Question on Stereomour T2/T14 Voltage Difference

[InfernoSTi]

Back when I built my Stereomour in Nov 2010, I posted these voltage readings for terminals 2 and 14:

Terminal     Expected     Actual

T2             230            207      
T14           230            194        

A year and a quarter later (different house, different tubes, all the tweaks I've done are pretty tame but they are different from base), I checked the voltage again and it was:

                                  2010      2012
Terminal     Expected     Actual    Actual

T2             230            207        206
T14           230            194        192

My concern is that the left/right channels are not at the same level of amplification (have always been out of balance).  I have previously always been in a small room with lots of room treatments and nearfield listening, so I've sort of naturally just adjusted the speakers and treatments to compensate, so this hasn't been a big deal. Now I'm in a large room with much less opportunity for treatments and speaker placement adjustment (a nice tradeoff, actually).  Additionally, my source and DAC have improved considerably since 2010, so with a more resolving system, I may be hearing new "weak links in the chain" if you will.  

My question is within the circuit, would the voltage difference between the left and right channels for terminal 2 and 14 be the cause of this?  And if so, which resistor value should I think about adjusting ahead of this to balance the two?  If that isn't the right way of thinking about this, what would be, please?

Thanks,
John

P.S. PJ did comment that the original design voltage was 200 for T2/T14 so the variance from 230 was not a concern at all.... :-)

[Jim R.]

Hi John,

You said that this has always been the case since building the amp?  Or did this come with any other upgrades?

Remind me of your source, commputer, software, etc. and maybe there's something in the chain I can point to?

Also, does swapping the output tubes have any effect?  Those voltage variations are so small I'm not sure they would really be responsible for an audible difference between channels.

You also took out the volume pot and replaced it with some fixed resistors, right?  These were fairly well matched?

Hey, I see the word "Trios" in your avatar text -- bigger place means bigger speakers? :-)

-- Jim

[Paul Joppa]

The voltage at T2 and T14 is subject to substantial variation due to tube parameters. If you have changed the 12AT7 then it is a startling coincidence you should end up with voltages so close to what they were.

The voltage will vary with mu (gain) and with gm (transconductance). Since gm is related to the very tiny grid-cathode spacing, and mu to the much larger grid-plate spacing, I expect the variation is mostly due to gm rather than gain. Measurement is the only way to be sure, but trial and error are a good way to find a solution.

First swap the output channels to eliminate room acoustics or speaker sensitivity from the equation.

Then swap the input channels to eliminate source unbalance from the equation.

Then swap the 2A3s to see if one of them has more gain than the other.

Finally, try several 12AT7s to find one with better balanced gain between sections.

[Jim R.]

Paul,

That's why I asked the questionsI did -- we're on the same path but I know John upgraded to the psvane 12at7 at some point, but I think he also said this has been an issue since the original build.

My first thought was the tube too, but I then remembered that they were changed, though I don't know about the 2a3s.

In my experience most of these kinds of issues have either been tubes or room acoustics -- once all other variables have been eliminated.

The one slightly quirky thing I'd check is that the trim settings in the audiomidi setup program on the Mac are both set to 0dB, and it's a long shot, but I don't know if the mac is still the source computer.  Of course there are also similar trim settings in foobar on Windows too and I have encountered this problem in the past (though not on any of my personal machines.)

There is sometimes a problem that can happen with the Tranquility SE dac in terms of channel balance, but Ialso don't know if John is using this dac or not.

-- Jim

[InfernoSTi]

Guys,

Thank you so much for the comments. I'm reading them and thinking it through.  I'll post a better description of what I am hearing and try tube swapping/measuring.  Most important thing: it isn't an emergency repair or anything.  This is a "tweak" level concern.  

Again, your comments have made me think this through a bit more....thank you so very much!

John

P.S. I'm working crazy hours (14+ hour days so I'm having to post slower than I normally would....just a function of time/energy).

P.P.S. For source, I'm using a Mac with Decibel, an HRT II+, and a Warpspeed Optocoupler for volume...the inputs are bypassing the volume pot in the Stereomour via and are taken back to ground with two 100K resistors (they match very well). 

I'm also using Hawthorne Audio SSI Trios with a bunch of mods so they are sounding good!  :-)

[InfernoSTi]

Hey, I see the word "Trios" in your avatar text -- bigger place means bigger speakers? :-)

-- Jim

Hey Jim,

Thank you for the additional question.  I'm actually stepping up to something that I believe may be the last speaker I consider for years to come.  This is a custom set of Hawthorne Audio References with an Air Motion Transfer horn (12"x16") crossed over at 1,000 Hz, two 15" SSI midwoofers per side, and two 15" Augies on separate towers per side.  These are in four hand built custom baffles made from Sapele wood (woodwork by a wonderful wood artist from the Seattle area, Mike AKA "Woodsage").  The main speakers will be reviewed by Bound For Sound before I get them (glad to help get the word out on these beauties).

Take a read if you have some free time:  http://www.hawthorneaudio.us/forums/viewtopic.php?f=2&t=4162

Here are the main baffles and matching plate amp cases:
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.hawthorneaudio.us%2Fforums%2Fdownload%2Ffile.php%3Fid%3D962%26amp%3Bmode%3Dview&hash=9123aeb97a0268eba11f532e7d5c00d15f8cf1b5)

Here is one of the two Augie towers:
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.hawthorneaudio.us%2Fforums%2Fdownload%2Ffile.php%3Fid%3D1055%26amp%3Bmode%3Dview&hash=7269b6096ed41130fa2dee39d99afc4c3c182e30)

Thanks, Jim...

John

[Jim R.]

John,

Wow, read the entire thread early this morning and yowza, as was mentioned somewhere in the thread, though in a different context, there's no replacement for displacement. :-)  With that much surface area the dynamics are going to be off the charts and the IM distortion absent.  Congrats -- those should bbe amazing to listen to.

BTW, I'm sort of on the same path -- a last pair of speakers to keep me happy for the long haul, though a very different approach.  See the Blumenstein Ultra Fi thread in the Speakers forum if you haven't already looked.

I also ordered a pair of caramelized Orccas and another pair of matching BUF subs for the living room system -- Clark and Mike seem to come at speaker building from a very similar artisan ethic, and let me tell you, even if I can't see them, the Nagas feel absolutely beautiful.  But Linda and everybody else has confirmed that they are truly works of art.

BTW, check your PM,

Jim

[2wo]

Very nice, I have a set of DIY Duets myself. I will post a pic, if you don't mind the off topic.

Try taking the Warpspeed out. You could be getting an imbalance there...John

[InfernoSTi]

Jim, thank you for the PM...I will e-mail the requested info directly (and with pleasure).  I think one of the things that is always said about Mike's work is that it feels better than it looks. Many people have commented on their habit of touching the wood when the speakers aren't playing. 

John, thank you for the comments and suggestions.  I'm with you on the concern...that was where I started with the WS because that is when I noticed it.  I traded amps out for a bit and it wasn't that way with a different amp, however, so I think the WS just has stronger separation than I am used to otherwise (however would be right).  I'm thinking a solution would be a two pot WS so I could adjust independently left and right.  Might be useful for more than one reasons, actually. 

Here is the group photo of the Hawthorne Audio References with AMT horns installed and 8 15" drivers....lots of displacement, Jim, you are absolutely right on all the benefits!

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.hawthorneaudio.us%2Fforums%2Fdownload%2Ffile.php%3Fid%3D1066%26amp%3Bmode%3Dview&hash=0467bce9eb5cd03c645975057d15918875a24b67)

Best,
John