Ono/Xono Circuit Experts needed!

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I recently modified my Xono deleting the output caps using trimmers to null DC offset and upgrading the polystyrene RIAA caps.
Problem-- the bass output seems deficient. I'm using this with balanced outputs.

Noticed 2 things & have an additional question:

1) Took out C6 the .033uF cap next to the RIAA network and did not replace it. Does anyone know what this cap does?
Could this be the cause?

2) Took out the 4 big 10uF film caps C7&9 but neglected to take out the C38 polypro 10uF cap in the inverter section. Consequences and possible reason for the rolloff?

3) If I put the C6 back in and take out C38 with a jumper wire, do I need to zero out the DC with trim pots again?

Is there any other reason I may have a problem. Any way to check this out? The sound is perfect but rolled off below 80Hz or so.

Just want advice before I plunge back into this. Thanks! Barry
The RIAA curve was put in place to decrease the amount of bass that went through the cutter heads onto the master. Of course the discs pressed from the master replicated that same signal. Just for fun, you might try listening to a record on a line input. You'll have to turn the volume way up because the signal won't get the boost in signal strength from the phono stage, but it's instructive to see what the phono stage actually does.
Cutting back the level of the bass frequencies (by 20dB or so ref. 1kHz) gives two main benefits: You can get more music recorded on the same amount of real estate, and it reduces the demands on the cutter and playback cartridge--imagine what an un-equalized version of Tchaikovsky's 1812 would do to a cartridge if the cannon shots were recorded flat.
On playback the phono stage boosts the bass back up to a nominally flat level. There are two ways to go about this. Passive RIAA circuits shunt a controlled amount of the high frequencies to ground. Active RIAA circuits use feedback to cancel the high frequencies.
The Ono uses the active strategy.
That means that the RIAA eq in the Ono must let through the proper amount of high frequencies (remember, these are going to cancel part of the signal) and block most of the lower frequencies (because you don't want to cancel them). The high frequencies cancel, which diminishes them, the bass frequencies zip through, uncanceled, so although it may take a moment of scratching your head to understand, the net effect is that more low frequencies come through than high...and all is right with the world.
Capacitors block low frequencies and allow high frequencies to pass. More to the point, removing C6 allows more low frequencies to cancel.
All of which being my long-winded way of saying that, yeah, you shot yourself in the foot when you took out C6. Put it back in and all will once more be right with the world.
I'll stay out of the DC blocking cap discussion. Just bear in mind that there's still an awful lot of amplification after the phono stage and a comparatively small amount of DC up front can cause an awful lot of mischief in back. If you really want to go that route, you might want to consider a servo. That way you'll have something actively watching your back if someone opens the door and a sudden draft of cold air happens to blow across the unit...thus changing your carefully set DC parameters. It's your system, you can do what you want.

Grey: Thanks for the explanation and help! I have run phono outputs through a line stage and understand the rationale for the RIAA process but I now appreciate active versus passive thanks to your explanation and for the Ono.

I'll restore C6 and think about a DC servo. I hadn't planned on going "topless" -- the thought of that is scarier than you could ever imagine.....

I read your posts all the time. I'm probably not alone when I say that the value of this forum owes much to your thoughtful posts and helpfulness.

Kindest regards, Barry
Joined 2003
Paid Member
AuroraB said:
Keep up the good work, Grey... I think quite alot of us really like your posts.

I do, too. When kept short. Sorry Grey, you asked for it. :D :)

I have bypassed the output caps as well and trimmed the DC with a fixed resistor.
Since my soundcard connected to the Ono seems to block the remaining mV's very well, I don't need a servo. Of course your setup may differ, certainly when you would be using a all no-caps signal path.

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