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Old 3rd January 2010, 07:23 PM   #11
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I'd recommend the rectifier and some capacitance in the PSU box, and then some more capacitance and the regulators in the Preamp box.

Jaycee Hi - above is a suggestion you made to me a few weeks ago.

....just one more question :
Can I have rectifiers, caps and regulators in both boxes ?
In other words, can I send rectified, smoothed and regulated dc from the PS box to the separate pre amp box which also has it's own rectification, caps and reg's on board ?

I have a ready built board that can be fitted to the PS box to perform the above tasks and for the correct 15 v ac to dc.
So, the pre amp will receive pre conditioned DC where it will be rectified, smoothed and regulated again - is this OK ?
Will this help with performance or simply make it more safe ?
Many thanks
Andrew
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Old 3rd January 2010, 08:15 PM   #12
AndrewT is offline AndrewT  Scotland
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Originally Posted by jaycee View Post
Another query, is the PSU box just the transformer? OR the transformer and rectifier? Any capacitance in it?

I'd recommend the rectifier and some capacitance in the PSU box, and then some more capacitance and the regulators in the Preamp box.
definitely send partly smoothed DC between the boxes.
Sending raw rectified AC between the boxes is just asking for a complicated hum/buzz problem to be identified and eliminated.

Can you guarantee that mains Live can NEVER get sent down any wire in the umbilical to the remote amp?

If you cannot give that guarantee then you MUST Safety Earth all the exposed conductive parts down stream of the PSU box.
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Old 3rd January 2010, 11:08 PM   #13
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Andrew Hi again !

Thanks for picking this up from the cold borders !
-7 down in Tyne & Wear right now and 5 inches of snow in 24 hours.

The pre made board I mentioned has diodes, caps and regulators and is fit for purpose, works well currently on a buffer and 50 K pot project and is now free as a result of my new creation.
Can I fire 18 v ac into it ( +, 0 , - ) and then via the umbilical send it into an opamp + diamond buffer based pre amp that also has on board diodes, caps and regulators ?
Or....do I instead remove the regulators from the board and take the now smoothed ac straight from caps to pre amp ?

At the moment I have no hum issues as I lifted the mains earth wire of the CD player mains cable - which I guess is not ideal.
I'm confident that my wiring and care taken with the insulation of the live, neutral and earth renders it safe.
Jaycee's suggestion then left me thinking about the additional smoothing.

At the moment mains earth is merely a bolt through the ps case and has nothing connected to it from the pre amp box - it's not a star in any way.
' 0 ' v centre tap of traffo is not connected to it either and instead goes up to the pre amp.

As is, it's the best thing I've done sonically compared to my past efforts and if I can leave it as it is I'd be pleased enough.

Safety first though.

I do find it hard to describe what I'm doing sometimes and probably try to over describe in too much detail - leaving you all even more baffled !
Hope this is not the case with this.

Best Wishes and thanks

Andrew
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Old 4th January 2010, 02:56 AM   #14
jaycee is offline jaycee  United Kingdom
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I'll try and describe it a little clearer. Read the Rod Elliot article I liked to though, it's good advice.

First, in the PSU Box:
We have the transformer, a bridge rectifier (or equivalent diodes), and some capacitance in here. Let's for the sake of argument, say 2x1000uF in here.

Suppose you have an IEC connector for the mains in. The live and neutral would go to the transformer (live via a fuse/switch as required). The earth from the IEC connector should go to a bolt that is fixed into the PSU case. The Rod Elliot article shows the best way to mount this bolt.

Suppose the transformer is a 15-0-15 secondary type. The two 15V taps would connect to your bridge rectifiers AC terminals. The 0V tap would connect to your star ground point - this is not the bolt mentioned earlier, but rather, a common point where all 0V wires should meet. This is typically between the capacitors. The + and - terminals of the rectifier would connect to the capacitors as appropriate.

For your 3 pin DIN plug, you have 3 terminals, and the "screen" terminal. You would use the 3 terminals to connect + and - coming from the capacitors (NOT from the rectifier), and also the 0v line which should connect to the 0V star point (at the capacitors - NOT the rectifier). The screen terminal should connect to the earth bolt.

Then, in your preamp box:
The 3 pins in the DIN plug should connect to a regulator circuit, such as your typical LM317/337 arrangement. In front of these regulator IC's, have some capacitance, say 2x1000uF again. The output of the regulators feeds your preamp circuit.

The signal grounds should connect to the 0V point inside the preamp box. You should have a similar star point as you have in the power supply. The phono sockets should be isolated from the case - at no point should there be any connection between 0V and the case of the preamp.

Finally, the "screen" connection of the DIN socket should connect to an earth bolt inside the preamp case.
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Old 4th January 2010, 06:36 AM   #15
Mooly is online now Mooly  United Kingdom
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I know from years of learning that hum problems were the bugbear of all my early amps etc.
You MUST think in terms of what "happens if a volt drop appears across that piece of conductor/print/wire etc" and will it contaminate or modify the signal in any way.

What many don't realise is that the charging currents of even a 470uf or 1000uf cap in a PSU can be huge, by that I mean the repetitive charging currents "topping off" the caps each half cycle. They can easily generate large (10's of mv) potentials that if they appear between say a feedback node and an input ground get amplified and appear as hum.
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Old 4th January 2010, 10:01 AM   #16
AndrewT is offline AndrewT  Scotland
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Originally Posted by Mooly View Post
What many don't realise is that the charging currents of even a 470uf or 1000uf cap in a PSU can be huge, by that I mean the repetitive charging currents "topping off" the caps each half cycle. They can easily generate large (10's of mv) potentials that if they appear between say a feedback node and an input ground get amplified and appear as hum.
AND THAT IS WHY THE STAR GROUND/AUDIO GROUND SHOULD NEVER BE BETWEEN THE CAPACITORS.
The audio ground can be adjacent to the capacitors if that is convenient, but the capacitors should be connected to the rectifier/s by short twisted pairs and a single wire taken from the capacitor zero volts to the audio ground.
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Old 5th January 2010, 12:55 AM   #17
jaycee is offline jaycee  United Kingdom
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hm ive never had a problem with it between the capacitors to be honest. I do note that most commercial gear seems to star all of the 0v connections to ONE of the capacitor's pins though. Strange.

For a power amp, which is where the big currents flow, ive typically got 4 capacitors on a board - 2 per rail. The rectifier output and transformer centre tap connect at one end of the board. The amps and all other 0v's are connected at the other end. This doesn't seem to hum.
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Old 5th January 2010, 06:19 AM   #18
Mooly is online now Mooly  United Kingdom
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I remember building JLH's 80 watt MOSFET amp many years ago.... when I was just starting out in electronics... and hum was always the problem.
I can remember now what it was like... the amp was silent untill you shorted the inputs and then shorted input grounds as well,
Metal case... din sockets (urghh) earthed to case... and all the earths taken to thick wire between the main caps. I had noted that any separation of wire along this connecting wire made things worse, I just hadn't put the final pieces of the puzzle together. Making that connection thicker altered the hum, and I could see the ripple along the wire with a scope. Even a couple of mm was all it needed for a very noticeable volt drop (ripple) to appear. I think I even used solder braid to reduce the impedance... when all it need was a spur off there and the input sockets isolating.
It was only when D Self's articles appeared all those years ago that I finally began to understand fully the importance of correct wiring and how you can make the effect of wires "dissappear" with correct layouts etc.

Last edited by Mooly; 5th January 2010 at 06:22 AM.
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Old 9th January 2010, 07:15 PM   #19
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Many thanks for the detailed and clear advice.
I've now got the screens of the din plugs/sockets connected to pre amp chassis and power amp chassis / mains earth.

Signal grounds are lifted from chassis ground/mains earth and are instead connected to '0'v now.
It's still quiet and works well.
I could not follow all of your instructions as the traffo is in a box of it's own and the bridge rect diodes, caps and recitifiers are all in the pre amp box.

RCA sockets were already insulated from the chassis so that was ok.

I think the screens of din's to mains earth bolts in both boxes gives me
peace of mind and am still absolutely confident that my insulation of the 15v feeds can not short to anything.

Thank you all very much for your help and patience once again

Andrew
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Old 9th January 2010, 11:56 PM   #20
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Quote:
Originally Posted by AndrewT View Post
AND THAT IS WHY THE STAR GROUND/AUDIO GROUND SHOULD NEVER BE BETWEEN THE CAPACITORS.
The audio ground can be adjacent to the capacitors if that is convenient, but the capacitors should be connected to the rectifier/s by short twisted pairs and a single wire taken from the capacitor zero volts to the audio ground.
From my experience you are 100% correct AndrewT. The star or audio ground should be a real zero voltage point. If you use a star between the caps and the wires to the capacitors are not exactly the same length and diameter and the caps are not eaxactly the same characteristics, the star ground is not a zero at any instance.

Very good observation Andrew, you must have found that out the hard way.

Regards

Nico
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