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Copyright
2008 FiveFish Studios
www.fivefishstudios.com
SC-1mk2 Microphone Preamp Kit
SC-1mk2 Troubleshooting Guide
Well, hopefully you won’t need this part.
“Measure twice, cut once”… as they say. Take your time assembling the kit, don’t be in a hurry, work carefully and methodically
and you won’t need this troubleshooting guide. I’ve built several prototype preamps in the course of testing and designing the
SC-1mk2 and each one of my prototype worked on the 1
st
try.
So I’ll try to imagine where are the “critical” areas where somebody might make a mistake.
Problem: No sound. No thump, no noise, no nothing.
Check if there is power. Check all wires of the power supply. Check that you have JP2 jumper in place. Check all IC chips are in
the correct orientation. Check that volume pot is not at minimum setting. Check that you did not swap the positive and negative
voltage regulators.
Problem: No phantom power. My condenser mic does not work.
Check if there is 48V power. Check that the transistor is mounted correctly. Did you turn on the phantom power switch? It should
be clicked fully to the left (i.e. pointing to the LED.
Problem: My condenser mic works when I switch on phantom power, but the LED is off.
Your LED is backwards. Re-orient the other way.
Problem: Sound is very faint. And very noisy. Distorted sound.
Check that all power supply wires (18-0-18) are connected to it’s power source. Check that you have both +18 and –18 voltages.
The THAT1510 chip is probably damaged. Replace IC1 with another THAT1510 chip and test again. Check that you have jumper
JP2 in place.
Problem: My FET condenser mic works, but the sound is very faint. I only hear the sound at the higher gains.
Check that you’re feeding the SC-1mk2 preamp with +48V. Some condenser mics will work with voltages as low as 14Volts or
18Volts. But some mics need a higher voltage than that. Measure the voltage between XLR pin2 and XLR pin1. You should be
getting close to 48Volts if phantom power is switched on.
Problem: My gain settings are out of order. It will get loud, very loud, then soft, then loud.
Check there are no shorts in the Grayhill selector pins. Check that the resistors nearest the Grayhill selector are the proper values
and that you did not “swap” any 2 of them.
Problem: I turn on phantom power but I don’t hear a thump and it takes a few seconds before I can hear sound.
This is normal. This is due to the soft-start phantom power. It takes anywhere from 15 seconds to 30 seconds to stabilize and
reach full power. I’m not applying the full 48V at full blast to your expensive mics. Rather, I feed phantom power to the mic
gradually and raise the voltage gradually. It also protects your speakers from the thumping sound, and your expensive mics.
Problem: I racked the preamp, but I’m hearing hum.
Did you use a good power supply? Are you using the PSU-1848/2448 power supply kit? Or are you using your own power
supply? IS your power supply well regulated and hum-free? Did you follow good wiring practices… keeping low level mic signals
away from the AC/high voltage lines? Is your rack grounded?
Problem: I don’t have any mic connected, but I’m hearing hissing noise at +66dB and +72dB gain.
First, let’s talk in real numbers instead of decibels (dB). A gain of 66dB is 2000x amplification. A gain of 72dB is 4000x
amplification. Without any mic connected to the preamp, you’re leaving the inputs unterminated. In normal use, the preamp
should see a 150-ohm load at the inputs, and there will be less noise/hiss. At these high gain settings, and without any mic or
load connected, what you’re hearing is called “Johnson noise.” You’re basically hearing the random movement of electrons in the
circuit. The hotter the temperature, the more the electrons are agitated, and the higher the hissing noise you’ll hear. See this
article in wikipedia.
http://en.wikipedia.org/wiki/Thermal_noise
. I’ve attempted to reduce this Johnson or thermal noise
phenomena by using metal film instead of carbon resistors as much as possible. Another method of reducing this type of noise
is cryonic freezing. Wear a warm jacket.
Problem: I want to record in stereo, but I only have (1) SC-1mk2 preamp.
Buy another kit
Problem: What do I need to power the SC-1mk2 preamp?
The SC-1mk2 preamp has on-board voltage regulators. This regulates the internal voltages to +/-18Volts as measured from
the TP+ and TP- terminals.
The ideal thing to do is feed the preamp with a voltage a bit higher than +/-18Volts. A Power supply that can give +/-20V to
+/-24V (or higher) will be good. I still recommend a regulated power supply. Using a cheap power supply, or unregulated power
supply will cause hum problems. Add to the mix the high gain capability of the SC-1mk2 and you can be amplifying problem
hum by 100x, 500x, 1000x, 2000x or 4000x time. Then you have a bigger hum problem. You can purchase my PSU-1848/2448
kit. Contact us.
