how much gain do I need for a guitar preamp to drive a line input? - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: February 03, 2020, 10:07:43 pm
content: basically I have a transistor output stage that I can easily drive from my phone, how many gain stages I need and how much gain to drive it from a guitar?I'm also looking for a LOT of gain.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: February 03, 2020, 11:47:12 pm
content: Okay, Moregain Freeman. Did you answer your own question?

author: gbaddeley
author_rank: Regular Contributor
timestamp: February 04, 2020, 09:41:50 am
content: A guitar pickup might output 100mv or less, depending on load impedance and how hard you play. A hot pickup could output over 1v if the strings are hit hard.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 04, 2020, 10:41:45 am
content: I believe in the case of guitars, the problem is not the level but the impedance. You might need a high impedance unity-gain buffer (current gain) more than any actual voltage gain.

author: Circlotron
author_rank: Super Contributor
timestamp: February 04, 2020, 10:48:04 am
content: High impedance, yes. I read that too low an impedance will dampen the strings. May only by a negligible amount though.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 04, 2020, 11:06:34 am
content: Quote from: Circlotron on February 04, 2020, 10:48:04 amConceptually, would make sense - low load impedance would have larger currents flowing and thus have a stronger counter-magnetic field form? Never noticed this though. Most pedals and amplifiers I know do something like use a 1Mohm at input into a high-input impedance buffer (some audio jfet opamp, or a tube)

author: rstofer
author_rank: Super Contributor
timestamp: February 04, 2020, 04:44:29 pm
content: Line Input voltage might go to 2V. In terms of the source, engineers would actually measure the voltage from the pick-up. Numbers are good! Guesses are meaningless.If we assume 100 mV (and this might be a meaningless guess) then you only need a gain of 20 or so. This can probably be done in a single op amp. Look at the gain/bandwidth spec for the op amp, assume a bandwidth of 20 kHz (way too high for a guitar) and see how much gain the device has. If two stages are necessary, no worries! Op amps come in 1, 2 and 4 per package.Manufactured units are dirt cheap, there's nothing to gain by building one:https://www.amazon.com/Pickup-Pre-Amp-Acoustic-Guitars-EQ-7545R/dp/B004P4PZHA

author: Prehistoricman
author_rank: Regular Contributor
timestamp: February 04, 2020, 10:12:37 pm
content: Quote from: Circlotron on February 04, 2020, 10:48:04 amYes negligible. The high impedance of a guitar pickup won't extract enough energy from the string to mute it at all, even if you short it.You will notice high frequency loss as you decrease the load resistance though. That would start to come into play at sub-100k load (at an educated guess).

author: langwadt
author_rank: Super Contributor
timestamp: February 05, 2020, 01:43:17 am
content: Quote from: ELS122 on February 05, 2020, 01:14:59 amsounds like most things tubes is a lot of hearsay and folklore that has been repeated so many times that is hasbecome unquestioned fact whether it has any basic in reality or not

author: Circlotron
author_rank: Super Contributor
timestamp: February 05, 2020, 12:32:43 pm
content: Before it hits the clipping point, a tube amp without negative feedback will *gradually* slide into more and more distortion. The harmonic structure of the resulting signal changes with signal level, somewhat comparable to pounding harder on piano keys makes a different sound. The amp is part of the instrument, just like the loudspeaker is. This is a perfectly valid use of tube amps. No reasonable person nowadays would say tubes are the best for everything, but for musical instrument amps, where you are trying toproducean original sound, notreproducean existing sound, they do seem to affect the sound in a desirable way, and give the results many people like, and pay good money for.Tubes for musical instrument amp = yes.Tubes for hifi = matter of opinion.I love tubes. Haven't got any tube stuff nowadays though.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 05, 2020, 07:36:07 pm
content: I'm Guy who mixes three bands at his place of worship on weekends + vocals.The answer is, "It depends on what you want to do". But you will have to do a lot of measurements to find what you need.One of our favorite pieces of gear has a switch on the input for +22,+4, 0, dBu and -10 dBV gains. Why? because there may be audio standards, but not every piece of gear follows them.On a guitar pickup. first stage, gain of +10 to +15 and watch your input impedance, coupling, and roll-off.Second stage, as much as you need to get to line level, and variable gain or fixed gain with a output potentiometer before the line driver.Why, because I can hear the changes as the load on the pick up changes on low end and prosumer gear used by our musicians. Hell, even a direct box can color the sound.I have one musician I will call "Dr. One Note Wonder", because of the horrible way his beat up old preamp / equalizer acts like a bandpass filter and peaks the middle of his main octave because of pickup loading. One day that box quit and we put him on a nice modern wireless sender with built in "classical music" grade preamp with very light loading. WOW, Night and Day.. Non technical persons walked up and asked him what he changed to sound so much better. However the old box is his "Security blanket" and he refuses to give it up.We keep a tube follower around for those who want "that" hollow state sound.Our digital mixer has a remote controlled variable gain preamp for each channel after the line receiver and then the slider on the IPOD we use for control. Both get used a lot on the guitars, and change a lot during rehearsals.No two guitarists or bass players get the same settings on the console, and I'm trying to convince leadership that we need a "pickup" clinic some evening to clean up the mess between say seven different guitarists and their instruments.Point of the matter, make your design very flexible.Cheaper to buy a few of them and see if they color your sound, versus building your own. Don't even get me started on the direct boxes that do not have a wideband transformer in them. I swear some of them use 60 Hz magnetics instead of audio transformers, and can see that on bench test when I sweep them.Don't get me started on the scratchy, cheap, passive adjustment rigs in some of the guitars.I'm not an Audiophool, my home speakers run on 14 Ga Zip Cord.Steve

author: obiwanjacobi
author_rank: Super Contributor
timestamp: February 06, 2020, 06:49:48 am
content: Quote from: ELS122 on February 03, 2020, 10:07:43 pmDo you mean "gain" as overdrive/distortion -or- as the amount the signal is amplified?Many guitar players use the word gain as measure of overdrive/distortion => so I'm just checking.My understanding:Input Impedance => look at pedal circuits that have a 'buffer'. That first stage is what you need. It makes sure that the impedance from the pickups is matched and the signal has no loss. After that a simple (non-inverting) opamp should do.If you also want overdrive/distortion: look at overdrive/distortion pedal circuits: notice the diodes in the amplifier stages.Note that a guitar amp has half the frequency range of the hifi frequency range. So connecting an overdriven guitar signal to an (hifi) audio input may make it sound crackling, thin and odd. A simple low-pass filter could fix that though.

author: Zero999
author_rank: Super Contributor
timestamp: February 06, 2020, 10:57:40 am
content: To answer the original question: not much gain is needed for a guitar pre-amp.A unity gain buffer is often used. A single MOSFET with in a source follower configuration can be used.https://circuitszone.com/guitar-preamp-circuit-based-fet/Another option is a BJT with bootstrapping to increase the input impedance.LTSpice Simulationhttps://www.eevblog.com/forum/projects/resistor-and-pot-in-parallel-not-working-as-i-expected/msg1401083/#msg1401083

author: sam[PS]
author_rank: Contributor
timestamp: February 28, 2020, 05:32:06 am
content: In short :For the electronic part of the question : line input should be set around 1dBu, that mean about 0.77V RMS. Output signal of a guitar is between 0.1V and 1.5V. So how much gain do you need : 0.5 to 10. For the input impedance 100K-1Mohm is the norm in almost all pedal/amp.Now for the sound part "LOT of gain" : if it's about distortion just clone/buy a mxr distortion+ and you should have all you need in there. If you want to use discret transistor, clone a big muff. Want tube ? clone a matchless hotbox... Really no need to reinvent the wheel there.Just my 2cts...