DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

Hi Owel,

I ordered a kit package yesterday, and I've been thinking about how to add a bypass switch for the input transformer. I came up with the circuit below, using an off-board 4PDT toggle. Do you see any problems with this? Please let me know.

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Best Wishes,
Jason Glass

Looks like it will work. Try it.

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Thanks, Owel. Will do.

_________________
Best Wishes,
Jason Glass

Hi Owel,

I built the switch into two SC-1mk2 units. It works. I can hear the jump in gain and the tone is nice.

The following are not complaints! The units will be fine in practical use, but I hope to get your thoughts or some troubleshooting help on these glitches.

One problem is that the output phase is reversed when the trafos are engaged. Hmmm. I poured over my schematic again (shown in my post above) and can only determine that the trafo would also reverse phase when installed normally. I hesitate to reverse the leads on the secondary without fully understanding how the 2K67 & 6K8 resistors function. I need to study your original schematic more closely.

There is also weirdness with one of the preamps, where the output and loud hiss fade in and out quickly, then fail with a thump, when the trafo is engaged and the input gain is cranked to 12. It occurs whether or not the input is terminated, and whether or not signal is present, but only with the trafo engaged. I doubt I would ever need to crank it that hard, but it is puzzling. I think an op-amp is freaking out. BTW, I reflowed all the PCB solder joints as a first strike at this one, but it didn't help.

There is also a matter of phantom power causing a loud 1-second burst of pink noise when accidantally engaged without a phantom-using load attached. For example, a dynamic mic. Both preamps do this. Hmmm.

Please let me know your ideas on this stuff!

Best wishes,
Jason Glass

_________________
Best Wishes,
Jason Glass

For the out of phase, You can swap pins 2 and 3 wires on the output XLR jack.



I don't normally use phantom power with a dynamic mic, but on few accidental occasions I switched PP on using an SM57, there was no burst of noise. It just behaved like nothing happened.



Cranked to 12? Could be overload?

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Hi Owel,



Yes, of course, but is the trafo version normally out of phase to the trafoless version? I'd like to correct it without installing a phase switch.



Me either, and especially not when I use my ribbon mics! However, when troubleshooting I test as many scenarios as I can.

With equal +48VDC on pins 2 & 3, common-mode rejection should null the voltage. I'll go back and try to figure out how DC is getting into the op-amp input. It seems like the most likely possibility.



If it only occurred with signal present, I would agree. I use a switched mic to test it, where the switch engages a dummy load when off. I switch the mic on, with only ambient room noise, crank to 12, and I hear oscillation of audio and hiss then thump and silence. With the switch off, then crank to 12, I hear oscillation of hiss noise, followed by the thump/silence.

If I switch the trafo out of line or twist the input gain back to 11, normal function returns. I must have goofed something up in my trafo switch circuits.

I am keeping in mind that it would be extremely rare for me to need +72 db of gain.

Hmmm...

_________________
Best Wishes,
Jason Glass

No, it's just jumpered across if you have the trafoless version.

What I meant to say is just swap pins 2 and 3 wiring on your Output XLR if you find it out of phase with your setup.



This is happening with your custom bypass switch, right? I would suggest keep your wiring short... it could be all those extra wires acting like antenna or causing oscillation?

From my testing, for example... the gain switch wired externally to a front panel (even using short 2" hookup wires) behaved noisier and prone to hum pickup than a PCB mounted gain switch.

The longer and more varied that path your mic signal has to take (through your bypass switching, etc...), the more chances for it to pickup noise and cause problems... and then you even crank it up to 12. Any little noise that gets picked up will get amplified thousands of times.

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Hi Owel,



It looks like the jumpers vs. trafo indeed swaps polarity. Check out the schematic below. Positive voltage across the primary should induce negative voltage on the secondary as shown.

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Best Wishes,
Jason Glass