FET opamp for condensor microphone?

[Svart]

I want to keep the potential of two ground sections of the mic at the same level but don't want the current/voltage/noise to flow between them. What the best way to isolate in your opinions? ferrite bead, resistor, cap or combinations

 

[Svart]

still wondering

 

[barefoot]

Ok, what if we're not encumbered by phantom power? We have bipolar power supplies for the op amp, a separate high voltage (48V or more?) supply for the capsule biasing, all very quite and able to deliver whatever current capacity we need. What would such a "no holes barred" circuit look like?

 

[Gus]

Svart

I don't understand what you are asking?

 

[barefoot]

Am I right that this is the situation in its most basic form? The time constant of R1 and the capsule capacitance determines the low frequency extension. And a larger bias voltage yields a higher signal to noise ratio, right?

 

[barefoot]

Maybe I also need a second resistor?

 

[Svart]

Hey Gus, I was trying not to give too much info away.. My circuit needs two grounds, one somewhat isolated from the other, I was wondering if I could use some old RF tricks to bring the grounds to the same potential but yet keep noise from coupling between them..

 

[Svart]

Ok the prototype does indeed work. It's noisy and needs to be tweaked for opamp supply and such but using an opa627 with a gain of 10 gives a nice hot level up there with my gefells, it just runs out of juice because I just have a resistor cutting the voltage to the opamp for testing and not a real supply. I also don't have much biasing on the capsule either.

When I get the hum/buzz/noise worked out, this thing will be HIGH fidelity for sure. I'm using a 797 capsule in case anybody is wondering.

 

[Gus]

http://recforums.prosoundweb.com/index.php/m/160341/0/?srch=m149+opamp#msg_160341

 

[Svart]

hmm the only real info embedded in that thread seems to be that they think the opa132 sounds better than the opa627 and others..

Since my mic is running at a gain of 10 right now I can try the opa637 too which is only stable above 5.

I suppose that I need to do this on a real PCB and put it in a chassis to cut the hum. I noticed that grabbing the ground and the base of the capsule mount together cut the hum considerably even though they already had a lead connecting them.

Gus, I'm still going to try the HV circuit we talked about, what do you think the best voltage for the capsule bias should be, most are around 90v right? This is a 3um capsule, 1".

 

[Marik]

[quote author="Svart"]Ok the prototype does indeed work. It's noisy and needs to be tweaked for opamp supply and such but using an opa627 with a gain of 10 gives a nice hot level up there with my gefells...[/quote]

Something is wrong here. With 797 you should get a nice hot level even with gain of two. Let's say you have 40V on the capsule. You are loosing about 6db compare to stock 80V (is it?). So (apart from sonic differences) you will need gain of 4 to get a good hot mic.

What is the input capacitance of the OPA627, and what is it doing driving a large input capacitance source?

 

[CJ]

Lundahl has a new transformer out that will do 6 meg. It's a bi-fi video jobby. As long as your going for a quick opamp, may as well have a quick transformer.

 

[Svart]

Hey Marik. This was just a proof of theory test so far, not a tweaking session. I had a little under 20v on the capsule because I didn't have the powersupply for the opamp working right nor did I have the HV for the capsule working yet. I just used the opamp V+ rail for the capsule to see if it worked first.

I'm going to do a quick layout for the next version so I can use the SMD parts I need to use to get the HV for the capsule and fit it into a case.

EDIT: I also just picked 10 as the gain as an arbitrary number, I expected to tweak this as I got more info from my prototype. 5k feedback and 500 shunt to get G=10.

what do you think?

 

[Svart]

hey CJ do you know which model it is?

 

[Gus]

Like I posted before overall gain of one

now with opamps the clipping might be more of a problem than a simple 1 device stage with stepdown or SF type. OR maybe the way the opamp clips could be a cool thing. Calculate the gain stucture of some of the neumanns most have feedback to control the gain some of the schematics even have Adjust(if I have the translation correct) marked on them.

 

[Svart]

so the opamp should have a gain of 1? is that assuming the trafo out or next stage would amplify at all? Wouldn't you want the first stage to do some amplification and the next stage to be a unity?

I like the trafo output idea with a single opamp driving it. simple and easy with clear advantages in sound quality at the expense of price..

 

[Svart]

anybody?

 

[Svart]

hey CJ where did you see that Lundahl trafo? I can't find it on their website.

 

[Svart]

what about using a small digital trafo? What are you guys using for replacements in other mics?

 

[Svart]

JT-MB-CA from jensen? 150:150...