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"WHAMMY" Pass DIY headphone amp guide
"WHAMMY" Pass DIY headphone amp guide
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Old 8th August 2018, 03:18 PM   #441
mod_sonic is offline mod_sonic  United Kingdom
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Default LM833 DC Offset

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Originally Posted by Nisbeth View Post
Most of the DC-offset probably comes from the opamp (due to something called input bias current, as explained here) The LM833 is bipolar input whereas the OP2134 is JFET and as JFET opamps have much lower input bias currents, they also tend to have a much lower output offset.

Your voltages looks spot on to me, so if you are concerned with the offset either switch to a FET-input opamp or consider going through the thread for suggestions on how to trim the output stage to null the offset.

Nisbeth

Thanks for your help and the pointer to Tangent’s site for how to deal with ‘cranky’ op amps. The problem is that Post #1 -6 implies that if you use the BOM, schematic and one of the listed op amps then no adjustments, including DC offset, will be required. Having used an LM833 from the listed op amps I was not expecting 24mV DC offset. I didn’t know if the problem was my bad build or the op amp.
A list of op amps that deliver < 5mV DC offset using the BOM, pcb, and schematic and a list of (‘cranky’) op amps that are known to have a high DC offset that will need tweaking would therefore be useful at the beginning of this thread.
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Old 8th August 2018, 04:11 PM   #442
jhofland is offline jhofland  United States
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Default dc offset problem

I think what Wayne is saying is that the input bias current on the LM833 will flow through the 100K resistor on the + input pin. The inputs are PNP as shown on the datasheet so the bias current will flow out of that pin and create a positive offset voltage on the pin. The typical bias current is about 300nA. Flowing through the 100K resistor gives a typical offset of 30mV, which isn't far from the 24mV you measure. You can reduce the input resistor to reduce the offset, change to a different op amp with lower input bias current, or add a DC servo circuit to cancel it out.
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Old 8th August 2018, 08:40 PM   #443
wayne is offline wayne  United States
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&quot;WHAMMY&quot; Pass DIY headphone amp guide
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Old 8th August 2018, 09:07 PM   #444
mod_sonic is offline mod_sonic  United Kingdom
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Default LM833 DC Offset

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Originally Posted by jhofland View Post
I think what Wayne is saying is that the input bias current on the LM833 will flow through the 100K resistor on the + input pin. The inputs are PNP as shown on the datasheet so the bias current will flow out of that pin and create a positive offset voltage on the pin. The typical bias current is about 300nA. Flowing through the 100K resistor gives a typical offset of 30mV, which isn't far from the 24mV you measure. You can reduce the input resistor to reduce the offset, change to a different op amp with lower input bias current, or add a DC servo circuit to cancel it out.
jhofland

Thanks for that : now I understand what I am aiming for. I am ordering an OPA2134, several more LM833s to see if there is any sample variation, and some trimmers to see if the offset will adjust to zero or a least within a few millivolts.
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Old 10th August 2018, 03:57 PM   #445
JamieMcC is offline JamieMcC  United Kingdom
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fyi if using the slightly larger Aliexpress enclosure there is enough space to try one of those inexpensive ebay stepped attenuators.

Click the image to open in full size.
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Old 10th August 2018, 04:28 PM   #446
jwjarch is offline jwjarch  United States
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Any impressions of the stepped attenuator JaimeMcC? Does it give good range? Any noise pickup?
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Old 11th August 2018, 09:40 AM   #447
JamieMcC is offline JamieMcC  United Kingdom
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Any impressions of the stepped attenuator JaimeMcC? Does it give good range? Any noise pickup?
Works well no noise good range. Just a bit of a hassle having to modify the thick face plate in order to mount it.
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Old 14th August 2018, 03:55 PM   #448
JamieMcC is offline JamieMcC  United Kingdom
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Sorry if this is a dumb question but anyone have thoughts on what might be a good way to reduce DC voltage at C1 & C5 to 0 (if achievable) I am curious to see how the Whammy would sound without these capacitors in the signal path capacitors?

I am not 100% sure if my interpretation is correct on this or if so how safe this is to do.
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Old 14th August 2018, 04:59 PM   #449
jhofland is offline jhofland  United States
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It certainly is doable to eliminate the input coupling caps C1 and C5 IF (caps intentional) you are confident that there is no DC component to the input signal. Any DC offset will be present at the output.
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Old 14th August 2018, 07:22 PM   #450
JamieMcC is offline JamieMcC  United Kingdom
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Currently I measure 0.04mV and 0.02mV would this be considered low enough to be no DC component if not what might I try to reduce this to zero?

Note this was with headphones plugged in and music playing measured with a UNI-T UT61E on the C1 and C5 lead outs.

Last edited by JamieMcC; 14th August 2018 at 07:28 PM.
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