Hi everyone:
When we hear the word ART we think of ART..... As You know the ProMIX is a little 3 channel Mic mixer Box with Phantom power.
One of these days It was humming connected to a DJ console Mic Input and I grabbed a 12V transformer from an AKG Mic and connected it to it. Unfortunately it didn't work since the polarity is reversed. (Negative TIP), so as I have a lot of those SMPS transformers I opened it and modded it.
By Investigating the beast, I've found out some strange things:
Op-amps are old RC4458 ones, no wonder it hisses a bit. I've tried NE5532 types and it seems it is a bit better now, but will try other low noise types. The thick wire You see is ground bolted to the cover. The ones on the right are for 18V battery power.
There are no 100nF PSU decoupling capacitors near the chips. The ones You see on the picture are in the 47pF range, (feedback path capacitors I presume). I added decoupling caps under the board between pin 4 & 8: The green capacitors on the lower left are connected to the dip switches for the low cut function.
There is no regulator on board. A single 1000uF capacitor. The transistors You see are for controlling the clip and phantom led. Not sure also what those diodes are..
There is a potentiometer inside the box, but set at max position. First I've thought it could be used for setting the gain as it is between the Op-amp output and the isolation transformer, but there is no hole on the box on that side, and reaching it via battery compartment is impossible as it is mounted backwards. It was removed and shunted. Nothing mentioned on the manual either.
The battery holder is a cardbox duct placed inside the box over the chips. I've glued it to the cover aligned with the hole.
Now the cherry on top of the cake:
Look at the phones / aux diagram printed on the box and on the underside of the board. Tip is connected to op-amp output, the same that goes to the transformer. Ring is connected to ground. Ground is floating / not connected. If You feed it into a balanced mixer it should work fine, but If You connect headphones, one channel is out of phase.
Forgive me but that's not Applied Research & Technology... Hey but works.
When we hear the word ART we think of ART..... As You know the ProMIX is a little 3 channel Mic mixer Box with Phantom power.
One of these days It was humming connected to a DJ console Mic Input and I grabbed a 12V transformer from an AKG Mic and connected it to it. Unfortunately it didn't work since the polarity is reversed. (Negative TIP), so as I have a lot of those SMPS transformers I opened it and modded it.
By Investigating the beast, I've found out some strange things:
Op-amps are old RC4458 ones, no wonder it hisses a bit. I've tried NE5532 types and it seems it is a bit better now, but will try other low noise types. The thick wire You see is ground bolted to the cover. The ones on the right are for 18V battery power.
There are no 100nF PSU decoupling capacitors near the chips. The ones You see on the picture are in the 47pF range, (feedback path capacitors I presume). I added decoupling caps under the board between pin 4 & 8: The green capacitors on the lower left are connected to the dip switches for the low cut function.
There is no regulator on board. A single 1000uF capacitor. The transistors You see are for controlling the clip and phantom led. Not sure also what those diodes are..
There is a potentiometer inside the box, but set at max position. First I've thought it could be used for setting the gain as it is between the Op-amp output and the isolation transformer, but there is no hole on the box on that side, and reaching it via battery compartment is impossible as it is mounted backwards. It was removed and shunted. Nothing mentioned on the manual either.
The battery holder is a cardbox duct placed inside the box over the chips. I've glued it to the cover aligned with the hole.
Now the cherry on top of the cake:
Look at the phones / aux diagram printed on the box and on the underside of the board. Tip is connected to op-amp output, the same that goes to the transformer. Ring is connected to ground. Ground is floating / not connected. If You feed it into a balanced mixer it should work fine, but If You connect headphones, one channel is out of phase.
Forgive me but that's not Applied Research & Technology... Hey but works.
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The '4458 is such a tame thing, that fat cap within a few inches is ample bypassing.
Regulation *may* be in the factory supplied wall-wart. Supply for the preamp/mixer is not critical: it will work to 7V or to 40V (rated 36V). Can't see the +48V scheme.
It *will* hiss. It is a 1-opamp diff-input. Noise Figure will never be less than 6dB even with an ideal opamp. May be 8-10dB with the '4458s.
Reminds me of something from ROLLS. ART's better stuff is really very good. I guess they have to place some products down in the pennies/unit range to keep their market share. (I know it retails at $99 but Banjo World take the lion's share of that.)
Regulation *may* be in the factory supplied wall-wart. Supply for the preamp/mixer is not critical: it will work to 7V or to 40V (rated 36V). Can't see the +48V scheme.
It *will* hiss. It is a 1-opamp diff-input. Noise Figure will never be less than 6dB even with an ideal opamp. May be 8-10dB with the '4458s.
Reminds me of something from ROLLS. ART's better stuff is really very good. I guess they have to place some products down in the pennies/unit range to keep their market share. (I know it retails at $99 but Banjo World take the lion's share of that.)
The factory supplied PSU is an ordinary transformer bridge cap. So, Line regulation. There is no 48V phantom. Only 12V or 18V with batteries I have several gear that uses the 4580 on Mic inputs. I didn't will try because I have only smd parts. My PC Sound card has detachable LME types. Will try them.
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