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Solid State Amplifiers => Amplifier Discussion => Topic started by: Koreth on January 01, 2010, 04:01:01 AM

Title: Modifying a First Act MA104
Post by: Koreth on January 01, 2010, 04:01:01 AM
Greetings all.

I have a First Act MA104 that I purchased from Wal-Mart years go when I first started playing guitar. I hated it's tone pretty quickly, as even with the gain knob all the way down, and the volume set very low, even vintage single coil pickups would send the amp into a nasty sounding breakup very quickly. When I found another amp (a Ross Systems 22) at a garage sale with a slightly better sounding distortion and the ability to clean up when the gain knob was turned down, the MA104 was shoved into a corner and forgotten about.

Fast forward to the present: The Ross 22 has since died and been cannibalized for parts and my main amp is a 100W Mesa/Boogie. The Mesa will remain my main performance and rehearsal amp for the foreseeable future, but a 100W all tube half-stack isn't very practical for bedroom practice, never mind the impracticality of lugging a big heavy head and cabinet across town twice a week for rehearsals. So the MA104 has been brought out of its corner and put to use again. Stock, it still sounds gross, but unlike years ago, I now own and know how to operate a soldering iron. :)

First Act wasn't forthcoming with a schematic, so I traced the circuit out and made my own in LTspice. It is attached. I'm pretty sure the diagram is complete, but I could have made some mistakes in tracing the circuit. So if anyone sees anything that doesn't make sense, please lemme know. Though the output chip is labeled "UTC2003", I'm pretty sure it is a TDA2003, which UTC makes. The power ratings and test circuit seem to be close to what's in the amp and First Act's claimed power output for this amp.

Thus far, I've made a few mods. I spliced a 1/4" plug onto the speaker out and used one of the 10" 40W 8Ohm speakers left over from the Ross 22. Overkill for a 4W amp, I know, but the 10" speaker sounds a lot better than the little 4" speaker it came stock with. With the help of the fellows at Music-Electronics-Forums, I lowered the gain in the 1st op amp stage. With the stock values, the gain of the 1st stage was over 300, enough for even a vintage single coil to overdrive the op amp hard against it's power rails. I changed R2 from 270Ohm to 1KOhm, and C4 from 10µ to 1µ. The gain knob has a much more usable range from clean to barely breaking up to full distortion now.

At this point I'm looking to achieve the following:

Hopefully all that isn't tl;dr. Your input on the above would be greatly appreciated.
Title: Re: Modifying a First Act MA104
Post by: J M Fahey on January 01, 2010, 01:20:20 PM
Hi Koreth.
Problem is, clearly the First Act designers made a little market research and found out that their average client would be a kid hooked on MTV (the old one that played music, including ACDC, ZZTop, Aerosmith, etc., not the new Disney Teen Channel type), had little or no $ to spend, wanted *that* rock sound, and bedroom volume level would be limited by Mom's patience.
So they built an excellent (for that purpose) distortion-pedal-straight-into-TDAsomething thing.
Great fun, but, what headroom?
*If* you have to replace speaker, preamp, power amp, cabinet and power supply, you are building a new amp.
If you only need a little more highs, replace C10 with a .0022uF cap, and solder a .022uF in series with a 1K resistor, across R10.
Anything beyond that merits a new amp.
What speaker does the First Act have?
Title: Re: Modifying a First Act MA104
Post by: Koreth on January 01, 2010, 03:40:20 PM
Stock, the MA104 comes with a tiny, 4" speaker of unknown manufacture.

I agree that replacing the power supply, preamp, power amp, and cabinet, I'm building a new amp. But that's not what I'm trying to achieve here. Thus far I've changed the speaker and replaced two components to give the gain knob a useful range. Hardly building a new amp, IMO. Now if I get to the point where I'm adding or removing op-amps in the preamp, changing the 2nd stage into a tonestack or something, replacing the power supply and changing the TDA2003 for something bigger, than yes I am building a new amp and would do better to start from scratch. But that's not what I want to do. What I want to do is reduce the output of the preamp slightly so it isn't overdriving the power amp, get some more highs and tweak the distortion character slightly. The latter I suspect I can do with a few dollars and a few minutes with my soldering iron. The former would require more parts, more time, more planning and knowing more than I do now to pull off successfully.
Title: Re: Modifying a First Act MA104
Post by: J M Fahey on January 01, 2010, 11:18:38 PM
Good, try those two mods I suggested.
The first one turns the "pedal" tone control more into an "amp" tone control; the second adds some more sparkle.
If you want to increase the amount of it, lower the series resistor to, say, 1K, and if you want ho boost some more high mids, double the capacitor; beyond that, you will get more hiss than useful sound.
Good luck.
PS: the single biggest "mod" would be, no joking, put a Jensen "Mod" there.
The 5" will very probably fit as-is; maybe you can use the 6", which is great; I doubt very much you can fit an 8" which would be killer.
Consider an 8" fits into a 22 cm circle but you should leave at least *some* wood around it.
The headroom brickwall that is stopping you is that *very* low powered TDA2003; any other, from TDA2006 up, would be in the ballpark of regular "Practice Amps".
Good luck.
Title: Re: Modifying a First Act MA104
Post by: Koreth on January 02, 2010, 01:07:31 AM
I was wondering about that TDA2003. Unless I'm misunderstanding things, I figure with 14V supply and a gain of 32 as it is currently configured, the hottest signal you could feed to it without breaking up would be in the neighborhood of 1.5Vrms. Basically, I could feed it directly with my guitar, and it would probably still breakup if I used the bridge pickup.

I took a look at the Jensen MOD series, nice. Unfortunately, there isn't even enough room to mount even the 5" speaker without building a new cabinet. A damn shame. I really like the sound of the 15W 6" version from the sound clips on the website.
Title: Re: Modifying a First Act MA104
Post by: phatt on January 02, 2010, 07:25:07 AM
Hi koreth,
             I'm afraid there is little chance of *Clean* while you have Diodes (D2,D3) permanantly strapped across the signal path.
Try lifting them and see how it sounds ,that should give it more signal headroom.

For more brightness try lifting some of those caps that hang across the signal path as well. (try C12)
if TO Bright then half the value,, try 500pF.
I'm guessing but as JMF has noted this is just a TS9 type circuit with a speaker output.
Phil.
 
Title: Re: Modifying a First Act MA104
Post by: Koreth on January 02, 2010, 01:14:33 PM
While I would generally agree with that, until their forward voltage the diodes should do nothing. So the 1N4148s should have no effect on the signal until the signal coming out of the op amp reaches 1.3-to 1.4Vpp. The bridge humbucker on my guitar isn't that hot as is evidenced by there being no clipping with the gain knob is turned down until the volume knob (located after the clipping diodes) is turned up. As such I want to look at the gain of the output stage or the opamp before it. But I'll try lifting D2 and D3 to see if that makes a difference.
Title: Re: Modifying a First Act MA104
Post by: phatt on January 03, 2010, 07:14:21 AM
Hi koreth,
           Just had another look at your circuit

Re "More clean headroom"
R2 is way to small,, try 2k ish for a more sane progressive gain.

Re "More highs"
C7 (after VR2) is also 5 mile to large, try 10nF.
that will get rid of the big hump at 100Hz.

Re "the distortion"
With or without diodes the trick is to setup the two opamp stages with just the right amount of *gain* then the amp will probably respond/sound ok.

It's debateable whether diode clipping sounds much better that just letting the signal slam into supply rails in a small amp like this,
Diodes or not you will have to tweak it.

BTW, you obviously have some kind of software setup which looks like it can simulate,,so why not run a few sims yourself and you will see this all happen.
Phil.
Title: Re: Modifying a First Act MA104
Post by: Koreth on January 10, 2010, 04:31:12 PM
I've done some more tweaking of the circuit. I first tried replacing C8 with a .01, thinking it would get me more highs. It did, slightly, but it didn't cure the muffled/muddy sound I'm still getting from the amp, and worse reduced the audible effect of sweeping the tone knob to almost nothing.

So I did some more reading and finally figured out how to use LTSpice to do more than just draw circuit diagrams. I made a schematic of just the tone stack starting at C7 and ending at the wiper of VR3, then did an AC analysis with sweeps for a few different positions of the wiper of VR3 to see just what the heck the tone stack is doing. I'm actually amazed by its simplicity. At one end of VR3's sweep the tone stack is a simple high cut filter. At the other end, it is a notch filter with the notch gradually shifting forward flattening out to smoothly transition into the high-cut filter. Thinking about how I run my EQ's on the other amp, it actually makes sense. Clean sounds tend to have a mid-range scoop to counter the mid-range emphasis that magnetic pickups supposedly have for a more natural, balanced sound. But when running heavy distortion, I tend to pull my highs back a bit to keep the distortion from sounding too harsh. So yeah, once again, the First Act designers knew exactly what they were doing.

By changing C8 to the same value as C10, I pretty much flattened out the mid-range notch. All lows and mids with rolled off highs sounds pretty muffled. Playing with a few values in the simulation I found out I actually want C8 to be a larger value than C10. Yes, a larger C8 rolls off more highs, but the greater the difference, the notch gets narrower and shifts it down in the frequency range. There are other places in the circuit I can get my highs back or reduce lows to keep the amp from sounding muffled. Also, changing C7 to a smaller value has a negligable (<-3dB) on the bass until the cap gets down into the .1µF range and lower. If I change C5 and C7 to 1µ, I get the bass down about -1.5 dB. I think I'd have to change C15 and C24 to 1µF to as well to see a -3dB reduction in bass. It'd be easier to change the input caps at each gain stage from .022µF to .01µF, methinks. So I'm not going to bother with C7 just yet. I'll play around with the values of C8, C10 and R7 in the simulation and see if I can't tweak the tone stack to my liking in the simulation, then I'll try it and report my findings.

Yes, I know I'm wasting my time. This is a fun learning experience.
Title: Re: Modifying a First Act MA104
Post by: Koreth on January 11, 2010, 01:57:16 AM
Success.  :)

C8 is now .047µF, and C10 is .0022µF. According to the simulation, this produces a notch -14dB down at about 330hz and spreading two octaves from center with the tone knob on 10. Perhaps not ideal, but it definitely gives the tone knob a usable sweep. I ran some more simulations and found that with C8 at .1µF, C10 at .001µF, and R7 at 6.8K (which I think is a standard value) I could make the tone knob approximate a Fender tone stack with the mid knob set at 2. I'm not sure how much more I want to play with the tonestack without changing to a better speaker.

Either way, the amp sounds much better and useful now.
Title: Re: Modifying a First Act MA104
Post by: Koreth on February 07, 2010, 11:20:11 PM
So I figured out a few more things with LTSpice, and even found enough models of parts used in the MA104 for a simulation of the whole amp to run. Yay!  :) Thus, I have spent most of my weekend making tweaks to circuit in the simulator and staring in fascination at pretty colored graphs of the resultant waveforms and voltages in various parts of the amp's circuit. Good times. I also redrew the old schematic, because it was a pain to try to follow. The new one is attached. I didn't draw out the output stage because I can't find a model for the TDA2003 and the arcane art of making models from scratch is still beyond my ken.

I made a few discoveries. Unless the 4558 model made by Texas Instruments is innaccurate, the 4558 can't swing all the way to it's voltage rails, it clips hard about 2V before them. I was tempted to blame the model first, but when I tried other opamp models that came with LTSPice, they did the same thing, clipping hard a volt or two before hitting the power rails. So I guess it is a behavior of op amps. I didn't know op amps did that, but it is a good thing to know.

However there's been some differences between the simulation results and the results of plugging a guitar in and playing the amp that are puzzling me. According to the simulation, with the gain, volume and tone knobs cranked, the highest voltage swing that could happen at the preamp's output is about 350mVpeak. The TDA2003's gain is presently set to 10, and it is getting about 13V from the power supply. So, unless I'm Doing It Wrong, a 700mV peak to peak times a gain of 10, means the TDA2003 should have a signal of about 7V peak to peak on it's output, well below it's power rails, and shouldn't be clipping, ever. However, when I plug in my guitar and strum as hard as I can, I can hear a bit of breakup on the attack with the gain knob set at 3.

Now, my big 100W tube amp has it's input stage biased at 1.5V. So the hottest signal my guitar could possibly put out without overdriving the input stage and giving me dirt on the clean channel (which it doesn't) is 1.5Vp. I doubt even the bridge pickup puts out that hot a signal. Now in my simulation, a 1.5v signal with the gain knob set at 3 doesn't even cause the opamp to clip. The clipper diodes have been lifted from the circuit for now, the 2nd opamp is set at unity gain, and the TDA2003 is set to a gain of 10. So where the heck is this distortion coming from?  >:(

Some thoughts. My simulation is wrong? (likely). If that's so, I'm not sure where to start looking. Maybe I have the taper of the gain pot set wrong? (i.e. What LTSPice thinks is 70% rotation is actually 30% on the pot itself). The TDA2003's gain is set too low and the chip is unstable? (maybe?) The chip gets warm enough to feel that it's warm when the amp is running, but it doesn't get HOT like I've read about in these forums when a chip oscillates. The datasheet specs a minimum closed loop gain of 92.3, but stock, gain was 32.1, I don't think it had any problems being set that low.

I wish I could find my multimeter and take some measurements, instead of making blind guesses like this. An oscilloscope would be nice too.

I suspect those of you who know what you're doing and know how to use SPICE are probably rolling your eyes about now. Fair enough. But your input would be appreciated.
Title: Re: Modifying a First Act MA104
Post by: Koreth on February 08, 2010, 01:01:47 AM
Let the record show that I am blind. My multimeter was sitting on the shelf where I last placed it all along. I just hadn't stared at the exact spot it was sitting hard enough.

I took a measurment of the gain pot. Let the record also show that I am an idiot. The taper in the simulation was way wrong.

An oscilloscope would still be useful to have.
Title: Re: Modifying a First Act MA104
Post by: rowdy_riemer on February 08, 2010, 12:47:06 PM
Get a $50 osciloscope. Check out this post on diystompboxes.com: http://www.diystompboxes.com/wpress/?p=96 (http://www.diystompboxes.com/wpress/?p=96)
Title: Re: Modifying a First Act MA104
Post by: Thebighat99 on March 02, 2010, 05:43:00 AM
For some reason I cant get your schematic to display. It keeps saying the file is corrupted. Would you please post it again or give me a link where I could get a copy. I have this amp and was also thinking of trying some modding. schematics would be a great help thanks.
Title: Re: Modifying a First Act MA104
Post by: J M Fahey on March 02, 2010, 10:31:31 AM
Me neither can open it, although I did a month ago.
Dear Koreth, please repost the schematic.
Thanks.
Title: Re: Modifying a First Act MA104
Post by: Thebighat99 on March 09, 2010, 10:04:54 PM
Ok I found the schematics on another post of koreth.  http://music-electronics-forum.com/t16417/ (http://music-electronics-forum.com/t16417/)  I was going to post them, but I think maybe I will just point to the other post. You have to be a member of that forum to see them as well.
Title: Re: Modifying a First Act MA104
Post by: DartPlayer170 on April 15, 2011, 02:49:05 PM
Attached is the schematic for the MA007, almost identical to the MA004 but with a few upgrades. It uses the TDA2030A instead of the 2003 and some changes in component values. Note that the reference designators are almost all different.

A few comments. It is common that op amps do not go rail to rail. This is especially true when the load is heavy. The term is output compliance and it is dependent on the load. Also, diodes are only linear at small signals. The 1N4148 for example, begins to show distortion on the scope at around 800 mVpp, much below the clipping voltage of 1200-1400 mVpp.
Title: Re: Modifying a First Act MA104
Post by: Dimi Pana on October 19, 2011, 04:57:23 PM
I recently made the mistake to purchase a First Act small amp as well.

It is the MA1248, shown here: http://www.toysrus.com/product/index.jsp?productId=3594388#showReviews (http://www.toysrus.com/product/index.jsp?productId=3594388#showReviews)

Yes, I was fooled by the reduction in price. I am guilty as charged, should have known better...

--

I though it would be usable but it is not. I was wondering if I posted some info about it you guys who all seem very knowledgeable with electronics could help me mod it to the point that I can:

1. Have a usable -full range- volume control. Now I cannot turn it up more than 50% if I do even with clean sound the guitar's bass strings produce a "farty" sound.

2. Have a usable -full range- gain control. Now I cannot turn it up more that 0%. Yes, zero percent, more produces noise and unpleasant distortion.

3. Have a usable -more noticeable- tone control. Now it ONLY makes the sound apparently brighter by cutting bass, how can I make it to REALLY add more treble for clarity.

4. Also, with all pots turned all the way to the left (e.g. down) I get noticeable noise (something between a hum and a buzz) when I turn ON the amp.

5. May be get a little more power out of it -say- 5 Watts (total) in order to try it with a bigger cabinet I 'm thinking to build (for fun) (lol).

--

I opened it up and it is using a JRC4558D, a TDA 2030A, and a transistor(?) which I believe is a 9013. Of course there's also a bunch of caps, resistors, diodes, etc. but even to a novice like me this circuit seems simple enough. Still, I can solder or de-solder components, I understand how to read (not by heart) resistors with a chart, I know some caps have polarity, I know diodes allow current flowing only one direction, I know about proper grounding techniques, etc. So if some one tells me remove this and put that in its place I am comfortable enough to do it. I would n't know why but if it sounds OK, I am happy.

I can also post some pictures of the pcb but I am not sure how to attach pictures to this message.

If you are interested please respond,

Thank you very much in advance.
Title: Re: Modifying a First Act MA104
Post by: joecool85 on October 20, 2011, 09:22:51 AM
Pictures would indeed be helpful.  To attach them, post a reply message.  Click the "Additional Options" link that is below the reply text box, then attach your files and click "post" as normal.
Title: Re: Modifying a First Act MA104
Post by: Dimi Pana on October 20, 2011, 01:07:48 PM
Hello joecool85 -

Thank you so very much for your prompt response.

I am attaching some pictures, one of the external appearance of this little amp, and three of the PCB from (slightly) different angles, in an attempt to show all components and their corresponding values on the PCB. Please let me know if you need anything else. Come to think about it I should have removed the PCB as well and taken a pic of the (etched) under side. Would that be helpful? I can do it in a snap!  ;)

Funny thing is in an earlier post by Thebighat99 the link he posted http://music-electronics-forum.com/t16417/ (http://music-electronics-forum.com/t16417/)  is a similar thread where another guy (I believe his nick is theWicked) has exactly the same amp like mine (MA1428) but unfortunately that thread never discussed this amp much neither did it finalize what changes can be made for Koreth's MA104 amp. I did contact "theWicked" (at the other forum) but he has not responded yet.

--

Well, it appears that this (simple amp) amp is not a bad design, it's just attached to a very bad speaker. Am I right to assume that as a first step to improving it, I could just try to hook it up to a better speaker, just as Koreth did in the very beginning? It should be OK with either a 4Ω, 8Ω or even 16Ω speaker providing a maximum of 4W, 2W or 1W of output power respectively, right? I mean even a 1W amp is good enough for me for home practicing, as long as it sounds OK, (not great just OK).

So, If I am not asking too much, I can work with anyone who is willing to share some ideas and has some free time to guide me to the right direction. No rush either, this is going to be my winter project. I have posted in my 1st message what this amp needs, please let me know at your convenience, where do I start?

Thank you very much joecool85 and anyone else who want to lend a hand. It's very much appreciated!

Cheers!
Title: Re: Modifying a First Act MA104
Post by: phatt on October 21, 2011, 04:59:15 AM
If you want cleaner sound ,,,try removing the diodes from the circuit.
Or maybe switch them to get a clean and dirty sound.

No this is not an Amp that will ever be famous,,, sorry it's very cheap and very nasty.

They are just a fuzz box circuit with a speaker and very cheaply made as well.

The best part is likely the box it all sits in.. :-X
Phil.
Title: Re: Modifying a First Act MA104
Post by: joecool85 on October 21, 2011, 08:31:14 AM
If you want cleaner sound ,,,try removing the diodes from the circuit.
Or maybe switch them to get a clean and dirty sound.

No this is not an Amp that will ever be famous,,, sorry it's very cheap and very nasty.

They are just a fuzz box circuit with a speaker and very cheaply made as well.

The best part is likely the box it all sits in.. :-X
Phil.

As much as this is true, that doesn't mean you can't make it sound nice.

As you suggested, Dimi, a speaker replacement is the first thing you should do.  It is designed for 4 or 8ohm speakers, 16 may work ok, but why bother since they are less common and would produce less wattage?

I'm not sure on your wattage calculations though if it is indeed a TDA2030A then it will have a maximum of 22w RMS with a 4 ohm load and 36v on the power supply.  On the data sheet it doesn't even list less than a 6w output and that is an 8 ohm load at 24v.  I'd be checking what your power supply puts out for voltage before buying a speaker.  Also, read up on the chip's datasheet here: http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1459.pdf

**edit**
Fixed the voltage ratings.
Title: Re: Modifying a First Act MA104
Post by: J M Fahey on October 21, 2011, 11:33:11 AM
I'm not sure, but I *think* the 2030 is a typo, and it actually has a 2003 inside.
May be mistaken, of course.
I don´t see a power transformer in the pictures, is it powered by a wall wart?
Title: Re: Modifying a First Act MA104
Post by: Dimi Pana on October 21, 2011, 11:52:12 AM
Thank you all for your feedback.

Yes, it is a TDA2030A. It's hard to see in the pics but that's what it is. Thanks, JoeCool85 for posting the data sheet, wow, it has a lot of info.

I thought Koreth had initially made a typo when he wrote 2003 but no, it appears there is a 2003 as well as a 2030, I am looking at it right now (lol).

Yes, JMF, it is powered by a 14 Volt DC wall wart rated at 500mA. However, when I tested it (without any load of course) the output is a little over 20 volts (!?!) 20.56 to be exact! It also says input should be 110 AC (duh!) with a maximum power of 10W.

I am on the road now, between clients, I have an interesting (I hope) story to tell about these amps, more later when I am at my home PC.

Thanks again!
Title: Re: Modifying a First Act MA104
Post by: joecool85 on October 21, 2011, 03:44:06 PM
Ah, well even if it is a 2030A, with that PSU you won't get much more than 4-5 watts.  If this is truly the case it would be interesting to "reverse engineer" the circuit and make sure it can take a higher voltage rating.  If it can't you could adjust it till it can, then use a larger PSU and put out substantially more power.  All this said, unless you need more power, it's probably not worth the hassle even though it would be fun.
Title: Re: Modifying a First Act MA104
Post by: Dimi Pana on October 21, 2011, 04:48:16 PM
Hello all and thank you again!

OK, this is all great stuff, I am confident this little toy some day -maybe- it will make an OK (not great, just OK) sounding practice amp.

First, responses to your feedback:

1. Sure more Watts is better, but I would still be very happy if this amp could output just 1Watt of usable (and somewhat musical) power. Right now it is rated at 4Watts into a 4Ω speaker, which I believe is rated for 8Watts. But the sound is not usable at least for me even for practicing. I am not a pro (probably will never be one) but I do have some minimum expectations. So when I did those calculations I was not really thinking of a single 16Ω speaker (yes, they are not so common and put out less power) but rather two 8Ω (or four 4Ω)(lol!) in series. Anyway, you get the picture. Again, this project is mostly for fun. I do not intend to spend too much time AND -of course- money on this.

2. The "5-leg" chip is indeed a TDA2030A copied straight from the top of it and it also says CXY 0908F below that. The other important component on this circuit, the "8-leg" IC has written on it this: 4558D then below it says JRC and below that it says 0058T. Finally, there is also a "3-leg" transistor which says 9013 on it.

3. Yes, the metallic enclosure this thing sits inside is probably the best thing about it, eventually I might use it for something else, i.e.: http://www.beavisaudio.com/projects/NoisyCricket/ (http://www.beavisaudio.com/projects/NoisyCricket/)

4. Question to phatt: When you say "remove the diodes" which ones specifically do you mean, or you mean ALL of them. And what do I do then? Just replace them with a jumper cable? And why is it that JoeCool85 then says that even though this is a correct approach it might not help improve the sound?

--

A little bit about myself. I am not an electronics expert, barely a novice, I am a foreign student in Chicago majoring in MIS (Management of Information Systems). I intend to take a couple courses on basic electronics starting next semester. I do know how to use a solder gun though but the most complicated project I ever successfully completed is a music metronome (did I mention I play the guitar?)(lol) and a moisture detector alarm (pls do not ask why). So, I have said this before, I am looking for some simple and straightforward advice like "remove this", "change that", "do this" or "do that". I am not saying this because I am lazy, but because even if you give me a lot of good and high level advice you will probably waste your time since I am by no means at a level that I can understand it.

Now here's some interesting info. Yesterday, I went to (you guessed it) Toys-R-Us and I purchased an MA104 which is Koreth's amp (who started all this) for $29.99 and the amp I am talking about the MA1248 for $4.98. I went home and took both apart. Here's what I found. These two amps are identical from how the cabinet is build to the pcb, the components and every drop of solder on it. Only one is black with black screen the other is brown with thatch (well resembling) screen. These two toys are identical. Of course the brown one is stamped as manufactured back in September of 2009 the black one in may of 2011. And both use a TDA2030A, a 4558 JRC and a 9013 transistor. Wonderful eh! This two items are identical but they sell at so different prices. [EDIT: I did return the MA104 back, in case you were wondering, but I kept the MA1248 to toy around with it. At $4.98 it's no big deal].

Anyway, to help me understand how this circuit works I will post my theory please correct me as needed:

The 9013 is part of the power supply and is there to create a buffer, something to provide steady and constant DC. It is not part of the signal. Then the JRC 4558D is the IC which acts as the preamp, and if I am right it has two stages. Then the signal is sent to the TDA2030 that is the power amp. Gain is applied on the IC and volume on the final output at the TDA. A tone control sits in between.

That's all for now, thanks to all for the input. I will start looking for a better speaker or speakers, BTW I'll search for an 8" or 10" at 8Ω, what do you think the maximum watts should be for the speaker?
Title: Re: Modifying a First Act MA104
Post by: phatt on October 22, 2011, 06:48:54 AM
Hello Dimi Pana,
                Yes the two diodes side by side will **Likely** be where all the distortion comes from. **as I don't have a schematic I can't be certain**

I repaired a similar circuit last week and it distorted on 1,, No clean whatsoever.
They know the kids today just want distortion and it seems the more it has the better they sell.

My guess is if you just desolder one end of both diodes and just pull them up from the board. Leave them standing,, other ends still soldered that way if you want to undo the mod you will not loose the diodes,, wink.

The diodes I've arrowed make the signal distort but it often robs the amp of clean power.
On such a small Amp I often find that they sound more REAL without diodes.

ps, If you are keen on learning Audio circuitry?
This is a golden oppertunity to teach yourself how it works as this is not a complex Amp circuit.
Heck you already know the black blob with 3 legs is a transistor,, so that is a good sign.

Lift the PCB hold it up to a strong light and you can then connect all the components.

Now with a pencil, Draw the schematic on paper.

Learn how to work in *Nodes* instead of imagery that will keep mistakes to a minimum.

*It will be hard at first* and you Will certainly waste paper but that is how I started learning about circuits,,, long before the internet.

So now you just use the net to find the pinouts of the devices and it won't take you as long as it took me.

Re, The 9013 transistor.
It's likely just a buffer stage. Go find the schematic of a TS9 and you will see same or similar.

Have fun,, Phil.
Title: Re: Modifying a First Act MA104
Post by: Dimi Pana on October 25, 2011, 03:13:52 PM
Thank you all for your feedback, it is very much appreciated.

I have followed your advice and started tracing the circuit on paper. It is not easy. The way I understand "nodes" vs. "imagery" is try not to visually transfer a picture of the board but starting from one side, move from junction point to the next one making sure I "transfer" all lines, components, etc., that start from it.

Another -more fun- thing I did is hook this amp up to different speakers. I found two old 6" (rated 6Ω at 20W) speakers from an old home stereo and tried the different combinations:

1. One speaker (6Ω) --> It sounded almost the same or maybe a little "better". I noticed less distortion from the speaker but it still rattled when played hard. Overall, I'd say it was marginally better.

2. Two speakers in series (6+6=12Ω) --> Sounded clearly "bigger", but noticed an output difference in the volume pot. There was also less speaker rattling, unless I played unrealistically hard, i.e. intentionally hit the strings hard in an ..."unmusical" way.

3. Three speakers in series (6+6+4=16Ω) --> This is exactly the previous setup (#2) with the addition of the original 4" speaker (4Ω , 8W) from the amp. Again the apparent sound was bigger, fuller and not as loud. I could not make the speaker cones rattle no matter how hard I played, but like I said this setup was noticeably NOT as loud as the previous ones.

However, one thing I noticed in all setups is that the amp generated distortion did not really change in timbre, and yes it still sounded cheap, not natural. Also bear in mind that I did not solder the wires to the speaker terminals but rather used wire with crocodile clips. Do you think this is a factor?

--

Finally, I was complaining that with no instrument plugged-in and all pots turned down, this amp produces a strange noise when turned ON, something between a hum and a buzz. I tested the wall wart output and found that even though the DC output is rated at 14V , 500mA , in reality it outputs more that 20 Volts. I then found a wall wart rated at 12V , 1.5A tested it and found in reality it outputs around 18V.  I then reversed the polarity (had to, the amp requires the pin to be negative and the sleeve to be positive) and used that instead of the FirstAct supplied one.

Guess what? With this PSU the noise is not gone but is significantly reduced. Any comments on that?

--

So it appears that a better(?) , cleaner (?) PSU will definitely help, as well as a bigger and better speaker/cabinet configuration.

I will continue tracing the PCB and get back to you with more. 'Till then thanks again to ALL of you!!!
Title: Re: Modifying a First Act MA104
Post by: DartPlayer170 on October 25, 2011, 11:11:05 PM
The 1.5A power supply is stiffer and so is more resistant to power supply humm. You could also reduce the humm by replacing the filter cap on the PCB with a larger value.

The 14V power supply is a nominal 14 Volts. It may be 20 or more V when unloaded but it is much less when fully loaded at 500mA. You can increase the wattage of the amp slightly by replacing the power supply with a higher voltage power supply. I am using a 24Vdc at 600mA on my MA007. The 2030A cannot handle more than 20W so I don't suggest going higher than 24Vdc.

If you want to increase the range of the volume control you can replace the resistor from the -ve pin of the 1st stage of the JRC4558 amp to ground ( through a series cap ) with a higher value. On mine it is reference designator R3 but it may be different on other models. On mine its a 270 ohm. Try replacing it with a 1 kohm. This will decrease the full scale clean gain from 400 to 100.
Title: Re: Modifying a First Act MA104
Post by: joecool85 on October 26, 2011, 08:46:53 AM
The 2030A cannot handle more than 20W so I don't suggest going higher than 24Vdc.

Actually the 2030A is rated for a maximum of +/-22v (44v total).
Title: Re: Modifying a First Act MA104
Post by: DartPlayer170 on October 26, 2011, 01:34:11 PM
Yes, you're right. I was more concerned about the 20W power dissipation but looking at the test circuit output power it should be able to handle more. It depends on the load though. At 4 ohms it reaches 20W at 34V. For 8 ohms it is above 40V. Don't forget that these are actual power supply voltages not the nominal values rated on the power packs.
Title: Re: Modifying a First Act MA104
Post by: Dimi Pana on October 26, 2011, 02:04:25 PM
So are you guys saying that with the original FirstAct wall-wart, or even a slightly better one, I mean with more milliamperes, this amp is pretty much "doomed" anyway to an output of (no more than) ~5 watts into a 4Ω speaker? This is not necessarily bad, I am not really looking for more power, but for a cleaner and more usable (thus musical) sound.

My intention originally was to hook it up to a 8Ω self-made cabinet and if I can scrounge a couple of speakers, perhaps build it as a 2x8" or 2x10" depending on the price I get. I mean who would n't want a 4x12" but in this case I think (lol) this might be overkill ...

So unless I can attach a real power supply to this toy-amp, are you saying my output should "see" a 4Ω speaker, anything else and I will probably blow the TDA2030A up?

Is this a correct statement?

Thanks again for your comments!   
Title: Re: Modifying a First Act MA104
Post by: DartPlayer170 on October 26, 2011, 05:41:53 PM
Your last statement is not correct. You can hook up a 4 ohm or an 8 ohm speaker to the 2030A.
You can replace the wall-wart to a 20W ( more would be a waste since the 2030A cannot handle it ).
The problem is to figure out how high of a voltage to get for the wall-wart.
There are two problems:
1 - the rating on the wall-wart is nominal. It may be more than the rating even at full load.
2 - the current drain of the 2030A is dependent on the supply voltage, the load and the input signal. And hence, so is the power dissipation.
 
The 2030A is rated for 44V max and 20W max. However, the current is dependent on the load. If you use a 4 ohm speaker then a 34V supply will dissipate 20W. I=P/V so the current is about 600mA.

What I can tell you is that I have replaced the wall-wart with a 24V 600mA and it works fine.

If you want to create a cleaner sound with the gain at 0, then you need to decrease the gain in the first stage of the amp. The designers set it too high. The clipping diodes that you were told to remove begin distorting the signal at about 800mVpp. You could just remove these diodes as suggested ( d1 and d2 ) if you don`t want to be able to use the distortion circuit. Another solution is to increase the negative feedback to the stage 1 amp. This can be achieved by replacing the feedback resistor with a higher value. The feedback resistor is in series with an electrolytic capacitor between either pin 2 or 6 ( depending on which one they are using as stage 1 ) of the JRC4558D and ground.

Title: Re: Modifying a First Act MA104
Post by: Dimi Pana on October 27, 2011, 12:10:46 AM
Your last statement is not correct. You can hook up a 4 ohm or an 8 ohm speaker to the 2030A.
You can replace the wall-wart to a 20W ( more would be a waste since the 2030A cannot handle it ).
The problem is to figure out how high of a voltage to get for the wall-wart.
There are two problems:
1 - the rating on the wall-wart is nominal. It may be more than the rating even at full load.
2 - the current drain of the 2030A is dependent on the supply voltage, the load and the input signal. And hence, so is the power dissipation.
 
The 2030A is rated for 44V max and 20W max. However, the current is dependent on the load. If you use a 4 ohm speaker then a 34V supply will dissipate 20W. I=P/V so the current is about 600mA.

What I can tell you is that I have replaced the wall-wart with a 24V 600mA and it works fine.

If you want to create a cleaner sound with the gain at 0, then you need to decrease the gain in the first stage of the amp. The designers set it too high. The clipping diodes that you were told to remove begin distorting the signal at about 800mVpp. You could just remove these diodes as suggested ( d1 and d2 ) if you don`t want to be able to use the distortion circuit. Another solution is to increase the negative feedback to the stage 1 amp. This can be achieved by replacing the feedback resistor with a higher value. The feedback resistor is in series with an electrolytic capacitor between either pin 2 or 6 ( depending on which one they are using as stage 1 ) of the JRC4558D and ground.

Alright, now I am lost...

1. Regarding the power supply: My intention is to hook this up to a 8Ω cabinet. So, are you saying that the original wall-wart will not be enough? If this is correct then, what are the specs for the proper PSU?

2. I thought phatt's advice about removing the diodes meant the gain pot is still working and you still get some distortion, you just get less distortion or "cleaner" distortion. However, from your last post, if I understand right, are you saying the gain pot becomes inactive and you basically lose ALL distortion this thing can produce? If that's the case, then no this is not what I am after. I do want the versatility of having a Gain control before my Volume. Question is can this Gain control be more gradual in response and smoother in performance?  

3. The 24V 600mA wall-wart powers your MA007 right? Are you using it as is or you modified it and play it through a different speaker?

Is this the MA007 ? Is it made of plastic? I'd be curious how it sounds!?!

Thanks!
Title: Re: Modifying a First Act MA104
Post by: DartPlayer170 on October 29, 2011, 01:10:53 PM
Quote
1. Regarding the power supply: My intention is to hook this up to a 8Ω cabinet. So, are you saying that the original wall-wart will not be enough? If this is correct then, what are the specs for the proper PSU?

I think earlier you had mentioned increasing the power. You don't have to, but it can handle a larger power supply if you want. The max the output op amp can handle is 20W and 44Vdc. For an 8 ohm load, you should be able to go up to 44V. However, my earlier warning was if you are going to try a 4 ohm load, then stick with 34Vdc because obvioulsy the current will be larger for a 4 ohm load than for an 8 ohm load. Increasing the power supply to 20W won't make a huge difference but it will be slightly louder at maximum clean output.

Quote
2. I thought phatt's advice about removing the diodes meant the gain pot is still working and you still get some distortion, you just get less distortion or "cleaner" distortion. However, from your last post, if I understand right, are you saying the gain pot becomes inactive and you basically lose ALL distortion this thing can produce? If that's the case, then no this is not what I am after. I do want the versatility of having a Gain control before my Volume. Question is can this Gain control be more gradual in response and smoother in performance? 

Essentially yes, but it is a bit more complex than what I explained. The diodes clip the output of the first stage and cause harmonic distortion. If you remove them you can still get distortion from overdriving the amp. But the purpose of the first stage is merely to overdrive the clipping diodes when the gain is set high. In my opinion the best solution is to simply reduce the maximum gain of the first stage so that the gain pot becomes more usable. However, you can try both solutions and see which one suits your needs.

Quote
3. The 24V 600mA wall-wart powers your MA007 right? Are you using it as is or you modified it and play it through a different speaker?

Is this the MA007 ? Is it made of plastic? I'd be curious how it sounds!?!

Yes. I did not mod the new wall-wart. My original intention was to mod the original wall-wart with a different transformer but it turned out to be simpler to just replace the whole wall-wart. Yes I have the MA007. I actually have two. One that I moded and I bought another to compare the original sound. They are different. But I don't have the ear to really say which is better. The moded one is slightly more powerfull with clean sound and my volume and gain controls are more usable. I also moded the tone stack to give it a brighter sound. And I put a large filter cap to reduce the humm.
Title: Re: Modifying a First Act MA104
Post by: Dimi Pana on November 13, 2011, 03:25:46 PM
Hello all!

Has anybody been able to open the MA104Final.png attachement Koreth has posted in the very beginning of this topic?

I am trying to compare his schematic with the one I have traced from my MA1248 amp and I cannot open the file, every program I tried say things like "invalid format", "corrupt file", etc.

Can anyone repost this file please?

Also, Koreth, is this your final, I mean FINAL, version?

I've compared my MA1248 with the MA107 (schematic posted earlier by DartPlayer170) and even a novice like myself can see there are differences, e.g. some R,C values are different but over all they are very similar.
 
I also know for a fact that the MA104 and the MA1248 are identical, at least two units I compared recently side by side. Koreth has mentioned that his is using the TDA2003 while mine is using a TDA2030 (and yes, as stated before this is not a typo) plus from another schematic (posted here: http://music-electronics-forum.com/attachments/7178d1260175055-ma104.png (http://music-electronics-forum.com/attachments/7178d1260175055-ma104.png)) it appears it's using two JRC4558 instead of one. So I am inclined to believe that the MA104 Koreth bought back in 2009 is slightly different than a MA104 currently sold by first act.

I reviewed both data sheets:

TDA2003 http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1449.pdf (http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1449.pdf)

TDA2030 http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1458.pdf (http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1458.pdf)

and other than 4Watt ouput power difference, both seem similar to me.

...

So what I am trying to do is compare side by side three schematics:

1) Koreth's (year 2009) MA104

2) DartPlayer170's MA107

and

3) Mine MA1248

Once I have the closest match I can produce a final version of the MA1248 schematic, which will help with all future discussion.

...

Anyway, is is possible -anyone- to repost the MA104Final.png schematic, please?   

Thank you all!   
Title: Re: Modifying a First Act MA104
Post by: Dimi Pana on November 13, 2011, 04:19:36 PM
OK, if you have not had enough of me today, just one last question, which I should have asked from day one:

What would be a FREE, easy to use, i.e. no steep learning curve, PC program to draw schematics with?

I mean, I've tried MS paint but -obviously- this program is not made for that.

Thank you!
Title: Re: Modifying a First Act MA104
Post by: J M Fahey on November 13, 2011, 08:52:43 PM
Hands down winner: DIY Layout Creator by Bancika (who besides his great work, also kindly contributes here) .
Use the earlier Windows version, because there are a zillion designs made using it, and you can learn from others.
To see some thousands of examples:
http://aronnelson.com/gallery/main.php
Title: Re: Modifying a First Act MA104
Post by: Bob E. on April 19, 2012, 05:52:03 PM
So, did anybody ever cut out those resistors? Did it work?
I just picked up a firstact MA2039 at a garage sale the other day. It's powered by two 9v's and it sounds like crap. Nothing but horrible distortion no matter the setting. The circuit looks similar, but different to the one pictured earlier. It still has those resistors only labeled D2 and D3 on the circuit board. I sure would be happy to get some clean sounds out of this thing.
(http://img.photobucket.com/albums/1003/Bob_E/IMG_3862.jpg)
Title: Re: Modifying a First Act MA104
Post by: Bob E. on April 20, 2012, 12:12:42 AM
If I could just get any clean sound at a low volume level out of this thing without much effort that would make me happy.
My speaker also has no markings. I do have a "manual" which is all of one sheet of copy paper, but it does have the specs:

Quote
SPECIFICATIONS
Output Power: 2 watts RMS
Frequency Rang: 60Hz ~ 6kHz
Speaker: 4" / 8W / 4 ohms
Input Sensitivity: 26 mV
Power Supply (not included): 14V DC 500mA or 2 x 9V batteries
Dimensions: (LxWxH) 6.75" x 3" x 6.75"
Weight: 2.5 lbs
SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE.
Title: Re: Modifying a First Act MA104
Post by: Bob E. on April 20, 2012, 11:06:01 AM
I can get a tiny bit of clean with a single coil bridge pickup, volume on the guitar at full, the gain on the amp rolled all the way off, tone all the way up and the volume on the amp turned down so low I can barely hear the amp over the sound of my solid body guitar. I can chase this same little window of clean all the way around the volume knobs until the guitar is rolled off and the amp is all the way up. But I almost have to hold the amp up to my ear to hear it. Anything beyond this and it starts to breakup. I wouldn't say it's not a musical sound. I'm pretty sure this is the way they built it. I paid $5 for the amp and no I don't want to invest any more into it for a speaker or anything. I was hoping it would be as easy as pulling out a couple of resistors, but if all that does is cut out the gain knob then I don't think it will help me. Yes I am in the US.
Thanks
Title: Re: Modifying a First Act MA104
Post by: J M Fahey on April 20, 2012, 11:59:02 AM
If the sound pressure level you get with (up to) 2W into the crappy zero efficiency speaker it has is not enough for you, there is no amount of tweaking that will do.
Simple as that.
Title: Re: Modifying a First Act MA104
Post by: Bob E. on April 20, 2012, 02:29:47 PM
Thanks for the advice.