Mic pre and phantom power PSU questions

[ddietz]

Hi folks. I have been working on a layout for a microphone preamp with 4-6 channels and switchable phantom power for each. I'm not completely sure yet of the preamp design. It will be balanced in, non-balanced out. Perhaps I would do an ina217 configuration or maybe OPA2134/2228 type opamp with differential on one side and inverted on the other side. Regardless, I am starting to think about the power supply for this and have a few basic questions. I'd like to stress that I want this to be very low noise floor.

I would need power for 48V phantom for each channel plus amp power for each channel. Most preamp designs I've seen on the web pull ~20mA, as does the opa2134 based amp I am testing now. The image below is my quick quick interpretation many designs I've seen on the web, using LM317T/337T for a +/-15vdc supply. Given the current draw of a preamp, am I correct in thinking I would have no problem running 6 channels off of this? Would powering 6 channels have any noise issues, how would I maximize the circuit to handle 6 channels, etc. Also, is this circuit correct, how could it be improved to minimize noise. I would be using a single, dual secondary Antec torroidal transformer, 0-15, 015, is 100VA enough, to much?


micprePSU by Dennis Dietz, on Flickr

Now for phantom power. It seems as though phantom is often taken right of of one secondary of the power supply transformer. Would this tend to lead to issues with a greater draw on one winding than the other. Would this lead to unwanted noise?

Last, would it be OK to use a basic LM317T circuit to boost the dc from the transformer (15*1.41-1.4=20vdc) to 48volts, as Minion linked in post #7 here, Diy Mic preamp with 48v phantom power Again, noise issues, heat issues, etc I should worry about.

[ddietz]

Er, and yes, I do realize I have the diodes at the bridge going the wrong way

[Minion]

What I used for Phantom power in my pre"s isn"t what I posted in that post you referanced but it is Simular ......

What I do is use a Voltage tripler right after the Secondary of the Transformer , when useing a 15v, 0v , 15v transformer I use one of the 15v AC taps and triple it to about 60v DC then regulate it to 48v with a 317.... as long as the transformer is able to handle the current draw there shouldnt be a problem with one side of the Transformer drawing more than the other pluss thats what regulators are for .......

[ddietz]

Fixed the diode direction and added a phantom part.

Do I need caps parallel on the bridge diodes?
What is a good value for the phantom large el cap?
Do all other cap values seem OK?


micprePSu2 by Dennis Dietz, on Flickr

[ddietz]

Thanks Minion,

Thant makes more sense to me considering I could not figure out how you could end with higher voltage than went in. I'd seen the voltage tripler with diodes on the green mic pre site (DIY Factory - green mic-pre) but had forgotten about that part. I really don't understand how that works though and would love an explanation.

Better???


micprePSU3 by Dennis Dietz, on Flickr

[Minion]

This site will explain how a Voltage multiplier works ......

[stratus46]

Originally Posted by ddietz

Er, and yes, I do realize I have the diodes at the bridge going the wrong way

If you're using plain old power diodes you may want to add a 1000pF cap in parallel with each diode. Certainly design in the components as you may want them. It got the noise down on my 1982 preamp which I still use.

Have you looked into the mic preamps from THAT corp (THAT 1510/1512), Analog Devices SSM2019 or TI (Burr-Brown) INA217 ?

G²

[ddietz]

I found a good, basic explanation of diode rectification and voltage multiplication here:
Voltage multipliers : DIODES AND RECTIFIERS

G^2, thanks for the advice. Inhad read it elsewhere and planned to add the ability for those caps when I layout an actual board. Would ceramic be suitable?

As for the dedicated chip, I'm not really opposed to them. I see them recommended frequently but rarely see anyone actually use them. Unfortunately, I've never heard any of the various options so I have nothing to work from. I'm not opposed to building a few different styles, but I want/need each to be useable and low noise from the beginning. I don't/can't waste the money building a channel that turns out bad sounding. That's why I've moved onto the powersupply for now. No matter that amp circuit(s) I use, they all will need a good powersupply of +/-12-18vdc and phantom power.

That reminds me.... I feel like I'm right but want to check. If I have 6 preamps (20mA idle and say 50mA load) plus 6 phantom lines (Rod Elliot suggests mics will only draw about 20mA each for 120mA total), that totals about 420mA. What then is a reasonable Amp rating for my transformer?

[ddietz]

Originally Posted by Minion

What I do is use a Voltage tripler right after the Secondary of the Transformer , when useing a 15v, 0v , 15v transformer I use one of the 15v AC taps and triple it to about 60v DC then regulate it to 48v with a 317.... as long as the transformer is able to handle the current draw there shouldnt be a problem with one side of the Transformer drawing more than the other pluss thats what regulators are for .......

How does the 317 handle that, I thought it had a 40V max voltage input?

[Minion]

Hi , the LM317 doesn"t have a 40v max voltage ...... It just has an Input / output differential of 40v ..... 60v down to 48v is only a 12v differential....

Cheers