Hi everyone!
I am a bass player, currently using a Vox Amplug2 Bass for practicing with headphones. I have to admit that the sound quality is really impressive for the price, and both tone and gain controls are nice to have (even if not mandatory). The rest, in my opinion is poorly designed and full of useless features (rythms, effects, ...) and lacking some key ones.
Thus the idea is to keep what's good and build myself the perfect device for practicing bass guitar in 2022, in headphones and/or tiny amps.
Key features/enhancements of the build:
1) Bluetooth audio receiver with optional dedicated volume knob. (3.5mm aux replacement)
2) 6.35mm jack input for placing the device at best, like belt, table or floor. (direct input replacement)
3) Rechargeable battery / power supply via USB type-C port. (disposable battery enhancement)
As a mechanical engineer I'd build everything myself but I am a total noob of DIY audio electronics and I know I need help for the design, details, and for choosing parts and components to include in the project. I have spent countless nights researching on the topic but I hope you could participate with your knowledge to the design and the build of the device.
I am attaching you a quick scheme of what I have in my noob mind, and a few fast design of how I see it. Any idea, suggestion, criticism, laugh is more than welcome.
Here a part list (with links) of what I've selected so far:
1) 6.35mm jack audio input
2) On/off switch with led for BT
3) Bluetooth receiver
4) AMP (x2)
5) Volume control (optional for BT)
6) 3.5mm headphone jack output
7) USB-c power module
8) Batteries
Thanks in advance for your precious help everybody!
Marti
I am a bass player, currently using a Vox Amplug2 Bass for practicing with headphones. I have to admit that the sound quality is really impressive for the price, and both tone and gain controls are nice to have (even if not mandatory). The rest, in my opinion is poorly designed and full of useless features (rythms, effects, ...) and lacking some key ones.
Thus the idea is to keep what's good and build myself the perfect device for practicing bass guitar in 2022, in headphones and/or tiny amps.
Key features/enhancements of the build:
1) Bluetooth audio receiver with optional dedicated volume knob. (3.5mm aux replacement)
2) 6.35mm jack input for placing the device at best, like belt, table or floor. (direct input replacement)
3) Rechargeable battery / power supply via USB type-C port. (disposable battery enhancement)
As a mechanical engineer I'd build everything myself but I am a total noob of DIY audio electronics and I know I need help for the design, details, and for choosing parts and components to include in the project. I have spent countless nights researching on the topic but I hope you could participate with your knowledge to the design and the build of the device.
I am attaching you a quick scheme of what I have in my noob mind, and a few fast design of how I see it. Any idea, suggestion, criticism, laugh is more than welcome.
Here a part list (with links) of what I've selected so far:
1) 6.35mm jack audio input
2) On/off switch with led for BT
3) Bluetooth receiver
4) AMP (x2)
5) Volume control (optional for BT)
6) 3.5mm headphone jack output
7) USB-c power module
8) Batteries
Thanks in advance for your precious help everybody!
Marti
Welcome to the forum, Marti.
I'm sorry you didn't get a response sooner; part of the problem is that this subforum ("Instruments & Amps") has been slowly dying for several years, with less activity each succeeding year.
The other part of the problem - at least in my mind - is that the project you're suggesting, is a pretty big bite to chew for a person entirely new to electronics. There are a number of different sub-systems to deal with.
Let me start by playing Devil's Advocate ("...one who argues against a cause or position, not as an opponent, but simply to determine the validity of the cause or position.")
A Fender Mustang Micro ( https://www.guitarworld.com/reviews/fender-mustang-micro-review ) seems to include almost everything you want, and much more as well (onboard effects, multiple amp models, EQ, ability to act as a USB audio interface).
The only thing the little Mustang Micro does not have, is a 1/4" input jack. However, this is very easily solved by buying a cable with a 1/4" mono plug on one end, and a 1/4" mono jack on the other. Plug the cable into the guitar, the Mustang Micro into the cable, your headphones into the MM, and away you go.
Here is a suitable cable: https://www.amazon.com/PNGKNYOCN-Ex...t/dp/B09BRDPW1G/ref=sr_1_20?crid=3K6QI229EZSL
Instead of headphones, you can of course simply use a 1/8" stereo male to 1/8" stereo male extension cable, and plug the output of the Mustang Micro into any suitable powered loudspeaker. I've used this exact setup many times for small jams with friends.
I'll add that I'm primarily a guitarist, not a bass guitarist. The MM does include a "studio preamp" model that is flat, neutral, and uncoloured, and that will definitely work with your electric bass(es). You may or may not like some of the other amp models in conjunction with bass guitar. Even if you only use the "studio preamp" model, you still get all those other nice features (onboard FX, Bluetooth, EQ, USB audio interface, etc).
So that's something to think about. Even though the price of the Mustang Micro has soared dramatically over the last two years, IMO it's still worth considering. Good sound quality, rechargeable battery onboard, onboard FX, onboard bluetooth receiver, drives headphones or powered speakers, not much bigger than a pack of chewing-gum, etc, etc.
But will it really deliver what you want? Only you really know for sure, but here are some of my thoughts:
1) Do you want any ability to EQ the bass? Or are you satisfied with the EQ/ tone controls already on the bass?
EQ circuits for bass guitar tend to be pretty complicated. If you need to build your own, that will be quite a complex project just by itself.
2) Will you need a very high input impedance (absolutely necessary for passive bass guitars)? Or will you only play active basses through this, so that you do not need such a high input impedance?
The amp module you chose probably doesn't have a high enough input impedance for use with passive bass guitars. I looked at the datasheet but didn't find it specified. If I'm right, that means you will need to add some sort of preamp stage, to go between your bass and the input of the power amp board. This will need to be designed, constructed, and added to your block diagram.
3) Used with headphones, the chip amp you chose can deliver enough volume to destroy your hearing almost instantaneously. One accidental "pop" from a loose cable could leave you with hearing loss for the rest of your life.
Personally, I take this issue very seriously. Deafness and tinnitus are very miserable conditions to have to live with, even more so for a musician.
I built my own headphone amplifier not long ago, and I powered it from a low 2.7 volts DC, which drastically limits maximum output power to headphones. I did this specifically so that it could not, under any circumstances, get loud enough to cause instantaneous permanent hearing loss.
4) The board you chose will not run on anything less than 7 volts. That means it will deliver dangerously high SPL through headphones.
5) How important is small physical size to you? For a DIY build, it's a lot easier to build something into a relatively big enclosure, than to engineer it into a tiny one.
If the Mustang Micro doesn't float your boat, and you want to DIY something, my suggestion is to start simply. First sort out the most important parts: the audio chain, from guitar input jack to headphone output jack. Do you need a preamp? Will you need tone controls? Will you need a compressor of some sort? Do you need any kind of overdrive capability? How will you keep your hearing safe in the face of the unpredictable nature of guitar signals and guitar cables?
Once you've designed and built something that meets all those requirements, then think about adding the Bluetooth.
When Bluetooth integration works, then think about adding the battery and subsystems. Note that Lithium batteries can and do burst into extremely high-temperature flames, which have destroyed many homes, burned many cars to the ground, and even taken many human lives due to toxic smoke inhalation and ancillary fires. This is a very serious risk with DIY electronics - it's a really bad idea to mess around with lithium batteries and their charging/discharging systems.
In recent years there are small companies selling microcontrollers and small digital electronics boards that are designed to plug into and run directly from a single lithium ion cell. While very convenient for Internet Of Things toys, I think this is quite a dangerous development, putting untested and uncertified lithium power systems into the hands of untrained hobbyists.
I don't mean to put you off from doing any DIY - but my suggestion is to scale back the project, start building and testing one key subsystem at a time, and power the whole thing with ordinary AAA or AA cells.
And please, keep in mind the need to protect your hearing!
-Gnobuddy
I'm sorry you didn't get a response sooner; part of the problem is that this subforum ("Instruments & Amps") has been slowly dying for several years, with less activity each succeeding year.
The other part of the problem - at least in my mind - is that the project you're suggesting, is a pretty big bite to chew for a person entirely new to electronics. There are a number of different sub-systems to deal with.
Let me start by playing Devil's Advocate ("...one who argues against a cause or position, not as an opponent, but simply to determine the validity of the cause or position.")
A Fender Mustang Micro ( https://www.guitarworld.com/reviews/fender-mustang-micro-review ) seems to include almost everything you want, and much more as well (onboard effects, multiple amp models, EQ, ability to act as a USB audio interface).
The only thing the little Mustang Micro does not have, is a 1/4" input jack. However, this is very easily solved by buying a cable with a 1/4" mono plug on one end, and a 1/4" mono jack on the other. Plug the cable into the guitar, the Mustang Micro into the cable, your headphones into the MM, and away you go.
Here is a suitable cable: https://www.amazon.com/PNGKNYOCN-Ex...t/dp/B09BRDPW1G/ref=sr_1_20?crid=3K6QI229EZSL
Instead of headphones, you can of course simply use a 1/8" stereo male to 1/8" stereo male extension cable, and plug the output of the Mustang Micro into any suitable powered loudspeaker. I've used this exact setup many times for small jams with friends.
I'll add that I'm primarily a guitarist, not a bass guitarist. The MM does include a "studio preamp" model that is flat, neutral, and uncoloured, and that will definitely work with your electric bass(es). You may or may not like some of the other amp models in conjunction with bass guitar. Even if you only use the "studio preamp" model, you still get all those other nice features (onboard FX, Bluetooth, EQ, USB audio interface, etc).
So that's something to think about. Even though the price of the Mustang Micro has soared dramatically over the last two years, IMO it's still worth considering. Good sound quality, rechargeable battery onboard, onboard FX, onboard bluetooth receiver, drives headphones or powered speakers, not much bigger than a pack of chewing-gum, etc, etc.
I think you basic scheme will work, in the sense that you can plug in a bass, and mix the bass guitar signal with audio from the Bluetooth receiver.
But will it really deliver what you want? Only you really know for sure, but here are some of my thoughts:
1) Do you want any ability to EQ the bass? Or are you satisfied with the EQ/ tone controls already on the bass?
EQ circuits for bass guitar tend to be pretty complicated. If you need to build your own, that will be quite a complex project just by itself.
2) Will you need a very high input impedance (absolutely necessary for passive bass guitars)? Or will you only play active basses through this, so that you do not need such a high input impedance?
The amp module you chose probably doesn't have a high enough input impedance for use with passive bass guitars. I looked at the datasheet but didn't find it specified. If I'm right, that means you will need to add some sort of preamp stage, to go between your bass and the input of the power amp board. This will need to be designed, constructed, and added to your block diagram.
3) Used with headphones, the chip amp you chose can deliver enough volume to destroy your hearing almost instantaneously. One accidental "pop" from a loose cable could leave you with hearing loss for the rest of your life.
Personally, I take this issue very seriously. Deafness and tinnitus are very miserable conditions to have to live with, even more so for a musician.
I built my own headphone amplifier not long ago, and I powered it from a low 2.7 volts DC, which drastically limits maximum output power to headphones. I did this specifically so that it could not, under any circumstances, get loud enough to cause instantaneous permanent hearing loss.
4) The board you chose will not run on anything less than 7 volts. That means it will deliver dangerously high SPL through headphones.
5) How important is small physical size to you? For a DIY build, it's a lot easier to build something into a relatively big enclosure, than to engineer it into a tiny one.
If the Mustang Micro doesn't float your boat, and you want to DIY something, my suggestion is to start simply. First sort out the most important parts: the audio chain, from guitar input jack to headphone output jack. Do you need a preamp? Will you need tone controls? Will you need a compressor of some sort? Do you need any kind of overdrive capability? How will you keep your hearing safe in the face of the unpredictable nature of guitar signals and guitar cables?
Once you've designed and built something that meets all those requirements, then think about adding the Bluetooth.
When Bluetooth integration works, then think about adding the battery and subsystems. Note that Lithium batteries can and do burst into extremely high-temperature flames, which have destroyed many homes, burned many cars to the ground, and even taken many human lives due to toxic smoke inhalation and ancillary fires. This is a very serious risk with DIY electronics - it's a really bad idea to mess around with lithium batteries and their charging/discharging systems.
In recent years there are small companies selling microcontrollers and small digital electronics boards that are designed to plug into and run directly from a single lithium ion cell. While very convenient for Internet Of Things toys, I think this is quite a dangerous development, putting untested and uncertified lithium power systems into the hands of untrained hobbyists.
I don't mean to put you off from doing any DIY - but my suggestion is to scale back the project, start building and testing one key subsystem at a time, and power the whole thing with ordinary AAA or AA cells.
And please, keep in mind the need to protect your hearing!
-Gnobuddy
Thank you very much Gnobuddy for your honest reply! I know it's quite a big project but I have no rush in getting it done. The aim is yes building something I can be proud of but also to learn a lot during the entire process. I hope there will be the change to receive guidance 
Anwering to your points:
- FENDER MM:
A Fender Mustang Micro ( https://www.guitarworld.com/reviews/fender-mustang-micro-review ) seems to include almost everything you want, and much more as well (onboard effects, multiple amp models, EQ, ability to act as a USB audio interface).
I do know FMM very well. It has very matching features (Bluetooth, Rechargeable battery) but once more it has a tonof feature I will never use (like CabSim, EQ, etc...). Also it's a great sounding headphone amp for guitars but lacks a bit for bass guitars, and that's why I went for the Vox AP2 BS.
The point here is the journey of learning while building something I really use daily, instead of buying. Hope that makes sense.
1) Do you want any ability to EQ the bass? Or are you satisfied with the EQ/ tone controls already on the bass?
EQ circuits for bass guitar tend to be pretty complicated. If you need to build your own, that will be quite a complex project just by itself.
I am super happy with the sound that comes out of my bass (a passive Jazz Bass), so I wouldn't need any EQ at the moment but a clean amplification of what's alreay there.
2) Will you need a very high input impedance (absolutely necessary for passive bass guitars)? Or will you only play active basses through this, so that you do not need such a high input impedance?
I'll take a look at that and get back to the thread, Thanks. I have a passive Jazz Bass.
3 and 4) Used with headphones, the chip amp you chose can deliver enough volume to destroy your hearing almost instantaneously. One accidental "pop" from a loose cable could leave you with hearing loss for the rest of your life.
I take the issue very seriously too. That's exactly when I need the most of help from experienced people, trying to understand wich circuit block could help avoiding these definitely unwanted problems. I am definitely ok lowering the Voltage of the system if there is no other way for protecting from pops.
Having a bit of power could help me using the device also with a little cab in small rooms, that's why I thought of it. Maybe placing two output could be an idea, one 1/8" protected for headphones and one 1/4" in case I connect to a little cab. Bad idea?
One of the schemes that I looked at is this MXR headphone Amp replica that was reverse engineered. Can't really tell where the protection is though (if any) but I gess it should be somewhere.
5) How important is small physical size to you? For a DIY build, it's a lot easier to build something into a relatively big enclosure, than to engineer it into a tiny one.
It's definitely one of the pro of the Vox, and FMM too. As I said before it's about the learning-by-doing process, so it will be definitely ok to start big (on using a breadboard) and go small in a second phase. As an engineer I totally understand it!
...Do you need a preamp? Will you need tone controls? Will you need a compressor of some sort? Do you need any kind of overdrive capability?
Preamp: no
Tone control: I like it on the Vox AP2 but it's not mandatory
Compressor: no
Overdrive: no
Finally, about batteries, that's the part where I feel confident. I am fully aware of risks involved and I know how to well protect batteries during charge and discharge. But what you say in once more right, that's the last step. I indend to start with alkaline batteries and then go designing and replacing the power bolck.
I hope I was able to explain well the spirit, reasons and expectations of the project. Yes, step 1 is chosing, designing and trying the audio chain. And I will highly appreciate to receive suggstions for starting the first phase with the right foot.
Thank you again for the time you dedicated to the response!
-Marti
Anwering to your points:
- FENDER MM:
A Fender Mustang Micro ( https://www.guitarworld.com/reviews/fender-mustang-micro-review ) seems to include almost everything you want, and much more as well (onboard effects, multiple amp models, EQ, ability to act as a USB audio interface).
I do know FMM very well. It has very matching features (Bluetooth, Rechargeable battery) but once more it has a tonof feature I will never use (like CabSim, EQ, etc...). Also it's a great sounding headphone amp for guitars but lacks a bit for bass guitars, and that's why I went for the Vox AP2 BS.
The point here is the journey of learning while building something I really use daily, instead of buying. Hope that makes sense.
1) Do you want any ability to EQ the bass? Or are you satisfied with the EQ/ tone controls already on the bass?
EQ circuits for bass guitar tend to be pretty complicated. If you need to build your own, that will be quite a complex project just by itself.
I am super happy with the sound that comes out of my bass (a passive Jazz Bass), so I wouldn't need any EQ at the moment but a clean amplification of what's alreay there.
2) Will you need a very high input impedance (absolutely necessary for passive bass guitars)? Or will you only play active basses through this, so that you do not need such a high input impedance?
I'll take a look at that and get back to the thread, Thanks. I have a passive Jazz Bass.
3 and 4) Used with headphones, the chip amp you chose can deliver enough volume to destroy your hearing almost instantaneously. One accidental "pop" from a loose cable could leave you with hearing loss for the rest of your life.
I take the issue very seriously too. That's exactly when I need the most of help from experienced people, trying to understand wich circuit block could help avoiding these definitely unwanted problems. I am definitely ok lowering the Voltage of the system if there is no other way for protecting from pops.
Having a bit of power could help me using the device also with a little cab in small rooms, that's why I thought of it. Maybe placing two output could be an idea, one 1/8" protected for headphones and one 1/4" in case I connect to a little cab. Bad idea?
One of the schemes that I looked at is this MXR headphone Amp replica that was reverse engineered. Can't really tell where the protection is though (if any) but I gess it should be somewhere.
5) How important is small physical size to you? For a DIY build, it's a lot easier to build something into a relatively big enclosure, than to engineer it into a tiny one.
It's definitely one of the pro of the Vox, and FMM too. As I said before it's about the learning-by-doing process, so it will be definitely ok to start big (on using a breadboard) and go small in a second phase. As an engineer I totally understand it!
...Do you need a preamp? Will you need tone controls? Will you need a compressor of some sort? Do you need any kind of overdrive capability?
Preamp: no
Tone control: I like it on the Vox AP2 but it's not mandatory
Compressor: no
Overdrive: no
Finally, about batteries, that's the part where I feel confident. I am fully aware of risks involved and I know how to well protect batteries during charge and discharge. But what you say in once more right, that's the last step. I indend to start with alkaline batteries and then go designing and replacing the power bolck.
I hope I was able to explain well the spirit, reasons and expectations of the project. Yes, step 1 is chosing, designing and trying the audio chain. And I will highly appreciate to receive suggstions for starting the first phase with the right foot.
Thank you again for the time you dedicated to the response!
-Marti
Sorry to have been gone so long; "long COVID" knocked me flat for weeks.
Most of us have been reduced to nothing more than empty consumers most of the time, even though evolution gave us intelligence and curiosity and the drive to create. This forum exists because of people like you, who have the urge to learn and create for themselves.
Then you will need a preamp (though it can be as simple as one single JFET or transistor). Your bass will want to "see" at least a 1 megohm input impedance, which the power amp chip doesn't provide. Inserting a simple preamp in between the two as a buffer will fix that problem.
Another question that needs to be answered is whether the power amp board has enough voltage gain by itself (i.e., will you get enough volume in your headphones). You may find you need a little additional voltage gain from a preamp.
There is certainly more than one way to accomplish this. After thinking about various options for my own headphone amp build, I decided that lowering the supply voltage was the solution I liked best, for reasons I'll explain later in this post.
Typical headphones today have a 32 ohm nominal impedance, and will generate 100 dB SPL at your ear when fed one single milliwatt of audio power. That corresponds to a peak-to-peak drive voltage of only 0.5 volts!
I'm sure you know that 100 dB SPL is already loud enough to cause hearing damage, particularly if your ears are exposed to it for a significant length of time. Musicians can and do expose themselves to such high SPL levels, and you may want your headphone amp to be capable of them, even though it's wiser not to listen at those SPL levels for long.
The problem with this approach is that it mucks up the frequency response of the headphones. Even though headphones may be rated at, typically, 32 ohms, the actual impedance isn't constant - it changes with frequency. When you insert a resistor in series (like R10/R11), the changes in impedance translate to changes in signal voltage across the headphones. There will be less voltage at some frequencies, and more at others; in a nutshell, adding the series resistors has not only limited maximum SPL (good), but also introduced unwanted EQ, boosting some frequencies and cutting others.
These days, headphones are designed to be driven from a very low output impedance - directly from the chip amp in your smartphone, MP3 player, or Vox Amplug. That preserves the frequency response of the headphones.
So: to keep your ears safe, we want to limit maximum output voltage to the headphones. To avoid ruining the frequency response, we don't want to put any large resistors in series with the headphones. In fact, we don't want to insert any resistance unless it's much, much less than the headphone's own resistance; a couple of ohms would be fine, but 10 or 20 or 100 ohms is a bad idea.
A very small resistance in series won't do any damage, but it also won't limit power to a safe level. We need to find another way.
That's why I chose to simply lower the supply voltage to the headphone amp. This automatically limits max power, without having to insert any resistors in series.
I used this little LM4853 board made by Sparkfun, which I bought from Digikey: https://www.digikey.ca/en/products/...=N4IgTCBcDaIHYHsCWBnAngAgMYCclYGsBTAFxAF0BfIA
The LM4853 is specified to work all the way down to a 2.4 volt supply voltage, at which point it cannot deliver more than about 20 - 30 mW to a 32 ohm load. If the phones have a typical sensitivity of 100 dB@1mW, this is sufficient to produce about 115 dB SPL at your ears!
115 dB is already deep into the red - it will cause hearing damage if maintained for long. But at least an accidental pop from a loose cable won't make you instantly deaf.
I built my headphone amp into a little FX pedal enclosure, with the intention that I would use some other FX pedals in front of it, and power it from the same 9V power supply. So I found a tiny little adjustable switching voltage regulator board, and built that into the enclosure; the actual headphone amp receives only 2.4 V DC, though 9V DC enters the box.
I can't find the exact regulator board I used, but it was similar to this one: https://ca.robotshop.com/products/123v-to-30v-step-down-buck-converter
If you eventually do use a lithium battery, a single cell provides 4.2 volts DC when fully charged. That's too much to protect your hearing, so you'll need to knock that down to 2.4 volts or so for the headphone amplifier, so you'll need some sort of voltage regulator (a very simple one will do).
This might be a good place to use an old-school linear voltage regulator, actually, rather than a switching one. Switching regulators make lots of high-frequency electrical and RF noise, and the power level here is so low that we don't need the additional efficiency of a switching regulator.
I'm attaching an excerpt from the LM4853 datasheet, showing output power into a 32 ohm load when powered from 2.7 Vdc. (The chip is rated down to 2.4 Vdc, but the datasheet doesn't include a graph at that supply voltage. You can extrapolate down from the 2.7 V graph.)
I'm also attaching a screenshot of a spreadsheet I just put together, showing drive voltage vs SPL for 32 ohm headphones rated at 100 dB@1mW sensitivity.
I'll also attach a pic of the headphone amp I built, just about a year ago. The LM4853 is a stereo chip, and my headphone amp has stereo inputs, which can be driven from, say, a stereo reverb pedal. The toggle switch on top is a Mono/Stereo switch - flipped to Mono, it connects the inputs of the two channels together, so that I still get sound in both ears even if I feed the headphone amp from a mono source.
(You can get the same result using an input jack with additional switch contacts for the left channel; I didn't have the correct jack on hand, but had the toggle switch, so I built it this way.)
I have actually played a bass guitar through this headphone amp a couple of times. Input impedance is too low for a passive bass, but, as it happens, my bass guitars have onboard preamps, so they can cope with the low input impedance of the LM4853. I didn't build in the ability to mix in a backing track, though, which is a huge weakness when used with bass guitar. I may have to try it with a bass guitar and a looper pedal one of these days.
My suggestion: a good starting point is a very simple preamp (I can help design one if you want), a very simple headphone chip amp board (that can work down to 2.4 volts DC or less), and two AAA cells, preferably the kind that only produces 1.2 volts DC each.
This will be a quick and simple project that has a very high chance of success, and will be usable right away. It will also be enough to tell you if, for example, you need additional voltage gain to get enough signal level in your headphones.
Once you have this designed, built, and working to your satisfaction, it will be time to add extra stuff (like Bluetooth audio, and a way to mix that signal in with the bass guitar signal.)
-Gnobuddy
Very much so. I think it's a great viewpoint.
Most of us have been reduced to nothing more than empty consumers most of the time, even though evolution gave us intelligence and curiosity and the drive to create. This forum exists because of people like you, who have the urge to learn and create for themselves.
Nice!
Then you will need a preamp (though it can be as simple as one single JFET or transistor). Your bass will want to "see" at least a 1 megohm input impedance, which the power amp chip doesn't provide. Inserting a simple preamp in between the two as a buffer will fix that problem.
Another question that needs to be answered is whether the power amp board has enough voltage gain by itself (i.e., will you get enough volume in your headphones). You may find you need a little additional voltage gain from a preamp.
The key thing is to ensure that the maximum drive voltage to the headphones cannot, under any circumstances, become sufficiently large to destroy your hearing.
There is certainly more than one way to accomplish this. After thinking about various options for my own headphone amp build, I decided that lowering the supply voltage was the solution I liked best, for reasons I'll explain later in this post.
Typical headphones today have a 32 ohm nominal impedance, and will generate 100 dB SPL at your ear when fed one single milliwatt of audio power. That corresponds to a peak-to-peak drive voltage of only 0.5 volts!
I'm sure you know that 100 dB SPL is already loud enough to cause hearing damage, particularly if your ears are exposed to it for a significant length of time. Musicians can and do expose themselves to such high SPL levels, and you may want your headphone amp to be capable of them, even though it's wiser not to listen at those SPL levels for long.
I don't think it's a bad idea, though there may be a simpler option: limit the headphone amp to very small output, and have a separate small power amplifier that sits on the cab; when you want to run into the cab, just run a cable from the output of your headphone amp to the input of the power amp on the cab.
The protection is done in a crude and simple way that was popular some decades ago. See R10 and R11 in the schematic? They are in series with the transducers in your headphones (one for the left channel, one for the right), and the resistors limit maximum current.
The problem with this approach is that it mucks up the frequency response of the headphones. Even though headphones may be rated at, typically, 32 ohms, the actual impedance isn't constant - it changes with frequency. When you insert a resistor in series (like R10/R11), the changes in impedance translate to changes in signal voltage across the headphones. There will be less voltage at some frequencies, and more at others; in a nutshell, adding the series resistors has not only limited maximum SPL (good), but also introduced unwanted EQ, boosting some frequencies and cutting others.
These days, headphones are designed to be driven from a very low output impedance - directly from the chip amp in your smartphone, MP3 player, or Vox Amplug. That preserves the frequency response of the headphones.
So: to keep your ears safe, we want to limit maximum output voltage to the headphones. To avoid ruining the frequency response, we don't want to put any large resistors in series with the headphones. In fact, we don't want to insert any resistance unless it's much, much less than the headphone's own resistance; a couple of ohms would be fine, but 10 or 20 or 100 ohms is a bad idea.
A very small resistance in series won't do any damage, but it also won't limit power to a safe level. We need to find another way.
That's why I chose to simply lower the supply voltage to the headphone amp. This automatically limits max power, without having to insert any resistors in series.
I used this little LM4853 board made by Sparkfun, which I bought from Digikey: https://www.digikey.ca/en/products/...=N4IgTCBcDaIHYHsCWBnAngAgMYCclYGsBTAFxAF0BfIA
The LM4853 is specified to work all the way down to a 2.4 volt supply voltage, at which point it cannot deliver more than about 20 - 30 mW to a 32 ohm load. If the phones have a typical sensitivity of 100 dB@1mW, this is sufficient to produce about 115 dB SPL at your ears!
115 dB is already deep into the red - it will cause hearing damage if maintained for long. But at least an accidental pop from a loose cable won't make you instantly deaf.
I built my headphone amp into a little FX pedal enclosure, with the intention that I would use some other FX pedals in front of it, and power it from the same 9V power supply. So I found a tiny little adjustable switching voltage regulator board, and built that into the enclosure; the actual headphone amp receives only 2.4 V DC, though 9V DC enters the box.
I can't find the exact regulator board I used, but it was similar to this one: https://ca.robotshop.com/products/123v-to-30v-step-down-buck-converter
If you eventually do use a lithium battery, a single cell provides 4.2 volts DC when fully charged. That's too much to protect your hearing, so you'll need to knock that down to 2.4 volts or so for the headphone amplifier, so you'll need some sort of voltage regulator (a very simple one will do).
This might be a good place to use an old-school linear voltage regulator, actually, rather than a switching one. Switching regulators make lots of high-frequency electrical and RF noise, and the power level here is so low that we don't need the additional efficiency of a switching regulator.
I'm attaching an excerpt from the LM4853 datasheet, showing output power into a 32 ohm load when powered from 2.7 Vdc. (The chip is rated down to 2.4 Vdc, but the datasheet doesn't include a graph at that supply voltage. You can extrapolate down from the 2.7 V graph.)
I'm also attaching a screenshot of a spreadsheet I just put together, showing drive voltage vs SPL for 32 ohm headphones rated at 100 dB@1mW sensitivity.
I'll also attach a pic of the headphone amp I built, just about a year ago. The LM4853 is a stereo chip, and my headphone amp has stereo inputs, which can be driven from, say, a stereo reverb pedal. The toggle switch on top is a Mono/Stereo switch - flipped to Mono, it connects the inputs of the two channels together, so that I still get sound in both ears even if I feed the headphone amp from a mono source.
(You can get the same result using an input jack with additional switch contacts for the left channel; I didn't have the correct jack on hand, but had the toggle switch, so I built it this way.)
I have actually played a bass guitar through this headphone amp a couple of times. Input impedance is too low for a passive bass, but, as it happens, my bass guitars have onboard preamps, so they can cope with the low input impedance of the LM4853. I didn't build in the ability to mix in a backing track, though, which is a huge weakness when used with bass guitar. I may have to try it with a bass guitar and a looper pedal one of these days.
Great! IMO, that will hugely increase your chance of success, as you can build and develop each individual sub-system until it works the way you want.
My suggestion: a good starting point is a very simple preamp (I can help design one if you want), a very simple headphone chip amp board (that can work down to 2.4 volts DC or less), and two AAA cells, preferably the kind that only produces 1.2 volts DC each.
This will be a quick and simple project that has a very high chance of success, and will be usable right away. It will also be enough to tell you if, for example, you need additional voltage gain to get enough signal level in your headphones.
Once you have this designed, built, and working to your satisfaction, it will be time to add extra stuff (like Bluetooth audio, and a way to mix that signal in with the bass guitar signal.)
-Gnobuddy
