Hi guys, I thought I would get some advice, since I feel stupid at the moment! XD
I am putting together a guitar amp for my niece to practice on using an old subwoofer enclosure as the cab, a daichi 8 inch 100instrument speaker
I am putting together a guitar amp for my niece to practice on using an old subwoofer enclosure as the cab, a daichi 8 inch 100instrument speaker
You have two options for electric guitar: build a valve amp (read this first) or build something that emulates a valve amp (select from something like these - they have sound clips to help you select).
For acoustic/classical guitar you don't need an amplifier (why is it that everyone at an "unplugged" gig is plugged in?)
For acoustic/classical guitar you don't need an amplifier (why is it that everyone at an "unplugged" gig is plugged in?)
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I don't advise hobbling together a valve amp unless your experience of working with valves lives up to your monicker! For your and your niece's safety I would recommend a solid state design.
A recent thread discussed the cost effectiveness of building a valve amp from scratch and threw up this bargain which is almost too good to be true!
Monoprice 5-Watt, 1x8 Guitar Combo Tube Amplifier with Celestion Speaker - Monoprice.com
Unlikely that you are based in the US though, or your monicker would be 'TubesOfSteel'! 🙂
A recent thread discussed the cost effectiveness of building a valve amp from scratch and threw up this bargain which is almost too good to be true!
Monoprice 5-Watt, 1x8 Guitar Combo Tube Amplifier with Celestion Speaker - Monoprice.com
Unlikely that you are based in the US though, or your monicker would be 'TubesOfSteel'! 🙂
If this is only for practise then an amp based on the LM386 will certainly be loud enough and easy to build. Check out the Ruby amp here ElectroSmash - Ruby Amp or the little gem here Little Gems .
Tubes of steel - they sell those at Home Depot. 😀
I lived in the US for nearly a quarter of a century, but still prefer to use the word "valves" when referring to our friendly hot glass bottles. This is in large part, because "tubes" is such a generic term. Fallopian tubes, metal tubes, my ball-point pen is a plastic tube, the plumbing in the house is all tubes, there are tubes everywhere, and not one of them has anything at all to do with electronics!
-Gnobuddy
That's a good way of looking at it Gnobuddy!
When I say tube, I insert a small 'y' sound before the vowel so that it comes out tyoob.
It seems that most Americans lose the 'y' and pronounce it toob - a process known as yod dropping.
This is reason enough for me to favour the word valve! 😀
When I say tube, I insert a small 'y' sound before the vowel so that it comes out tyoob.
It seems that most Americans lose the 'y' and pronounce it toob - a process known as yod dropping.
This is reason enough for me to favour the word valve! 😀
There are other interesting pronunciation differences I noticed when I first moved to the USA. "Nuclear" becomes "nuk-e-lar", swapping the order of the last two syllables, "creek" becomes "crick", "clothes" are "close", "pen" is "pin", and "Notre Dame" is pronounced to rhyme with "otter lame", the "tre" inexplicably becoming "ter".
But I try not to take pronunciation too seriously, as *everyone* has an accent, and there's always someone else who finds yours hilarious or ridiculous or stupid. 🙂
I also like "valve" because it actually describes what all "toobs" - starting with vacuum diodes - do: they're one-way valves for the flow of electric current.
Huh. I thought that was what pigeons did. When I park too close to a lamp-post, I come back and find yod droppings all over my car.
-Gnobuddy
Absolutely!
My son, the Youtuber, often has members of his American audience comment on his 'sexy' Scottish accent and his quaint use of the word 'choob'!
It's all a bit of harmless fun! 🙂
Wow thanks for the replies guys! I had actually asked the mods to delete this as I accidentally hit send about 10% into the post! Since it posted anyway, bugger it, here we go.
The idea I have is to use a 60w (running a little undervolted, estimated at about 45W or so.) solid state class D amp that runs off around 21 volts (18.5 or so in my case to keep things cool mostly) to provide the power to a 100w daichi i8000 8 inch instrument speaker, which I have tested and sounds really good in the old subwoofer box I have it all in.
As the input stage I have purchased and built a $10-15 tube pre with 6j1 tubes in it, just to get a nice tube flavour into the signal before the power amp stage. I figure that this will keep things nice and safe (highest tested voltage in the tube pre was about 30v and it runs off a 12vac light PSU.) and the tubes are literally about AU$2 or so on ebay, I can buy a bulk lot of them and throw out the microphonic ones pretty easily if need be. I had the idea to use both sides of the pre (it's a stereo unit) as a double mono boost with the tone control in the middle of the tube sandwich (BTW I will use tubes and valves interchangeable, we Aussies will take Brit and American phrases and mash them more often than not)
I tested out one side of the tube pre with a headphone out sig from an old radio, and might I say, it sounded pretty damn good through the power amp and then the surprisingly great quality for a AU$50 speaker! So I makeshifted a bandaxall tone control just to test things out, I have a tone control PCB coming and plan to change the treble cap to make it better for guitar, but it hasn't arrived yet so I used copper tape and solder to make tracks on a piece of thin particle board from the back of an old speaker and soldered in the parts to make the circuit, which wasn't exactly a James version but I work shopped in Tone Stack Calculator and got something useful with the parts I had to work with.
Anyway, long story short, I hooked it all up and got not much more than buzz, I don't discount that I may have made the tone circuit wrong, as it's my first rodeo with such things, so I will probably take photos of everything and post them later this weekend if anyone wants to inspect my (admittedly a bit crap and makeshift) handywork.
My big question is: If the pre is running off 12vac, and the power has one side hooked to the signal "ground" on both the ins and outs, like a rail, won't that put 12vac into the signal path? Since the AC has no "ground" so speak of? This didn't seem to be a problem when connected with the RCA connectors it came with, but seems to be now that I have the sig going through the left channel, out and back in the right channel through the tone control and then out to the amp. I am basically going to take it all apart again some time this weekend and test each part on its own again to make sure I didn't fry something with a hot soldering iron when I was desoldering some parts to connect things with permanent wires after I had a good test. I used pickup wire to hook everything up, with the earth going to a foil inner jacket and one wire for the signal (cut all other wires short) and each run is only about 10cm or so between components, so I don't think the buzz is coming from the interconnect, but once again, first rodeo.
I may go back to just using one side of the pre and then tone control straight into the power amp I think, seems the double boost is not great.
But, should I disregard the earth connections between devices because of the AC section? and just use a single signal wire to cut down on earth loops etc? I would greatly appreciate any advice you guys can give.
I will check back later tonight!
Thanks in advance.
The idea I have is to use a 60w (running a little undervolted, estimated at about 45W or so.) solid state class D amp that runs off around 21 volts (18.5 or so in my case to keep things cool mostly) to provide the power to a 100w daichi i8000 8 inch instrument speaker, which I have tested and sounds really good in the old subwoofer box I have it all in.
As the input stage I have purchased and built a $10-15 tube pre with 6j1 tubes in it, just to get a nice tube flavour into the signal before the power amp stage. I figure that this will keep things nice and safe (highest tested voltage in the tube pre was about 30v and it runs off a 12vac light PSU.) and the tubes are literally about AU$2 or so on ebay, I can buy a bulk lot of them and throw out the microphonic ones pretty easily if need be. I had the idea to use both sides of the pre (it's a stereo unit) as a double mono boost with the tone control in the middle of the tube sandwich (BTW I will use tubes and valves interchangeable, we Aussies will take Brit and American phrases and mash them more often than not)
I tested out one side of the tube pre with a headphone out sig from an old radio, and might I say, it sounded pretty damn good through the power amp and then the surprisingly great quality for a AU$50 speaker! So I makeshifted a bandaxall tone control just to test things out, I have a tone control PCB coming and plan to change the treble cap to make it better for guitar, but it hasn't arrived yet so I used copper tape and solder to make tracks on a piece of thin particle board from the back of an old speaker and soldered in the parts to make the circuit, which wasn't exactly a James version but I work shopped in Tone Stack Calculator and got something useful with the parts I had to work with.
Anyway, long story short, I hooked it all up and got not much more than buzz, I don't discount that I may have made the tone circuit wrong, as it's my first rodeo with such things, so I will probably take photos of everything and post them later this weekend if anyone wants to inspect my (admittedly a bit crap and makeshift) handywork.
My big question is: If the pre is running off 12vac, and the power has one side hooked to the signal "ground" on both the ins and outs, like a rail, won't that put 12vac into the signal path? Since the AC has no "ground" so speak of? This didn't seem to be a problem when connected with the RCA connectors it came with, but seems to be now that I have the sig going through the left channel, out and back in the right channel through the tone control and then out to the amp. I am basically going to take it all apart again some time this weekend and test each part on its own again to make sure I didn't fry something with a hot soldering iron when I was desoldering some parts to connect things with permanent wires after I had a good test. I used pickup wire to hook everything up, with the earth going to a foil inner jacket and one wire for the signal (cut all other wires short) and each run is only about 10cm or so between components, so I don't think the buzz is coming from the interconnect, but once again, first rodeo.
I may go back to just using one side of the pre and then tone control straight into the power amp I think, seems the double boost is not great.
But, should I disregard the earth connections between devices because of the AC section? and just use a single signal wire to cut down on earth loops etc? I would greatly appreciate any advice you guys can give.
I will check back later tonight!
Thanks in advance.
That will deliver roughly 17 watts into an 8 ohm speaker, or twice that into a 4 ohm speaker, if the supply voltage is well regulated. (Ignore the claimed 60W power output, which is invariably fantasy in these sorts of products. Actual power depends on only two things, the power supply voltage and the speaker impedance, and that will result in roughly 17 watts or 35 watts into an 8 ohm or 4 ohm speaker, respectively.
That will probably work. You might need a volume control in between the two tube stages, too, to prevent excessive gain.
I've done almost that exact thing in a pinch, many years ago. The only difference is that I used a scrap of Masonite. I think this stuff is called hardboard in Australia.
It's trouble-shooting time!
Tubes don't run on AC (except for heaters), so that 12V AC is being rectified and voltage-doubled to generate the 30-odd DC volts you measured on the tube plates.
I don't understand the question. The two channels of your preamp board almost certainly already share a common earth, which is fine, and necessary to use them in series as you're doing.
It might help to see a pic or two at this point - I have no idea what you've actually done. (You cannot get an earth loop from a signal wire, only from two or more ground wires.)
-Gnobuddy
Cheers mate, I will take some pics tomorrow and post them, it's a bit of a mess at the moment as I don't want to mount anything until it's all working! Good to know about the power thing too, I am using an 8 ohm speaker so the 100w rating on it is more than more than enough I guess! XD
And yes I will add a volume to that tone control I think, as the volume knob on the tube pre works on both sides of the stereo pre at the same time, so it gains like crazy at a certain point and (I think) the power amp cut out on me from too much input!? My theory anyway since it cut and then came back within a second.
I might even just figure out a good permanent resistor to use on the input to cut it down, though the tone circuit is passive so it does lose a few dB just running through that in between I would think.
By the earth wires I mean all of the components have a sig + and - on them, so 2 wires to hook up the signal path between the bits, I just wondered if that might create ground loop issues between the devices because the signal ground seems to be connected straight to one side of the AC on the pre! I will have to double check that though it might be only on the DC rectified side and I derped out while looking at it last.
One thing I most certainly agree on, trouble shooting time! I am thinking I will hook the radio output back up to just the power amp to start, then one channel of the pre, then one pre channel and the tone circuit, see where I am going wrong.
My theory is that I have balked the tone circuit somehow, either I have an earth connected to the signal or missed a connection somewhere... Either way I will post the board and a pic of the tone circuit in tone stack calculator for reference of what I used. I had to change some parts to match values I had (I only have 500k linear pots at the moment that I got at the good price to use in guitars) so the sweep on the pots is a little weird because of that, and it cuts something like 15dB or so even with everything in the middle I think, so I figured that might negate the double pre if I put it in the middle! Anyway I will do some checks in the morning and report back with a few pics for your perusal.
Thanks for the advice so far.
Quick question before I head to sleepy bies for the night, You say that you can't get more voltage to speakers than you put into the amp, I agree that you can't get any more power out of the amp than the voltage at the power rails into the speaker load, and having looked at the specs of the chip used in these amps, it seems they use a feedback circuit to get more perceived volume at lower power I am guessing, which could make very little difference in the end, but my question is this:
Could the board be using some kind of step up converter to put more voltage on the chip rails (on a better amp, not this one as the specs on the chip state 24v max) to get more power to the speaker at a lower input voltage? assuming that the PSU has enough amperage rating to handle the extra amps needed to make the conversion?
Just a thought I had in my groggy state before I flake it for the night.
Thoughts?
Could the board be using some kind of step up converter to put more voltage on the chip rails (on a better amp, not this one as the specs on the chip state 24v max) to get more power to the speaker at a lower input voltage? assuming that the PSU has enough amperage rating to handle the extra amps needed to make the conversion?
Just a thought I had in my groggy state before I flake it for the night.
Thoughts?
For aftermarket car audio, this was not uncommon, because you're stuck with a 12 volt supply rail, which means a maximum of maybe 15 watts RMS to each 4 ohm speaker using a bridge-mode amplifier.
Cars usually have speakers at four corners, and 60 watts RMS in their very small interior volume is more than loud enough for the average listener - more than loud enough to damage your hearing, in fact. But the young men hoping to lure teenage girls with extremely loud thumping noises in their car want much more than 15 watts RMS per speaker, and the only way to get that is with some kind of switching boost converter powering the amplifier. So aftermarket car amplifiers typically did have a switch-mode boost converter in them.
-Gnobuddy
Good to know that it's possible at least!
I figured it would be pretty easy to do as long as you have the amps at the source to drive out the extra power required for the up convert, which I guess car batteries and alternators have in spades!
The specs on the chip say that it can run in stereo at 50w a channel but into a 4 ohm load, the specs on the mono 100w spec say it's into 2 ohms! Does that mean that the amp should take 2 ohms of speakers with not too much trouble? Possibly adding a large heatsink? I could make a 4x10 cab with 4 of these speakers in parallel and get a 2 ohm load, but I don't want to overtask the amp at the same time!
I guess I should be looking for an amp that runs something like 30v at the rails then! At least that should give me 112 watts or so to an 8 ohm load. (It is V squared over the load resistance isn't it?)
Anyway, new day, off to have some brekky and do a few tests before I head to my mate's place to help him try to reflash his WRX ECU. 🙄 I'll try to post the pics before I have to head out.
Sorry I couldn't answer this fully yesterday. It was a very busy day for me!
Anyhow, the attached image shows how this is most probably wired up.
I'm guessing that the circuit uses a voltage doubler to turn that 12V RMS AC (about 17 volts AC peak) into roughly 30 volts DC. The type of voltage doubler shown in my image shares one of its DC output wires with the incoming AC wire, which I think is what has been bothering you.
There are indeed two wires running to each input, but one of them is the ground wire, and I expect the two grounds are in fact one single ground wire (copper trace) on the PCB. This means you will not need to connect a ground wire from the output of the first triode to the input of the second - that's already on the PCB.
-Gnobuddy
Attachments
That's what the specs say, indeed. But are the specs honest, or did the manufacturer use the smallest cheapest heatsink that kinda-sorta almost did the job? 😱
I tend to be conservative with these inexpensive boards by unknown manufacturers, because I just don't know if they're up to spec. I've received more than one of these amps with a heatsink that wasn't even tightened down onto the chip!
On the plus side, when you need 10 watts, it's pretty cheap to buy a 50 watt chip, and then you have some hope that the thing won't fry!
(You mentioned your chip cut out briefly when turned to full volume; this might have been a case of overheating, followed by thermal shutdown to avoid frying. It would come back on once sufficiently cooled down.)
That's exactly right when you apply DC voltage to a resistor. Things are a wee bit more complicated when AC is involved, though!
Here's how I estimate output power from a solid-state power amplifier:
1) First, subtract two volts from the power supply voltage. (This is to allow for voltage lost in the output MOSFETs. It's approximate.)
2) Second, square what's remaining.
Now we come to a fork in the road. Most of these little class-D amps actually work in bridge mode - they are really two amps per channel, wired so that the speaker "hangs in between" the outputs of the two amps. The giveaway is that neither speaker wire is connected to power supply ground.
3a) IF your amp is operating in bridge mode, multiply your speaker resistance by 2.
3b) If your amp is not in bridge mode, multiply your speaker resistance by 8.
5) Take the number from step 2, divide by the number in step 3 (a or b as appropriate.) This is your approximate output power.
As an example: 18.5V DC power, 8 ohm speaker, bridge mode amp;
Subtract 2 to get 16.5 V.
Square that to get 272.
Multiply 8 ohms by 2 to get 16.
Divide 272/16 to get 17; this is your approximate output RMS power in watts.
Ah, yes. This is the part where he takes a perfectly good engine, and turns up the boost until flames and pistons and connecting rods shoot through the air, right? 😀
-Gnobuddy
I see where I went wrong in those calculations now! Starting with assuming a perfect amplifier! That's my high school physics class (that I loved) coming back I guess haha.
I just took some pics I will email them off my phone to my computer and post em on imgur right now and re post.
To answer a few things first though:
I don't really need a huge amount of power from the amp, it is for a 12 year old to practice on after all. I started out on a 10w piece of tish with a really crappy 8 inch speaker, so the fact that I got an actual PA speaker than can handle the power in spades and actually sounds good, and 17 watts or so from the amp, I am pretty happy with that to be honest!
I also got a mini electric guitar with a single humbucker from my mate, because my niece is tiny, and I have been upgrading it over the last couple of months with a hot rail, a coil split switch, new hardware etc, and I also painted the cavities with nickle paint to earth it all for noise reduction. I went all out, I figure I am about the only guitarist in the family, and she showed interest, so it's my job to get her something good to practice on! Still, she is just starting out, so a $1000 rig is a bit of overkill, hence I am fixing up the best I can from cheap bits and pieces, and also learning about amps and guitar setup for a possible retirement business making and fixing both the guitars and the amps for later in life. This is my motivation for this whole project, and I have been collecting bits for a couple of months slowly, but now it's crunch time and I have about a week or two to get this thing finished up! I think her birthday is the 10th of August!
On the subject of heatsinks, the amp didn't come with any headsink at all, and the auction recommended using one if you ran on high voltage, I put a small 1cm aluminium heatsink on the chip, glued on with some heatsink paste, the chip is buried between some caps or I would have gone bigger, but at 18 volts it doesn't even really get warm! When it started cutting I was using 24 volts raw from the PSU as the auction stated it was rated for such with a heatsink, and checking when it cut the heatsink was indeed warm! Also that was basically my assumption at the time.
The reason for the voltage shenanigans is that I also wanted to put a 9v power supply into the amp with a negative out on a standard pedal power plug so that a simple double ended lead will run a 9v pedal (I have a spare line 6 pocket POD that I plan to give her and it runs off 9v pedal power) so I had 2 buck regulators running off the 24v 4A supply I got, one was stepping down to its max voltage, which turned out to be 18.5v from 24v and not the 21v the auction stated, but that is neither here nor there I am happy enough with 18v for the safety and cool running margin, and I had another one stepping down to about 9.2-9.4v regulated, then through a couple of caps and inductors in an ebay noise reduction circuit rated 50v (the rating on the caps coincidentally lol) to run pedals etc from.
Anyway because I was using a pedal socket to power the whole thing, as that is what the 24v supply came with, and the same socket for the 9v power out, I of course ended up plugging the 24v into the 9v out socket and absolutely smoking the 2nd buck regulator and the power conditioner board by running 24v in reverse through them both, thankfully the buck took most of the hit and smoked the chip, but to try and keep the 9v out after my stupidity, I ran the 24v direct into the power amp and used the first buck wound down to 9v to try to keep the pedal power, I think I will have to rethink this now though, and remove the 9v side for now till I can get another buck converter and conditioner. That way I can still run the amp at 18v stepped down and keep it from cooking!
The 12v AC for the preamp comes from a cheap lighting transformer, I also found a 12vAC 1A wall wart with a giant brick that I was using for testing, so if I keep getting noise from the lighting power I will probably switch to that.
I have a 250v 2a fuse wired to the active in case something does go wrong, and a switch on the whole thing on the 240v side. All of this will be inside the amp behind the speaker of course, so it won't be accessible usually.
Also the ratings I was quoting came from texas instruments! I would figure that they wouldn't exaggerate too much! I could be wrong of course, and even if I am not, who is to say that this chip isn't some cheap chinese copy of the TI chip that hasn't been rated for the same power!?? All of which makes me think that 18v in and 17-18 watts out is just fine by me!
Anyway I have to head out soon so I will stop waffling and get these pics uploaded.
Thanks so much for the help so far! And yeah, the worst part about flashing ECU's is that the software doesn't seem to want to play with windows 10, (multiple BSOD trying to install the cable drivers) so now we might have to get an old XP laptop just for tuning! Greeeeeeeat!
I just took some pics I will email them off my phone to my computer and post em on imgur right now and re post.
To answer a few things first though:
I don't really need a huge amount of power from the amp, it is for a 12 year old to practice on after all. I started out on a 10w piece of tish with a really crappy 8 inch speaker, so the fact that I got an actual PA speaker than can handle the power in spades and actually sounds good, and 17 watts or so from the amp, I am pretty happy with that to be honest!
I also got a mini electric guitar with a single humbucker from my mate, because my niece is tiny, and I have been upgrading it over the last couple of months with a hot rail, a coil split switch, new hardware etc, and I also painted the cavities with nickle paint to earth it all for noise reduction. I went all out, I figure I am about the only guitarist in the family, and she showed interest, so it's my job to get her something good to practice on! Still, she is just starting out, so a $1000 rig is a bit of overkill, hence I am fixing up the best I can from cheap bits and pieces, and also learning about amps and guitar setup for a possible retirement business making and fixing both the guitars and the amps for later in life. This is my motivation for this whole project, and I have been collecting bits for a couple of months slowly, but now it's crunch time and I have about a week or two to get this thing finished up! I think her birthday is the 10th of August!
On the subject of heatsinks, the amp didn't come with any headsink at all, and the auction recommended using one if you ran on high voltage, I put a small 1cm aluminium heatsink on the chip, glued on with some heatsink paste, the chip is buried between some caps or I would have gone bigger, but at 18 volts it doesn't even really get warm! When it started cutting I was using 24 volts raw from the PSU as the auction stated it was rated for such with a heatsink, and checking when it cut the heatsink was indeed warm! Also that was basically my assumption at the time.
The reason for the voltage shenanigans is that I also wanted to put a 9v power supply into the amp with a negative out on a standard pedal power plug so that a simple double ended lead will run a 9v pedal (I have a spare line 6 pocket POD that I plan to give her and it runs off 9v pedal power) so I had 2 buck regulators running off the 24v 4A supply I got, one was stepping down to its max voltage, which turned out to be 18.5v from 24v and not the 21v the auction stated, but that is neither here nor there I am happy enough with 18v for the safety and cool running margin, and I had another one stepping down to about 9.2-9.4v regulated, then through a couple of caps and inductors in an ebay noise reduction circuit rated 50v (the rating on the caps coincidentally lol) to run pedals etc from.
Anyway because I was using a pedal socket to power the whole thing, as that is what the 24v supply came with, and the same socket for the 9v power out, I of course ended up plugging the 24v into the 9v out socket and absolutely smoking the 2nd buck regulator and the power conditioner board by running 24v in reverse through them both, thankfully the buck took most of the hit and smoked the chip, but to try and keep the 9v out after my stupidity, I ran the 24v direct into the power amp and used the first buck wound down to 9v to try to keep the pedal power, I think I will have to rethink this now though, and remove the 9v side for now till I can get another buck converter and conditioner. That way I can still run the amp at 18v stepped down and keep it from cooking!
The 12v AC for the preamp comes from a cheap lighting transformer, I also found a 12vAC 1A wall wart with a giant brick that I was using for testing, so if I keep getting noise from the lighting power I will probably switch to that.
I have a 250v 2a fuse wired to the active in case something does go wrong, and a switch on the whole thing on the 240v side. All of this will be inside the amp behind the speaker of course, so it won't be accessible usually.
Also the ratings I was quoting came from texas instruments! I would figure that they wouldn't exaggerate too much! I could be wrong of course, and even if I am not, who is to say that this chip isn't some cheap chinese copy of the TI chip that hasn't been rated for the same power!?? All of which makes me think that 18v in and 17-18 watts out is just fine by me!
Anyway I have to head out soon so I will stop waffling and get these pics uploaded.
Thanks so much for the help so far! And yeah, the worst part about flashing ECU's is that the software doesn't seem to want to play with windows 10, (multiple BSOD trying to install the cable drivers) so now we might have to get an old XP laptop just for tuning! Greeeeeeeat!
okay I posted on imgur.
Amp Shenanigans - Album on Imgur
Comments to clarify included, which is why it took me a half hour!
Okay I gotta get some lunch and head to a job interview. Be back later this arvie (that's Aussie for afternoon) to check in and see if anyone had any ideas, though to be honest I will probably disconnect it all and test from the power amp back to make sure I didn't fry anything when I did that bonehead 24v into the 9v out thing... I will hopefully have more test info or maybe even a working amp, if I am really lucky (haw haw haw), by nest post.
Amp Shenanigans - Album on Imgur
Comments to clarify included, which is why it took me a half hour!
Okay I gotta get some lunch and head to a job interview. Be back later this arvie (that's Aussie for afternoon) to check in and see if anyone had any ideas, though to be honest I will probably disconnect it all and test from the power amp back to make sure I didn't fry anything when I did that bonehead 24v into the 9v out thing... I will hopefully have more test info or maybe even a working amp, if I am really lucky (haw haw haw), by nest post.
That's what I would suggest, too. It's either that, or pray to the guitar gods, and gods are known to be capricious. They might steal your dog or turn you into a bull instead of helping you. 😱
I am a little scared after looking at your photos. I know you're planning to tidy up some of the construction, and I couldn't be sure from looking at the photos, but please do make absolutely 100% sure that the guitar input jack ground terminal is securely connected - through the AC mains cable - to the mains ground (3rd pin.)
This is absolutely mandatory for safety. Imagine what happens if this ground is missing, and if the insulation of the 12V transformer breaks down, and 240V AC bleeds through to the secondary wires? Your poor niece will be holding onto a guitar that has suddenly become an electric chair, and may never be able to let go. Don't even want to think about that. 😱
So the guitar input jack MUST be grounded securely to mains earth. That way, even if something goes wrong, the guitar strings remain at zero volts, until the fuse blows or your circuit breaker trips off.
Also, I assume you will seal up that mains fuse, in such a way that little fingers can never touch it?
I hope the job interview went well!
-Gnobuddy
The imperfect amp costs you a volt or two (really, modern MOSFETs are darn near to perfect for this sort of job.) But the remaining problem is the relationship between amplifier supply (DC) voltage, AC peak voltage, AC RMS voltage, and AC RMS power.
In bridge mode, a perfect amp would have a peak voltage equal to the supply voltage. Square that, and divide by (twice speaker resistance) to get RMS output power.
If not in bridge mode, a perfect amp would have a peak voltage of only half of the supply voltage. Half the AC voltage means it delivers one-quarter of the RMS AC power to the speaker.
Bridge mode amps used to be rare, because they used twice as many expensive transistors, and twice as much expensive heatsink. Times have changed, MOSFETs are dirt-cheap when they're inside an IC, class D is so efficient you only need a tiny heatsink, and now bridge-mode class D chips are everywhere.
And I'll keep my fingers crossed that she stays that way! Show here these two videos, please:
1) Molly Tuttle's blazing flat-picking and beautiful singing: YouTube
2) Larkin & Poe, two sisters who are awesome guitarists and singers: YouTube
Agreed, but I bet the 50W rating is with a big fat heatsink, no?
That works out to 102 dB SPL at 1 metre with your 90 dB@1W@1m speaker. Incredibly loud...loud enough to damage your niece's tiny ears permanently, in fact. So I agree, 17 watts is far more than enough.
The official authorities in the USA and Canada say that anything over 85 dB SPL starts to damage your hearing after a few hours exposure, but I looked up the original research paper, and 85 dB for 8 hours is only "safe" if you spend the rest of your life (16 hours every day) in utter silence! If you're exposed to normal urban noise levels before or after that 8 hour day, then your ears do suffer progressive damage.
I know that feeling, we have Windows 10 being thrust down our throats at work, and a bunch of software my co-workers use is about to stop working. Happy-happy-joy-joy. 🙄
-Gnobuddy
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