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Old 4th January 2010, 11:40 PM   #1
stwhite is offline stwhite  United States
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Default Transformer for Velleman K4003 amp and K8084 Preamp


Just a quick question about the Velleman K4003 stereo amp and the K8084 preamp kits and and their power needs. I have both kits and the transformer (24V CT 3A Power ) recommended for the amp and want to know if the transformer has enough guts to power both units (I want to build an integrated amp) or do I need to have second transformer for the preamp also? If it can, what needs to be done as far as current requirements etc. if anything for the preamp? It wasn't clear in any of the threads I read.

Thanks in advance for the input.

Stew White
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Old 4th January 2010, 11:49 PM   #2
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I have both of those and I use the same transformer for both. Even though they don't sell the pre-amp at Parts Express, the techs were familiar with it and told me the same transformer can be used for both. No need for a second transformer.
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Old 4th January 2010, 11:49 PM   #3
Minion is offline Minion  Canada
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The preamp will draw much less current than the power amp maybe 100ma if it"s got a lot of opamps and/or transistors but probably much less ... You should be fine .....
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Old 5th January 2010, 01:02 AM   #4
stwhite is offline stwhite  United States
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Default Thanks guys

Thanks for the info guys...I was hoping that would be the case. I built my first amps ( 2 Dynaco ST 150's bridged as monos and a Dynaco ST 70 tube amp... which are all used semi regularly), about 18 years ago. Looking forward to see what the chip amp will end up sounding like. I think my little B&W's might be efficient enough. I know my Maggie IIC's will not. Again thanks for the help.

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Old 4th August 2012, 09:10 AM   #5
Vostro is offline Vostro  South Africa
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I have a Velleman K8084 Preamp kit,
Id like to know if it would work properly with 9V-0-9V AC instead of 12V-0-12V AC,
Lets say max output of preamp should be 1Vrms.

Documents show the preamp regulates supply to about +/-8.4V,
Will I get away with a lower voltage transformer, if not what changes do you advice.
Thinking of replacing R8,R11 with wire links and lowering R7,R10 to 300R,
and using 1000u as filtering, as I already have some small 16V and 25V 1000u.


Last edited by Vostro; 4th August 2012 at 09:24 AM.
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Old 4th August 2012, 11:29 AM   #6
Mooly is offline Mooly  United Kingdom
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I would see what the actual voltages are first before doing anything. A 9v tranny will probably give a little more when lightly loaded anyway.

You could reduce all the resistors a little say to 560 or 680 ohm. Its all very uncritical. The transistors should be high gain to make the most of low bias currents.
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Old 4th August 2012, 12:30 PM   #7
Vostro is offline Vostro  South Africa
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Its a 9-18V 15VA 265724D504E266C753D656E266D6D3D6D61746368616C6C267 06D3D5E5C647B337D5B5C732D2F255C2E5D5C647B332C347D2 426706F3D313426736E3D592673743D52535F53544F434B5F4 E554D424552267573743D3232332D373736352677633D4E4F4 E4526
I was planning to use the two windings in series to make 18Vrms giving approx 24Vdc,
Its to power my mains 24V relay and two 12V speaker protect relays in series,
With 5V regulator supplying microprocessor.

Thats why Im wodering if I could use the same transformer to power the preamp,

My project is still In planning so Ive not built the protect board yet.
So Ive not actually measured what voltage Ill get from Transformer.

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Old 4th August 2012, 02:41 PM   #8
sreten is offline sreten  United Kingdom
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Lookong at the supply it appears you only need to reduce the voltage
value of the Zeners to get the regulators to work properly with a
lower AC input voltage. Seems 6.8V zeners would be about right.

The op-amps work down to +/- 3.5V apparently, should be fine.

rgds, sreten.
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