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Old 10th January 2008, 01:43 AM   #1
decky is offline decky  Australia
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Default Ci and DC offset ??

Hi guys,

I have just completed my first gainclone 3886 kit (Brian GT). It is a dual mono configuration with two trafos 160VA each (25V supply).

I decided to ommit the Ci capacitor to avoid any capacitance in the signal path. The measured DC offset without load is 97mV on one chanell and 120mV on the other. The sound is pretty good but I keep wandering is this DC offest good enough and is the 23mV difference between chanells something I should worry about. Would the DC offset and the noise floor be any lower with 47uF Ci?

Second question - how dangerous is driving 6ohm speakers with this amp considering that the supply is chosen for 8ohms. Is it only thermal dissipation I should worry or there are some other issues.

Thanks and sorry if I am raising same old stupid questions but I could not find answers by seraching.
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Old 10th January 2008, 02:28 AM   #2
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That is a lot!,I build mine (LM3875)and I have 8mv in one channel and 10 mv in the other channel.you should check all wires and connections again. I think it is way too high.
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Old 10th January 2008, 06:49 AM   #3
decky is offline decky  Australia
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I thought that a rule of thumb (for the worst case scenario) is 10mV * Gain (33 in this case). This is a non-inverting kit and Brian suggested DC offset <100mV - I am more worried about the 25mV differential.

Any suggestions about the 6ohm speaker load?
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Old 10th January 2008, 08:13 AM   #4
Nuuk is offline Nuuk  United Kingdom
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The differential is of no consequence although I prefer not to have DC offset of more than around 70 mV!

The DC offset can vary a lot with the chip, even of the same type.
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Old 10th January 2008, 10:24 AM   #5
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When you tested the DC offset did you short the input on them? This makes quite a difference and it is the accepted way to test it.
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Old 10th January 2008, 11:06 AM   #6
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Those values are for no-signal state. However, you should expect much more DC leaking in with signal present, so I think omitting decoupling capacitors in this design is not a good idea.
Instead of lossy and distorting 47 uF electrolytics, you can use 4,7 uF MKP (1/10th of design value for elecotrlytics) without losing the bottom end response.
As for the impedance matching, it depends on how your speakers behave and what is their minimal impedance. Most of 8 Ohm loudspeakers will go down near to 4-5 Ohm on some frequencies and this is taken into account in amplifier designs. It is reasonable to suspect that a 6 Ohm loudspeaker would go even lower and this can significantly drain your output stage and possibly kill it.
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Old 10th January 2008, 11:58 AM   #7
decky is offline decky  Australia
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No - the input was open circuit. Would that make any considerable difference??
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Old 10th January 2008, 12:10 PM   #8
Nuuk is offline Nuuk  United Kingdom
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Well short the input and make sure you have a load (10 ohm resistor will do) on the outputs and then measure again.
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Old 10th January 2008, 12:50 PM   #9
decky is offline decky  Australia
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I really have to be completely anal now. Lets go back a step. DC offset defines the noise floor if I understand everything properly. The Ci is a filter capacitor to serve as a high pass filter together with R3 (680 ohm in Brian's layout) adjusted to very low frequency - 47uF and 680ohm leads to f=1/(2PI*R3*Ci)=5Hz - therefore 4.7Hz will filter below 0.5Hz so might be not as efficient. The LM3886 data sheet aims at 7.2Hz with Ri and Ci combination of 1kohm and 22uF respectively.

Also I am not sure that I can physically fit an MKP capacitor on the board. Would tantalum or at least NP electrolytic be any better?

Also, bare in mind that the sound from the amp is pretty awesome. I have a bit of noise only on my CD modified output stage. Everything else works as a charm.

Regarding the speaker - here is the impedance curve

http://www.stereophile.com/standloud...41/index4.html

It does not look too bad
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Old 10th January 2008, 12:50 PM   #10
decky is offline decky  Australia
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Quote:
Originally posted by Nuuk
Well short the input and make sure you have a load (10 ohm resistor will do) on the outputs and then measure again.
Ok Ok - right channel 17.3mV - left 16.8mV - but which method is more relevant.

Also considering that I have bit of a noise when using CD player is it really worth while cramping a big MKP cap on such a small board?
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