Feeler: Classic Solo

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
I placed it to ground .
Black probe to ground, red probe to 48V pad.
Measure DC voltage.
That is because the 48V is not referred to ground. It is only connected to pin 1 of J1. Try between the 48V pad and pin 1 of J1 with nothing plugged into J1.


oh my god ... I'm so dumb.
With pin 1 at J1 and 48V pad I get 48,6vdc.
So it is as it should be.
Okay ... now I've to figure out, why it not work with the toggle board.
What is your advice to connect the led ?
No you are not dumb. A wise man once siad "the only people not making mistakes are those doing nothing"

At least the phantom power is good.

If you want to use the 4toggle PCB to operate the LED then you will probably need to modify the PCB. However, I just looked at the PCB layout and to be honest this is not really practical. It would be easier to design a new 4 toggles PCB with a switch that operates both the phantom and the LED. Let me have a look at this and I will get back to you.


Hi Ian, this has peaked my curiosity! I installed LEDs on the switch as per your advice in this thread last year.

Depends what you mean by active. The usual method is just to connect the LED via a suitable series resistor (10K should work for many LEDs) to the switch that turns on the phantom power so when phantom is turned on the LED comes on too. I do not recall the total current the Classic Solo power supply can provide from the phantom supply but I do not expect an extra 5mA would worry it too much.



All seemed good and I tested the build with a condenser mic which worked fine. Since then I've only been using the pre with dynamic mics so haven't encountered any problems, however after your last posts I've measured the voltage and with the LED connected the phantom is only 36v at the XLR, if I disconnect the LED it is 48v as expected. So, I've disconnected the LEDs for now, but I would also be interested to know if there's a good way to include the LEDs without affecting the voltage at the XLR.

I think you were lucky. I don't use the circuit on the 4toggle PCB any more The 4toggle version means that when phantom is off there is nothing connected to the mic input. The more normal way of doing it is to leave the two 6K8 resistors connected and just disconnect the 48V from their junction. This mean that with phantom off there is an additional 13K6 connected between hot and cold but its effect should be minimal. At first glance I cannot see a workaround on the existing 4toggles PCB without some serious track cutting a wire linking but I will look into the options.


I have created a new version of the 4toggle PCB that incorporates switching for a phantom LED. I have changed the phantom switching so the 2 x 6K8 resistors are permanently in circuit and their junction is simply connected to 48V to turn on the phantom power. I retained the DPDT switch so I could switch the 48V to the LED separately. Without it, the LED and its feed resistor would be connected to the 2 x 6K8 resistors which remain connected to the mic hot and cold input. With the DPDT the LED is isolated from the mic input circuit when phantom is off. I have ordered 3 fast turn around PCBs from Aisler. I expect them at the end of next week. I will send you guys one each and keep one for reference. Then, assuming it has no errors, I will open source the Gerber files.


I’m getting ready to connect power to my Classic Solo and I have a couple questions. I’m using the Hammond 369AX, which has center tapped secondaries for ht and heaters.

1) should I ground the center taps to the chassis or let them float?

2) Is it OK to mount the mains transformer in an external box and, if so, should I run those ct’s into the chassis (assuming they are to be grounded and not floating)?

It sounds like trying to use the 50v bias tap for phantom power is potentially problematic, so I’m planning to just bring phantom power from elsewhere. Plus, that regulator seems to be in short supply these days, so I just didn’t populate that part of the board.

Thanks in advance!
I always find Hammond mains transformer specs confusing. They say the heater winding is 6.3V CT which is fine and from which we deduce the winding is actually 3.15-0-3.15. The 369AX HT secondary is listed as 250V CT which must therefore be 125-0-125 if their definitions are consistent. From this we conclude that for use in the Classic Solo the centre taps are not required and must be left unconnected. It is puzzling however because a bias tap implies it is intended for a tube power amp but who 125V HT in a power amp?

So, just to be sure I am not getting myself confused, I have just popped over to my workshop. There I have a unit I built for an American customer which he has sent back to have a REDD EQ fitted. The point is I used a Hammond transformer in its power supply which I know needs 250V HT. I just checked the transformer number which is 369JX - just one letter different to yours. I looked it p and sure enough it is listed as 500V CT which is what I needed for that project because it has a rube rectifier.

From the above you are correct in surmising that the 50V bias tap cannot be used for phantom power.

So, bottom line, do not connect the centre taps.