OEP vs Lundahl transformers in the 1176

[glide 1]

[quote author="gevermil"]Ive got that big old freed 1:1 on the out and it works well
the oep dose tend to break up when you crank it
What ratio is that A262A2E wired on the H version , 1:2 ?
ID like to try another freed 1:1 in maybe[/quote]

i think it's 1:2

 

[Dr nEon]

Sorry folks to drag up this ancient thread , but there is a significant question here regarding the correct value for resistor R84 when using an OEP 262A2E output tx ...

It is suggested that the load resistor r84 ,after the Tx ,be changed (from 1k) to a 2K4 , to suit the OEP , for better frequency response . The tests mentioned on the previous few pages of this thread look interesting , but I am surprised , after lots of searching, to find no talk of anyone implementing this...

I have an OEP in my latest 1176 build, which I fired up for the first time last nite...I was absolutely floored at how good it sounded, or should I say , how good it felt , compared to my Lundahl/pana fc/superclean/super spineless version..

The OEP feels way better in the low end and mids ...and just generally much more exciting , like a (decent)1176 should.

Although the OEP top end roll off isn't by any means nasty , I really want to explore the simple R84 resistor mod ,which is hinted at earlier, and see how good this OEP unit can get...

However , I don't fully understand what are the implications regarding output impedance, and how this mod would affect the way in which the 1176 will interface with the next unit in the chain, whatever that may be..

So for that reason I'm cautious , and also surprised that the mod isn't mentioned anywhere else (BOMs etc),if it really is such a no brainer!

If anyone can confirm that this R84 mod doesn't cause any nasty side effects , then I will be a very happy bunny indeed!

DIY rocks!

nEon

 

[Dr nEon]

..Anyone??

Sorry to bump , but I imagined a lotta folks out there may have gone for the OEP option, and may also be interested in what became of this mod..

Cheers

nEon.

 

[gyraf]

I haven't tried the OEP option - but it makes sense with the 2K4 termination..

Jakob E.

 

[Dr nEon]

Great , thanks as ever , Jakob !

I still get nervy about deviating from published values , in parts of circuits that i don't fully understand ...but in this case it looks like the 2k4 is technically more correct than the 1k anyway..

So I'll give the 2k4 a go,and see how it sounds!

I can see that I still have a lot to learn about transformers...just printed off rod elliots exhaustive articles...should keep me goin for a bit!

Many thanks again

nEon

 

[rodabod]

People here should listen to MNats, as he's done what each of you should do if you want to investigate with the transformer loading here.

Remember that the secondary load resistor will end up in parallel with you next piece of gear's input impedance (eg. commonly 10K).

Ideally, I'd set up an audio test set and place a proper Zobel network by following Jensen's procedure

Roddy


[quote author="mnats"]
On the other hand, when did DIY become wait for someone else to do it?

Here's just a quick and dirty test I did: I put a 10k pot on the secondary of the OEP, fed a test signal in at a mid-frequency to set a reference output on the 'scope. Then I cranked up the test frequency to 20kHz and futzed with the pot and output gain until it gave the same output voltage from about 1k to 20kHz. When I measured the value of the pot I got 2.77k.

2.4k gave -0.6dB down at 20k

1k was -4.93dB down at 20k
[/quote]

 

[Dr nEon]

Hi rodabod,

Yes it was MNats earlier post that got me interested in this..

Remember that the secondary load resistor will end up in parallel with you next piece of gear's input impedance (eg. commonly 10K).

So are you saying that this is a good thing or a bad thing? This was the exact bit that I was worried about , in the earlier posts...I don't understand.

Unfortunately I don't (yet :wink: ) have an audio test set , or a scope ,even ...

..just a dmm and a great willingness to learn ! :grin:

cheers

nEon.

 

[rodabod]

[quote author="Dr nEon"]
So are you saying that this is a good thing or a bad thing? This was the exact bit that I was worried about , in the earlier posts...I don't understand.
[/quote]

It's just something you need to consider. So, if you think about it, the secondary load will actually be your load resistor in parallel with say, 10K.

So, If MNats says that a 2.4K load gives good response, then what resistance in parallel with 10K equals roughly 2.4K?

Anyway, experiment and see what sounds good.

 

[veermaster]

I know nothing about TX but I checked my g1176´s FreqResponse (OEPs at Output) before and after the "MNats Mod": BEfore I had the same bass-rolloff everybody seems to experience. Sounds cool in a way but I better like the "modded" version with the flat FreqResponse, so it works!
emre

 

[Dr nEon]

So, if you think about it, the secondary load will actually be your load resistor in parallel with say, 10K.

BING!!...(lightbulb appears..) ..GOT IT! Thank you Rodabod!

Yeah I'll experiment...probably I'll measure my tx ,the way mnats did , and then calculate my resistor value taking into account the assumed 10k ...

..And LISTEN of course :grin: ..very carefully!


Hey Veermaster!..

BEfore I had the same bass-rolloff everybody seems to experience

You mean HF rolloff , right?..

Pleased to hear you did the mod and were happy!...so what value did you wind up using in yours..just outta interest..?.

Great stuff..thanks guys!

nEon

:guinness: :sam:

 

[veermaster]

Sorry, of course I meant HF rolloff!
I used 2,8k ...
Good luck, emre

 

[raysolinski]

I have to chime in..I purchased Matta's 1176 that has an OEP output transformer and found it to be very dark....did the r84 mod to 2k7 and now it it much more flat but still has color and balls..might be cool to put a 5k pot in there and dial in how much hf rolloff you want :green:

Cheers,
Ray

 

[kenrinc]

I finally had time to go back and check R84 on my dual unit. First channel has a 2k in R84 and 2nd channel as 1k which explains why I thought ch2 was a bit dull, not by much but just enough to be noticeable. 2k works for me so I'll move ch2 over to one.

What was the difference in going with 1:1 on input as opposed to 2:1 stepdown? Using OEPs.

Ken-

 

[goldenGeek]

I`ve been searching for days for the pinout/wiring for using OEP output in my 1176. I know I had it on a webpage earlier on, but that`s not there anymore. I`m building on a PCB I got from Gustav about a year back and these boards don`t have the possibilities of dumping the OEP straight in, so I`ll have to wire it to the right connectors.  So my question is basically - which pins do I solder together and which pins goes to where on the PCB?

 

[YLab]

Here it is!

Hub

 

[mathflan]

Hi,


I'll plan to build a dual 1176 but I hesitate between two configurations :

OEP transformers at input and output section

OR

Electrical input and lundahl transformer at output section

I do not have the budget to build a 1176 with lundahl transformers at input and output so
What do you think?

thanks a lot

 

[mathflan]

I will put OEP trannies at input and output section,

But Which I have to choose for INPUT : A262A2E o A262A3E?  :-\

thanks

 

[rodabod]

But Which I have to choose for INPUT : A262A2E o A262A3E?   :-\

A2E,

 

[mathflan]

ok thank 

 

[goldenGeek]

I know this is an old post, but I just tried this mod on my 1176 with OEP OT (switchable between 1k and 2k4), and it doesnt do anything at all. The switch, cable and resistors are fine, i measured that right before I soldered the cables to the board. Its built on Gustavs board, does this have anything to do with it? I'm feeding some noise through it and my meter does not show any change in hf response (or any other frequencies as well). It's the R84 right next to x-y pionts and the trafo, right?