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Author Topic: Modifying a First Act MA104  (Read 33441 times)

DartPlayer170

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Re: Modifying a First Act MA104
« Reply #30 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.

Dimi Pana

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Re: Modifying a First Act MA104
« Reply #31 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!   

DartPlayer170

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Re: Modifying a First Act MA104
« Reply #32 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.


Dimi Pana

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Re: Modifying a First Act MA104
« Reply #33 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!
« Last Edit: October 27, 2011, 12:17:15 AM by Dimi Pana »

DartPlayer170

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Re: Modifying a First Act MA104
« Reply #34 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.

Dimi Pana

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Re: Modifying a First Act MA104
« Reply #35 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) 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

TDA2030 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!   
« Last Edit: November 13, 2011, 04:20:59 PM by Dimi Pana »

Dimi Pana

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Re: Modifying a First Act MA104
« Reply #36 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!

J M Fahey

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Re: Modifying a First Act MA104
« Reply #37 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

Bob E.

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Re: Modifying a First Act MA104
« Reply #38 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.

Bob E.

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Re: Modifying a First Act MA104
« Reply #39 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.

Bob E.

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Re: Modifying a First Act MA104
« Reply #40 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

J M Fahey

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Re: Modifying a First Act MA104
« Reply #41 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.

Bob E.

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Re: Modifying a First Act MA104
« Reply #42 on: April 20, 2012, 02:29:47 PM »
Thanks for the advice.

 

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