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Building a class A audio amplifier - no audio out

Started by Unknown May 11, 2009
I tried to build this Class A amplifier circuit as a learning
experience:
http://www.techlib.com/electronics/audioamps.html#class-a

with the following modifications:
1.  I have no MPSW45, so I built a Darlington out of a 2N2222 and
TIP31A.
2.  150k resistors: I only had two 100k resistors in my collection.
But as long as they're identical, should work...???
3.  speaker:  I had only a 10W, 4 ohm car speaker readily available.

Powered up with a 6V battery, input was the headphone out from a
portable radio.  Nothing except for an impressive Thump when I
connected power to the circuit.

What could have gone wrong?

I'm not sure if I would have 200mV p-p on the radio headphone out.
Would that be it?

Thanks,

Michael
On Sun, 10 May 2009 23:47:30 -0700, mrdarrett wrote:

> I tried to build this Class A amplifier circuit as a learning > experience: > http://www.techlib.com/electronics/audioamps.html#class-a > > with the following modifications: > 1. I have no MPSW45, so I built a Darlington out of a 2N2222 and > TIP31A. > 2. 150k resistors: I only had two 100k resistors in my collection. > But as long as they're identical, should work...??? > 3. speaker: I had only a 10W, 4 ohm car speaker readily available. > > Powered up with a 6V battery, input was the headphone out from a > portable radio. Nothing except for an impressive Thump when I > connected power to the circuit. > > What could have gone wrong?
You chose a crap circuit. Analogue isn't exactly my strong point, but the circuit appears to be relying upon the MPSW45's relatively low hfe (25) to draw enough base current through the 150K resistor to avoid saturation. The 2N2222/TIP31A pair has a combined gain of 2500, so the transistors are probably saturated. A more realistic circuit would use an emitter-follower resistor to stabilise the DC operating point, bypassed with a capacitor so that you still get AC voltage gain.

mrdarrett@gmail.com wrote:

> I tried to build this Class A amplifier circuit as a learning > experience: > http://www.techlib.com/electronics/audioamps.html#class-a
Why why why ? It puts DC through the speaker is is horribly badly biased. It's a JOKE. Find some decent websites. Graham
Eeyore wrote:
> > mrdarrett@gmail.com wrote: > >> I tried to build this Class A amplifier circuit as a learning >> experience: >> http://www.techlib.com/electronics/audioamps.html#class-a > > Why why why ? > > It puts DC through the speaker is is horribly badly biased. It's a JOKE. >
And how is a beginner supposed to know? Do you have any suggestions for a site to look at, or a book? -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
 On May 11, 7:32=A0am, Tim Wescott <t...@seemywebsite.com> wrote:
 > Eeyore wrote:
 <snip>
 > > Why why why ?
 >
 > > It puts DC through the speaker is is horribly badly biased. It's
a JOKE.
 >
 > And how is a beginner supposed to know?
 >
 > Do you have any suggestions for a site to look at, or a book?
 >
 > --
 >
 > Tim Wescott

Lots of good stuff here

http://sound.westhost.com/

G=B2
On May 11, 8:16=A0am, stratu...@yahoo.com wrote:
> =A0On May 11, 7:32=A0am, Tim Wescott <t...@seemywebsite.com> wrote:=A0> E=
eyore wrote:
> > =A0<snip> > =A0> > Why why why ? > =A0> > =A0> > It puts DC through the speaker is is horribly badly biased. It's > a JOKE. > =A0> > =A0> And how is a beginner supposed to know? > =A0> > =A0> Do you have any suggestions for a site to look at, or a book? > =A0> > =A0> -- > =A0> > =A0> Tim Wescott > > Lots of good stuff here > > http://sound.westhost.com/ > > G=B2
Ah, thank you for the reply. One thing I noticed is, the larger amps on westhost.com (10W+) need split power supplies (+/-). I'd like to start with something needing just 0V-6V or 0V-12V. Thanks again, Michael
mrdarrett@gmail.com wrote:
> On May 11, 8:16 am, stratu...@yahoo.com wrote: >> On May 11, 7:32 am, Tim Wescott <t...@seemywebsite.com> wrote: > Eeyore wrote: >> >> <snip> >> > > Why why why ? >> > >> > > It puts DC through the speaker is is horribly badly biased. It's >> a JOKE. >> > >> > And how is a beginner supposed to know? >> > >> > Do you have any suggestions for a site to look at, or a book? >> > >> > -- >> > >> > Tim Wescott >> >> Lots of good stuff here >> >> http://sound.westhost.com/ >> >> G&#2013266098; > > > Ah, thank you for the reply. > > One thing I noticed is, the larger amps on westhost.com (10W+) need > split power supplies (+/-).
That's because if you want high power you need high voltage swing, which you get either with a pair of amplifiers in a bridge configuration, with a split supply, or with a honking big electrolytic capacitor. That capacitor has to be able to flow all the current from that high-power audio through an 8-ohm speaker while not interfering with the sound -- it's going to be big and expensive.
> I'd like to start with something needing > just 0V-6V or 0V-12V.
So consider an amp in a bridge configuration, but consider also all the extra work you'll do to build two (fairly complex) amps instead of one fairly complex amp and one dirt-simple power supply. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
On May 11, 8:40=A0am, Tim Wescott <t...@seemywebsite.com> wrote:
> mrdarr...@gmail.com wrote: > > On May 11, 8:16 am, stratu...@yahoo.com wrote: > >> =A0On May 11, 7:32 am, Tim Wescott <t...@seemywebsite.com> wrote: > Ee=
yore wrote:
> > >> =A0<snip> > >> =A0> > Why why why ? > > >> =A0> > It puts DC through the speaker is is horribly badly biased. It'=
s
> >> a JOKE. > > >> =A0> And how is a beginner supposed to know? > > >> =A0> Do you have any suggestions for a site to look at, or a book? > > >> =A0> -- > > >> =A0> Tim Wescott > > >> Lots of good stuff here > > >>http://sound.westhost.com/ > > >> G=B2 > > > Ah, thank you for the reply. > > > One thing I noticed is, the larger amps on westhost.com (10W+) need > > split power supplies (+/-). > > That's because if you want high power you need high voltage swing, which > you get either with a pair of amplifiers in a bridge configuration, with > a split supply, or with a honking big electrolytic capacitor. =A0That > capacitor has to be able to flow all the current from that high-power > audio through an 8-ohm speaker while not interfering with the sound -- > it's going to be big and expensive. > > > =A0I'd like to start with something needing > > just 0V-6V or 0V-12V. > > So consider an amp in a bridge configuration, but consider also all the > extra work you'll do to build two (fairly complex) amps instead of one > fairly complex amp and one dirt-simple power supply.
Ah. I'd neglected to say that a 1W amp would be fine for learning purposes. If I just wanted power I'd go for an LM3886 or just buy an Onkyo system. I'd like to understand how the amps work. Any comments on this circuit as a beginner project? http://www.redcircuits.com/Page33.htm Thanks, Michael
mrdarrett@gmail.com wrote:
> > Ah. I'd neglected to say that a 1W amp would be fine for learning > purposes. > > If I just wanted power I'd go for an LM3886 or just buy an Onkyo > system. I'd like to understand how the amps work. > > Any comments on this circuit as a beginner project? > http://www.redcircuits.com/Page33.htm >
Looks ok to me. You have to pay heed to the instructions, especially about adjusting the quiescent (no-signal) current. I'd recommend mounting the two output transistors Q3 and Q4 on a small heatsink, making sure that they are electrically isolated from each other and from ground and other components. The two transistors can theoretically dissipate nearly 1W each with a 4-ohm load. That may seem small, but it's enough to get a bare TO220 transistor quite hot. One 0.47-ohm 1/2-watt resistor in series with the emitter of each output transistor will aid stability. Q2 dissipates about 0.25W which will also cause the small transistor to run quite hot to the touch. I suggest using a BD135-16 or a BD137-16 instead of the BC337. Choosing a sub-type with the -16 suffix ensures that it will have about the same gain level as the BC337.
On May 11, 11:32=A0am, "pimpom" <pim...@invalid.invalid> wrote:
> mrdarr...@gmail.com wrote: > > > Ah. =A0I'd neglected to say that a 1W amp would be fine for learning > > purposes. > > > If I just wanted power I'd go for an LM3886 or just buy an Onkyo > > system. =A0I'd like to understand how the amps work. > > > Any comments on this circuit as a beginner project? > >http://www.redcircuits.com/Page33.htm > > Looks ok to me. You have to pay heed to the instructions, especially abou=
t
> adjusting the quiescent (no-signal) current. > > I'd recommend mounting the two output transistors Q3 and Q4 on a small > heatsink, making sure that they are electrically isolated from each other > and from ground and other components. The two transistors can theoretical=
ly
> dissipate nearly 1W each with a 4-ohm load. That may seem small, but it's > enough to get a bare TO220 transistor quite hot. > > One 0.47-ohm 1/2-watt resistor in series with the emitter of each output > transistor will aid stability. > > Q2 dissipates about 0.25W which will also cause the small transistor to r=
un
> quite hot to the touch. I suggest using a BD135-16 or a BD137-16 instead =
of
> the BC337. Choosing a sub-type with the -16 suffix ensures that it will h=
ave
> about the same gain level as the BC337.
Ah, thank you very much. I'll heatsink the transistors, and thanks for the tip about emitter resistors. Michael