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Old 19th September 2004, 04:55 PM   #1
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Question adding xtra transformer to cdp need help!

hi there, heres my problem my denon dcd 520 has only +/- 5v supplys powering my op amps which i have installed ad8620s as replacements which were an improvement over the originals. the problem is, the ct transformer doesn't put out enough volts to where i can raise the voltage on my op amps to the +/- 12volts i would rather have, to have more choices in op amps. also, the transformer is the pc board soldered in type with lots of terminals and would be hard to replace. i have a preamp for parts which used to be used powering +/- 15v. regulators for its op amps. it also has a small ps board with stealth diode bridge built on it plus dual 2200uf filter caps. i have enough room i could install transformer and board which, also has protection fuses on it in cdp and use maybe lm317/lm337 regulators to get my +/- 12v. my main question is, the ct of transformer should it be connected to original point on the added ps board and also, a wire from it to center tap ground point already in cdp ps board? or how should i go about this? i just want to add these xtra components just to power the op amps with more voltage options by disconnecting original op amp voltage supplys and leave the rest of the player to be powered as usual from the original ps. would be verry greatful for any guidence here. thanks crippledchicken
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Old 19th September 2004, 05:19 PM   #2
OliverD is offline OliverD  Germany
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Ground connection should be made near the output connectors.
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Old 19th September 2004, 05:28 PM   #3
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Default Wire it with the CT tied to ground.

I could not follow exactly what you are asking, but moving a small transformer into the box should be easy if there is room.
Tied the CT to circuit ground. This will keep the rails balanced. The diodes and regs need to be tied to player ground also.
Using the board you already have should speed to building up. I would built a new board myself. Using the 317/337 regs is a good way to go. If you can find some, I like running a small cap on the output of the regs, then a small choke to big caps. Then feed the opamps with this.
I no nothing about the Denon, but am surprised they are running the opamps with +/- 5 volt rails. If the parts are SM, swapping is a pain. The DIP 8 parts allow easy swapping around. I used the 8620 in a Sony 222ES and went back to 2604's, this was with 8.6 volt rails, perfect for the 8620. The 2604 responds better normally to higher voltages.

George
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Old 19th September 2004, 05:31 PM   #4
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hi OliverD, could you be a little more specific on where the center tap goes on the added transformer sorry, this is my first attempt at doing this and i want to be sure to do it right so i don't blow anything. thanks again, crippledchicken
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Old 19th September 2004, 05:49 PM   #5
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hi panelhead, what i'm really needing to know is, if i add the new tranny and the ps board it's connected to now which has bridge rectifiers and two 2200uf caps should i leave ct from tranny where it's at now between the two caps on board i'm adding and run a wire from that point to ground point between the two caps on cdp original ps. the regulators i will have to add but they also should connect to ground point between ps caps on original main board? in other words, everything should ground to that one point between ps caps on original main board? sorry if i'm confusing you this tweak i've never done before. thanks crippledchicken:xeye :
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Old 20th September 2004, 11:56 PM   #6
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If I understand correctly, you have replaced the original +/- 5V op-amps with some better +/- 12V op-amps. However, in order to get the +/- 12V sources, you are going to use a second power transformer with center tap. Sounds fine to me.

BTW the second transformer doesn't HAVE to have a center tap.

You can use a second bridge rectifier and ground the center tap to circuit ground. Use two filter caps for each rail.
Now, if the transformer is centertaped, then you need at least 10VAC from each side of the secondary, or a 20-25VAC centertaped secondary. If it is not, than only 10-12VAC is required. Just make a half wave rectifier. (actually two half wave rectifiers, + and -), like a voltage doubler.

You could use an LM317 and LM337 if you can find a pair, or use a zener diode. If you use a 12.6V zener diode, put a resistor in series with it from 1 rail to ground and you get 12.6V across the diode. Take that voltage into the base of a BJT transistor and connect the collector to the rail, maybe through a current limiting resistor, and the emitter will output exactly 12V. The current needed through the diode resistor to the base is only the output current divided by the beta of the transistor, much smaller. Do this with the other rail, just use opposite polarity transistor. This is a cheaper route than the 317 & 337.
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Old 21st September 2004, 02:02 AM   #7
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hi cunningham, great post and info. i got it done what i did was take the extra tranny which is 18v. from ct to each leg i guess you would consider it a 36v. tranny? i used the same small ps board that was with tranny in a preamp i had for parts. the pc board also had headphone jack and op amp to drive it so i cut that section of the board away making the board even smaller. the pc board has two 2200uf 50v. elna filter caps and i used 4 amp stealth diodes replacing, the 1n4003 diodes. i have some lm78/79 regulators and also some lm317/337s but just to speed things up for now, i just used the 78/79s and installed them close to the ad8620s. i may fix up some of the others when i round up some more resistors and see what the difference is. it sounds much better now at 12v. instead of 5v. they had everything running off of same +/- 5v source protected with icp-n15 ic protectors i tried to add a little more capacitance for the faster op amps and one of the protectors opened. that's when i decided to try and do something with the ps after i replaced the ic protector. now, i guess i'm going to have to add a second switch in order to turn on and off the added parts. the original tranny on main board has primary circuit on when plugged in as well as the one i added. the main power switch i guess just shuts off the secondary to turn it off. i guess i could add a switch to the primary circuit of the tranny i added. should i unplug the cdp when not in use so op amps don't receive power untill i get another switch added? thanks verry much for your help, crippledchicken
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Old 21st September 2004, 10:44 PM   #8
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Does the +/- 5V source turn off when the power button is pressed? If this voltage source is switched on or off with the power switch, then just use a relay where the coil is powered by the +/- 5V switched source and the contacts connect the primary of the new transformer to the line. Use normally open configuration. BTW: you might want to put a small spike dampaning capacitor, or varistor across the relay contacts. When the relay switch is opened, there will be a voltage spike which could arc and weld the relay contacts together.

Or.....Just use a solid state relay.....no clicking.
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Old 22nd September 2004, 02:30 AM   #9
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hi cunningham, i'm not positive but i think the+/- 5v. is controlled by the on-off switch and i will surely check and see if so, that sounds like a good way of going about that problem. why couldn't i have thought of that? darn it he he. the original on-off switch that was used to cotrol the primary on the new installed tranny in the preamp i took it out of has a blue disc cap across it's switch contacts marked 472 i guess would mean 4700pf? would that cap be alright to use on relay? thanks, you've been a great help crippledchicken ps. would i use the +5v and circuit board ground to power relay coil?
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