Help with amp

Discussion in 'The Projects Forum' started by shorteddiode, Aug 11, 2013.

  1. shorteddiode

    Thread Starter New Member

    Apr 6, 2013
    6
    0
    I want to add a reverb tank to this circuit but not sure where to tie it in. I want it adjustable and my first thought is to put it before the input, is that right?

    The schematic calls for an 8 ohm speaker, mine is 4 ohm. Can I just add a resistor to the out put going to the speaker?
     
  2. #12

    Expert

    Nov 30, 2010
    16,321
    6,818
    You need a driver circuit for a reverb tank, and a recovery circuit, and this circuit has neither. The good part of that statement is that it clarifies that you can not install a reverb tank before the input.

    A 4 ohm resistor will protect the amplifier from too low of a load impedance, but it will waste half the power that the amplifier generates. Obviously better to use the right speaker.
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Hello shorteddiode.

    Your question about adding a reverb circuit makes me wonder if you may be thinking guitar amp. You need to know that you can't plug a guitar directly into this. You will need a guitar preamp, which is also where you would put the reverb circuit.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Why? Guitar or pedal signals easily have a 1v p/p signal or more, and that amp only needs about a gain of 80 to start clipping with an input of 1v p/p.

    I'm more worried about the distortion and reliability issues of trying to squeeze "150W" out of two power transistors which are crudely driven and crudely biased.
     
  5. Shagas

    Active Member

    May 13, 2013
    802
    74
    Either put a 4ohm resistor in series with the speaker to satisfy the amp but as # said you will be wasting half the power as heat in the resistor.
    Or you could put a second 4ohm speaker in series with the first and you got yourself 8ohms
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    The input impedance is way too low for an electric guitar with passive pickups. That will affect the tone quality and load down the signal. Many guitars have less than 1v p-p signals.

    Yes, I agree the amp circuit has issues. Should have 2-3 times the output transistors. 150 watts @ 8 ohms is not even possible with 90 volts rail and a bit for transistors losses. More like 120-125 watts max at 8 ohms. In my opinion, for good reliability, about 50-60 watts per pair of (200-250 watt rated) outputs is a good rule of thumb.

    EDIT: Those are 125 watt, 100 volt darlington transistors, so there will be plenty of crossover distortion, along with a short life span. (The BC558 is only 80 volts.)
     
    Last edited: Aug 12, 2013
    THE_RB likes this.
  7. shorteddiode

    Thread Starter New Member

    Apr 6, 2013
    6
    0
    Thanks guys, Yes I am looking to build a guitar amp. So any ideas on a build for a good guitar amp. I am a newbee guitar player
     
  8. #12

    Expert

    Nov 30, 2010
    16,321
    6,818
    Look at schematics of known, good amplifiers. Choose your sub-sections like, preamp, tone controls, tone recovery stage, reverb driver, reverb recovery stage, phase splitter, and power output stage...or just copy a whole schematic. Building one for yourself won't violate any laws that I know about.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Thanks, I missed that. To me a "guitar" signal always comes after one or more effects pedals (which have a low output impedance). Every guitarist I know has 5-10 effects pedals in use, or some larger compound type effects system.

    I have no idea why someone would plug a raw guitar pickup into a crude "150W" amp... I suppose there are twangy country and western people who might want to plug raw guitar sound straight into an amp, but it's a very uncommon situation. ;)
     
  10. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Yep. One of my buddy's guitar "amp's" is actually an effects pedal which has programmable sound effects, guitar amp emulation, midi control etc... He runs this direct through the PA. The only thing it doesn't do is play the guitar for him.... yet.

    A preamp also gives the capability for extreme over drive and lovely distortion. But the amp circuit posted doesn't need any help generating distortion. :rolleyes:
     
    Last edited: Aug 14, 2013
Loading...