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Active preamp, no EQ
- Thread starter KingRazor
- Start date
sure, that should be a good one. I have used the TLE2072 in designs.
OPA134 or OPA1641 would be a couple of good choices too.
Aren't C3 and C4 redundant - you don't need two caps in parallel - just buy a bigger cap.
I like the specs on the OPA1641, would it be better than the tle2071? Looks like Mouser doesn't have a thru-hole version of the OPA1641, only surface mount - D'oh. Since the OPA1641 is a new product, may they will get a thru-hole version in the near future.
they do different things. The large electrolytic acts like bulk storage. When the op amp has to deliver large amounts of signal, instead of yanking on the power supply it gets the current from the cap. The small ceramic placed as close the to power pins of the signal carrying device as possible keeps high frequency garbage out. For this application it might be a bit overkill but good practice.
I have seen some pretty nasty distortion and noise in a signal path only to be eliminated by adding these parts and providing proper power supply decoupling.
Thanks, I'll look into adding those.
What do I need to do on the negative feedback side to get 3dB (2x) gain? IIRC, it's a matter of putting in 2 equal resistors on the negative feedback loop so the negative side gets 1/2 the output signal. Will this cause any oscillation issues? I recall there being a general concern about unity gain stability on some opamps and the one I've chosen doesn't note unity gain stability as a feature (the OPA136 has that design feature). Would that help?
What do I need to do on the negative feedback side to get 3dB (2x) gain? IIRC, it's a matter of putting in 2 equal resistors on the negative feedback loop so the negative side gets 1/2 the output signal. Will this cause any oscillation issues? I recall there being a general concern about unity gain stability on some opamps and the one I've chosen doesn't note unity gain stability as a feature (the OPA136 has that design feature). Would that help?
If you want a voltage gain of 2, +6dB, you are right - 2 resistors of same value. Typically one resistor from output to inverting input and the other from inverting input to GND.
This does add some complication though due to the GND reference not really being GND. The resistor that goes from inverting input to GND may need to be connected to GND via a large value cap. Or instead of feeding the bias voltage to the non-inverting input, connect the bias voltage to the inverting input.
I'll look into this. Of the 2 ways I have attempted to describe, both of them will work (I think)...just one may be better than the other in some way.
Oh, the noise problem you have in your current set may benefit from a 0.1uF ceramic right at the power pins of your ckt. That part is really necessary when your power supply is more than several inches away.
oh, as far as unity gain instability...
typically a very small value cap will work, something around 22pF ceramic.
also, anytime you have a resistor in negative feedback it's good practice to have the same cap in parallel.
I'll try to get a couple schematics to show these scenarios. may not be until tonight though.
typically a very small value cap will work, something around 22pF ceramic.
also, anytime you have a resistor in negative feedback it's good practice to have the same cap in parallel.
I'll try to get a couple schematics to show these scenarios. may not be until tonight though.
So, to be clear, I want:
An 8 pin DIP or IC socket.
It looks like about five "electrolytic caps" (I have no idea what these are just FYI) with values somewhere around 47uF to 100uF and rated to at least 10V (25V if for some reason I want to go 18V). I may want to put a small value resistor at the power input rated at around 4.7 ohms (I also know nothing about resistors). It looks like I want a fairly high value for C1 & C2 so I won't lose too many frequencies.
"R1 & R2 set up the bias voltage so that your signal sits halfway between 9V and GND. Make those about 100k." What's the actual name of these parts? When I go into radio shack to buy these what do I tell they guy? "Hey, I need a 100k _" ?
Whatever R3 is it should be around 1M?
C4 should be a 0.1uF ceramic cap.
For the pot, can I just use the one in my bass already?
Will I be able to use the balance pot that's in my bass or is there a chance that won't work?
yep, that's what you want. click on the link that says, "Project instructions link for this pcb".
On that page you will see, "IC Buffer Bill of Materials". This give you a list of parts to buy.
Also on the page you will see, "IC Buffer Schematic". This is the schematic. It is different from the one I posted but should work just the same.
I did suggest a few parts additions that are power supply decoupling improvements - post #78.
(Oh, if the right op amp is chosen and the appropriately rated caps you can run this preamp at 36V.
)
Alright, add these to the part list and replace the TL071 with...
?
you can certainly use a TL071. I think radioshack carries those, or maybe a TL081?
The second quote lists a few higher quality op amps that you can sample from the Texas Instruments web site. Go to the website, search for the those part numbers and there should be a button or link to sample those parts. Look for PDIP or DIP parts.
That is why I recommend the 8-pin IC socket (radioshack should them). So anytime you come across a good, low-noise, jfet input op amp you can try it out and see if you like it w/out turning on a soldering iron. Just make sure you put it in the right way.
No worries, it will be a couple of days before the PCB and parts arrive. I can walk to radioshack for the various small caps, small resistors and 8 pin socket you've mentioned.
Regarding opamps, since I've already ordered the TLE2071, I'm going with that. From the spec sheets, the OPA1641 is tailor-made for this but Mouser only has the surface mount version. Once a thru-hole version is available, I'll probably do a swap. The opamps cost less than $3 so experimenting isn't a big expense. Mouser shipping hasn't been outrageous either although $7 shipping for a $3 part isn't great. Total expense for this, PCB and parts, is still less than $30. The great thing about this is I've learned a helluva lot along the way, have the satisfaction of DIY, and still haven't spent a whole lot.
just noticed that my post was not very clear. That 22pF cap mentioned is in negative feedback (from output to inverting input) in both cases.
I was looking for a power gain of 3dB which is equivalent to 6dB voltage gain for a given resistance.
If the cap is in parallel with the negative feedback resistor, won't that make the gain unity for all but DC?
22pF is too small of a cap and only really effects high frequencies. That is exactly what it is there for high frequency stability. May not be needed, but is required for some op amps.
the TLE2071 is a good op amp. I have used the TLE2072 in a couple products I have designed.
"Better" in what regard? You can study specs all day, but better specs do not necessarily mean it will sound better.
I know the OPA134 will sound good. It's a pretty standard op amp in audio designs. The AD823 is also a very good sounding op amp but AFAIK there isn't a single package version (I have of these in my EBMM SR4 right now). I also like the OP275 for what it's worth, but again I don't think there is a single package version.
I keep saying this, but use a socket. I find it fun to try new op amps.
I can't remember what they are called right not but they do make little pcb's that "convert" smt to thru-hole.
