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Old 14th June 2015, 09:56 AM   #1
tfboy is offline tfboy  United Kingdom
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Default Opamp input buffer choices

I'll carry on with more searching and reading up, but in the meantime...

I'm trying out Hypex's new Ncore500 units. They come without any buffering so I want to add my own. Their test rig (which I've also picked up) comes with a LM4562.

But it looks like there are as many solutions as you could dream of, and some of the suggestions in their datasheets are rather juicy, particularly the LME49724.

So has anyone integrated one of these (or other opamps) with an NC500?

Any ideas or thoughts to share?
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Old 14th June 2015, 10:06 AM   #2
AndrewT is online now AndrewT  Scotland
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What is the input impedance of the Ncore500?

What about that gives you reason to think you need some "buffering"?

The more usual problem with interconnecting audio equipment is the inability of the Source equipment to drive the cables.
It's generally the Source that may need a Buffer and this Buffer is inside the Source equipment.

It is very rare that Receiving equipment would need a Buffer.
an example is converting a single ended or unbalanced input to balanced. In this situation the Cold feed to the -IN pin can see a very low impedance.
R.Cordell designed and posted details of the jFET Buffer to meet, exactly, this duty.
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Old 14th June 2015, 10:48 AM   #3
tfboy is offline tfboy  United Kingdom
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Hi Andrew.

The amp is a differential / balanced audio one and the input impedance is relatively low; 25Ω per leg, i.e. 50Ω in total.

I think most modern electronics can drive that without too much difficulty, it's more trying to cover "weaker" pre-amps which would have too much voltage drop.

I've not dabbled with pre-amp side of things before so it's a learning experience for me
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Old 14th June 2015, 12:26 PM   #4
AndrewT is online now AndrewT  Scotland
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I don't believe this:
Quote:
the input impedance is relatively low; 25Ω per leg, i.e. 50Ω in total.
Check what you have read and ensure you understand what the specification means.
Can you post an extract from the spec here?
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Old 14th June 2015, 12:34 PM   #5
tfboy is offline tfboy  United Kingdom
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Oops. Sorry, before the compulsory morning coffee.

It's 1.8kΩ

Click the image to open in full size.

Additional note: The INH/INC inputs form a differential pair. Note that the input impedance is fairly low meaning that minimalist discrete circuits or valve input stages won't work. All op amps commonly used in audio can handle them though.

Last edited by tfboy; 14th June 2015 at 12:40 PM.
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Old 14th June 2015, 12:37 PM   #6
AndrewT is online now AndrewT  Scotland
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2.2Vac = 3.11Vpk.
that voltage into 1k8 is ~1.7mApk
Any Source should easily drive 1.7mApk into a resistive load. Even an iPad can do this.

It's the cable capacitance that can be the far bigger current demand.

edit,
Av is +12.4dB requiring a maximum input of 80Vpk/4.17=19.2Vpk

The input will draw ~10.6mApk if driven to near clipping.
That will require a good pre-amp capable of ~14Vac & ~14mApk

It's a mini power amp rather than a pre-amp and will probably operate on +-24Vdc to +-30Vdc supply rails. Opamps cannot look at this duty.
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Last edited by AndrewT; 14th June 2015 at 12:53 PM.
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Old 14th June 2015, 12:41 PM   #7
EssB is offline EssB  United Kingdom
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Are you sure? 25/50Ω is ludicrously low input impedance. More likely its 25/50 KΩ.

Can't find any data on the NC500 as it appears to be an OEM unit, can you link to a datasheet?
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Old 14th June 2015, 12:42 PM   #8
EssB is offline EssB  United Kingdom
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OK , that's more like it.
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Old 14th June 2015, 12:53 PM   #9
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Quote:
Originally Posted by AndrewT View Post
2.2Vac = 3.11Vpk.
Can't see a reference to 2.2V in the DS included in post 5.

I do see though that the output power into 8R is typically 400W. This requires 56VRMS, 80V peak. The voltage gain is stipulated as 12.4dB so *4.16. Dividing the output voltage by this gain gets us to 19.2V peak. Probably a bit less in practice coz the 400W figure was clipping slightly (1% distortion). So it looks like any buffer at the front end needs to do 10mA.

There must be a mistake somewhere because an LM4562 is only rated to 17V on each rail so won't drive this into clipping. Unless of course the input is balanced which would mean each side only needs to swing up to 10V or so.

<edit> Ah I see AndrewT and I have eventually reached pretty much the same conclusion. I am pretty sure Bruno's a fan of balanced inputs so opamps will handle it but you'll certainly want to bias the outputs into classA with CCSs to VEE.
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Last edited by abraxalito; 14th June 2015 at 12:59 PM.
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Old 14th June 2015, 12:57 PM   #10
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by abraxalito View Post
Can't see a reference to 2.2V in the DS included in post 5...................
He had not included any Ncore 500 spec details.
I took a guess at a typical CDP maximum voltage and showed an example of how to calculate the current requirement of the source.

Picking up from Abrax comment:
Quote:
Unless of course the input is balanced which would mean each side only needs to swing up to 10V or so.
two channels of phase inverted output for an effective balanced impedance source would require ~10Vpk and ~14mApk from each of the phases (hot to pin2 and cold to pin3).
A pair of Opamps operating on +-15Vdc to +-18Vdc could manage this duty.
Bruno Putzeys' balanced vol pot could probably be used if modified to have an actively driven second phase.
I think the second phase is balanced impedance passive without a signal voltage.
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Last edited by AndrewT; 14th June 2015 at 01:14 PM.
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