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Help with Autotransformers

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Posted on June 13, 2019 at 18:26:06
Cory M.
Audiophile

Posts: 1168
Location: Midwest
Joined: April 10, 2005



I picked up two vintage Jensen Autoformers that I'd like to try out with my OTL amp, before (if) I invest in the Zeros. I hooked them up the way I thought that they should be connected, after doing a bit of research (and not finding a whole lot), and the outcome was just a very faint sound.

The connection that tried was: positive from amp to "common", 16 ohm tap to the speaker positive terminal, and negative straight from the amp to the speaker.



Cory


 

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RE: Help with Autotransformers, posted on June 13, 2019 at 19:37:39
hitsware
Audiophile

Posts: 2553
Location: N. Calif.
Joined: December 30, 2004
Try:
amp common and speaker common to 'common'
amp output to 16 ohm
speaker in to 4 or 8 ohm

 

RE: Help with Autotransformers, posted on June 15, 2019 at 17:26:35
Cory M.
Audiophile

Posts: 1168
Location: Midwest
Joined: April 10, 2005
Thanks, hitsware. That does make more sense now, as it's not just a simple resistor in series.

I did try it in that configuration, but ended up with slightly lower volume, and less bass. Wondering if I should try the amp side into 4 or 8, and the speaker into 16?


Cory


 

RE: Help with Autotransformers, posted on June 15, 2019 at 17:50:01
hitsware
Audiophile

Posts: 2553
Location: N. Calif.
Joined: December 30, 2004
Why not ?
Barring an accidental short, I don't think
you can hurt anything ?
( Though some tube amps ( may ? ) oscillate into
no load .

 

RE: Help with Autotransformers, posted on June 16, 2019 at 12:49:49
Caucasian Blackplate
Industry Professional

Posts: 8312
Location: Seattle
Joined: June 18, 2004
Yes, you should have lower volume, were you expecting different results?

If you use the autotransformer to step the speaker load down, then you are defeating the entire purpose of doing this in the first place in the context of OTL amps!

 

RE: Help with Autotransformers, posted on June 16, 2019 at 14:06:49
Cory M.
Audiophile

Posts: 1168
Location: Midwest
Joined: April 10, 2005
Hi CB,

Thanks for the reply. I wasn't quite sure what to expect, actually- possibly more effortless dynamics, maybe a greater sense of "ease-" both qualities that this amp/ speaker interface already excels at. Definitely not less bass and lower volume, though.

I'm new to the OTL party, and have read that a few folks have had success with autotransformers (The Zeros, in particular) to match their speaker's reflected impedance to the tubes more closely.






The builder of this OTL, Craig Merz, spec'd power ratings at 100 WPC into 8 ohms, and 75 WPC into 4 ohms. This amp uses 10 6AS7Gs per channel in a modified totem-pole configuration, so the output stage's impedance may already be where it needs to be to drive my 8 ohm nominal KEF LS-50 speakers at their best.

I'm planning on trying it with 'stat panels soon, which may be an entirely different story.

Best,




Cory


 

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