Phono II advice Please!

[Lyndon Johnson]

I downloaded to general Mundorf data sheet and as for as I can make out it is 2.4 Ohms (at 100Hz) for the nearest equivalent.

Vishays ordered - I'll report back on how they sound.

Lyndon

[Lyndon Johnson]

Here's what I found...
Firstly I decided my KLP1 was letting things down too much. I had noticed a rather 'closed in' sound since reinstating the CF circuit. I put this down to the additional components now in the signal path. So I replaced R9 with Audio Note Tantalum non-magnetic (small improvement)and C2 with Hovland Musicaps (huge improvement).
Happy my system was back on-song I fitted the Vishay low ESRs to the PSU2 and yes, I felt I could easily hear a most welcome 'silkiness' to the presentation. Definitely a worthwhile move. After a few days listening I then fitted the 30H choke. I can't say I heard a significant difference here. Maybe a bit more emphasis in the upper registers, but that could have been my imagination.

Before I move on from the PSU I would appreciate anyone's thoughts on whether there is any more upgrade scope. I have heard talk of fast recovery diodes for example....
Lyndon

[bob orbell]

Yes, Shottcky diodes are the chosen best, but I would only fit them to the HT side, bypass each diode with a small capacitor 400v 10nF polyester, this is best done on the underside of the board, if you fit the new diodes your HT voltage will rise a small amount. I think the reason for the new choke not having much influence on the sound (in your case)(and mine) is because a choke needs to draw a minimum load for it to work correctly, if this currant is not achieved, then the choke reverts to pulse charging, I found no improvement so returned to the original that was most probably chosen to work with one or two units. Bob

[Lyndon Johnson]

Thanks Bob. I presume when you say 'only the HT side' you just mean to leave alone the rectifiers that belong to the DC heater supply?
Lyndon.

[bikerhifinut]

I might just try the bypass caps first before swapping the diodes for fast recovery types, they are supposed to damp down the switching spikes that can cause noise on the HT supply and bypass capacitors on their own might be enough.
I wouldn't use Polyesters, nothing wrong with that but if I was buying a set then i'd use ceramic jobs, and I'd also use slightly higher working voltage jobs say 630 or even 1kV dc WKG, remembering that there will be an AC element across the capacitors. ceramics because they usually are physically smaller so take up less space. They aren't passing any audio and are simply there to damp the spikes that can inject noise into the supply rails.
As to using Schottky barrier diodes, sure why not but just check the switching speed of a rectifier diode when shopping for them as some aren't always described as Schottkies. You are looking for fast recovery times measured in uS and also look for a description of the diode as a "Soft recovery" device, this indicates there isn't a sharp cutoff that can cause the switching spike.
I stuck with the UF (stands for ultrafast I think) 5408 in my KT88 amp and just did the bypass capacitor trick which I think took the noise floor a wee bit, this basically is what you are trying to achieve to get a clean supply and no noise on the audio. Those UF diodes are pretty good and streets ahead of the older silicon rects in their old 1N**** equivalents.
I don't think there will be any meaningful increase in HT voltage if you do go to Schottky or other diodes with a lower Vf as the difference will be in the region of a volt or 2 which is about as good as saying no difference at Valve HT voltages.
Bob has a point about chokes and current, but the only real way to see whats going on is to get a scope on it and you'll see if theres any beneficial effect. I personally think fiddling around with different chokes is a bit hit and miss and If you're really intent on cleaning up your HT then the effective fix is a regulated HT PSU.
I think Bob and I see eye to eye on this if nothing else.
If you haven't got a copy its worth laying your hands on Morgan Jones' valve amplifiers book, I don't know what edition its at by now but a second hand 2nd or third edition might be found for cheap? It's got a very good chapter on PSU design and theory, well worth the read even if some of the maths gets a bit tricky.

Oh and Bob does mean leave the DC heater supply as it is, it's debateable how much benefit can be got from DC supplies once you are past the line stage signal level, routing and care in heater wiring and referencing to ground takes care of a lot if not all hum issues. Of course there are good reasons to use DC in Phono stages especially if you are using valves that may not have quite as good heater to cathode insulation etc as Valves expressly designed for Audio.

Andy.

[bob orbell]

As always Andy wrights a better picture than me , but I would not recommend ceramic capacitors, in M Jones book he gives these a big no no, dam good book though. Bob

[Lyndon Johnson]

Thanks both of you. I will try the capacitors (polyester 630vDC) with my existing diodes and let you know what I can hear.
Lyndon.

[bikerhifinut]

It may also be worth putting a small value resistor in series with the diodes as this will help form a low pass filter as I understand it. Apparently Schottkies and similar really fast diodes can shut off so fast that they can show parasitic oscillations up as high as 50kHz and the addition of a series resistor can alleviate this. My regulated HT psu has 10 ohm resistors in series with each HT diode in the bridge and I wondered what that was all about, now I know.
A.

[bob orbell]

Andy, you should put the resistor, capacitor and diode all in parallel to alleviate switching spikes, that is what is in M Jones book, and he say's 1meg 2watt. Bob

[John Caswell]

Hi all,
Ceramic capacitors are ideal to add across elcos as they have low ESR, low self inductance, and high losses, hence their use in FM and TV RF sections for decoupling HT lines, G2 supplies and cathode bias resistors.
I suspect MJ in his book doesn't recommend them as coupling caps, and quite rightly so, as that is not what they were designed for!
Go ahead and use them as extra bypass caps on elcos they will help with noise on HT lines.

John