Building the Tiny Giant amp

[bluebunny]

Well, I finally got around to building this over the weekend.  Nice board, Taylor.

I have to say it's very quiet.  Very quiet indeed.  Like, totally silent! 



So here goes with the debug...  I'm getting a healthy 11.69V out of the LM338.  This is being delivered correctly to the TL072 and TDA7240A, and both are grounded OK.  There's no tricky continuity between the LM338 tab and ground, nor to either speaker connection.  The volume control does indeed sweep to ground in one direction, and away from it in the other.  The standby switch does indeed do its job grounding pin 2.  So far, so good.

The next bit seems a little off, though.  Whilst the unused half of the TL072 is producing half the positive rail (5.84V) as you might expect, the voltage divider made up of the two 1M resistors is not.  They have 3.9V between them (which is then the bias going into pin 5 of the TL072).  I would have put money on closer to 5.84V.  Anyway, looks like the TL072 is working, despite the strange bias.  But the TDA7240A is perhaps looking suspect?   

Apart from power and ground at pins 6 and 4 (and 2 when the mute switch is thrown), I'm getting 0V at pins 5 and 7 (the speaker outputs) and 0V at pin 3 (the input).  Pins 1 and 2 (with the mute open) look like they're discharging slowly through their capacitors to ground.

Other than that, there's no smoke, no heat, no emergency services on their way.  Any thoughts?

I guess the next step for me is to see if and where there's any signal...   

[Jdansti]

Shall we assume that you've checked that your guitar's volume is turned up and you have a good cable?


You ready know the stuff I'm about to say, but sometimes it helps to review it:

-Check the signal and ground connections on the input jack
-Check cap values and polarities
-Check resistor values
-Check that TL072 isn't backwards
-Check for cold solder joints and bridges
-Check that the speaker works

Also, shouldn't you be getting around 14V from the LM338 instead of 12V given the resistor values selected by Taylor?  The amp would still run on 12V, but I wonder if this is pointing to the problem.

[bluebunny]

Ha ha!  Yes John, guitar turned up...   

OK, nice jangly guitar getting all the way to pin 3 of the TDA7240A (i.e. the input).  Absolutely nothing at the output pins (tried the audio probe across the speaker jack).  Speaker is working.  All component values and polarities checked.

Have I scored a bum 7240 off eBay??

[bluebunny]

P.S. The one good thing to come out of this is that the audio probe I built a few weeks ago works like a dream!   

[bluebunny]

Also, shouldn't you be getting around 14V from the LM338 instead of 12V given the resistor values selected by Taylor?  The amp would still run on 12V, but I wonder if this is pointing to the problem.

Taylor's build docs say 11.6V, so I was quite chuffed with my 11.69V!   

[Jdansti]

Sorry!     Missed that and the paragraph about changing the resistors to get more power. I just saw the note on the board photo about the 14V take off for other external things.

I guess it's possible you got a bad amp chip. One option would be to carefully remove the 7240 from the PCB, breadboard it with the minimum components shown on the data sheet, and see what happens.

[PRR]

> voltage divider made up of the two 1M resistors is not.  They have 3.9V

A 1Meg||1Meg= 500K divider is a HIGH impedance. What kind of meter do you use?

A good VTVM or typical DMM is usually 10Meg input, which will cause a 5% loading-error on a 500K source.

Needle-meters, and some specialty DMMs, have lower input impedances.

In either case, in this circuit, poking the output of the opamp should give the true divider voltage (since the amp is wired for unity-gain at DC, and the opamp output is plenty strong).

Futher obvious check: the output of a POWER stage has to be halfway between the supply rails. With 12V supply you really want to find 5V to 7V at the output. This design uses a Bridged output, really two outputs; both should be very similar to each other and awful close to 6V.

This chip has a feature: if an output is shorted to ground (got the car-speaker wire pinched in bodywork) both chanels will shut-down nicely. (Older designs would spark the wire until something burned.)

> I'm getting 0V at pins 5 and 7 (the speaker outputs)

Disconnect the PCB from your wiring. Now what's the volts? Study the PCB for blobs. Temporarily unsolder the 0.1u(?) output loading caps (in case they shorted). Assuming a good chip, it's got to be a short on an output.

I hope you noticed that the speaker jack may NOT be grounded (no metal jack; or fiber washers).

> Have I scored a bum 7240 off eBay??

Odds may be 50:50. Is there a reputable supplier?

[bluebunny]

> voltage divider made up of the two 1M resistors is not.  They have 3.9V

A 1Meg||1Meg= 500K divider is a HIGH impedance. What kind of meter do you use?

A good VTVM or typical DMM is usually 10Meg input, which will cause a 5% loading-error on a 500K source.

I'm using a DMM, nothing special.  I'm less bothered by this reading now (though still interested why it's apparently "low"), since I'm getting a good signal either side of the TL072 buffer (and the output is pretty much mid-rail).

Futher obvious check: the output of a POWER stage has to be halfway between the supply rails. With 12V supply you really want to find 5V to 7V at the output. This design uses a Bridged output, really two outputs; both should be very similar to each other and awful close to 6V.

This chip has a feature: if an output is shorted to ground (got the car-speaker wire pinched in bodywork) both chanels will shut-down nicely. (Older designs would spark the wire until something burned.)

> I'm getting 0V at pins 5 and 7 (the speaker outputs)

Disconnect the PCB from your wiring. Now what's the volts? Study the PCB for blobs. Temporarily unsolder the 0.1u(?) output loading caps (in case they shorted). Assuming a good chip, it's got to be a short on an output.

Hmmm... yeah, both outputs are "zero" wrt ground, but apparently have no continuity to ground.  I'll recheck carefully when I get home.  PCB work looks neat enough, but it doesn't hurt to take another close look.

I hope you noticed that the speaker jack may NOT be grounded (no metal jack; or fiber washers).

Yep!     Speaker jack connected to nothing but the output pins.

> Have I scored a bum 7240 off eBay??

Odds may be 50:50. Is there a reputable supplier?

I may find out soon.  Found another from a local vendor this time.  Will try a substitution when that arrives.

Thanks for your input, Paul.  John too.  Will get this sorted: it's too neat a little project to let go.

[bluebunny]

> Have I scored a bum 7240 off eBay??

Odds may be 50:50. Is there a reputable supplier?

Looks like I found one of each!  I swapped out what turned out to be a rogue amp chip.  The new one works like a dream - sensible voltages, gets warm, and lots of nice, loud guitar coming out of it.  Just tried it with my BSIAB2 and most likely upset the nieghbours.  

Thanks for your help, guys.  This is a neat project - thanks, Taylor.  Sounds great.  Everyone should build one!  

[Jdansti]

Glad you got it working!

[bluebunny]

Glad you got it working!

Me too!  Thanks John.   

[Taylor]

Very exciting when it finally fires up and gets so much louder than you're expecting, eh? 

[bluebunny]

Very exciting when it finally fires up and gets so much louder than you're expecting, eh? 

Oh yes.   

[MDK002]

Hi,  just powered up my tiny giant today and i'm having an issue.  When I turn it on, there is a rhythmic popping sound.  When I play something on the guitar, the sound goes away mostly. 
Also, when I turn the volume up more than about 2/3 the sound really breaks up and distorts to the point it's horrible.
I've checked for grounding issues.  No problem there that I can find.
power supply is rated 16V -- 4.5A
10K pot
built it exactly as noted on the build sheet.
Where should I start looking for problems?

thanks,

[MDK002]

a little more info on the popping sound.   with the volume turned all the way down....popping is faster.    turn the volume to 3/4 and it just about stops...  turn up all the way and it pops a little faster.   the popping sound is pretty much the same volume all the time.   now when i play the guitar it just keeps popping.

[MDK002]

Also, the standby switch has no effect on the amp...sound comes out in either on or off position

[Jdansti]

Can you provide voltages on all of the pins of the ICs and regulator? Place your black probe on a ground point and use the red probe to check the components.

[MDK002]

Hi John...thanks for helping
TL072
pin 1     5.84
pin 2     5.85
pin 3    5.57
pin 4   0
pin 5   5.57
pin 6    5.84
pin 7   5.85
pin 8   11.75

LM338
16.5 v in
11.81 v out
10.56  adj

TDA7240
pin 1  9.3
pin 2  11.9
pin 3  9.06
pin 4    0
pin 5  8.17
pin 6   0
pin 7   8.70

[Jdansti]

^ Pin 6 of the TDA7240 is supposed to have power on it and you're showing 0v.

Are you using the board from MusicPCB, or some other board or perf?

[MDK002]

I'm using the board and components from Music PCB.