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I have built the following electric guitar preamp which is mounted inside the instrument. The primary reason for doing is that there is a piezo pick up in the bridge with a very high output impedance which never sounds good plugged into the amp passively. I aimed for 0db net output gain with linear bandwidth within the 20-20k range so as not to impede the attack and brightness of the piezo. While the circuit is tested as functioning as intended, there is audible spurious HF interference noise, a +/- 6 to 8 kHz ring or whine mixed with static in the background, which becomes audibly loud during pauses when distortion is used in the main amplifier. All individual pickups have shielded connections to the main switch inside the guitar cavity which is additionally completely shielded with conductive paint. This was a quiet electric guitar without the preamp. The components are very tightly fitted on a 2.5x4 cm piece of hobby board with star grounding. The potentiometer terminals are connected via +/- 5cm wire jumpers all within the cavity.

enter image description here

  1. I might have gone overboard with the HF bandwidth. Will the reduction of the HF bandwidth by increasing the values of C3, C9, C11 achieve this and which of these capacitors are most relevant?
  2. I have read that introducing a modest +/- 1k series resister prior to the output can improve transmission through the guitar cable. Is this true and are there any other simple modifications to optimize the transmission through an unbalanced guitar cable?
  3. The tone controls are a bonus but not necessary per se. Would removing the tone stack make any difference to the problem?
  4. Any other suggestions are appreciated

Thanks!

BTW: I don't have an oscilloscope but I do have a loudspeaker testing setup. With tone controls set at unity, the circuit is linear to +/-40k with overall low distortion

This circuit is also public on CircuitLab: https://www.circuitlab.com/circuit/52475zw4edt5/opa134-based-guitar-preamp-with-tone-control/

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  • \$\begingroup\$ If you're building this level of circuitry, you need an oscilloscope. :) \$\endgroup\$
    – rdtsc
    Feb 9 at 20:00
  • \$\begingroup\$ Can you be more specific than "spurious HF interference noise"? Can you record an audio snippet to MP3 and post? Can you record it to your computer and use an audio editor program to look at the waveform? \$\endgroup\$ Feb 9 at 20:07
  • \$\begingroup\$ I will do that. FWIW, it sounded like when a DACT telephone came too close to a wireless mic a few years ago \$\endgroup\$
    – p-we
    Feb 9 at 20:32
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    \$\begingroup\$ Another possibility is that your circuit itself may be oscillating at rf frequencies. It is important to have power supply decoupling capacitors right at the op amp terminals.These should go to ground. Also, all the grounds should be short. \$\endgroup\$
    – user69795
    Feb 10 at 2:05
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    \$\begingroup\$ It is the inductance of the long wires at MHz frequencies that cause a problem. 0.1 uF capacitors with short leads bypassing the power pins to ground is the solution. \$\endgroup\$
    – user69795
    Feb 10 at 22:04

4 Answers 4

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This is the most common RF interference. It is the cell phone pinging the tower on a slow periodic basis for connectivity tests. It occurs from the EM coupling to the wires on higher impedance lines. What is being heard is the carrier demodulation of microwave carrier on the input diodes or transistors.

Choosing an RF cap is not that difficult but and only needs to be placed near the amplifier input to ground. Typcially this is in the 50 pF ballpark. NPO and film have lower ESR but may not be necessary compared to the high kohm audio source. If there is no series resistance between sensor cable and amplifier input, this can improve RF attenuation without necessarily affecting audio attenuation due to the breakpoint of input of 1/T = 1/RC

It's not clear to me is which cable and amplifier is picking up the interference, as you output is also high impedance. So the treatment of this solution depends on which input is affected the most. (pre-amp or power amp)

enter image description here

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  • \$\begingroup\$ Thanks for that. Is that 1k under the yellow circle? \$\endgroup\$
    – p-we
    Feb 9 at 21:10
  • \$\begingroup\$ It was left out as not critical compared to 1M and may not be necessary with 47 pF being < 10 ohms at carrier microwave frequencies compared to Zo of cable and impedance of coupling from cell phone. But if there is no cell phone near by, it must be something else ! \$\endgroup\$ Feb 9 at 21:20
  • \$\begingroup\$ I just plugged in here but could not reproduce the issue as we heard it clearly on a friend's gear and not present with other guitars with identical settings. It sounded different to your example in the link. It was more of a high pitched whine I would estimate between 6 to 8k mixed with variable static lower in frequency. It also indeed varied in tone and amplitude when I waved my hand around that part of the guitar. Touching the metal hardware made no difference. I will add your mod plus Audioguru's series output resistor recommendation then try to reproduce and will record if unsolved. \$\endgroup\$
    – p-we
    Feb 10 at 0:05
  • \$\begingroup\$ The filter was primarily for the mobile RF interference and the proximity symptoms of oscillations are an unknown cause. Are there other pickups nearby? Do they share a ground somewhere?but if lucky the filter might work. Audioguru's series R neglected the fact that the Pot is already there as a series R so no driver capacitance load oscillation. \$\endgroup\$ Feb 10 at 0:47
  • \$\begingroup\$ There are 3 pickup's (piezo + 2 electromagnetic) switched prior to input. Long before this project I replaced radial dual wafer selector switch with quad type to completely short out any pickup which is not selected. This has always helped with hum/noise as even a pickup that is not connected to + output can still send noise to the ground. About the series resistor, would that still hold true if the 25k pot was wide open? I would think then that the series R would only be a few ohms or so. So 100 ohm doesn't seem like much but I guess more is not needed? \$\endgroup\$
    – p-we
    Feb 10 at 6:41
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Interference on a circuit that has a very high input resistance is usually caused by wiring that is not shielded and the circuit has its parts too far apart. Maybe the preamp should be in a metal box that is connected to 0V and the cable from the piezo transducer should be a shielded audio cable.

I hope you bought the excellent OPA134 opamps from a real electronic parts distributor like Digikey and not fakes from "over there".

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    \$\begingroup\$ The capacitance of a shielded audio cable connected directly to the output of an opamp frequently causes it to oscillate, therefore a 100 ohms series resistor is used between the opamp output (or from your volume control output) to the hot internal cable wire. \$\endgroup\$
    – Audioguru
    Feb 9 at 19:55
  • \$\begingroup\$ Thanks Audioguru. I edited above to indicate the pickup inputs and output have shielded wires internally as well as the internal cavity where the circuit is. The layout is tight on a 2.5x4 cm piece of hobby board with star grounding. Appreciate the output advice. The OPA's should be real. From Jaycar in Aus/NZ. \$\endgroup\$
    – p-we
    Feb 9 at 20:55
  • \$\begingroup\$ its possible your series resistor tip has proven itself valid. I haven't modified anything yet but yesterday listened carefully to volume fully cracked open and slightly dialed back and there is a change/degradation of clarity even before there is a noticeable volume difference. Ironic because in standard passive guitars this is always the other way around \$\endgroup\$
    – p-we
    Feb 10 at 20:09
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    \$\begingroup\$ Guitars are odd since the preamp in them has the volume control at the output of the preamp causing the opamp at the output of the preamp to oscillate if the opamp directly feeds the cable capacitance. Volume controls are usually at the input of an amplifier, not at the output. \$\endgroup\$
    – Audioguru
    Feb 10 at 21:19
  • \$\begingroup\$ Thanks, will trial that and easy to change. I guess will ideally have to then use standard 500k guitar pot \$\endgroup\$
    – p-we
    Feb 11 at 6:06
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I've done some testing based on the very helpful feedback here:

  • With no pickup attached a background high pitched ring could be heard at high gain amp settings which was solved by the suggestion of @user69795 to bypass the V+/- with 0.1uF caps. Those jumpers were running very close to the bottom of the PCB
  • There was more background noise when EM pickup's were again attached to input where the piezo made no real difference. The suggestion of @Tony Stewart EE75 to LP filter the input cleaned up the background noise floor subtly when clean but obvious with overdrive and compression.
  • Following advice from @Audioguru, I compared output signal between PotVol wide open and bypassing PotVol and there was definitely more clarity overall. I was worried about putting a potentiometer at the input due to the piezo's high output Z but the choice is clear. I will try a 1M input pot in the next version
  • Back in 2020 I could not source nickel print to shield the electricals cavity. I found some acrylic 'conductive paint' which had good reviews and I tested beforehand to only have 10 - 15ohm ohm across +/- 10cm. What a mistake! The stuff had become a bit gummy and is now measuring about 600 ohm. So the shielding has become sub-optimal over time possibly causing some of the noise as well.

I will rebuild from scratch using the new schematic below:

  • PotVol now prior to input using RC LP filter and 4.7M bias resistor instead of 10M hopefully reducing noise a bit more
  • I will locate the DC connections and 100uF PS cap's closer to the op-amp V inputs plus 0.1uF decoupling cap's directly to V pins
  • Tone circuits changed to cover wider bandwidth
  • I removed C4 as I don't think it is necessary. Glad to hear if I'm wrong about that.
  • I also added another LP filter to the input of OA2 thinking that possibly the tone circuit could also introduce noise or UHF resonances?
  • The cavity will be re-shielded with something better

EDIT: This is the intended final circuit

enter image description here

I am sincerely grateful for the expertise provided on this!

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Your power supply has an active ground (between the two 7.4v batteries). You should use that as Ground for the rest of your op amps instead of capacitively coupling to that ground.

All of those capacitors take a few cycles to establish a center voltage once you start strumming. You can run some jumpers (short jumpers!) and simply short those capacitors (c2 and c8).

Also, a hot pickup can have 4v or more peak voltage input. Each op amp has gain of 1.5 overall 2.25 so you may be saturating your signal which may sound buzzy when using +/-7.5v rails.

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  • \$\begingroup\$ Thanks for your input. AFAIK electro harmonic pickups do not exceed 1v even at peak. This s an example. sound-au.com/articles/guitar-voltage.htm. The Fishman piezo however is a beast. I currently have a 470k-470k voltage divider on it individually before going to the input switch to get it close to parity with the others. Are you suggesting to effectively get rid of c2 and c8? \$\endgroup\$
    – p-we
    Feb 11 at 21:13
  • \$\begingroup\$ I have star grounded the whole lot including PS. There are a few ground jumpers to achieve this none of which are longer than 3cm. I guess that might not be ideal. Being a semiconductor circuit novice I can't follow your advice. Could you elaborate? \$\endgroup\$
    – p-we
    Feb 11 at 21:37

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