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PostPosted: 14 Jun 2009, 16:34 

Joined: 13 Feb 2009, 23:51
Posts: 4
Location: Buenos Aires, Argentina
Hi All,

I've built a preamp based on the "Beauty" Schematics. Actually my friend built the thing as he has the experience with circuits.

The construction followed pretty much everything on the original design, except for the power source. I'm on Argentina, and we have 220V, so the switching ps wasn't necessary. Instead he built a standard 220V - 50 hz source. Another difference is that we used plug connectors instead of RCA.

I tested it with my bass guitar with an Acoustic transistor-based amplifier. The sound is spectacular. It gives so much body to the sound. And we added a potentiometer to the circuit. And when in max position it can be used as a very interesting warm distortion. I'm very happy with it. Thank you for making the Beauty available.

Now the problem I had.

I wanted to connect my bass thorough the preamp to my m-audio duo external sound card to record and do some more specific tests to the pre-amp capabilities.

So I did:
Bass > plug cable > pre-amp > plug > m-audio duo > usb cable > pc

When I turned on the pre-amp I saw in horror how the usb cable that was plugged onto the pc sparked and my pc turned off :eek: . It never turned on again. My motherboard was burnt. :firefighter:

My friend did some research and told me that (I don't know how to translate some of the technical terms he told me in spanish but I will give it a try) some valvular equipments use a a transformer with galvanoplastic insulation to prevent this kind of things. It seems the conflict was between the computer and the pre-amp circuits.
He told me it may conflict with devices with digital inputs like usb.

Any thoughts on this?
Any help would be greatly appreciated.

Thanks for reading!

PostPosted: 14 Jun 2009, 19:59 
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Joined: 28 May 2008, 21:53
Posts: 4588
Location: Winnipeg, CANADA
Hi Pedro,

Based on your description, I am wondering if you used an isolation transformer? If you did not use an isolation transformer, what you have created is very unsafe :!:

What can happen when you do not use a transformer is that if you reverse the power plug (hot and neutral) the mains voltage potential can appear between the signal and the chassis - particularly with grounded equipment such as a computer.

If this was the circumstance, consider yourself very lucky that it was not your body that got in between the preamp and the computer.

[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]

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