Attenuating Unbalanced to Balanced Signal. Instrument level to Mic Level

[Colortone]

It doesn't answer the desire of the OP for attenuation, whether it's still necessary or not? The design brief seems to be a moving target.

True, attenuation is required. I dont believe my brief has changed and the title to the thread remains true - Im just finding the right language to describe it whilst also trying to understand the many varying replies.
For now im sticking with the design I have and attenuating the output with a U Pad to the desired level.

 

[Whoops]

Bo Deadly's Simple DI JFET / Impedance Balanced / Dual 9V

Could it be fed from Phantom power?

Is the output Mic Level or Line Level?

 

[Colortone]

Could it be fed from Phantom power?

No. Not this version anyway

Is the output Mic Level or Line Level?

The output is Mic Level as the thread title suggests.

 

[Bo Deadly]

Could it be fed from Phantom power?

Is the output Mic Level or Line Level?

No. Bo Deadly's Simple DI is NOT for a console mic input. Most people don't have consoles. And even if they do, there would be an inclination to just run into an audio interface or some other line-level gear. Phantom / attenuation + amplification is not necessary.

 

[Whoops]

No. Bo Deadly's Simple DI is NOT for a console mic input. Most people don't have consoles.

Most people have Mic Preamps so having a console or not it's irrelevant.
Also in Live Sound all inputs on stage are Mic Inputs.

A DI box to me, it's still a converter from Hi-Z unbalanced Instrument Level to Low Z Balanced Mic Level that's what more than 95% of the devices that have DI in the name are.
For devices that have Line Level output I can totally understand it's usefulness for smaller home setups where an hobiest only has a compact soundcard, but I personally don't call them a DI, and in the studio I prefer to inject my instrument signals through my Mic Preamp of choice.

That's also what the Op explained in the first post that he is looking for, Mic Output...

 

[Bo Deadly]

A DI box to me, it's still a converter from Hi-Z unbalanced Instrument Level to Low Z Balanced Mic Level that's what more than 95% of the devices that have DI in the name are.

That's also what the Op explained in the first post that he is looking for, Mic Output...

I'm not trying to have a semantic debate and I wasn't really trying to directly answer the OP question. I posted the circuit because it shows how simple it can be to get directly from an instrument to a destination and that using a transformer to step down only to amplify again is simple unnecessary these days. Even in a live environment there are line level inputs. Having a passive box with galvanic isolation is good in some scenarios. But not so much in a studio (home or otherwise). A transformer is not ideal for a guitar. The impedance is not really high enough for some cases, the primary will have significant capacitance and you could easily get LF distortion if you're not careful or the transformer is a cheap one. Bo Deadly's Simple DI will capture a completely transparent signal with a through connection to an amp so that you can re-amp.

 

[JohnRoberts]

This topic has been well inspected over the years... I guess the forum search isn't fruitful enough.

Historically DI boxes used step down transformers to reduce the relatively hot voltage from MI (musical instruments) down to mic level voltages appropriate for mic input stages. The transformer step down has the additional benefit of transforming impedances by the turns ratio squared. A good thing for both not loading down MI sources, and being compatible with typical mic preamps.

The broad DI category has been bastardized by numerous variant products that add sundry features and confuse classic definitions.

A few of us here have designed commercial DI boxes. One inexpensive one I did for Peavey last century (EDB-1) used a bifet opamp (TL072) to deliver decent input impedance, with a resistive pad on the output to step down to appropriate mic levels and source impedance.

JR

 

[ccaudle]

Here's a circuit that I just whipped up that I think would satisfy most environments and be a lot more effective over a passive transformer circuit:

View attachment 100368
Bo Deadly's Simple DI JFET / Impedance Balanced / Dual 9V

I have pointed this out before, don't want to be a broken record, but it seems that people still don't quite grasp that the output coupling capacitor is dominant in the output impedance. The build out resistors are 100 Ohms and matched on hot and cold leg, which is a minimal requirement, but at 60Hz the 22uF coupling capacitor has a reactance of over 120 Ohms, so the two legs are actually unbalanced by over 120%. Not quite single ended output mismatch, but certainly not impedance "balanced" in a usual understanding of the word.

 

[Bo Deadly]

I have pointed this out before, don't want to be a broken record, but it seems that people still don't quite grasp that the output coupling capacitor is dominant in the output impedance. The build out resistors are 100 Ohms and matched on hot and cold leg, which is a minimal requirement, but at 60Hz the 22uF coupling capacitor has a reactance of over 120 Ohms, so the two legs are actually unbalanced by over 120%. Not quite single ended output mismatch, but certainly not impedance "balanced" in a usual understanding of the word.

If you really want to get pedantic about CMRR then add the cap to pin 3. But that's really not necessary IMO. The purpose of the impedance balanced output is not so much about CMRR and more about ground noise cancellation which, if the other end is properly balanced, should be very effective.

Although my circuit should have a ground-lift switch.

 

[ccaudle]

not so much about CMRR and more about ground noise cancellation

Same thing, ground noise is common mode noise.

 

[Bo Deadly]

Same thing, ground noise is common mode noise.

True. But for a "simple" DI I don't think it would make enough of a difference to justify adding the 22uF (and drain resistor) to pin 3.

 

[Somkereki]

Ezt a témát az évek során jól átvizsgálták... Gondolom a fórumkeresés nem elég eredményes.

A hagyományosan a DI-dobozok lecsökkentő transzformátorokat használtak, hogy csökkentsék a viszonylag meleg feszültséget az MI-ről (hangszerekről) a mikrofon bemeneti fokozatoknak megfelelő mikrofonszintű feszültségekre. A transzformátor lelépésének további előnye, hogy az impedanciákat a fordulatszám négyzetével alakítja át. Jó dolog, ha nem tölti le az MI forrásokat, és kompatibilis a tipikus mikrofon előerősítőkkel.

A széles DI kategóriát számos termékváltozat rontotta el, amelyek különféle funkciókat adnak hozzá, és összezavarják a klasszikus definíciókat.

Néhányan közülünk terveztek kereskedelmi DI-boxokat. Az egyik olcsó, amit a Peavey-nek készítettem a múlt században (EDB-1), bifet opamp-ot (TL072) használt a megfelelő bemeneti impedancia biztosítására, a kimeneten egy rezisztív betéttel, amely a megfelelő mikrofonszintre és forrásimpedanciára csökkent.

JR

Én is így gondolom, 40 év hangmérnöki (és elektrotechnikai) gyakorlattal! Nincs is jobb egy (Jó) transzformátornál! (És még a CMRR is tökéletes!) Az egyetlen gondom a FET bemenettel rendelkező opampok használata! Tapasztalataim szerint a hangzás nem valósághű (persze a rockzenénél nem fontos) De dolgoztam rockzenekaroknál és különböző műfajú zenei fesztiválokon is. Saját, jól felszerelt laborom van, emellett stúdióberendezéseket fejlesztek és javítok. Tapasztalataim szerint a vibrációs zaj rontja a hangot, ezért kerülöm a FET-eket. 20 évig fejlesztettem a magyar színházak hangtechnikáját is. Ott kritikus a zaj. Sok sikert a munkádhoz, sajnos már 67 éves vagyok. Egy akkreditált laboratórium mérései szerint a 10 GBit sebességű Cat 6 kábel is közel 10-szeresére javul,

 

[Somkereki]

Én is így gondolom, 40 év hangmérnöki (és elektrotechnikai) gyakorlattal! Nincs is jobb egy (Jó) transzformátornál! (És még a CMRR is tökéletes!) Az egyetlen gondom a FET bemenettel rendelkező opampok használata! Tapasztalataim szerint a hangzás nem valósághű (persze a rockzenénél nem fontos) De dolgoztam rockzenekaroknál és különböző műfajú zenei fesztiválokon is. Saját, jól felszerelt laborom van, emellett stúdióberendezéseket fejlesztek és javítok. Tapasztalataim szerint a vibrációs zaj rontja a hangot, ezért kerülöm a FET-eket. 20 évig fejlesztettem a magyar színházak hangtechnikáját is. Ott kritikus a zaj. Sok sikert a munkádhoz, sajnos már 67 éves vagyok. Egy akkreditált laboratórium mérései szerint a 10 GBit sebességű Cat 6 kábel is közel 10-szeresére javul,

I think the same, with 40 years of sound engineering (and electrical engineering) practice! There is nothing better than a (Good) transformer! (And even the CMRR is perfect!) My only concern is the use of opamps with FET inputs! According to my experience, the sound is not realistic (of course it is not important for rock music) But I have worked for rock bands and also at music festivals of various genres. I have my own well-equipped lab, and I also develop and repair studio equipment. In my experience, flicker noise makes the sound worse, so I avoid FETs. I also improved the sound technology of Hungarian theaters for 20 years. Noise is critical there. Good luck with your work, unfortunately I am already 67 years old. According to the measurements of an accredited laboratory, even the 10 GBit speed Cat 6 cable improves almost 10 times, even though it is no longer a sound spectrum!

 

[abbey road d enfer]

There is nothing better than a (Good) transformer! (And even the CMRR is perfect!) My only concern is the use of opamps with FET inputs! According to my experience, the sound is not realistic

So you think a transformer connected to a passive electric guitar is "realistic"?
I can see much more performance alteration from a transformer than from an opamp (JFET or else).

 

[Newmarket]

Same thing, ground noise is common mode noise.

Yes. But I think the differentiation is between "Ground Noise" due to varying "Ground" voltages between units and common mode interference from external sources eg H-field from a mains transformer. The former is generally dominant in a studio environment.
fwiw I don't see it as a problem to fit the extra cap' and resistor especially if it's DIY for own use as the cost is small. Although bear in mind that electrolytics are usually +/-10% tolerance and may age differently.
Alternatively (or in addition) - use a larger cap'(s). 22uF seems a bit on the 'mean' side to me.

 

[Newmarket]

. According to the measurements of an accredited laboratory, even the 10 GBit speed Cat 6 cable improves almost 10 times, even though it is no longer a sound spectrum!

Improves what ?

 

[tinn]

Very simple HiZ adapter

This is a very simple adapter which fits into a XLR connector.
Not ideal with transformer mic inputs (biasing).
Otherwise very usable, especially if the artist has 20 Piezo Picups on stage.
Attenuation is around 26 dB. Needs Phantom Power P48.

 

[JohnRoberts]

Back in the 80s I designed a simple phantom powered JFET DI buffer. It fit inside an 1/4" connector shell.

old post

JR

 

[Whoops]

Very simple HiZ adapter

This is a very simple adapter which fits into a XLR connector.
Not ideal with transformer mic inputs (biasing).
Otherwise very usable, especially if the artist has 20 Piezo Picups on stage.
Attenuation is around 26 dB. Needs Phantom Power P48.

Very nice, thanks for that

 

[abbey road d enfer]

Very simple HiZ adapter

This is a very simple adapter which fits into a XLR connector.
Not ideal with transformer mic inputs (biasing).
Otherwise very usable, especially if the artist has 20 Piezo Picups on stage.
Attenuation is around 26 dB. Needs Phantom Power P48.

I'm very suspicious about the noise performance of this arrangement with high impedance sources.