PH6 Board Question

Question about the schematic that came with my PH6 kit. It shows C2, C1,and R14 where you would expect them to be, at the low voltage side of R15 but when I looked at the circuit board, they are all at the high voltage side of R15. It is hard to see what they are doing there, as it leaves R15 totally un-decoupled. This also negates the oportunity to squash any remaining PSU hum. Does anybody know the reason for this topology, and why it was changed from what the circuit indicates?

This is really interesting. I built a PH6 for my brother and it sounds very good, but it's not anywhere I can look at it.

I don't understand the electrical engineering well enough to comment on the difference, I'm not even sure what the difference between a coupling capacitor & decoupling capacitor. If anyone wants to get up on their soapbox... I for one am all ears!

Goddlediddles
good question, but I did remove the schematic diagram as it is proprietary to Tubes4HiFi.
The answer is the schematic of the circuit is correct as it should be, when the board was laid out R15 was placed incorrectly and connected incorrectly. This doesn't prevent the circuit from working properly or sounding as good as it should,
but you are correct that it negates the purpose of R15 to act as an RC filter on the B+.
So I've corrected the PCB for future customers. You could cut a trace and add a jumper to fit it, and it MAY reduce a small amount of B+ noise.

@ tubes4hifi,
OK, but a 'scope reveals a few millivolts of PSU noise at the B+ where it goes onto the board and it's there also on my speakers, admittedly at a low level, so I'm going to make it sound even better with a small choke on the B+ line before it enters the board. I will also remove R15 and jumper the gap. Thanks for your help and Seasons Greetings.

I'm getting to grips with the problem at last. I have decided not to start cutting and jumpering the board but as a test I have inserted an old single-ended triode O/P transformer winding, which measures 17 Henrys, into the B+ line to the PH6 and this brings down the 100 Hertz PSU Hum appreciably (power outlet supply in Europe is 50 Hz). On the strength of this crude test I have ordered a Hammond 155C, 60 Henry, 8 mA choke from Italy. In the meantime here is screen-shot of the offending 100 Hz PSU hum at the O/P of one of the channels without the trafo winding inserted. It measures 15 mV peak to peak with the volume up full, but as can be seen, the noise is very low level. It's also obvious that there is only a small 50 Hz component on the trace (every second cycle). This is great and bodes well for the final outcome; fingers crossed.

yes, a choke should help. Most all of my better preamps also use a HV regulator chip, which reduces PS noise below 1mv

@ tubes4hifi. Yes indeed, and it is on one of these better pre-amps with the high voltage regulator that this problem exists (SP9 with PH6) due to the faulty supplied board. The measured ripple on the PSU is indeed in the region of 1mV but unfortunately amplified to 15mV peak to peak at the O/P because of the board fault. I suppose it's my own fault for trusting and not checking the traces before starting the assembly and requesting a replacement board, but things are expected to be correctly done by the supplier. The PSU ripple is now non existent due to the addition of the 60 Henry Hammond choke and no guntering-up of the assembled board required. I pity anyone who has not got the 50 years of electronics experience and a raft of test equipment who has to deal with this problem. One final thought: I think that a choke should be supplied with the PH6, regardless of the the PSU high voltage regulator because the 4.7K resistor and 22mF capacitor are not sufficient to eliminate all PSU ripple even with the corrected board.

the PH6 is my low budget phono board, many many very happy customers with it, but as with any product, improvements can be made.
That's also why for very little more, there is the PH10 (with high voltage regulator on board), the PH12, PH14, and PH15 models.
Anyone with an older PH6 can do a simple cut and jumper, I'll provide those directions upon request via email, that's a no cost solution
VS buying a choke for $20 or more, or a voltage regulator for $2 which works even better than the choke does.
The limitations of the PH6 were so that it is a plug-in replacement for the PAS preamp, but so is the PH10 board which is 10X quieter!

@ tubes4hif. My PH6 is supplied from the PSU on my SP9 which has the High voltage regulator on board. It consumes only 4 mA and the regulator does it's job. The reason I put in the choke was to fix the problem created by the faulty board without pulling the guts out of my completed build. The original design on a properly made board could be improved indeed with a second voltage regulator but that would mean reducing the HT voltage to bring the regulator within range before hitting the dropout point. I think we might be talking cross-purposes here. Anyway all's well that ends well.

goddlediddles wrote:@ tubes4hif. My PH6 is supplied from the PSU on my SP9 which has the High voltage regulator on board. It consumes only 4 mA and the regulator does it's job. The reason I put in the choke was to fix the problem created by the faulty board without pulling the guts out of my completed build. The original design on a properly made board could be improved indeed with a second voltage regulator but that would mean reducing the HT voltage to bring the regulator within range before hitting the dropout point. I think we might be talking cross-purposes here. Anyway all's well that ends well.

I have a PH6 board/kit that I purchased along with my SP10 kit - the second week of December. Since I purchased the whole kit before Christmas 2012 - when Roy said he would correct the problem on future boards, I suspect my board has the same problem. Since I haven't assembled anything yet, could you share a bit of a sketch of what circuit cutting and jumpering is needed to correct the circuit trace problem. I would much appreciate it!

Pete

Hi Pete, Roy said above that he would eMail the details to anybody that requested them but I decided to put in the 60 Henry choke because I didn't want to rip up my finished board and the choke, in combination with the two 22uF capacitors in parallel, gives almost 60dbs of PS ripple rejection. It works so well that I can't see any trace of PS hum on the 'scope; it's well below the noise which itself is only barely audible at a distance of 6 inches from the speaker (with the volume control up full). At the end of the day, I'm very happy with the PH6.

goddlediddles wrote:Hi Pete, Roy said above that he would eMail the details to anybody that requested them but I decided to put in the 60 Henry choke because I didn't want to rip up my finished board and the choke, in combination with the two 22uF capacitors in parallel, gives almost 60dbs of PS ripple rejection. It works so well that I can't see any trace of PS hum on the 'scope; it's well below the noise which itself is only barely audible at a distance of 6 inches from the speaker (with the volume control up full). At the end of the day, I'm very happy with the PH6.

Could you say Choke features you used.
thanks goddlediddles.

Hi Pillo, the choke is the Hammond 155C, 60 Henry, 8 mA. Mouser were out of stock when I looked so I hasd to buy it in Italy here: http://shop.tubes.it/index.php?lang=it

Thank goddlediddles.

I've been following your thread with interest, in reference to the card PH6.
I have a Dynaco PAS which has the same symptoms of noise phono board, very faint noise coming up with the volume, I have interest in replacing the plate by a PH6 phono.

I have a spare Choke Dynaco ST70, model C-354, with 55 Ohms, 1.5 Hz and 200 mA. and it would be interesting to test before replacing the plate, if not Choke with those characteristics would be valid.

Hi Pillo, you don't say what Inductance the choke has, but if it has a resistance of only 55 Ohms I would say that it is too low. Do you mean 1.5 Henrys?

It is this: http://www.dynakitparts.com/store/schematics/C-354.pdf

Pillo69 wrote:It is this: http://www.dynakitparts.com/store/schematics/C-354.pdf

1.75 Henrys is far too small. You need at least 20 Henrys.

Thank you.