Hi, friends, I’m very new here so I hope I’m posting in the right place.
I’m building a four channel mic preamp that uses the INA217 and a power supply from five fish audio. I’m still waiting on my enclosure to arrive, but I’m trying to plan the xlr inputs and trs outs. I want to put as much distance between the power transformer and these IO, so would it work to place the trs output right next to/slightly under the xlr in? Would there be any interference in the signals? Also, even if I do this, the IO will be ~4 inches away from the power supply and trafo, but should I even be concerned about noise from it? If so, what can I do to shield the power supply?
Thank you!
I’m building a four channel mic preamp that uses the INA217 and a power supply from five fish audio. I’m still waiting on my enclosure to arrive, but I’m trying to plan the xlr inputs and trs outs. I want to put as much distance between the power transformer and these IO, so would it work to place the trs output right next to/slightly under the xlr in? Would there be any interference in the signals? Also, even if I do this, the IO will be ~4 inches away from the power supply and trafo, but should I even be concerned about noise from it? If so, what can I do to shield the power supply?
Thank you!
Successful mixers like the PV8 have the transformer outside the box. I reworked a Herald RA-88a that the hum became apparent once I took the hiss out by replacing the op amps. The transformer was banded, but right next to the high gain circuits. So was the 120 vac power switch. I moved both of those out of the box, replacing with a wall transformer. That and double pi inlet filtering of the 18 vdc in got rid of the hum.
It is instructive to look at successful commercial designs. The dynaco PAS2 RIAA preamp uses an internal steel bulkhead to isolate the sensitive tube base wiring from the power transformer, which is steel wrapped. The Peavey MMA-875T mike/line input mixer has a banded EI transformer, but they pushed it into a corner and wrapped it with a U of steel. The AC switch and line cord entry are over in that corner. The only circuits near the transfomer are the filters, regulators, and then the the gain 1 tone controls. The possibly high gain mike input circuits are 14" away at the other end of the box, with a steel bulkhead running from top cover to bottom cover behind those circuit boards. The XLR input jacks are on the back surface of that internal box. So sheet steel is your friend in the war on hum. Be sure your screws or brackets effectively ground internal steel shields. The external case is connected to the AC safety ground of course. The negative of the XLR's or RCA jacks should not be.
Happy building.
It is instructive to look at successful commercial designs. The dynaco PAS2 RIAA preamp uses an internal steel bulkhead to isolate the sensitive tube base wiring from the power transformer, which is steel wrapped. The Peavey MMA-875T mike/line input mixer has a banded EI transformer, but they pushed it into a corner and wrapped it with a U of steel. The AC switch and line cord entry are over in that corner. The only circuits near the transfomer are the filters, regulators, and then the the gain 1 tone controls. The possibly high gain mike input circuits are 14" away at the other end of the box, with a steel bulkhead running from top cover to bottom cover behind those circuit boards. The XLR input jacks are on the back surface of that internal box. So sheet steel is your friend in the war on hum. Be sure your screws or brackets effectively ground internal steel shields. The external case is connected to the AC safety ground of course. The negative of the XLR's or RCA jacks should not be.
Happy building.
Last edited:
Be sure to keep that power transformer as far away from any audio transformers as you can, especially poorly shielded mic transformers. It should be completely doable to have in the same chassis; it's been done many times. You may consider powering it up and very carefully moving that transformer around to find the quietest location and orientation. And yes, shielding is your friend.
So the circuit I’m using doesn’t use any IO transformers, but I’ve considered adding some later if I want to. But is it really so simple as enclosing the power transformer with grounded sheet steel? I wish I could construct an external power supply, but I don’t know how to even approach that. What sort of connector would go from the power supply into the main unit? Would the transformer be by itself in the other box or would I include the filtering and regulation circuit in there, too? I just don’t have any reference on that, since most preamps I see are all in one.
It is a hard battle when going at very low noise. Shielding of course, but you must firstly reduce hum picking as much as possible. That is: Good ground routing, absolutely no ground loop. Signal loop areas as small as possible, especially where there is low signal levels and high impedances. Good PCB routing is the essence.
Phantom power ground ( the -48V ) should not be connected to signal ground, it should be connected only to XLR pin 1 = chassis ground.
Phantom power ground ( the -48V ) should not be connected to signal ground, it should be connected only to XLR pin 1 = chassis ground.
The PV8 mixer uses this wall transformer https://www.newark.com/triad-magnetics/wau16-2400/adaptor-ac-ac-120v-16v-2-4a/dp/96M0302
But you have to go to parts-express.com to get the mating barrel receptacle 2.1 mm x 5.5 mm. I've found no way to pry it out of newark.
The rectifier and filter caps are inside the PV8 mixer chassis.
Actually the PAS2 preamp has a both steel wrapped transformer and also steel bulkheads inside the box to keep flux away from high gain wires. Peavey equipment wraps the EI transformer with a copper band to reduce flux, but they also put it behind a bulkhead or U wrapper.
In the switcher supply CS800s, peavey puts the power supply inside a separate steel box inside the main chassis, to keep the RF flux shorted out.
You can't buy copper banded or steel wrapped transformers anymore. Buy you can buy junk equipment which has them, often for $60 or less including freight. Which is what I do. the circuit boards and IC's you can get cheaply. The mechanicals and connectors are hard to get & expensive on the new market. As for equipment above, dynaco equipment is pricey, even the badly designed ST120. Dead Peavey equipment covers the south. Stay away from numark, their stuff is all surface mount and ribbon connectors. 70's to 90's stuff can be leaded components.
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.