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Old 22nd December 2005, 11:23 PM   #1
mnaglik is offline mnaglik  Poland
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Unhappy OPA2134 with BUF634 preamp

Hi,
Hello to everyone reading this forum.

I am building some kind of headphones amplifier and am completely newbe at this topic. I just want to recreate the schematic I found (attached).

I have menaged to get all parts except film 4.7u capacitors C13, C4. Its really difficult to get them for a reasonable price here in Poland.

Can I place an electrolical cap instead of them? Or is there any way to simplicity (low cost) the circuit?

Thanks for the tips, and sorry for that idiot question.
Cheers,
Marcin
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Old 23rd December 2005, 12:26 AM   #2
lineup is offline lineup  Sweden
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Avoid to use electrolytic in input of amplifier.

You can use one 2.2uF film cap, will work almost same good.
Maybe even one 1.0uF can be good.

Or parallell several 1uF/2.2uF film.

2 parallell 2.2uF = one 4.4uF

2 parallel 1.0uF = one 2.0uF
4 parallel 1.0uF = one 4.0uF
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Old 23rd December 2005, 12:41 AM   #3
digi01 is offline digi01  China
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mnaglik,
opa2134 as input opamp,you can remove coupling caps.dc offset is very small.

Zang
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Old 23rd December 2005, 07:55 AM   #4
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Default Re: OPA2134 with BUF634 preamp

Quote:
Originally posted by mnaglik
I am building some kind of headphones amplifier and am completely newbe at this topic. I just want to recreate the schematic I found (attached).
You could check out my QRV-04 which consists of OPA134 + BUF634. As Zang says the offset is very small so you don't have to use any big caps.
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Old 23rd December 2005, 10:48 AM   #5
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Default Re: OPA2134 with BUF634 preamp

Marcin, you don't need those coupling caps, unless you are connecting the circuit directly to the voltage output of a dac. Are you? Schematic says 'Dac_L'.
Also, what you are doing with the two halfs of an OPA2134 you could do with a single step, only one half (or a single op-amp). Less is more.
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Old 23rd December 2005, 04:11 PM   #6
lineup is offline lineup  Sweden
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OPA2134 is used to filter the output of a DAC.
According to schematic. (which was not there, when I first posted)

That is different.
There are two 4.7uF parallell = 9.4uF in feedback loop of OPA2134.

I can not understand that circuit.
The best guys on this circuit
would be in Digital forum
Jocko Homo and others.
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Old 23rd December 2005, 05:47 PM   #7
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Quote:
Originally posted by lineup
OPA2134 is used to filter the output of a DAC.
According to schematic. (which was not there, when I first posted)

That is different.
There are two 4.7uF parallell = 9.4uF in feedback loop of OPA2134.

I can not understand that circuit.
The way it is 'organized' can be confusing, but it's really simple.
Those caps are not on the feedback loop.
C13 and C4 (2x4.7uF) are between the output of the first half of the op-amp (Out A) and non-inverting input of the second half (+In B), with 100k to ground (R23).
Easy.
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Old 23rd December 2005, 05:54 PM   #8
lineup is offline lineup  Sweden
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I see.
If been draw with two Op-amps, I would have seen it.
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Old 25th December 2005, 04:28 PM   #9
mnaglik is offline mnaglik  Poland
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Unhappy Re: Re: OPA2134 with BUF634 preamp

Quote:
Originally posted by carlosfm
Marcin, you don't need those coupling caps, unless you are connecting the circuit directly to the voltage output of a dac. Are you? Schematic says 'Dac_L'.
Also, what you are doing with the two halfs of an OPA2134 you could do with a single step, only one half (or a single op-amp). Less is more.
Unfortunatelly I AM conecting it to voltage output of DAC. I know that I can use single step opamp (i.e. OPA134). I just decided to use OPA2134 and have made PCB for it (maybe I will change it to AD8066 in the future).

Just wondering how using the lower capacitor (i.e. one 1uF) would impact the circut (sound)?
Could you give me a hint or route to any page, document etc. regarding this topic?

Thanks.
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Old 25th December 2005, 04:58 PM   #10
lineup is offline lineup  Sweden
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Default Re: Re: Re: OPA2134 with BUF634 preamp

Quote:
Originally posted by mnaglik
Just wondering how using the lower capacitor (i.e. one 1uF) would impact the circut (sound)?
Could you give me a hint or route to any page, document etc. regarding this topic?

Thanks.
As the bias resistor of second op-amp is as high as 100kOhm
I find no reason at all to use 2x4.7uF.
The lower freq, bass rolloff, will be low enough using an input
1uF + 100kOhm ( 1uFx100kOhm= 100uFkOhm )

Sometimes, when for example input resistor is 10KOhm, I would use 2.2uF give 'only' ( 22uFkOhm )

formula
f = roll off freq
f = 1 / (2pi x C x R)

with CxR = 1 uFx100kOhm
I get f = 1.59 Hz
So at this frequency you have a -3dB rolloff

I woudnt think you will find many loudspeakers in this world, or even headphones,
with -3dB roll off at 20 Hz

Is it good enough with 1.59 Hz?
I think so!
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