A circuit diagram from PCB layout and components

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Hi all,

I was wandering if anyone is up for a challenge :). Here is a picture of a preamp power supply with the exception of the transformer itself: http://www.edmundsworld.net/Edmunds_World/Preamp_Rectifier_Circuit.html.

I cannot turn it around to see the bottom, since that would be too much violence. I have a basic understanding of power supplies, so I can understand the rectifier, the cap on top of it and the big cap filter thing, but I'm not experienced enough to figure out what the surface mounted stuff is and how it all works together. If anyone feels he/she can come up with the suspected circuit design, that would allow me to understand what and how to modify (I want to connect another transformer or a complete power supply), this would be great learning.

Thank you for your time,

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Can't open the link... often happens. Try adding an image like this,

Too add a photo,
First click "go advanced" in the box below the "quick reply" message box. Doesn't matter if you decide half way through a message to do that, it carries it foward.

Then click "Manage attachements"
Click browse in the first box at the top and find your picture. Repeat for any more pictures.
Click upload... a message appears "uploading"
When complete, scroll down to the bottom of page and click "close this window"

The pictures should now be attached and when you post will appear. I don't think they show in message preview... they never used to anyway.

Make sure your pics aren't too big, a couple of 100k is plenty, and many object when they are massive and it alters the margins
It tells you in the attachments window what max sizes are allowed.
Joined 2007
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Can see it now...

Well the small sm caps all look to be just decoupling and bypass. The four round the bridge means one cap is across each diode in the bridge for RF suppression. Bit hard to make out exactly where the other go but I think one will be across that large electroylitic, and the other perhaps across that diode or from one end to ground... but they are all just extra bypass caps.
Joined 2007
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You say you want to mod it or add an external PSU.

Can't tell from the piccy whether it is a single or dual rail supply. What does that diode on it's own feed ?

If there is only that one large cap and only two wires from the transformer then it's probably single rail.

Might help to know the make and model... ultimately though any alterations have to be done carefully or you could end up with inferior performance.

You can work backwards from the main rail voltage as the DC voltage across the main cap is 1.414*the AC voltage applied to the bridge.

Thank you for your reply.
What are "decoupling and bypass"? :)

It is a single rail - separate power for the preamp. I don't know if that comes in two rails at all, but this certainly isn't. I have included all the parts that participate in that party in the picture, I think, but I can do a full picture of the amp and psus as well, if you think that would help. It is Logitech z5500.

I'm a but confused about the voltage. Would you expect a preamp to run on something like 18-20VDC? Isn't it more likely to be 12VDC? Can the single diode be used to reduce the voltage maybe? Other than that, I have no good explanation for the diode either.

I will do some more testing and multimeter work tomorrow night :).

Joined 2007
Paid Member
18 to 20 volts sounds reasonable... 12 volts would allow only limited output (no headroom)... but it could be though. The preamp will almost certainly have a regulator following the bridge and reservoir cap.

Decoupling/bypass refers usually to small value caps placed across power supplies to help maintain a low impedance and so minimise high frequency noise on the rails. The four caps over the bridge eliminate commutation noise as the diodes come into and out of conduction... this can cause a burst of hf energy that might find it's way into sensitive circuitry.

The other diode may produce a secondary rail... impossible to tell from picture. Perhaps used for a quick mute function on power up/down.
I differentiate between decoupling and bypass.

I interpret decoupling as the fast source for transient current demanded by the circuit. The little XnF caps across the supply pins of an opamp and the bigger YnF caps across the output transistors of a power amp.

I see bypass as the cap (or other component) to pass signal from source to receiver better than what is there already. This could be a plastic film and foil cap passing VHF in parallel to an electrolytic cap that does not pass >100kHz very well.

Overhead for signals passing through the sound system.
Let's start with the signal required to drive the Power amplifier to maximum output power. For this example, we will assume this is 1Vac (= 1.414Vpk = 2.828Vpp).
The source must be able to pass at least 1Vac to be able to let the Power Amplifier send maximum Power to the speaker if the music signal demands that transient peak.

I see two reasons for applying an overhead margin in the source.

1.) The output of the source is often a single ended (SE) ClassA stage. As output signal approaches the maximum signal level of that stage the distortion increases. I think that applying an overhead of +6dB to +12dB in the signal capability ensures that the SE stage stays well clear of this increasing distortion output level. This requires a maximum output capability for an SE stage of ~2 to 4Vac for the hypothetical Power Amp.

2.) interference spikes superimposed on the signal can sometimes be large enough to clip in the source. These spikes then tend to become more squarewave with many HF harmonics. The subsequent RF filters can attenuate the spike and it's harmonics but can never eliminate them.
I believe that passing the interference spike unclipped to the input filter of the next stage makes for less audible interference.
I further believe that this ability to pass interference without clipping requires at least +10dB of voltage overhead and more the better. +20dB of overhead may still show some benefit.
This requires the output capability of the source to be at least 3Vac and preferably approaching 10Vac.

Now we look at the supply rails necessary to achieve these potential outputs.
Our 1Vac is equivalent to 2.83Vpp. This will require supply rail of ~4Vdc to pass that 1Vac signal.
2Vac requires ~8Vdc
3Vac requires ~12Vdc
4Vac requires ~15Vdc
10Vac requires ~30Vdc.

All of theses supply voltages can be handled by opamps (ClassAB output stages) and by low voltage high performance discrete semiconductors.
It costs little extra to provide a 30Vdc supply in lieu of a 4Vdc supply.

I think it is good value for money to aim for +20dB of voltage overhead in source equipment.

A 1Vac maximum input signal Power Amp can use a +-15Vdc powered pre-amp and retain good value for the money spent in achieving that +20dB of voltage overhead.

In closing, I have been ridiculed in this Forum for suggesting that a +20dB voltage overhead be considered on the grounds of potential sound quality improvement. Find other recommendations and try them. Then decide.
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