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Questions about circuits around OP-Amps and its effects to sound itself.

post #1 of 28
Thread Starter 
Greetings!
I recently got into "DIY scene" by building my first amp, CMoy (the most basic version of it). I'm planning to build a new one (and "better") with rail splitter, vground buffers, powered from wall psu and 2 opamps/channel. But some questions I still do have, that I have not managed to quite figure out.


Firstly, here is a picture to the most simply looking circuit of what I had in mind for the circuit of opamps:
http://i43.tinypic.com/15s6d5v.png

1
I've heard that using more than one different sized (big & small) bypass caps in power circuit is better than only one alone, would there be any dis/advantages to use 2 or more ac-coupling capacitors (C1) in parallel for the input of the opamp? (eg. 0.22uF & 0.68uF)
As those AC coupling capacitors along with R1 creates highpass filter, I would guess that atleast the response on the low frequency would be better, would there be any other effects to sound?

2
For the output resistors in the OP Amp, should they be connected like in the picture, or so that the resistors were in "loop" with opamp output pin and inverting input pin? (moving the connection point for inverting input to the other side of resistors R2 & R3)
In CMoy they are other way as in my picture, but I've seen it done like this way when using more than one opamp.
And would 10 Ohms resistors be sufficient?
If I made the other resistor higher valued than the other, would the lower valued op amp affect the sound more than the one with higher valued resistor?


And a question regarding to this type of circuit:
http://i42.tinypic.com/t8kfpe.png

3
In some schematics I've seen a capacitor added to the OP Amp gain stage like this. What is the purpose of it and how does it affect to sound itself?
I had a guess that since capacitors have higher impedance in low frequencies than higher frequencies, it would boost the Bass or dynamics along with the resistor thats connected parallel with it?
As when lower frequencies kicks in, the impedance goes higher over the capacitor forcing the opamp to make more gain?


Another pic regarding to next question:
http://i43.tinypic.com/kdaafb.png

4
In OP Amp input I've seen a capacitor (decoupling capacitor?) added like this with a resistor in parallel. I'm wondering what does it do and how does it affect the sound?


If you actually managed to read this all, I thank you for your time and would apperciate any answers to the previous questions.
post #2 of 28
1.

The 2 capacitors (in a PSU) are normally of greater difference in size, e.g. 10000uF and <1uF, with the larger being electrolytic and the smaller being a film cap of some type. Use a single film cap in the position shown in your cmoy, a polystyrene type is good.

2.

1 ohm resistors outside the feedback loop are suitable. Make them both the same size. Don't use 10 ohm.

3.

The purpose of this capacitor is to roll off the high frequency gain, if for no other reason than to prevent oscillation. A suitable value should have no effect on the sound.

4.

This shunt capacitor provides a path for high frequency signals (radio signals) to ground, preventing them from getting into the amplifier. The resistor forces the non-inverting input, and hence the output to zero volts (ground).

w
Edited by wakibaki - 11/12/13 at 3:18am
post #3 of 28
Thread Starter 
Thanks for the good answers!
Quote:
Originally Posted by wakibaki View Post

3.



The purpose of this capacitor is to roll off the high frequency gain, if for no other reason than to prevent oscillation. A suitable value should have no effect on the sound.

w

 


For this, I actually managed to find a spare 0.22uF capcitor and paralled it with a 4k resistor, and the other resistor was 2k. It boosted the lower frequencies, I'd say around <500Hz..
As 0.22uF cap at 120Hz has an impedance of 6k Ohm, together with the paralled resistor of 4k it would make a total impedance of 2.7k Ohm. Resulting to a gain of 2.2x at 120hHz with the other resistor being 2k. And at 500Hz 1.5x, and so on.
I guess for using it to roll off high frequencies, the cap value should be lower to maintain higher impedance (so that the overall paralled impedance is almost identical to the resistor value of 4k at <20kHz) on a wider frequency scale, so it would not so easily lower the gain at mid and higher frequencies, but only at extremely high frequencies.

But, with suitable resistor and capacitor values this could also be used to boost bass?
Edited by P701 - 11/13/13 at 9:49pm
post #4 of 28

Here is the circuit you have built:-

 

 

This is how bass boost is normally implemented...

 

 

This leaves some gain above the rolloff frequency.

 

w

post #5 of 28
Thread Starter 
Ah, I see. Although I'm not really sure what the dotted line is picturing, maybe some sort of phase?

May I still bother you a little, and ask whats the name of the software you used? It would look really handy to play with to find some nice values to components for some circuits (just like for this bass boost).

Much appreciated, many thanks!
post #6 of 28

Yes, the dotted line is phase with the scale on the right side of the graph.

The software is LTSpice.

post #7 of 28
Thread Starter 
Alright, thanks for letting me know!

One thing I almost forgot, was that if I wanted to make the amplifier to Class-A with 2 OP-amps, I'm not 100% sure if I got the connection done right.
Here's what I came up how it probably should be like:
Image: http://i.imgur.com/QxK2cge.png

This is the site where I looked for examples http://tangentsoft.net/audio/opamp-bias.html but there's also always a buffer after an op-amp.
Would it matter, if I won't be using buffer in my circuit after the op-amps?
Edited by P701 - 11/18/13 at 12:40am
post #8 of 28

 

The class "A" bias mod will be ineffective in the situation you have drawn.

As you might have guessed, it needs to be in between the op amp and

a buffer to have any effect. Even then, the effect varies by op amp.

post #9 of 28
Thread Starter 
Quote:
Originally Posted by Avro_Arrow View Post

 

The class "A" bias mod will be ineffective in the situation you have drawn.




As you might have guessed, it needs to be in between the op amp and



a buffer to have any effect. Even then, the effect varies by op amp.


 


Ah, alright. Thanks for answering.
I then must reconcider, if it will be worth to also add a buffer in the circuit to make it work like that.
I'd guess, there would also be no benefits using 2 op-amps in parallel if I added a buffer. Or atleast not that much.

For OP-amps I was thinking OPA2134's.
I have read that Class-A type amp sounds a bit more "dynamic" tho.
post #10 of 28

Yes, two op amps in parallel would not be beneficial driving a buffer.

Driving headphones, yes, that would be an improvement.

 

The buffer could be another op amp (or two), an IC buffer

like BUF634 or LME49600 or a discrete circuit.

post #11 of 28
Thread Starter 
Hmm... Thinking further, couldn't a buffer be used just alone instead of an op-amp, when no gain is needed?
As it has better current output capability than op-amp, and probably has a better slew rate and settling time and also no signal would be lost in op-amp to noise.

EDIT: This question does not relate to previous "Class-A" amp discussion. But speaking, as in general, simple op-amp based circuit, if you replaced it with a buffer.
Edited by P701 - 11/18/13 at 9:42am
post #12 of 28

 

You could use a buffer by itself, but with no feedback loop, offset could be an issue.

post #13 of 28
Thread Starter 
Ah, got it.
Thanks to both of you, seems I've gotten answers to anything I wanted to know, and also even a bit more. Appreciate it alot!

Now only thing I'm wondering, if it'd be better to use single op-amp/channel with buffer and class-a bias mod, or just two op-amps in parallel.
I would guess the first one deliviers all the benefits of the latter one, but without any additional disadvantages? (Well except that class-a mod would constantly drain some current)
post #14 of 28

 

A dedicated buffer like BUF634 or LME49600 are much more powerful

than a single op amp like OPA2134. The buffer also draws more current

from the power supply than an op amp as well so the small current

from the class A mod is not much of an issue.

 

The only drawback is the fast buffer needs more careful layout

for the components than the slower op amps.

Two OPA2134 in parallel would be easier to build for a beginner.

 

Happy building!

post #15 of 28

building audio electronics can be its own hobby - but really the 1st question should be for what headphone - they vary by orders of magnitude in required power for the same SPL, have different impedances, the ratio of I to V,  this changes amp/buffer device and power supply choices

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