I really need help with this preamp!

[mwmitchell]

I need a preamp for my recording studio at home. I have been tinkering with this design for a while, although I haven't built it yet. My problem is, if this doesn't look like it's going to work well, I'm going to sell my parts and just buy a preamp instead. I really want to build it, I just don't want to be dissapointed.

My plan is to have a seperate box for the power supply, probably the Allen Wright SuperReg, and of course I'll need phantom power for the condenser mics.

My main questions are these:

• What would the gain be of this preamp?
• What is the gain of each stage?
• How can I implement a SIMPLE optical limiter? (Silonex LDR modules?):
http://www.silonex.com/audiohm/application.html

• What about a simple EQ?
• How would I plug a guitar into this circuit?

I'm afraid that the Limiter and EQ might steal too much gain.

My idea for the limiter was to just strap the LDR across the inputs to the 6H30, and then build a control circuit. This would bring the 2 phases together and cancel/reduce volume.

Please help with this. Any tips or comments are welcome. I'm not really a pro at electronics, I just love building things and elctronics, so I'm sorry for some of the questions if they are too basic.

Thanks!

Matt

 

[Joel]

• What would the gain be of this preamp?
• What is the gain of each stage?

How can we know until you fill in the circuit values?

 

[mwmitchell]

Hey Joel. Sorry about that. Here is a revised schematic with values. Thanks for your reply!

Matt

 

[mwmitchell]

Do I really need a regulated supply for this?

I'm wondering if I really need a regulated supply for this. It's all balanced so noise shouldn't be an issue, and as far as the supply sagging from transients, can that really happen with a mic preamp?

Matt

p.s. anyone feel free to have a look at the schematic and PLEASE let me know as much as you can about it. Thanks.

 

[Bas Horneman]

Hi Matt,

Also try posting at K&K audio at the AudioAsylum distributor of Lundahl in the US.

Hate to see you sell everything and just buy a preamp!

Cheers,
Bas

 

[mwmitchell]

Yeah, believe me I don't want to do that. I just want to make sure that this is a good design first.

I figured this would be the best place to do it. Thanks for the info on KK forums. I'll do that.

Cheers!

Matt

 

[Sch3mat1c]

Oh yeah, I think I was gonna add something to this thread...

What are the cathode resistors for?
How is the output going to be balanced if it's not a CT output winding?

Tim

 

[mwmitchell]

Well, the center tap on the secondary would be grounded. So it would be balanced. I messed up the drawing!

As far as the cathode resistor, I don't need one before getting to the constant current source? Please tell me why? Would the current source determine the current all by itself?

Please let me know what would be best. I was thinking about using Kevin Kennedys current source, but also thought about an LM317 with its adjust tied to ground and the output tied to ground thru a current limiting resistor. the input would be tied to the cathodes.

If you have any input on this at all, please let me know. I'm ready for all comments! More than a completed preamp, I want to learn!

Matt

 

[Sch3mat1c]

Well, the center tap on the secondary would be grounded. So it would be balanced. I messed up the drawing!

😉 I see.

As far as the cathode resistor, I don't need one before getting to the constant current source? Please tell me why? Would the current source determine the current all by itself?

Uhm....well....uh.....that's its job by definition, um yea.... *choking back snickers* J/K.

Please let me know what would be best. I was thinking about using Kevin Kennedys current source, but also thought about an LM317 with its adjust tied to ground and the output tied to ground thru a current limiting resistor. the input would be tied to the cathodes.

Personally I'd probably jump for a signal pentode CCS, but I don't have experience in anything else 😉 A HV transistor or MOSFET based CCS would work as well.

If you have any input on this at all, please let me know. I'm ready for all comments! More than a completed preamp, I want to learn!

Ok! 🙂

Tim

 

[mwmitchell]

Thanks for the response.

I get the current source thing. OK, that's funny. So, the resistor is not needed because the CCS is there, and no signal is being taken from the cathode. Is that right? Couldn't you adjust gain with a cathode resistor in this configuration?

Please understand, I'm NOT a pro. I just pic up info from different places. I'm hoping I can learn something from this, and maybe somday I'll teach you something! 🙂

Please people, tear me up if you have to, but share the knowledge!

Matt

 

[JZatopa]

with out reading this post I am going to say to buy the paia tube mic pre (or regular tube pre's) and do some mods to them (many mods online). This will give you an amazing pre w/o costing an arm and a leg.