[monty]

Please indulge me on a verbose first post!

I've owned a Stratos HT-3 (with cap upgrade) for about 8 years now. When I first got it, I reported some hum/buzzing to Klaus, but he said the Stratos were very quiet amps. At the time my speakers weren't particularly sensitive, and I was using a home cinema receiver as a preamp.

I've since upgraded speakers to more sensitive ones. Somehow in a new room layout, the buzz is audible 18 feet away and is really too much.

Here is the first weird thing: With the left (only) input on the Stratos wired up, there is no hum from any speakers. When I connect the right input on the Stratos, both left AND right speakers hum. If I connect the left preamp output instead to the centre channel on the Stratos, there is no hum. I could also swear that the right channel is quieter than the left.

Countless careful speaker, cable, and equipment substitutions have shown consistent results. The preamp output is dead quiet and the screened Chord XLR cables are without fault.

Here is the second wierd thing: If I use the left and middle channels for stereo, there is no audible hum/buzz. But the sound is definitely lacking something. Also, if I feed a quiet 1 Hz signal into both (which I did to check phase initially), I hear silence from the left (just a gently heaving speaker cone) but a strange rhythmic groan from the centre channel. Again, it's not the speakers, cables etc. Of course (a) this may be less broken in that the other channels - only used for a few hundred hours of films and (b) has less capacitance than the stereo amp doing only one channel.

So my HT-3 currently offers me one good channel, one poor one, and one noisy one.

So here's my conundrum. With a bigger room (and therefore a temptation to turn up the volume) I'd like to think that Stratos Extreme Monos would fit the bill nicely. But it's a lot of money (especially to the UK) given I've not had the best experience to date. They would also leave the HT-3 doing only a single channel. If I convert the HT-3 to (say) a mono, I've got to be absolutely sure that all problems go away. Any plan that involves sending the amp back to the US is expensive and slow.

And yet I do love the sound.

Any similar experiences or advice very welcome.

Thank you.

Monty

Equipment:

Lossless audio on PC -> RME digital audio card -> Stello DP200 -> Chord Chorus 2 XLR -> Stratos HT3 -> Chord Epic Super Twin -> Tannoy D700 MK II + ST200 supertweeters. Velodyne DD-15 for 10-30Hz. Tannoy D750 for center speaker. Own-written room correction equalizer used for some recordings. Buzz/hum manifests itself without PC and Velodyne connected - they are irrelevant.

[aaron.luebke]

Have you tried it using the RCA's instead of XLR's?

[monty]

Not previously. Thanks - I'll try RCA. I had hoped that balanced cable would help, but on peering at the Stratos PCB it's clearly a single-ended amp.

I found a previous thread describing very similar symptoms:

http://www.audiocircle.com/index.php?topic=76682

This seems to suggest a fault (bad solder joint etc.), or very bad luck with how the ground voltage nodes and antinodes are distributed around the chassis/circuit. Lifting a ground lead on one cable could be a solution, but for me is a last resort for the danger and asymmetry.

My stratos also buzzes much more when dimmers are used (in a distant room). So I don't think it's properly isolated from interference on the mains.

[smata67]

I have recently found out how sensitive the odyssey amps to interaction with preamp/speakers.  My guess is that you have also.  The amps apparently like low impedance preamps.  I suggest you experiment with different preamps, aiming towards the low end <2K ohm output impedance.  This should could be your problem.  Of course, a call to the Master is always in order for the ultimate advice on these.

[Speedskater]

If you have tried different brands of XLR cables and a different brand of source unit. Then weirdness like this suggests mis-wiring or a defective solder joint.

Are all the XLR panel jacks, pin #1's connected to the chassis rather than the audio circuit boards?

[monty]

Thanks guys. 

smata67: I don't have an alternative preamp to hand, and rely on the Stello's integral DAC so I can't just swap to anything. I'll try to find out the Stello's output impedance - and whether it's above your 2Kohm rule of thumb.

Speedskater: Klaus talked me through a wiring mod to short the 'black' pin of the XLR to the XLR ground. I've done this. It *has* helped - especially on background hum when all other electrical equipment is off. I've double checked all cable connections (I had a bad solder joint some years ago and just like you suggested it caused the same symptoms). They all test good. The problem I think is that with sensitive speakers and good gain in the Stratos it probably only takes a millivolt AC at the Stratos input to be audible 18 foot from the speakers.

The (probably worse) demon I'm now battling with is low voltage transformers (fitted EVERYWHERE in the building). Removing these is a last resort, but they induce an incredible amount of noise in the pre-amp/Stratos/speakers when 50% to 90% dimmed (whereas they don't in my Onkyo home theater amp). The lighting is obviously on different mains circuits to the Hi Fi, but I've heard that the radio interference is not quelled by this.

If anyone's had any success in eliminating dimmer noise (other than replacing the dimmers!) I'd be very glad to hear from you. I already have an RF filter in the preamp mains cable - makes no difference.

[Speedskater]

It was in past times, standard practice to connect the XLR pin #1 to the audio circuit ground.
Then in 1995 it was first documented that a much better practice was to connect the pin #1s to the chassis near the connector.
The entire audio circuit should only have a single connection to chassis.
It took a long time to get all mfg.s on-board.

[geowak]

You can type in "hum" and search the odyssey forum and you will have excellant reading
on this problem

or start here
http://www.audiocircle.com/index.php?topic=76682.0

After many, many months of trying to eliminate hum with TWO odyssey pieces, I sold it all and went with McCormack gear.