Velleman K8084 pre-amp kit question - diyAudio
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Old 28th January 2011, 03:40 AM   #1
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Default Velleman K8084 pre-amp kit question

I bought this kit to provide a pre-amp for a old Yamaha reciever where the control circuits blew out but the main amp still works.

I am also very new at DIY stuff and have some questions.

First, there are some criptic notes about controlling the amount of input and output amplification by controlling the ratio of certain resistors. The question is what do I set them too? I want to connect in a phono preamp, CD player and a MP3 player via headphone jack.

Second question: I would like to move the pots off the circuit board. Can I just add jumper wires and twist them together to minimize noise?

schematic is here:
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Old 28th January 2011, 04:12 AM   #2
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Join Date: Oct 2007
Location: Santa Cruz, California
It's set up as a unity buffer stage i.e. the gain is 1 or 0dB.
The gain is (1,000/100,000)+1 =1.01

This is probably a good starting point. If you need more gain then increase Rf or decrease Ra. Just make sure that Ra is always >100ohms or so and keep the values sensible (don't use 1megohm)

regarding off boarding the pots. You'll have to try it. Twisting the wires is good protection. If that fails, try shielded. After that you need to look at the power supply.

keep posting and we'll help you out.
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Old 28th January 2011, 10:55 PM   #3
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Thanks for your help. It will probably be a week or so before I get it all together, but I will let you know how it works out
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Old 25th February 2012, 09:47 PM   #4
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Location: Easton, PA
This is an easy kit to build. Follow the directions carefully. BE SURE to triple check polarity and placement of the TL072 ICs before applying power. Plugging these in backwards will blow the ICs, transistors and Zener diodes (speaks the voice of sad experience). Also, plan to plant this board in a metal or shielded (lined with copper or aluminum foil) case, since this is a high impedance circuit and picks up power EMF like its going out of style. Best of luck. Despite low cost, this is a very high quality preamp, with design similar to much more expensive units.

I recommend 200K and 100K for Rf and Ra. 6 dB of voltage gain is enough that you can compensate for source differences in line level. You can up the gain much higher but the noise increases, too. I have used 300K and 100K on onr of these, still with good results.

To decrease hum pickup, consider adding a bridge rectifier and filter capacitors (1000 uF at 25V) to recommended 12-0-12V transformer. This works without modifying circuit--the onboard bridge passes the DC through, regardless of how you connect polarity, and thus is a safety factor for feeding DC.

BTW Radio Shack sells 12.6-0-12.6V 2A transformers that work just fine with this kit.
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