Stereomour OPT upgrade (part 2, please bear with me)

[Fred_P]

I had already received valuable advice in another thread regarding my Stereomour OPT upgrade
which helped me narrow down the options to   

1) MQ EXO-45 full nickel   
   or   
2) Sowter 8983 full nickel.

Now I am asking for a little more of your patience since I have received the quotes for these as well, so it is finally ordering time.

Some background information on the setup:
the amp will be converted to 45, I will keep the stock choke, and bass is planned to be crossed over at 200Hz to active woofers.

Price wise the EXO-45 has a distinct edge, but customs duties + VAT will reduce the difference somewhat for me. (I am located in Europe)
The Sowter is more expensive, even if it comes from within the EU.

Now the obvious question is how they compare.

The MQ irons seem to have an excellent reputation here in the forum.
I have found less information about the Sowter...
Not sure if someone has any experience with it (or at least with the standard 30% nickel version)

My current preference would be the EXO-45,
but I would also be ready to pay the extra for the Sowter if it looks justified.

Any advice is appreciated.

[Doc B.]

They are both very good transformer winders. I have experience with the EXO-45 but not the 8983, so I cannot offer any useful comparison.

[Paul Joppa]

The coil design philosophy is quite different between the two transformer makers - the measurable differences occur well above 20kHz but are nevertheless audibly apparent to many listeners. I have not heard enough Sowters, in carefully enough controlled circumstances, to have any judgement to offer. For what it's worth, my own designs are closer to the Magnequest approach.

The nickel that Sowter uses is mumetal, around 80% Ni. Magnequest uses the lower 50% nickel called permalloy, which has much better power handling but lower permeability and less hysteresis. There will be a clear difference, but I hesitate to predict which any one person might find preferable. The one thing I can say is that you must use a high pass crossover filter before the power amp if you are using the Sowter in all mumetal. (It wasn't clear from the context here whether you have a line-level crossover for the 45 amp).

It's sort of a "should I get the Lotus or the Bentley?" question.

[Fred_P]

Thanks for the insight...

To keep things simple I did not plan any line level crossover, it is simply the open baffle speaker that rolls off at 200Hz,
while the active woofers are connected at the line level to an LDR volume control and go up to 200Hz.

So yes, this way the bass still goes through the amp.

On the other hand, the original 8983 with only 30% mumetal laminations is rated at 5W.
Here is what Sowter told me regarding the custom full mumetal 8983:
"Thanks for your enquiry regarding the 8983. We’ve made some adjustments to the 8983 using our design program and are happy that an all-Mumetal core would be able to handle the 2W of your specification. We have had to make some alterations to the numbers of turns on the windings but this is purely to maintain signal levels as per the original 8983."

Some research also confirmed the lower Bs and higher permeability of the mumetal compared to permalloy.
The only thing I did not really find was a lower Hc of permalloy compared to mumetal, which would be in favor of permalloy.

Anyway, currently my gut feeling is to be adventurous and go with the Sowter...
Once the whole thing is completed, I plan to share some impressions and may be some pics.
But that will take some time...

Thanks
 
Fred

[Grainger49]

Paul J. is saying to high pass to your Stereomour so you don't waste power on frequencies being handled by the sub.  You get more headroom and a cleaner, lower distortion, sound from the amp when the frequency range is optimized.

[Fred_P]

Hi Grainger,

Yes I do see the point.
But I am reluctant to do so.

The reason is that I am trying to keep the amp as clean and  as simple as possible.
To this end I will remove the source selector switch (this feature is provided by my DAC anyway) and will replace the volume pot with an LDR volume control that does not have any moving parts in the signal path.

Btw. I tried the LDR vol control already with my SEX amp, and even my wife noticed the improvement immediately.

Putting an extra crossover filter into the amp seems somewhat contradictory to all this.

Do you have any suggestion for a simple solution?

I was also thinking about using the coupling caps for this purpose.
I could choose smaller values to increase the cutoff frequency, but I still need to keep it low enough not to introduce a phase shift issues above 200 Hz.
Could this be an idea?

Thanks

[johnsonad]

Fred, just my two cents. OPT swapping makes a difference but a small one compared driver stage, coupling cap and Parafeed cap changes. I expect you will find a 5-10 percent change in sound at most. Paul's OPT designs are really well done and from my own experience switching to custom MQ iron made a small difference.

[Doc B.]

To extend Aaron's thoughts a bit, a big issue is what you consider an improvement. Some people like more treble response because for them it enhances the detail and resolution. Others like ample bass for the foundation, while still others prefer a midband emphasis for its added sense of dynamics.

This is painting with a very broad brush (OK maybe a roller), but the higher perm materials tend to give me a little better sense of extension at the frequency extremes, sometimes at the expense of a little midband compression when the transformer is pushed hard and the material saturates (I guess). I find that the grain oriented steels tend to be more forgiving and you can get away wth pushing them a lot harder if you want to rock out, but they tend to have a little more of the midband emphasis kind of sound.

The methods used to balance this are:
Make the high perm core f***ing huge - $$$$$$
Make the core out of cobalt steel lams - $$$$$$$$$$$
Mix a small amount of high perm material with a lot of grain oriented steel, to give it the "tickle factor" that Mike refers to, meaning enhanced low signal sensitivity.

This last approach is not as expensive as the others and when compared with a straight grain oriented steel core it does seem to show a little better resolution.

[Fred_P]

Hello Aaron,

Very true.

I do not expect day and night differences from the OPT upgrade.
And may be I blow the whole topic out of proportion....

I just installed the stock Stereomour iron in my SEX 2.0.
The only significant change to my ears was the improved bass.

But I am still crazy enough to do it.

Regarding the driver stage upgrade you mentioned...
The changes I came across are 1) the tube itself   2) the R1 on the C4S board   
I have already replaced the volume pot with an LDR volume control.
Is there anything else you would change in the driver stage?

Thanks

Fred
 

[braubeat]

Not to stir the pot too much but Lundahl makes parafeed outputs which can be gotten in amorphous cores. Which I think contain cobalt.

[JamieMcC]

I have been following the discussion with interest and wondered about the annealed steel transformers? I have seen a few offered as options which are said to have this grain orientation and also a lower carbon composition ratio? As its not something I have any knowledge of it seemed like a sensible time to ask about any experiences using these annealed sheet transformers and how they might perform in comparison to the stock steel(non annealed) and higher end Nickle options being discussed here if anyone has any experience with this type they might share?

[Fred_P]

Hi Braubeat,

Do you have the exact type of the Lundahl?
The only Lundahl parafeed I see is the LL1930, which is a preamp line output transformer with mumetal core.

A German maker called Silvercore has an amorphous core line output tranny which he would alter for me to have 8Ohms output impedance instead of 150Ohm.
Or alternatively he would make a toroidal parafeed OPT with an amorphous core. This would cost about the same as the Sowter 8983 full mumetal version.
But to me this seems the most adventurous option for a number of reasons.

Does anyone have something to share about the amorphous core?

[braubeat]

You order any of their outputs with a PPZ suffix, this means no airgap.
See the following:
http://www.audioasylum.com/cgi/vt.mpl?f=kandk&m=651
I have been reading about amorphous cores for years but still do not understand exactly what they are.

Michael

[Paul Birkeland]

Yes I do see the point.
But I am reluctant to do so.

The reason is that I am trying to keep the amp as clean and  as simple as possible.

You can change out a pair of resistors in the stock circuit to move up the low pass filter created by the coupling cap a bit higher.

[Fred_P]

@Michael
Thanks for the hint.
The LL1663-AM-PPZ seems to be a good candidate.
I just mailed Lundahl to confirm.

Btw I found some background information on the PPZ transformers here:
http://www.jacmusic.com/lundahl/tech-papers/PPZ-SPECIFICATIONS.pdf


@PB
Thanks for confirming the high pass filter question.
Would 22k be a good starting value?
That should move the -3dB point from 6.4Hz to 72 Hz if I am not mistaken, which would result in a 20° phase shift at 200Hz.