Input-Output jack wiring


I am building an audio amp, which is built and I just wanted to ask what kind of input output sockets would fit best. My input jack wiring now looks like this:


I was thinking about putting the same socket on the output. Is it a bad idea/wrong idea? What would you do to make it work best?

You have not given any specs, or context, for us to know how to evaluate what is “best”. Members of this Forum are generally friendly and helpful, and will probably give you some opinions, but you may get better answers on a Forum which is oriented toward audio circuit design such as “Electronics 101” or “DIYAudio”.



I have some vague memory of this type of socket being used for power inputs connected to more than one source. When plugged in then the other source gets disconnected. Maybe in audio it is for some kind of ‘hum’ suppression when unplugged?

As shown you might be able to get away with a cheaper version of the jack since they are tied together. But as @dchisholm states. we would all be guessing.

The symbol implies some variety of 3-contact, R-T-S (Ring, Tip, Sleeve) jack with transfer contacts. The particular style isn’t defined; could be the old 1/4" “phone” jack, a more modern 3.5mm miniature jack, the venerable WECO 310 jack, etc. They have a long history in audio applications - disconnecting your stereo speakers when headphones are plugged in, connecting incoming calls to the desired extension on a PBX switchboard, etc. There are several factors to consider - electrical, mechanical, logistical, and psychological - when advising @Uros_Jankovic about the suitability of his choice.


I am sorry for not giving more info. This is my circuit, and my question is, can I use the same jack and connect it the same way for input and output? And is it advisable to use it in such a situation?

PCs use the same socket on both inputs and outputs, just different colours

If you do this, you must make sure that mixing up or inserting plugs live won’t blow up your amplifier, ie the output must be short circuit protected. This normally means series resistance.


Or device contacts that are specified as make or break during connection.

For me to understand this symbol it would require that the datasheet for the actual part was also provided.

The jack will bridge across contacts during insertion, so a short to ground and between L and R is inevitable


I do not give your circuit much hope of working if you tie the bases of Q3 and Q32 directly to their emitters.

Q5 and Q52 also do not like a working circuit. Your intention may have been to use PNP transistors for these.

Put a bit more separation between the 2 circuits. The power connections of +15V and -15V are overlapping.

After you’ve gained a bit more experience with KiCad, you can look into “hierarchical sheets”. With these you only have to draw one channel of your audio buffer, and the other can be duplicated, by including the sheet with the amplifier twice in your circuit.

C3 also looks suspicious…

About the original question:
This circuit looks like some headphone amplifier, except for the use of the big power transistors and low value emetteer resistors.

For low power amplifiers (such as for headphones) the concentric 3.5mm plugs are useually fine, but these are not suited for higher power stuff.
First the socket themself are not made for high currents, and secondly, during insertion of the plug momentary shorts are created between various contacts of the connectors. These will damage transitors if too high currents are allowed to flow, and may also lead to emitted sparks which will scare you. (and damage the connector furfaces).

For higher power audio amps, usually screw connectors are used.

1 Like

Have you tried to simulate this circuit?

I will wager that it is based on somebody else’s published design. (That is OK! :grinning: The great majority of circuit designs start from a previous design that is known to be usable, or at least the shortcomings are well documented.) Can you share the source of your inspiration?

At the very least, the output stage is drawn incorrectly. The multiple “dots” near the R8/R9 junction suggest that the circuit may not have everything connected as you intended. I am guessing you intended the output stage to be a complementary pair, but the 2N3055 and the BD440 are far from being complementary! Did you read the first page of either device’s Data Sheet? The 2N3055 and BD440 are not even close in their ratings for collector current or power handling capability.

(I was surprised to find that the 2N3055 is still in production, and even affordable, since the device was old when I was young. The metal TO-3 package has been essentially obsolete for 20 years or more. On the other hand . . . . Like the 6L6 and 12AX7, the 2N3055 may be immortal. In 1970 the things were used in so many designs that they almost littered the galaxy. )

Or maybe you intended this to be a quasi-complementary output stage. They were popular from the late 1960’s into the 1970’s, before manufacturers learned how to make silicon PNP transistors that performed well at ratings more than a few watts. That design topology probably has more value for its nostalgia rather than a serious contender for current performance standards.

How much output power do you expect from this amplifier? The bias circuit seems to provide some rather high idling currents in the output devices, but it lacks the stabilization and temperature compensation circuitry that is essential amplifiers rated at more than a few watts. Again, simulation is a valuable tool for helping you pass the initial smoke tests.

As I mentioned back in my first response, there are other Forums and discussion groups better suited to answering questions about your circuit design. The folks at DIY Audio, and those who have built solid-state guitar amps, can probably recommend basic, proven good, circuits that are suitable for somebody learning to lay out PCB’s.


1 Like

Ok, here is the deal with the circuit. I used LTSpice for making schematic and simulating it, and I just wanted to make a pcb design in KiCad. I used different Q5 and Q52 transistor that I didn’t find in KiCad. I think I will just change Q4 and Q42 to be more complimentary to Q5 and Q52.

As for the positioning of Q5 and Q52, I see now that it is wrong, they should be flipped, and also yes, they’re pnp.

This circuit is taken from the book “Art of Electronics” (Horowitz, Hill), with added bootstrapping. I also changed the transistors since the one used in the book were not able to satisfy my expectations (to say it like that).

The average power gotten from LTSpice simulation is around 10W and AC Analysis looks fine as well.

Thank you all for answers and helping, I will contact folks at DIY Audio, since I’m also interested in guitar amps, pedals etc.

I have not heard from you for a few days. Here is a link to a thread about amplifiers using the 2N3055 transistor. The thread has been running for over 16 years(!!) and has almost 900 posts. If you are REALLY serious about using that particular device, I’m sure there are viable designs in that thread.


1 Like

No, I did not simulate it.
Base and Emitter of Q3 are hard shorted to each other, how could that ever work?

Uros mentioned he did put it in Spice though???

My apologies for the confusion, Paul. Although my post is a reply to yours, I really meant to address my questions to @Uros_Jankovic , the thread initiator.

(Actually, I meant to supplement and expand your comments regarding errors and questionable connections in the schematic in Post #5.) I hope we hear from @Uros_Jankovic before long, because I think it could be satisfying to help him walk through this project.



Maybe Russell Edwin Nash can help us understand each other with his great prize he won in 1986. :slight_smile:


sorry for not being here in a couple of days. So, I replaced two diodes with 15 Ohm resistor. I saw the mistake of Q3 and I changed it. I was thinking about removing output 3.5mm jack and placing just two holes for loudspeaker connector. Also, I need to check current through 0.33 resistors and also I need to look at datasheet of transistors to check the power. One more thing that i noticed is that I miss is DC socket connector, which I will add. Another thing in mind, I should maybe rearrange my input and output. for example put it on top, to make my PCB more compact and also I think I should leave some place for the radiator. I put transistors BD439 and BD440 on the output. I have to make sure everything is good, I might need to change my transistors again, but I don’t have much time in these days to get more information.

And also, here are the simulations taken in LTSpice.

Input vs. Output:

AC Analysis:

Output power:

Audio simulation:

1 Like

The schematic is a poor place for this sort of re-layout. What you have looks good. Inputs on the left, outputs on the right. Clear left to right signal progression. Remember, the visible schematic is to describe how your circuit works, not necessarily how it is built.

Once you get to the PCB layout is when you need to worry about where to put connectors to connect to the outside world, what large components or obstacles you need to avoid, etc.

Yes, I meant for the PCB, sorry for not mentioning. this is my PCB layout right now.

One more question about my circuit, my 1000uF capacitors are for 16 V DC, is it enough for this circuit? And also, how would I know what power dissipation should be for my resistors?

Since I am currently working on my circuit, I want to ask about my output transistors. Power dissipation is 36 W, with Ic = 4A and Vce = 60V. Is the power dissipation too large for this kind of circuit?