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pjanda1 20th January 2011 04:18 AM

Opus, meet Jensen: how to implement an OPT?
I believe I have a pair of these headed my way (link to a Jensen 11ssp-6m datasheet, for those averse to downloading PDFs). I'd like to use them to convert the opus output to SE. I've reviewed the OPUS thread, and looked at the thread re: the Buffalo and opts. I've seen a few ideas, but much of the discussion is a bit over my head. I also don't see a great deal of consensus. I gather that I'll be choosing either 600:600 or 600:150, probably the former. (No worries about output impedance, its feeding a tolerant tube stage). And I'll need some resistors, at least on the secondary. I'd love some specific recommendations from folks that know more than I.


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Skip Pack 20th January 2011 03:44 PM

I have been using Lundahl 1544As on the output of my Opus for
a couple of years now. Work beautifully. I have them configured
at 1:1 with a 4PDT switch between the Opus outs and the transformers
to flip phase.


pjanda1 20th January 2011 03:47 PM

Why do you flip phase? Are you using any resistors or RC networks?

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Skip Pack 20th January 2011 03:59 PM

I just wanted to be able to reverse absolute phase without
swapping speaker leads. I have open baffle Altec 605s with
2-way crossovers, and the effects of changing absolute
phase and relative phase (highs versus lows) on the OBs
is interesting. I did not load the outputs of the transformers
beyond that of the amplifier it's driving, which is about 15K
at present.


pjanda1 27th January 2011 08:54 PM

1 Attachment(s)
Hooked 'em up and they work. I only listened for a couple of minutes, but it sounds good! The guy I bought the trafos from sent a little PCB that had resistors and caps. I added a resistor across the output jack. So, the transformer should be seeing 600 ohms at each end.

Should I be using the G connection somehow from the DAC board? I know I could could connect it to the "center" tap of the primary (have a two primaries wired in series), but is there any reason to? Should I be worried about DC? I'm not using output caps on the OPUS, nor do I want to. In Jensen's most relevant app note, they use a pot to balance it. I'm not in favor of excessive complexity, but I don't want to magnetize the transformer cores either.

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pjanda1 28th January 2011 04:26 PM

I hooked the G to the center taps and put it back together. Definitely a less "romantic" sound than running it SE through oil filled caps! The output level difference is less pronounced than I expected (based on the position of my volume knob, I didn't measure it). It does sound cleaner and faster, better defined. Probably a worthwhile upgrade for me. Others might be happier with the caps (which, FTR, were some little old motor runs).

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io 28th January 2011 08:11 PM

1 Attachment(s)

opus recommended output transformer design LL1527

pjanda1 28th January 2011 08:44 PM

Why use an opamp and an OPT? I'm sure it is technically sophisticated, but it also seems a little like the worst of all worlds: the sound of an opamp and the added distortion of the transformer.

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io 29th January 2011 12:48 PM

2 Attachment(s)

I do use it, not OPA
WM8742 DAC 1+1:1,75 a TP PCB
now test hearing

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