help diagnosing a buzz after adding tube preamp

Chris_B · « **on:** 3 Oct 2018, 11:47 pm »

Hi,

I recently replace my tube preamp with a higher end tube pre from the same manufacturer. When I added the new pre, I induced what I would describe as a buzz/hum in both speakers that is audible from the listening position when the room is quiet. I then went through the usual ground loop hum diagnosis steps.

The amps are Odyssey Audio Stratos mono extremes so high power solid state
The preamp is Linear Tube Audio's new MicroZOTL pre
The speakers are Spatial Audio's X2s (Air Motion Transformer (AMT) that cover both mid and treble 101dB sensitivity)

Here is what I did with results:

1) took the system apart and then connected amps to the speakers - no hum
2) added the preamp (single ended cables) - hum came back both channels ( no source components connected yet)
3) mute / volume doesn't change the volume of the hum
4) plugged the amps and pre into the same power strip - no change
5) lifted the ground using cheater plugs - no change
6) added a 12dB inline attenuator between the preamp - amp - significantly lowered the volume of the buzz/hum. Not audible from the listening position any more

I haven't rolled tubes yet but plan to soon. Do you guys have any conclusions based on the info above or recommendations for how else to trouble shoot?

I'm left with either a 1) problem with the tube preamp that is accentuated by the high power amps and sensitive speakers 2) no equipment fault though the system is not well matched ( tube pre/high power solid state amps/sensitive speakers).

I did hear the actual preamp at the manufacturers location paired with roughly equivalent speakers and a tube amp and didn't hear any buzz/hum but I'm not entirely sure. LTA is an awesome manufacturer and I'm 100% confident that I can bring the pre back and ask them to check it out. Before doing that, I'd like to run down as much of my potential system induced issues first which is why I'm asking for you guys to check my work.

FWIW, when playing music, soundstage is awesome, instruments are in their own space and the PRAT is crazy good. I just hate the buzz and want to figure out how to get rid of it.

Thanks,

Chris

FullRangeMan · « **Reply #1 on:** 4 Oct 2018, 12:38 am »

You could check impedance mismatch:
LTA preamp output is 50 ohms
Odyssey Candela Preamp output impedance is <2 kOhm what is value Stratos could expect.

Tyson · « **Reply #2 on:** 4 Oct 2018, 12:39 am »

I've had that problem a lot. I've noticed its usually a ground loop hum, and it's induced in the preamp, which is then amplified by the main amp. And it's usually independent of the turning up/down the volume pot. I've tried a bunch of different solutions, including several different power conditioners and various other attempts to deal with ground issues. Let me save you a lot of time and heartache and just recommend that you get a Jensen ISOMAX - it will fix the ground loop and it's quite transparent sound wise.

Chris_B · « **Reply #3 on:** 4 Oct 2018, 12:54 am »

Quote from: Tyson on 4 Oct 2018, 12:39 am

I've had that problem a lot. I've noticed its usually a ground loop hum, and it's induced in the preamp, which is then amplified by the main amp. And it's usually independent of the turning up/down the volume pot. I've tried a bunch of different solutions, including several different power conditioners and various other attempts to deal with ground issues. Let me save you a lot of time and heartache and just recommend that you get a Jensen ISOMAX - it will fix the ground loop and it's quite transparent sound wise.

Tyson - is this what you were talking about: https://www.amazon.com/Jensen-CI-2RR-IsoMax-Isolator-Jensen/dp/B00ASVWYCS ?

BTW - you were absolutely right about the Spatial X2s.

Tyson · « **Reply #4 on:** 4 Oct 2018, 01:22 am »

Quote from: Chris_B on 4 Oct 2018, 12:54 am

Tyson - is this what you were talking about: https://www.amazon.com/Jensen-CI-2RR-IsoMax-Isolator-Jensen/dp/B00ASVWYCS ?

BTW - you were absolutely right about the Spatial X2s.

Yeah, those X2's are da bomb! There are things that high efficiency speakers can do with ease that other speakers just can't touch. But the downside is any noise in your system becomes way more of an issue.

The CI 2RR is a really good choice and should fix the issue.

One other thing you might try is the PC-2XR Pro-to-consumer isolator. It's really useful if you find yourself in a situation where you have a high gain source, a high gain preamp and a high gain amp. With really sensitive speakers like yours, having a bunch of gear that's all high gain will give you problems because you really end up hearing the noise floor of each piece of gear amplified. The PC-2XR will lower the gain between the preamp and the amp (in addition to breaking the ground loop) so your system is better matched for your very efficient speakers.

Or you might want to check out a different amp with lower gain.

avta · « **Reply #5 on:** 4 Oct 2018, 02:16 am »

You might try changing your cables. I had a similar problem once and it turned out to be the cables between pre and amp. I was using quality cables but didn’t match the stup I had.

JackD · « **Reply #6 on:** 4 Oct 2018, 02:34 am »

Chris

Did you try the cheater plug on both the amp and preamp or just the preamp? Also just to make sure swap both pairs of tubes left to right to re-seat them though the tubes are not all four bad but it is an elimination step. If you were using the MZ2 in this set-up before and that is the only swap you made then you shouldn't have a gain issue as the two LTA preamps are basically the same gain. But you can fix that issue by swapping in 12AU7's.

Chris_B · « **Reply #7 on:** 4 Oct 2018, 09:59 am »

Quote from: JackD on 4 Oct 2018, 02:34 am

Chris

Did you try the cheater plug on both the amp and preamp or just the preamp? Also just to make sure swap both pairs of tubes left to right to re-seat them though the tubes are not all four bad but it is an elimination step. If you were using the MZ2 in this set-up before and that is the only swap you made then you shouldn't have a gain issue as the two LTA preamps are basically the same gain. But you can fix that issue by swapping in 12AU7's.

I used cheater plugs on both mono amps and the preamp with no noticeable change. I also swapped in 12AU7 (good NOS RCA ones) also with no real impact. The inline attenuator did reduce the volume of the hum (as would be expected) but didn't eliminate it. Swapped cables too with no change.

I'm hopeful that the Jensen CI-2RR will fix it. It should be hear early next week and I'll report back.

Tyson · « **Reply #8 on:** 4 Oct 2018, 02:47 pm »

If the Jensen doesn't fix it all the way, try using the Jensen and the inline attenuator together.

Chris_B · « **Reply #9 on:** 4 Oct 2018, 04:16 pm »

Quote from: Tyson on 4 Oct 2018, 02:47 pm

If the Jensen doesn't fix it all the way, try using the Jensen and the inline attenuator together.

That's the plan. With just the inline attenuator, the hum is close to inaudible from the listening position. I'm hopeful that the Jensen will solve the problem. I'm fearful (probably unfounded) that putting filter/attenuation in the signal path will impact dynamics.

Tyson · « **Reply #10 on:** 4 Oct 2018, 04:43 pm »

Quote from: Chris_B on 4 Oct 2018, 04:16 pm

That's the plan. With just the inline attenuator, the hum is close to inaudible from the listening position. I'm hopeful that the Jensen will solve the problem. I'm fearful (probably unfounded) that putting filter/attenuation in the signal path will impact dynamics.

The Jensen definitely will not affect dynamics, I know that from personal experience. In fact, if you think about dynamics in terms of both loud and soft, then you begin to realize that being able to hear very soft music is just as important as the jump factor with very loud music. By lowering noise in your system, you actually expand your dynamic range significantly.

This is why system matching is very important. You have a speaker that's more dynamically capable than just about anything on the market, with the exception of things like large horn based speakers. This will shift a little bit your optimal choices for electronics, mostly the amp/preamp. IME, amps and preamps with a bit lower gain often sound better than amps/preamps with higher gain. From a technical standpoint, it seems to be a lot easier to design a really great medium/low gain amp/preamp than it does a high gain version.

Just food for thought. Your speakers are opening up a whole new category of amp/preamp that simply wasn't feasible with a lower efficiency speaker. And that's a good thing IMO - I have found that it's much easier (and cheaper) to find an amazingly great medium/low power and medium/low gain amp/preamp. Because of your speakers, you have the luxury of being able to focus on quality and not worry too much about quantity (watts and gain).

Rusty Jefferson · « **Reply #11 on:** 4 Oct 2018, 05:27 pm »

If your original LTA Microzotl is silent (same amps and speakers, ie nothing else changed.) and the newer model isn't and they have the same gain, there's an issue. Dave prides himself on making his designs quiet, amps and preamps. Contact LTA rather than inserting things into the chain. Don't accept noise where there was none, or try to mask it, imho.

Chris_B · « **Reply #12 on:** 4 Oct 2018, 06:07 pm »

Quote from: Rusty Jefferson on 4 Oct 2018, 05:27 pm

If your original LTA Microzotl is silent (same amps and speakers, ie nothing else changed.) and the newer model isn't and they have the same gain, there's an issue. Dave prides himself on making his designs quiet, amps and preamps. Contact LTA rather than inserting things into the chain. Don't accept noise where there was none, or try to mask it, imho.

I was thinking the same thing which was why I wanted to try to eliminate everything else before calling Mark at LTA (who is a great guy btw). I'll try the Jensen in between the preamp and amp first to see what happens and then ask Mark for his advice. I'm sure that they will take a look.

Rusty Jefferson · « **Reply #13 on:** 4 Oct 2018, 07:31 pm »

Do you still have the MZ? Can you go back and forth? I assume you're in the DMV? I'm in Annapolis and have an MZ if you're close.

rollo · « **Reply #14 on:** 4 Oct 2018, 07:39 pm »

Quote from: FullRangeMan on 4 Oct 2018, 12:38 am

You could check impedance mismatch:
LTA preamp output is 50 ohms
Odyssey Candela Preamp output impedance is <2 kOhm what is value Stratos could expect.

Agree unless previous preamp is 50 Ohm as well. Call LTA ASAP. We showed together at NYC show great people.

charles

kphooligann · « **Reply #15 on:** 4 Oct 2018, 10:24 pm »

What sources are connected to the preamp? Did you try removing them one by one?

Chris_B · « **Reply #16 on:** 5 Oct 2018, 02:23 am »

Quote from: Rusty Jefferson on 4 Oct 2018, 07:31 pm

Do you still have the MZ? Can you go back and forth? I assume you're in the DMV? I'm in Annapolis and have an MZ if you're close.

I’m in Arlington, VA but thanks for offering. I had the MZ2 preamp previously (output impedance 2 ohms). My current MicroZOTL preamp output impedance is 50 ohms.

Chris_B · « **Reply #17 on:** 5 Oct 2018, 02:26 am »

Quote from: kphooligann on 4 Oct 2018, 10:24 pm

What sources are connected to the preamp? Did you try removing them one by one?

It is currently humming with no sources connected.

FullRangeMan · « **Reply #18 on:** 5 Oct 2018, 02:39 am »

Quote from: Chris_B on 5 Oct 2018, 02:26 am

It is currently humming with no sources connected.

This looks hi noise floor or something related to sensitivity/gain/impedance imo.

Rusty Jefferson · « **Reply #19 on:** 5 Oct 2018, 05:40 pm »

Quote from: Chris_B on 5 Oct 2018, 02:23 am

I’m in Arlington, VA but thanks for offering. I had the MZ2 preamp previously (output impedance 2 ohms). My current MicroZOTL preamp output impedance is 50 ohms.

No problem, you're not far as the crow flies, but could take 2 hours in the car.

So it sounds like you don't have the original MZ any longer? I'm headed that way on the 13th if you haven't nailed this down.

I'm not familiar with the suggested noise reducer but glancing at the specs it appears the output impedance is 4.65k ohms. That's high, and definitely high for driving amps with only 22k of input impedance. The new MZ now with 50 ohms can drive any load/cable. Plus you'll be adding another set of interconnects.

Call Mark or Dave to discuss changes they made that might be causing this issue.