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Old 7th February 2011, 06:22 AM   #1
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Default cmoy improved?

Hey Guys
to date i have built two different amps, the Headbanger (HeadBanger Headphone Amp Construction Kit) and The CMoy. Headbanger was based on LM386 while the CMoy on OPA2132PA.The question is about input and output caps and DC offset.

Cmoy has input caps (i selected 1mF and 0.1mF) and i get some DC offset on my headphones all the time.(headphones plugged in and not plugged in)

But the Headbanger Click the image to open in full size.

has Caps on the output stage, and as soon as any load is put onto the amp (my headphones for example) the DC offset goes to 0.

i read articles at tangentsoft about weakness of caps when in signal lines (Bass Roll Off, Phase distortion, Cap Distortion)
Input Capacitors for Headphone Amps

now my question is, when caps are placed in the output stage, is there no Bass Roll-Off, Cap distortion and phase distortion? if there isnt, is there a way to implement caps at output stages rather than on the input lines in the cmoy? would that be beneficial?

Both Schematics are attached
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File Type: gif headbanger.gif (10.7 KB, 552 views)
File Type: gif cMOY.gif (16.7 KB, 666 views)
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Old 7th February 2011, 06:54 AM   #2
Mooly is offline Mooly  United Kingdom
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The circuit you posted runs on a single rail so the output is at half supply. With no phones plugged in you will read voltage on the H/phone socket as the cap has been unable to "charge". As soon as you plug phones in it will drop to zero.

As long as the caps are sized appropriately for the load there will be no audible roll off.
470uf is fine.

The CMOY should have an offset in the millivolt range using FET opamps. If using Bipolar then there will be significant offset as the opamp input bias currents (resistors) are not equal.

So if there is an offset you need to find out why.

What is the offset measuring ?
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Old 8th February 2011, 08:16 AM   #3
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hey mooly
my amp is based on OPA2132PA, not using electrolytics as input caps, also my offset, without headphones loaded is 6mV at max doesnt cross this limit

i was actually impressed by the headbanger (lm386 based chipamp) and how the offset went down to a total 0 when the headphones were loaded. i'd like to ask though, is there any way to not have input caps but employ them in later stages as in the Headbanger?
HeadBanger Headphone Amp Construction Kit
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Old 8th February 2011, 10:26 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
there are no charging/discharging resistors for the output caps.
You could add some high resistance from the output side of the caps to ground.
10k would do, but any value around there.

When Cmoy is OFF the 10k holds the tip and the ring at the same voltage as the ground. You can plug in headphones and no "click" to be heard.

When Cmoy is ON, the 10k takes any leakage current to ground leaving just AC signal on tip & ring. Again no "click" as you insert the phones.

When Cmoy is changed from ON to OFF or from OFF to ON the output capacitor has to change charge (voltage) from old state to new state. This requires current to flow. At the moment one side of the cap has all routes blocked with capacitors. No DC current can flow (until you plug in your phones). Change the circuit. It is a bad design. Add the two 10k resistors and sort what should never have been published.
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Old 8th February 2011, 10:55 AM   #5
Mooly is offline Mooly  United Kingdom
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The CMOY as you have posted will work OK with the input cap shorted... but it's not advisable because you rely on the feed into the amp being at 0.00 volts. Any slight offset would be amplified by the CMOY depending on the volume setting.

Andrews suggestion is good for adding a resistor to each output of the LM386 to ground (after the cap). Any amp that's AC coupled at the output will appear to have zero offset, so that in itself doesn't mean anything.
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Old 8th February 2011, 12:22 PM   #6
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If you are interested in a high performance headphone amp without caps in the signal path, take a look at this one:- John Conover: Direct Coupled Stereo Headphone Amplifier

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Old 30th March 2011, 03:57 AM   #7
ddietz is offline ddietz  United States
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As a dense noob, I'm curious about AndrewT's post above. I'm going to add my little story to this thread since they seem very similar.

The CMoy has no output capacitor. Are you talking about adding an output capacitor and resistor to ground for each output one OPA2132PA CMoy, or was that referring to the LM386 based amp?

I was just building a cmoy based headphone amp for a friend, after making one for myself. My works great and has very nice sound (IMO). I'm using 0.1uf 200V polypro input caps in that and have good bass response.

The new amp has almost no bass and distortion at volume spikes of transients. The new amp differs from the old in a couple ways. I'm using a OPA2134PA instead of the 132PA on the first. I read that these might be a little finickier. Also, the input caps on the new are 0.15uF polyester (the little green rounded rectangle kind) pulled from the parts bin.

Aside from bass loss (capacitor related) I get some DC at the outputs. One side is 0.6mv max, the other is 1.6mV max.

Bypassing the input caps leads to a better sound. Its much louder and has less distortion, but there is still a little on the heavy bass. My sources so far have low enough DC that I felt safe trying it bypassing the input caps.

Input is MP3 from ipod. I'm using 66R headphones and cheap earbuds. Distortion (crackly at louder parts and higher volume- but not that high) is apparent in both.

Should I try changing the opamp chip, different input caps, or bypass the input caps and add output caps with a grounding resistor? Obviously I'd like to try all but...I can get different caps locally but have to order the chip so I'd like to try and get this chip working. Also, I have cleaned the flux and checked all my solder joints.

Thanks for reading!
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Old 30th March 2011, 09:07 AM   #8
AndrewT is offline AndrewT  Scotland
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I think you have current clipping.
What is the impedance of your headphones?
What is the average voltage you listen at?
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Old 30th March 2011, 12:46 PM   #9
ddietz is offline ddietz  United States
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Thanks for the reply.

Headphones are cheap apple earbuds (don't know impedance) and 66 ohm ATH 3O.
Clipping happens at all volume of source level. Not sure the listening voltage but lll try to check it.
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Old 30th March 2011, 01:35 PM   #10
Mooly is offline Mooly  United Kingdom
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0.15 caps all things being equal will give "more" bass than 0.1's when used as input caps.

The DC offset at the out is minimal and should be an issue.

Have you tried add a small cap of say 10uf directly across the opamp supply pins ?
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