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DIY Combo amp for Guitar/Bass  

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ImCoKeMaN
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04/09/2020 10:07 pm  

So my quarantine project was learning guitar, and I enjoyed it enough and progressed to where I've decided to get a Bass as well.  I've ordered a combo amp that can do some modeling and works with Bass and Guitar, but it is on backorder.  This got me thinking, what would it take to build one?  Seems most amps have an open back, or semi open.  I've played with chip amps, but they take Line in not an instrument level signal.  

Just thought I'd throw this out in case anyone else had looked into it, or thought it would be worth a project.  

Edit: I did find this http://www.valvewizard.co.uk/glassblower.html


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TVOR-Ceasar
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05/09/2020 12:29 pm  

Give a look at RedFreeCircuits (website) for related music amps and inputs, and also DIY Guitar Pedals on YT. He has a 10 minute amp you can put together plus many other input devices. There's a few other places I go to, but I'm not at home atm.

-Charlie


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123Toid
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06/09/2020 10:48 pm  

I honestly wish I knew enough to give some good advice here, but I do not.  I hope someone else can.  This would be a really cool project.


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TVOR-Ceasar
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07/09/2020 12:36 am  

Actually, the Fender BassMaster has been used for years as a regular guitar amp and was actually sought after for a long time. It just needs the right speaker to play for the correct instrument. I believe that a good pair of 12's will do either justice. 

Like I said, Red Free Circuits has a Music related Circuits tab - even the Home and Auto tabs have some ideas that can be adapted. And here is DIY Guitar Pedals YT channel. Take a poke around both and you might see something that will pique your interest.

To the cab, it depends on the drivers. Most instrument speakers work best as open baffle / open back. You could make the cabinet with a door that can be closed to tighten up the bass response, that is make it quicker. Of course, it will all come down to experimenting to see what works best.

-Charlie


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ImCoKeMaN
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08/09/2020 8:19 pm  

Ok, so I see this one, no pics but in my head I'm thinking it could be like one of those little ammo can size boxes http://www.redcircuits.com/Page145.htm   

Now realistically, how different is this instrument source to amp vs plugging an instrument into the aux/line in on another amp for example the dta-120bt and speakers.    I realize without a preamp to take it to line level it would be quieter, but if I was only looking for say 10w anyway and to save my speakers would this work?

I guess I'm looking for the sanity check of "No, idiot you'll blow your speakers" or something before just plugging in cuz I'm curious.  The other thought was just a preamp to that. 

Although I'm intrigued, I'm sort of thinking it's hard to gain much when I can get a modeling amp for $125 or so to put too much effort into designing and building my own which wouldn't really "save" anything factoring time.  However... if I were to build say a 100w cab with a larger woofer....  =)


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TVOR-Ceasar
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08/09/2020 10:37 pm  

@imcokeman

Really, it is usually okay, to an extent, to just plug a guitar into an regular input, albeit it usually has less than stellar results. Usually It is a 2-fold reason:

  1. There is an Impedance mismatch between the guitar/bass and the input of the amp
  2. There isn't enough gain in the input stage to drive the rest of the circuit into the desired "overdrive" mode so often desired or there's too much signal going into a low input on the amp, which can have the obvious bad results. You can try it out by starting with very quiet strumming to see how it reacts. If it sounds horrible, just stop and move on.

What the FET input does is raise the impedance so the instrument has a better match - more transfer with less distortion due to the mismatch. Distortion is what should come later in the amp chain.

The Input also raises the signal by a factor of ~3 X's. This gives enough extra voltage to work through the tone stack and then drive the main amp input to full volume and then into clipping - the part where you get distortion. Plus, if you have decent pickups, the input will start the clipping/distortion so the main amp uses that before hitting it's limit. And FET's tend to sound more tube-like than standard bipolar transistors. 

The main plus to doing something like this is that you've built it. It tends to be a foundational source of pride that leads to more intimate working with the device, allowing expansion, experimentation, and a freedom to customize to your heart's content. This Audiophiliac video actually nails this sentiment on the head.
The downside is you have a long way to go to get to the paid level of a bought amp. Especially one that does modeling. The best you can do with one of those is to find the signal path and work from there - changing the tone stack (if available) and the actual amp itself along with the speaker. And this all depends on how far you want to dive into the learning curve of electronics and whether or not the warranty is done or means anything to you.

In short, it won't blow your speakers, but there is some possibility that there may be amp damage if your guitar/bass has hot pickups. Follow my initial step at the end of #2 above to test things out.

-Charlie


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123Toid
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09/09/2020 2:06 am  

@imcokeman

I have to admit, I haven't kept up with this, but I have been looking into making a diy build.  I think this would be really cool.  I noticed that Eminence sells what many believe is a real nice 10" speaker  specifically for this type of build.  They sell a few different ones, but this the Lil Buddy really caught my eye. 

I just wish I had a good idea for an amplifier.  Granted it is high efficiency 99db with just 1 watt.  So maybe even just pairing it with something like this 5w guitar amplifier. Although that doesn't seem like a lot, it would be more than enough for a small room or practice amp.  The SPL should reach over 100db quite easily.  Even at 20ft away it should still be over 90db. 


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ImCoKeMaN
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09/09/2020 7:35 am  

Thank you both for the replies.  I'm definitely intrigued by this idea.  I was mostly thinking of this for my Bass because I didn't want to run that on my guitar amp which wasn't designed for the lower frequencies, and well I have "extra" subwoofers 😉  This would mean I don't need that overdrive bit, but I understand the need for the input impedance so the pickups work.  My Bass does have a 9v in it with some active Tone/Bass boost in it so I'm actually curious if that in itself has enough to solve the impedance issue. 

I might try it out on a low volume just to see what happens at some point here =)


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TVOR-Ceasar
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09/09/2020 8:48 am  

Yep, looked at the Footnote a number of times. Seems to be a decent setup with some really nice features you wont find on way more expensive amp heads. Uses the same output chip as the one ImCokeMan looked at above.

Ultimately, the real magic happens in the preamp section and then the selection of driver(s). That's why so many drivers are just one of a handful of brands that are used. Music Creation and Music Reproduction are two different beasts.

-Charlie


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TVOR-Ceasar
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09/09/2020 9:00 am  

Just finished up the reply I started last night and saw you replied in between. 😀

If it tends to work, just build a second cab for the bass and switch between the two. Really, most amps will work for both, it's the cab/built-in speaker of the combo that you need to worry about. Yes, excursion, but more the frequency response of the driver itself, it has to go lower than a standard guitar driver yet still have enough top end to hear the string attack and other nuances that come with the stringed instruments.

-Charlie


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ImCoKeMaN
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09/09/2020 5:06 pm  

So maybe a footnote, a GRS 8pf8 or 10pf8 woofer, and if not loud enough mod amp like on this page? https://www.guitarscanada.com/threads/footnote-electric-guitar-amp-11-99.236186/

Those speaker choices would just be for the bass because I have em, nothing special.  Guessing they wouldn't like open baffle though and I'd have to learn something about cabinets.

haha the shipping is like 30% of the cost, and that makes me went to spend enough to get to $49 which defeats the purpose lol


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TVOR-Ceasar
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10/09/2020 12:45 am  

It looks like it would be good to go with at least the gain mod. Add in the resistor, a couple values of capacitors around what he stated (larger and smaller to experiment with), maybe add in this Bluetooth Media Module for more music accompaniment options, and a decent 12V power supply (never mind the plug, you can direct wire it). I'm sure you could find enough stuff to fill out the $49 requirement. 😉

If it were me, I'd do that and take the 9V board out, put in a bunch of switched stereo 1/4" jacks, wire up so they start at the left and travel through the right, make up the same number of 1/4" stereo plug to two 1/4" plug - in and out of the guitar to pedal - easily mix and match up to 8 pedals to get the best tone without messing with the guitar cord, with the guitar input routed through them first and then back to the the next stage of the amp. Plus, since the head cab will have plenty of room, you could get an IEC cord/jack and hook the 12V adapter up inside the cab, and give you the option to add a 9V adapter inside to also give back the 9V rail, also inside the cab.

I know, the mind starts running rampant...  

 

Almost forgot, I like the 8" specs over the 10" for smaller cab size, especially since they have the same XMAX and SPL. Might give up just a few Hz, but it'll fit life a heck of a lot better. Need any help with boxes, just give a shout.

-Charlie


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123Toid
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10/09/2020 1:38 am  

@imcokeman

I wasn't thinking about this being a bass.  Another woofer to consider would be the Legend.  Very high reviews.  But PE has a bunch that should fit the application here.


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TVOR-Ceasar
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11/09/2020 11:46 pm  
Posted by: @tvor-ceasar

....

I know, the mind starts running rampant...  

Oh dang! I think I've got a project on my own hands now. My youngest is thinking about picking up guitar and was asking about something like this a month or so ago.

I've got a nice AlNiCo 6" speaker in it's original box that would make for a really good bedroom amp/combo. I knew I saved it for a reason.

-Charlie


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ImCoKeMaN
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12/09/2020 10:12 am  

That's awesome, it's cool watching my daughter learn guitar!

 

Edit:  So I realize I'm pretty lame still when it comes to actually making anything audio myself at this point, but I was able to get an Irig2 pretty cheap and had an old iphone laying around.  I think this might suffice for my needs in addition to my loaner amp and the Peavy vyper VIP1 on backorder for the bass while I have a small electric amp as well.  I may be interested in building a more powerful head/box at some point too.

I am still staring at that footnote each day though.  And I just realized I have a 6 cell laptop battery I could probably rig up a charging circuit for and use in the amp says it is ~10v


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