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Old 23rd August 2004, 11:43 AM   #1
MikeW is offline MikeW  United States
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Default Cap across rails?

Does anyone put caps across the +/- rails? Say 70 volt 30,000 uf.cap across +/- 25 volt rails.
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Old 23rd August 2004, 01:17 PM   #2
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As long as the cap is rated for the voltage, you'll be okay. Ground can be determined by a resistive divider as long as you're not drawing a lot of current and it's evenly distributed across both + and - rails.
This trick is best when you've got some "regular" filter caps in the mix--meaning the kind that meet in the middle at ground. Take your ground contacts from there and use the rail-spanning caps as an added bonus.

Grey
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Old 23rd August 2004, 02:32 PM   #3
eLarson is offline eLarson  United States
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Why would I want to put a cap across the rails?
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Old 23rd August 2004, 02:37 PM   #4
MikeW is offline MikeW  United States
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To reduce the ripple. I only had one big cap, not one for each rail.
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Old 23rd August 2004, 02:40 PM   #5
Prune is offline Prune  Canada
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A capacitor across the rails will only reduce common mode noise.
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Old 23rd August 2004, 11:21 PM   #6
byteboy is offline byteboy  Netherlands
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I was thinking to do the same for my AlephX with 16-18 Volts rails.

If you already have or could buy them at a voltage of minimum twice the rail voltage, (also often there is little or no price difference between caps of say eg. 25 Volts or 40 Volts) you could even use them IMO with an advantage?

Say you have 2 caps of 40 Volts and C uF, Urail = V.

If both capacitors are connected across both the rails, the 40 Volts caps would get charged at double the rail voltage V, thus doubling the charge (Q = U x C):

Qtotal = 2 x V x 2 x C= 4VC

This charge would have to be shared by the two rails, so has to be divided by 2 to get the charge available "per rail":

Qrail = 4VC / 2 = 2VC

as opposed to Qrail = V x C= VC, with one capacitor per rail to ground?

Is this correct? Any drawbacks?

Further, would a center tap (full wave rectifier with 2 diodes for each supply) on the transformer be enough to use as a 0 Volt / ground?

Or maybe use two extra lower capacitance (cheaper) caps from rail to center tap / 0Volt/ground to take care of the (small-) currents flowing thru gound in the AlephX?
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Old 24th August 2004, 01:23 AM   #7
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I believe you'll find that there's a virtual ground in the middle of the cap. In other words, each rail sees half of the capacitance. This is the same principle that you see in so-called flying EQs in balanced phono stages, where the cap values have to be adjusted (doubled) in order to achieve the proper RIAA curve.
Still, it will work as energy storage and if you've already got the caps on hand, why not put them to use? Just be sure to lock ground down with resistors or the usual two-caps-and-ground or it might tend to wander a bit.

Grey
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Old 24th August 2004, 01:46 AM   #8
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Default Re: Cap across rails?

Quote:
Originally posted by MikeW
Does anyone put caps across the +/- rails? Say 70 volt 30,000 uf.cap across +/- 25 volt rails.
Mike,
I am using this technique in the first stage of my amplifier. I used it to help isolate ground noise. And save a little chassis space. This is a dual rail supply so to me it should be the same as two caps used at twice the value.
Since the two stages of capacitance following are conventional the ground does not need a resistor pair to establish a psuedo ground.
Works for me.

George
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Old 24th August 2004, 02:22 AM   #9
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I would only do it for a bridged amp. With a non bridged amp and the neg speaker terminal connected to ground, the single rail to rail caps has no direct return connection to the neg speaker term and contributes nothing to driving the load.
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Old 24th August 2004, 07:04 AM   #10
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Hello.
I allwais was interested in this aproach of PA: single supply which make one secondary and one bridge. If necesery to get the half of supply on Res/caps divider. I did a few experiments breaking the middle zero going from PSU to amp itself and discovered that the only one design which run stabilly on such a condition is Aleph-X.

I aggree with Brian that in no bridged amps single supplt is point less.

When I started to analise the X, I figured that after making some alterations it schould run on single supply. I disconnected the PS middle zero from the amp's ground, disconnected R1,4,44,45 from ground , leaving them together acting as an 62 Ohm resistor hanging paralelly to the speaker, mooved the bottom end of R17 to negative rail (R17=4,7kOhm). With the R19 & R29 ends, that were previously grounded I made a node of virtual zero, linking this node to positive and negative rails with 220 Ohm/2W resistors paralleled with WIMA 220nF MKP10 caps. The input GND connect to this node too. After a while of hesitation I swiched this on and. . . it worked without smoke at all

Now I was curious how this modification influences the common and diferential DC offsets. Having crocodile clips wires I connected back the grounds returning to oryginal design while monitoring ofsets voltages: No changes at all!

Then arrived time for listening test for this mod. The audience could hardly find any diference, pointing that with floating ground the base is a little more open, free or bigger than comparing to original circuit on which the base sounds more tight.

Piotr
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