Mini amp circuit

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Robert Reed
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Re: Mini amp circuit

Post by Robert Reed » Sat Sep 12, 2009 8:20 am

Yea, there is so much hype on audiophile components that it boggles my mind - and the prices!!
From an engineering standpoint there is a certain amount of truth in what they say, as an 0.00000001% improvement will occur- at a cost of 100's or 1000's of dollars. From a human stand point its doubtful that any improvement can be discerned. What is hi-fi reproduction? A seemingly distorted sound may well be what the recording artist intended even though it may be annoying to the listeners ears. And the best of quality sound reproduction will suffer if not played back in the right environment (room acoustics,etc.). I have always heard that the loudspeaker may have up to 7% distortion and the recording devices approaching that figure. Now of what value is that amp spec'ed at 0.005% harmonic distortion! I think as long as all equipment specs are moderate and reasonable and played back in a PROPER ENVIRONMENT, not much improvement could be made with incredible specs, monster cables,audio capacitors and on and on. What it really comes down to is the enjoyment and pleasure to the listeners ears, not some bar room bragging rights.
As to audio quality capacitors take a look at this site http://www.reliablecapacitors.com/pickcap.htm .
These are respected gentlemen in their feild, but I don't know if I would go to these great lengths in audio design.

Ceasar
I see a lot of concern for low end response in your postings. Maybe I am wrong, but I envision your design as a lo power amp driving a small, cheap, hi efficiency, low power speaker. Would such a speaker even be able to handle very low frequency audio even if the amp could?

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CeaSaR
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Re: Mini amp circuit

Post by CeaSaR » Sat Sep 12, 2009 9:08 pm

Bob,

By all means, take care of the little woman. She's much more important than some guy
who has a beginner question on a hobby. Hope you get it all done swiftly and safely
(and most importantly, in the dry!). When things are under control, I'd love a tutorial on
audio amplifiers - they have been my passion since before I learned what a transistor was.
The days of my early youth were spent looking over the old living room radio - it was really
a large piece of furniture! It had soooo many buttons to push on AM and several SW bands,
I think it was too early a manufacture to include FM. All tube and coils, paper 'lytics and a
big honkin' speaker, the NON permanent magnet (electromagnet) kind. I remember listening
to it when it worked, what magic! From there it was a slew of transistor radios up through
the hand-me-down stereos until I could afford to buy my own. Listening to music has been
a huge part of my life for 40+ years. I think maybe now would be a good time to actually
learn about audio amplifiers. I thank you for the kind offer.

Robert,

I agree on the fact that, yes, these things may bring an infintesimal improvement, but "we"
humans cannot really detect the difference. I always wondered about that old guy in ?Stereo
Review?, who is probably 80+ years old now, who said he could hear these differences at his
age. I wonder if he's had his hearing tested to see if his perceived range is actually there.
That magazine is way too subjective to give any real advice on components.

As for the low end response, what you are seeing in my posts is my infatuation with having a
flat frequency response out of the design regardless of maximum output capabilities. I guess
you could match the lower and upper limits of the amp to the speaker and make it more efficient.
A guy has to have his standards, right? Besides, when I go for a speaker, I don't want some cheesy
bit of paper staring me in the face. I prefer something with a bit of substance. Smallest I will go
is the replacement speakers I bought a few years back to install in my kitchen. The local Ollie's
had what looked like the faceplates off of some boomboxes that have ~2.5"x4" full range with a
tweeter. I tried them with an old amp I had and they sounded decent when placed in a rough
enclosure (cheap test boxes can be made of cardboard). They probably don't go below 100 HZ
but they'd be a good match for something low power like this. Most other speakers I have around
run from 30 - 60 HZ on up to 18-20k Hz (2 or 3 way). I have an old 6" AlNiCO magnet speaker complete
with whizzer cone that would be great in a low power guitar amp. I used to drive this one from
an old GE clock radio and it would be so loud in my office area that I could only turn it up when I
was alone, otherwise the boss would get p*ssed at me. That one kicks some tush!

Anyway, I stated earlier that I am not sure how to set up my copy of CircuitMaker to do such tests
and I am basing my opinions on what the voltage/power is at a particular test point and what the
waveform looks like. Once I find a max input on a design I'll change the frequencies at the input and
check the output voltage/power/waveform. It takes a while to run a full range, but I can change them
pretty quick and write them down.

Sofaspud,

I like that link and have bookmarked it. I already have ESP saved and have been going over his stuff too.
I like his El Cheapo. Those 2N3055's sure look good when mounted out in the open.

Thanks again guys,

CeaSaR
Hey, what do I know?

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sofaspud
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Re: Mini amp circuit

Post by sofaspud » Sat Sep 12, 2009 10:20 pm

With the volume control connected as a rheostat, I believe that changes the input impedance
with variations in volume, and the frequency response in action with C1. Better to connect
the pot across the input, with the wiper connected to C1.

I agree with what Robert said, as my bottom line is "if it sounds good to you, go with it." Not
that objective data isn't valuable, but electronics design is full of compromises, likes, and
dislikes. There's no way around it.
You may be familiar with the "green Sharpie CD tweak." I was sent a catalog by one of the
sellers of audiophile gizmos. Their green Sharpie was priced at $20! I guess the ink molecules
were specially aligned or something. I suppose their are benefits to having a medical-grade
AC outlet, but they're outweighed by the bank debit.
I wasn't aware of the COG ceramics, but it's worth keeping in mind. They seem to be priced
reasonably with the likely-needed capacitance values.

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CeaSaR
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Re: Mini amp circuit

Post by CeaSaR » Sun Sep 13, 2009 9:27 am

Schematic #3 - latest schematic on page 6, all the way at the bottom.

Image

Here's the latest version repleat with scope shot and power output across the speaker.
Scope shot is taken between C3 and the speaker. There is a very slight deformation of
the peaks and valleys at this level, but if I reduce it down to the ~550 mW level, the
scope shows a beautifully shaped sine wave. This particular setup allows for an adjustable
input from between 10 mV to 1 V for full output.

My next question is this: Should I use the reading across the speaker to determine output
or the peak to peak scope reading times the output transistors current capacity?

In this case, the reading says 632 mW, but the scope says 6.3 V * 600 mA = 3.8 W max.
Which one is correct?

Thanks again,
CeaSaR
Hey, what do I know?

Robert Reed
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Re: Mini amp circuit

Post by Robert Reed » Sun Sep 13, 2009 10:26 am

6.3 V p-p = 2.23 v rms
P=E sq./R
P = 2.23 v rms sq./8
P=622 mw across speaker terminals

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CeaSaR
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Re: Mini amp circuit

Post by CeaSaR » Sun Sep 13, 2009 1:05 pm

So,

P-P -> RMS = 1/2 P-P *0.707 = 0.5*6.3*0.707= 3.15*0.707= 2.22705= ~2.23 V RMS
P = (RMS^2) / ohms = 2.23^2 / 8 = 4.9729/8 = 0.6216125 = 0.622 W = 622 mW

Gotcha!

And if the amp could handle more current, ie a lower speaker impedance, then a 4 ohm speaker
load would result in approx. 1.244 W and a 2 ohm load would be approx. 2.488 W. Conversely, a
16 ohm load would be 0.311 W and a 32 ohm load would be 0.1555 W.

Thanks,
CeaSaR
Hey, what do I know?

Robert Reed
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Re: Mini amp circuit

Post by Robert Reed » Sun Sep 13, 2009 7:36 pm

Right, but (and going strictly fom memory here) the 44xx's are rated at 400 mw max. dissapation, your probably pushing the current limits with an 8 ohm load. Higher Z-yes. Lower Z-probably not good.

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CeaSaR
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Re: Mini amp circuit

Post by CeaSaR » Sun Sep 13, 2009 8:34 pm

Robert,

Understood on that point. I was just throwing out hypothetical calcs, hence the "And if
the amp could handle more current, ie a lower speaker impedance," disclaimer.
The specs I have on the 4401's is "Si 625mW 40V 600mA 250MHz GenPurp pkg:TO-92B"
- straight from CircuitMaker. So you'd probably derate it down to 400 mA to be safe.

One thing I never quite understood is how to determine what is the best impedance
match for a particular output. Oddly enough, I was playing around with the beginning
post's schematic and found that the output could be increased from ~6 mW to ~23 mW
just by changing the speaker impedance up to 32 ohms instead of 8 ohms. That was the
best match out of all the standard speaker impedances. According to that finding, there
is more to the final power calculations than stated "simply" earlier.

I obviously have a lot to learn.

Thanks again guys,

CeaSaR
Hey, what do I know?

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CeaSaR
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Re: Mini amp circuit

Post by CeaSaR » Fri Sep 25, 2009 11:39 am

Hey Bob,

How's the wife's car? The dishwasher? The rain.... Feel like talking audio amps - the really short course
in the basics thereof? I'm really interested in what you know about them.

I should've gone to Tech School and taken electronics way back when. All these wasted years!!! :mrgreen:

CeaSaR
Hey, what do I know?

Robert Reed
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Re: Mini amp circuit

Post by Robert Reed » Fri Sep 25, 2009 7:50 pm

Ceasar
"One thing I never quite understood is how to determine what is the best impedance
match for a particular output."

To a large extent, the impedance of the output amplifier collector is the collector load. Think of the amplifier as a constant current source where the collector is completely isolated from the input circuit and supplys current to any load that is presented to it. Of course this will follow current source rules and limitations and also the transistor has to be rated for the voltages and currents that the load requires. There are more details that have to be worked out, so this is more of a generalization than an exact circuit design.

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Bob Scott
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Re: Mini amp circuit

Post by Bob Scott » Fri Sep 25, 2009 8:42 pm

The collector of a transistor is a constant current source, infinite output impedance, so the characteristic output impedance of a transistor amplifier would be solely equal to the resistance of the resistor connected from that collector to B+, if that amp has no negative feedback. All audio amps these days have negative feedback to make the outputs be constant-voltage, that is low low output impedance.

In the olden days people matched load impedance to the source impedance of audio amplifiers in order to get maximum transfer of power to the load. These days with constant voltage output stages, you can load the output of an audio amp to the maximum current rating of the amp output. Current capability of the amplifier is now the only limit to the available power.

Loudspeakers are designed to be driven by a constant voltage source (zero output impedance) in order to work at their designed "Q" (see Thiele Small parameters of speaker design).

Caesar, your amp doesn't have a good feedback loop, so your output voltage is varying with the speaker's impedance.
-=VA7KOR=- My solar system includes Pluto.

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sofaspud
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Re: Mini amp circuit

Post by sofaspud » Sat Sep 26, 2009 4:28 pm

I gotta give a plug to our host.
If you have your N&V back issues, you may want to have a look at Ray Marston's eight-part series
'Bipolar Transistor Cookbook' beginning July 2003, but particularly the October '03 and January '04
installments. It doesn't answer all questions, but does provide lots of what, why, and how.

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CeaSaR
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Re: Mini amp circuit

Post by CeaSaR » Sat Sep 26, 2009 4:39 pm

Sofaspud,

I came on board with N&V when Poptronics went under in the beginning of '03,
so I should have those issues stored away upstairs. I'll have to dig them out again.
Thanks for the heads up.

CeaSaR

Speaking of PopTronics - Dean, I miss reading your answers in PT's Q&A. Maybe N&V
could get some of the "old gang" together and do a special column to let the readers
know what other writers helped all the "Electronics Magazines" out through the years.
I, for one, would like to see what has happened to the collective alumnus. Maybe one
question could be asked and each writer would submit their take on the answer for
that column. I bet it would make for an interesting comparison. Here's to the future!
Hey, what do I know?

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Bob Scott
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Re: Mini amp circuit

Post by Bob Scott » Tue Sep 29, 2009 9:19 am

How about this?

Image

or this? Heat sink required. (4X more power; fewer parts)

Image

Do not substitute transistors unless they have similar gain to TIP41 and TIP42. ie: no high gain.
These are untested. Some tweaking may be required. I did make the top version work ~1980 as an monitor amp for tape editing, but I think I may have used a 741 op-amp. I don't think the TL07X or TL08X series are guaranteed to work at 12V, but I know that in the circuits I have built, they do.
Use only single op-amp per package type with quiescent (idling) current ~ 1-2mA
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CeaSaR
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Re: Mini amp circuit

Post by CeaSaR » Tue Sep 29, 2009 10:18 am

Hey Bob!

Since we are headed this way, I have some LM324's that I was going to use for a guitar amp.
I have 1 spare pair of amps left on the last one (IC3-c and IC3-d) that I was thinking of using
in a similar manor. How about substituting them for the '071's and setting them up in a BTL
configuration?

As for the original intent of this thread, I'm still interested in the discrete version I last posted
(or one of those variations thereof). The knowledge gained by learning from the ground up can
only enhance newer studies (IC designs).

Thanks,

CeaSaR
Hey, what do I know?

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