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Jensen ISO-Max CI-1RR Review (Isolation Transformer)

pma

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Link transformers need deep and thorough understanding to give their best. This applies to source and load impedance, stray capacitance, floating/grounded sources and balancing of source and load impedance. Please see the trivial example where the link transformer is put into a soundcard loopback. The card has balanced input with 18k + 18k input impedance and SE output with 1k output impedance.

1) direct loopback, i.e. the transformer is sourced from 1k impedance
Loop_trafo_1kohm.png

Please note the 50Hz multiples with 100Hz the highest.

2) soundcard output first goes to the audio buffer with gain = 1, output impedance = 50 ohm
Loop_trafo_50ohm.png

Please note that the 50Hz multiples have disappeared. This is because the 1k source resistance in (1) caused gross impedance unbalance of the transformer input. Its impedance is complex, with capacitive component that brings the errors if heavily unbalanced. With the 50 ohm source impedance the situation is much better and the result is completely satisfactory. Everything can be explained.

Edit: This is even more pronounced when the high sensitivity microphone input is used. Again, a loop with transformer and 1k card output impedance and then the buffer with 50 ohm output impedance is added. The suppression of mains components is considerable and their origin is only because of the transformer parasitic impedances mismatch.

Loop_trafo_1kohm_xlr.png


Loop_trafo_50ohm_xlr.png
 
Last edited:

DualTriode

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Hello All,

Back in 2017 when I first put an APx555 on my bench I went on a quest looking for the “Best” tube headphone amplifier I could build.

With a up to 600 Volt Keysight bench power supply and switching in and out different cathode resistors it was pretty straight up selecting the optimal operating voltage. I could dial in the sweet spot for harmonic distortion and tube noise.

I tried various types of voltage regulators and did well at removing the power supply noise. Except no matter what I tried there was always a FFT peak at the 60Hz mains frequency.

I came to the conclusion that the 60Hz peak on the FFT was mains frequency leaking into the output transformer.

See the attached FFT of a 12B4A headphone amplifier.

Thanks DT

12b4 with filters FFF.jpg
 

levimax

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Hello All,

Back in 2017 when I first put an APx555 on my bench I went on a quest looking for the “Best” tube headphone amplifier I could build.

With a up to 600 Volt Keysight bench power supply and switching in and out different cathode resistors it was pretty straight up selecting the optimal operating voltage. I could dial in the sweet spot for harmonic distortion and tube noise.

I tried various types of voltage regulators and did well at removing the power supply noise. Except no matter what I tried there was always a FFT peak at the 60Hz mains frequency.

I came to the conclusion that the 60Hz peak on the FFT was mains frequency leaking into the output transformer.

See the attached FFT of a 12B4A headphone amplifier.

Thanks DT

View attachment 134915


Looks good! .... I doubt -115dB @ 60 Hz was an audible concern :)
 

Robbo99999

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The test for "does this eliminate the ground loop" is: Listen to your audio system. Do you hear a hum and/or buzz? OK you probably have a ground loop.

Disconnect ALL the inputs to your amp or preamp. The hum/buzz should be gone. Reconnect one of the inputs through this isolation transformer -place this transformer in line with one of the inputs to your amp or preamp. Did the hum or buzz go away? OK you eliminated the ground loop.

Does the humm/buzz come back when you plug the other inputs in? Then EACH ONE could need one of these isolation transformers. Elseways, try to identify the source of that ground loop and eliminate that source.

I found I needed an isolation transformer on the input to my Cable Box from the cable TV company - an RF isolation transformer. Had a nasty ground loop buzz in my A/V system until I put that transformer in there. I used this $15 job from Amazon => https://www.amazon.com/dp/B08FRGH4X2/ref=cm_sw_em_r_mt_dp_WFH7D1T1ANF0WE9BCGE6
Same here, I had to put an isolator on my TV aerial, only cost a couple of quid though.
 

Rottmannash

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I asked because I once had a cheap stereo isolation transformer that certainly didn't have flat response at low levels. Cylinder type with RCA leads coming out from both ends. I'll search if I still have it somewhere and do some measurements.
I have one of those I used years ago on my car stereo systems which did produce quite a bit of hum at times. Looks like a black cylinder w/ RCA ins and outs.
 

Spocko

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So buy 100 of these, take it a part, put it inside a nicer fancier larger "high end" case, then charge $2,000 and call it a dual "Speaker Riser & Noise Eliminator" all in one solution, then provide these measurements for John Atkinson to verify and publish, and you're on your way to making it on Stereophile's Recommended A-List accessories.

After the first 100 are sold, create a limited edition triple-threat $4,000 version that includes a small DAC (repurpose the THX dongle) with meters that randomly move whenever anybody walks by.
 

SIY

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Is there any way to measure CMMR for this? That is the one area transformers really shine and would help put it's performance into perspective. I have a DIY tri-amp systems with a computer and unbalanced connections which had ground loop issues (no surprise). I could have converted it all to balanced at great expense in money and time but instead I got a stereo version of an Iso-max and it works perfectly... dead quiet even with my ear next to speakers. Transformers, while old school, still have a place for some applications.
Here you go! 100k termination, no Zobel, 40R source impedance, 100k load impedance (thus the rise in frequency response in the top octave).
Jensen JT11-P1 CMR.png
 

Veri

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I solved the problem by using XLR to RCA cables from my DAC to my amp
Most of the time this can't really solve anything though. Your SE/RCA output must have been wired very badly for XLR-RCA cable to have any benefit. It's not like you get any balanced signal benefit through such a cable, that's not possible due to single ended wiring.
 

pma

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Simulated CMR of signal transformer with source resistance as a parameter.

primary winding resistance = 40 ohm
secondary winding resistance = 70 ohm
primary/secondary capacitance = 2 x 1nF
secondary parallel capacitance = 200pF
source resistance = 1 ohm, 50 ohm, 1 kohm
load impedance = 4 kohm (2 x 2 kohm vs. ground)

The best CMR would be with source resistance = 0 ohm. However, at least at higher frequencies we can get much better CMR with electronics. The biggest advantage of the transformer is that it can handle high CMV that would overload input of electronics.

CMR_trafo_SIM.png
 

Mulder

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There are manufacturers who build dedicated volume controls based on transformers, so-called transformer volume control (TVC). Does it degrade the sound? I assume that what is said here also applies to these constructions. In general, I have been thinking about whether it would be a good idea instead of a preamp if you have a turntable that you want to connect to an end stage in addition to DAC, CD-players, etc.
 

Pietro

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Wow, I did not expect such a level of performance...
using a symmetrical version would probably eliminate the induction of mains voltage
 

KxDx

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Most of the time this can't really solve anything though. Your SE/RCA output must have been wired very badly for XLR-RCA cable to have any benefit. It's not like you get any balanced signal benefit through such a cable, that's not possible due to single ended wiring.
I can only tell you that it worked. It wasn't a typical ground loop, it was noise being introduced from the PC. Perhaps some kind of coil whine. I started by buying a new sound card, but the new one didn't help, so I went external. I tried coax, optical, USB. I bought several DAC's. I replaced the computers power supply. I bought power conditioning devices. I ran drop cords to plug different devices into outlets in different rooms. The amp's Ground Lift switch did nothing. I cannot begin to tell you how much time and money I put into that issue.

The only other thing that stopped the noise was a cheater plug where I removed the ground pin, but that's not safe. But after all else failed, going from the balanced out on my DAC eliminated the noise. I may never understand the technical reason that worked, but I'll take a win when I get one.
 

TimoJ

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The only other thing that stopped the noise was a cheater plug where I removed the ground pin, but that's not safe.
I have used Hum X type DIY ground "lifters" with great success (HTPC, active speakers, preamp). It removes ground loops but still has working safety ground.
 

PeteL

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Looks good! .... I doubt -115dB @ 60 Hz was an audible concern :)
But his signal is at -20 dB, so in this case it's -95 dB, for this signal that we don't know how it translates in Volts. Quite alright too but a graph like that without references, in his case he was just showing us the leak, and said it's a working measurment so it don't matter but just shows that we have to be careful with the fine prints when we read graph without evidence of how it's calibrated.
 

MakeMineVinyl

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I use a Jensen JT-11 transformer at the input of my active crossover which isolates my preamp rack from the crossover/amp rack. I've found this transformer to have excellent performance, even down to 10Hz flat. I also use one of these transformers in my tube preamp (my own design) on the input from my DAC's balanced outputs. Since my system is very complex (a recording studio basically) I need foolproof protection against ground loops which may arise through connecting different gear through my patch bay.
 

SIY

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I use a Jensen JT-11 transformer at the input of my active crossover which isolates my preamp rack from the crossover/amp rack. I've found this transformer to have excellent performance, even down to 10Hz flat. I also use one of these transformers in my tube preamp (my own design) on the input from my DAC's balanced outputs. Since my system is very complex (a recording studio basically) I need foolproof protection against ground loops which may arise through connecting different gear through my patch bay.
I also use them, which is why I happened to have one handy. At 1V, distortion in midrange and treble is below the AP residual.
 

DualTriode

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But his signal is at -20 dB, so in this case it's -95 dB, for this signal that we don't know how it translates in Volts. Quite alright too but a graph like that without references, in his case he was just showing us the leak, and said it's a working measurment so it don't matter but just shows that we have to be careful with the fine prints when we read graph without evidence of how it's calibrated.

You know exactly how it calibrates to volts.

The scale is dBV, as in; 1Volt = 0 dBV.

Thanks DT
 

PeteL

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You know exactly how it calibrates to volts.

The scale is dBV, as in; 1Volt = 0 dBV.

Thanks DT
Yeah, noticed after the dBV, sorry, Performance is great, nothing to say here, but isn't -20dBV a bit low to caracterize performance, Useful but non optimal? Not putting in doubt your methods here, just curious. Measurments here are against 2 Volts, yours against 0.1 V
 
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