WAD Phono2 power supply.

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Andrew
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#31 Re: WAD Phono2 power supply.

Post by Andrew »

Shane, the NiCad is one way to avoid degenerative feedback on the cathode of the first valve and thus not impact the equalisation or reduce gain. There's no easy answer to this problem in a general case unless you have access to some Black Gates unobtainium capacitors. I would build it as per circuit then experiment, this is a contentious area, LEDs, other types of diodes, resistors with a quality capacitor in parallel are all accepted approaches.

Interestingly, in this case, Nick has requested a constant current source on that valve and as a result the feedback is often reduced, its a impedance related thing...when you get to this point, you may find a just a resistor is enough. This is how I solved this problem on my version of Thorten's simple phono, use a current source.
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#32 Re: WAD Phono2 power supply.

Post by IslandPink »

I'm going to dig my heels in here because I think it's not worth using good components or a shunt-reg power supply if you're just going to slap a NiCad in the cathode of the 1st stage. Finding a transparent CCS for that location is another question, I'm not 100% sure that's possible ( haven't heard Nick's phono in my own system ) but let's leave that one for now :D
I would suggest calculating the resistor value then finding a suitable pair of these :
http://www.hificollective.co.uk/catalog ... -5289.html
..nose to tail across the resistor. They are lower distortion than most electrolytics and rumour has it you get further transparency gains from the Audio Note-style pairing. I used a pair of these ( 1uF ) for the input to the TDA7297 chip amp, as recommended, and the midrange is very clean. I haven't had chance to test them elsewhere in a critical location but was discussing this re. 300B cathode caps ( sub for BG 'NH' ) , where we could use two pairs of the 50V ones across two resistors.
The value of the caps would depend on the resistor. As a guide I use a 1000uF across a ~100R resistor on my phono.

As Andrew says maybe you can avoid a cap. The output impedance of the valve will increase, maybe it's just a case of decreasing the first resistor it drives for the RIAA ?
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Nick
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#33 Re: WAD Phono2 power supply.

Post by Nick »

I'm going to dig my heels in here because I think it's not worth using good components or a shunt-reg power supply if you're just going to slap a NiCad in the cathode of the 1st stage. Finding a transparent CCS for that location is another question, I'm not 100% sure that's possible ( haven't heard Nick's phono in my own system ) but let's leave that one for now :D
I wasn't suggesting that anyone did anything, I just posted in response to a question a circuit I built 14 years ago.

I would like to think that we have all moved on in that decade and a half.
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shane
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#34 Re: WAD Phono2 power supply.

Post by shane »

Are the changes to the Phono2 circuit itself necessary for use with the shunt psu or are they "standalone" upgrades, for want of a better expression? The Phono2 I have is a fully built up PCB, I'm only building the supply. If that works ok, then I'll think about modding the phono.
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Nick
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#35 Re: WAD Phono2 power supply.

Post by Nick »

Yes, that shunted supply will be fine with the original circuit.
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shane
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#36 Re: WAD Phono2 power supply.

Post by shane »

:D
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#37 Re: WAD Phono2 power supply.

Post by IslandPink »

I wasn't aiming comments 'at' anyone - just against the use of NiCads in the signal path where there's current modulation. Does the phono 2 have the battery ?
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Nick
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#38 Re: WAD Phono2 power supply.

Post by Nick »

No. the original is a unbipassed resistor and a 100k (if I remember) anode load. TBH, looking at that circuit, feels a bit like seeing photos of yourself in the 70's, and wondering "what I was thinking!". Actually, thats a bit unfair to that circuit, its basically ok.
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shane
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#39 Re: WAD Phono2 power supply.

Post by shane »

This is the original:

Image
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shane
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#40 Re: WAD Phono2 power supply.

Post by shane »

Nick wrote: Tue Feb 28, 2017 1:53 pm Two 60va 9v transformers are £38.30 from Rapid, less than a purpose would transformer.
Just a quick one; do I need a 60va rating for this? I can get a couple of 15va toroids which have the advantage of being a bit smaller, but would they be up to it?
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Nick
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#41 Re: WAD Phono2 power supply.

Post by Nick »

Try them, but the heater on its own is close to 1A so its a bit marginal I would have said.
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shane
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#42 Re: WAD Phono2 power supply.

Post by shane »

Thanks Nick. If it's marginal, I'll go for the big ones, I think.
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shane
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#43 Re: WAD Phono2 power supply.

Post by shane »

Just putting together a BOM, and there's a couple of issues.

RHRD4120 and MJE350 appear to be obsolete. Are there equivalents, preferably from Rapid?

Is L78S05CV suitable for the 5v regulator?

Will any old LED suffice across 2N3906 and MJE350?
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Nick
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#44 Re: WAD Phono2 power supply.

Post by Nick »

UF4007 will be fine for the diodes

I dont think MJE350 is in any way obsolete, but I can't see any high voltage PNP's from rapid. Try ebay.

That reg will be fine, and any LED should be ok. I would be tempted to use green ones.
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shane
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#45 Re: WAD Phono2 power supply.

Post by shane »

Thanks for that, Nick. Green for aesthetic or practical reasons?
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