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Old 2nd April 2012, 06:10 AM   #11
godfrey is offline godfrey  South Africa
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Wow, nice score with the scope!
The tantalum caps all sound like 0.1uF 35V. They'd be ideal for supply decoupling close to the chips. IMHO, the electrolytics are too small for output coupling, but could be put to good use smoothing the supply. Both the electrolytics and the tantalums are polarised, so have to be put in the right way round.
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Originally Posted by Bobbias View Post
What effect does output capacitors have vs resistors?
Ideally, you don't want either. If used, high value caps or low value resistors will have the least affect on the sound.

Output caps would protect your phones by preventing any DC getting through to them in the event the amp fails catastrophically. They also roll off the bass below a certain frequency. The -3dB corner frequency is given by: F = 1/(2pi * RC). So for 100uF and 32 ohms, about 50Hz.

Unfortunately, electrolytic caps need some DC voltage across them to work properly. That's easy to organize if you use a single ended power supply, but then you get a big "Thump!" through the headphones (which could damage them) when switching the amp on or off.

There is a way to get round the problems but it requires more caps. In the scheme shown below, the caps charge up slowly through the resistors, with almost no current through the headphones (ideally none, but the caps won't be perfectly matched).

If you're using 2 or more opamps in parallel, then you need resistors at the outputs to ensure they share the current equally, and to prevent them getting into an argument about exactly what the output voltage should be. The resistors probably also help with stability.

They will have some effect in the frequency response since the output resistance and the headphone impedance forms a voltage divider, and the headphone's impedance varies with frequency.
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Old 2nd April 2012, 03:39 PM   #12
Bobbias is offline Bobbias  Canada
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Quote:
Originally Posted by godfrey View Post
Wow, nice score with the scope!
The tantalum caps all sound like 0.1uF 35V. They'd be ideal for supply decoupling close to the chips. IMHO, the electrolytics are too small for output coupling, but could be put to good use smoothing the supply. Both the electrolytics and the tantalums are polarised, so have to be put in the right way round.
I've always been very careful with caps because I know what happens when things go wrong with them.

I also knew that these tantalum caps were too small. I just wanted to mention that I did have them and could use them. If I'm going to be decoupling near the chips, where should they go?

I realize the more questions like this I ask the more I end up using someone else's ideas to do this, rather than my own, but my electrical theory is rather weak right now because it's been ages since I've been taught most of it, and
to be honest they never taught us anything about **** like pull up resistors, decoupling, etc. etc. I think half of my knowledge that I'm actually using here comes from wikipedia and reading application notes and datasheets online.

Quote:
Originally Posted by godfrey View Post
Ideally, you don't want either. If used, high value caps or low value resistors will have the least affect on the sound.

Output caps would protect your phones by preventing any DC getting through to them in the event the amp fails catastrophically. They also roll off the bass below a certain frequency. The -3dB corner frequency is given by: F = 1/(2pi * RC). So for 100uF and 32 ohms, about 50Hz.
Makes sense once I think about it. I don't like the idea of bass roll off, considering the Grados don't have amazing bass to begin with.

What affect does the resistor have on the output, other than limiting the current?

Quote:
Originally Posted by godfrey View Post
Unfortunately, electrolytic caps need some DC voltage across them to work properly. That's easy to organize if you use a single ended power supply, but then you get a big "Thump!" through the headphones (which could damage them) when switching the amp on or off.

There is a way to get round the problems but it requires more caps. In the scheme shown below, the caps charge up slowly through the resistors, with almost no current through the headphones (ideally none, but the caps won't be perfectly matched).
That's an interesting problem, and solution. It's too bad caps are hard to match well.

Quote:
Originally Posted by godfrey View Post
If you're using 2 or more opamps in parallel, then you need resistors at the outputs to ensure they share the current equally, and to prevent them getting into an argument about exactly what the output voltage should be. The resistors probably also help with stability.

They will have some effect in the frequency response since the output resistance and the headphone impedance forms a voltage divider, and the headphone's impedance varies with frequency.
Is there a way to model this well? I've got a copy of LTSpice that I don't know how to use, and that sounds like a good use for the program. It would be cool if I could lay this out in LTSpice and get a decent simulation of what happens, but I have no idea how to use the program properly.

Also, since I have a bit of money I might head over to the local electronics place and grab some more caps... and hopefully a male to male headphone jack jumper since I seem to have forgotten that that would be a good idea.

I'll update you on whether I have anything new or not after I get back from class today.
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Old 2nd April 2012, 11:32 PM   #13
Bobbias is offline Bobbias  Canada
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Picked to 6 470uf caps on my way home from class, so I have those too now.
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Old 3rd April 2012, 12:52 AM   #14
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If you are going to use two op amps in parallel, the standard resistor/capacitor
power splitter used in the CMoy will not be enough. You need your ground return
to be able to handle the current. If, for example, you have op amps that can put out
a total of 60mA per channel, you should be able to handle 120mA back into the ground.

Thats why using a split battery supply is so attractive.
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Old 3rd April 2012, 01:38 AM   #15
Bobbias is offline Bobbias  Canada
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Quote:
Originally Posted by Avro Arrow View Post
If you are going to use two op amps in parallel, the standard resistor/capacitor
power splitter used in the CMoy will not be enough. You need your ground return
to be able to handle the current. If, for example, you have op amps that can put out
a total of 60mA per channel, you should be able to handle 120mA back into the ground.

Thats why using a split battery supply is so attractive.
If I was planning on making it ultra-portable, I'd be fine with that, but I don't want to have to keep replacing the batteries. I'm perfectly fine with my amp being plugged into the wall at all times. I mostly plan on using it at home and at my girlfriend's place.

Is there a way I can use my transistors to give the added current capabilities?
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Old 3rd April 2012, 02:06 AM   #16
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You would need both a 2N2906 and 2N2904 to make a voltage divider/buffer.

Something like the virtual ground in this example:
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Old 3rd April 2012, 03:12 AM   #17
Bobbias is offline Bobbias  Canada
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I can only find PNP transistors...

I just scavenged a few from an old microwave control board, but they are all PNP.
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Old 3rd April 2012, 03:21 AM   #18
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PNP is what you need to go with your NPN 2N3904s. What is the part number?
This page may be worth a look. It has a low parts count op amp circuit with transistor output, and the author even tried it with a TL071 op amp.
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Old 3rd April 2012, 04:09 AM   #19
Bobbias is offline Bobbias  Canada
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Hmm, I had thought the 2N3904 was PNP lol. Guess that's what happens when I'm used to everything being PNP.

I've got 1 single A1273 and 4 A1266. The 2N3904 can switch 200mA, the A1266 can handle 150mA and the A1273 can handle 2A.
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Old 3rd April 2012, 05:05 AM   #20
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I'm used to most everything being NPN. So anyway, you could make the amp using a single biased transistor, using 2 unbiased transistors, or using 2 biased transistors (the latter two options are shown on the linked page, and the first is shown in most any active component textbook).
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