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Old 27th March 2013, 12:55 AM   #1
Spec is offline Spec  Australia
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Join Date: Apr 2012
Location: Wollongong, Australia
Default Help with component selection.

Im building an opamp buffer for my Icepower 50asx2, as suggested by the designers manual.

Im having trouble deciding what type of cap's and resistors I need to buy for the circuit, The costs are starting to add up rather fast.

I'm planning on running 2 OPA2134's

What sort of caps should I be using (mylar, polly ect)

and what wattage resisters should I also use.

The circuit is going to be fed from the 25v +- aux supply via a 25-12v +- regulated power supply once again, from the designers manual.

Here are some schematics of said circuits.

Thanks !
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Old 29th March 2013, 12:15 AM   #2
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Join Date: Sep 2004
Location: New Zealand
Resistors considering this is a preamp need only to be 1/4 watt, which are readily available.
You could also use surface mount if these are available and you have skill to solder them ie 0805 size

Caps if saving cost can be electrolytics, otherwise Jaycar sell crossover part poly caps. Ideally with audio path you need to avoid having polarity change with caps, which can be achieved with 2x 22uf electrolytics in series + - - + to make 11uf, or poly caps which are bipolar. Other smaller caps can be MKT types.

Suggest you make separate boards for PSU and opamp components.

Hope this helps

Last edited by Chris Daly; 29th March 2013 at 12:17 AM.
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Old 29th March 2013, 05:35 PM   #3
Spec is offline Spec  Australia
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Join Date: Apr 2012
Location: Wollongong, Australia
Yeh. I did a heap of reading and two days searching sites for parts. I ended up grabbing some nice quality stuff from rs components they seemed to be the cheapest. Spent just over $60 worth with the opamps and the regulators and heat sinks.

The other pain was finding the connectors to suit my 50asx2. I noticed they were JST items but there were so many different types thankfully the designers manual had drawing of some evaluation wiring kits which had the connector and pin part numbers so I grabbed them from element14
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Old 1st July 2013, 09:12 AM   #4
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Join Date: Jul 2013
Default Do you already made PCB layout of the mentioned preamp?

If you do can you share?

Also there's a very nice preamp which I assembled and put there LT1364 OP amp and it sounds great.
It is here as DIY:

and costs only US$5

Last edited by tamazi; 1st July 2013 at 09:26 AM.
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Old 4th July 2013, 10:47 AM   #5
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Join Date: Jul 2013
Default A very scary question

I know I'm asking a very stupid question, but what the hack do we need the buffer?

I knew that pre-amplifier is just amplifying input signals to app. 1V level ,i,e. let's say 5:1 amplification.

what for
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Old 4th July 2013, 08:31 PM   #6
Zero D is offline Zero D  United Kingdom
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Join Date: Aug 2009
Hi, you don't need the other OpAmps & components just to buffer !

You are dividing the signal by 2, then amplifying it by 2 = same out as in

Here's all you need, unless you Actually need some gain ? If you do, just say how much & i'll let you know what to do.
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Old 5th July 2013, 10:13 AM   #7
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Add an RF attenuating Filter to the Input of post6.

For the widest range of deep bass you need the pre-amp at least an octave lower than your power amp.
regards Andrew T.
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Old 10th July 2013, 10:54 AM   #8
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Default Thanks for support

I actually need low-noise preamplifier with amplification of 5:1 to connect to IcePower 50ASX2.

The reason is that the 50ASX2 has only 20.5 db gain and it's obviously not enough for some devices.

I also need volume control in front of 50ASX2.
This preamp (I replaced ne5532 with lt1364) sounds good,but is a little bit noisy:

I also read somewhere in this forum, that if you connect volume pot directly at the input of 50ASX2 the input impedance will be substantially jumping along the audio frequency range....

So, any alternatives are more then welcome

Last edited by tamazi; 10th July 2013 at 10:57 AM.
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Old 21st March 2015, 01:06 AM   #9
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i am also preparing to build this opamp buffer. my question is if i want to add volume control, can i just add a pot between buffer and the source?
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Old 21st March 2015, 03:23 AM   #10
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Join Date: Jul 2014
Originally Posted by tamazi View Post
I know I'm asking a very stupid question, but what the hack do we need the buffer?
There are no stupid questions, but there are certainly teachers who don't get it.

Generally speaking, in audio the purpose of a buffer might be one of two, and they're related:
  1. To get from high impedance to low impedance.
  2. Separation between stages, to prevent components over there from affecting components over here.
And that's about it. The whole point of a buffer is that it has unity gain (not a "gain of one," but unity gain), very high input impedance (Zin), and very low output impedance (Zout).

This very high Zin is accomplished by running 100% feedback, which is called "a Beta of 1." To give a for-instance, an NE5532 all by itself has a worst-case Zin of some 30k. But wired as a buffer its Zin shoots up into the megohm region. A TL072, which already has megohm Zin, wired as a buffer effectively has infinite Zin.

Note that only 100% feedback and unity gain identify a buffer. Others might argue this point.
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