Noise Floor / Completing Upgrades

[onelivewire]

Some months back I purchased a BeePree2 used with partial upgrades. After upgrading to 350W monoblocks, a raised noise floor is apparent when the preamp is powered on. Of course I love the tone imparted by the BeePre (as with my Crack/SEX/Mainline).

I believe the preamp has:
1) CSS but without balanced outputs (no XLR)
2) Hybrid Shunt Regulator without volume pot upgrades (Original volume and balance pots present)
Pictures: https://imgur.com/a/Ow1b7VD

What would be the best course of action to solve for my noise floor issue?

Thanks in advance!

[Doc B.]

It would help if you could describe the noise floor. Hum? Hiss?
It's not so much the power of the amplifier that matters with respect to this issue. It is the sensitivy of the amp that matters. Generally the thing you will find in this situation is the amp has a pretty sensitive input (lots of gain). It would follow that you would find you are setting the volume knob pretty low on the preamp to compensate.
Usually the best solution is to install inline attenuators like the Harrison Labs ones between the Beepre2 and the amps. That cuts down any noise floor from upstream components and gives you a little more range on the volume control.

https://www.parts-express.com/Harrison-Labs-12-dB-RCA-Line-Level-Attenuator-Pair-266-244?quantity=1&utm_source=google&utm_medium=cpc&utm_campaign=20344069331&utm_content=150561967189&gadid=664791375060&gad_source=1&gclid=Cj0KCQiAwbitBhDIARIsABfFYIKnKg9bIysh1fVSoV0a6Ga9tY6u4K6jsqGpaDIzZzBKAwqTPnxXWHMaAm7kEALw_wcB

[onelivewire]

I would describe the noise as a hiss present almost exclusively in the tweeters. I basically cannot hear anything coming from the woofers. 

I keep the volume on the BeePre around 40% at all times and adjust volume digitally at the source, as I find this keeps microphonics to a minimum, and thus would prefer not to raise it above this level. There appears to be a hiss that goes up and down with the preamp volume along with microphonics, but this hiss is not an issue at the 40% attenuation level. Rather, the hiss I'm taking issue with is present regardless of the preamp volume.

Is an inline attenuator still the ideal first step for solving here? I have not tried lifting the ground at this point, although this isn't the type of hum I would associate with a grounding issue.

[Paul Birkeland]

I keep the volume on the BeePre around 40% at all times and adjust volume digitally at the source, as I find this keeps microphonics to a minimum

Microphonics will be at a minimum when the volume on your source is all the way up and the BeePre is used as the level control.  This is actually a design choice we made between BeePre 1 and BeePre 2 to address sensitive systems with large solid state amps.


Do you have the second upgrade with the 6CM7 tubes in the back 9 pin socket holes?  There is perhaps one small modification I can suggest which might buy you anther dB or two of reduced noise.  If you do not have the upgrade with the 6CM7 tubes in the back 9 pin socket holes, there's a different modification I might also suggest.

Is an inline attenuator still the ideal first step for solving here? I have not tried lifting the ground at this point, although this isn't the type of hum I would associate with a grounding issue.

Yes, this is a very good idea.  It would help to know which speakers you're using as well as the model of the amp, then we might be able to suggest the most appropriate attenuator for your specific setup. 

[onelivewire]

Certainly understood on the source end of things - I try to manage everything remotely (12V triggers and such), so the goal was 'how low can I keep the preamp volume while allowing upstream volume control'. ~40% has done it for me there. Running the recommended flask weights for isolation there, as well as a heavy quartz console with a quartz plinth for the preamp.

9-pins are indeed 6CM7s. Sounds like the XLR outputs are not necessarily helpful here, though as a side note I am interested in going to balanced. Maybe there's little to nothing to be gained given the short run on those cables. Also interested in the stepped attenuators which I've been pleased with on the Mainline, but again, understood this will not likely do anything to address this issue.

The amps are PS Audio M700s and the speakers are KEF Reference 3s.

[Paul Birkeland]

So you have the second upgrade but not the first upgrade?


https://www.mouser.com/ProductDetail/Panasonic/ECQ-E2225KF?qs=3ERjQB%2FgCVI2gYm7OT416A%3D%3D  Buy two of those.  On the C4S board installed with the 6CM7 upgrade, install each of those caps with one leg in the pad marked with an X and the other end in Breg.  That may or may not make a difference for what you're up to.

The PS Audio has 30dB of gain and the speakers are about 83dB sensitive.  I think this is a pretty good place to put a pair of the -12dB Harrison Labs attenuators. 

[onelivewire]

I believe the Constant Current Source was installed without the stepped attenuators, and the Hybrid Shunt Regulator without the balanced outputs. In case it's helpful, I've added a full under-chassis picture in addition to those linked in the original post here: https://imgur.com/a/Ow1b7VD

The original owner made it sound like he was able to special order just those partial part sets from you for those upgrades, which I thought odd but not entirely unfeasible.

I'll take a closer look tomorrow and see where those locations are for adding the caps. I'll admit I'm hesitant on the attenuators, but understand that may simply be the best path.

[Doc B.]

The original owner made it sound like he was able to special order just those partial part sets from you for those upgrades, which I thought odd but not entirely unfeasible.

We only sell complete kits.