Audio Amp Component Selection

[vne147]

A CD sounds perfect.
An MP3 and an FM stereo radio sound very good.
A high quality cassette tape deck with Dolby noise reduction and high bias tape sounded pretty good 20 years ago.
But a lousy old AM radio sounds awful.

Most people can hear frequencies as high as 20kHz. But an AM radio rolls off frequencies above only 3.5kHz like a telephone. An AM radio also has high distortion and lots of noise.

I am still not understanding how this applies to what I'm building. Once my circuit is built it will be hooked up directly to the speaker through a DPDT switch so my friend can select between the radio and the iPod. It will bypass the old AM radio completely. Am I missing something or is my project for some reason I'm not understanding doomed to poor AM level sound quality? I'm ready to order the parts from Digikey but I don't want to if I'm about to do something wrong.

 

[schmitt trigger]

You are welcome!
And thanks for the images.

Your friend's radio is a newer model than my friend's. In your case everything is in a single chassis, in my case two chassis were involved. Nevertheless, it follows a similar circuit design, it requires a mechanical vibrator to step up the voltage. In your photos, the vibrator is missing, the empty socket can be clearly seen on image 100-1006.

The speedometer, is it in MPH or Km/H? I'm pretty sure my friend's was Km/H.

 

[vne147]

Okay, I saw the empty socket but wasn't sure what it was for. I guess I just assumed a tube amp when there. The speedometer is in mph. According to my friend this VW was manufactured for export to the US.

I'm about to order all the parts. I'll start laying out the board and when that's done I'll post it on here for everyone to tear it apart. Im also thinking of using an aluminum enclosure for the finished project. I want the enclosure to be sealed because he is going to put it in the trunk. I think the aluminum will help with the heat dissipation. I'm also planning on going with all surface mount components. Thoughts?

 

[schmitt trigger]

Indeed, SMT components are better for switchmode power supplies.
When you lay out the board, include a ground plane. This means at least a 2-layer board.

 

[vne147]

This means at least a 2-layer board.

Would a 2 sided board meet this requirement?

 

[schmitt trigger]

Would a 2 sided board meet this requirement?

Yes, that is what I meant

 

[Ross Craney]

1)



We don't know if the speaker is rated in Watts or in Whats. .

AG - I've never heard the term "Whats" What is it ? (excuse pun)

 

[audioguru]

AG - I've never heard the term "Whats" What is it ? (excuse pun)

Many amplifier and speaker manufacturers lie about their max allowed power.
They say "peak power" which is simply double the real power.
They blast the amplifier and speaker with the volume control turned up way too high so that the output is severely distorted square-waves (but the measured power is more).

I call the lieing numbers Whats, which are exaggerated Watts.

 

[vne147]

OK, so here is my schematic for the boost converter and amplifier. It is pretty much right out of the data sheets.

For the boost converter stage, I changed a few things from what National’s software told me I should do. Digikey was out of the 47 uH inductors so I’m using a 56 uH one instead. Also, the recommended value for the Cin capacitor was 330uF but I had a couple extra 150uF caps that I am going to use in its place. Also, similar deal with the Ccomp cap were I’m using two .47uF caps in place of a single 1uF one. I’m also going to leave the sync and the frequency adjust pins floating.

For the amp stage of the circuit I didn’t change anything except I was uncertain if I needed the Rx and Cx components and what the values should be.

Any comments, corrections, suggested improvements you have for my circuit are as always much appreciated. I’ll wait to hear everyone’s inputs before I design the board. Thanks.

 

[schmitt trigger]

Couple of things: from the component datasheet DO CHECK that you can leave the sync and the frequency adjust pins open, or whether you have to tie them to gnd/Vdd.

Although higher inductance is usually not a problem in a SMPS, I would still simulate it and compare it against the performance of the original value.

Also...The 56 uH inductor, what is its current rating as compared to the 47uH?

 

[vne147]

Couple of things: from the component datasheet DO CHECK that you can leave the sync and the frequency adjust pins open, or whether you have to tie them to gnd/Vdd.

Although higher inductance is usually not a problem in a SMPS, I would still simulate it and compare it against the performance of the original value.

Also...The 56 uH inductor, what is its current rating as compared to the 47uH?

The data sheet shows boost converter application examples where the freq adj and sync pins are left floating (refer to page 22 in the data sheet, link below). However, the schematic generated by the software shows the sync pin floating and the freq adj pin tied to ground through a Radj resistor of undefined value. Not sure what that's all about.

The substitution of the 47 uH inductor with a 56 uH one was one of the options offered within the software when I clicked on edit component. The differences between the two inductors are:

L= 47uH / 56uH
R= .022Ω / .024Ω
I= 6.5 A / 6.2A
P= .377 W / .412W

Thanks for the prompt response, let me know if you need any more info.

LM2588 data sheet link:

https://www.electro-tech-online.com/custompdfs/2009/12/LM2588.pdf

 

[schmitt trigger]

Ok, got you.

The Radj is only used if you want to modify the base frequency. I would advise, when you create the layout, to leave an empty placeholder there. So you can either leave it empty, or if the need arises to use the resistor, you can solder it without resorting to an ugly dead-bug.

 

[vne147]

Ok, got you.

The Radj is only used if you want to modify the base frequency. I would advise, when you create the layout, to leave an empty placeholder there. So you can either leave it empty, or if the need arises to use the resistor, you can solder it without resorting to an ugly dead-bug.

OK, good suggestion. I'll leave a place holder for a resistor just in case. Maybe I'll even do you one better and leave a place holder for a potentiometer. I think I have a few extra 100K SMD pots lying around.

 

[vne147]

Still haven't heard from Audioguru or anyone on the amp stage of the circuit. I'm unsure about the Rx and Cx components. This data sheet says they are for "upper frequency cutoff" but I don't what frequency is appropriate. I assume that I would not want the amp to cutoff below any audile frequency but I'm not sure. The data sheet instructs you to calculate values for Rx and Cx using:

Rx = 20*R2
Cx = 1 /(2*Pi*B*R1)

I'm guessing that B is the cutoff frequency in Hz? Also, the application example in the datasheet shows those components in dashes. That indicates to me that they are optional but once again I don't know.

I'd like to get moving on designing the board. I guess for now I can make the board using those components and just not include them later if someone tells me they are unnecessary.

Thoughts anyone?

 

[audioguru]

I think the ST Micro datasheet is a bad translation from their native Italian.
I don't know what is "B".

 

[vne147]

I think the ST Micro datasheet is a bad translation from their native Italian.
I don't know what is "B".

The table on page 4 shows B to be:

B
Frequency response (-3 dB)
Po = 1W RL = 4W
40 to 15,000 Hz

Maybe that's a different B? If it's the right B, 40 - 15K is a big range.

Do you think it would be OK to just not include those components?

 

[audioguru]

The TDA2003 is pretty old. The newer TDA2004A and TDA2005 don't use those components.

 

[vne147]

OK, I'll just design the board leaving space for those components but not plan to include them at this point. Thanks!

 

[vne147]

Been kind of busy with work but should have time to layout the board in the next few days. I had a question first. I'm planning to use a die cast aluminum emclosure for the finished product. Right now I'm thinking that I'll just connect the enclosure to the circuit ground. Does anybody see a problem with this? Thanks.

 

[audioguru]

Of course the metal enclosure should be connected to 0V as a shield.