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  #1  
Old 4th January 2011, 07:46 PM
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David Roberts David Roberts is offline
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Default 3 Series smoothing

Out of curiosity, and apropos of nothing in particular, does anyone know if there are specific reasons for dividing the HT smoothing over separate stages?

The PSU itself provides 100uf of smoothing after L1, then both Pre and Phono 3 contain a further 100uf in their respective enclosures. Are there any advantages to this arrangement as opposed to, say, just having 200uf in the PSU?
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Old 4th January 2011, 09:19 PM
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Default Re: 3 Series smoothing

There may be reason of minimum impedance (of the umbilical) by putting the cap in the Pre/Phono boxes to stop oscillation in the power supply. This would be motorboating (LF) but there is also another interesting advantage.

The filter factors from each filter stage multiply.
The voltage drop usually limits the resistor that can be used but if you use it in several smaller amounts rather than one large hit you can get more filtration;

Example,

1K series resistor with 100uF shunt cap at 100Hz gives filter factor of 16/(16+1000) = x0.016 (16 is cap reactance at 100Hz)
put in another section the same gives 0.016 x 0.016 = x0.000256 filter factor

Now use the same total resistance 2K and total cap 200uF in one filter, gives 8/(8+2000) = x0.004 filter factor (8 is cap reactance at 100Hz)

How the factor is applied is simply that if you had 2V ripple, in the first case the 2 stages would reduce it to 0.0005V, and in the second 0.008V, so 16x better.
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Old 5th January 2011, 12:05 AM
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Default Re: 3 Series smoothing

Richard, do you think splitting the smoothing also helps stop any HT modulation caused by one stage "polluting" other stages running from the same supply?

Phil.
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Old 5th January 2011, 12:41 AM
John Caswell John Caswell is offline
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Default Re: 3 Series smoothing

Yes,
It is called "common impedance coupling". In all stages of amplification there are slight variations in current flow and if these all travel down the same wire then the current flow through one stage can and will affect the current flow through another stage varying the gain and often giving rise to "motorboating"
It can be problematic/serious in SS amps if the input current flow and the output current flow are in the same piece of wire.
It doesn't often happen in audio, but at RF freqs it can be very difficult to remove so careful earthing and stage gain adjustment often effect a cure.

John
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Old 5th January 2011, 09:52 AM
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Default Re: 3 Series smoothing

Some users of Phono2 reported motorboating and John fixed it for Phono3 by moving cap C17, below, to the Phono case, separated by 1K at R35. This proved enough of a fix for that problem.

Interstage decoupling was taken a stage further with Phono3S when R29/30 1K was added (not sure if John did that too). The signal will take the path of least impedance. The problem is that cap reactance (AC resistance) rises with lowering frequency;

10uF 20Hz 800 ohms
10uF 2Hz 8000 ohms
100uF 20Hz 80 ohms
100uF 2Hz 800 ohms

So we can see what effect the 1K of R29/30 might have. There may be a case for increasing C13/14 which may have been set with 20Hz in mind, I don't know.

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Old 5th January 2011, 01:57 PM
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Default Re: 3 Series smoothing

Quote:
Originally Posted by Richard View Post
There may be a case for increasing C13/14 which may have been set with 20Hz in mind, I don't know.
LF roll-off/warp filtering possibly? Maybe. Or summat, just an idea

DTB
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Old 5th January 2011, 03:07 PM
John Caswell John Caswell is offline
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Default Re: 3 Series smoothing

Hi all,
Yes I added quite a lot of extra smoothing/decoupling both to my Phono II and Phono III see attached drawings. The values are different due to differing HT voltages and were calculated to give the WD figures on the various anodes. Although the drawings state Phono II they equally well apply to Phono III as HT wise the circuit is to all intents and purposes identicl
If you decide to do this bear in mind that tracks will need to be cut and the extra components need to be mounted somewhere.
I also did away with first stage bias and fitted NiCd battery bias, which I think improved it no end, and also fitted Hi quality metal oxide Rs for the anode load.
But everyone to their own taste.

John
Attached Files
File Type: pdf PhII:IIIDecoupling1.PDF (22.5 KB, 102 views)
File Type: pdf PhII:IIIDecoupling2.PDF (18.3 KB, 57 views)

Last edited by John Caswell; 5th January 2011 at 03:10 PM. Reason: Added info
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Old 5th January 2011, 03:54 PM
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BOONDI BOONDI is offline
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Default Re: 3 Series smoothing

Quote:
Originally Posted by Phil Y View Post
Richard, do you think splitting the smoothing also helps stop any HT modulation caused by one stage "polluting" other stages running from the same supply?

Phil.
Hi Phil, Richard, John, all....

I had this exact problem with a SS design (Shame on me i know:p), but instead of motorboating, I had ringing in the transformer. It was cured by dividing the capacitance up into 3 separate caps and taking several wires from the tx solder tag.

Hmmmm gone to have a look at my PSU3 now......
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  #9  
Old 5th January 2011, 06:49 PM
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Default Re: 3 Series smoothing

Yes,
It is called "common impedance coupling".


Ah good, i'm glad you agree John as that is exactly what I did with my PhonoII.

Phil.
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Old 5th January 2011, 08:15 PM
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Default Re: 3 Series smoothing

Quote:
Originally Posted by Phil Y View Post
Yes,
It is called "common impedance coupling".


Ah good, i'm glad you agree John as that is exactly what I did with my PhonoII.

Phil.
Hi Phil, John, all......

How difficult was it to do the mods and get everything in the case? Is there room enough for physically larger caps. I only ask as I have 2 PSU3 cases waiting on components and solder. I will try to incorporated this thinking into the "improved" PSU design that I am planning. It will be valve rectified, and as suggested by Dr John, I will use EZ81. I was going to choke load it, but I don't know if I can get the volts back up to 250V needed for the Pre3, phono3. I can cap load it and get about 280V at the first cap. Then I can throw some away......

Which mod sounds best though? The one that is for 250V or the one for 280V......
What does the panel think????

My Tx gives 250 - 0 - 250V @ 50 mA per side, so I can get 250V or 280V with cap loading. Choke loading is a different matter though.....
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Last edited by BOONDI; 5th January 2011 at 08:24 PM. Reason: Question needing asked?????
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