question for electronic types

JonB · #1 04-01-2006, 10:04 AM

I just built two versions of Don Tillman's preamp. One is battery powered, the other phantom. They are pretty much identical. All the voltage readings are close, and the resistors and caps measure the same on both.
The phantom one is way louder than the battery one, this with them running into the same preamp, same bass, same settings.
I've tried changing the transistor in the battery version, but no difference.
My question is this; Is it because there's more current available with phantom power? The voltage supply is higher (15v) but I've brought that down to the 9v with a resistor.
Thanks for your help.

mje · #2 04-01-2006, 11:29 AM

I suspect it's variability in the transistors you used, but short of swapping the two transistors, I can't say. The voltage determines the headroom, but not the gain, unless I'm mistaken... which I often am when it come to engineering questions. ;-)

I've built both versions as well- I like using the preamp cable for my DB w/Fishman pickup. Putting a 10M buffer right at the pickup really smooths out the response. The cables are cheap to build, too, although building them takes all my fly tying skills and then some.

JonB · #3 04-01-2006, 12:55 PM

Thanks, mje,

So far, I've tried 3 different transistors in the battery one, and they all sound pretty much the same. I'm hesitant to switch out the phantom one, as it's working so well. (if it aint broke...)
I suppose I could try switching the power supplies. Hmmm.

fdeck · #4 04-01-2006, 01:34 PM

I wonder if the additional series resistor has increased the gain. Did you bypass the low side of the resistor?

mje · #5 04-01-2006, 03:32 PM

Quote:

Originally Posted by fdeck

I wonder if the additional series resistor has increased the gain. Did you bypass the low side of the resistor?

Or perhaps the resistor in the PS or the source resistor are not as spec'd?

JonB · #6 04-01-2006, 04:02 PM

Wait, I'm lost....happens all the time.

What I'd like to do is increase the gain on the battery one like it is on the phantom one. Sorry if I wasn't clear on that. The phantom version sounds pretty good, and is about the amount of gain I was hoping for.
Fdeck, the resistor is bypassed on the high side, but not the low. What would you suggest?
Mje, by source resistor, do you mean the 10m on the input?
Thanks for your help!

mje · #7 04-01-2006, 04:16 PM

Quote:

Originally Posted by JonB

Wait, you guys lost me - it isn't hard to do.
The original circuit has a 6.8k between the battery + and the transistor. I increased this to 33k to drop the phantom voltage from 15v to 9v. What else should I have done? (or did I need to do that?)
Thanks for all the help.

That's it, then.

fdeck · #8 04-01-2006, 04:40 PM

Yes, going from 6.8k to 33k increased your gain by about 13 dB. You could try the 33k on the battery powered version.

Source resistor refers to the resistor connected to the Source terminal of the JFET... in Tillman's circuit, it is the terminal closest to ground. Just FYI, the other terminals are: Drain is the one going up to the power supply and Gate is the input terminal. On some JFETs, the source and drain are actually interchangeable.

In very general terms, you can increase gain by increasing the drain resistor (what you did) or decreasing the source resistor. Working with both resistors allows you to maximize the output voltage amplitude achievable with a desired gain. The circuit is pretty forgiving, and amenable to experimentation. Also, the JFET can tolerate the 15 V phantom, if you want to get everything back to the same gain by sticking with 6.8k.

JonB · #9 04-01-2006, 05:26 PM

Thank you, gentlemen!