Home Forums General DIY Pedal Discussion 18VDC Operation for circuits

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  • #3047
    Dominick
    Participant

    I am interested in increasing the supply voltage to a few of the circuits for operating a bass guitar signal chain. The bass that I am building is running an onboard preamp to 18VDC using two batteries. It seems that running any circuit at 9VDC defeats the purpose of having two batteries on the bass.  So I am interested in the following pedal circuits to build a 3 parallel effects chain for the bass player I am supporting.

    Combo Pedal 1 will have 1 Paramix and 2 BuffNBlend

    So I am considering the Paramix, Buff N Blend, Bass Fuzz, Bass Drive circuits to run them at 18VDC.  Maybe some others.

    Paramix uses TL074 OpAmp circuit which I know can support >18VDC operations and the circuit is pretty simple so this is easy to up the voltage:  Bass Guitar Input, Insert will have the Cali76 Bass Pedal for compression of fundamental. So the Insert is the first of 3 parallel paths for the circuit. This will be driven directly to output.

    I plan to use two Buff N Blend to get two buffered signals of the bass input. Probably post Insert is where I will tap this signal versus directly on the input as the Paramix uses. One path will go to a Fuzz/Distortion/Octave/Pog Send/Return loop. The second path will support  Chorus, Delay, Reverb, Envelope and maybe a few others.

    I want to have a volume mixer to mix the amplitude of the 3 return signals direct to output.

    Oh and I will have a third buffered output for tuner with a master mute switch on the combo pedal. Probably using the Silent Tuning circuit.

    So can the BuffN Blend be set to run at 18VDC. I assume that this is not going to require changes.

    For the fuzz/OD path, I am considering using two of the following:  Bass Overdriver, Color Tone Base Fuzz, Guitar/Bass Driver or Mastodon Fuzz. Which all seem to be capable of running higher supply voltages but I may need to use higher trim pots and/or higher fixed resistors on the output of the transistors to ensure the same current through the transistor with higher supply.  Perhaps someone has a recommendation for modifications to specific component values to run the supplies of these circuits at 18VDC versus 9VDC.

     

    #3048
    wilkie1
    Moderator

    Here are some basic thoughts about changing voltages.

    FIrst, as TONMANN posted several years ago, increasing the voltage does not increase headroom.  I have noticed a change in the tone of circuits when doing this.

    Second.  The components must be capable of operating at the higher voltage.  Electrolytic capacitors should have a rating that is at least 1 1/2 times the operating voltage to be safe.  Most opamps and transistors used in these circuits should be able to handle 18V.  However.

    Third.  The higher voltage will change the bias for all circuits that use transistors.  Some of thee circuits do not have a way to adjust the bias voltage and may not operate properly.

    Last.  I do not think that the bass preamp operating at 18V will have any effect on circuits downstream operating at 9V.  I suggest you try it first and evaluate the results.

    #3051
    Barry
    Keymaster

    To help I am copy and pasting Tonmann’s original article from the old site here. Unfortunately the excellent Images he had provided from his Scope are no longer available which really helped understand his points. Keep that in mind when reading.

    Courtesy of Tonmann:

    The gain for op amp and transistor circuits have nothing to do with the power supply, both are set by circuit components.

    Distortion is where the output signal is not a faithful reproduction (but usually just a larger amplitude) of the input signal; while clipping can be considered as distortion, distortion is not necessarily clipping:

    This is distortion in a transistor circuit which was done by reducing the base voltage. The input signal (green) is perfectly symmetrical above and below the 0V line, the output signal is not symmetrical – the positive going part of the signal is flattened and has a lower amplitude than the negative going part of the signal.

    This is clipping in a transistor circuit which was done by increasing the gain of the transistor stage (bypass capacitor across the emitter resistor).

    Notice that the signal peaks do not reach the power supply voltages (9V and 0V), this is due to the internal workings of the transistor.

    I’ve put in the above information so you can see what I mean when I say that op amps don’t distort,at least not in the sense of the first diagram, but they do clip as in the second diagram.  Of course you can make an op amp distort by adding extra components such as diodes to the circuit.

    The level at which op amps clip is dependent on the power supply, consider the following diagram where the power supply is 9V with the output biased to 4.5V and a “standard” op amp is used:

    Again notice that due to the internal workings of the op amp the signal never reaches the power supply rails (9V and 0V) but fall about 1.5V short.

    Doing the simple math it should be plain to see that the maximum signal level before clipping occurs is about 3V.

    If you increase the power supply the maximum signal level before clipping occurs also increases e.g. for a 12V supply you would get roughly a 4.5V signal.

    Main Point:

    A 3V signal is much too hot for guitar amplifiers or other effects pedals – why would any one want to increase the power supply to get a higher clip free signal when it would have to be attenuated by a larger amount to feed an amplifier?

     

    #3054
    Dominick
    Participant

    Ok thank you both for your information.  I think I understand your responses specifically when it comes to blindly wanting to increase the supply voltages for these circuits. While I want to increase the headroom, I am not just trying to increase the supply voltages blindly. I guess the information you provided is to suggest that increasing the headroom on a pedal above 9VDC is not necessary at all. But neither is a 600 horsepower motor in a Corvette when you can only drive 65MPH…

    For this I am really just concerned with having enough headroom on the fundamental tones of a bass guitar specifically for the lower strings which have higher voltage levels in their output. While it may not seem necessary, the bass guitar that he uses has a preamp which is running from two batteries and has a headroom up to 18VDC. Why wouldn’t the player want the remaining path have similar headroom?

    So my objective is to ensure that the path for the fundamental tones has as much headroom as possible. And I am also thinking of adding high pass filters for the 2nd (OD/Fuzz) and 3rd (Chorus/Phaser/Flanger/etc) paths so perhaps the pedals operating on the higher frequencies will not require 18VDC. I get that.

    So in general, the OpAmp based circuits using 18VDC that would carry the fundamental tones would likely benefit (at least in theory). In my example, I may opt to run the Paramix and BuffN Blend at 18VDC while running the rest at 9VDC.

    FY, that I see that the preamp/eq section of the GK bass head he will use has power supply circuit of +/- 15VDC. The power amp section is at higher voltages as well. Here is a case where the 500W power available to the GK head is analogous to the 600HP Corvette. You may not need 500W to power the stage when the guitar player has a 22W Deluxe Reverb. But having the headroom makes sure that the lower frequencies will not clip. Driving to the bank in a Corvette is much cooler than a Pinto!

    I do understand that the BJT and FET based circuits used in the overdrive and fuzz PCBs can not just automatically use a higher power supply voltage without having their input and output bias voltages being adjusted to support more symmetrical operation. So I may start with the Paramix and BuffNBlend and analyze it under the player’s load using a scope and see how it lands. Maybe it will be a waste of time. We’ll see.

    #3059
    wilkie1
    Moderator

    An 18V supply is not the same as a headroom of 18V.  As Tonmann discussed, the output SIGNAL level will not exceed 2-3 V depending on the input level that the amp first stage can accept.  The supply voltage sets the rails of the power supply, not the output voltage.  If the signal level at the LAST pedal in the chain is around 1-2 V, that should be plenty of signal level for the amp to do its job.

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