Phono II - 3.18uS mod

Hi all !

After building my WAD Phono II and accompanying PSU II towards the end of last year I have finally got round to doing the 3.18uS mod mentioned on the forum in the WAD FAQ section (http://www.world-designs.co.uk/forum...ead.php?t=145). I found the mod more than subtle - the effect was to open to sound stage and also focus the treble.

I used an EAR 384P previously and although happy with my Phono II felt that it lacked something when compared to the EAR - the mod has definitely tipped the balance!

I also changed the electorlyic caps for Blackgates but this had far less effect that the 3.18uS RIAA time constant mod - a great deal at 36p for the resistors from Maplins!

Hope this will encourage other to do the mod.

Steve

[pre65]

Hi Steve-i see you have bettered the 834P,were you aware of the following guide to modding it http://www.goodsoundclub.com/Forums/...px?PostID=2052

Philip

[Paul Barker]

Mark sold me his throsten moded 834 and it was not a lot better than standard. Neither are a patch on a wad phono stage.

YMMV.

[Greg.]

Yep, I agree. The EAR can't even light it's candle let alone hold it up to the WAD Phono II. In comparison, the EAR left me completely underwhelmed, and the one I listened to was at the top of the range.

Best wishes,

Greg

[Paul Barker]

otoh if you have never heard another valve phono stage or against a Muscical Fefility valve phono stage the ear sounds good value for money.

It's another one of those things like the rb300 rb250 scenario where the world seems to rate it and it's fairly priced, so you buy it and are very happy with it until someone with a better one lets you hear it, then you take advantage of the ignorance of the market place and sell it almost for what you payed because the world rates it, and buy a wad phono II kit with the money and a secondhand unipivot arm off ePay.

[John Caswell]

Hi all,
If you are doing this mod it is worth checking the 200pF cap associated as I have found that this will affect the sq wave respose seriously and therefore the RIAA curve.
I finished up in the end with a 180pF//50pF trimmer and adjusted for best sq wave response.
Yes I know a lot of you out there don't believe in measurements, 'fraid I do, and this is one area it pays off. I use Hagerman and Reg Williamson reverse RIAA filters, both give very similar results

John aka Dr John

[NealG]

John, can you talk me through this? I don't have the reverse RIAA filter but can generate square waves on the PC and apply reverse RIAA there.

[NickG]

You may be limited by the upper frequency you can get out of a PC.

I still have the resistors needed to build a inverse RIAA, you just need to get 4 caps to build the Hagerman, let me know if you need the R's

http://www.hagtech.com/pdf/riaa.pdf

[John Caswell]

Hi all,
The best way I found to do it is thus:
Remove stepup tx if fitted and wire PCB direct to I/P socket leaving the loading R (10K?) in place. Using a reverse RIAA filter on the input inject a 1Khz sq wave, view o/p on scope (set to DC coupling) and adjust i/p to give you about 1V p-p o/p. If the RIAA filter is correct you should see a nice clean sq wave with slight rounding on the leading edge.
I found that the 220pf C (C 20/21) gave wildly differing results depending on type, as did the following valve depending on make, so decided to do an AOT (Adjust On Test).
I removed C20/21 and put a couple of PCB pins in their places and then across these in parallel I wired a 180pF Cap and a 50 pF trimmer and with the sq wave input adjusted the trimmer for best sq wave with minimum rounding or overshoot/ringing. So far so good!
Then using Morgan Jones info from his book I made a Zobel network for the input tx, refitted the tx, and twinced the C in the zobel network once again for best sq wave response.
Obviously you will have to reduce input level further when you use a stepup tx to avoid core saturation and depending on the tx you may get some LF tilt on the sq wave. I found the original 10:1 Sowter 8055s very good with little tilt but with the 20:1 Lundahls it was more pronounced, but not noticeable audibly. I may replace them with the Sowter 20:1 9575 variety

Thats about it really. All questions will be answered here if possible

John aka Dr John

[NealG]

John many thanks for the explanation. I have tried the MJ correction previously for the Lundahls and achieved a 'perfect' SW response. It sounded worse! Dynamics where sat on and it generally sounded 'dry' so I ripped it out. But I'll try C20/21 adjustment.

Nick, that's kind of you, I'll PM my address to you. Many thanks.