The Rockman Stereo Chorus Delay module runs at 6V+, 6V-. I have confirmed this. The information I found gives the mains transformer used as 10V CT. What I don't understand is how this works. It uses a 7806 and a 7906 which are not LDO versions. The voltage drop for these is around 2V each, plus a voltage drop across the full wave diode bridge. How could this work given that after rectification the DC voltage would be around 7.1V per side ignoring the diode drop?

I've made 2 of these for Nobles sound studios, same +_6v. Your unloaded transformer voltage is always over the stated voltage. So 10vct is going to be more than 5v each side. Never had a problem with low voltage. Depends on your transformer current i suppose, use 12vct if you think you are going to have problems.

Yazoo, your approach and question are on target. The short answer is probably that (1) the output current is significantly less than the 1A that the 2V dropout voltage is specified, so the real dropout voltage is less, if imponderable from the datasheets and (2) the transformer is oddly or incompletely specified, so it's giving more voltage out than expected.
The engineer-for-reliability approach is as you expect: take the output voltage needed, add to that the regulator's specified dropout voltage, add to that the size of the filter capacitor ripple voltage peak to peak, add to that the voltage drops in the rectifier diodes. This gets you to the minimum peak voltage you need from the transformer >> at the minimum incoming AC line voltage <<. Now specify the transformer secondary voltage, including any resistive losses in the windings, at the maximum DC load current.
This generally results in  bigger transformer than the accountants want to buy but it works in all cases and all AC line conditions.

Transformers are specified for the output voltage they give at nominal AC line voltage and maximum current. Since small transformers have a lot of resistance losses in their windings, they often sag by 20-30% (!!) from the no-load condition when fully loaded. This means a "10Vct" transformer might be a 13Vct transformer at light or no load. It's confusing if you expect the manufacturer to tell the whole truth and nothing but the truth.

10Vct, if accurate, gives +/- 7.07V peak when full wave rectified; Two diode drops come off that, down to 5.6 V peak at the filter caps. If you put in enough filter cap, you can get the ripple down to maybe half a volt or less. Now you're flirting with 5Vdc at the filter caps, and it won't run the 7806/7906 at full spec, just as you were thinking.

But maybe the designer was playing games. Maybe the circuits are all opamps that work fine from +/-4V to +/-12V and have lots of power supply ripple rejection, so they're OK with the regulators being in dropout most of the time as long as the regulators stop higher voltages at times. It would mostly work. I personally would not do it that way, but I've seen similar lines of reasoning.


You're probably familiar with it, but here's my run-down on normal transformer power supplies.
Power Supplies Basics for Effects

Quote from: Yazoo on December 22, 2024, 08:59:18 AMThe Rockman Stereo Chorus Delay module runs at 6V+, 6V-. I have confirmed this. The information I found gives the mains transformer used as 10V CT. What I don't understand is how this works. It uses a 7806 and a 7906 which are not LDO versions. The voltage drop for these is around 2V each, plus a voltage drop across the full wave diode bridge. How could this work given that after rectification the DC voltage would be around 7.1V per side ignoring the diode drop

When things are pushed a little close like this you need to be very specific about the details.   Just using general numbers aren't accurate enough and may or may not show it working.

Quote from: mozz on December 22, 2024, 01:23:54 PMYour unloaded transformer voltage is always over the stated voltage. So 10vct is going to be more than 5v each side.

So that's one detail which helps.

Small transformers can have anything from 10% regulation to 25% regulation.  When you buy a transformer you actually don't know what that is unless the manufacturer specifies it.   

*If* the nominal transformer rating is 10VAC ct that means the output is 10V at full load but the circuit is only pulling a fraction of that so it could be 10% to 25% higher.   Suppose it's 20% regulation then the light load output could be 12VAC ct.

If you want to scrape details the 7806 regulator dropout is around 2V at full load.  At light loads the dropout might only be 1.5 to 1.7V.

Based on the 10VAC ct nominal 20% regulation transformer:
With that scenario we need the *bottom of the ripple* to be more than 6+1.5 = 7.5V.
With 12VAC ct and one diode drop for full-wave we get 1.414*1.2*(10/2)-0.7 = 7.8V.

For reliable operation we might want to factor in 10% main voltage drop, which could drop that down to 6.9V.     And we should also allow for the regulator output voltages at maximum.

So we can see it could possibly work if the details are in our favour.
However,
- It's unlikely the 10VAC nominal is accurate.
- The 20% regulation estimate is only an educated guess.
- If you were to buy an off the shelf transformer you cannot rely on such fine details.
If transformer nominal rating was 12VAC ct then it's more likely to work.
Even at 15% regulation, the peaks at 10% low mains would be,
1.414*0.9*1.15*(12/2) = 8.8V
which is going to give you far more confidence of it working.

There is still one detail to be checked.  We need the *dips* of the mains ripple to be greater than 7.5V.
We the need load current of the circuit and filter cap values to determine that.

If you went to 15V ct it makes things easier again but then you might need to consider the regulator temperature rise.

Here's the factory one from the Nobles sst-1, a rockman clone but i don't know how exact it is and/or if they draw the same current.There was a lawsuit so i guess it's fairly close. First transformer i used was 7.2vct 150ma., nicad battery wall wart. I cracked it open and there was a center tap, so i used it and got my 6vdc +_. Second one was a radio shack 12.6vct 450ma, that works fine also with no heat sinks either.
Am seeing Scholz Regulated Power Supply for rockman listed as 200ma.

Quote from: mozz on December 22, 2024, 04:17:43 PMere's the factory one from the Nobles sst-1, a rockman clone but i don't know how exact it is and/or if they draw the same current.

Notice the diodes in the ground leads of the regulators.  That means the regulator output is 6.6V or so, not 6.0V.
It also means the input voltage needs to be higher than the figures above (which as based on 6.0V output).

The Rockman had 6.4v+ and 6.0v-. This clone appears to be higher? I don't have the transformer specs. It is loaded with LF353's instead of TL072 that the Rockman used. On the outside it says 6 & -6. They both took 8 AA cells. I wonder if they did that for a reason and the led's were on the higher side. Anyway dual 6v will work fine. I would tell the OP to just get a 12.6vct transformer and build with that. Maybe i should lift the gnd legs .6v on mine to give it a little boost. Anyway, on the Nobels, right after the power jack there are diodes. So it is dropping said increase anyway! Don't know if the Rockman did the same or not.

Quote from: mozz on December 22, 2024, 04:59:31 PMAnyway, on the Nobels, right after the power jack there are diodes. So it is dropping said increase anyway!

That makes some sense.   They added polarity protection diodes to the pedal then they compensated by increasing the regulator outputs from 6.0 to 6.6.    So if you discard all the diodes and build a +6V/0V/-6V with straight 6V regulators it will help reduce the chance of dropout.

Quote from: mozz on December 22, 2024, 04:59:31 PMI would tell the OP to just get a 12.6vct transformer and build with that.

Yep, sounds good.

There's still a chance it could be close.    If the pedal current is 100mA the ripple would be just under 1Vp-p which is probably good enough for it to work.  I suspect the pedal current could be less than 100mA.

This clone is loaded with 10 pcs of 220uf on the rails, they put them everywhere.
The Rockman Stereo Chorus Delay is what the OP has, i was thinking he had the regular rockman. Anyway, i think they specs the same power supply for a lot of the variations.NOPE.

EDIT: way off track here, it seems to be self contained.
Hobart PSD 310 10VCT @ 0.25A
Hammond 162E10 Transformer, low voltage PCB mount, low profile, 115/230V, 2.4VA, 10V C.T. @ .25A

Quote from: mozz on December 23, 2024, 06:18:12 AMHammond 162E10 Transformer, low voltage PCB mount, low profile, 115/230V, 2.4VA, 10V C.T. @ .25A

https://www.hammfg.com/files/parts/pdf/162E10.pdf?v=1713801852

Open circuit voltage (with nominal mains input) 6.124V
(implies regulation 18% or 22% depending on definition used)

1.414*6.124 - 0.7 = 8.0V peak     ;without ripple or mains regulation
1.414*6.124*0.9 - 0.7 = 7.1V peak ;without ripple but with mains regulation

So technically could work with nominal mains but the output is too low if the mains fluctuates.

Can't find the website right now, they are both the ones listed as replacements, so i guess they work? Maybe it's running 5V rails?

Quote from: mozz on December 23, 2024, 12:45:44 PMCan't find the website right now, they are both the ones listed as replacements, so i guess they work? Maybe it's running 5V rails?

I suppose there's quite a few unknowns:
- We don't know the original transformer details.
- Did the original units handle main fluctuations anyway?
  (Not all product are designed 100% correctly.)
- Hard to know if the replacements are better or worse than the originals in that respect.
- Is one of the replacements better than the other?
  So far I cannot find any no-load voltage or regulation info on the Hobart PSD 310.
  (We might find info elsewhere.
   It does seem there could be possible similar non-replacement models to the PSD-310:
   Speculative prefixes:  P-  flying leads, PD- solder tabs, PSD- PCB pins)

To me it seems the replacements, and perhaps the originals, could work correctly "most of the time".

Going further, if the regulator did drop out, does the pedal misbehave?
Maybe it still works with slightly poorer hum specs.   
Plenty of audio products operate from non-regulated supplies.

I found this document which appears to originate from Rockman.
If I'm reading it correctly they state the originals used Hobart transformers.

Click download PDF to get full doc,
"Rockman modules, Guitar Compressor, Smart Gate, XP100, XPR, PGE2, Sustainor, Midi-Octopus, Chorus/Delay, Instrument Equalizer User manual "
https://manualzz.com/doc/23161699/rockman-modules--guitar-compressor--smart-gate--xp100--xp...?p=14

There's a picture of one with a PSS-316
16VCT @ 0.15A

Thanks everybody for your advice. It's the Hammond 162e10 transformer I bought. I also ended up with a 12.6 CT transformer which was delivered by mistake on a previous order. I was told I could keep it. I'm tempted to use that. It's rated slightly lower at 200 mA versus the 250 mA for the 162e10. I always get into this cycle of doubt at the end of a project!

And Happy Christmas/Seasons Greetings to everyone.

Quote from: mozz on December 23, 2024, 06:48:02 PMThere's a picture of one with a PSS-316
16VCT @ 0.15A

I wonder where that fits in.  Looks more suited to the +/-6VDC and +/-8VDC supplies.

If we repeat the calculations in this thread for each of the transformers listed in that document (ref: post #12) then the +/-8VDC supplies and +/-12VDC supplies have considerable excess input voltage on the DC side.  Which is in strong contrast to the lack of excess DC input voltage on the +/-6VDC unit.

Without knowing the currents drawn on each of the pedals we don't know if the transformer voltage used for the calculations need to be close to the nominal rating (corresponding to full load) or some factor higher according to the transformer regulation (corresponding to light loads).

With such inconsistencies it's very hard to second guess anything or see any patterns.

And the beat goes on...

I did another search and the same query about the transformer came up on the rockman gearup bb - no resolution though. However there is a photo showing a chorus delay module with a different transformer, a Hobart PSS-316, 16v CT at 0.15 amps, the one referred to in Mozz's post.

I've decided to work on the basis of that and use the 12.6 volt CT transformer. I'll use Schottky diodes for the diode bridge and maths-wise that will fit. Crossed fingers as always!