Preamp Noise Problems, Question.

[mkruger]

So I'm building my first DIY preamp and I'm wondering what internal electronics are putting out enough EMI to interfere with the signal path. Is the power transformer a common problem for noise? Is that why people tend to put them in their own little boxes? or is it just for looks?

 

[gyraf]

In diy preamps, the foremost source of hum is the ground path. Use toroid power transformer - that gives lowest magnetic stray field.

Jakob E.

 

[Ethan]

I don't mean to hijack the thread but this is kind of related.
Is there any reason why not to have Audio Transformers near power supply filter caps?

Thanks.

 

[owel]

I dont think its the power supply caps per se, but everything in that area... i.e. from the AC lines, to the rectifier, to the unfiltered DC (which basically looks like mountains in an oscilloscope), which gets smoothen at the filter caps. At this point, you still have some ripples left (i.e. the peak of those mountains), which gets smoothen out at the voltage regulators.

If you have voltage that varies up and down (i.e. AC and the unfiltered DC), and it's near an inductor (i.e. the audio transformers), you're going to induce voltage (which becomes hum) in that transformer.

This is called Faraday's law.

 

[Ethan]

Ahhh, almost forgot about the law of induction!

 

[mkruger]

well i actually had a transformer custom made from PC Transformer Corp. for this tube project. the tranny puts out 48v 12v and 320v. How can I tell what it's made of?

I will implement a star grounding. However I was wondering if i could possibly use a grounding rail. Like what you might see in a circuit breaker box in your house. where there are about 20 or so individual holes on the same rail. I suppose this might defeat the purpose of a star grounding... but it would be a neat feature.

 

[NewYorkDave]

Actually an alternating current can induce a current flow in a nearby inductor. So if you have a filter cap with a ripple current flowing through it, it can be coupled into a nearby audio transformer even though the ripple voltage across the cap may be small.

A practical example of an inductor being used to sense current through a conductor is the popular "clamp on" current probe used by electricians and others.

Short story is, keep audio transformers and inductors away from power supply components.

 

[gyraf]

A bit OT -

Canford has a dedicated mic-pre powersupply transformer on sale - 6.5£:

http://www.canford.co.uk/commerce/productdetails.aspx?productid=23-503

http://www.canford.co.uk/commerce/resources/catdetails/2741.pdf

17-0-17V 60mA and 44V 30mA.

Jakob E.

 

[mkruger]

NYD - how far away? i mean typically people use 2u or 3u rack spaces. So are we talking an inch or so, how far do these magnetic fields reach? Or should I seriously consider using a metal cover on the inductor(10H 200mA) and transformer?

 

[NewYorkDave]

You can't answer the question of "how far" without knowing some important factors that affect strength of field among other things.

One thing I can tell you is that the strength of field falls off at the square of the distance.

 

[Gus]

In one issue of an Speaker builder there was a article on crossover networks. The person tested air coils for interaction IIRC they use a audio amp to drive a coil and were able to pickup the signal a good distance away depending on the way the coils were placed. At 90 degrees you could get very close with min pickup.

I wounder if we could drive a power transformer with a music etc signal and more around any of the devices that might pickup its field. With music etc it might be easier to find the min points because you would hear around other souces of 50 or 60Hz