IOaudio Inventor of the parrallel 408a tube configuration in U47 Clone ?

[micaddict]

a few time ago , i was fixing a vintage hifi integrated tube amp : a Hitone H300
a very high end french product of the 60's with two marvelous Chretien output transformers
anyway, i focused on a quite huge overshoot on 20khz square waves...
i analyzed that i was probably the 12pf (only) on the Volume Pot
when i removed it, the overshoot disappeared and the sex appeal of this amp at the same time..
so i've quickly putted back the capacitor at its original place
in theory the overshoot (or parasitic over-oscillation) frequency is "wa....ay beyond" audible range

same story with C4 in a M49c, put a too big capacitor and bye bye the mojo...

just to say that theory and measurements are sometimes misleading

Exactly.

(See reply #35)

 

[Matador]

a few time ago , i was fixing a vintage hifi integrated tube amp : a Hitone H300
a very high end french product of the 60's with two marvelous Chretien output transformers
anyway, i focused on a quite huge overshoot on 20khz square waves...
i analyzed that i was probably the 12pf (only) on the Volume Pot
when i removed it, the overshoot disappeared and the sex appeal of this amp at the same time..
so i've quickly putted back the capacitor at its original place
in theory the overshoot (or parasitic over-oscillation) frequency is "wa....ay beyond" audible range

same story with C4 in a M49c, put a too big capacitor and bye bye the mojo...

just to say that theory and measurements are sometimes misleading

I honestly can't figure out what this has to do with the discussion at hand:  are you arguing that a change to circuit A somehow proves or disproves an unrelated change in circuit B?

How many purely capacitive sources appeared in that HiFi tube amp of yours?

 

[granger.frederic]

ok let's come back to the point

purely capacitive source : yes but i can't understand ...
an added capacitance to the capsule lowers the sensitivity of course but this has nothing to do with the miller's effect inside the tube
the grid sees an ac voltage swing as usual...
there's no esoterical process in here...

how can you justify your theory when there's a coupling cap between the capsule and the grid ?

 

[o3misha]

C1 is working not just a filtering cap but as a coupling cap. Its old discussion, but I am sure that I am right!

 

[granger.frederic]

???

 

[Matador]

how can you justify your theory when there's a coupling cap between the capsule and the grid ?

It's pretty easy: the coupling cap is in series with the capsule.  Caps in series combine like resistors in parallel, which means that any coupling cap lowers the effective capacitance of the capsule slightly (80pF into 1000pF looks like a single 74pF).  This would lower output a tiny bit.

Obviously cap non-linearity can come into play here, but it changes none of the analysis.

Miller effects must be evaluated in-situ:  it's as much of a circuit interaction as it is a separate concept.

 

[granger.frederic]

you act like the capsule was only a capacitor
but it is off course not only one...

it is a transducer which converts air movement into  a significant voltage swing

let's say it very simply:
a capsule is a special fixed biased capacitor
when the air around moves, the membrane moves, the capacitance changes, and the voltage changes instantaneously above and below the bias
after the coupling cap the signal is pure AC voltage and the tube is needed for current gain with voltage constant

no mystery here
i don't know why all the usual parasitic effects on audio signal shouldn't be present ?

 

[Matador]

you act like the capsule was only a capacitor
but it is off course not only one...

it is a transducer which converts air movement into  a significant voltage swing

let's say it very simply:
a capsule is a special fixed biased capacitor
when the air around moves, the membrane moves, the capacitance changes, and the voltage changes instantaneously above and below the bias
after the coupling cap the signal is pure AC voltage and the tube is needed for current gain with voltage constant

no mystery here
i don't know why all the usual parasitic effects on audio signal shouldn't be present ?

Sorry, I'm not following this...are you claiming that the Miller effect is somehow modifying the mechanical properties of the capacitor?

I don't think there are too many mysteries here:  the fundamental (electrical) action of a capsule comes from the definition of capacitance, Q=CV.  The change in charge is just current (dQ/dt = i), so if we want a change in capacitance to cause a change in voltage, then Q must be held constant.  We can minimize current by using V = IR, or I = V/R.  So to minimize I with a fixed V, we make R -> infinity.  Since we don't have an infinite resistor, we choose something "very high".  This is why we use "very high" resistors to bias the capsule.

I believe most of the other effects are happening in the mechanical realm (e.g. the delta change in capacitance is actually frequency dependent), but the Miller effect isn't in play here.  I won't comment about the mechanical properties, as I'm not a mechanical engineer and would have to leave that to the capsule design experts.

Has anyone ever tried lowering the "very high" bias resistors downward to see what happens?  The lower the polarizations resistors, the more that the change in charge compensates to the change in capsule capacitance, so the voltage change starts to reduce in magnitude.  Once the polarization resistors fall down below about 10M then the signal from the capsule essentially disappears and becomes a constant voltage system (rather than constant charge).

 

[granger.frederic]

"Sorry, I'm not following this...are you claiming that the Miller effect is somehow modifying the mechanical properties of the capacitor?"

???

the miller effect is universal and present in each tube or transistor , in fact in every active device.

cascodes are often used to limit this effect .
http://www.seas.upenn.edu/~ese319/Lecture_Notes/Lec_11_Miller_Effect_08.pdf

parallel tubes are adding their input capacitance, that's why that type of design was used in phono Riaa Preamp : the capacitance helps to load the cartridge correctly, and in this case the noise decreases in the same time:

http://www.tubecad.com/articles_2002/RIAA_Preamps_Part_2/RIAA_Preamps_Part_2.pdf

in a mic, changing the grid resistor from 10Mohms up to 300Mohms mostly changes the low end response, beyond i can't hear any significant difference, and the capsule becomes more and more sensible to plosives.In theory, the capsule distortion decreases...

a good book about microphones (not only) for those who loves that :

http://books.google.fr/books?id=eh60Ue_K2QkC&printsec=frontcover&hl=fr#v=onepage&q&f=false

 

[Melodeath00]

The U67 pad though, with capacitance across the capsule, does increase distortion.  There's an old article by Bob Schulein on the Shure SM81, and IIRC also by Sennheisser, in AES that explain the evils of stray capacitance.

Do you have any links to these articles, or any more info? I see in the U67 that C10 is connected to the front diaphragm and the backplate. How is this different from the U48, which has a cap between the front and rear diaphragms? Or would the U48 also feature this "increased distortion" from stray capacitance?

 

[ricardo]

The U67 pad though, with capacitance across the capsule, does increase distortion.  There's an old article by Bob Schulein on the Shure SM81, and IIRC also by Sennheisser, in AES that explain the evils of stray capacitance.

Do you have any links to these articles, or any more info? I see in the U67 that C10 is connected to the front diaphragm and the backplate. How is this different from the U48, which has a cap between the front and rear diaphragms? Or would the U48 also feature this "increased distortion" from stray capacitance?

I don't have detailed circuits for U48 but if their caps are connected as you describe, the effect is the same.

The Shure paper is http://www.aes.org/e-lib/browse.cfm?elib=1379

I think the Sennheiser one is http://www.aes.org/e-lib/browse.cfm?elib=8001 ...

.. but I have been a beach bum for nearly 2 decades so might be wrong.

There's also the Neumann PDF book by Peus on their website and also one by Jo Wuttke of Schoeps.  Both are essential reading for gurus, pseudo gurus and wannabe mike gurus alike.  They may have a discussion of this distortion.

IIRC, the Sennheiser paper was the most comprehensive.

This type of pad is common.  Yes it distorts but this might be part of the 'sound' of some famous mikes.  I didn't use this at Calrec for the distortion and other reasons.

 

[Melodeath00]

The U67 pad though, with capacitance across the capsule, does increase distortion.  There's an old article by Bob Schulein on the Shure SM81, and IIRC also by Sennheisser, in AES that explain the evils of stray capacitance.

Do you have any links to these articles, or any more info? I see in the U67 that C10 is connected to the front diaphragm and the backplate. How is this different from the U48, which has a cap between the front and rear diaphragms? Or would the U48 also feature this "increased distortion" from stray capacitance?

I don't have detailed circuits for U48 but if their caps are connected as you describe, the effect is the same.

The Shure paper is http://www.aes.org/e-lib/browse.cfm?elib=1379

I think the Sennheiser one is http://www.aes.org/e-lib/browse.cfm?elib=8001 ...

.. but I have been a beach bum for nearly 2 decades so might be wrong.

There's also the Neumann PDF book by Peus on their website and also one by Jo Wuttke of Schoeps.  Both are essential reading for gurus, pseudo gurus and wannabe mike gurus alike.  They may have a discussion of this distortion.

IIRC, the Sennheiser paper was the most comprehensive.

This type of pad is common.  Yes it distorts but this might be part of the 'sound' of some famous mikes.  I didn't use this at Calrec for the distortion and other reasons.

Thanks! I will read those papers tomorrow. I should probably explain that the U48 cap is not for a pad. It connects the rear diaphragm to the front diaphragm (blocking the 105V, but passing the AC). Perhaps it will say in the material, but does it matter if the cap is connected in series versus in parallel? That's the only difference I can think of between the U48 cap (which is not a pad) and the U67 pad cap. I could be wrong about all of this.

 

[EvLoutonian]

Interesting reading - thanks for sharing thoughts & insights on these topics, folks.
  ..now back to setting up for tomorrow's session for me..

 

[Pip]

Please stop. Please. It is just bad blood now. What has happened has happened. This is a small little group that you should respect. Again who is right and who is wrong doesn't matter. Good luck with your business. May you do well. But stop just trying make a very tense situation right it just is what it is. The truth is very few of us have really come up with anything new in the area of audio electronics. We just like dabbling is all. No one is getting rich off of any of this. So your need to win some abstract contest is just taxing. What would really happen if you all of a sudden found out, yes you are the rightest person here. Would it change anything at all?

 

[tubestation]

Hello,
I am very sorry and I don t want to  pour Oil in the Fire but I can t hesitate to say something .
As a longtime Member ( not enhanced and postactive , still a Novize but  I always tried my best to help where I can  )
i really  followed the Development of the MK47 and the parrallel 408 .
As Said before Nobody Here  invented  the use of parallel Tubes in  general  , to half Rp ,to increase Inputcapitance  or to match Heatervoltage.
But I am 100% sure that Io Audio was the first who came with a working, practical Concept to sub the VF 14 with two 408. Nether Saturn who never used  double 408 s ,not  Anybody Else nor Oliver  had a Product in Offer  using this Concept. Oliver used the EF 80/800 as a possible  and praised Sub, no ?
iO Audio put a lot of labourintensive Work into his Baby , has best technical Support and is a helpfull and  explaining  Member here, and now he makes some Money with a Product that makes People more than happy.
There is another Inovation that was invented by IO that You, Mic & Mod , stole.
The Pattern change with a Relay, or was this Your Invention ? No ! The whole Design of the Powersupply was stolen as well. As said before it doesn t  change anything , this may be True in Your Case but. it s disgusting how You bring up an old Thread just to link Your Selling Side where You heavy lie that You invented  this Mic and  diss this Pleace, where probably almost everything You make Money with was invented or created , Bäh ! It s one Thing to do something but than do it and leave this Place in Peace. When I started to read this Thread I wanted to stop because I don t like Gossib but than a  lot  of interesting  Information and Science was posted and even it was a little controverse and rough Debate it was about the Subject and on a quiet high technical Level. Did You, Mic and Mod  said  ever something  Usefull or helped Anybody with a Problem ?
Sorry again, this was the most and only unfriendly Post I  wrote here and I don t have any Relationship with Io Audio nor have any financial Interest .but I really like this Place and it is a Pain to see it destroyed  by People like Mi& M , who only have bold  financial Interests. I am also the Last one who  critcs the more and more upcoming Selling  and Painting by Numbers Mentality here,  even If I don t like it  and i am looking  foreword  how this Place develops and really hope  that not more  nice People leave the Building .
Please M& M , don t answer me because I don t want to debate with You, I don t believe You anyhow, doesn t matter what You say.
I am really sad to say such negative Things and If somebody is angry now  , I am sorry , or If You  think  it s unfair, You can report  to the Moderators, and if they  don t like my  Post , I may risk to get ereased, but I had to say it.
Now something to the Subject . If talking about pF in high Impedance Circuits the Tube Socket Capitance may  come in  Place witch also lies parallel to the Grid to Cathode Capitor.
Peace,
Lothar

 

[abbey road d enfer]

I don't think there are too many mysteries here:  the fundamental (electrical) action of a capsule comes from the definition of capacitance, Q=CV.  The change in charge is just current (dQ/dt = i), so if we want a change in capacitance to cause a change in voltage, then Q must be held constant.  We can minimize current by using V = IR, or I = V/R.  So to minimize I with a fixed V, we make R -> infinity.  Since we don't have an infinite resistor, we choose something "very high".  This is why we use "very high" resistors to bias the capsule.

No. There are two factors that motivate the choice of a very high value resistor in this position. The first is that the capsule capacitance, combined with the bias resistor, create a High Pass Filter. In order to get good frequency response, and given the low capacitance of the capsule, the resistor must be very high. With an LDC (about 80pF) and a desired  -0.2dB response at 20Hz, the resistor must be >450Mohm; that is the total impedance produced by the bias resistor AND the grid/gate resistor. That is typically the case when using two 1G resistors, one for bias, one for grid/gate.
If the same response was expected from an SDC (about 20pF), the resistors would be about 4Gohm each.

Has anyone ever tried lowering the "very high" bias resistors downward to see what happens? 

Are you genuinely posing this question or is it a naïve teaser? Indeed, many have tried using lower value resistors, for the basic simple reason hat 1G+ resistors are costly and require special attention in manipulation.
This has been used in commercial products where the frequency response was on purpose limited in the low frequencies (close speech applications). The main issue there is that noise increases as the resistance is decreased. Using 100Meg resistors instead of 1 G, would increase electronics noise by 20dB. In most cases, the electronics noise is significantly lower than the intrinsic capsule noise (noise described as "acoustic radiation impedance noise" or "thermal molecular agitation noise"). With 100Meg resistors, the electronics noise would be higher than the instrinsic noise.
Some tinkerers have tried to use the bootstrap effect in order to increase the apparent resistance seen by the capsule, but the laws of physics are stubborn and don't want to be twisted...

 

[ruairioflaherty]

Excellent post Abbey, thanks.

 

[tonycamp]

I musta missed this thread somehow, i own 2-MKU47's and 1-MK47, they sound great, i like the MKU's better than the MK's for a number of reasons. The main reason i posted was to address the "softens transients" and "noisey" ??? that were stated. I've rolled dozens of different brands of 408's through my mics, some brands are almost all noisey, some quiet and grainy, some smooth and noisey, and some sound smooth beautiful and dead quiet. I've also tried different resistor types, different capacitor brands/types from styrene, polyprop, paper in oil, i've messed with head basket damping and up to 6 top of the food chain capsule choices. All that stuff has made from small to significant differences to the MKU47 mic character, transients have proven more and less "soft" with different configs, the idea that the parallel tube design is the exclusive culprit to "softening transients" and "making more noise" appears to be an agenda driven assertion with no correlation to my lived 3x over reality, the final results of all that tweaking has given me incredible unobtanium mics with low noise and beautifully well represented transients.

 

[AusTex64]

Tony, I gotta ask - what capsule, resistors, transformers and coupling caps did you end up settling on for use in your unobtainium U47 inspired mics?

 

[tonycamp]

Tony, I gotta ask - what capsule, resistors, transformers and coupling caps did you end up settling on for use in your unobtainium U47 inspired mics?

I ended up with bluelines in the MKU's and a K7 in the MK, the reason i stayed with the K7 was because it was dark, rolled off and thicker sounding and is exclusively for kick or bass cab, it sucks on vox and about anything else, but thats ok because it makes 5k down super weighty, i'm going to change the polar pattern to supercardioid pretty quick here to more resemble a 47fet pickup, i also used some brown bean vishays for metal films, NOS beyschlag for carbon resistors, the tranny's are IOaudio's, and I used some hybrid poly/paper in oil coupling and non styroflex polystyrene bypass caps in the mic and supply. The swapping/listening testing of parts is fun to me, and the combinations are almost limitless, so if you have the time and compulsion to do this, i'd suggest keeping a log of the changes you hear over a number of days of living with specific set ups as they can be pretty subtle, then move on and reference back, it's pretty fun and the end results can be totally rewarding. I've done the same with a C12 that is just amazing sounding to me, I've also collected a ton of NOS for my elam pair, including original haufe's, beyschlags, wet tants and a pair of NOS perfect measured and matched 1953 GE 5* red inks, i'll use TC12's in these, i'm super excited about the forthcoming 251's!