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Old 28th April 2013, 07:04 PM   #11
JMFahey is offline JMFahey  Argentina
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Agree and add: real parts have tolerances.
Even if each one looks small ... they add up.
One feedback resistor 5% higher, another other 5% lower ... now you have 10,5% gain diference.
And **measuring** itself has tolerance
Some typical cheap multimeters have 5% tolerance , or a number that *sounds* smaller until you realize what it really means: "guaranteed accuracy 1% (or 2%) of full scale range" which means: if you are in a 200V scale , error may be up to 200V * 0.02=4V which if you are measuring, say, 180V is not much (it still is *almost* 2%) but if you are measuring, say, 26V , those 4V are *a lot*.
And why would anybody measure 26V on a 200V scale?
Simple, it overflows the next lower scale (20V).

Digital meters give a false sense of security if you don't consider these facts.
Simulators are far worse, the happily tell you that an amp has "158.457W RMS"
Indeed?
They must be taken for what they really are: excellent tools to save time, try ideas, but the final word comes from what you've built and sits in your bench.
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Old 29th April 2013, 05:00 AM   #12
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Quote:
Originally Posted by Mooly View Post
I would look a little harder at your built amplifier and compare the circuit and the passive component values to those used in the simulation.

The gain should be identical (to within minute limits) of sim vs actual build for a simple design like this . There will be a real reason why the two differ . . .
I agree. The mid-band gain in this circuit should track very closely between simulation and measurements. If you are constructing with 5% tolerance resistors I wouldn't expect more than about 5% difference.

Can you share details of the procedure you are using to measure the gain?

As a quick first step to troubleshooting I would measure the values of the two gain-setting resistors with a multimeter. If that looks OK, I'd look at the DC operating point voltages (no signal applied) on each of the transistor connections, and compare to the simulated values. Since transistor characteristics vary widely even within a given type number you may find differences as high as 20% or so, but certainly anything greater than that would make me look for a construction error or a defective component.

Dale
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Old 2nd May 2013, 02:06 AM   #13
Noobert is offline Noobert  United States
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Also, if I wanted to add fuses where would they go? I realize I don't need them for this particular circuit,but if I were to add them where would they go?

Thanks for all of your replies.
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Old 2nd May 2013, 01:40 PM   #14
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You've got an output capacitor to the speaker so you don't need a fuse there. Of course one puts a fuse in the AC input to the transformer or switching power supply in case the diodes short out there. One can also put fuses between the output filter capacitor of the power supply and the rails, where the voltage source is shown in the schematic diagram. These fuses are outside the feedback loop so the amp should compensate for any errors they cause by being non-linear. The rail fuses could also protect any other output transistors if one of them overheats and fails.
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Old 2nd May 2013, 02:55 PM   #15
Noobert is offline Noobert  United States
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Quote:
Originally Posted by indianajo View Post
You've got an output capacitor to the speaker so you don't need a fuse there. Of course one puts a fuse in the AC input to the transformer or switching power supply in case the diodes short out there. One can also put fuses between the output filter capacitor of the power supply and the rails, where the voltage source is shown in the schematic diagram. These fuses are outside the feedback loop so the amp should compensate for any errors they cause by being non-linear. The rail fuses could also protect any other output transistors if one of them overheats and fails.
Thanks for the replies. I will be adding those fuses today.

Quote:
Originally Posted by Mooly View Post
I would look a little harder at your built amplifier and compare the circuit and the passive component values to those used in the simulation.

The gain should be identical (to within minute limits) of sim vs actual build for a simple design like this . There will be a real reason why the two differ. It could be a component value error or even something like a wiring error where the feedback signal is getting "modulated" or modified due to "real world" wiring having resistance. Are you measuring the gain at "mid band" frequency where the capacitors reactive component is negligable ?
Yes I believe I am measuring at midband. I checked again and I am getting a gain of 9.6 peak to peak today. I don't know what I was looking at yesterday .

Now I am working adding a potentiometer for volume control, and possibly tone and balance control. I have the volume control working okay with a logarithmic potentiometer, but it isn't perfect. The potentiometer should be wired into the input correct? Any suggestions on picking the proper potentiometer for my circuit?

For tone and balance it would just be 2 more potentiometers correct? How would you wire/set up the potentiometer for tone control? For balance?

Thanks.

Final build:
https://www.dropbox.com/s/vnmajwl1vd...138ebuilt1.asc
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Old 2nd May 2013, 04:57 PM   #16
Mooly is offline Mooly  United Kingdom
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Some reading for you Volume and balance controls.
Potentiometers (Beginners' Guide to Pots)

Tone controls are normally a separate stage in the preamplifier (although they don't have to be) and some audio power amps do actually incorporate tone controls with the feedback networks but its generally regarded as a "oddball" solution.

Simple tone control,
Hi-Fi Preamplifier
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Old 2nd May 2013, 11:42 PM   #17
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For a simple integrated sample amp to copy look at the schematic of the Armstrong 625 on this thread Overhaul of an Armstrong 625 gone wrong! Please help me :) post 48. It has the tone controls, high level and magnetic phono level inputs, just about anything you want or you can leave anything out. Biggest thing I see wrong with this amp is that if the output transistor bias control potentiometer breaks, the idle current runs away. If I build one I might put a clamp diode across the bias control pot. 40636 transistor was a specially selected 2n3055 for high voltage: sort of an early manufacturer's only 2n3772.
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Old 9th May 2013, 02:15 AM   #18
Noobert is offline Noobert  United States
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Okay thanks for all of your help guys.
Also, how does one go about designing something like this on their own? How did they know how to bias each bjt? When you bias one it certainly must change the values of other things in the circuit. Any background info on designing something like this would be appreciated
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Old 9th May 2013, 06:21 AM   #19
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Noobert View Post
Okay thanks for all of your help guys.
Also, how does one go about designing something like this on their own? How did they know how to bias each bjt? When you bias one it certainly must change the values of other things in the circuit. Any background info on designing something like this would be appreciated
Designing successful circuits from scratch only comes when you have a thorough grounding and understanding of the theory and practice. For many that means having chosen electronics as a career.

Learning is a progressive thing though and you don't need to be at that level to take a design and modify or tweak it
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Old 9th May 2013, 09:04 PM   #20
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Since the 2n3055 was invented people have been biasing TO3 transistors about 20 ma each pair. See the Armstrong 621 amp circuit above from the early 70's. They say to increase bias current until the turn on edges disappear from the oscilloscope trace on sine waves, but I don't have a sine wave generator or a scope. Djoffe designed a 20 ma closed loop bias circuit for my dynakit ST120, "deformed son of the Leak Stereo 70", and it sounds better with that much. Doesn't overheat, either. I can measure bias current with a voltmeter.
I fool around with old capacitor coupled circuits like this because if you make a stupid mistake it doesn't burn up your $600 pair of speakers. The speaker coupling cap stops the rail current before it hurts anything expensive. The latest design of an amp with speaker capacitor is the Sakis Geezer amp the G amp ....
If you want to understand the terms around here, last week google found for me the build instruction document for the "honey badger" amp with an explanation of each transistor and what it does. The "honey badger" is a sticky thread at the top of "solid state" forum, sold as a kit by diyaudiostore, and is supposed to be your starter amp. Other than trashing your speakers if you make a mistake, I suppose it is a good choice. I've been wondering 2 1/2 years what a "long tailed pair" and "Vbe multiplier" are, and how a "CCS" or current source works. Not to mention the mysterious "VAS stage". The build instructions for the "honey badger" spells out those acronyms and points out which transistor is which, finally. Honey badger has got way more connections than I want to assemble point to point, but using $.08 transistors to stabilize current to signal amp stages makes economic sense, instead of using a $5 transistor and $8 heat sink to regulate the power supply like the Dynakit ST120 does.
If you're really a newbie, a good textbook I found surplus at Goodwill from the local community college was "Electronic Devices, the Electron Flow version" by Thomas L. Floyd. Great explanations of power supplies and how to use a meter, some basic amplifier circuits. It is a way better book than what I started on, "Electronics for Scientists (& dogs)", 1968 version, and "GE transistor manual 7th edition".
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Last edited by indianajo; 9th May 2013 at 09:25 PM.
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