A Pre for your Ribbons

[clintrubber]

[quote author="guitarmaker"]If you can get by with a little lower secondary http://cinemag.biz/mic_input/CMMI-7C.pdf has a 50 ohm tap.

Steve[/quote]
Hmm, nice suggestion, makes me realize the prim-CT of a Beyer TR/BV could be used as well for 50 i.s.o. 200.

I'm thinking of:

TR/BV 3.10.210.005 1:10 200:20k bif split prim

(Should check if it has enough Henries left.)

Bye,

Peter

 

[pstamler]

[quote author="enthalpystudios"]i already have a small handful of o-1's is why i ask, maybe i'm wrong about 50 ohm taps..... for some reason i thought that it was a good move to bridge the very very low impedance of most ribbon mics.[/quote]

Actually, classic ribbon mics were designed to see a high preamp impedance, a transformer coupled to a tube grid with no terminating resistor, which was the usual design back in the good old. See Scott Dorsey's review of the new AEA preamp, designed for ribbons.

Peace,
Paul

 

[NewYorkDave]

Here's a good AES paper:

On Electrical Loading of Microphones
Werner, Richard E.
JAES Volume 3 Number 4 pp. 194-197; October 1955

Abstract:
Electrical loading of microphones by preamplifier circuits has not been of great concern to the audio engineer, heretofore. In the past, the input impedance of preamplifiers has been so much higher than the output impedance of microphones that the effect upon the performance of microphones has been indeed negligible. The recent appearance of transistorized preamplifiers whose input impedance is sometimes quite low, and the growing use of high-sensitivity ribbon microphones which have a highly frequency-variant output impedance has stimulated new interest in this loading problem. A study of the Thévenin equivalent circuit of certain common types of broadcast microphones discloses that the input impedance of a preamplifier must be maintained at a value at least five times the nominal impedance of the microphones with which the preamplifier may be used-in order to avoid undesirable alteration of the microphone's frequency-response characteristic. This applies unless the preamplifier has been designed for a particular microphone.

 

[clintrubber]

Thanks for adding both :thumb:

That (familiar) factor of five is well paid attention to in the specs of
my ribbon mic (and the only ribbon for a while I expect):

impedance < 200 Ohms, recommended load impedance: 1000 Ohms

So 1000 or more I expect; I guess that say 2k, 20k or 200k loading won't make any difference anymore.

OK, so better no sec-termination of the input-TX, unless peaking requires a Zobel, right ?

 

[Marik]

Question: what is the 6SN7 White CF output impedance? around 150-200 Ohm? With NFB even lower. Then with a 4:1 output xformer we have less than <10 ohm output impedance. Do we really need it THAT low and lose db?

[quote author="clintrubber"]

That (familiar) factor of five is well paid attention to in the specs of
my ribbon mic (and the only ribbon for a while I expect):

impedance < 200 Ohms, recommended load impedance: 1000 Ohms

So 1000 or more I expect; I guess that say 2k, 20k or 200k loading won't make any difference anymore.
[/quote]

The factor of five works well in condensers, but in ribbons can be somewhat unpredictable.
It works better for lower quality mics, where some loading helps to damp resonance modes in the motional impedance of thicker ribbons on lower freq., or supress some xformer ringing.

With quality ribbons I found unloaded (i.e. factor much higher than 5) operation gives much more natural and uncongested sound. You will hear a big difference between 2K and say, 20K loading.

 

[clintrubber]

[quote author="Marik"]Question: what is the 6SN7 White CF output impedance? around 150-200 Ohm? With NFB even lower. Then with a 4:1 output xformer we have less than <10 ohm output impedance. Do we really need it THAT low and lose db?[/quote]
Hmm, I had even an Edcor 10k:150 in mind (I have these, not the :600) so that would even make things more tilted.

If the 150-200 is true then the TX could be skipped altogether I guess,
just like was said about the ECC82/12AU7-output of the
analag tube-opto-comp (Zout of that one said to be 225 Ohms).

OK, that's less iron then, but OK.


Hmm, since there's a HT-supply anyway, thoughts popped up to combine that analag tube-opto-comp with this mic-pre. Maybe put a 10k:10k in between for 'regaining iron'.

 

[clintrubber]

[quote author="Marik"]The factor of five works well in condensers, but in ribbons can be somewhat unpredictable.
It works better for lower quality mics, where some loading helps to damp resonance modes in the motional impedance of thicker ribbons on lower freq., or supress some xformer ringing.

With quality ribbons I found unloaded (i.e. factor much higher than 5) operation gives much more natural and uncongested sound. You will hear a big difference between 2K and say, 20K loading.[/quote]
Thanks for that info Marik, most informative. So I better make the input-Z not too high.

And so while it'll may look just fancy and me-too, adding a 'impedance'-control does makes sense then.
Even when not as a configurably input-TX but in the form of a few switched load-resistors.

Bye,

Peter

 

[clintrubber]

[quote author="mikka"]12SN7..... :green:

ECC 33/34/35 ..... if you're rich or lucky

5692 .....

VT-231 .......

12SX7 .... specially selected

The 6SN7 is basically two 6J5 triodes in one glass envelope....(Wikipedia) Are these just around the corner from you?
http://cgi.ebay.com/4x-NEW-6J5-USA_W0QQitemZ320091134585QQcategoryZ7275QQssPageNameZWDVWQQrdZ1QQcmdZViewItem[/quote]
Hmm, I actually was lucky, the only octal double-triode in my junkbox happened to be a 6SN7 :thumb:
Not sure if it is still OK, so I keep an eye on those nearby 6J5's.

 

[NewYorkDave]

What's the purpose of the positive feedback from U3 to the cathode of U2?

Did you build this thing?

 

[clintrubber]

[quote author="NewYorkDave"]What's the purpose of the positive feedback from U3 to the cathode of U2?[/quote]
The 56.2k & 5.1uF branch ?

Stated to be:

The second stage is somewhat tamed with the omitted cathode bypass cap and current feedback from the White CF.

... but I'm not sure either.

Did you build this thing?

A pic would be nice. It's not that I don't believe it, it's just that these CAD-drawings don't have the vibe of a turret-board :wink:

 

[valvehead]

[quote author="NewYorkDave"]What's the purpose of the positive feedback from U3 to the cathode of U2?
[/quote]

On top of that there is also feedback to the screen of U2. The cathode and screen appear to be forced to the same AC voltage. Is this not also positive feedback?

 

[mikka]

I don't know that the circuit is exactly correct..... I doubt if Analag would spoon feed us to that extent ....... :green:

I assume that because he's leaving the cathodes unbypassed for negative feedback he's introduced some positive feedback to bring the gain back up. Whether the loops have been returned to the right point ..... I dunno ....

 

[solder_city]

i think it was meant to be negative feedback but he goofed and brought it to the wrong point. the amount of feedback is so low (look at those resistor values) that his simulator didnt freak out and he called it good. just my guess. im sure if he responds hell say he meant to do it that way and mikka is right on the money :razz:

pstamler already brought up the positive feedback long before nyd did, back on page 1 and analags reply to him is completely wrong- as if he either doesnt understand whats happening in the circuit or he didnt pay attention to the question. but you guys were too busy pissing yourselves with delight to take a closer look and see that pstamler was right.

im still amazed at how enthusiastic you guys get about this vaporware and dont you even take a good look at the circuits before you heap on the praise and start ordering parts to build?

i know it looks like im picking on analag but what im really picking on is how unquestioning and uncritical a lot of you are. its cool that the guy posts ideas and i never try to stop a brother whos doing a creative thing- but its a diservice to the spirit of the drawing board to just take everything at face value and not apply your critical faculties. if someone gets all hurt and bitchy and personaly offended over a defect in a circuit then thats his personal problem

 

[analag]

Mine has no feedback, and I did goof up this schemo. I'm rewiring it this way to run some tests.
http://www.twin-x.com/groupdiy/albums/userpics/RibbonPre%7E0.JPG

I can't even get mad at solder_city this time. Thanks for pointing out the error guys.

analag

 

[solder_city]

ah thats more like it.

i might do couple things different but this one looks overall good to me.

 

[analag]

[quote author="solder_city"]ah thats more like it.

i might do couple things different but this one looks overall good to me.[/quote]

Well since I got it opened up, all suggestions are welcomed.

analag

 

[clintrubber]

[quote author="solder_city"]im still amazed at how enthusiastic you guys get about this vaporware and dont you even take a good look at the circuits before you heap on the praise and start ordering parts to build?

i know it looks like im picking on analag but what im really picking on is how unquestioning and uncritical a lot of you are. its cool that the guy posts ideas and i never try to stop a brother whos doing a creative thing- but its a diservice to the spirit of the drawing board to just take everything at face value and not apply your critical faculties.[/quote]

I take your remarks as the result of a growing impression over a few of his designs, not just this one. I mean, I assumed the original circuit of this thread was also built, working & being used. So it didn't look vaporware to me.

I agree that one shouldn't jump without some thinking about what's going on. But since this is obviously a PTP-thing and I liked the idea of it... corrections are easy then.

You must be a difficult person to live with, I don't recall much friendly posts from your side. Even when your remarks would be right to the extend that you present them to be, please try to express it in a more constructive manner next time.

Thanks,

Peter

 

[clintrubber]

[quote author="clintrubber"]I assumed the original circuit of this thread was also built, working & being used. [/quote]

Hi Analag,

To get things clear, was my assumption correct ?

Thanks

 

[clintrubber]

[quote author="analag"]
http://www.twin-x.com/groupdiy/albums/userpics/RibbonPre~0.JPG[/quote]
I might be goofing, but I don't see how we now get more than some 40dB of gain.

 

[mikka]

Redd 47 is 46 dB of gain.....