The Phonoclone and VSPS PCB Help Desk

[Ed LaFontaine]

Thanks Richard,

My VSPS boards arrived AOK

[toque_101]

One question:

What is the minimum transformer VA recommended for the VSPS and the phonoclone?

-steve

[rjm]

A tough question but one I've been asked often enough to be able to fudge an answer:

Technically anything above even 5 VA will be sufficient to get the job done, but 50 VA is a good "sweet spot" : decent price, relatively compact, and commonly available - while still packing a reasonable punch. I use a 160 VA toroid, but mainly only as I prefer to be able to interchange my power supplies between gainlclone and preamp circuits, and the Plitron model is conveniently supplied with electrostatic screen and magnetic shield. Or at least that's the excuse. Truth is even 160 VA is not necessarily overkill.

So, minimum recommended is 35 VA. Bigger is generally better, but quality counts, too.

-rjm

[cjunk]

Hello,
in the attached pictures you can see my phonoclone prototype.
My choice of components is almost compliant to the most common audiophile obsessions (1W metal film resistors, Elna Cerafine for power supply decoupling, polystyrene caps for RIAA, Blackgate N as output cap), and I have fitted the boards to a small aluminium enclosure. Power is +/- 18 V from an external source that I already had (choke filtered LM317 circuit).

After testing the psu, I mounted the opamps and connected the thing... OUCH! I got a serious problem with RFI (I can hear voices coming from the loudspeakers if I listen close to them) and some hiss from the tweeters.
However, I could not resist to put some records and, despite this noise, I liked a lot what I heard (very dynamic and detailed sound).

Any suggestions to fix the RFI? I followed the gounding scheme proposed by Richard and the circuit is grounded to the metal case.

p.s. I would like to thanks Richard for having shared this project

[rjm]

The RFI is most likely picked up from one of the cables, either the phono interconnect or power supply umbilical. I guess my approach would be to first unplug the phono interconnect from the Phonoclone input, and/or make sure the noise isn't present with the unit connected but powered off.

Try also connecting the phono amp chassis to earth. After that its a game of disconnecting or rearranging things until the problem goes away.

Twisting the lead wire might help, though in a shielded case as you have it that shouldn't matter...

A photo of your power supply arrangement might be informative.

-rjm

[cjunk]

Originally posted by rjm
The RFI is most likely picked up from one of the cables, either the phono interconnect or power supply umbilical. I guess my approach would be to first unplug the phono interconnect from the Phonoclone input, and/or make sure the noise isn't present with the unit connected but powered off.
Try also connecting the phono amp chassis to earth. After that its a game of disconnecting or rearranging things until the problem goes away.

Hi Richard, I have already tried both things you suggest with no success. However, I just replaced the my old phono (SUPA Phono by Hormann Audio) with the clone. The SUPA was very quiet using the same cables and power supply. This would suggest that the problem may be in the case (but a similar aluminium case is used in the supa) or somehow in the circuit itself...

Twisting the lead wire might help, though in a shielded case as you have it that shouldn't matter...

I will try...

A photo of your power supply arrangement might be informative.

Richard, it is just an external box connected by a shielded 3 conductors cable. I can provide a picture later.

Few months ago I had a similar RFI problem with a Gainclone: in that case, as suggested in the LM3875 datasheet, I added a small cap (200 pF or so) between the inverting and non inverting inputs of the opamp and this solved the problem. I was wondering whether this could be appropriate for the Phonoclone...

carlo

[rjm]

The Phonoclone isn't especially sensitive to RFI - the circuit bandwidth is only 200 kHz or so - but then I would have said the same thing about the Gainclone, too. Unlike many commercial designs, the Phonoclone has no inbuilt anti-RFI measures of any kind. Its up to you to add whatever might be needed in this regard.

I'd definitely give that trick with the 200pF cap over the inputs a try. Perhaps some ferrite beads near the input and/or outputs might help.

Richard

(I'm still pondering on how you can get RFI with the inputs disconnected and everything shielded in a circuit with such low bandwidth. Parasitic inductance? I'm no expert here so its a llittle beyond me...)

[rjm]

This might help, its a bit over-simplified but...

http://internet.aca.gov.au/ACAINTER....359:pc=PC_1264

"SOME METHODS FOR ELIMINATING CITIZENS BAND RADIO TRANSMISSIONS FROM YOUR HI-FI STEREO OR AUDIO EQUIPMENT"

You'll have to copy-paste the URL. I couldn't link it.

-R

[cjunk]

Hi,
the RFI problem is gone with the addition of a 220 pF capacitor between the inputs of the first opamp (terminals 2 and 3).
This seems also not having degraded the sound which is still very open, dynamic and detailed (and I still have to wait finishing the break in).

I wish good luck to the other builders of the phonoclone, this project is really good!

Carlo

[stueyh]

Ive just completed mine and Im very pleased indeed. However I do have a couple of question;

Using the phono pre on my valve amp (uses a simple passive pre) I have to really turn the volume up. Ive set the gain to the standard 40dB. Will changing this to 50dB via R2 & 3 solve this without causing any other probs? My other question is relating to the cartridge itself. At the mo Im just using a cheap AT95 but Im looking at a new cartridge. Most are recommending the Denon DL110 or 160. These give an output of 1.5mV, how Im I best adapting the phono pre to suit?