LM4950TA - Help me make a practice amp? Updated - HELP! ;)

[javacody]

Here is the app note:



Help!

What do I do with the boxes in the schem (shutdown control, bias) and what is the bypass for? Any suggestions converting this to a practice amp?

It runs on 12V and puts out 7.5 watts mono.

[mikeb]

You've got the full datasheet, right? There's info on how to hookup the shutdown control pin using a SPST switch, as well as info on the bypassing caps in there - as well as a suggested PCB layout (ok, so you'll need to convert the double-sided one to a single-sided version). If you use a dual-gang pot to replace RinA and RinB then you'll have a volume control - again, it's all in the app notes. Shout out if you need help with interpreting the details, it can be a pain to dig through all the info sometimes.

Mike

[csj]

I don't have a SS amp either and I'd like one to run some of these ROG pedals into so I've been fooling around with what I had around the house. I'm not familiar with 4950 except for the app/docs.

I had an old LM383, a tda2002, a tda2030 and several transistors I'm going to try to use in a bootstrap configuration to get a push/pull effect going.

The problem I've had so far with the 2002 and the 2030 is that the input sensitivities are so low that they can't really stand a whole lot of preamp gain. A regular guitar signal drives them fine, in fact it's about all they can take. Things get ugly real fast when you try to drive them harder. This one here is the tda2003. It's gain is set at around 75. I'll try to bring that lower and see what I can do with it then.

I like these components though because they make easy/simple builds based upon basic circuits which I can't screw up and they sound pretty good.

[javacody]

Does this look like it'll work to you guys? Any help/advice would be appreciated.



Here is the datasheet.

http://www.national.com/ds/LM/LM4950.pdf

I'm trying to figure out some kind of gain control or attenuation of the signal to the speaker (like the Little Gem has).

[Peter Snowberg]

Add a resistor between pins 1 and 3 and add a 100K resistor between pins 2 and 6 and you should have it. I would use the same 20K values for those four resistors that they use on the datasheet though. You could always just leave the switch out.

Take care,
-Peter

[javacody]

Thanks Peter, I missed that 100k resistor completely and meant to include the 20k. I went back to look at the app note (you must be pretty fast at absorbing app notes!) and noticed that 100k you were talking about. Anyway, here is v2.



Also Peter, any idea how a volume control is usually implemented with this type of chip?

[Peter Snowberg]

(you must be pretty fast at absorbing app notes!)

It's all in figuring out what to ignore.

Also Peter, any idea how a volume control is usually implemented with this type of chip?

Exactly the way you did it. :mrgreen:


Take care,
-Peter

[javacody]

Well, I can't take credit for slapping RunoffGroove's Fetzer Valve onto the front of this chip. I just hope it sounds good.

Any suggestions for improving the odds of this sounding good as a guitar amp?

[Chris S]

Can I just say Baking tray = great idea for a big stompbox!

[AdamOn6thStreet]

Can I just say Baking tray = great idea for a big stompbox!

Hmm, as soon as I saw that picture I thought "that would be a great chassis for a project tube amp!"

[Pablo]

javacody,

did you get this amp working?

if so, how does it sound?

could you post pin voltages?

TIA

[javacody]

This amp is NOT working. I'm not sure what I've done wrong, but the chip is getting very hot. Should I be supplying negative voltage to this, the same way that I do the JFET?

[javacody]

bump. Does anyone know if these chips are supposed to get so hot just run on 12 volts?

[javacody]

OK, re-read the appnote, I think that part of the reason the chip is getting so hot is that I'm using a 4 ohm load rather than an 8 ohm load. I definitely need to heatsink this puppy. When I applied 12 volts to this chip, the chip pulled the voltage down to ~9.5 volts somehow? Also, there were about 9 volts on pins 3 and 7, that can't be right, can it? Could it be a short somewhere?

At any rate, I'm starting over. I had tried to reuse a piece of perf that still had some solder on the pads, and I went to the RatShack today and got some new perf. I just read the appnote on using a heatsink, so back to the ratshack I go tomorrow. Maybe one of these old PC heatsinks I have can be made to work?

Anyway, if any of you folks have any ideas that could help ensure success on this, I'd appreciate it. I almost gave up and bought a Frontman 15 on ebay today.  

Still looking at the app note, I think that I'm also supposed to have a 100K resistor going to ground from pin 2.

Last but not least, I have a 10 uF cap between the speaker lugs, would this cause problems for this BTL 7.5 watt chip?

Edit If I replace RinA and RinB with lower values, will this raise the gain? Also, what effect do the Feedback resistors have? What happens if I raise and lower the feedback resistors?