So, I found out something that indicates a problem. I measured both inverting and non-inverting inputs voltage referenced to the circuit GND, and what I got was about -9,5 or -10V on the both. The interesting thing is that, when I disconnect the -12V (-Vs) input from the PSU, the both inputs show a still 0V.

The first pin of the left IC is connected to the input signal connector only, so I don't think it is shorted to the -Vs line or similar.

Is there a common cause of this problem or you have something specific on mind, because I checked the whole schematic again and I really can't find any errors. All the components are connected properly as well!

PS. I tried using the left IC only to see if there is any output. No, nothing whatsoever. The output is at constant 0V.

PPS. Should I mention that I have omitted some components that seemed like optional:

http://prntscr.com/184fr6

I don't know if there are really required, as they are marked differently than the other components and don't have a specific value assigned...

(btw, the whole datasheet.... http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1458.pdf )
Thanks!

 

Do you have +12 vdc on pin 5?

 

Okay, at this point I made some real progress. I have found several issues:

1. I noticed that the right IC had it's +12V connection disconnected. That wasn't the cause of the problem, but nevertheless is now solved.

2. The both ICs were inserted the wrong way, and now are correct.

After fixing these, I powered up the circuit and used the left IC only, with having the R5 resistor, which is the connection between the two ICs, disconnected. That resulted in total success, the power and the sound quality turned out to be great.

Similarly, I used the output of the right IC only, and got small buzz + weak sound from the phone I use as an input. (that should be expected to some point..?)

Now, I powered up the circuit again, but this time with the R5 resistor connected, and the speaker connected across the both outputs. That resulted in a very loud 'hisssss' sound. Tried the same with the speaked connected across each speaker output and GND, nope, nothing changed. Constant 'hisss'.

I measured the voltage across the inputs and outputs of the right IC, and there is ~ +10V at the output and inverting input, as opposed to 0V at the left IC.

Could I have burned the other IC, which was exposed to +12V at the 1st pin when I had it connected the wrong way (as opposed to the other IC which wasn't connected to +12V, as mentioned at the beginning of the post), or now I have to recheck the whole schematic again and look for the issue there?

Thanks again.

 

If I'm following, you did get good sound with only the left IC connected, but weak sound with the right IC connected the same way, and as soon as you connect R5 you get the hiss.

Yes you could have damaged the right IC with the crossed connections. I would check the 1n4001 diodes for shorts and if you have a diode test check them that way also.

 

The diodes checked OK, so I replaced the IC and now everything seems to be functioning properly. I am using a 15W speaker, so that should be the reason I get the same volume when I use one vs. the both outputs. I'll post back if I need help, until then, thank you for everything!

 

Hey. I seem to have issues again. It's that whether I connect the speaker across the left IC output and GND or across left and right ICs, the sound volume stays exactly the same (and the tone as well). I am using a 50w speaker so it's not a speaker limitation, and I don't have idea why this happens. It's sure that the both ICs are OK.

Thanks!

 

Hey. I seem to have issues again. It's that whether I connect the speaker across the left IC output and GND or across left and right ICs, the sound volume stays exactly the same (and the tone as well). I am using a 50w speaker so it's not a speaker limitation, and I don't have idea why this happens. It's sure that the both ICs are OK.

Thanks!

Do you get the same sound level if you connect the speaker between the right IC and gnd ?

 

No. Not even close. I have to get really close to the speaker to hear very weak sound.

Just to mention, I measured for DC voltages at pin 1,2 & 4 and got almost 0V. (~0.1)

 

No. Not even close. I have to get really close to the speaker to hear very weak sound.

Just to mention, I measured for DC voltages at pin 1,2 & 4 and got almost 0V. (~0.1)

For this test, you are still running the input signal thru the coupling capacitor into the left IC and R5 is connected between the IC's as shown in the schematic (middle, post #41), correct ?

In this bridging circuit both IC's should output the same signal to the speaker, with the right IC signal inverted with respect to the left IC.

 

For this test, you are still running the input signal thru the coupling capacitor into the left IC and R5 is connected between the IC's as shown in the schematic (middle, post #41), correct ?

Yes. I was thinking to try out the right IC with R5 disconnected, and the input signal connected to the noninverting input thru a coupling capacitor. That should work fine assuming the right IC circuitry is correct, so that would rule out the option that I have a PCB mistake in that area. (correct me if I am wrong, or suggest something else if you have a better idea).

 

Yes. I was thinking to try out the right IC with R5 disconnected, and the input signal connected to the noninverting input thru a coupling capacitor. That should work fine assuming the right IC circuitry is correct, so that would rule out the option that I have a PCB mistake in that area. (correct me if I am wrong, or suggest something else if you have a better idea).

I agree, that's a good test as long as you use a coupling cap.

 

It turned out to be a dry solder on C6, without which the input can't get to the second IC. Once I fixed that, everything seems OK.

I have one more issue though: the left IC heats up MUCH more and faster than the other one. The heatsinks I use for the both ICs are identical, and are properly fastened to the chip.

Could this indicate a problem?

 

Are they getting hot just idling, or only when you are running a signal through the amp? They should not get hot while no signal is supplied.

Is the speaker load 8 ohms ?

What size are the heat sinks ? Are the IC's mounted to the heat sinks using heat sink compound or a thermal pad ?

Another cause of unusual heating is high frequency oscillation - upper limits of hearing or beyond. That could be caused by ground issues wire routing, and/or lack of bypass caps on the power supply. (the small value caps from the V+ and V- to gnd, in parallel with the PS filter caps.) These should be physically close to the power supply pins on the IC's.

 

So, I verified the soldering points of all the caps on the board, and found another dry solder, this time on C2. Now not only the chip stays cool all the time (even when loaded), but also the volume has increased dramatically. So now I can say it is working properly.

But now something else has occurred. Whenever I plug in the cellphone, I get LOUD humming noise, and that continues until I unplug it. Even louder hum (like noise from a big transformer) occurs when I touch the input wire with a bare finger. I have no idea where this comes from.

 

Have you been using this same cellphone and connecting cable all-along ?
Is the cable shielded, with the shield connected to ground on the amp ?

 

Have you been using this same cellphone and connecting cable all-along ?

Yes. I also tried with another cellphone and got the same result.

It's funny because this wasn't an issue till I fixed that dry solder.

Is the cable shielded, with the shield connected to ground on the amp ?

No. I am not sure if I find a shielded cable with a 3.5mm jack, but I'll give it a go.

 

It seems that this will wait for some time. In the meanwhile, I am working on the preamp. I made this circuit here ( http://www.circuitstoday.com/guitar-pre-amplifier-using-ua-741 ), and now looking for a good distortion solution. I found a preamp circuit on this forum, and I am thinking to use a part of that for the distortion.

Namely: http://prntscr.com/19r9i1 (so, the preamp circuit above + the marked part of this circuit).

Here is the catch. Instead of using that opamp (U10) in the circuit, I want to know if it is possible to use the diodes only. Something like this: http://prntscr.com/19rae5

If it is OK, then where can I add a gain pot, to modify the distortion level?

Thanks!

 

First of all, use vacuum tubes (only even harmonics are amplified). The amplified tones will have a more melodious quality. The best and most economical basis I have ever found for a guitar amplifier is the Masco amplifier circuits: Photofacts (check Ebay)...Class B Push-Pull 6L6 tubes give lots of power with low distortion. I used to build these for up and coming poor musicians during my highschool and Penn State years.

Cheers, DPW [Everything has limitations...and I hate limitations.]

 

It seems that this will wait for some time. In the meanwhile, I am working on the preamp. I made this circuit here ( http://www.circuitstoday.com/guitar-pre-amplifier-using-ua-741 ), and now looking for a good distortion solution. I found a preamp circuit on this forum, and I am thinking to use a part of that for the distortion.

Namely: http://prntscr.com/19r9i1 (so, the preamp circuit above + the marked part of this circuit).

Here is the catch. Instead of using that opamp (U10) in the circuit, I want to know if it is possible to use the diodes only. Something like this: http://prntscr.com/19rae5

If it is OK, then where can I add a gain pot, to modify the distortion level?

Thanks!

Your top preamp circuit is OK, but use one of the recommended op-amps (or similar) for the solid state preamp I posted in post #21. You will get very poor performance from the 741. It's input impedance is not suitable and the frequency response is poor. In fact, it will color the guitars sound, so it would be interesting to compare the sound between the various op-amps.

Gotta go, more later.

 

Your top preamp circuit is OK, but use one of the recommended op-amps (or similar) for the solid state preamp I posted in post #21. You will get very poor performance from the 741. It's input impedance is not suitable and the frequency response is poor. In fact, it will color the guitars sound, so it would be interesting to compare the sound between the various op-amps.

Gotta go, more later.

Okay, will use one of the opamps you mentioned over there. Just I need to know what about the distortion part..