Don´t care about "other" stuff you own, THE standard Guitar Effects pedal connector for at least 20 years has been:
Quote:
The dimensions are 5.5 mm x 2.1 mm with a negative center.
That nice and very useful Mic Preamp is not a PA/Recording studio rackmount product but has been designed to give control to the performing Musician himself, so it sits in his pedalboard, within arm´s reach, and lets him plug a microphone , either stand mounted or headset, amplify it to line level, and send it to a remote mixer or even a Keyboard/Guitar/DJ/Home Theater amplifier if nothing else is available.
VERY useful.

So they designed it to be able to work with the same power supply used by all other pedals in the pedalboard ... what else?

And on the contrary, it does not force you to buy a brand name expensive PSU but it uses the most common and inexpensive ones.

Only "special" need might be that it requires a 12/15V single supply instead of the more common 9V one, because it uses standard Op Amps, not a special low voltage or rail to rail one ... although I bet in a pinch it will still "reasonably" work with a standard 9V one .

To boot, and this is personal experience, cheapest "9V" supplies are unregulated and often are ~ 10/12V unloaded, dropping to 9V only under full load, say 500mA

And as Enzo said above, it simplifies switching from internal 9V battery to external supply, center negative *always* goes to ground, and body positive replaces battery positive.

So again, don´t rewire yours, just use it with a cheap "9v" pedal power supply ... make that 12/15V if dedicated just to this preamp.
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Last edited by JMFahey; 16th April 2019 at 06:34 AM.
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Old 16th April 2019, 12:43 PM   #13
ICG is offline ICG  Germany
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Quote:
Originally Posted by JMFahey View Post
Please DO NOT REWIRE IT.

Don´t care about "other" stuff you own [...]
I don't see any harm in re-wiring it. There's a serial diode in the circuit, the worst what can happen is it doesn't work with the polarity. You could add a 'wrong polarity' LED or a switch if you don't want to use a bridge rectifier.
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Old 16th April 2019, 07:09 PM   #14
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by ICG View Post
I don't see any harm in re-wiring it. There's a serial diode in the circuit, the worst what can happen is it doesn't work with the polarity. You could add a 'wrong polarity' LED or a switch if you don't want to use a bridge rectifier.
The harm is that this is a pedalboard Mic preamp and after rewiring:

1) it will not work any more with other pedals on same board ... or any other

2) PEOPLE DO NOT READ LABELS, so inevitably somebody will plug a normal/standard one there and either blow a diode if in parallel or plain not work if in series.
In any case your show/performance is ruined for that night.

3) if your special positive center supply dies or is lost, you will have to repair it or make/buy another, instead of using a cheap and plentiful (and probably available on the spot) negative center one.

All in exchange of no practical advantage.
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Old 17th April 2019, 12:42 AM   #15
ICG is offline ICG  Germany
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Quote:
Originally Posted by JMFahey View Post
The harm is that this is a pedalboard Mic preamp and after rewiring:

1) it will not work any more with other pedals on same board ... or any other
That's nonsense! That only happens if you use the same power supply and plugs/pin configurations for all pedals. Some need different voltages so you can't say

Quote:
Originally Posted by JMFahey View Post
2) PEOPLE DO NOT READ LABELS, so inevitably somebody will plug a normal/standard one there and either blow a diode if in parallel or plain not work if in series.
In any case your show/performance is ruined for that night.
I already said that it got a serial diode. And I also said you could add a WRONG POLARITY LED. Just one LED and a resistor needed. There are even LEDs with blinking electronics embedded in the LED case, to make it even more obvious. And I already said you could use a bridge rectifier too, which makes the polarity of the plug meaningless as long as it works with the voltage drop.

Quote:
Originally Posted by JMFahey View Post
3) if your special positive center supply dies or is lost, you will have to repair it or make/buy another, instead of using a cheap and plentiful (and probably available on the spot) negative center one.
If your 'special positve center power supply' dies, the rectifier still works, the LED still works and if you're considering repairing it, you can also change it on the pedal or the wire or in the power supply as well.
I'm highly annoyed by your intentionally provocative ignorance and unwillingness to actually read or understand what someone wrote or what actual problem and needs he has.

Quote:
Originally Posted by JMFahey View Post
All in exchange of no practical advantage.
Well, for him it's the exact opposite of your logic, because..

Quote:
Originally Posted by amplitode View Post
I am curious to understand why the power supply input is tip negative while everything else I own and everything I can think of has tip positive power connection. The inversion is inconvenient when setting up battery powered systems with common power. Someday I will use the wrong plug.
  1. Everything I suggested is cheaper than a different power supply (aside from the fact that he doesn't want to use a battery powered system).
  2. Everything I suggested is easy to use, non-dangerous neither for person nor equipment (unlike your suggestions as his other equipment already got the opposite polarity on it, it will not damage the Rolls preamp but probably one of his other equipment).
  3. Everything I suggested fits the needs and wishes of amplitode (unlike you practically demanded the exact opposite
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Old 17th April 2019, 12:52 AM   #16
5th element is offline 5th element  United Kingdom
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JMFahey is right though in that the rolls device is wired with an industry standard. If he changes it to only accept centre positive then it could cause an issue of compatibility if someone else wants to use it.

Of course this is up to the discretion of the OP and the diode bridge seems like a good solution that gives you the best of both worlds
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Old 17th April 2019, 01:28 AM   #17
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by ICG View Post
That's nonsense! That only happens if you use the same power supply and plugs/pin configurations for all pedals. Some need different voltages so you can't say
You clearly have no clue or experience on Guitar effect Pedals or pedalboards in general.
One supply for all is the norm, for very good reasons, and using something different is the exception.
So rewiring something standard into something exactly the opposite for no good reason is .... can I use the word "stupid"?
Just google "pedalboard supply" and you will have a heart attack.
A couple samples:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
and so on and on and on, dozens of pages showing the same.
because the standard use is:
Click the image to open in full size.
Click the image to open in full size.

Quote:
I already said that it got a serial diode. And I also said you could add a WRONG POLARITY LED. Just one LED and a resistor needed. There are even LEDs with blinking electronics embedded in the LED case, to make it even more obvious. And I already said you could use a bridge rectifier too, which makes the polarity of the plug meaningless as long as it works with the voltage drop.
None of your "solutions" will make it work, so my "ruining your night" comment still applies

Quote:
[*]Everything I suggested is cheaper than a different power supply
You are forcing a different, additional power supply, because his setup then will be incompatible (reverse polarity) from any and all other pedals.
Using the general purpose supply already on the pedalboard is cheaper than *anything* else.
Nothing is cheaper than zero expense.

Suggest you read a little on pedal and pedalboard supplies before posting or you´ll keep repeating same mistakes.

Google is your friend
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Old 17th April 2019, 02:33 AM   #18
Enzo is offline Enzo  United States
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It was mentioned that a series diode will drop the voltage - which may or may not cause issues. But I must point out that a bridge rectifier causes the loss of TWO diode drops.
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Old 18th April 2019, 01:43 AM   #19
ICG is offline ICG  Germany
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Quote:
Originally Posted by JMFahey View Post
You clearly have no clue or experience on Guitar effect Pedals or pedalboards in general.
One supply for all is the norm, for very good reasons, and using something different is the exception.
I do have experience on pedals. And that amplitode got everything else exactly the opposite way is reason enough to actually change it! You've actually read he got no equipment whatsoever with the polarity of a 'normal' pad. No matter how many examples you post, none of it apply to amplitode's setup and needs! Really, shut up and go away since your 'advice' does not help him in any way!
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Old 18th April 2019, 01:52 AM   #20
ICG is offline ICG  Germany
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Quote:
Originally Posted by Enzo View Post
It was mentioned that a series diode will drop the voltage - which may or may not cause issues. But I must point out that a bridge rectifier causes the loss of TWO diode drops.
Beside I mentioned it already: No, it doesn't. One diode drop is already in the factory circuit built in as it got a reverse polarity diode in the circuit, which I already posted. If you use a Schottky diode rectifier, the voltage drop is just 0,4V to 0,6V instead of the usual silicium diode drop of 0,7V. That means, the voltage drop of a Schottky diode or -rectifier would be even lower than the initial silicium diode used in the circuit. No matter how you turn it, there's no argument against it.
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