Mic amp idea

[NewYorkDave]

I never got much into the internals of condenser mics, and therefore I'm not familiar with all the circuits out there. But has anything like this ever been used?

The source impedance seen by the second stage is too low to be optimal for noise, but it seems to me that the low input capacitance of the first stage would be a good thing.

 

[Svart]

What valve are you planning to run in there? something regular or something unique? I think that may make a difference eh?

 

[alk509]

Are you sure about that gazillion ohms resistor? If my math is right, it's off by a few shaba-laba-dillions...

Peace,
Al.

 

[NewYorkDave]

Svart, I'm only asking if the topology has been seen before in a condenser mic. There are others here who are much more acquainted with various mic circuits than I am. My focus till now has been mostly on the equipment that comes after the mic.

 

[Larrchild]

but it seems to me that the low input capacitance of the first stage would be a good thing.

Determined by Miller, no?

Anyway, make the polarizing resistor to the capsule about a 1/10 of a gazillion.=)

 

[NewYorkDave]

Yes, and the grounded (at AC) plate means no Miller :wink:

 

[Larrchild]

So V1 is all about the impedance conversion, and V2 is a grounded-grid, cathode driven amp with sorta low gain. Is that the intent? Does the common cathode R provide degenerative feedback also?

 

[NewYorkDave]

V1A is a cathode follower to provide a high-Z, low-C load for the capsule. V1B is a grounded-grid stage to provide some amplification compared to a simpler cathode follower-only circuit. The common cathode R is for coupling the signal between them.

A single pentode would accomplish pretty much the same thing--except partition noise then becomes a factor, and the plate resistance would be very high. A cascode would not have the partition noise but the plate resistance would still be too high for transformer coupling.

 

[Larrchild]

Does the common cathode R provide degenerative feedback also?

Neeeevermind, it's a follower. So this circuit has an in-polarity condition all the way thru (I almost said phase, :shock: )

And the bias for V1b is held by the grid leak circuit of V1a?

 

[Marik]

Dave,

Haven't we talked about this circuit having a beer? I don't remember exactly...
Anyway, you took the schemo out my mouth :shock: :grin:

AFAIK, nobody ever used it (at least commercially), despite many obvious advantages. The only commercial ground grid amp I ever saw was a Countepoint pre.

I was planning to try it some day to see how it sounds.

 

[NewYorkDave]

I believed that I had just thought of it this morning... But now I'm wracking my brain to try to remember if we did talk about something like this. But all i can remember for sure is the beer! :shock:

(Getting old sucks, but I guess it beats the alternative!).

I do think we talked about grounded-grid stages in preamps, don't recall if we covered putting a cathode follower in front (in a diff-amp configuration). Does the Counterpoint circuit look like the one I drew?

 

[northsiderap]

Well, I did something sort of similar at this link with solid state stuff, except I'm not sharing my schematic for fear it will get 'slightly changed' and manufactured by someone else.

http://www.groupdiy.com/index.php?topic=162792#162792

My circuit boards are encased in black epoxy so ripoff artists cannot reverse engineer them.

An ultra low parts count, and the marketing writes itself.

I've been working on this mic prototype for 2-3 years now. Check the sound clip on page 2.

 

[Marik]

[quote author="NewYorkDave"] But all i can remember for sure is the beer! :shock: [/quote]

Hahahaha! :grin:
Dats fo shoor! :guinness: :thumb:

[quote author="NewYorkDave"]
I do think we talked about grounded-grid stages in preamps, don't recall if we covered putting a cathode follower in front (in a diff-amp configuration). Does the Counterpoint circuit look like the one I drew?[/quote]

It has been awhile since I last saw that Counterpoint schematics (about 9 years). What I remember, it was 6SN7 with a cathode CCS and a FET as an input device. IIRC, there was a switch to exclude the FET from the circuit, for work with low source impedances.

 

[northsiderap]

Solid state version in the flesh. Prototype 8 - omni.. Well almost.

 

[NewYorkDave]

Well, I've never done microphone DIY, but I guess ya gotta get your feet wet eventually. I have a cheap Marshall cardioid LDC someone gave me a while back, I never use it 'cause I never liked the sound of it. I suppose I could use that as a platform to test, since it wouldn't be a big deal to me if I messed up the mic somehow in the process. Now I just need to find a source for suitable connectors (7-pin XLR).

 

[Marik]

[quote author="NewYorkDave"]
... it seems to me that the low input capacitance of the first stage would be a good thing.[/quote]

The biggest advantage of the CF input in a mic is its high input impedance.

Don't discard the 1G resistors from your MXL--you will need them. I have plenty of 7-pin XLRs. Let me know and I can send you.

 

[NewYorkDave]

[quote author="Marik"]
Don't discard the 1G resistors from your MXL--you will need them. [/quote]

Yeah, I figured as much. I'll save the resistor. I'm planning on point-to-point construction to keep the capacitance low (and it's easier than making up a circuit board, anyway).

I have plenty of 7-pin XLRs. Let me know and I can send you.

Thanks. It looks like I already have a source for the connectors I need for this mic project. But if it's successful and I decide to do another I'll certainly be looking for more eventually. :sam:

 

[northsiderap]

Where are you getting your connectors if you don't mind me asking?

 

[Larrchild]

I have a 47fet head sittin here on top of my hv supply. Maybe I will fashion this also. I promise not to go into production. :wink:

Can't really do any kind of open-faced sandwich DIY with mics, hum's gonna dominate. Gotta shield em. Guess i'll make a raw pc board made carridge and wrap it in foil for the test.

 

[Gus]

been thinking about this most of the day.

It looks like it will be a pain to get a good CF and common grid operation point

I am trying to figure out its good points. I have not found one yet.