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Old 9th May 2015, 12:08 PM   #1
diyAudio Member
Join Date: May 2015
Default Load output resistors in parallel opamps amplifier

I have portable headphone amplifier based on this scheme:
only my is:
Master - OPA128SM
4x Slave - OPA627AU biased 5mA to Class A over resistors
How much are minimal safe and stable values for output resistors (R2,R3,R4 on picture)? Original 47ohm resistors I replace with 10ohm and sound is much better. I need low output impedance of headamp too for IEM.
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Old 9th May 2015, 04:00 PM   #2
wwenze is offline wwenze  Singapore
diyAudio Member
Join Date: Mar 2008
Hmm... have you simulated that schematic to make sure it works?

EDIT: Ok, I went to the website on the picture and did a search,

You will build a light meter that uses all four OP-Amps in a LM324 to form a ladder comparator.
But they can provide output current of 20mA only.
So ya, that is a comparator, hence the positive feedback.

For an audio amplifier try this instead

Click the image to open in full size.

Don't worry about output impedance since the output resistors are in the feedback loop so they don't affect output impedance too much, as long as there is enough power supply and gain headroom (in most audio amps will have lots of).

Minimum value for the resistors is the minimum load the op-amp can drive - imagine a new load current that flows from the output of on op-amp to the output of the other, due to minute differences of output voltage.
That said, there isn't a solid value since op-amps don't suddenly fail when load is too low, but rather the distortion level rises as the load impedance gets smaller. So it's a balance of that versus output impedance.

That said, doesn't matter too much. I would just try a 100 (or 1k since it's OPA627) ohm and calculate the resulting Z-out, if it's below 1ohm I'll forget about further calculations.

Last edited by wwenze; 9th May 2015 at 04:23 PM.
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Old 10th May 2015, 03:19 PM   #3
diyAudio Member
Join Date: May 2015
Thank You very much for answer and next idea.
Here is really schema of this amp:
e10headamp.jpg at ImageHosting.cz - Hosting pro tvoje fotky a obrazky
Opamps was not biased, biasing is my modification.
I make simulation - with 47ohm output resistors is output impedance about 12,5ohm. Now is output impedance 2,9ohm.
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