Matching pairs of OPT's?

[Dr. Toobz]

Is there any easy way to match pairs of output transformers? I bought a bunch of Specos recently, and they all seem to vary quite a bit. While I could listen to each one next to another, is there a way of measuring DCR or something to get a reasonably close match between two random transformers (for stereo)?

[JC]

I don't know an easy answer to your question.  In what respect(s) do you find that they "vary quite a bit"?

[Paul Joppa]

I don't know an easy answer to your question.  In what respect(s) do you find that they "vary quite a bit"?

I think that's the right question. They won't vary in turns or turns ratio - all that is done with mechanical counters and is likely to be precise. That leaves variations in the lamination stacking, specifically the EI gap spacing, variations in the metallurgy if the processing is not consistent, variations in the space between laminations if the punch leaves edges, variations in the wire thickness (DCR will tell this, but it is unlikely to be enough to make a discernable difference) and variations in the capacitance between winding layers.

The lamination variations will affect the bass extension and distortion; variations in the capacitance will have an effect on the treble. In the case of the Speco, the capacitance is relatively small - not enough to affect treble extension within the limits of normal hearing, but as is always the case with capacitance of real world materials it might affect the character of the treble.

You can count the laminations, but they are also probably done by machine and would be identical in all samples. That leaves an inductance bridge to test for LF inductance and losses, a distortion analyzer plus test fixture to test for LF lamination distortion, and some kind of high frequency reactance measurement to look at capacitance. Or of course the listening test - but be sure you are comparing transformers that are equally broken in; usually 50-100 hours of music will do that job.

[Dr. Toobz]

Basically, what I refer to is this: one transformer will often be louder or softer than another, pulling the stereo image off center. In addition, when I've played low bass tones through them, one channel will sound different than the other - for example, the bass may be less solid and will sound more as a buzzy, second harmonic than a fundamental tone (in this case, 30Hz or 40Hz). I first noticed this with the Specos that originally came with the S.E.X. amp - I couldn't figure out why one channel always seemed a little louder than the other and the stereo image pulled a bit to that side.

Perhaps this really does have to do with variations in the laminations from sample to sample. These are $3 transformers, after all! I can't measure inductance, so it looks like I'll stick with the listening test to match pairs.

[Thoburn]

Hi,

Pardon me as I don't have anything constructive to contribute to your issue, but I have an observation of my own system. Because of the layout of my living room, my speakers are offset. The room width is 16 feet and as much as 55 feet deep. The left speaker is just under 3 feet from the left wall where as the right speaker is almost 5 feet from the right wall. The right wall is dominated by 8' wide by 5' tall windows. The left is all solid wall. In my system the image is offset from center (relative to the speakers) by about 6 inches. It is rather annoying, but I try to ignore it. I've reversed the speakers. I've swapped the speaker cables left from right and the amp outputs as well. But, the image is always off set to the right. I even have a Bosch laser level device that lets me aim my speakers with high precision. Yea, I really do 8^). I guess it is just a sonic characteristic of my room which I would rather live with than try to fix by inserting a balance circuit into the system (The WAF has nixed room treatment options). I have seen the Bottlehead pre-amp that has seperate right and left volumn controls. That might just do the trick. Right now I have a Stereomour which I am very pleased with. So no pre-amp is forseen in my future.

[JC]

Thoburn, I think that if I were you, I would be very tempted to put dual mono level controls on that Stereomour.  There looks to be more than enough room.

[Grainger49]

Thoburn,

How about a picture in the gallery?  Come to think of it I'm starting a pictures of your listening space thread.  Here:

http://www.bottlehead.com/smf/index.php/topic,1251.0.html

[Doc B.]

Asymmetry in the room shape and in the speaker positioning relative to the walls can definitely cause that kind of imbalance. You might start by working on killing the first reflections off of the side walls.

[Thoburn]

Hi JC,

I am considering dual volumn controls. I think there is more to it. Because of my room I think the accoustics on the sides are very different and may not be symetrical in frequency response.

Hi Granger,

Will do with the pictures.

Hi Doc,

Thanks for the reply. I do have a GIK Tri Trap in the left corner. My listening room also happens to be our living room, so I have WAF to deal with. I will try to post some wide angle pictures of the space. I have been toying with the idea of some easily setup and tear down room treatments that can be stored out of sight when not needed. I saw some nice ones that are staggered depth chambers made by layering different width pieces of wood. They sit on their own stands and can be placed where needed. They actually look pretty good to.