White passive EQ with boost and cut

[bluebird]

Thanks Marik!

Yea I put a lot of work into that thing. I don't think I'd trade it for anything...well thats not entirely true. I don't really use all six at one time, I just don't have the setup for analog mixing yet. I wish I could just break you off a couple!!! Boy I'd love some of your ribbon mics.

Dave, putting in the step down transformers is tempting. I have a 1K resistor on the input grid of the SRPP stage. I'm pretty much at unity gain.

If I were to put in the step down transformer, and up the value of the 1K do you think the SRPP stage would provide enough gain?

 

[NewYorkDave]

If you increased the value of the grid resistor, the EQ would cease to work properly, since that resistor forms part of the voltage divider that is the basis of the whole circuit.

(Note to others: Bluebird scaled his EQ for 1K, whereas my component values are scaled for 600 ohms. His grid resistor is performing the function of "RL" on my diagram, posted earlier).

I don't know if you can wring an extra 12dB out of your SRPP stage. I don't know what circuit you're using.

An SRPP is cute because it provides some gain and a relatively low output Z in a single "package", but it isn't the best output driver for low-Z loads.

 

[gyraf]

Blu,

The G-pultec SRPP output stage gives 18-20dB gain as is. There's not much to do to change this significantly.

Jakob E.

 

[bluebird]

I'm guessing to get the full 20db gain out of the SRPP I would have to have a 1m resistor on the grid. if that would mess up the impedance of the circuit then I'll just have to stick with the 1:1 transformer.
seeing as the circuit is at unity now a 4:1 step down might be too much.

its kind of cosmic how I just put this circuit together and it happened to be at unity. a little DIY luck.

the mids and lows boost plenty. its just the very high frequencys, say 8K and above could use a little more in shelf mode. for normal eq correction it has enough and definitely for mastering theres plenty of gain.

thanks again Dave for a cool design.

 

[NewYorkDave]

The value of grid resistance does not directly affect the gain of the tube. You might be confusing its function with that of the plate resistor.

You could get about twice the gain out of that SRPP by using a 12AV7 in place of the 12AU7. You might have to play with the values of the cathode and plate resistors to optimize performance, but it would likely work pretty well as a direct drop-in, in this case anyway.

I managed to find your schematic just now... It looks like you altered the value of R19 (R30 on Jakob's schematic). Try Jakob's original value of 1.5K--this might net you some extra gain, although probably not enough to make a huge difference.

 

[bluebird]

Thanks Dave,

there is a slight difference between the SRPP stage in the pultec and the G9.

one uses the 1.5K and one uses a 1K. so your saying the 1.5K might yeild a bit more gain.

ok I'll try and find a 12AV7 and throw a 4:1 well actually more like a 3:1 transformer in there and see how it goes.

 

[NewYorkDave]

Cool.

The 1.5K may or may not make a difference. I'm too lazy to do the analysis right now :wink:

Don't pay more than $2 or $3 for an NOS 12AV7. They're common as dirt. If there are any old TV repair shops in your town, they probably have a few sitting on their back shelves.