How to build a 9V AC output Regulated Power Supply?

[KMS]

I picked up a Digitech RP7 Valve Multi FX pedal.

I know this is not our typical stombox dialog but I couldn't pass up the deal... the RP7, and Ibanez RX240, and a 2x12 Crate Club 50 all for $250.00.  The RP7 will make a nice addition to my router and other DIY FX.

The RP7 has no power supply (PS).  I called Digitech and they sell the PS for $39.00 plus shipping and tax it is $50.00.

Their service guy told me that the output on the PS is 9V regulated AC at 1.33 amp.

I want to build my own PS.

I can find no such transformer but I can hook two 1 amp 9V AC transformers together (in phase of course) and have a 2 amp 9V output (actually I measure 10 V AC on the transformer with no load)

I hooked it up and it works but I am concerned about the voltage spikes due to non regulated PS.

The guy at Digitech told that 9.75 V AC is the max voltage allowed.

How would I regulate the AC voltage and is their some way to drop the Voltage output by 1 volt?

Thanks in advance.

[Paul Perry (Frostwave)]

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=320258&pa=320258PS

In case that url doesn't work, that's a 1.5 Amp 9v AC wall wart from www.jameco.com part# 320258PS
and, it costs $8.95. Can you say, "%^&* you Digitech"? 

[cd]

Are you absolutely sure you need *regulated* AC?  AC wall warts are a dime a dozen but regulated ones... if the Digitech one is regulated, it's probably worth the $50.

[Paul Perry (Frostwave)]

I would be VERY suprided if the Digitech AC output supply was 'regulated'. It *IS* possible (using special alloys & windings and core geometry) to make an AC transformer that gives a regulated AC output, at least over a certain input range.. you see them sometimes as heater supplies for 1950s scientific instruments... but on a FX supply in 2006? I don't think so!!
True, if you measure the output (under load) of a transformer that is designed to give 9v at 5A, and you are running it at a much lower current, the voltage will be higher. In fact under NO load it might be 14V say!!! but, I would not hesitate to use this Jameco wart, judging from what you say.

[cd]

I would be VERY suprided if the Digitech AC output supply was 'regulated'. It *IS* possible (using special alloys & windings and core geometry) to make an AC transformer that gives a regulated AC output, at least over a certain input range.. you see them sometimes as heater supplies for 1950s scientific instruments... but on a FX supply in 2006? I don't think so!!

I'd be surprised as well, hence my skepticism.  The Digitech guy was probably thinking of 9VDC regulated supplies for typical pedal FX. 

[KMS]

Thanks guys....your input is a lot of what I was already thinking.

I read somewhere tonight (surfing all over) that Microprocessors need a regulated AC supply.

I don't even know if micro processing is used in the RP7.

I guess there must be something in the unit that uses a sine wave and that some consistency is needed.

I hooked up the 9V AC transformer and the RP7 worked,  I did not check out all the functions as I was trying to minimize the time on the transformer just to make sure it worked before I bought it.

The delay function and fl anger worked flawlessly, but as I scrolled though some of the Bank functions and stumbled on distortion.....woe!!! it got real loud.  Maybe it was just a loud preset.

The guy at Digitech was a service technician and he said, "do not go less than 1.3 amp and no more than 9.75 volts.

The 1.3 amp is not a standard transformer at all, but I guess 1.5 amp would be the real rating on the transformer.

I just don't know enough about digital stuff to have any certainty about whether I would cause a fatal error to the unit without regulating the PS.....It might be the no damage is possible...just bad performance.....or.....it is all BS to sell the darn PS.

How do us DIY deal with this...I've been reading all night to find an answer.......nada so far.

I did find a patent on an AC regulator and an abstract description.....the patent details the use of a comparator where the voltage of the input is compared to the out put and sends signals to relays to switch between the leads on the secondary (or something like that).

Wouldn't it be easier to just regulate DC and then convert back to AC?

[Cliff Schecht]

Thanks guys....your input is a lot of what I was already thinking.

I read somewhere tonight (surfing all over) that Microprocessors need a regulated AC supply.

I don't even know if micro processing is used in the RP7.

I guess there must be something in the unit that uses a sine wave and that some consistency is needed.

I hooked up the 9V AC transformer and the RP7 worked,  I did not check out all the functions as I was trying to minimize the time on the transformer just to make sure it worked before I bought it.

The delay function and fl anger worked flawlessly, but as I scrolled though some of the Bank functions and stumbled on distortion.....woe!!! it got real loud.  Maybe it was just a loud preset.

The guy at Digitech was a service technician and he said, "do not go less than 1.3 amp and no more than 9.75 volts.

The 1.3 amp is not a standard transformer at all, but I guess 1.5 amp would be the real rating on the transformer.

I just don't know enough about digital stuff to have any certainty about whether I would cause a fatal error to the unit without regulating the PS.....It might be the no damage is possible...just bad performance.....or.....it is all BS to sell the darn PS.

How do us DIY deal with this...I've been reading all night to find an answer.......nada so far.

I did find a patent on an AC regulator and an abstract description.....the patent details the use of a comparator where the voltage of the input is compared to the out put and sends signals to relays to switch between the leads on the secondary (or something like that).

Wouldn't it be easier to just regulate DC and then convert back to AC?

I would be really surprised if your RP7 didn't have regulated power onboard... There is most definetly a microprocessor which does require clean power, but I don't think they would just stick that in without some sort of filtering. I almost guarentee you can get a usable power supply from a Goodwill or similar.

As far as your noise problems with the overdrives, it's probably because the transformers grounds weren't isolated.

[Transmogrifox]

The main concern is the ratings on the input caps & DC regulators.  Assuming it's standard stock, you're probably ok with any old cheap 9V ac wall-wart that's reasonably close to 9V.  If you're operating at 11V rms, then you may have a problem, though it's not going to show in the form of digital glitches or volume levels.  It'll just go dead after something heats up and smokes in the power supply regulation section.  I'd take a stab in the dark and say the power supply capacitors are 16V rated, and would be the most likely to roast first.

If you're not confident, you can go searching for a DIY low power amplifier design with 2 amp capacity (32W into 8 ohms) and make a DIY 60Hz bandpass twin-T filter.  Run the input AC into a back-back diode to ground clipping configuration (like a Dist + circuit) and run it through the filter.  Adjust the gain so that you get 9VAC on the output of the amplifier.  Then you'd have a real well regulated and filtered power supply.  Quite a bit of trouble and expense compared to $9 for a Jameco supply.

[Paul Perry (Frostwave)]

No harm saying something three times: (or shouting) GET THE JAMECO 1.5A 9V AC WART.
NOW.
And if you really want to do some DIY, well I expect you will have to change the plug on it.

[KMS]

I hear ya.....and shouting is good for me.

Thanks.

I am going to try the 2 amp rig I made and check the settings out on the distortion....now that I have read the manual and know how to check everything.

I'm thinking........they have to make DC out of AC and why in the (&()_+ would Digitech take the easy job of regulating DC and make it more complicated and more expensive by regulating the AC?

These guys are the masters of inexpensive and minimum labor......it is not logical for them to regulate the AC.....and the unregulated AC worked fine with my DIY PS except it was "loud" on distortion......but I did not look at the settings of the distortion.

I let you folks know if it works.

Thanks

[KMS]

Thanks a lot guys for giving me some certainty that Digitech service employees just want to sell their PS.

I hooked up one 9V AC 1 amp Transformer and have been playing since I made the last post.....tested all the functions on the RP7 and they all work flawlessly and the distortion settings were set up on high output and that is what was wrong.

No regulated AC is needed.......also....1.33 amp is bull crap too unless my transformer has the 1.33amp output for which I don't know....and at this point it makes no difference.

I can also see no clipping on the clip indicator LED.

One thing is for sure....the RP7 Valve has put me into a whole new FX capability and it sounds awesome.....I don't think I could build any pedal of this quality and if I could I would not be able to do it for  anywhere near the price you can get these on E-bay.....$125 buy-it-now price tag.

I still need a Lesli sim

And I love  all of my DIY pedals.

The delay on the RP7 is fantastic but it does not have that cool sounding analog distortion that builds on each repetition.

The Octave functions and Harmony on the RP7 are out of this world!   Damn.....I have a whole orchestra! and 2 Octaves up with 6 Octaves down is like cheating.

Thanks again!

[Transmogrifox]

No regulated AC is needed.......also....1.33 amp is bull crap too unless my transformer has the 1.33amp output for which I don't know....and at this point it makes no difference.

This is probably what they mean by "regulated".  The transformer would be rated to a 9V output only if the current required was 1.33 amps, which is probably what the RP7 draws.

With a 1.5Amp rated AC transformer, the voltage will appear higher than its nominal rating until you load it to 1.5 Amps.  If you load it to 2 Amps, then it will be giving a voltage that is less than the nominal.  If you load it too much over its rating, it will get hot and burn up.

So...I could draw up a circuit that would effectively load the AC transformer until its output voltage is what you want if you even care.

Sounds like it's working fine, so I guess it would be a waste of time anyway.

[KMS]

Yes, I do care.....the PS is important to me.......to be as much like the original as possible.

The transformer did get very warm...almost "hot".

So I went back to using two 1 amp transformers and the temperature is cooler now but still moderately warm (normal).

I can't help but wonder why....why this guy did not have the original transformer....he told me he had it and then said he could not find it.

This gets interesting....price negotiations.......ethics are about getting a low price from KMS.

The guy I bought the RP7 from is a coworker...and two years ago he told me he bought the unit for $60 (I'm sure he did not remember but I did).

I told him last week I wanted to buy the RP7 and he said he would throw it in for free if I bought his whole rig (Ibanez RX240, Crate Vintage Club 50 2X12 combo). He told me $400 and let me take it home overnight to test out.....I told him I "might be able to find a PS or rig one up fro the RP7)

I called Digitech (from my work) to get a price quote and the specs on the PS. I told my coworker before I went home with his rig that Digitech said it was $50 for the PS and for him to dig deep and find the PS (he is and engineer) and he asked what the specs were on the PS so I told him.

I rigged up a temporary PS at home and I fired it up and it worked (short story version).

I tell him (next day) that what I really wanted was the RP7 and that I could not get a PS in town because it is a very complicated PS and I could not build it for less than what it cost to buy commercial so I will just have to take your word that the thing works and buy the PS from Digitech....I'm on my own and if I loose money I will make it up when I am done with the amp and sell it ...SO....how about $250 for all of it?

He said, "I know.....I looked everywhere on the web for a comparable PS"......but he said nothing about looking through his house to find the old one.

He said OK you can have it all for $250.

Here is what I think......He never had a PS! He has never use the RP7! and I can tell by the way he had his strings wound on the Ibanez pegs that he knows nothing about a guitar...much less knowledge about pro audio equipment.

So I'm back to....why?...  why did the guy who originally had the correct PS not sell the unit with the PS and take such a loss at $60.

Maybe the PS was lost.....or maybe it is fried.......It is a good chance that it fried.

If you have time to draw the circuit I would be very happy to make use of it.


Thanks,

KMS

[bancika]

check out my site, I built similar thing with 1.2A transformer I found in old speaker set.

[Paul Perry (Frostwave)]

For the fourth time.......
oh, forget about it

[KMS]

Played all day Sunday with two 1 amp 9VAC transformers....no problems.   The link bancika provide only has a DC Power Supply....this application if for AC Power Supply.  Thanks for the advice Paul.....I am basicaly using the same thing.