Phantom resistor buffer

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nacho459

Well-known member
Joined
Jul 20, 2004
Messages
339
Location
Pasadena CA
I should know this, but what value resistors should I use between the +48 line from the PSU and pins 2 & 3 on the input XLR of a mic pre. A 10K would do shouldn't it? Or would it mess with the power level and or input impedance?

(pin 2)--////---(+48)---////--(pin 3)
 
I don't know if I have any 6.8k laying around, maybe they have them at Radio Shack, I doubt 1% though. Is the value critical? If so can you explain why?

I hate to have to put in a Mouser order for just a few resistors.

Thanks, Jon "Nacho"
 
The resistors should be as close a match as you can do in each channel. The 6.8k value might be important for some microphones that use the S circuit the 6.8ks are EF resistors for the output section of that circuit. Alot of transformerless china microphone use that circuit SD and LD.

Other microphone circuit might not "care" as much. It all depends on the microphones.

I will hand match with a meter for the channel. I like to leave phantom on to keep the input phantom power blocking electros formed even when using tube condencers with a transformer out and dynamics.
 
[quote author="Gus"]I like to leave phantom on to keep the input phantom power blocking electros formed even when using tube condencers with a transformer out and dynamics.[/quote]
HIJACK:

Gus, I've read you saying this before. May I ask why? Just so the electros last longer? How do you deal with the pops when plugging/unplugging mics? Turn volume down like usual?


Nacho,

Better matching yeilds better CMRR. As mentioned the DIN/IEC standard says 6.81k because two of these in parallel makes 3.405k. We divide 48V by this value and we have a current limit of 14.1mA. Very few mics need even this much. Some people apply phantom with a single 3.3k feeding both pins or through the input TX primary.
 
Not so the electros last longer.

I do it to keep the electros formed. Electro caps get a bad rep from not being used correctly. To my ears electros need forming voltage. This is nothing new, its how the oxide layer forms to make the electro a cap and not a mess.

Do this test if you have time using a dynamic or condenser tube microphone with output transformer (ext ps) Use a mackie or green or other transformerless preamp with electro phantom blocking caps turn the phantom off (1/2 hour) and try the microphones turn the phantom on and try the microphones, Try to do this with headphones.

I think at the old place a few others posted about leaving phantom on all the time.

FWIW all the electros in the green pres I built have forming voltage(I had to add some parts).
 
Gotta find that Bateman article in Electronics World.

Greg Timbers at JBL has some speakers with batteries inside keeping a d.c. voltage on back-to-back 'lytics in the crossover network ;-).

The thing about maintaining the voltages I think has more to do with keeping things at equilibrium with regard to leakage current. Most modern high-quality 'lytics don't degrade like the old ones did when open-circuited, as far as gross loss of capacitance. But they will still take minutes to reach the minimum leakage current. Note what cap manufacturers specify for measuring their products' leakage: you are told to bias it for minutes before the measurement. Even then their leakage specs are usually wildly worse than actual parts, which poses a dilemma for designers.

I guess one thing you could do would have a pair of 'lytics biased all the time to the anticipated loaded phantom voltage, and connect them when you employed the phantom powered mic. In the meantime you would use another pair for input coupling with the lower voltage bias associated with unpowered mics. This would trade one problem for another, namely managing all those mostly "dry" contacts. Mercury-wetted reed relays perhaps? Of course you can't get those anymore other than surplus I believe (?). Or, just good switches with lots of wiping action located close to the circuit.

Sounds like a really large common-mode range d.c. coupled preamp wouldn't be a bad idea either---there's no capacitor like no capacitor as they say. Unfortunately some of the mics assume that you will be rejecting any d.c. differential at their outputs, so your capacitors get smuggled back in as d.c. servo or interstage coupling caps :(

Brad
 
Thanks for that link Peter.

FWIW I saw the AP curves of the before and after output biasing of an LM833, a pretty good for fairly cheap op amp. The differences were not subtle, although the class B output wasn't too bad to begin with. I did not see the FFT however.

Output loading-based biasing is different from offset voltage tweaking. In principle the output biasing will result in only the tiny change in d.c. level associated with the finite output R and d.c. open-loop gain of the amp. No coupling caps required.

I liked the account in the link of sync-ing the 7kHz to the 1kHz. It is strange and surprising that western music hasn't embraced the 7 multiple in music, although there are a few eccentric exceptions like Partch and modern "microtonalists". Britten uses 7 in the Serenade for Tenor, Horn, and Strings when he asks the horn player for an open tone at the "real" overblown pitch. A lot of performances have the hornist lipping or stopping this a bit because it sounds like a sour note, but some bravely let it all hang out.

Brad
 
Output loading-based biasing is different from offset voltage tweaking. In principle the output biasing will result in only the tiny change in d.c. level associated with the finite output R and d.c. open-loop gain of the amp. No coupling caps required.

Thanks Brad for pointing out that difference. You're right, 'pulling into class A' implies loading and I understood what RN did was to offset the outputs by means of simply driving the opamps inputs. I meant the latter but wrote the former ! :wink:

This all looks cool & simple & cheap - since it seems to improve sound with a worthwhile amount I'm really amazed why it isn't commonly done.
OK, idling current is higher, that might be a showstopper for some of applications. But still, this is a low/non-cost improvement.
Am I missing something here ?

Regards,

Peter
 
Peter, it may be that it is done more often but not talked about.

I recall a friend that worked originally for B*s* , sworn to secrecy on some of "their" ideas, who thought that it was something that had occurred only to him or to them, to put pulldown R's on single-supply LM324's---turning a truly crappy amp with hideous amounts of crossover distortion into a barely passable one. He was a bit disappointed when I told him lots of people knew about that.

Note that peranders, who makes a Jung buffer product, includes current sources on board at the inputs, for situations where you want to load the outputs of a preceding op amp (see the thread Discrete Buffer Design).

Brad
 
[quote author="clintrubber"]This all looks cool & simple & cheap - since it seems to improve sound with a worthwhile amount I'm really amazed why it isn't commonly done.
OK, idling current is higher, that might be a showstopper for some of applications. But still, this is a low/non-cost improvement.
Am I missing something here ?[/quote]

I was told by a tech at Ficusrote that they were using this technique in their Silver Series channel strips. The high-level signal processing in this line, at least the ones I looked at, is done with bog-standard 5532 chips, but they sound a lot more transparent than usual; the tech credited the improved sound to the output stage biasing, which they did with a pulldown resistor.

I wasn't particularly impressed with the mic pres in these units, even after they replaced the defective one I originally got for review (leaky coupling cap someplace), but the processing sections were mighty good for the price, and messed up the sound a lot less than usual. Pretty good for one resistor per amp.

Peace,
Paul
 
from Brad:
Peter, it may be that it is done more often but not talked about.

I see, yes that will be the situation.

With all those schematics around here it barely seems to turns up though -do you know of any examples ? There'll be a few that passed by unnoticed - it will be nothing more than a strategically placed resistor so that's easily overlooked, but still, I don't know of any boxes we've seen here that use it.

Regards,

Peter
 
Peter, true, I haven't seen it in here to speak of---but I am still a newbie by most of you folks' standards. I've seen a few occurences in Audio Amateur and its later incarnations (in fact I bitched about one where I thought the guy didn't understand how current sources/sinks worked).

For driving really low Z loads it does have the disadvantage of taking away from the maximum available current in one polarity. But who really drives loads that low very often? We all sleep better knowing we can perhaps, then drive a little cable C with maybe 10k at the other end, mostly.

It also is just shifting the region of crossover distortion to a different output voltage, unless output loading is really light---only so much you can do if the IC has crossover distortion to begin with. Intuitively that should be where the signal is such as to make that distortion less audible. But I think Self argues somewhere that rich class AB isn't a good idea---that if you're going to do class A, go pretty much whole hog.

(An approach that avoids some of the disadvantages of class AB, without the dissipation of class A, keeps a bit of quiescent current flowing in both sections of the output stage, regardless of output current, to keep the transistors' Ft's from going to zero. But it is tricky, and usually involves diodes somewhere with their own turnon/off behavior).

I say if it is really loud when you get to turning one half of the output off, who hears it anyway---but then I'm usually reaching for earplugs at that point anyway, or running from the room.
 
from Gus:
FWIW all the electros in the green pres I built have forming voltage(I had to add some parts).
I'm curious to what you did with the biggo 4700uF-cap @ the gain-setting. I guess it won't be two times 10.000uF and a tap to a higher voltage, right ?
BTW, this cap is not bypassed, unlike all the other elco's. I could understand that because of the required huge film-cap value. But would it make sense to do at least something ? Like adding say 4u7 in // ? I guess this 4700uF is now the most 'present' elco in the whole story, right ?

Want to share your mods Gus ?

Thanks,

Peter
 
[quote author="clintrubber"]I checked, nothing like that in the 'Rot-6'-box. Just curious, which one has it ?[/quote]

The VoiceMaster and ToneFactory, the first offerings in the Silver series. Possibly later Silver series as well.

Peace,
Paul
 

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