Weird tube to transformer coupling cap question

I have a mic the late Oliver Archut built for me that has the coupling capacitor on the minus/ground side of the output transformer primary to ground, instead of between the plate and the + side of the input transformer. Not sure I understand all the implications of that. I've never seen another mic built like that. Usually the coupling cap is between the plate and the + side of the output transformer, in order to block DC from the plate. BTW this is a plate follower circuit. Just seems like it might not a good idea to subject the tiny windings of the mic output transformer to any DC, no? But I also figure he had his reasons for doing that. Or maybe he was just in a hurry. Curious what you guys think. Thanks!

It doesn't matter!
It is just a series connection.
Capacitor-Transformer is the same as transformer-capacitor...

Oh, i'm waiting for the "voodoo people" to come out of the woodwork and contradict you... ;D

RuudNL said:

It doesn't matter!
It is just a series connection.
Capacitor-Transformer is the same as transformer-capacitor...

Click to expand...

RuudNL said:

It doesn't matter!
It is just a series connection.
Capacitor-Transformer is the same as transformer-capacitor...

Click to expand...

That's what I thought too. The LC filter formed by the cap and transformer isn't affected by the order of components either, right?

My concern is if there is no difference electrically or sonically, why take the risk of passing DC through the transformer primary, where it could potentially arc or short with the secondary that references ground? Those are some VERY tiny wires on that transformer. If DC is being blocked by the cap BEFORE the transformer primary, isn't that "safer" from a transformer reliability standpoint?

AusTex64 said:

But I also figure he had his reasons for doing that. Or maybe he was just in a hurry. Curious what you guys think. Thanks!

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Two possible explanations come to mind:

• The layout was easier that way.
• Connecting the cap groundside eliminates potential problems with stray capacitance; big capacitors have big stray capacitance. I wouldn't think it's a big issue with a voltage-follower.

That's how they frequently did it in the 1930's.  There are some texts (RDH3?) that address the benefits.  Don't recall exactly.  And yes, some people do think it sounds better. 

Well, considering there are people who claim (certain) mains cables "sound better"... Nothing should surprise me (anymore)...

emrr said:

That's how they frequently did it in the 1930's.  There are some texts (RDH3?) that address the benefits.  Don't recall exactly.  And yes, some people do think it sounds better. 

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At the time I used to work for a radio station, we had microphone cables in two colors: black and gray.
They were made by the same manufacturer.
One collegue was sure that the gray cables sounded better!
I suppose he once made a recording that sounded good and on that occasion he used the gray cables.
He even went so far that if he had to make a recording, he would hide some gray cables the previous day, to make sure that they were available!  ;D
Those audiofools...

Now I come to think of it, the same person asked me to add a coaxial digital output to his DAT recorder. (The DAT recorder only had an optical output.) In order to do this, he gave me a short length of teflon isolated coaxial cable and asked me if I could use a gold plated connector, because this would make a difference. Digital signals!!!

RuudNL said:

Now I come to think of it, the same person asked me to add a coaxial digital output to his DAT recorder. (The DAT recorder only had an optical output.) In order to do this, he gave me a short length of teflon isolated coaxial cable and asked me if I could use a gold plated connected, because this would make a difference. Digital signals!!!

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As long as you make sure the DAT machine is located above the amp/mixer it is connected to, you are OK. Making gravity help electrons flow is paramount for good sound.  :-[

Khron said:

Well, considering there are people who claim (certain) mains cables "sound better"... Nothing should surprise me (anymore)...

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Off topic, and I suppose you've listened yourself?  I haven't yet, but I own preamps with both connection orientations, from different eras.  Both certainly work. 

Since no one else has pointed this out explicitly, there is no DC current in the transformer primary either way. 

If that seems counterintuitive, think of this: having a DC blocking cap between the other end of the transformer primary and ground is comparable to lifting the transformer ground connection for DC.  What path could DC current take through the primary of the transformer if that end wasn't grounded?

I vaguely remember a hi-fi circuit that I think was in one of the esoteric single-ended triode magazines in the nineties that used shunt feed and placed the DC blocking cap between the bottom of the transformer primary and the tube cathode (rather than to ground).  I think the argument was that it kept the AC signal current through the transformer primary from having to travel the "long" path back through ground connections and through the cathode bias resistor, but that's just off the top of my head...

Adadan said:

Since no one else has pointed this out explicitly, there is no DC current in the transformer primary either way. 

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Nobody really questioned that. The ambiguity is the OP mentioned "subject the tiny windings of the mic output transformer to any DC"; Undoubtedly there is no significant current (only the cap leakage, same in both configurations), but there is voltage, admittedly much lower than the breakdown voltage of the wire's insulation; I think he was concerned with the possibility of arcing due to wear or corrosion.

I vaguely remember a hi-fi circuit that I think was in one of the esoteric single-ended triode magazines in the nineties that used shunt feed and placed the DC blocking cap between the bottom of the transformer primary and the tube cathode (rather than to ground).  I think the argument was that it kept the AC signal current through the transformer primary from having to travel the "long" path back through ground connections and through the cathode bias resistor, but that's just off the top of my head...

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Audiophools experiment in many ways, most often not really analysing what they're doing; in that case, that would produce some positive feedback that may (or may not) be pleasant.

abbey road d enfer said:

Nobody really questioned that. The ambiguity is the OP mentioned "subject the tiny windings of the mic output transformer to any DC"; Undoubtedly there is no significant current (only the cap leakage, same in both configurations), but there is voltage, admittedly much lower than the breakdown voltage of the wire's insulation; I think he was concerned with the possibility of arcing due to wear or corrosion.

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I went back and re-read the original post, and see your point.  I read that too quickly and made assumptions, usually not the best practice. 

Yes, I can see how with wear/moisture/corrosion there could be some possibility of arcing between the primary and secondary/internal shield/core, etc., whatever is at ground potential. 

It does seem like things inside the transformer would have to get pretty bad for this to be a problem at typical mic voltages, but better safe than sorry, I guess, especially if the mic sounds great as-is.  Who knows whether an equivalent transformer would be easy to source after some future failure...

Adadan said:

I vaguely remember a hi-fi circuit that I think was in one of the esoteric single-ended triode magazines in the nineties that used shunt feed and placed the DC blocking cap between the bottom of the transformer primary and the tube cathode (rather than to ground).

Click to expand...

abbey road d enfer said:

Audiophools experiment in many ways, most often not really analysing what they're doing; in that case, that would produce some positive feedback that may (or may not) be pleasant.

Click to expand...

See RCA 41-B preamp (1934), Collins 6P preamp (1946), etc, etc, etc.  Old widely used method, definitely mentioned as another way to squeeze slightly more gain out of a circuit.  Probably as well combined with very low bypass cap values such as to EQ the system.  They must have had some good reasons to do these things, well paid people in laboratories designed these equipments that cost on scale with automobiles at the time.  Many of those reasons had to do then with the assessment of parts quality, particularly capacitor reliability.  Just because we aren't used to seeing 'things that work' like this in use now, does not mean they are wrong. 

<I think he was concerned with the possibility of arcing due to wear or corrosion.>

Exactly. Perhaps overly concerned. Then again, might be better to be safe than sorry. I figure there's a reason almost every name tube mic manufacturer puts the blocking cap between the plate and the + side of the transformer primary.

AusTex64 said:

I figure there's a reason almost every name tube mic manufacturer puts the blocking cap on the + side of the transformer primary.

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Primarily because it's the only version they've ever seen used at a quick glance.  History proves otherwise. 

emrr said:

Primarily because it's the only version they've ever seen used at a quick glance.  History proves otherwise.

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Perhaps I give Neumann and AKG too much credit in this regard...

AusTex64 said:

Perhaps I give Neumann and AKG too much credit in this regard...

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They are certainly only a very small percentage of the many companies that have designed and sold similar commercial amplifier circuits successfully.  Their fame rests on their capsule implementations rather. 

It is curious to think Oliver Archut would not have a well founded reason. 

emrr said:

Old widely used method, definitely mentioned as another way to squeeze slightly more gain out of a circuit.

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That's giving too much credit to an arrangement that is not critical (at least should not be; if it was, that would mean something very wrong with the design). Moving the cap cannot change gain anyhow.

  They must have had some good reasons to do these things,

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When someone is faced with an alternative of equal performance, is there really a lot of thinking? It may appear as a decision based on a particularly intricate motivation, just because it is a tad unusual, when in fact it's probably one of the two reasons I mentioned earlier.

Just because we aren't used to seeing 'things that work' like this in use now, does not mean they are wrong.

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It doesn't mean either that it's more astute.

<It is curious to think Oliver Archut would not have a well founded reason.>

Agreed.  Especially since it’s so different from all his other designs.

But from what I’m seeing inside the mic, it might have simply been ease of layout. Sure wish he was here to ask....