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Author Topic: Modifying a First Act MA104  (Read 34219 times)

Thebighat99

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Re: Modifying a First Act MA104
« Reply #15 on: March 09, 2010, 10:04:54 PM »
Ok I found the schematics on another post of koreth.  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.
« Last Edit: March 09, 2010, 10:57:20 PM by Thebighat99 »

DartPlayer170

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Re: Modifying a First Act MA104
« Reply #16 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.

Dimi Pana

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Re: Modifying a First Act MA104
« Reply #17 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

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.
« Last Edit: October 19, 2011, 05:10:32 PM by Dimi Pana »

joecool85

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Re: Modifying a First Act MA104
« Reply #18 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.
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Dimi Pana

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Re: Modifying a First Act MA104
« Reply #19 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/  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!

phatt

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Re: Modifying a First Act MA104
« Reply #20 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.

joecool85

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Re: Modifying a First Act MA104
« Reply #21 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.
« Last Edit: October 26, 2011, 08:47:26 AM by joecool85 »
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J M Fahey

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Re: Modifying a First Act MA104
« Reply #22 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?

Dimi Pana

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Re: Modifying a First Act MA104
« Reply #23 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!
« Last Edit: October 21, 2011, 11:55:17 AM by Dimi Pana »

joecool85

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Re: Modifying a First Act MA104
« Reply #24 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.
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Dimi Pana

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Re: Modifying a First Act MA104
« Reply #25 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/

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?
« Last Edit: October 22, 2011, 12:54:10 PM by Dimi Pana »

phatt

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Re: Modifying a First Act MA104
« Reply #26 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.

Dimi Pana

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Re: Modifying a First Act MA104
« Reply #27 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!!!

DartPlayer170

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Re: Modifying a First Act MA104
« Reply #28 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.

joecool85

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Re: Modifying a First Act MA104
« Reply #29 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).
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