A Stereo Instrument Preamp/Headphone Amp (formerly "a JISBOS-with-gain Headamp")
Dec 24, 2009 at 5:05 AM Post #77 of 109

Zaubertuba

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Any reason why something like the interstage solutions from this (see fig. 2.2 and 2.3) wouldn't work? It'd save me the cost of two transformers.

BTW, I'm finally starting to get the parts together for this project: I just snatched up a pair of stereo Penny & Giles 4000 series faders off eBay. Cheap.
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Dec 24, 2009 at 5:28 AM Post #78 of 109

Steve Eddy

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Quote:

Originally Posted by Zaubertuba /img/forum/go_quote.gif
Any reason why something like the interstage solutions from this (see fig. 2.2 and 2.3) wouldn't work? It'd save me the cost of two transformers.


Well, 2.2 is using an output transformer.

2.2 would work, though keep in mind that the 11P1's secondary has a bit of imbalance due to the winding techniques used.

Quote:

BTW, I'm finally starting to get the parts together for this project: I just snatched up a pair of stereo Penny & Giles 4000 series faders off eBay. Cheap.
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Sweet!

se
 
Dec 24, 2009 at 6:44 AM Post #79 of 109

Zaubertuba

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Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif
2.2 would work, though keep in mind that the 11P1's secondary has a bit of imbalance due to the winding techniques used.


Well, I'm thinking about trying this with those Ampex transformers if I can get my hands on them.
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Dec 24, 2009 at 7:00 AM Post #80 of 109

Steve Eddy

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Quote:

Originally Posted by Zaubertuba /img/forum/go_quote.gif
Well, I'm thinking about trying this with those Ampex transformers if I can get my hands on them.
wink.gif



Hehehe. Well you're likely to find the same with other good quality input transformers.

And it's not a huge imbalance, so in the grand scheme of things, you'll probably still be fine.

se

Or what you can do is flip the trannie around, using the primary as the secondary. The primary is balanced. Just that you'll take a slight hit in CMRR.

se
 
Dec 26, 2009 at 6:06 AM Post #81 of 109

Zaubertuba

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Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif
...2.2 would work, though keep in mind that the 11P1's secondary has a bit of imbalance due to the winding techniques used.


You meant 2.3, right?
wink.gif



Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif
Or what you can do is flip the trannie around, using the primary as the secondary. The primary is balanced. Just that you'll take a slight hit in CMRR.


Well, the original idea was to utilize these for both balanced and unbalanced sources, with the lion's share of use with balanced (at least at this moment in time). I guess my thought is I wouldn't want to negate whatever benefit I would get with coupling the balanced source with the transformers.

Of course, perhaps this is a moot point. Should I be thinking about using the transformers exclusively on the unbalanced inputs? I was thinking it would be "better" to use them if I have them for unbalanced--balanced conversion anyway, but perhaps this would be overkill?

Also, I found this rather exhaustive tome on calculating an RC network, but I don't have ready access to an oscilloscope. Is there another way? If not, I suppose I could hit up my Physics Prof. friend at the University to help me out.
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Dec 26, 2009 at 6:23 AM Post #82 of 109

Steve Eddy

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Quote:

Originally Posted by Zaubertuba /img/forum/go_quote.gif
You meant 2.3, right?
wink.gif



Oops. Yes. 2.3.
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Quote:

Well, the original idea was to utilize these for both balanced and unbalanced sources, with the lion's share of use with balanced (at least at this moment in time). I guess my thought is I wouldn't want to negate whatever benefit I would get with coupling the balanced source with the transformers.


Then go with 2.2.

Quote:

Of course, perhaps this is a moot point. Should I be thinking about using the transformers exclusively on the unbalanced inputs? I was thinking it would be "better" to use them if I have them for unbalanced--balanced conversion anyway, but perhaps this would be overkill?


I'd definitely use them for unbalanced as well. That's one of the nice things about an input transformer. It will just as easily take input from an unbalanced source as a balanced source. And even when fed from an unbalanced source, they'll still give you excellent common-mode rejection as well as galvanic isolation.

Quote:

Also, I found this rather exhaustive tome on calculating an RC network, but I don't have ready access to an oscilloscope. Is there another way?


Sure.

You can just stop feeding square waves into transformers and freaking out if you see some ringing.
atsmile.gif


Seriously, in order for a resonance to ring in the first place, it has to be stimulated by something (transformers don't resonate on their own).

If the resonant point is up around 100kHz or higher, I wouldn't even worry about it unless it was REALLY nasty or something.

se
 
Dec 28, 2009 at 5:15 AM Post #83 of 109

Zaubertuba

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Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif
Sure.

You can just stop feeding square waves into transformers and freaking out if you see some ringing.
atsmile.gif


Seriously, in order for a resonance to ring in the first place, it has to be stimulated by something (transformers don't resonate on their own).

If the resonant point is up around 100kHz or higher, I wouldn't even worry about it unless it was REALLY nasty or something.



LOL - I like pragmatic. Sure simplifies the wiring, that's for sure.

Also - After watching the frantic bidding war on the Ampex's (someone must be refurbishing a tape recorder somewhere), I just found these online. You have any experience with them?
 
Dec 28, 2009 at 5:58 AM Post #84 of 109

Steve Eddy

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Quote:

Originally Posted by Zaubertuba /img/forum/go_quote.gif
Also - After watching the frantic bidding war on the Ampex's (someone must be refurbishing a tape recorder somewhere), I just found these online. You have any experience with them?


Never heard of 'em.

Their line isolators seem to be simple bifilar output transformers, same as CineMag's CMOB series, and their prices don't appear to be any better than CineMag's.

se
 
Jan 16, 2010 at 2:29 AM Post #85 of 109

Zaubertuba

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Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif
Never heard of 'em.

Their line isolators seem to be simple bifilar output transformers, same as CineMag's CMOB series, and their prices don't appear to be any better than CineMag's.



I haven't been able to find anything about CineMag's pricing, but if it's the same or close I'll sure go for the CineMags.
biggrin.gif


Got my kits from Jeff today. Woohoo! Man they're tiny!

After thinking it through I'm a little concerned about using just two transformers now. Take a look at this simplified sketch:

odysseyunbalpath.jpg


Running balanced (top circuit), everything looks fine. But running in "Unbalanced/Dual Headphone" mode, it doesn't look like there'd be a valid return signal path. Am I right on that? Seems like the transformer would need to be part of the same circuit in both modes...which isn't actually possible with just two transformers.

Which would bring me back to this, at least for the input stage:

jodysseyblockdiac.jpg


Would this work for both modes?
 
Jan 16, 2010 at 3:16 AM Post #86 of 109

Steve Eddy

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Ok, you've just reminded me that another reason I recommended using two transformers per channel was to deal with that funky mode switching you want to do that I had since forgot about.

Using two transformers per channel would take care of everything. Balanced and unbalanced inputs, as well as being able to easily implement your mode switch.

se
 
Jan 16, 2010 at 4:16 AM Post #89 of 109

Steve Eddy

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Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif
I like. The only thing I wouldn't want from that configuration is gain.


Why's that?

It's the same configuration I'm using for this, except I'm using step-up trannies instead of the 1:1's I drew for Zaubertuba's circuit.

iron_horse.jpg


se
 

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