ONO output cap choice

[woodturner-fran]

Hi all,

I have a diy clone of an ono phonostage. Recent experiments with a valve phonostage (valve itch) show me that the ono is not quite as transparent as the valve one. This of course may be just due to the circuit, but looking again at the boards I can't help but think a little tuning may be in order.

So, first on the list are the output caps C7 and C9. Since I only use single ended output, I need only change 2 of these. My question is.... what to use?

I have a range of caps that might be suitable:

cornell 4.7uF film
Russian K73-16 6.8uF
Russian MBGO-1 in both 4uf and 10uF
bennic film 10uF

The originals are Cornell CDET 10uF/250V. I've also seen that I could add some trimmers arounf R4 (I think) to remove offset, but I also read that there is serious offset until the phonostage warms up, and I'm not too keen on that.

Anyone any ideas for what to try?


Fran

[MRupp]

Did you measure the offset voltage at the output of your ono (before the coupling cap off course)? Mine has 50mV untweaked (will try to tackle this in a while) and I get away without a output coupling cap. Maybe you could try this as a test. At least you would be able to get an idea on what your existing coupling cap does to the sound. Btw., what exactly do you mean by transparency, are you lacking details, etc.. ? What exactly is the difference you hear.

[woodturner-fran]

I will test the offset. The difference is the soundstage is flat in comparison, with seemingly less resolution of details.

It's not poor sq - in fact it sounds great, but I can't help think I could improve by trying other caps. The is already a salas shunt.


Fran

[woodturner-fran]

?I have ~100mV on one channel and ~60mV on the other.


Taking one for the team.....

First up for installation is the Russian K73-16 6.8uF. Its only been in for half an hour, but first impressions are that its fairly neutral. Very quick impression though and not long after turn on....


Fran

[h_a]

I get away without a output coupling cap.

Please be aware that the reason for the cap is not the offset value itself (that can be tweaked by paralleling a pot in parallel to the drain resistor of the jfet differential pair).

The offset is sensitive to temperature and jumps quite a bit (due to the large gain) after turning on the phono stage, even some volts have been seen by some members.

Happy experimenting!

Hannes

[woodturner-fran]

Yes, that is what I was worrying about......

Anyway, I took out the Russian K73-16 6.8uF after listening for a while last night. Sound was clear and smooth, even throughout. Next in are some Russian MBGO-1 4uf. From what I can make out these are PIO caps.

I forgot I have some BG N 4.7uF 16v here as well - I will try those too.


Fran

[MRupp]

The offset is sensitive to temperature and jumps quite a bit

Understood, but the offset voltage in my setup stays pretty constant and does not cause any issues, even when switching to other sources. Btw. if you balance the current through both halves of the input diff pair (assuming they are identical in VGS, otherwise you have to unbalance slightly again) you should be able to get the offset down to very low values, one of the next steps I will try by adjusting the current source under the diff pair.

[MRupp]

've also seen that I could add some trimmers arounf R4 (I think) to remove offset

Overlooked this detail: Yes I think this is what some people have done but then you have a trimmer in the signal path and if you are concerned about the quality of caps you might also be concerned about the quality of resistors. Personally I prefer to change the current by adjusting R67. In addition you could lower the value of R4 slightly or add a parallel resistor (rahter than a trimmer) to have a better balance to start with and you could increase the current through Q3 to ca. 10mA by lowering R53 to 100 ohms and so raise its VGS.

[Marra]

I'm following this with interest as I used Wima MKP4 caps on the output and am interested to hear peoples thoughts on other possibly better ones. I used Ralph Stens excellent boards but had to change a couple of resistor values to alter the voltage around the led this also helped with dc offset which fluctuated quite a lot; now down to a few mv's. The boards have trimmers in unison with R4;R59 & R25 but I could never get the offset to stabilise so left them at the original resistor values. This phono stage has excellent sq but is slow to stabilise; I leave mine on 24/7 and it sounds better for it.

[woodturner-fran]

Just an update:

Over the last few nights I have tried out:

BGN 4.7uf 16V. Nice even tone, very clean on highs and lows, but ultimately a little flat sounding.

Bennic 10uf PP cap, bypassed with a russian K42Y-9 0.1uF: Pretty much as for the BGN above.

The cornell PP cap - this wasn;t great really, sounded a bit muddy on the highs and lows, lost some of clarity of the sound, like in the way when there's some fluff around the stylus (although not as much as you would get in that situation).

The winner so far is the Russian bathtub MGBO-1 4uF. There is quite amaxzing clarity with this cap, soundstage takes a nice jump forward and the bass seems tighter than with the other caps. They look like this one:




Now the only thing is that the sibilance is titled up ever so slightly, not annoying (and I am pretty sensitive to sibilance) but if it calmed down a little bit this would be the perfect choice I think. It only has about 2 or 3 hours on it, so it might just settle down over time (I hope!)


Fran