I recently built a Genera phono stage and have since been exploring the mysteries of cartridge loading. Once it had been burned in, I can confirm that the standard 220pF capacitance gives an excellent sound with a moving magnet cartridge (Sontra 500EX), running either into a Rogers HG88 or HiFi World KEL84 amplifier. 

However my normal cartridge is a Denon DL103 through a HiFi News Blackhead stepup transformer (10x) and this has proved more problematic. I have two DL103s: one with the standard stylus and one with an Expert Stylus Paratrace stylus and lightweight sapphire cantilever. With the standard stylus 100pF capacitance sounded good but a little edgy at the top. At 220pF  the sound balance was too bright; it reduced the edginess but lost crispness as well. 150pF gave a reasonable but not perfect compromise. However with the Paratrace stylus there were problems. At first, tracking was great, detail was great but there was a false brightness on some records and a spittiness and edginess. Also, although it tracked perfectly, it sounded strained and downright unpleasant on some records. The first stage of the solution was to play around with alignment and tracking weight to get the best basic sound out of the cartridge (line contact styli are very fussy compared with a standard spherical). However although this got rid of the spittiness, some records still sounded terribly strained and unpleasant. I knew that CD sounded fine through the amplifier, so the problem had to be something to do with the Denon and the Genera. 

Having run out of other options, I tried adding a parallel 500 ohms on the 'cartridge' side of the stepup transformer, in order to reduce the input resistance seen by the cartridge from 47k/100 = 470 ohms to 240 ohms. The result was miraculous: suddenly the glare and strain disappeared, leaving only sweetness and light. 

I appreciate that it is probably not common to feed a Genera with a low output moving coil and stepup transformer and the Denon DL103 is notorious for its setup and loading foibles. However anyone else who runs into similar problems may find the above experience helpful. 

However what was going on technically? I previously used the Denon DL103 Paratrace and stepup transformer to feed the Rogers HG88 phono input without any problems, so there was nothing wrong with the Paratrace stylus. Also why was the Paratrace DL103 so much worse than the DL103 with a standard stylus? I can only assume that there was something different about the Paratrace DL103's output which was causing trouble - and this was somehow eliminated by reducing the input impedance across the DL103. 

The main effects of lowering the resistance fed by a cartridge are to increase high frequency damping and also (working in series with the cartridge internal inductance) a roll off of output at high frequencies.  

Could it be that, compared with the standard DL103, the lightweight Paratrace stylus was generating more output at supersonic frequencies, e.g. due to record cutter ringing, and either this was upsetting the Genera itself or else it was being passed on faithfully by the Genera and upsetting the main amplifier? If so did the lower resistance solve the problem by reducing the cartridge's supersonic output to a level that did not cause trouble? Any thoughts?

Alasdair

PS

After writing this I found that the problem had not gone away and a rather hard unpleasant sound returned. Following a lot of fruitless fiddling around I decided to try taking out the 150pF input capacitors I had installed in the Genera and put back the 100pF capacitors which had sounded good originally with the conical stylus DL103.  Back came a really nice sound. Sorted!

Therefore rather than an obscure high frequency problem in the Genera, the problem may have been either the 150pF capacitors I had bought (branded Evox Rifa but 400v rating rather than  the 100v rating of the capacitors supplied by Graham Slee) - or else possibly my soldering of them. 

Regards,

Alasdair

Dear Graham,

Thanks for your reply. The 150pF capacitors I bought are marked as Evox PFR 150pK 400-. They look identical to your 100pF and 220pF capacitors, except for the higher voltage rating. 

Regards,

Alasdair

Oops, there was a typo in my message. The 150pF capacitors are marked EVOX PFR 150pJ 400-

Sorry about that.

Regards,

Alasdair

Dear Graham,

Thanks for taking a bit of 'weekend time' to think about this. As everything is now sounding OK I am going to leave well alone for the time being and provisionally put the problem down to experience. After all, it could just have been duff capacitors or maybe I was a bit hasty when I soldered them in. If the problem recurs I'll investigate further but otherwise I'll just listen to the music ...

Best wishes,

Alasdair

Alasdair

Dear Graham,

After dinner and some further thought I decided to try replacing the 100pF capacitors with the standard 220pF ones and ... you've guessed it ...  it sounded even better. In other words, once I had got everything else in the setup right, the standard recommendation of 220pF is best, even for a low output DL103 feeding a Genera via a stepup transformer. (However the DL103 does benefit from 500 ohms in parallel across the transformer input.)

Thanks for your patience and help.

Best wishes,

Alasdair

Thanks for the comment Fatmangolf.

Actually the story did not end there - it had a twist in the tail and in the end the solution was nothing to do with capacitors. No sooner had I congratulated myself with solving the problem by installing the 220pF capacitors than the harshness gradually returned yet again. Before resorting to more drastic action, I decided to completely remove the cartridge and reinstalled it from scratch - and in the process discovered that a washer which should have been under the head of a mounting bolt had somehow managed to end up in between the top of the cartridge and the underside of the headshell. No wonder everything was rattling about a bit. 

With the washer restored to its correct position and the DL103 carefully remounted and realigned, all was sweetness and light and I was confident that the problem was finally solved. Interestingly, the loading of the cartridge was no longer as critical as previously: it sounded fine with no additional loading resistors in parallel with the transformer input and the effect of adding 390 ohms to reduce the impedance seen by the cartridge to about 210 ohms was now very subtle. I haven't bothered to play around any more with the Genera input capacitance but I wouldn't mind guessing that with the cartridge set up correctly the differences between 100pF, 150pF and 220pF would probably be similarly subtle.

The moral of the story? The cause of a problem may not be what you think it is and until you look in the right place you won't find the true solution. It also confirmed the importance of getting everything else (effective mass, mounting, alignment and tracking weight) right first before worrying about fiddling with electrical loading. The latter still has its uses as a means of 'fine tuning' the sound - but only after everything else is absolutely right. I thought I had already learned this lesson well and got everything right before bothering Graham with queries about capacitors - but then I was beaten by a stray washer. 

Ho hum, one to put down to experience methinks.