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Old 2nd June 2009, 05:22 PM   #1
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Default Unbalanced to balanced transformer before UCD?

I have a UCD180HG fed from a single ended preamp. The differential input on the UCD180HG is wired as per Hypex's instructions for this scenario.

Would there be any advantage in using an unbalanced to balanced transformer between the preamp and UCD?
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Old 2nd June 2009, 11:14 PM   #2
Javin5 is offline Javin5  Switzerland
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A potential advantage could be the isolation you get via the transformer, i.e. no ground loops. Be sure to bypass the red-brown input caps if you use an input transformer, otherwise you will have two time constants affecting your low-end response. Make sure the input transformer is of high quality (Luhndal, Jensen, etc.) and has a flat response at the low end down to at least 20 Hz.

Kurt
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Old 3rd June 2009, 09:24 AM   #3
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Thanks Kurt - the added isolation would be an advantage.

Not knowing how the input stage of the UCD works, I am wondering whether it performs better (lower s/n ratio?) with a balanced source.

If there were advantages to be had, and those advantages outweighed the disadvantages that would come with sticking a transformer in the signal path, then it would be worth doing.

Anyone understand the inner workings of the UCD enough to comment?
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Old 3rd June 2009, 09:39 AM   #4
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I discussed to the guys at Hypex a while ago as I had the idea on how to connect a transformer. As I thought the amp could be used with its input OP and capacitors removed like an instrumentation-amp, ie with the transformer secondary balanced. Zin of the UCD is then 3,6kohm. Senstivity is 9Vrms.
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Old 3rd June 2009, 10:03 AM   #5
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I hadn't thought about replacing the onboard input stage, but it is good to know those values. I was thinking of simply adding the transformer between the preamp and the UCD and connecting into the UCD in the normal way.
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Old 3rd June 2009, 10:09 AM   #6
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The whole idea is to exchange components, ie out with OPs in with transformers .

Adding another will not help much more than to get galvanic isolation. Doubt it is worth it. Good "see-through" transformers cost......
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Old 3rd June 2009, 11:32 AM   #7
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Quote:
Doubt it is worth it. Good "see-through" transformers cost......
I see the attraction in bypassing the opamp on the UCD, but that's not what I'm looking to do. I'd like to know if the on-board input stage performs better with a balanced input signal - if so, it may be worth adding a good unbalanced to balanced transformer before the UCD module. You say you doubt that it would be worth it. I'm grateful for your input, but am really looking for someone who fully appreciates the workings of the UCD input stage to comment.

You may well be right :-)
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Old 3rd June 2009, 01:14 PM   #8
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Maybe I was to quick to diss the transformer-before-opamps-idea.

IF.... UCD180 has input-caps to the OP-amps(donīt remember at the moment) it is a great idea to remove them and go balanced via a small 1:1 transformer like Lundahl amorphous LL1544A or LL1690.
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Old 3rd June 2009, 05:01 PM   #9
Javin5 is offline Javin5  Switzerland
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The UcD180 HG has a LM4562 bipolar opamp at its input. The input impedance is 100k on each input. If you bypass the caps, as I suggested earlier, the windings of your input transformer will reduce the original 100k resistance to a small fraction, which should result in lower input offset. Not that this is a problem in the original design, but lower is always better.

There is at least one amp manufacturer who uses the UcD with an input transformer; unfortunately I can't find the link anymore. Some will say the sound improves, others will disagree. I'm using mine dc-coupled, i.e. without the caps and without a transformer. Only you can decide which configuration you like best.

By the way, there is a huge thread on the Hypex UcD on this forum, maybe a search will reveal additional info on the benefits of an input transformer.

Kurt
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Old 3rd June 2009, 05:25 PM   #10
cbdb is offline cbdb  Canada
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As already stated the main advantage of a transformer is isolation. If you dont have ground problems you probably dont need it. IMO anything in the signal path thats not necesary is a bad idea. You will be adding some distorion, eq, noise etc. you will color your sound (more or less depending on the tranies). Some people like the color. And good transformers are expensive.
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