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Old 10th May 2011, 04:52 PM   #11
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Well thanks metalsculptor . I don't know if I need to go as far as an actual active amplifier just yet, though yes its crossed my mind. I did order that PAC SNI-1 I posted a link to as it was only a couple of bucks. and yeah it did have a little low end dropout. It got a little better after I hacked it and removed some input coupling capacitors, but the transformers are a bit tiny. I think with a little larger transformers I'll get decent wide band coupling with acceptable loss. I've got some "real" audio isolation transformers on order. We'll see how they work out.

To answer your question though, it's kind of just something audio has gotten boxed into. On the one hand, grounding is primarily a safety issue, with standards created and set fourth by electric companies. So we have all this methodology that really is ill suited for audio. Add to that the simple fact that balanced signal transmission adds the cost of 3 conductor cables and additional components (like transformers), and what we have is a case of every piece of equipment made with an eye to "pushing" the ground isolation issue off to someone else to handle.

It should be getting easier, because at least for me, 100% of the equipment I use in performance works off little wall transformers and DC converters that isolate pretty well. But anytime you have a stage 100 feet from the sound board, there are always surprises. I'm convinced Murphy's first name must be Groundloop. So all I can do is keep adding to my bag of tricks, and hope to get one up on him more often then not. In fact I have such a performance coming up tonight. Wish me luck! :-)
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Old 10th May 2011, 11:17 PM   #12
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Good luck, avoiding a ground loop in a performance setting with long cable runs and multiple power sources will need a bit of luck. In the industrial instrumentation field we either locally sample and digitize then send the data over ethernet or use shielded twisted pair to differential inputs. Anyway going to a better quality signal isolation transformer would probably improve the low end, it is not easy to make a transformer give a flat response over 3 decades of frequency.
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Old 11th May 2011, 01:03 PM   #13
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Metal: Well last night we ended up doing pretty well. It was a good test, but then again the sound guy was well equipped with the essential ingredient, DI boxes! Essentially they allow you to convert one or more of the low impedance balanced microphone input to the sound board, and interface to a line level out using an impedance matching transformer, and (I believe) some added passive electronics. Those boxes even have ground lift switches which surprisingly gave pristine quite results in the GROUNDED position! The sound was great, and our tracks do include some very high quality drum samples and string bass. I was impressed, and I think it would be wise for me to discover exactly what components and circuitry are inside one of those boxes.

Fenris: you're right... sorry. :-) Bad eyes!
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Old 13th May 2011, 09:00 AM   #14
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DI boxes come in several flavours; passive (which I prefer for low z sources such as synthesizers and mixers as, with no power requirements, they can give total ground isolation should it be needed) transformerless active, and transformer coupled active (better for pickups and really high impedance sources.

The former consist of merely a transformer, maybe a pad switch so you can take power amp outputs, a capacitor to remove any DC offset on the incoming signal, and a grounding switch, either on/off, or with added "resistive" link.

The hum rejection and audio quality depend entirely on the quality of the transformer (as does, unfortunately the price. No-one seems eager to make a really good cheap transformer). I have in the past used mic input transformers recuperated from old gear, with very acceptable results. They almost always step down the signal, assuming you want highish impedance input, but in your case this is not essential, and if you found a good pair if one to ones – but they will be bigger and heavier than stepdowns.

Ruddy Behringer techspecs; they give output drive and impedance for main outs, control room outs, headphone outs but nothing for the recording out. It is quite possible (likely, even) that it's lower level and higher impedance than the main outs, to match better to the domestic apparatus for which they are designed. In which case taking out of the jacks through a reasonable DI could end up with as much signal (and a better low end response; transformers work better driven from a lower impedance) as a one to one on the RCA connectors.
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Old 13th May 2011, 12:18 PM   #15
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@ chrispenycate Thanks! I have some (hopefully) good 1:1s on order, and I am hoping that "passive" will be sufficient. It will be driven from my mixer output which puts out a clean signal up to +10db, so I'm sure I'll be able to compensate for any loss, but we'll see. As far as low end response, I had already tried a "cheapo" product that had some much smaller transformers, though really they were no smaller then I've found in microphones in the past, and I thought the low end dropout was significant.

I think I may have made an error in my testing though. My mixer is one of those that has a feature that drives me nuts... it has separate MAIN and MONITOR output level controls, but the MAIN control affects the monitor too. I wanted to change that behavior, but rater than do it by hacking the mixer itself, I made a separate passive level control box to pass the MAIN output through. It contains a simple dual 10K pot, and it allows me to change the MAIN level if I need to, without affecting the monitor mix. Anyway, when I tried that inexpensive dual transformer isolator, I connected it between the output side of my passive MAIN volume control, and the amplifier. Since that control was set somewhat lower than MAX, I was driving my transformer with what was likely a much higher Z source than if I'd fed it directly from the mixer. And since you're saying transformers work better driven from a low Z source, that may account for a lot of the low end loss I was hearing!

So I'll have to re-try my experiment. But even so, the transformers I have on order have at least 4X the iron mass of those in this cheap unit I bought, so I'm sure using them will be a wiser choice. If I do continue to use my external passive level control, I'll put it on the output side (though I'm seriously considering saying the hell with the warrantee and modifying the mixer!).

You mentioned isolation capacitors to filter any DC in the output, and I'm very curious what might be the best way to do that, or if its even necessary. Again, referring to the "cheap" unit I originally experimented with, it did have perhaps 10uF in series with the input side of each transformer, but I've always been skeptical of using a single polarized electrolytic capacitor in this way. On the other hand, the only place I've seen non-polarized electrolytic used has been in tweeter circuits in loudspeakers. So I would like to know what is the best capacitor type there, and how I'd choose a value. And bottom line, I measure less than 1mV dc offset on the output of either channel on my mixer, and seriously wonder whether I should even bother with a capacitor. What do you think?

Last edited by Peter Pan; 13th May 2011 at 12:22 PM.
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Old 13th May 2011, 04:31 PM   #16
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chrispenycate, and others: I just got my EDCOR PCB-E1050 isolation transformers in the mail. Geesh... these transformers weigh in at about 2.3 oz each! If there's any loss of low end audio through these, I'll be shocked! :-)
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Old 14th May 2011, 07:48 AM   #17
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I would check out the transformer specifications for any permissible DC, while grain oriented silicon steel might be able to soak up 0.1% DC some of the nickel alloy cores would have trouble with this.

If the DC still worries you, put a resistor in series with the transformer and look at the unloaded current drawn by the transformer with a oscilloscope when driven by a low frequency sine wave. If it is asymetrical compared with the output voltage voltage then DC might be an issue
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Old 14th May 2011, 02:20 PM   #18
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Well the transformers I got are rated 1/4 watt, and as i said, the DC voltage I measured was less than 1mV. The question remains though, if I do wish to add some DC isolation, what kind of capacitor should I use, and how do I choose a sensible value?
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Old 15th May 2011, 03:48 AM   #19
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Quote:
Originally Posted by Peter Pan View Post
how do I choose a sensible value?
You need to know the load impedance and the desired lower roll off frequency. A rough indication of the load impedance is the input impedance of the mixer times the transformer turns ratio squared. The capacitor should have an impedance of say 1/2 or less of this value at the lower roll off frequency. The formula for the impedance of a capacitor is 1/2piFC where C is the capacitance in farads and F the frequency.

If this capacitance turns out to be 4.7uF or less use a film capacitor else aim for an impedance of 1/5 of the input impedance and use a bipolar electrolytic capacitor. The rationale here is that a low impedance behaves like a short circuit and even a non linear short circuit is still a short circuit. I don't believe that electrolytic capacitors are as bad as people claim but they do dry out over time.
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Old 15th May 2011, 01:16 PM   #20
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Metal: Yeah this whole impedance thing has been both the reason I was slow in getting transformers for isolation, and now, the reason I've no idea how to choose a capacitor value for DC isolation. Now I've tested my circuit with no capacitor with the mixer I'm intending to use to drive it, and I think I'm safe without it. But really, this whole impedance things is turning into a real mystery for me.

For one thing, one person in this thread replied that with a 1:1 turns ration, impedance is "reflected" through a transformer. Well I know the point of all this is to interface a line level output from my (known) mixer, to the line level input of another (unknown) mixer. I chose some fairly hefty transformers (1/4 watt rating), which happen to be 10K, and 1:1. (OK, they are center tapped, so I could theoretically make them 5K 1:1). But what is the output impedance of my mixer? It doesn't say in the specs. And as to the input Z of the mixer I'll plug into, remember the point of all this is ground loop isolation in a performance situation, which means I'll never have a clue what I'm interfacing to.

I do know that for signal level (not power) audio equipment, the output Z is supposed to be very low, and input Z is usually high. But who knows... The sound man may often pull out DI boxes when they see our setup, just to have a convenient way of connecting our line level outputs to low Z microphone inputs, and those DI boxes have their own transformers with (once again unknown) impedance.

Bottom line, I've made my isolator box with switches to put the transformers in or out of the circuit, and I just have to hope for the best. I've tried it with the output side connected to a variety of amplifiers accepting line level input, and they all seem to sound the same with or without the transformer. So that's a good result. Did I make a wrong choice buying 10K 1:1 transformers? Should I worry about a capacitor with less than 1mV DC offset? I don't know anymore. It all seems to sound good and nothing gets warm :-). I WOULD like to better understand how to have better planned on the correct impedance, but it seems like there are too many unknowns in a situation like this, and so I'll just have to cross my fingers and hope this works for most situations I run into.
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