Bob Blick preamp-noisy!

[Noplasticrobots]

I've got the Bob Blick preamp on my breadboard. When running it through headphones or into my Tascam it's pretty noisy. I've done the usual checks but I've got everything in the right place. Is there anything I can do to reduce the noise? I'm using an LM741. 

[Noplasticrobots]

Is it possible to drop in a different op amp, like a 386?

[toneman]

no the 2 are not interchangable.
the 386 is Not an OpAmp/
The 741 is not recommended for modern audio applications.
The 741 WILL work fine in a fuzz circuit.
For "less noisy" OpAmps, try the 5532, or 5534.
google for the datasheets.
Tone

[Jeremy]

It might be noisy just because it's on a breadboard; it's not shielded at all. 

[Noplasticrobots]

That was one of my other concerns also, Jeremy. I guess I can always solder it together anyway and sub the 741 out whenever I order some new op amps. Thanks!

[David]

Hey, NPR!  For audio purposes, a 741 is pure crap.  Sub a TL071 or a 5534 instead.  That was the first preamp I tried to breadboard and my near-nonexistent skills at the time just could not get it to work.  Keep us informed.  I'll be interested to know how that kludge actually sounds...

[Noplasticrobots]

Another problem with this circuit that I forgot to mention is when I select between pickups, I get a LOUD pop that overloads my Tascam. Is the crappy op amp causing this?

[David]

I haven't looked at that thing in a dog's age.  Does it have blocking capacitors and pull-down resistors on input and output?  IIRC the popping indicates that DC is getting into what you're using for an amplifier.  Not a good thing.

[Noplasticrobots]

Here's the schematic:
http://www.bobblick.com/techref/projects/preamp/preamp.html

It has a pull down resistor on input but I don't see one on output. Where would theblocking caps be?

[David]

I've always thought that that thing was cursed somehow.  How important is it to you to build that one?  There are other equally-simple preamps that are proven designs.

[Noplasticrobots]

I am in no means tied to this preamp. I'd easily give another preamp a shot, the only requirements being that it can amplify a guitar to use for computer recording; the easier the better!

I've seen this schematic but don't know if it will work. Looks similar to a FV to me:
http://www.till.com/articles/GuitarPreamp/

[David]

Oh, yeah!  That one's confirmed!  Check Paul Marossy's site!
Also check out the AMZ MOSFET Boost, Dragonfly's Sparkle Boost, and the MXR Micro Amp on the GGG site.  Heck, for that matter, just build half of an Adjusticator on the GEO site.  That thing boosts AND cuts!  What more could you want?

[Noplasticrobots]

I tried the FET preamp listed above at the till.com site, but it didn't work the way I thought it would. I can't plug headphones into it to hear myself playing, so I couldn't use it as a pre for recording.

The Adjusticator looks interesting...I've got a TL082 laying around. After searching the forums it seems like I could use the TL082 to build it so I'll give it a shot.

[Mark Hammer]

The Blick preamp looks fine.  It also looks like it is crying out for a TL071  As noted in the DEVICE op-amp shootout, the 5534 supercedes Bi-FETs in noise specs when the input and feedback resistances are low-ish, but the Bi-FETs pull ahead in noise performance when the resistances are high, as they are bound to be in this case.  Note that Rg will be 100k if you want a gain of 5-6 and even greater if you want more gain than that.

A 5532 has also been recommended.  This is a dual 5534.  Good chip but: a) not the same pinout as a single op-amp, and b)not likely to have as decent noise specs in this instance as a Bi-FET like a TL071.

Noise performance can also be helped by shaving off any unneeded bandwidth at the high end by placing a small cap in parallel with Rg.  For instance, if Rg = 100k, a 150pf cap in parallel will provide a high-end rolloff starting around 10khz.  180pf will bump that down to 8.8khz.  The guitar only gives so much high end, and all "treble" above that is simply hiss.  Who needs it, right?  The formula for calculating rolloff is F = 1/(2*pi*R*C) where F is the rolloff point, R is the value of Rg in megohms, and C is the value of any parallel cap in uf (so 150pf = .00015uf).  If you use an Rg value other than 100k, you can figure out any possible rolloff with other R and C values.

Note that the 22k/1uf combo provides for a low-end rolloff around 7.2hz.  If you reduce the 22k resistor, that will raise gain, but it will also start to raise the low-end rolloff point, according to the formula above.

Note as well, that you can have a secondary RC combination, in parallel with 22k/1uf that can produce another gain/bandwidth relationship.  If there were a 10k/.01uf series combination that ran to ground in parallel with 22k/1uf, AND Rg was 100k, you'd have a gain of (100k+22k)/22k = 5.5 for content below around 1.6khz, but a gain of (100k+10k)/10k = 11 for content above 1.6khz.  This differential gain can be used in combination with well-chosen treble-trimming caps in parallel with Rg to achieve select little "bumps" in the frequency response that make the preamp behave like a preamp/EQ combo with selective resonant boost in the lower treble or upper mids.

There should not be any popping with this unit since both input and output caps have a path to ground via terminating resistors

[Noplasticrobots]

Dumb question: can I use just one section of a TL082 for the Blick preamp (you mention I can use a 5534 which is a dual)? I don't have any TL072s laying around and really can't wait for one as I have to get this to a friend ASAP.

[Mark Hammer]

Yes.  Better yet:
1) use a socket so you can swap devices when you "chip comes in" (couldn't resist that one)
2) take a look at the parametric EQ article/s over at geofex and use the 2nd half of the chip to support a single or dual  EQ section for tone tweaking, OR
3) use the 2nd half to produce a unit-gain inverted version so you can send a balanced output.

[Noplasticrobots]

Awesome! I'll get to work on that! Thanks. Nice joke too!

[Noplasticrobots]

I definitely would like to run a balanced line, but my buddy just wants to hook his guitar up to his PC's 3.5mm audio input.

Dumb question number 2: How do I make a 3.5mm TRS balanced?

Regarding the unity gain dual op amp setup:

If it's possible and I understand this correctly, then I need to connect the 470 ohm resistor from pin 2 (output A) of the first opamp to pin 6 (inverting input B) of the TL082? What value resistor would keep the singal at unity, 470 ohms since that's the input from pin 2?

[Mark Hammer]

Ok, I definitely want to run a balanced line, but is that possible with a 3.5mm mono line? My buddy just wants to hook up his guitar to his PC's 3.5mm audio input.

Most such sound card inputs - unless it's a high end card expressly for recording - have unbalanced inputs.  As such, providing a balanced output for your buddy doesn't really do him any good.

If it's possible and I understand this correctly, then I need to connect the 470 ohm resistor from pin 2 (output A) of the first opamp to pin 6 (inverting input B) of the TL082? Then from there to set unity gain I would place what value resistor to keep it unity, 470 ohms since that's the input from pin 2?

If *I've* understood things correctly, to turn it into a balanced out, one would do the following:
1) the output of the first op-amp would feed the inverting input of the second op-amp via a 4.7uf cap into suitable-value resistor (let's say 100k if you use a TL072/82, and 10k if you end up using a 5532)
2) An identical value feedback resistor is used between the inverting input and output of the second op-amp
3) The noninverting ('+') pin of the 2nd op-amp is tied to the same junction between the leds and 2.2k resistor that the 1meg resistor is.

The output of that second op-amp is now an inverted same-level version of the output of the main/first op-amp.  If that output and the other one are fed over a stereo cable for a long distance into a balanced input somewhere, whatever hum they pick up should cancel.

[Noplasticrobots]

Edit:

Nevermind!