First preamp design critique  - Page 1
Forum section: electronics / beginners

author: derationalize
author_rank: Newbie
timestamp: February 16, 2018, 02:53:26 pm
content: Hi, everyone! I just finished designing a small preamp to fit into an electric guitar. I breadboarded the design and everything seems to work fine. I'm posting this here for you to critique for any improvements or corrections that can be made before I go to PCB and install it. This is my first real electronics project completed so I may have missed something critical.The design takes two 4 conductor leads of a 2 humbucker equipped guitar. The DPDT switch selects either series or parallel out of phase wiring. The signal then goes to a highpass Fliege filter with a pot for cutoff and one for Q. Then a simple lowpass tone knob followed by a master volume pot. Thanks for checking it out!

author: Audioguru
author_rank: Super Contributor
timestamp: February 16, 2018, 04:52:47 pm
content: Your circuit has a very low input impedance for an electric guitar preamp that usually has an input impedance of 1M or more. The TL072 opamps have a very high input impedance that is not being used in the circuit. The low input impedance of your circuit cuts high frequencies to make muffled sounds.The highpass filter cuts lows to make tinny sounds and the lowpass filter pot is connected wrong. If it is connected correctly then it will make the 2nd opamp oscillate at a high frequency with a capacitor to ground shorting its output.Your schematic drawing software is a nightmare of crossed lines all over the place.

author: derationalize
author_rank: Newbie
timestamp: February 16, 2018, 06:10:16 pm
content: Regarding the input impedance, does that mean R1 and R2 should be upped to 1Meg? Do I need to adjust C1 to 10uf with that?As for the treble knob, is the proper way to wire that with the wiper connected to the active filter, leaving one leg for the cap shunt to ground and the other to master volume?Would it have been a better idea to use vias / named wires to avoid all the crossings on the drawing?Thank you for your input! I'm looking forward to learning more.

author: paulca
author_rank: Super Contributor
timestamp: February 16, 2018, 06:16:40 pm
content: Use labels for the power, usually tidies things up.Where you going to find 50.6k resistors? It's cheaper and easier to accept "close" values that are easily available. 50.6k could be a 47k for example.

author: derationalize
author_rank: Newbie
timestamp: February 16, 2018, 09:40:31 pm
content: Here is a new version redrawn in a different program. I also removed the switching stuff from it to help simplify it. Is this better, Guru?

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 16, 2018, 10:07:59 pm
content: Quote from: derationalize on February 16, 2018, 09:40:31 pmMuch improved. However, four-way connections (with a dot) are a bad practice except for the rare case of illustrating a single-point ground requirement.

author: derationalize
author_rank: Newbie
timestamp: February 17, 2018, 04:42:00 am
content: I just realized I'm missing a cap for the lowpass. Should I connect that between P3 and ground?

author: Audioguru
author_rank: Super Contributor
timestamp: February 17, 2018, 05:21:30 pm
content: The new schematic is much clearer.R1, R2 and C1 are at the battery, not at the input so they do not affect the input impedance.The input impedance is only 21.8K when P2 is zero ohms. Use an added buffer opamp for an imput impedance of 1M or more.Will the circuit oscillate at certain settings of P1 and P2? I didn't calculate it. SCREEEECH!When P3 is turned down to zero ohms then the opamp driving it will get extremely hot if there is a signal.The volume control input and output are backwards.Why do you want to cut highs to make muffled sounds and to cut lows to make tinny sounds?

author: derationalize
author_rank: Newbie
timestamp: February 18, 2018, 06:03:29 pm
content: So in something like this you want the input going into a passive high pass RC filter before your first buffer, whose input impedance is determined by the resistor to ground? I've seen some schematics with capacitors at the end or a resistor to ground right before the output. Is that necessary?How do you determine an appropriate value for the voltage divider resistors and capacitor for the virtual ground?As to why I want to cut highs and lows to such extremes, it's more of an effort to sculpt sounds which are very different from each other. I want a sweepable mid control, but the only potentiometers I have with shafts long enough to fit through the guitar are B500k. State variable filter is out then since all my dual gang pots are too short to be mounted in the guitar.I soldered this up and stuffed it in the guitar. It works and there are some really cool sounds I can get with it, though there's also a lot of noise and screechy useless stuff. I suppose it's back to the drawing board.