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Mini amp circuit

Posted: Mon Jul 20, 2009 6:49 pm
by CeaSaR
Looking for more quick little projects and found this: ... amplifier/
Ran a sim and I think the guy switched R1 and R2. As described, with a 50 mV
input, the output is approx. 5 mW with a chopped sine (on the neg. swing).
Swap R1 and R2 and drop the input to 5 mV and you get just about 625 mW
with a nice sine wave, albeit the swing is only from 622 mV to 626 mV. I just
don't see how this will drive a small speaker without wasting energy as a DC

Any comments on this circuit?


Quick reference - latest schematic on page 6, all the way at the bottom.

Re: Mini amp circuit

Posted: Mon Jul 20, 2009 8:12 pm
by Robert Reed
How did you calculate a 625 mw output when the p-p ac across the speaker is only 4 mv?
Without reaching for my calc, I get a 1/4 mw ac output approx. and a 'bonus' of .65 watt of battery sagging useless disapation for a grand total of 0.0004% efficiency.

Re: Mini amp circuit

Posted: Mon Jul 20, 2009 9:16 pm
by CeaSaR
I did a sim in CircuitMaker and when I clicked on the speaker it gave me the power
across it. I was rather suspect of the loss there and decided to post here. Like I
said, there seems to be a "bit" :roll: of DC wasted in this tiny little amp. Maybe I'll
just breadboard it and see what happens.

According to another guy who did an instructable there, he used 2 of these for a
stereo mini-amp. ... -amplifier
Unfortunately, his schematic is "slightly erroneous". Backwards power connections, OOPS.

Tomorrow's another day. Thanks Robert.


Re: Mini amp circuit

Posted: Tue Jul 21, 2009 7:30 am
by Bob Scott
CeaSaR wrote:Any comments on this circuit?
Ceasar, this might have been an acceptable beginner experimenter project 40 years ago, but you can make a much better amp today with the LM386 IC that does not suffer from these drawbacks:

The discrete circuit shown has no negative feedback. Expect heavy audio distortion, and expect an unreliable and unpredicatable DC voltage level at the output that drifts with transistor temperature.
The wasted DC current and power is to be expected in a class A amplifier (one output device driving a resistor/speaker coil.)

The LM386 has been around for years, is class AB so it won't waste as much power, works down to 4 volts, implements feedback for low distortion, and is easier to put together with less parts than that inferior "hacker level" discrete amp. Check the sample circuits in the datasheet:

If you poke around at the and websites you will find even better audio ICs developed more recently, although they may only be available in surface mount. I don't know if you just want to keep it simple, but if you're concerned with efficiency, consider class D. There is theoretically no power wasted in a class D amp. All the power goes to the load.


Re: Mini amp circuit

Posted: Wed Jul 22, 2009 4:51 am
by Dave Dixon
I built a cool 80mW stereo amp from a free sample I obtained from Maxim of their MAX4410. Biggest parts on the board were the 2 audio jacks! PCB I made was 1.8" X 0.7".
Also required: 8 surface mount caps, 4 resistors. Got a schematic and layout in EagleCad if you'd like me to send them to you.

From the datasheet...
The MAX4410 stereo headphone driver is designed for portable equipment where board space is at a premium. The MAX4410 uses a unique, patented, DirectDrive architecture to produce a ground-referenced output from a single supply, eliminating the need for large DC-blocking capacitors, saving cost, board space, and component height.

The MAX4410 delivers up to 80mW per channel into a 16Ω load and has low 0.003% THD+N. A high power-supply rejection ratio (90dB at 1kHz) allows this device to operate from noisy digital supplies without an additional linear regulator. The MAX4410 includes ±8kV ESD protection on the headphone outputs. Comprehensive click-and-pop circuitry suppresses audible clicks and pops on startup and shutdown. Independent left/right, low-power shutdown controls make it possible to optimize power savings in mixed mode, mono/stereo applications.

The MAX4410 operates from a single 1.8V to 3.6V supply, consumes only 7mA of supply current, has short-circuit and thermal overload protection, and is specified over the extended -40°C to +85°C temperature range. The MAX4410 is available in a tiny (2mm x 2mm x 0.6mm), 16-bump chip-scale package (UCSP™) and a 14-pin TSSOP package.

No Bulky DC-Blocking Capacitors Required
Ground-Referenced Outputs Eliminate DC-Bias Voltages on Headphone Ground Pin
No Degradation of Low-Frequency Response Due to Output Capacitors
80mW Per Channel into 16Ω
Low 0.003% THD + N
High PSRR (90dB at 1kHz)
Integrated Click-and-Pop Suppression
1.8V to 3.6V Single-Supply Operation
Low Quiescent Current
Independent Left/Right, Low-Power Shutdown Controls
Short-Circuit and Thermal Overload Protection
±8kV ESD-Protected Amplifier Outputs
Available in Space-Saving Packages
16-Bump UCSP (2mm x 2mm x 0.6mm)
14-Pin TSSOP

Re: Mini amp circuit

Posted: Tue Aug 11, 2009 7:31 am
by CeaSaR
Ok, I finally threw this little thing together and:

1. It does work, but at 3v it is pretty anemic.
2. Boosting voltage does increase volume, but the DC component is visible in the speaker.
3. An ouput DC blocking cap blocks all output (that a human can hear, anyway).
4. Yes, fidelity is not a strong point of this circuit.
5. Input level increase without serious distortion is only tolerated with increased power supply
voltage (see #2 above).

All in all, a fun little thing to play with, but not something to keep for everyday use. If I were
(and no I am not) to keep this, I would want to use at least 6V and maybe run it off a DC wall
wart so as not to burn through the batteries. I'd also have to use heavier duty speakers than
called for in the original schematic, although I'd be skeptical as to their longevity due to the
constant DC present.

Still on the prowl for discrete component low power (1-10W), low parts count amps at this point.
Will check in later.


Re: Mini amp circuit

Posted: Wed Aug 12, 2009 4:43 am
by Bob Scott
CeaSaR wrote:Still on the prowl for discrete component low power (1-10W), low parts count amps at this point. Will check in later.
Hi Charles.
That is a tall order. Why are you insistent on discrete semiconductors when op-amp IC's are simpler to use? With op-amps you just add power and feedback to control the gain. There is no problem with biasing because biasing is built in with sophisticated multi-transistor constant current devices.

With discrete transistors you need 3 or 4 resistors for reliable DC biasing per stage, and that gets you a power hungry class A amp that is sensitive to PS noise.

The schematic of the mini-amp that you posted has no feedback. There is no guarantee that a circuit with the simple bias of one base resistor and no emitter resistor will actually work at all. It may only work some of the time but maybe not with the next batch of transistors with the same part number, or if the temperature drifts. It is guaranteed to distort. I think that requiring BOTH "low parts count" AND "discrete" are counter to the performance and efficiency that I understand that you are expecting.


Re: Mini amp circuit

Posted: Wed Aug 12, 2009 5:07 am
by Dave Dixon

Just thought I'd share!

Re: Mini amp circuit

Posted: Wed Aug 12, 2009 8:09 pm
by CeaSaR

I insist for I am really Don Quixote and it is my destiny to do so!

Actually, in all my years of putzing around with electronics I have never built a discrete audio amp
from scratch and would like to do that before going forward with such modern things as IC's. That,
and my current situation precludes purchasing of lots of new stuff (even the cheap bits at "The Shack").

Back in my formative youth the most exotic thing you could buy was a 555. There weren't any Amp chips
then that you could readily buy. Most DIYer's built with discrete parts and that is something I want to do
before I lose that chance again.

Perusing the latest issue of N&V, there are 3 articles based on old style technology, the Lightning
Screen, the Nixie Neon Clock and the Testmaster Quiz Box. Last month had the discrete Digital Clock.
Why did the authors build them that way when they could have used more up to date parts (I'll leave
the Lightning Screen out of this question)? Because they wanted to do it that way. And a few issues ago
a guy built a water alarm with older parts that fit his criteria better than most anything new.

Of course N&V has alot of newer tech in there, for if we do not embrace the newer stuff, we will stagnate.
I just want to brush the moss off of this old wish and do it so I can move on. As for schematics, I have found
a few that actually look promising, if only I could remember where I found them. Praise be for "History".

As for the original Mini-Amp, I stated above that I would not be keeping it and it will be off my breadboard
tomorrow. I do appreciate all the help and comments you and everyone else have given to my posts.


From the looks of the SMT parts and previous posts (somewhere, I'm not sure where) that looks like the amp
someone described as (paraprhase) "the biggest thing on the board is the jacks." Can't remember who or where
I saw it. Any description for it?

(EDIT: Duh! that was your description above.)

Thanks again guys,


Re: Mini amp circuit

Posted: Thu Aug 13, 2009 4:42 am
by Dave Dixon
I can totally relate.... I have a nixie clock on my bench here at work!!! Something to be said for doing it the "old fashion" way and blowing out the old cobwebs. I did PM you the board and schematic jpg's of that new fangled one that I had fun building... I had most of the components laying around, and the chip was a free sample. I cut the PCB from left-over material, so total cost was next to nothing, and I sold it on Fleabay for $25 when I got tired of playing with it! Hope you get the attachments on your PM okay.

Re: Mini amp circuit

Posted: Tue Sep 08, 2009 6:40 pm
by CeaSaR
Hey guys, I've been messing around with this one and have noticed that:

1. The input is very sensitive and I would like to be able to push more voltage
(approx. 100-200 mV) into it without overdriving it ito serious distortion. Any
suggestions? (A 50 kohm VR inline before C1 will allow input from 10 mW - 150 mW)

2. As depicted, CircuitMaker says that the output is ~ 570 mW at an input of
10 mV at 50 HZ. When I raise the frequency up the scale, the output really drops
off - 1 kHZ=101 mW, 5kHZ=13 mW, 10 kHZ=7 mW. How can I help flatten out the

3. Power output is taken as a reading when I click on the speaker. clicking on the
wire between C3 and the speaker shows a voltage swing of 6v +- p-p, so I should
be able to get ~3 W out of it at 50 HZ. The 8 ohm speaker eats up ~2.5 W?
Electrical power vs. acoustic power?

Original schematic - latest schematic on page 6, all the way at the bottom.

'Nuff for now,


Re: Mini amp circuit

Posted: Tue Sep 08, 2009 7:39 pm
by Robert Reed
Hello Caesar
C5 looks like a probable suspect for poor freq. response. Should be much lower in value, if used at all. Also R8,R10 seem to be too high in value- maybe 1/10 or even less of what is stated. Are you sure you copied those values right?
A volume control of 5-10 K should allow you to input any level desired. Better yet you could add a fixed attenuator at that point and in conjunction with the control wired as a hi-lo switch.

Re: Mini amp circuit

Posted: Tue Sep 08, 2009 8:52 pm
by CeaSaR
Here's the link to the original schematic:

I was playing around with the components and that's what I came up
with based on the original. I'll check it tomorrow.


*Rick's site is no longer available. Here is another link to his schematic:
Rick Andersen's amp

**I have found Rick's new site. The amp can be found here.

Re: Mini amp circuit

Posted: Wed Sep 09, 2009 8:11 am
by Robert Reed
Well, that print makes a little more sense as far as load shunt cap values and output biasing goes.
The 39xx's have a 300 mw max. rating and the 44xx's somewhat higher. other than that their specs are very similar. In either case, I would suppose you can get a couple hundred mw of non hi-fi output and thats ample power for close range.

Re: Mini amp circuit

Posted: Wed Sep 09, 2009 9:58 am
by Bob Scott
CeaSaR wrote:Here's the link to the original schematic:
In this diagram, on the collector terminals of Q1 and Q2, notice the parallel capacitor and resistor. There is a frequency where the gain of the high end starts to climb at 6dB/octave. This frequency where the treble starts its climb is where the capacitive reactance of the capacitor becomes equal to the value of the resistor, and can be calculated using the equation:

f=1/(2 * Pi * C * Xc)
where the value of the resistor is substituted for Xc, the capacitive reactance. They are both equal at the corner frequency.

The frequency calculates to ~3,400 Hz. Because there are TWO of these, the treble is boosted a the 12Db/octave. Why would you want so much treble boost? The HACKER (I can't be forced to call him a designer) probably wanted to compensate for the loss of treble due to the lack of any sort of negative feedback, and the frequency where the amp naturally starts to lose HF gain from transistor limitations is quite low. There is no limitation on the gain of these two stages other than the maximum gain at DC and the high frequency dropoff of each transistor. So the frequency that the transistor gain starts to fall down towards its zero gain at Ft (frequency where transistor current gain drops to zero dB) starts way below the frequency Ft, and the hacker is trying to compensate for the dropoff in high frequency gain by boosting the treble back up again.
Trouble is, every transistor's gain is different. Look at the data sheet for the 2N3904. The typical gain can vary ~two to one. If you get a near-flat frequency response out of this diagram, it will be a fluke.

The 2N3904 is probably not the original transistor used by this diagram. 2N3904's response is way up there, like 300 MHz. The REALLY original circuit probably had transistors with a much lower Ft.

These hacker schematics makes me cringe every time I start to analyse them. Negative feedback is not complicated at all. It is just like a resistor divider, just wired from the output (collector) to the input (base). At least, use an emitter resistor! The hackers have no idea what they are doing, not even a clue.