9v Battery in 1w Power Amp (TDA7052) :: Voltage drop issue..

[chptunes]

To confirm (visually), this is what I'll be working on tonight.. omitting the 4.7k Input Resistor and adding a 1M Resistor from Pin 4 to Ground.  As Gurner says, this 1M Resistor on Pin 4 should yield full Gain.  I plan to control the output volume with pedals, but if the Punch amp is too hot at full gain I'll reduce this Resistor or replace it with a Pot.

More to come..  -Corey


image ru

[PRR]

> my 9v battery is only sending 1.9v to Pin 1 of the TDA7052.  Disconnected, my battery measures 8.9v..

My first thought is: dead battery.

8.9V is low, 9.3V is typical for a fresh 9V.

A severe drop on heavy load is also typical for a weak batt, even though it may come up to near full voltage after a rest.

[chptunes]

> my 9v battery is only sending 1.9v to Pin 1 of the TDA7052.  Disconnected, my battery measures 8.9v..

My first thought is: dead battery.

8.9V is low, 9.3V is typical for a fresh 9V.

A severe drop on heavy load is also typical for a weak batt, even though it may come up to near full voltage after a rest.

Thanks for that thought PRR.. I'll crack open a fresh one for tonight's endeavor.  Also, I use Carbon Zincs mostly but maybe I should use an Alkaline for this little Power amp.?..

I'm gonna measure the current draw, after she's working properly.

-Corey

[Gurner]

A couple of points....

The 1M isn't necessary on pin 4 (that 1M will set it to full gain, but essentially without the resistor it's set at full gain!)

Unless you need the extra power, consider using two speakers in series totalling 16 Ohms (longer battery life)

Only use alkaline batteries.

I'd imagine the current draw of that circuit is somewhere between 25 to 40mA (no signal)

[PRR]

> maybe I should use an Alkaline for this little Power amp.?..

The best Alkies you can get.

> I'd imagine the current draw of that circuit is somewhere between 25 to 40mA (no signal)

No signal, perhaps. (The sheet says less?)

That's not a heavy strain on a healthy 9V batt.

But he wants to PLAY it.

My estimate of the Full Power demand at 9V 8r is nearly 0.5A 500mA. That's a HEAVY load for any pocket 9V battery.

The good old LM386 *was* designed for good performance and long life on 9V batts. Use a 16 ohm load, expect 0.3W-0.4W out. True, that's for generic carbon batts. You can do better with the best bunny-batts. But 1W must be the upper end of what may be reasonable for a pocket 9V batt.

FWIW: all the specs are given for _6V_ supply, and that gives the whopping 1.2 Watts. 9V scales to 2.7 Watts (unless overheating causes shut-down), an incredible amount of power for the small 9V.

BTW: D-cells are far more energy/dollar than 9V. Four Ds would gig for days. Even AA cells are better buys than the 9V. The 9V is about convenience, decent radio gain, modest power without OTs or BTLs, and convenience. But if you can change chords, you can change four cells.

[Gurner]

> I'd imagine the current draw of that circuit is somewhere between 25 to 40mA (no signal)

No signal, perhaps. (The sheet says less?)

The sheet says 7 to 12mA but that's at 6V....but that's with an infinite load!  (also that quiescent figure is likely with 0V on pin 4 the volume pin.... however it's being run at full gain above) ...then add in the FET current draw  ...so I reckon 25mA in total for the whole circuit, which I agree isn't gonna break the battery out in a sweat, but as soon as he plucks a string it will!

As you say, 'D' cells are the way to go here.

[Gurner]

duplicate post - self deleted

[chptunes]

First of all.. ..IT LIVES!!

The revised schematic that I posted above, with only my Esquire plugged in and a 12" Weber speaker, produces a stronger-than-conversation-level output.  Loud enough that I don't hear the plucked strings.  The tone is sweet too.. not steril and boring.. definately has an interesting quality in the sound.  But, it is very saturated in this wide-open mode.. and the bass response is smeared and squishy.

I put a 470k Resistor from Pin 4 to Ground.  The output level is still comparable to before, but the response is more clear and solid across the frequencies.  Nice.

Next, I measured the current draw of the circuit with only a guitar lead on the input.. 23mA.  I wonder how many milliamps I could save by using a 16 Ohm speaker.  And, I wonder if the tone would be noticeably darker and more compressed with an impedance mismatch.

Thanks for the loads of help guys.. tca creates some great designs.. so far, I've tried his Mu-Valve and Punch amp and I plan to try the TRANFET overdrive circuit next.

Mu-Valve

TRANSFET

-Corey

[PRR]

> how many milliamps I could save by using a 16 Ohm speaker.

No signal, nothing. (A few mA because it's never NO signal, and there's some DC offset and current in the speaker.)

FULL power, 16 ohms is about half the current drain, but also near half the Watts output.

You should monitor the power voltage when you strum hard. If it sags a lot it's gonna be squishy.

One thing about going to 6V: the typical FET will not be at its best.

_My_ leaning for battery systems around 1 Watt would be single-end cap-coupled output (LM386 LM380 topology). That seems a better fit. However it seems that a lot of available LM386 are real cheezy, aimed at modem beepers etc, and not clean either soft or loud. LM380 may be worth a try because AFAIK they have not been common in decades, and the early ones were built well.

[chptunes]

> how many milliamps I could save by using a 16 Ohm speaker.

No signal, nothing. (A few mA because it's never NO signal, and there's some DC offset and current in the speaker.)

FULL power, 16 ohms is about half the current drain, but also near half the Watts output.

You should monitor the power voltage when you strum hard. If it sags a lot it's gonna be squishy.

Very cool.. a worthy experiment.  I'll need to check for voltage sag.. but, of all things, I just wanted to play guitar through the amp last night.  I know, I know.. insanity!   

Seriously, I do want to save a few milliamps so that the battery option stays viable.  One idea:  What if I wire an 8 Ohm Resistor (rated for 2w or more) in series with the Output.. so that I have a constant 8 Ohm load plus whatever load that I connect.  I could even put this Resistor on a switch.. a "Power Conservation" switch, no?..

Thoughts?

Link:  3w Resistor at Mouser

_My_ leaning for battery systems around 1 Watt would be single-end cap-coupled output (LM386 LM380 topology). That seems a better fit. However it seems that a lot of available LM386 are real cheezy, aimed at modem beepers etc, and not clean either soft or loud. LM380 may be worth a try because AFAIK they have not been common in decades, and the early ones were built well.

I've read up on the LM386 circuits a bunch.. but, tca's TDA7052 Punch amp appealed to me much more.  This amp just provides clean power, and handles my AMZ Tweed Stack Preamp really well.

-Corey

[chptunes]

Numbers!

At idle, my Alkaline is sending 8.6v into Pin 1.. which still sounds nice and clean with my [single-coiled] Esquire.

A crashing chord sags the Voltage to 8.3v.  Clicking on my Tweed Stack Preamp (with Gain and Volume set near 50%) sags the Voltage to 8.0v.  A 600mV swing, eh.

I'm not sure how these Voltage numbers compare to what someone would expect from this type of Power Amp circuit.

-Corey

[tca]

LM380 may be worth a try because AFAIK they have not been common in decades, and the early ones were built well.

It is funny you mention the LM380. Just found this today:
- http://sonic.net/mjones/paia/products/1750.html
- http://home.comcast.net/~synthdoc/P/1750.pdf

Have to build it!

Cheers.

[PRR]

10% sag (8.6V-7.8V) is actually quite small.

Power amps often sag 15%-20%.

I have seen 12V at idle sag/crash to 3V when worked hard. It did not sound happy.

[chptunes]

10% sag (8.6V-7.8V) is actually quite small.

Power amps often sag 15%-20%.

I have seen 12V at idle sag/crash to 3V when worked hard. It did not sound happy.

Thanks for the insight PRR.  I'll be setting up to test the tone and current draw with a 16 Ohm load.

Numbers soon!!