Determining correct potentiometer

Replies to This Discussion

Permalink Reply by Bob Lumpkin on January 18, 2012 at 9:24pm

Hi Hal:

 I've used several of these. If you're starting with the kit, just leave off the trim pot & solder 3 wires in it's place (different colors so u can keep them straight) Use a A10k audio pot in it's place. Be sure to keep your wires straight. If you need more info, just e-mail me & I'll send you some drawings, can't do it tonight. 

Good luck;

Bob

 

Permalink Reply by Habanera Hal on January 18, 2012 at 10:42pm

Thanks, Bob.  I purchased the already-assembled kits, but did unsolder the trim pot on the first one and replaced it with an A500k pot I had laying around.  Wasn't happy with the result, so I'll pick up some A10k's at Radio Shack tomorrow.

Is there any reason not to just wire it in to the speaker line?  Lots less soldering.

Since you have some experience with these units, maybe you can tell me if there's a simple way to add a tone control.

Permalink Reply by John Wilson on January 19, 2012 at 6:03pm

You can't wire it in to the speaker line because the resistance of the speaker is so small compared the the resistance of the pot.  You may be able to buy a rheostat at radioshack and use that for a volume control on the speaker line. 

As for a tone control, with this circuit I've tried to add an integrated tone control and haven't had any luck yet.  The datasheet on the TBA820M seems to indicate that it is possible but when I wire up my test circuit I don't get any change in the tone whatsoever.  So your best bet is to put a passive tone network in front of the entire amp.

Permalink Reply by Mama Mojo on January 19, 2012 at 10:12am

that diagram makes my head hurt....

Permalink Reply by petey twofinger on January 20, 2012 at 2:25pm

you can ohm out a pot too , if you put the probes on the far pns it should tell you the total resistance of the part , if you do not know hat the value is .

check this out , been wanting to try one of these circuits :

http://www.beavisaudio.com/techpages/BigMuffToneControl/

as far as that schematic goes :

the triangle represents the chip (the active piece in the circuit , the amp) looks like it prolly has 8 pins .

the saw tooth squigles are resistors .

c1 and all like it are electrolytic caps (capacitors) cause it lists polarity (positive) so ya know which way they "connect" .

c3 c5 c6 are non polarized caps , or ceramic disks , those lil orangies . 

the speaker goes on the left , the lil plus and minus ,the big plus and minus on the left is the battery .

i dig that wine crate amp hack Hal !!

Permalink Reply by Ellwood T. Bear on January 20, 2012 at 6:19pm

Ok, Guys, I am going to jump in here. The 10 K trim pot is NOT for adjusting the Volume to the speaker. it is for adjusting the level of the INPUT to the amplifier meaning what ever you are plugging into the amp.  "On board PCB trim pot style input level adjust. "  A volume control goes between the Output of the chip and the speaker.

You will br better off purchasing the amp kit from C.B. Getty.

Permalink Reply by John Wilson on January 25, 2012 at 9:51am

Ellwood, that is not how modern amplifiers work.  At least, not the ones you plug in to the wall.  They do not make volume knobs that can handle that much power.  I won't say it is impossible, but it is certainly quite impractical.

When you adjust the level of the signal coming in to the amplifier, you are, effectively, adjusting the output volume.

Permalink Reply by Ellwood T. Bear on January 25, 2012 at 12:07pm

John, here is the link to the site that he purchased the kit from, http://store.qkits.com/moreinfo.cfm/FK602

After being in the medical field (Biomedical Engineering) repairing life support systems for 30 years I do know how to read schematics. Plus I do have a degrees in electronics.

This 2 watt mono amplifier kit is based on the TBA820 8 pin DIP

3vDC = 200mW 4 ohm speaker
6vDC = 750mW 4 ohm speaker
9vDC = 1.6W 4 ohm speaker
12vDC = 2W 8 ohm speaker
S/N ratio : 80dB (A weighted)
frequency response : 20Hz to 20kHz (-3dB)

On board PCB trim pot style input level adjust.
Power supply can range anywhere from 3V - 12V DC.
The speaker shares a common ground connection.
Dimensions: 1 5/8 X 1 3/8 X 3/8"
(41 x 35 x 10

I will stand by what I stated before.  With out the trim pot adjust you could Overdrive the IC  and blow it.

Permalink Reply by Habanera Hal on January 25, 2012 at 12:21pm

Elwood -

Are you saying to leave the 10k trimpot attached and NOT to replace it with a 10k potentiometer to adjust the volume?  If anything, it seems to me that it would be "safer" to adjust the input level - would put less stress on the circuit rather than having the trimpot turned all the way up all the time (the "set it and forget it" thing)  Isn't that why most amps have their volume controls wired into the main circuit rather than at the speaker?  What am I missing?

Permalink Reply by Ellwood T. Bear on January 25, 2012 at 3:28pm

No, I did not say that, you can replace it with a 10k pot.  What the 10K pot is used for is to adjust the input level (On board PCB trim pot style input level adjust. ).