You have two issues.
Issue one is that your guitar cannot drive a 10k load. Get a DI box to match the guitar to the amp or build a suitable preamp.
Issue two is the speaker with 88 dB/W/m. That is the sensitivity of a hi-fi speaker, not an instrument speaker. A guitar speaker is likely to have a sensitivity of 95-100+ dB/W/m, which means it will need ~1/10 of the power yours does for the same volume.
Issue one is that your guitar cannot drive a 10k load. Get a DI box to match the guitar to the amp or build a suitable preamp.
Issue two is the speaker with 88 dB/W/m. That is the sensitivity of a hi-fi speaker, not an instrument speaker. A guitar speaker is likely to have a sensitivity of 95-100+ dB/W/m, which means it will need ~1/10 of the power yours does for the same volume.
Shooting from the hip, since I haven't spent any time with the 3886 datasheet, couldn't he just jack the value of R3 up by, say 5x or 10x to 330K? The extra gain will come at the cost of added distortion, but hey - it's a guitar amp, right? Also might try a higher resistance input pot or just replacing it with a 100K resistor and using the volume control on the guitar as an experiment to see how much the 10K load affects the guitar output.
The right approach is to put some kind of preamp in front of it, but it doesn't hurt to experiment. (as long as your parts budget permits it.)
Jim
The right approach is to put some kind of preamp in front of it, but it doesn't hurt to experiment. (as long as your parts budget permits it.)
Jim
Even when I plugged the amp into the computer with the volume on max I get only 8-10V at the speaker terminals. Is that normal?
I tried replacing the pot with a 500k one but the volume does not get much louder.
BTW can anyone help me on how to calculate the speaker voltage depending on the impendance and wattage.
Thanks
A single LM3886 isn't enough if you want to amplify the guitar signal. You MUST have a preamp of some sort with 20-40 dB gain.
24 VAC transformer will result in 30-32 V peak into your speaker and this is 21 Vrms. Right now you have 30 dB gain ( 34 times). => 617 mV for full output. Your guitar will probably generate a few mV (I don't know) so you must have something which amplifies the signal.
24 VAC transformer will result in 30-32 V peak into your speaker and this is 21 Vrms. Right now you have 30 dB gain ( 34 times). => 617 mV for full output. Your guitar will probably generate a few mV (I don't know) so you must have something which amplifies the signal.
Last edited:
What are your volume control settings on your pc? Depending on your operating system and the sound chip or sound card there may be some settings that are turned down a bit. remember to get 50 watts you need 20 volts on 8 ohm speakers, you're getting about half of that, close but no cigar. Most volume controls are logrithmic ie at 1/4 volume the signal could be as low as 1/100th, at half volume the signal is 1/10th. How are you measuring the voltage, most DMMs will average out the reading you get. They do not respond to short peaks. Some DMMs will monitor the voltage over a period of time and display the min max voltages. Music is dynamic in nature and it is not unusual to have music that varies 20db or more ( x 10) in signal. The tone control settings will also effect your results.
P = E x I Power = volts times Current ( amps)
E = I X R voltage = amps times ohms
I = E/R current ( amps) = Voltage / ohms ( resistance)
so substituing we get
P = Esquared / R
or
P = E x E / R
in your case
P= (20 X 20) / 8
P = 400/8
P= 50 watts
P = E x I Power = volts times Current ( amps)
E = I X R voltage = amps times ohms
I = E/R current ( amps) = Voltage / ohms ( resistance)
so substituing we get
P = Esquared / R
or
P = E x E / R
in your case
P= (20 X 20) / 8
P = 400/8
P= 50 watts
Triple check your component values and make sure there are no mistakes. Mistakes are the #1 reason why an amp doesn't do what it supposed to.
I'm not sure what the SPIKE proection does to the output if it engages, but it might be a factor. You want to make sure that the chip is well heatsinked and there should be an output inductor in series with the speaker load (a 10 ohm, 2 resistor wrapped with 10-12 turns of #22 wire).
I've got it! You're using the wrong volume control. You need one that goes to eleven.
Google tells me that guitars put out between 100mv and 2v RMS. With that information and the formulas that multisync posted in #26 you should be able to figure out the gain you need to hit the power levels you want.
If you build the preamp that John posted (post #18), make sure you put a DC blocking capacitor between the preamp output and the amp input. I'd seriously consider building the FET preamp posted directly above. It looks like a winner although there's only 3db of gain, so you'd have to jack up the gain of your amp a bit. The gain of the amp is approx. the ratio of R1/R3. As you increase the gain of the 3886, distortion will rise, especially at the high end, but I would guess it would be OK to try a 100K which would triple your voltage output and jack up the power output by 9X.
Jim
Google tells me that guitars put out between 100mv and 2v RMS. With that information and the formulas that multisync posted in #26 you should be able to figure out the gain you need to hit the power levels you want.
If you build the preamp that John posted (post #18), make sure you put a DC blocking capacitor between the preamp output and the amp input. I'd seriously consider building the FET preamp posted directly above. It looks like a winner although there's only 3db of gain, so you'd have to jack up the gain of your amp a bit. The gain of the amp is approx. the ratio of R1/R3. As you increase the gain of the 3886, distortion will rise, especially at the high end, but I would guess it would be OK to try a 100K which would triple your voltage output and jack up the power output by 9X.
Jim
Last edited:
Nice, but not enough gain.
My schematic is missing a couple of GND symbols. Should be pretty obvious where they go.
Nice preamp:
Clean channel from the Ampeg VH140c. Doesn't run on a 9V battery though and a bit more gain.
I am using Windows 7 and a HP laptop. All the volume setting i could find were turned up all the way. And I am using a DMM set on the AC setting.
Thanks for the formulas. They have made things clearer for me.
It would be normal, if your input signal was 0,235-0,294 V at the time of measuring. Try to use a steady test tone with a low frequency (200 Hz or less). Measure the amp input and the output. You should get a ratio of ~34. Increase the volume and see where you get to.
It would also be normal, if the rails sag too much under load, e. g. due to too little smoothing capacitance. In that case you would probably notice distorted sound. Well, more distorted than usual with an electric guitar.
By the way, 8-10 V is not that much quieter than the maximum output you can expect from that amp.
It makes an ugly sound and won't go unnoticed.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.