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I received a request to add a 1/4" stereo headphone jack to the Marshall AS100D. My initial reply was to just use something like a Behringer HA400 that can be bought for around $25 as that will be the easiest and cheapest option. Customer said no as he doesn't want a separate piece of gear. He wants to be able to plug his headphones into the AS100D and have it silence the amp's speakers. He's willing to pay to have the functionality added.
In doing some research, I finally found the schematics for this amp, along with the manual. On page 6 of the manual, you can see the block diagram of the amp. I'm looking at a couple of paths that I could go down and wondering what your thoughts/guidance are on them. I believe I have two problems to solve. First is the adding of the headphone jack. Second is finding a way to mute the main speakers, possibly made more difficult by the 1/4" jacks not available in a TRRS variety.
Option 1: Tap into the as10_62_02 schematic circuit at the SPK_REDL and SPK_REDR points and add a resistive network to scale the signal down, but this doesn't mute the speakers.
Option 2: Tap into the as10_61_02 schematic circuit OUT_R and OUT_L signals after the volume pot, but before they hit the power amp and then feed them into a new amplifier circuit mounted on a small veroboard or similar. For the circuit, maybe something like what Marshall did for the MG-100HDFX amp, but duplicated for right and left channels since this amp is stereo. I could then use the switching third leg of the 1/4" jack to activate the mute circuit. I found T1 on the power amp schematic for the mute circuit, but to me it looks like it only has a brief mute during power up to prevent speaker popping sounds. Controlling this, or this same point on the two power amp chips would have to be figured out.
Option 3: There's an existing headphone amp circuit board that can be added internally?
Option 3: Thoughts? Suggestions?
Thanks!
In doing some research, I finally found the schematics for this amp, along with the manual. On page 6 of the manual, you can see the block diagram of the amp. I'm looking at a couple of paths that I could go down and wondering what your thoughts/guidance are on them. I believe I have two problems to solve. First is the adding of the headphone jack. Second is finding a way to mute the main speakers, possibly made more difficult by the 1/4" jacks not available in a TRRS variety.
Option 1: Tap into the as10_62_02 schematic circuit at the SPK_REDL and SPK_REDR points and add a resistive network to scale the signal down, but this doesn't mute the speakers.
Option 2: Tap into the as10_61_02 schematic circuit OUT_R and OUT_L signals after the volume pot, but before they hit the power amp and then feed them into a new amplifier circuit mounted on a small veroboard or similar. For the circuit, maybe something like what Marshall did for the MG-100HDFX amp, but duplicated for right and left channels since this amp is stereo. I could then use the switching third leg of the 1/4" jack to activate the mute circuit. I found T1 on the power amp schematic for the mute circuit, but to me it looks like it only has a brief mute during power up to prevent speaker popping sounds. Controlling this, or this same point on the two power amp chips would have to be figured out.
Option 3: There's an existing headphone amp circuit board that can be added internally?
Option 3: Thoughts? Suggestions?
Thanks!
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