Sequential Circuits Pro-One – transformer replacement

STATUS: Everything working (Nov 12, 2015). All in all a pretty easy mod – but always be careful with high voltage stuff! There are some unused holes I need to plug, bu that is purely cosmetic.

The original transformer in my Pro-One hummed a LOT (I mean physically, not from the Audio jack).

I replaced it with a toroid, which is supposedly better for audio applications. I also ditched the 110/220 voltage selector, and installed a standard IEC power cable chassis connector so I could ditch the annoying and potentially dangerous permanent power cord.

Here is what it looks like before any mods:

(It seems someone has already done some work on it, the original fuse holder is not connected and there is a new additional one mounted).

Step 1 – ripping out the old stuff
Old stuff ripped out
I ditched the voltage selector and old unused square fuse holder, as my Pro-One had a new round one mounted.

Step 2 – mounting a standard IEC power connector
I modified the unused square hole from the old fuse holder.

Sawing a little
Sawing a little
Filing a little
Filing a little
Mounting IEC power jack 3

Haven’t decided how to cover the remaining unused holes yet.

Step 3 – mounting new toroid transformer and connecting it
All connections done

All soldering done and heat shrink tubing in place.

The “middle tap” from the transformer should supposedly be connected to ground somewhere on the circuit board and NOT the metal case to avoid ground loops.

I measured the voltage once more to be sure everything was ok, and it seemed fine: around 20V on each pin against the middle tap/ground and 40 V between the two pins in the white connector. (The transformer is a 2*18V 30 VA model)

Hooked it up to the main PCB and fired it up… sweet music – no hum!


Transformer size: I was curious about the data for the original transformer and found that the company still exists. I emailed them and got a data sheet back in less than an hour – amazing! The transformer model is DP-241-7-36, and it is a 56 VA model. That’s a lot of juice.. but does the Pro-One need it?

No. Syntaur is selling Pro-One replacement transformers that are only 12 VA! I guess Sequential got a good deal on these bigger ones and just used them.Considering that the first 1500 Pro-Ones had a small transformer mounted on the PCB, there is no way that was a 56 VA model. The voltage regulators on the PCB also have very little heat sinking, and a seasoned tech told me the currents were then probably close to 200 mA or so. My 30 VA model can deliver 830 mA. So even that is bigger than needed.

Nov 16, 2015: Wrong fuse? My Pro-One didn’t have the original fuse holder, a new one had been added. There was also a sticker that said ‘250 mA’. Looking at the Pro-One schematics, it says “1/4A” (250 mA) when running it at 110V and “1/8A” (125 mA) when running at 220V. So I think the person who fixed this before missed that info.. Will change it to a 125 mA ASAP.


10 thoughts on “Sequential Circuits Pro-One – transformer replacement

  1. sooperb, thanks for confirmation, I’ll do all the same mods to my Pro One, transformer, IEC, capacitors, keyboard (tho Doepfer). I also have a Prophet VS which hums like crazee, I’d guess a transformer swap for a Toroidal will resolve that. Did you manage to get the Amp/Voltage data from the existing Pro One transfomer and use that as a basis for choosing a new Toroid transformer?

    Liked by 1 person

    1. Hmm I figured since the pro-one fuse is at 125 mA (for 230 V), a transformer that can deliver 800 should be more than enough. It is also physically much bigger. But I have to admit I haven’t researched the exact power consumption of the pro-one. I’ll try and find the time to do that soon!


    2. I emailed the company who made the original transformer and got a reply with a data sheet! Amazing. My old transformer is marked DP – 241 – 7 – 36. According to their data sheet this is a 56 VA transformer.. Still doesn’t tell us if the Pro One actually NEEDS that much juice. Will measure the actual current and find out.


  2. Hi again, do you have a close up pic of your old transformer?

    Here’s is the pic of the transformer from my SCI VS maybe it’s the same brand which I emailed while back but they said they haveno data sheet on it.


    1. Looked at the schematics now. The VS has a much more complicated power supply circuitry. Beyond my skills but probably a piece of cake for a tech repair person. The “7” in the transformer number indicates 56VA, and it looks like it has double secondaries, two blue and two red, schematics say one ’32VCT’ which means 32V with center tap (or 16-0-16 or 2*16V) and one ’16VCT’ (8-0-8 or 2*8V). The 16V lines are used to create +/- 12V, and the 8V lines to create +/- 5V.


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