P27 Pre + LM3886 Power + MP3 In + Headphone Out

[exztinct01]

I built myself a P27 Preamp (Rod Elliott's design) and an LM3886 Power amp (also Mr. Elliott's P19). It works well except some noises w/c I'm still trying to figure out how to remove.
Now, I'm planning to add a CD or mp3 input to be able to play with backing tracks, and a headphone out for playing at night. Of course I want the headphone out jack to mute speakers so that I won't need to use another switch.

Can anyone please give me a schematic for these circuit additions?
Or can I use those sub-circuits in the Marshall MG10?
Link for schematic is http://www.electrosmash.com/marshall-mg10. If yes, what values should I use for R20, R21, R22, R41 and C17?

[exztinct01]

Anyone? I know this is not even about stompboxes but most of you here knows electronic circuits more than I so I still am in the right forum

[M.A.P]

Hi Stephen

The MP3 mixing part of the Marshall circuit should be no problem. But I would rather add an additional OP-Amp stage for the mixing in front of the power amplifier.

If you built the LM3886 amplifier with an voltage of +/-35V you can't use the headphone output like shown in the Marshall schematic. The Marshall amp only runs on +/-15V. You could use amp style audio jacks like these http://amprepairparts.com/j503.gif They have an SPST normal close switch which could open the ground connection of your speaker when headphones are connected to mute it. Look at other Marshall schematics. I'm sure that's the way they do it.

Hope this was helpul for you.

Greetings from Germany
Marcus

[exztinct01]

Thanks Marcus. I am actually looking at many schematics for that headphone out but most of them have opamp stages in between the headphone out and the power amp out and I don't want those complicated circuits there if I can do it without them.
Anyway, here's a link for Rod Elliott's Headphone Adaptor http://sound.whsites.net/project100.htm
This one can handle high wattage amps but it is not switchable by the jack. Instead, I would need an SPDT switch. How about it?
Would it be okay for a guitar amp?

[M.A.P]

You could use that schematic from Rod Elliott and put it directely after your power stage. You have to connect the ground of your speaker to the SPST of an amp style jack which breaks the connection when headphones are connected. There is no need for an SPDT switch like shown in the schematic when you do it with the SPST from the jack.

You can see that on the following Marshall schematic on page 2 around the field D4. Ground of the speaker is connected to the SPST from the sleeve connection (S).
http://www.drtube.com/schematics/marshall/vs30-60-02.pdf

I'd draw you a schematic of what I mean but I'm not at home atm. Hope you understand what I mean anyway

[exztinct01]

So can I do it like this? 


I didn't draw the other parts like the 22k b/w Pin3 and Pin9, also the supply. But is my schem correct?

[M.A.P]

Yes that's exactly what I meant

I'm not an expert in grounding but I think you don't need an extra star ground. Just connect the headphone sleeve with the 0R1 resistor to the amp ground. Or you try both ways and decide wich to use if you could hear a difference.

Keep in mind your headphones are stereo. So you need a stereo audio jack and connect tip and ring together like shown in the Marshall schematic.

[exztinct01]

I'm not an expert in grounding either but I saw some amps using that 0R1 5W resistor from Speaker (-) to the Star GND and another separate path to the Power Amp GND.
And for the stereo jack, I just didn't include the ring coz it won't fit in my drawing in b/w the tip and sleeve
Thanks BTW 

[R.G.]

I'm not an expert in grounding either but I saw some amps using that 0R1 5W resistor from Speaker (-) to the Star GND and another separate path to the Power Amp GND.

I've posted several long explanations of grounding here. You could probably turn them up by searching.

[exztinct01]

I've posted several long explanations of grounding here. You could probably turn them up by searching.

Right away Mr. Keen 

How 'bout those sub circuits for headphone out and MP3 in Sir? Do you have anything to say? I still feel like I need your good advice regarding those 

[teemuk]

Headphone input from speaker terminals will likely sound awfully hideous, or at least nothing like what you hear coming out from the loudspeaker. The loudspeaker/cab introduces significant alterations to overall frequency response. Basically they tend to have limited bandwidth between upper low and upper mid-range frequencies while a generic headphone speaker is more a "full-range" driver by nature. The difference is not particularly nice if you tend to like any "distortion" in your tone since all those "nasty" high order harmonics are no longer significantly attenuated by the transducer. Works fine with "clean" acoustic instruments though, since they do not introduce continuous bombardment of high order harmonics in the signal like "distorted" tones do.

Anyway, most people will shortly discover they need some kind of "cabinet emulation" circuit before their headphone output to tame the obnoxious "fizz" of unfiltered power amp output. It can be passive and at speaker output but "active" circuit solutions tend to be less "simplified" (they sound more like a real speaker cab), less expensive, and contain less exotic parts like specially wound inductors, which you probably find from nowhere. Also, you can't get true stereo output from a "mono" power amp. Drawback of solving these issues is that you need separate "power" amp for the headphones. But if you study some schematics you will discover its not such a big deal. Single OpAmp stage maybe, two for stereo, and the "cabsim" circuit still is probably five times as complex as the headphone amp stage.

Use switching headphone jack. Use the switch to either "open circuit" loudspeaker load, or to activate circuitry that mutes power amp input signal. The LM3886 has a "muting" feature, which should be quite trivial to incorporate to such design if wished.

If it was me, I would stick to an option to mute just the "power amp" while leaving headphone amp(s) functional. Now you can use the headphone out as headphone out, silent (stereo) recording output, or you can toggle the loudspeaker and use the headphone out as line level output. Very nice feature if the thing happens to have that "cabinet emulation" circuitry. Can be a life saver at some venues or studios.

It's worth to note that other signal sources will sound "dull" if running through cabinet simulator. You would need to mix the auxiliary stereo signal post any "cab sim" circuit.

[exztinct01]

having a separate power amp for the headphone would be costly though unless I use LM386? Now, can I still test my stompboxes in front of that LM386?