1:1 transformer (for audio isolation)

[snk]

Hello
I'm willing to build a passive audio isolation transformer to get rid of some possible hum on some FX chains.
So, i need to buy some transformers, and I am looking for a sensible balance between "sonically transparent enough" and "cheap".
I am not willing to buy expensive Jensen or Lundhall transformers : i'm looking for sub-10$ transformers.
Some Neutrik or Monacor are in the 10-15$ ballpark and well rated among users, but i am wondering if the mega cheapo chinese transformers from Ebay would be good enough for my needs ?

My questions are the following :
- how do you know the audio tech specs of a transformer (harmonic distorsion, frequency range, etc) ?
- Could you recomend any model/reference/brand of "cheap but good enough" (For instance the Bourns LM-NP-1001-B1L is very cheap, too) ?

[peterc]

Hi SNK

The cheaper audio transformers a re generally intended for telephony useage which means a limited frequency response and low signal level handling

If you have a look at the datasheet for the above Bourns transformer
https://docs-emea.rs-online.com/webdocs ... 09a5c7.pdf
the freq response is 200Hz to 3.5 kHz which would really impinge on your signal.

Also the distortion is .1% at 0dB @1kHz, which sounds OK on paper, but as soon as you apply valve amp levels, you could be sending +10 dB or more out there. Added to the fact that the distortion down at 100Hz will be 10x worse than that at 1kHz.....

So, unfortunately you gets what you pays for. The cheaper brands to look at are OEP and Edcor (WSm and XSM ranges). In the region of $15 - $20 apiece.

The small Neutrik is very small and does not handle high levels very well, but might be worth trying.

Hope this helps

Peter

[snk]

Hi, Peter,
Thank you for your reply. In the meantime, i have found the datasheet of those cheap transformers, and found that indeed, they're not suitable for "transparent' audio applications.
I think i will be buying a couple Neutrik transformers, they should be fine.

[phatt]

Why not simply just break the shield on one end of interconnect cables,, ground lift.
I'd try that before messing with transformers.
Phil.

[snk]

I never tried, i may try : how do you do ? You just take a jack plug, and unsolder the shield on one end ?
Is it safe ?

[phatt]

Yes break the shield at one end of cable. That is the easy bit ,, your big problem is working out exactly what and where the hum is coming from.

As a general rule of thumb; IF 2 audio systems are joined together and they are both grounded at the wall outlet then the shield of a mono cable can be open at one end and the signal will still pass as the units already have a ground return path.

"The idea is to not have more than one ground path".

As I have no info on your signal path or how it is powered then I can only give general advice.
The hum might be a weak psu for the pedal chain leaking onto the signal.
IF so then plug your guitar into the first pedal before amp then keep adding pedals one by one,,, Try to isolate which pedal is causing the problem.

Some pedal circuits are noise prone by nature and it takes some talent to work out how to solve these issues.
Hum can be picked up right on the pedal boards because of inductive coupling. i.e. psu cord right next to signal cord.
If the signal leads are cheap (not well shielded) then they can pick up hum.

If using switch mode psu for pedals they can cause a nightmare of interference and buzz.
Phil.

[snk]

Yes break the shield at one end of cable. That is the easy bit ,, your big problem is working out exactly what and where the hum is coming from. .

Thank you, Phil, i will try.
Ground loops/humming doesn't happen very often, so i don't need to find the source (as my effects chains change all the time, and most of the time the signal is very clean). I will try to make a custom "ground loop killer cable"
But i will also make a little box with a couple transformers, as i will also use one of them as a buffer (impedance converter - of course, in that case, it wouldn't be 11).

[roseblood11]

Good transformers for high impedance signals aren't cheap. I used the Lehle LTHZ in some pedals, it's the same that is used in the Lehle P-Split II isolation box. The P-Split is always in my guitar case, every guitarist should have one...

[snk]

Thank you, Roseblood;

[Dirk_Hendrik]

What I found rather interesting for this application were the Wurth WE-DD inductors that were recently put to my attention:
http://katalog.we-online.de/en/pbs/WE-DD

As an example the 744878101 is quoted 2.16 eur/pc at Mouser.

[plush]

Wurth WE-DD inductors

Using power chokes for audio isolation?
What is their frequency response?

[snk]

Thank you Dirk for the tip (and for your great website, as well), i will consider these inductors!

[Dirk_Hendrik]

Wurth WE-DD inductors

Using power chokes for audio isolation?
What is their frequency response?

Depends on the application. But in stompboxes, where we use so many parts in their non-linear region, I consider nothing sacred.

[plush]

Wurth WE-DD inductors

Using power chokes for audio isolation?
What is their frequency response?

Depends on the application. But in stompboxes, where we use so many parts in their non-linear region, I consider nothing sacred.

Typical chokes have pretty low inductance, it will ruin the signal if we use low power dual inductor as a 1:1 transformer.

[grrrunge]

What I found rather interesting for this application were the Wurth WE-DD inductors that were recently put to my attention:
http://katalog.we-online.de/en/pbs/WE-DD

As an example the 744878101 is quoted 2.16 eur/pc at Mouser.

With DC-resistances ranging from 0.03Ω to 1.4Ω, i guess you'll have a hard time finding a reasonably priced op amp capable of driving those
They look like they're intended for use in low power flyback converters. In that case high frequency response shouldn't be a problem, but you'll quickly run into core saturation issues at low frequencies.

[plush]

With DC-resistances ranging from 0.03Ω to 1.4Ω, i guess you'll have a hard time finding a reasonably priced op amp capable of driving those
They look like they're intended for use in low power flyback converters. In that case high frequency response shouldn't be a problem, but you'll quickly run into core saturation issues at low frequencies.

Yep, I've managed to get some response from similar inductor somewhere around 1-1.5 mhz (the whole idea of me trying this was pretty retarded).

My personal verdict: unchoochable.

[robinbowes]

Good transformers for high impedance signals aren't cheap. I used the Lehle LTHZ in some pedals, it's the same that is used in the Lehle P-Split II isolation box. The P-Split is always in my guitar case, every guitarist should have one...

[ Image ]

Any idea where to buy the LTHZ?

R.

[plush]

Good transformers for high impedance signals aren't cheap. I used the Lehle LTHZ in some pedals, it's the same that is used in the Lehle P-Split II isolation box. The P-Split is always in my guitar case, every guitarist should have one...

[ Image ]

Any idea where to buy the LTHZ?

R.

Gewgl Ebay

[robinbowes]

Crikey, they're not cheap.

I was considering making a homemade Radial IC-1 ( http://www.radialeng.com/icecube.php ) but it's not worth it at that price.

R.

[mauman]

I've successfully used the Xicon 42TM018-RC transformer, which is 1:1 with 10k impedance on both sides. Data sheet: http://www.mouser.com/ds/2/449/XC-600134-1212477.pdf I got mine for less than 4 bucks each at Mouser. The data sheet says the 3dB frequency response is 300Hz to 3.4kHz, but don't be afraid of that - after adding enough gain to bring it back it unity, my application measured quite flat to 10kHz. I got the idea from this circuit: http://www.geofex.com/FX_images/TransformerSplitter.pdf , you could adapt it from a splitter to a single in/out for isolation. Regards, Mike