ARC Ref 6 Tube Microphonics

Hi All, I purchased a used Ref 6 preamp a few months ago, initially everything was fine, tube hours indicator said ~950 hours but not sure if that was accurate. After about 50 hours of use, turning the volume past 0 and the mechanical relay kicks in, a chime is heard coming through the speakers. I assumed one or more tubes had become microphonic. I replaced all tubes with new ARC select from Upscale, 7 Sovtek 6H30Pi, 1 Sovtek 6550WE. I let it warm up for a few hours, turned volume past 0 and the same chime is still there. On the positive side, the SQ improvement was immediately noticeable and outstanding, I love the preamp in my setup but the microphonic chime is annoying. ARC service said that some chime is normal but it still bothers me. Any advice? Ignore it? A new set of tubes direct from ARC? New tube rings? Send in to service? Thanks.


Interesting, I am using the Ref 6 connected to a Hegel H20 and then to a pair of Harbeth 40.3XD. I believe the H20 has higher than average gain at 32dB. Also the output/input impedance matching of the Ref 6 and H20 appear to be just barely OK with the avg. output of the Ref 6 being ~600 ohms and the input of the H20 being 9.4k ohms, I am using XLR. JA’s measurements set a recommended lower limit of 10k ohms for amplifier matching so maybe this isn’t a very good match.

@agbrace That 32dB amp gain is definitely higher than average, though not unheard of. This is certainly boosting your audible "chime"! In my case I was running Rogue Apollo Dark monoblocks with ~33dB gain. At times it felt like ungodly amounts of gain. Anyways, inline attenuators (say 10dB) placed between preamp and amp can reduce the chime proportionally if you have the gain to spare (you do, between 14dB Ref 6, plus 32dB Hegel) - BUT most ready-made attenuators are RCA, and you have to deal with the old "does this extra connection & resistor reduce sound quality" thing we all torture ourselves with here (in reality, it’s a negligible impact).

That Hegel’s 9.4K input impedance (XLR) is almost shocking. Much lower than I’d expect or want. That said, it’s a marginal mismatch, but not enough to have bearing on the tube chime issue. The Ref 6 is cap coupled, so maaaaybe you start losing some bass power due to the low input impedance, depending on the value of those caps. Which is a shame because the Ref 6’s "proper" bass is amazing.

The single ended connection between the Ref6 and H20 appear to be much more cooperative at 300 ohms / 50k ohms. 

The single ended connection between the Ref6 and H20 appear to be much more cooperative at 300 ohms / 50k ohms. 

Wow, that's a world of difference. I believe Ref 6's circuitry is optimized to normally be best from XLR outputs (whether using SE or balanced sources), but yours might just be the rare scenario where SE wins. You might ask ARC what they think of using a Jensen ISO-MAX XLR-to-RCA transformer box to potentially get the best of both worlds here. That still won't address the tube chime issue lol. 

I call it the 6H30 Zing… not a worry! Enjoy the music!

Doesn’t the Ref 6 have essentially a built in attenuation? My LS 26 has low, mid, and high gain settings via the remote. I know the Ref 3 and 5 do as well. It also has the ability to choose SE and Bal connections. ARC pre’s are notoriously over driven given the range of “volume” they allow. Some actually send them in and have ARC reduce the gain in the entire pre.

@agbrace   What do u mean u hear a chime ? Can u explain that chime sound you are hearing coming through the speakers? I have owned the Ref 5se , the Ref 6 and now the ref 6SE. I was using a Krell FPB 600 amp now I am using the Audio Research Ref 750s. I have NEVER heard anything but dead silence