Guitar Level To RCA Line Level Preamp

Thread Starter

Paul Bober

Joined May 11, 2024
6
This is my simplified schematic, that brings up the -20 db guitar signal to the -10 db level, required by RCA Line Level Inputs. These Inputs are found on P.A. , D.J. , and Home Stereo Equipment. Do not connect the audio output of this preamp to Phono, Microphone, or 600 Ohm Line Inputs. I hope this circuit is useful for Electronics Engineering Instructors/Students, Hobbyists, and Guitar Players.
 

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Ian0

Joined Aug 7, 2020
10,277
55kΩ is to low an input impedance for a guitar. The usual is 1MΩ but 220k is better, because it damps the resonance between pickup inductance and cable capacitance.
The LM1458 being a dual 741, probably needs at least 10V power supply to work properly.
You would be much better with a JFET input opamp.
I also think that a guitar output is more like -40dBm than -20dBm.
 

LowQCab

Joined Nov 6, 2012
4,309
This can be done with a Single-Op-Amp and half of the Components.

I would highly recommend an Input-Level-Pot,
these "supposedly-standard" Input and Output-Levels vary all over the place in the real world.
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Thread Starter

Paul Bober

Joined May 11, 2024
6
This can be done with a Single-Op-Amp and half of the Components.

I would highly recommend an Input-Level-Pot,
these "supposedly-standard" Input and Output-Levels vary all over the place in the real world.
I chose to use a 2nd op amp, to cancel any noise that may be present on the d.c. output of the power supply.[/QUOTE]
 

Audioguru again

Joined Oct 21, 2019
6,785
I agree that the MC1458 is an antique dual 741 opamp.
The first opamp adds noise from the old opamp.
The battery needs a parallel filter capacitor instead.
 
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LowQCab

Joined Nov 6, 2012
4,309
For extremely Low-Noise super-low-Distortion,
use one of these OP-Amps.

But generally Noise and/or Distortion is not a big problem with Guitar-Amplification.

The preferred Power-Supply is two 9-Volt Batteries, but it will work just fine with one, 2 will probably last longer.

To better understand the effects of Input-Impedance check out the Graphs below.
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Fender Guitar Pickup Freq Resp Load Effects .png
 

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Ian0

Joined Aug 7, 2020
10,277
MOSFET = Metal Oxide Semiconductor Field Effect Transistor
IGFET = Insulated Gate Field Effect Transistor
It is the Oxide that Insulates the Gate
They are the same thing, and it is static sensitive, and also not very good for audio because of a high 1/f noise corner.
 

Ian0

Joined Aug 7, 2020
10,277
For extremely Low-Noise super-low-Distortion,
use one of these OP-Amps.

But generally Noise and/or Distortion is not a big problem with Guitar-Amplification.

The preferred Power-Supply is two 9-Volt Batteries, but it will work just fine with one, 2 will probably last longer.

To better understand the effects of Input-Impedance check out the Graphs below.
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View attachment 322111
Guitarists are always complaining that their guitars sound wrong on stage, and tend not to believe it is because they are using a much longer lead which has moved the resonance down to a frequency where it causes a problem.
Your plots seem to confirm that 100k is about the right input impedance, not 1M.
 

Thread Starter

Paul Bober

Joined May 11, 2024
6
This is my simplified schematic, that brings up the -20 db guitar signal to the -10 db level, required by RCA Line Level Inputs. These Inputs are found on P.A. , D.J. , and Home Stereo Equipment. Do not connect the audio output of this preamp to Phono, Microphone, or 600 Ohm Line Inputs. I hope this circuit is useful for Electronics Engineering Instructors/Students, Hobbyists, and Guitar Players.
The voltage gain of this circuit = .909 × ((1+(56,000/22,000)). 3.222 as shown. 10.16 db. This is because 9.1% of my audio input signal is buffered by A2, and applied to the inverting input of 1458.
 

LowQCab

Joined Nov 6, 2012
4,309
That's good to know. Mosfets are static sensitive. I would think that igfets are also static sensitive, but I might be wrong.
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Sensitivity to Static is not a deciding factor as to whether or not
a Chip is going to be "reliable", or good or bad, in any practical way.
Most MOS or CMOS Chips have built-in Over-Voltage-Protection on the Inputs,
requiring only that the surrounding Circuitry limit the
maximum Input-Current with some form and level of Resistance.

MOSFET Transistors, ( with rare exceptions ), don't have any Input-Protection,
and yet they are quite often the preferred choice,
even though they sometimes may need an external Zener-Diode for Input-Protection.

The Specification-Sheet will almost always have
a section on protecting the super-high-Impedance-Inputs of MOS-Devices.
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dovo

Joined Dec 12, 2019
72
Even though the 1458 is an antique, I like the fact that it is not static sensitive.
The 1458 is indeed static sensitive.

https://www.ti.com/lit/ds/symlink/lm741.pdf
Even though the 1458 is an antique, I like the fact that it is not static sensitive.
I would not describe the 1458 as not static sensitive. It has a particularly low HBM rating (see datasheet, 6.2) of 400 V (note that this rating is from an LM741 datasheet). ESD protection to 8 kV per IEC 61000-4-2 would be more appropriate and requires additional ESD protection circuitry.


LM741 Datasheet https://www.ti.com/lit/ds/symlink/lm741.pdf

HBM and IEC 61000-4-2 ESD Immunity
https://www.renesas.com/jp/ja/docum...ce-between-hbm-and-iec-61000-4-2-esd-immunity

System Level ESD Protection Guide
https://www.ti.com/lit/sg/sszb130d/...49728&ref_url=https%3A%2F%2Fwww.google.com%2F

ESD Protection for I/O Ports
https://www.analog.com/en/resources/technical-articles/esd-protection-for-io-ports.html
 

LadySpark

Joined Feb 7, 2024
194
Guitarists are always complaining that their guitars sound wrong on stage, and tend not to believe it is because they are using a much longer lead which has moved the resonance down to a frequency where it causes a problem.
Your plots seem to confirm that 100k is about the right input impedance, not 1M.
The norm for amps is 56K-63K because they want a linear response from all pickups. A tube amp preamp stage is most likely 1M input impedance, but that is the bias and not the input impedance the guitar sees. It only sees the 56K-63K feed resistor.

You can use a lot of different things for a guitar preamp. I used a 4558 before, but I would now these days use a TLE2072.

But regardless if it was transistor or op amp, I would put in the reversed biased diodes in the input circuit to shunt the spike from plugging it in.
 
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Ian0

Joined Aug 7, 2020
10,277
This wasn't a yes or no question. But now I know you don't understand how the unbalanced input circuit works and interacts with a voltage source device like a pickup.
If you are going to join the conversation when it has been going a while, how about taking the trouble to read previous posts? especially posts #7 and #11.
 

LadySpark

Joined Feb 7, 2024
194
If you are going to join the conversation when it has been going a while, how about taking the trouble to read previous posts? especially posts #7 and #11.
Never heard complaints from anyone about that, and the range where the frequency peaks in the graph is where its typically rolled of in a mix.
 

Audioguru again

Joined Oct 21, 2019
6,785
The norm for amps is 56K-63K because they want a linear response from all pickups. A tube amp preamp stage is most likely 1M input impedance, but that is the bias and not the input impedance the guitar sees. It only sees the 56K-63K feed resistor.
I also disagree because all toob amplifiers for electric geetars had a 1M input resistance because the speakers (shriekers) had cone breakup boosted mids (they liked that sound) and a poor high frequency response. Phono preamps for magnetic cartridges (remember them?) used an input resistance of 47k ohms for a flat frequency response.
 

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