T&L Publications

Bob Scott wrote: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 really must take more time reading posts, that's the second one I missed in this thread.
I'm going to see if I can get out tomorrow for the parts.

CeaSaR

Bob, I thought you had me on that one and that I was wrong. But you're using 0.01µF as the cap value to get 3386 Hz and the value listed for C5 on the schematic is 0.1µF which will get you a frequency of 338.6 Hz for 4700 ohms. Is there another skem after that first one on Page 1 where the value was changed?

Dean, I guess there are multiple schematics from Charles in this thread with differing values for this capacitor. I assumed you had analysed the schematic that appears in a link in Ceasar's message that appears immediately before your response. Here's a copy:

CeaSaR wrote:Hi Dean,

I found the basis for this on the web here..................

In this link is a schematic and a circuit description. The cap is .01 in this one. He says that the cap is for "cutting the highs and mellowing the sound a bit, for more comfortable listening."

Here's the URL:
http://www.tricountyi.net/~randerse/amp.htm

Best Regards. 73

Sorry about the confusion Dean, I do tend to continuously update and repost newer schematics
when I get them ready. As Bob said, the one I based my very last design (on page 3 about 1/3
of the way down and reposted below) came from the site Bob relisted
(http://www.tricountyi.net/~randerse/amp.htm). If you look at that one and mine,
you'll see similarities AND differences.
Schematic #4 - latest schematic on page 6, all the way at the bottom.

Most notably the removal of the first preamp stage and the addition of a low power
quasi-complementary output stage. The reason I used the transistors listed is because I actually
have them in my parts bin.

Hope this clears up any confusion.

Thanks again guys,

CeaSaR

Gasp! You mean that we all have to "be on the same page"?

It didn't help that I'd originally managed to hop into the middle (time and pages) of a 2-month-old discussion without realizing it!

Why Dean, that's a novel concept!
I wish I had thought of that...

No problem about jumping in "late", as it were. That's what the Q&A dialog is all about, if something
is not understood or unclear, just ask. Hopefully the answer will clarify the point(s) under question.

My posting style can sometimes be a bit hard to follow, especially when the topic reaches lengths of
more than 2 pages (within the forum). I prefer to leave the older schematics for historical reference.
Perhaps I should go back through and edit the older posts to relabel the pictures something like:
Original, Trial 1, etc. Then I could point to the most current version, as I did above:
"Page 3 about 1/3 of the way down."
At least that would help disperse any confusion about which schematic is to be referred.

Whaddya think?

CeaSaR

EDIT: Relabeling done!

Ceasar
Thanks for the link to THE AMPLIFIER INSTITUTE. I read this with great interest and the section titled PSEUDOSCIENCE IN AUDIO was especially interesting. It seems that in no area of engineering is subjectivity more prominent than in the feild of audio reproduction. In reading this it reinforced all my collective thoughts about audio and the BS that retailers push on the market. One thing in particular caught my attention - EXPERIMENTOR EXPECTATIONS. In looking back, I think that I have been guilty of this on more than one occasion. It tends to make a person "fudge" the test results when their is an area of uncertainty to be more in line with what one expects. Have you read that section yet? It is a must read!

Robert,

I skimmed the site when I found it. That was one paragraph I missed. Oh boy, subjectivity. I've had my fill
on that particular tangent a long time ago. That is why I have no real expectations on my projects except
that they function. How well they perform is a product of long term use. I let time tell for me, not some fluff
headed so called golden eared expert. When I want to find out about an audio product, I review it in person.
I wouldn't trust the audio critic any more than I would trust any other paid critic. There have been way too
many electronics/movies/books etc. that have been panned by the talking sped heads that turned out to be
gems in my eyes.

CeaSaR

Since I have been busy with other things lately, I haven't had much time to futz around
with this circuit or thread in general.

Bob,
I went down to the local "The Shack" and, although they had LM741's, they didn't have
complementary pairs of either sets of TIP's. They had '31's and 42's, but NOT '32's or '41's!
Typical...

As to my original topic, I went back to the schematic labeled Test 3 and placed (2) 0.5 ohm
resistors in the emitter/collector paths of Q5/Q6 (leading to C3) and found that the output
only dropped by 1 mW while the whole circuit current dropped by almost half. Also, I changed
the value of R7 to 10 ohms and upped the output power to ~ 1 W.

I have a 32V 900 mA source and would like to try it on this setup. If I change the load resistors
from 0.5 ohms to 1 ohm, I sim ~9 W audio power at 403 mA overall load. Should I attempt it with
the 4401's or should I add a pair of heavier output transistors? TIP31/41 or 2N3055?

CeaSaR

Hi Ceasar,

First, try reducing the number of diodes in series. You have 3 transistor junctions in series (2 on the Q3 + Q5 darlington power transistor arrrangement and only 1 on the Q4 + Q6 beta multiplier pair, so you should only need 3 of those 1N4007 diodes in series in order to bypass those 3 transistor junctions. More than 3 will cause too much bias current to flow through the outputs (shoot-through). The whole purpose of the diodes is to limit excess current. In commercial amplifiers, the diodes are usually mounted to the same heat sink as the output transistors, so the diode junction temperatures track the output transistor temperatures. Sometimes they use two diodes on the circuit board, and one thermistor on the heat sink in place of the third diode.

If you need more gain, try adding a 10uF electrolytic cap across R7. Hmmm..maybe too much gain, but I bet the output swing gets a lot bigger. Right now the amplification factor of that stage can be calculated as approximately X2.2, or (R6 / R7). Believe it or not, R7 is part of a negative feedback "loop" because the emitter of a transistor is not only the + output terminal, but also the - input terminal.

If you add the cap, it effectively shorts out R7 for AC purposes, increasing the gain to the transistor's limit. However, The gain is stopped by the R3, R4 pair at a gain of R4 / R3, or X14.4. That would be about 6.6 times more gain.

[EDIT] Oh, I reread your last message and you've already changed the value of R7, so you already have the added gain.

There is one more improvement to add. It is a boost for the + output because R6 runs out of current to drive the output when excursions get high, just when higher current is needed. I'll have to make the change in a schematic.

What software did you say you were using to make the schematic?

I have updated improvements to my circuit too. I'll upload but not right now.......

I'm using CircuitMaker student version. It is no longer available, but I'll send a copy to anyone that
would like to have it. There shouldn't be any problem with that as it was free to begin with and the
company that made it (Protel) was bought by another company and CircuitMaker is no more. Therefore,
what I have is abandon(free)ware.

Playing with the sim and doing your suggestions, I can't quite seem to get it right. I can get more power
out of it (actually very close to my original "high output") but at the expense of reducing input. Yes, I can
increase the size of the input VR (R9), and that really isn't a problem. Removing 1 (or even 2) of the diodes
reduce the power before distortion appears, even when changing R7. Interesting quandry.

I also notice that with R7 set at 1k, the output swing is centered above 0V, while reducing R7 centers the
output swing below 0V. Is this normal? I'm finding more questions than answers, it seems.

Let me know what you think,

CeaSaR

Note: this is the official post #50,001 of the General forum.

CeaSaR wrote:Removing 1 (or even 2) of the diodes reduce the power before distortion appears, even when changing R7. Interesting quandry.

OK, but you should notice that current draw from the power supply is reduced when one diode is removed/shorted.

I also notice that with R7 set at 1k, the output swing is centered above 0V, while reducing R7 centers the output swing below 0V. Is this normal? I'm finding more questions than answers, it seems.

Where are you taking the output measurement from? The schematic does not indicate. If ground is zero VDC, then the output measured at the + end of C3 should ideally be centered at about 6VDC, so the waveform has the room for excursion between +12 and ground, or +/- 6V in either direction. The output measured at the - end of C3, or the top loudspeaker terminal should be centered at 0VDC because C3 isolated any DC voltage from the speaker.

At the (-) end of C3. I'll have to take some screen shots to show you. It's really wierd.

What software do you use? Do they have a free or student version? I'd like to get a bit more
current (should I say complete?) with respect to components. CircuitMaker is missing quite
a few of the well used actives that are called for in many designs.

CeaSaR

EDIT: Now that I think about it, centered may not be the best term. I see a sine wave that has
more amplitude on one side as described before. It may vary from ~ 0.2V to more than 1V. I think
that explains it better.
For example: in the schematic referenced above, Yc = ~3.222 and Yd =~-3.044. I say approximate (~)
because I have to manually move the respective Y's to the edges of the waveforms.

Bob,

Yep! Overall circuit current goes from ~355 mA (w/4 diodes) to 10.01 mA (w/3 diodes). I can crank the
power output back up by changing the input divider (R9) percentage to less than 75% or push more voltage
in (>200 mV). Big difference. Makes it worthwhile from the power consumption and heat generation concerns.

I have a question that has been bugging me about this particular design since I first found it. Why isn't there
a larger voltage swing at the output? It shows just a little over +- 3 V. That's a 6 V swing from a 12 V supply.
Mayhap you could shed some light on that subject.

Wonderingly,
CeaSaR

CeaSaR wrote:Yep! Overall circuit current goes from ~355 mA (w/4 diodes) to 10.01 mA (w/3 diodes). I can crank the power output back up by changing the input divider (R9) percentage to less than 75% or push more voltage in (>200 mV). Big difference. Makes it worthwhile from the power consumption and heat generation concerns.

When you do this, please check to see what the DC voltage is at the junction of Q5, Q6, and C3 ... at the + end of C3.

I have a question that has been bugging me about this particular design since I first found it. Why isn't there a larger voltage swing at the output? It shows just a little over +- 3 V. That's a 6 V swing from a 12 V supply. Mayhap you could shed some light on that subject.

OK, it would be more efficient using as much of the available output swing as possible. First, let's give the circuit some capability for higher output swing. This trick is used in most of the older discrete consumer hifi audio amps. R6 supplies current to drive the base (input) of Q3. Trouble is, the higher the output swing, the less voltage across R6, and the less current flows through R6, and less current drive is supplied to the outputs. This is right when R6 needs to supply as much current as possible. So try this:

R6 is 2.2K. Add another 2.2K resistor in series with it. You'll then have two resistors in series where R6 was. Then add a 10uF (or higher) cap with the + end connected to the middle of those two resistors in series. Then connect the - end of the 10uF cap to the wire that leads to + end of C3. That should increase the high drive capability. On another matter, let me know what the DC voltage is at the + end of C3. We want it to be about half the power supply voltage.

[Edit] If the + end of C3 is lower than 6V, add a resistor from Q2 base to ground. Play with the values of this resistor in your simulator and see what value brings the voltage at C3 up to ~6V. Maybe um, 10K to start?

Meanwhile I'll see what the gain should be for increasing the peak-to-peak output.

Pls post a new schematic with all updates. I remember you changed R7.

I don't use simulator software. I use a calculator. I think Bigglez recommended LTSpice.