Phono 3 noise issues

[bob orbell]

Yes, I heard a lot about that Philip, but it was all about your mains input voltage, mine is always very high and never had that problem, also I think some transformers were not well made giving rise to a low LT voltage and consequently the regulator dropping out. Bob

[Damien]

Well guys I've just spent an hour writing an update with the details & whilst trying to add the pictures its disappeared.

Any suggestions?

Damien

[Richard]

That's annoying and happens on some sites. Best to type it up in any word processor and then copy and paste it in so you don't lose it. There is a time limit and it has just logged you out. If you are still online when it loses it don't close the browser but follow the instructions on screen and log in again and you will often find it continues.

[Damien]

Thanks Richard

I'd just come to the conclusion I'd create something in word & then copy & paste into a reply.

Is there a restriction on the number of characters as I did go on a bit?

Regards

Damien

[Richard]

5000 characters from memory and 2 pictures/pdf per post. I usually just try and paste the lot and if it's over it will give you the character count so select a smaller amount to paste then paste the rest into another post straight after which will also give you 4 pics in total etc. Select go advanced in the message box for more options or go User CP top left Home page > settings and options > edit options and set message editor interface to Enhanced Interface then SAVE

[Damien]

Hi all.

I thought it may of interest to give an update on progress & recap on the background for those who are interested.

I'd just like to point out at this stage that I'm not a Design Engineer but have a basic understanding of electronics & work with a colleague who is & who shares my passion for HIFI.

I would also like to say that this is not in any way intended to be a criticism of WD products, but merely an overview of how we solved a problem I was having with my particular unit.

I inherited what I thought was a PSU III & Phono III back in November & being a bit of a vinyl junkie decided to give it a try.

It sounded lovely but was quite noisy & I had some disturbing deviations on the bass cones of my speakers even with no signal at the input.

To put this into context the hiss came in at about 35% volume & the low frequency movement which was quite random was there even at low volume settings.

So knowing nothing about valves I contacted the Forum for advice.

Bob kindly clarified what I had & Greg sent me the build instructions to allow me to get started on finding the issue.

So I have a PSU II & Phono II in III cases with the 10K resistor & 100uF capacitor at the HT input of the phono stage.

I started by checking the design voltages from the build instructions & after a false start with my meter everything checked out as intended, albeit at the high end of the range.

The incoming supply at this point measured 240VAC.

I decided to replace the electrolytic capacitors & re touch the solder joints in both units as some looked dubious but still no improvement.

I also tidied the wiring up a bit.

So having made no impact I decided to take them into work & get the scope on them.

We started at the output of the phono stage & worked backwards to the PSU where there were some worrying dips in the supply rail which seemed quite random & were around 2 – 3 Volts.

These dips were getting into the signal path of the phono stage & being amplified causing the movement in the speaker cones.

So we basically came up with a simple Zener current limit circuit which would regulate the HT supply rail & avoid variations in the mains supply getting through.

We checked the voltage either side of R6 which is the 15K resistor at the HT output. This was 340V & 280V respectively.



So the idea was to drop the output voltage from 280V & regulate it at 250V with a suitably rated Zener Diode.

The Vishay BZT03 series was selected capable of handling 3W.

We checked the additional demand on the 15K resistor which would now be dissipating 0.54W rather than the original 0.25W but still well within its capabilities.

The current in the Zener Diode calculated at 6mA so would be dissipating 1.5W again well within the specification of the device.

We had a 220V version in stock BZT03C220 so popped it in to prove the theory & result. The HT output was now regulating nicely at 250V DC.

You may ask why the voltage was 250V using a 220V Zener but I believe they are what’s called soggy & tend to operate above the rated voltage in use. Don't ask me to explain that though

The beauty of the mod is there’s an unused position on the board for C4 which allows you to drop the Zener in place.

The output was still a bit noisy so I've ordered some 150uF capacitors to replace the original 100uF at C1 & C2 & will also replace the same device at the HT input to the phono stage.

I'll update once they arrive.

The hiss was cured by replacing the input valves & I now have a minor mains hum but only at volumes high enough to not really be a problem. I've re soldered all the connections & tightened the nut on the chassis but no improvement.

I've attached a photograph of the PSU with the component fitted along with a picture of the amp in case anyone can spot an issue which might be causing the hum.

I had a few question myself but I'm over on characters so will send a separate post.


Best Regards

Damien

[Damien]

Hi All

Further to my earlier post this is the remainder I couldn't include: -

Several people have suggested that the PSU III & Phono III incorporated improvements & Richard has kindly posted the details for comparison.

A few people have suggested I should build up a new PCB base on the upgraded designs.

Having had a look I have a couple of questions which someone may be able to help with.

PSU Upgrade

It seems that the only difference is the regulator which instead of creating 6.2V gives 6.7V.

Is this just to cope with low incoming mains voltage?

There has also been a suggestion that the upgrade removed noise but don't understand how?

Phono Upgrade

The changes appear to be :-


1. the addition of switch-able cartridge loading. Mines fine at 47K.

2. The addition of a variable capacitor at C9a/C10a which I believe just allows correction of the slight error in one of the RIAA time constants due to original the use of nominal values.

3. The addition of R31/R32 & a capacitor at C15/C16 to add some decoupling to the anode at the final stage.

I might be able to add the RC network to my existing board if people think it would help?

Other than that the changes don't really seem significant enough to invest in a complete new build but I'd welcome any feedback about other improvements. Especially with regard to improving the ground structure.

Best Regards

Damien

[John Caswell]

Hi all,
Changing the regulator to an LT/LM1084 was my decision as we were getting this strange 120Hz sort of noise in some phono stages which was due to the regulator "dropping out" of regulation on low mains voltages. As the regulator was a standard 3 pin type, I looked around for a suitable "Low Drop Out" type and the LM1084 fitted the bill perfectly, hence the change although it did mean extra resistors to set the voltage. I note that yours is a bit high so you may want to change the lower resistor a little to reduce it - to be measured on the ECC83 valve heater pins please.
The large decoupling capacitor now on the HT line in Phono2/3 was once again mine to stop common impedance HT coupling due to longer cables between PSU and Phono.
The variable capacitor in the equalisation was again my idea, as I had measured a good many phonos and found some to sound a bit "dim". This was due to the input capacitance of the cathode follower output valve. Fitting this capacitor allowed some control of square wave response measured via an inverse RIAA filter.
With the extra HT decoupling you, shouldn't really need anything on the anode of the final stage, but belt and braces won't harm. I would suggest a 1K - 2K2 and a 10-22µF capacitor.
All theses changes help as do fitting high quality resistors to the anodes and cathodes of the first and second stages to reduce "Johnson" and "Shot" noise.
My own Phono3 has bulk foil resistors (deep pockets required) fitted here and noise is non existent at well above normal listening levels.
Contrary to what Bob Orbell believes, regulated PSU are not the be all and end all, sorting why you have the problem is always the first step, do not let "The tail wag the dog" so to speak.

John

[Damien]

Hi John

Many thanks for the explanation.

I'll certainly looking at fitting some higher quality resistors on the first & second stages but might not be able to stretch to bulk foil

Best Regards

Damien

[bikerhifinut]

Damien,
I take it you no longer need my spare PSU3 PCB?
No problem as it's given me a bit of impetus to rebuild my PSU3 from scratch and see what difference that makes.
I've a few high quality 100uF x 450V F & T capacitors that look destined for it.

The regulated PSU board won't go wrong as I have a spare toroid from a different project that will be pressed into service to power other preamp/ low current projects. That'll be interesting as the heater winding is 6.3V but a word with it's previous owner suggests that I should be able to put an extra overwinding on the toroid to get the volts up enough to regulate the heater DC, I've a drawer full of LDO regs.

As previously noted I have gone over to the dark side of late with some silicon op amp based MM circuits that have surprised me beyond all expectations, but real care was taken with the EQ which although debateable as to how much difference we can actually hear, given that even the most expensive cartridge/arm combinations can give quite high distortion figures, I figure you still have to get everything as "right" as possible in order to keep things in the ball park.

I think I should also have a good butchers at the PCB on my Phono3, I didn't assemble the board, but i did have to sort out some howlers on the earthing that took me ages to track down, there's a thread here about it in the distant past.
John C's words have given me food for thought about the high level of noise and hiss on my example. Theres visible corrosion or perhaps its a crusty flux deposit around the valve socket pins, but it makes me think a reflowing of the soldered joints wouldn't go amiss and perhaps check out the resistors on V1 too.
Must check out bulk foil prices, Morgan Jones also advocates them in noise sensitive areas of valve amps.

Andy.