Carl Martin Compressor/Limiter Power Mod

[Berzons]

Hi,

Apologies if this is posted in the wrong section...

Is there any way of converting the old Carl Martin Compressor/Limiter from the hardwired 12v AC to DC to make the pedal more pedalboard friendly?



Thanks in advance!

[ElectricDruid]

Can we have a few photos of the inside? I don't suppose there's a schematic available anywhere?

Is that a mains cable going into the box, or does it have an external transformer? How do you know it's 12V AC?

It's probable that the 12V AC is converted to 12V or 9V DC inside the box, in which case it should be fairly simple to take out the power supply components, and put a direct DC input in instead. What voltage that is remains to be seen, and if you don't have other 12V stuff on your pedalboard, it might not entirely fit the description of "pedalboard friendly" but we'll see. Deal with that when we get there!

[duck_arse]

and welcome to posting.

[Berzons]

Hi,

That is just a stock photo I posted above. I acquired this pedal a few years ago and it was wrecked. Didn't have a switch, housing had been cut in half, etc. so I had re-house the pedal and add a switch.










Cheers!

[ElectricDruid]

Ooh, interesting! A THAT4301...you don't see those everyday.

There's some stuff about this on the other site:

https://www.freestompboxes.org/viewtopic.php?t=9287

And there's THAT Corp's page about the chip:

https://thatcorp.com/that-4301-analog-engine-dynamics-processor-ic/

The THAT Corp page has a "Design notes" tab which includes an absolute ton of interesting stuff about how to design compressors, limiters, noise gates, and so on with their chips. It's very likely this circuit is mostly taken from one or other of those notes.


...and finally, as Duck said, "Welcome!". Sorry, I didn't notice it was your first post.

[Berzons]

Thank you for that. I'll be sure to have a read through. I've been building pedals on PCB, Vero and perf for a few years now just at a hobby level so I'll have to up my game and do some studying of the electrical side 

I take it it's not as simple as removing the transformer, rectifier, etc. and just installing a boss syle jack input then?

[ElectricDruid]

I take it it's not as simple as removing the transformer, rectifier, etc. and just installing a boss syle jack input then?

It looks like it uses a bipolar power supply, so +/-9V or +/-12V. To derive that from a 9V input, you'd need either a LT1054 chip or one of the commercial modules that does a similar thing.

So no, unfortunately it's not as simple as just pulling those parts and sticking a Boss DC jack in. Pity, it *could have* been that easy.

[Berzons]

Thanks for the info. That's a pity it's not so easy to do. Interesting nonetheless.

[Rob Strand]

Is there any way of converting the old Carl Martin Compressor/Limiter from the hardwired 12v AC to DC to make the pedal more pedalboard friendly?

Not 100% sure without the being able trace the bottom side but I'm fairly confident the power supply is:

Transformer 2x15V AC, bridge rectifier, 2x filter large caps.
Then that feeds 78L12 and 79L12 TO-92 package regulators
which produces a bipolar power supply with +12V 0V -12V outputs.

Very much like this:
https://circuitspedia.com/wp-content/uploads/2018/07/12v-regulated-dual-power-supply..jpg

So two things against you with the conversion 12V and bipolar supply.

If you wanted a 9V DC input you would need to replace the transformer with a DC/DC converter with a dual output of some sort.   Some solutions could keep the filter caps and regulators, and others would replace those entirely.  It can be done.   There are DC/DC converter modules which will fit easily however sometimes you end up having to chase down noise issues.  (Pretty much what ElectricDruid said.)

It might be possible to do a lesser mod and run the device from +9V and a charge pump to generate -9V.    However, without knowledge of the circuit I'd be a little hesitant doing that since compressors use the supply rails to set to the threshold controls.   A change to 9V might change the behaviour of the compressor slightly.

[ElectricDruid]

Rob, there's a schematic and layout here that looks pretty much exactly like OP's board. And yes, the power supply is exactly like you imagined.

I would propose at least trying one of the DC convertor modules that do this job:

Possible candidates on Mouser

These are +9V in, +/-12V out. They make life pretty simple if they work ok with the compressor. Here's the datasheet for the Traco range:

https://pt.mouser.com/datasheet/2/687/tec2_datasheet-3049819.pdf

These have a 100KHz switching frequency, so that should be ok (well outside the audio range), and the relevant part accepts up to 470uF/470uF on the outputs (the existing circuit uses 100uF/100uF on the regulators) so that's ok too.

[Berzons]

Thanks for all the info, it's all very interesting. It gives me hope! 

Here is the underside of the board...





Cheers,
Brian

[Mark Hammer]

There are generally three reasons why a pedal would have an onboard transformer and power cord:

1) It needs far more current than a 9v battery could supply for very long.

2) It uses/relies-on a supply voltage higher than +9V (e.g., +15V)

3) It is designed around having a bipolar (+/-) supply.

Occasionally something is designed around the latter, but doesn't use a transformer.  I'm in the midst of "modernizing" an older Rivera buffer pedal ( https://www.effectsdatabase.com/model/rivera/buf/iv ) for someone that uses a pair of 9V batteries in +/-9v manner.  This required plugging into two jacks in order to "turn on" the batteries.  Naturally, given its age and design, it provides no external power jack.  To make it a little more pedalboard-friendly, I've installed a little board with an LT1054 charge pump and a 2.1mm jack, so that it can be powered from the main +9V supply.  I did manage to find a phone jack in my bin with some SPDT switches that would have permitted the owner to turn the batteries on by only having to insert one plug into the input jack, but he said he didn't need to have the batteries available as backup.  The pedal provides for footswitch selection of boost, A/B output selection, and engaging an eternal loop, none of which have any indicators.  So I drilled some holes for status LEDs, and the current available from the 1054 should suffice to power the original circuit plus indicator LEDs.

[amptramp]

There is another solution:  use two regulated 12 VDC wall wart power supplies and connect them with the positive of one going to the negative of the other.  Then you have +12, 0, -12 VDC power from commercially available components.  Even if you use unregulated 15 VDC wall warts and put the regulators in the box, you have a variety of wall warts to choose from rather than rare regulated ±12 VDC devices.

[Berzons]

There is another solution:  use two regulated 12 VDC wall wart power supplies and connect them with the positive of one going to the negative of the other.  Then you have +12, 0, -12 VDC power from commercially available components.  Even if you use unregulated 15 VDC wall warts and put the regulators in the box, you have a variety of wall warts to choose from rather than rare regulated ±12 VDC devices.

Good solution but the point of the mod is so I can do away with additional plugs/wall warts and power the pedal through my pedalboard power supply.

[Berzons]

@Mark Hammer - Side note. The first DIY pedal that I built was the Nurse Quacky with your mods    It was great!

[Mark Hammer]

Thanks.  More credit goes to Jack O., though, for the changes that made the Dr, Quack possible with a broader range of dual op-amps.

[Rob Strand]

Thanks for all the info, it's all very interesting. It gives me hope! 

Here is the underside of the board...

Here's a trace of the PSU:







The idea would be to remove the small black TO-92 package 78L12 and 79L12 then wire the +12V/0V/-12V DC/DC converter to the points marked with the red, green and blue dots.

The two large capacitors are no longer used.  You could removed those to create space.

I've left the two smaller capacitors in circuit.    Not sure what value they are on the actual PCB.    You might need to replace those with smaller values if they exceed the spec of the DC/DC converter.

If you have noise problems you might need to leave the existing caps in there then put small value resistors between the DC/DC converter and the +12V/-12V connection points on the board.  This will form an RC filter to removed some noise from the PSU rails.
Something else, for noise reasons it's a good idea to put at least a 100uF cap across the input terminals of the DC/DC converter.

[Berzons]

Thanks for all the info, it's all very interesting. It gives me hope! 

Here is the underside of the board...

Here's a trace of the PSU:







The idea would be to remove the small black TO-92 package 78L12 and 79L12 then wire the +12V/0V/-12V DC/DC converter to the points marked with the red, green and blue dots.

The two large capacitors are no longer used.  You could removed those to create space.

I've left the two smaller capacitors in circuit.    Not sure what value they are on the actual PCB.    You might need to replace those with smaller values if they exceed the spec of the DC/DC converter.

If you have noise problems you might need to leave the existing caps in there then put small value resistors between the DC/DC converter and the +12V/-12V connection points on the board.  This will form an RC filter to removed some noise from the PSU rails.
Something else, for noise reasons it's a good idea to put at least a 100uF cap across the input terminals of the DC/DC converter.

Hi Rob. That's brilliant. Can you recommend a DC/DC convertor that I could use, possibly from the list that ElectricDruid posted above. This is all still pretty new to me.

Cheers!
Brian

[PRR]

Looks like 25ma-35mA each rail. (THAT chip and dual opamp.) Too muggy here to do the shopping.

[Rob Strand]

Hi Rob. That's brilliant. Can you recommend a DC/DC convertor that I could use, possibly from the list that ElectricDruid posted above. This is all still pretty new to me.

Cheers!
Brian

I don't have a lot of experience with those smaller Traco models in audio projects.

For a 9V DC input the device to choose is:
TEC 2-0922       9VDC (4.5V to 13.2V)   output  +12V / 0V / -12V   83mA
(There's also TEC 2-1222WI which handles wider input ranges).

The datasheet and applications notes are here:
https://www.tracopower.com/series/tec-2

The application notes show some filtering.   However, the recommended parts are really only suitable for SMD parts.

If you look at the circuit here it's more friendly to through-hole parts (this one is 15V but just use the 12V part number)
https://aionfx.com/app/files/docs/eclipse_documentation.pdf

These DC/DC modules have isolated outputs.   On that schematic the input side gnd is not the same as gnd on the output side.    There's no need to connect the two grounds.   Technically you should add a small cap between the grounds, as recommendeded in the Traco applications notes.

For the parts shown in the power supply block on that schematic, you want short wires between the traco unit and the caps & inductors.

Referring to the my previous post, you can leave caps C11 and C12 in circuit,
https://www.diystompboxes.com/smfforum/index.php?topic=132274.msg1288044#msg1288044

It would be wise to locate the DC/DC module away from the audio signals and the footswitch.    Put as much distance you can between the audio and the module.   Maybe put it in the corner.

For the DC input jack you will want to isolate it from the case, if you can.