Building a small combo amp for a Digital Multi-Efect Modeling Pedal

[ElRay]

I have a Digitech RP150 that I'd like to build a combo amp for those times I want to play away from the computer or headphones.  So, I need something that's more of a high fidelity amp that's closer to a PA or keyboard amp than the typical guitar amp.  Also, talking to Digitech Tech Support, if I use the 1/4" TRS output jacks (left(mono) and right), they should go into a high-Z input.  Googling turned-up some folks that were having good results with a Ruby clone and I found these: Tang Band W3-1053SC 3" Full Range Driver for the speakers.  The specs seem pretty decent, especially if I put them in the right sized, ported enclosure.  So the plan became building an enclosure and allowing space for a Ruby to be built in.

Then I started thinking some more: Since I'll need a High-Z input for the RP150, I could also plug the guitar directly into the amp, which would mean that I'd like keep some of the "grit" as I increase the gain and I wouldn't want something that was clean all the way through it's volume range.  So, again, the description of the Ruby seems spot on; however, the Tang Band speakers are 15W, 8ohm.  Is 1W output "enough" to dive them?  Should I up the power stage?

The TDA2030 looks pretty good because with 12v, one-sided input, into 8ohm, it would give 8W of power. But how much of the Ruby's "great, natural overdrive" is due to the behavior of the LM386 being saturated at lower volumes?  Would I have to keep the buffer gain and guitar volume very low to get some break-up from the TDA2030 at reasonable volumes?  Would the TDA2030 even give me something resembling "great, natural overdrive", or just hard clipping noise?  Similarly, I could use a TA7222AP, TDA1519 or a TA2020.  Those last two can even be wired so I could use the RP150's stereo outputs.

(Un)Fortunately, I'm in Afghanistan, so I can't tinker.  Which means, for better or worse, I'll have several months to plan this project for my return.  So, is the Ruby, Noisy Cricket, etc. really the best starting point for a High-Z input, single-sided 9-12v DC input, good cleans with some decent overdrive amp?

Ray

[Brymus]

Yeah re-amping the "Ruby" or 386 is a good starting point.
I cant say its the best because its not,but if you like the clipping characteristics of the 386 chip(like I do).Then its good and simple.

[J M Fahey]

Hi ElRay.

(Un)Fortunately, I'm in Afghanistan

Ugh!!, all I can tell you is that I wish you a happy return home a.s.a.p.
Returning to your amplifier, if you have only 12V available, a TDA2003/LM383 will work better than a TDA2030.
It can give you 10 clean watts into 2 ohms, easy to get with two 4 ohm speakers.
If you dare, you can use a TDA2005 bridge amplifier, which will give you *very* good 20W RMS into 4 ohms, although it gets more complicated than a straight 2003.
Brymus was lucky with his TDA1519 which has a lower parts count.
You can build a mini "Twin" lookalike.
The speaker you quoted *can* work, but is not very efficient, strictly a bedroom proposal.
You will get much better results with one (or two) salvaged old car radio speakers, which you can get for free.
Somebody posted here an amp with two 6x9" which was very cool, they were mounted forming a "V" and the amp resembled an "alien" head.
If you get your distortion from an overdriven LM386, the power booster needn't distort itself.
Good luck with your projects.

[ElRay]

Hi ElRay.

(Un)Fortunately, I'm in Afghanistan

Ugh!!, all I can tell you is that I wish you a happy return home a.s.a.p.

Thanks.  I'm in the home stretch, hence the planning for "me" projects when I get home.  The "house" and "family" projects are planned out for the first nine months after my return easily.

Returning to your amplifier, if you have only 12V available ...

12v is a design choice -- The RP150 already uses a 12v wallwart, and that's also 8 AA/C/D batteries if I wanted some portability

The speaker you quoted *can* work, but is not very efficient, strictly a bedroom proposal.

I thought these were reasonably efficient, but I've been out of this realm for a while.  Isn't the efficiency: SPL: 85 dB 1W/1m?  So, If I'm 1m away from the speaker, and I've got the full 1W from the 386 going through the speaker, the volume should be about 85 dB, correct?  The amp isn't meant to be too loud, but I'd like a bit of "reserve" if I needed it.  But then again, a lower impedance speaker would allow the 386 to put out a bit more power.  I'll have to check the spec sheet, maybe the 1W was into 4 ohms and not 8 ohms.

If you get your distortion from an overdriven LM386, the power booster needn't distort itself.

I was hoping to get the distortion from the power stage, like the Ruby.  I wasn't really wanting to add another stage after that, unless adding another stage is the best bet.

Thanks.

Ray

[ElRay]

Yeah re-amping the "Ruby" or 386 is a good starting point.
I cant say its the best because its not,but if you like the clipping characteristics of the 386 chip(like I do).Then its good and simple.

That's the thing.  I can do the circuit design, enclosure design, etc., I just have no idea of what the other amps will sound like over driven, and for the first project, I'd like something that's more of a sure thing.

Based on what I've been able to hear over the web, the distortion and cleans from a 386 are good enough.  What I have no feel for are what the distortions would be like for the other IC's, nor do I have a feel for what the output volume would be like at the gain needed for the distortion.  I'm trying to avoid too much trial and error right off the bat.

Thanks.

Ray

[J M Fahey]

Hi elray.
Maybe rather than trying to cover all bases from the beginning you just could start by building a straight ruby as-is, or even simpler, just the most basic LM386 application, straight from the datasheet (2 or 3 parts only, not even a volume pot), since you´ll be driving it from your pedal.
No enclosure, nothing, plus any salvaged speaker in a cardboard shoebox.
If you like what you hear (you will), you can add whatever you want: a TDA20xx power booster, a cabinet, better or more speakers, whatever.

[ElRay]

... No enclosure, nothing, plus any salvaged speaker in a cardboard shoebox.  If you like what you hear (you will), you can add whatever you want: a TDA20xx power booster, a cabinet, better or more speakers, whatever.

Yeah, that's close to what I've been thinking.  I should breadboard it and then thinker until I find something that will hold me over until the next project and then hard wire what I've put together.  I've become too used to planning everything out in as minute detail as possible before starting.  Actually, as I've been reading more posts here, and I'm typing this, I'm beginning to realize how many of the details are subjective and I actually may have hit the "as detailed as possible" with a lot more "We'll worry about that when we get there" items than I'm used to.

That said, I'll probably build the enclosure as I go.  It's design and construction is fairly trivial.  I could always squeeze that in among the other projects around the house:  Cut the pieces from left over MDF when I've got the table saw fired-up, cut slots when I've got the biscuit jointer out, prime it when I'm priming something else, etc..

Ray

[ElRay]

Has anybody experimented with the "bass boost" on the LM386's?  The idea is to run a 25k pot in series with a 2kR and a switch between pins 1 and 5 so that I can tun the boost on and off, and adjust the level.

When I get home and start to build, I likely will experiment with this, but I don't want to re-invent the wheel (or beat a dead horse) if folks have already been down that path.

Ray

[J M Fahey]

Hi ElRay. It sure works, but the LM386 low power means it only makes sense in a earphone output or making a table radio sound less thin, it definitely won't turn it into a bass amp or something like that.
Anyway it's simple enough. Just test it and you tell us. 

[ElRay]

Just test it and you tell us.

Will do.  I've already picked-up that this is the MO around here.

Ray