Voltage Regulator Noise

[MarshallPlexi]

Several years ago I was working on a voltage regulator for an amp that would put out 6VDC. I had the regulator mounted right under the preamp circuit board and it took me WEEKS to figure out that the horrible noise I was getting in the amp was due to the placement of the voltage regulator. I moved the regulator to the far side of the chassis and it solved the problem. No noise and super clean DC feeding the preamp tubes heaters.

Why did this happen? Is there anything I could have done apart from moving the location of the regulator to reduce the noise? Was I picking up switching noise? Years later it's not an issue, but still I'm curious.

Thanks!

[Merlinb]

I doubt it was switching noise, because I assume you were using a linear regulator. It may have been EM radiation from the input wires, carrying the heavy ripple current of the reservoir capacitor.

[loudthud]

What kind of noise was it? Many times 3 terminal regulators are put in circuits that don't have enough overhead voltage for them to regulate properly. The result is spikes in the output where the chip runs out of input voltage. Grounding can also be an issue if there is a common ground between the heater circuit and the signal ground.

[Enzo]

Also where you grounded the thing matters. If the regulator ground return shared copper with a preamp circuit, its ripple currents would be impressed on the signal.

[MarshallPlexi]

If memory serves it was a horrendous buzzing 120Hz noise. Moving the grid wires did make a huge difference, but nothing solved the problem until the regulator was on the far side of the chassis. There was also a Full Wave Bridge on the same PCB as the regulator. Any ideas?

[Dave H]

If memory serves it was a horrendous buzzing 120Hz noise. Moving the grid wires did make a huge difference, but nothing solved the problem until the regulator was on the far side of the chassis. There was also a Full Wave Bridge on the same PCB as the regulator. Any ideas?

If it was a 120Hz hard buzzing sound rather than the usual warm hum it was probably from the diode's reverse recovery current. You could reduce it by using ultra fast diodes eg. UF series but it's best to keep the diodes well away from the preamp circuit.

Dave H.

[MarshallPlexi]

Here is the signal path of the regulator board.

10V @ 2A winding on the power transformer.
BR310 Full wave Bridge
3000uF Filter Cap
L78S75CV (7.5V @ 2.0A)
BYT03-400 Diode (Used to drop voltage down to 6.3V and power the preamp tube heaters.)
2 1N4007 Diodes (Used to drop voltage down to 5.0V and power the relays.)

The hum is a 120hz hum that is really hard and obnoxious; not soft in any way. Is there a way to add caps to the power supply somewhere that will limit this hum? I would really like to continue to use this power supply board since it's very small and seems to work perfectly if I could just get this noise issue to go away.

Also the regulator is mounted to the chassis and if I recall correctly, the ground lug on the regulator is internally tied to the physical ground of the unit.

Any ideas?

Thanks!

[loudthud]

From the data sheet of the regulator it takes 10.5V for the chip to regulate at currents of 1.5A and less. A simulation of the power supply using 16V with 1 ohm bulk impedance and 3000uF gives a minimum input voltage of 13 volts at 2 amps. From this I have to say I see no reason why the regulator shouldn't produce clean DC although there is 4V peak to peak on the input of the regulator.

How much current does the regulator need to supply? What is the resistance of the transformer winding? I didn't look up the rectifier. What are it's ratings?

[MarshallPlexi]

How much current does the regulator need to supply? What is the resistance of the transformer winding? I didn't look up the rectifier. What are it's ratings?

I was wrong. The PT winding is 10V at 2A. The regulator is supplying 1 EF86 and 3 12AX7's for a total of 1.2A. The BR310 Bridge Rectifier are 1000PIV and 3A.

With a 10V winding the Bridge should produce about 14.14V. The regualtor does give really smooth power with hardly any AC component at all as measured on my Fluke Multimeter. My concern is the noise being radiated off the units. Will bypass caps help at all?

[loudthud]

The old RMS * 1.414 formula only works in the classroom. In the real world, losses eat your lunch and you have to tip the waitress and buss the dishes. A simulation shows the valley of the ripple with 3000uF is going to be 9.2V at nominal line voltage. That's not enough for the regulator to maintain regulation. With 10000uF you'll have 10.8 volts, again at nominal line voltage. When the line voltage is lower, you'll have less. You are going to need a low dropout regulator or more voltage from the transformer.

[MarshallPlexi]

The old RMS * 1.414 formula only works in the classroom. In the real world, losses eat your lunch and you have to tip the waitress and buss the dishes. A simulation shows the valley of the ripple with 3000uF is going to be 9.2V at nominal line voltage. That's not enough for the regulator to maintain regulation. With 10000uF you'll have 10.8 volts, again at nominal line voltage. When the line voltage is lower, you'll have less. You are going to need a low dropout regulator or more voltage from the transformer.

WOW!! Thank you so much for your time! That is great information! I'll hook up the regulator to a bench DC power supply and see how high I need to run it to get the clean power I need. Thanks again for all your help. I'll let you know how it goes.

[loudthud]

OOPs! Simulation was run at 50Hz. That means the voltages will be higher than predicted in the US. Too account for more of the losses, you need to know the ESR of the cap you are using and the losses of the transformer. I just took a wild guess. The biggest improvement will be changing the regulator to a low dropout type with the voltage set to 6.3V.

[MarshallPlexi]

I'll have to check out the voltage under load and see what the regulator is being fed. If the voltage is in excess of 10.5V and I'm still getting 120Hz hum, what do you suggest? Snubber caps across the Diodes of the FWB or even the dropping diodes?

[loudthud]

It should be easy enough to find fast recovery diodes to fit your layout for the FWB. They usually don't need any type of snubber caps. Some regulators require some type of cap across the output for stability. Other than some type of grounding issue this thing should work.

[MarshallPlexi]

loudthud -

Man you were spot on! My power supply was putting just 10.25VDC. When I boosted the voltage using a variac the buzz went away when the voltage hit 10.5V!

Thanks so much!