Is there aDIY mic pre equivalent of the Chip Amp?

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Does a ribbon pre not needing phantom power make it easier to design quiet, ie, can you lower the input impedance?

Or does that make it noisier? :confused:
 
apl said:
Does a ribbon pre not needing phantom power make it easier to design quiet, ie, can you lower the input impedance?

Or does that make it noisier? :confused:
Click to expand...

I don't think it matters much. Any noise on the phantom supply is common mode. The phantom supply generally is decoupled from the gain stage, so it doesn't impact impedance, other than the phantom supply resistors presenting a small load to the AC signal.

I did a direct phantom-coupled preamp once, but that was solely for use with my own mics. Since I could define the current draw, I knew what the resulting bias would be at the opamp (this was a single-ended supply). But it never made production, and a real-world general-use preamp would never consider doing that.

Anyway, a dedicated ribbon pre would want to have relatively high input impedance--I think Royer's is 25K. This is to keep the ribbon unloaded to preserve level and response. I would also argue that an ideal ribbon transformer be high-impedance output (that is, overwound, or more than 1:40 ratio), to increase output level to overcome preamp noise. That requires a medium-impedance load, such as the Royer pre.
 
Ok, guys, so...

I am probably going to be using this chip to do my project.

Do you guys have any links to some good projects done with this chip?

I really just need to get it working, but I'd like to get it working well enough to use it. :) So I'm planning phase switch, possibly transformer balanced inputs, phantom, etc.

Any links are much welcomed. I'm googling and searching groupdiy as well. :)

Thanks guys!

Brandon
 
Ok, guys...

First off... what transformer are you guys using to go from unbalanced to balanced inputs? I'd love to use the Jensen JT-11-DM, but needless to say its not cheap.

Secondly, anyone have experience with FiveFishStudio's power supply and transformer kit? I definitely think that would be the best to way permanently power my pre (and probably a few extra channels :) ).

Thanks!

Brandon
 
NeveSSL said:
Ok, guys...

First off... what transformer are you guys using to go from unbalanced to balanced inputs? I'd love to use the Jensen JT-11-DM, but needless to say its not cheap.
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I have two Jensen knockoffs I pulled from my M80 you can have for the price of shipping . . . you might need to clean some solder off . . .
 
My neighbor back home is a ham guy... I've cleaned parts up a bit to use before. :D In fact, I got some NOS 12AX7s from him that he had from his Navy days! :eek:

Anyway, I'll send you a PM.

Brandon
 
Hey there Brandon, I recently did a mic pre project for school, nothing exciting, but I was marked really well, so I thought 'd throw in my 2c. The project was specifically regarding opamp configuration, so noise/ sound quality wasn't the biggest issue, and no phantom power was used, unbalanced output etc. It was a matter of understanding concepts of the instrumentation amp (you may be well passed that already).

Using three channels of a TL074, I built a variable gain instrumentation amp (like the 217), this then went to a "trim" type control then into the last channel of the 074 as a buffer. It had a decent enough freq response and something like 72dB of gain. I've never used this pre in anger (ie recording) but did plug it into the PA at rehearsal and was actually quite surprised at the results.

As for 217 aside from your input protection design, and output coupling, there really isn't a lot of room for variation on design!

I did a quad pre project using 217's which I am planning to incorporate into a little four channel mixer to record drums onto my mobile pre. Probably going to use a AD797 or NE5534 as the summing amp, but I can;t decide whether to transformer balance the output or to do it with a couple of op-amps.
 
You actually don"t need anything very elaborate to create a Ballanced output...It can be done with as little as 2 resistors a cap and a ferrite bead...

If you look at this schematci from the jensen transformers site and look at the output it will show you how to make an Impedance Ballanced output for you unballanced output preamp without useing a Ballanced line driver or Transformer....

http://www.jensen-transformers.com/as/as017.pdf

There aren"t many Variations of the INA217 Preamp other than the Usually options Like Input output transformers and the various options that are allready in the 217 Datasheet like DC Servo , Hot plugging Protection ect...

One option you could try is Maybe useing the Input stage of this Preamp:

http://sound.westhost.com/project66.htm

And put it before the INA217 Chip and Maybe use 2 10K reverse log Gain pots so you have a pre Gain and a Post Gain and it would also Give you a Huge Gain boost over the INA217 By it"s self for Ribbons or any other type of low sencitivity mic....

Just a Thought....Cheers
 
Great link to the jenson schemo! That really simplifies things... I might go for that option, It seems quite logical electronically speaking.

I do have a query though, why would Rane utilise an active balanced line driver stage in the Ms1b, surely the 217 has enough fire power to drive something like the passive circuit in the jenson schematic. This would mean less components therefore less $$$ and I guess that less active components means less noise... So what are the advantages/ disadvantages of active and passive balancing circuits? (sorry for the hijack!)
 
dr.colossus said:
Great link to the jenson schemo! That really simplifies things... I might go for that option, It seems quite logical electronically speaking.

I do have a query though, why would Rane utilise an active balanced line driver stage in the Ms1b, surely the 217 has enough fire power to drive something like the passive circuit in the jenson schematic. This would mean less components therefore less $$$ and I guess that less active components means less noise... So what are the advantages/ disadvantages of active and passive balancing circuits? (sorry for the hijack!)
Click to expand...

Well, it's questionable. But noise is not a big issue after the gain stage, so another active stage is no real penalty there.

You get a bit more level from an active balanced output, but that's not very compelling in a typical situation. It's actually a little more convenient to do impedance balancing in a lot of situations, because you can easily convert the output to unbalanced with a TS cable; whereas in an active balanced layout, that would short one amp to ground, which will waste power.

Note that INA217 does not "drive" the passive impedance balanced output. Indeed, it is not connected to it at all. The impedance balanced output presents an equal source impedance to interference signals on both conductors to a differential amp at the other end, which thus cancels them.

Also note that the ferrite bead is on the output to help reject RF noise, but strictly speaking, it is not part of an impedance balanced output. That is, ferrite beads will serve the same function on an active balanced output, or an unbalanced output, but don't have to be used with any of them.
 
"Note that INA217 does not "drive" the passive impedance balanced output."

What I meant to say was that the 217 would have no problem supplying sufficient current to drive a load via the passive circuit... That to me would be the only reason to apply an active balancing circuit.

Thanks for the comments. I guess this is one of those situations where it my be easier to test the circuits and listen to the results rather than speculate.
 
dr.colossus said:
"Note that INA217 does not "drive" the passive impedance balanced output."

What I meant to say was that the 217 would have no problem supplying sufficient current to drive a load via the passive circuit... That to me would be the only reason to apply an active balancing circuit.
Click to expand...

Well that would be a power amp stage then, not a line-level amp. The typical active balanced circuit routes the output to the inverting input of another opamp, which provides the inverted output. So the previous stage is still driving the noninverted output; there is no difference in the load on INA217 in that case.
 
Which one of Green pre and SSL 9k pre is better?
I think that the winner is the SSL but am not sure.
 
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