This is my first post - I hope I have chosen the right forum section. I am designing a valve pre-amp for acoustic guitars - the pre-amp will fit inside the guitar just like the many other guitar pre-amps out there. I am thinking of using the DRV632 DirectPath audio line driver as the output device but am wondering how to interface the valve stage to the DRV632. The valve stage has an output impedance of approximately 22.5k ohms and is unbalanced. Are there any recommendations on how I should drive the balanced input of the DRV632 from the valve stage? Should I put another op amp with balanced outputs in between the valve and the DRV632? Any thoughts would be welcome.
Not sure where you posted this, but it got moved into the Audio Amp forum, so no worries.
I vaguely recall going to Radio Shack with my father to test valves as a young child... That is the extent of my experience with them...
Check out section 4.3 of the EVM Users Guide for a sample single-ended input circuit. My only concern is if the input impedance of the op amp will load down your valve. You could try bumping up the resistors, but then the noise will go up, too, but if you're using valves, I guess noise isn't a primary concern.
We also offer a SPICE model for this part if you want to spice it up to see about the loading.
Audio Applications Engineering Manager
Dallas, TX USA
Thanks for the pointers. I've taken a look at section 4.3 of the EVM Users Guide. A couple of questions have come out of that:
1. With respect to loading down the valve, since this is a guitar pre-amp and hence mono, could I use one channel of the DRV632 as a unity gain follower like this:
I'm guessing that the input stages behave just like regular op amps...
2. I also downloaded the FilterPro desktop application to calculate the component values for the active filter. The first thing I tried to do was calculate to get the same component values as specified in the EVM Users Guide. The EVM Users Guide says that the component values are calculated for a "30kHz, second-order, active Butterworth filter...multiple feedback configuration..." and the values for various gains are reproduced here:
However, when using the FilterPro application, I cannot get component values anywhere close to those shown above. I would like to be able to reproduce the above values in FilterPro as a baseline reference before I customise it. However, for a unity gain, second-order 30kHz Butterworth (first row in Table 3) I get values of: [E96 series]R1=R2=3.48K, R3=2K, [E48 series]C1=1nF, C2=4.02nF (I don't have a C3 - the third capacitor in my diagram (C7) is there based on a recommendation in the last paragraph of section 4.3). As can be seen, my calculated values are not even close to those shown above. Am I reading the EVM Users Guide wrongly and there is no connection between the values shown there and the FilterPro application?
Many thanks in advance for any insights...
One thing jumped out at me-- if the plate voltage is turned on or off, a high voltage transient will be coupled through the coupling capacitor into the op amp input, destroying it. I'd suggest putting a couple of Zeners in to clamp the voltage to a safe level. While you're at it, add a resistor from that input to ground so that there is an input bias current source.
Regards, Neil P. Albaugh ex-Burr-Brown
Got it. Many thanks.
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