Rarely Seen WAD, Andy Groves Designed, IC Phonostage

[JohnG]

Sorry Richard for causing confusion, the RCA Resistor sockets were not being used on my Phonostage, they were being used on another Phonostage that was
playing a MC Cartridge, the method caught my attention through another attendee talking about how it was a simple and practical solution. I did not investigate the concept much more on the day, but the method has stuck, even if i'm still to learn the process.
I like the idea of a switchable device, as I can then put it head to head against my recently purchased MM/MC Phonostage, with a variety of cartridges.
As I already have a dedicated MC Stage I can,t see the need to use SUT's, especially when a MC conversion with the WAD, seems achievable.
I,m glad John has a very positive experience, with similar thoughts of my own with this stage.
I have also been supplied a few checks to be looked into, in relation to the Schematic. I am not that savvy, but if anyone would like to view the comments, I would happily paste them onto this thread.

[Richard]

Thanks John, I understand now. Those RCA resistors would be fitted from signal pin to ground shell. That resistor then appears in parallel to the 100R loading resistor inside the amp and lowers the load value. For example a 47R in the RCA and 100R inside would give 32R. Yes please paste the checks and comments

[JohnG]

Below is the supporting mail I received from another individual with a experience of building this Phonostage.
The circuit he attached to the mail has not pasted to this reply unfortunately.

I built my first MC Phono stage using the same circuit having bought the relevant magazine in the ‘90s. I did not see any subsequent articles so was unsure if a pcb was produced; is yours a World Audio one or did you produced it yourself?

I built mine on Vero board and powered it by 12 volt batteries. Testing it revealed two problems. Firstly, the RIAA filter response was way out and I have recently realised that the two filter capacitors shown on the circuit diagram needed reversing; the filter is now quite accurate (some fine tuning of the two resistors is required). I don’t know if any subsequent magazine articles corrected this error. You can check the values using this RIAA Calculator - https://www.kabusa.com/riaa.htm

My circuit is shown below with the corrected RIAA filter configuration –



The second error I had was that when switched on the output would sometimes sit at the + supply rail. An engineer at work thought this was due to the capacitive load on the output of the first IC causing instability. I fitted the AD797 IC as I used the phono amp with a MC cartridge so this device may be more susceptible than the one used in the MM configuration. This was overcome by adding a DC Servo which additionally meant I could remove the output capacitor (a DC servo removes any DC voltage from the output by feeding back a correction signal to the input of the first IC).

One other change I made was to remove the LF roll off capacitor as I was keen to replicate low frequencies (my speakers go down to 30 Hz and as the Lenco produces little very little lower frequency artefacts, speaker flutter has never been an issue).

[JohnG]

A copy of the schematic missing from the previous mail
WAD-Schematic-4b.jpg

[Richard]

Certainly the supplied kits were sometimes revised/corrected from the earlier magazine articles (Phono2 for instance) so compare his circuit to the one you got with the kit John and the magazine.

The quickest way to check yours would be to run a frequency sweep through a reverse RIAA filter which I can do for you or perhaps John Caswell has already swept his and can help here?

[JohnG]

I,ve opened the box up and have had a look at the parts, with a help from a magnifying glass and manipulation of the positioning to a light source , i have discovered my IC's are Plug In Types and IC 1, is a AD743 JN and
IC 2, is a TL071CN.
The C1, is a 100uF Blackgate and I have seen that R3, is 5K62.
That's about all I can decipher against the info,s made available in the last few days.
I,ll be sourcing AD 797,s and other components to go the MC route.
I stand by the WAD being out performed by the Clones Audio PA1 in the MM version, when being fed by a Lin Axis + Akito + A&R 77, I would not know which alterations would be required within the WAD, to achieve a MM performance to rival the Clones.
I will compare them with the GP 390 Ceramic, and then the MC.
The WAD will make old bones with me, it is just changing its function, and becoming a device I will experiment a little with.