Velleman K8084 pre-amp kit question

[benhumphries]

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: http://www.vellemanusa.com/downloads/0/illustrated/illustrated_assembly_manual_k8084.pdf

 

[Iain McNeill]

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.

 

[benhumphries]

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

 

[stujonesmd]

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.

 

[tedrick]

Hi Group - I would like to have a go at one of these Velleman pre amps -
Stu - I'd like to try adding the bridge rectifier and filter caps to the power supply -
Which rectifier diode would you recommend that I use in this instance?

Thanks!
Ted

 

[Oystein]

If you take the same k8084 kit it has already the rectifier diodes and filter caps.

If you feel you want to "move" them to a separate pcb at a different place why not use these parts as they are already in the kit and designed to be good enough for this board.

If you want to beef up the capacitors with higher value because of the increased startup / charging currents things need to be re-evaluated

 

[tedrick]

Thanks Oistein - I wasn't sure if Stu was suggesting adding a 2nd rectifier/filter ahead of the rectifier already on the kit's pcb --

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.

would it be problematic to add another bridge rectifier in front of the pcb?
Ted

 

[Oystein]

The added bridge rectifier does not cause any problems.

The only thing what happens is that you "lose" an extra 0,7 Volt either side through the second set of diodes but the tranformer of 12 Volt is sufficient to cover this extra loss.

 

[tedrick]

hmm... do you think the added filtering would improve the PS?
Thanks!
T

 

[Oystein]

Not that much ...

How the BC547 and BC557 transisitors are implemented in the power supply using the 9,1 Volt zener diodes already make a clean and stable supply for the amplifier stages.
Of course there are better designs but then you will need to put in a complete seperate supply, the Jan Didden super regulator comes to mind.

Going this route is probably ooverkill but a good start if you want to explore DIY further and maybe build another (better) discrete pre-amp in the future.

The Superregs | Linear Audio NL

Super Regulator – diyAudio Store


P.S. there is nothing wrong with your initial idea and listen for yourself if you can hear any improvements with or without the addiotinal rectifier and added capacity.
One additional step could be to remove the diodes D1-D4 and put in 2 resistors at position D2 and D4 with a 4,7 ohm value.
But then you must be sure you get the polarity correct from the external recitifier.
+ to the D2 resistor and - to the D4 resistor.

 

[tedrick]

excellent! Thank you very much!

I think I will do that --
start with the kit and modify the ps if needed --

Thanks for the links to the other ps resources!
Ted

 

[tedrick]

kit and 12 - 0 - 12 transformer has been ordered -
$40 from jameco --

wife wasn't happy about how much is cost, but the end goal is for her to get her little Denon bookshelf receiver back - I'm currently using her Denon as a preamp for another power amp --

 

[tedrick]

progress report

Hi Group -
well my build was going along just fine until the very last step

I tested the device without the IC chips in the 2 sockets and the device powered up normally -

the directions referred to a notch in the NE5532 IC - however, on my chips there was no notch -

I guessed putting the chips in the sockets with the lettering running left to right - the device powered up -- ok -- so far so good --

I connected a source to the device input and sent signal out to my test amp --
powered up and I got a bad hummm sound --

So my first thought was, the IC chips must be in wrong way - so I flipped them --

bad to worse - now the device does not power on either with or without the chips --

I'm presuming I fried the two IC chips when I put them in flipped the wrong way -- but that the power led doesn't light up either now with the chips out is concerning -- did I burn out the transistors too?

Any insight would be appreciated - would Jameco or Velleman send me new chips since their directions did not match the actual parts?

I think the loud humm I had when I first powered the device up was probably just a lack of grounding with the center tap wire.... the instructions could have made that a bit more clear....

 

[takitaj]

The chip will have either a notch cut out of one end or a dot in one corner, either cut into it or printed on it. That notch or dot goes toward the top of the board, the long edge the diodes and transistors are on. The directions do warn you to be careful you put it correctly so I doubt they will send you new ones, but maybe. Fortunately the parts are cheap. The transistors and maybe the zener diode should be replaced too.

Not sure where you are but here's links to the parts at Mouser.com:
NE5532 NE5532P Texas Instruments | Mouser
BC557 BC557ATA ON Semiconductor / Fairchild | Mouser
BC547 BC547CTA ON Semiconductor / Fairchild | Mouser
Zener diode TZX9V1D-TR Vishay Semiconductors | Mouser

 

[tedrick]

Thanks Takitaj - yes - there is a nice little dot printed at one end -- I did have them in correctly the first time --

I wish I would have come in and asked for guidance before making the mistake of trying to switch the orientation of the chips - my fault

I will order out the parts that I most likely damaged --

Thanks!
Ted

 

[tedrick]

a couple questions - before I order replacement parts - would it be worth desoldering and testing the diodes and transistors to see if they are still good?

for the IC chip - I wouldn't need to replace the interconnect base, would I?

Thanks -
Ted

 

[tedrick]

I went in and desoldered the zeners and transistors -
the zeners test OK but the transistors are definitely toasted

547 transistor
Emitter to base infinite
Base to emitter infinite
Emitter to collector infinite
Collector to emitter infinite
Collector to base 2.5K
Base to collector 2.3K


557 transistor
Emitter to base infinite
Base to emitter infinite
Emitter to collector infinite
Collector to emitter infinite
Collector to base 0
Base to collector 0

 

[asuslover]

Carefull with the cheap board traces , I used bd139/140 instead of bc547/557 when I had mine done.

 

[tedrick]

hmmm! interesting! are these transistors interchangeable? better performance with bd139/140?

Thanks!

 

[asuslover]

Not better, bd 139/140 have higher power rating.