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Old 17th February 2013, 06:43 PM   #21
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10-12mV is excellent for DC offset. Typically, anything below 40mV is fine, 25mV or below is plenty good enough as it becomes academic, like rows of zeros in a distortion measurement. When there is no provision to adjust offset (and for reliability, I don't think there should be either), the match of Q1:Q2 Vbe determines the current balance in the LTP (input pair) and hence the degree of offset voltage at the output.

Swap them or try other similar matched pairs and get a different offset or none at all, if you must have it.

The ratio of R10:R19 sets bias current, much as the pot usually placed there does in many other designs. I'd warn against experimenting with trying the next closest resistor you have to see what happens. The circuit will be more sensitive than you think and a toasted output stage of really good output transistors is not going to be nice to replace.

The double Szlikai OP stage is featured in Douglas Self's Amplifier Design Manual as the "Load Invariant Amplifier", meaning it performs almost as well on 4R as 8R loads.
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Old 17th February 2013, 07:00 PM   #22
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Uhh..no... sorry...R13 and 20 are the "collector" resistors in the output stage, so you are measuring Bias current - I guess you have quoted for each channel and across both resistors?
The schematics show different resistors; 0.22R in your posting Evenharmonics and 0.1R in mine. If ohms law is still true, you need to calculate the current from the voltage across the actual resistor(s) you are measuring. Across both R13 & 20 is more accurate.

That calculated current should match the specification on the website documentation. Which, IIRC, is 20-40 mA
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Old 17th February 2013, 07:15 PM   #23
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Quote:
Originally Posted by Ian Finch View Post
10-12mV is excellent for DC offset. Typically, anything below 40mV is fine, 25mV or below is plenty good enough as it becomes academic, like rows of zeros in a distortion measurement.
Does it matter whether it's positive or negative DC voltage? Mine shows negative when red (positive) lead of DMM is connected to + of speaker terminal and black lead to - of speaker terminal.
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Old 17th February 2013, 07:22 PM   #24
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Originally Posted by Ian Finch View Post
The schematics show different resistors; 0.22R in your posting Evenharmonics and 0.1R in mine.
Mine shows "OR 1" and on the surface of actual resistor shows "0.22 Ω".
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Old 17th February 2013, 07:35 PM   #25
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Originally Posted by Evenharmonics View Post
Does it matter whether it's positive or negative DC voltage? Mine shows negative when red (positive) lead of DMM is connected to + of speaker terminal and black lead to - of speaker terminal.
I thought 10 and 12 mV was what you measured across the output transistor collector resistors?

Well, if it was the voltage across the output terminals, that's fine - it's the offset voltage and it doesn't matter what polarity it is. In simple theory, if you swap Q1 and Q2 the polarity should simply reverse.

Re: the resistors: Yes the 0R1 is the same on the schematic I showed but for yours, these are all changed to 0R22 for Rev-2.
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Last edited by Ian Finch; 17th February 2013 at 07:41 PM.
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Old 17th February 2013, 08:39 PM   #26
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Originally Posted by Ian Finch View Post
I thought 10 and 12 mV was what you measured across the output transistor collector resistors?
That was the bias voltage. DC offset from output terminal is -24mV DC.

Anyway, for $19 pre-assembled module, this thing is a bargain!
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Old 3rd April 2013, 08:44 PM   #27
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Bump.
Quote:
Originally Posted by Ian Finch View Post
10-12mV is excellent for DC offset. Typically, anything below 40mV is fine, 25mV or below is plenty good enough as it becomes academic, like rows of zeros in a distortion measurement. When there is no provision to adjust offset (and for reliability, I don't think there should be either), the match of Q1:Q2 Vbe determines the current balance in the LTP (input pair) and hence the degree of offset voltage at the output.

Swap them or try other similar matched pairs and get a different offset or none at all, if you must have it.
I'm guessing that you meant Q6 and Q8 (1015 transistors).
Click the image to open in full size.

I had little extra time and those transistors cost about 12 cents each at Mouser so I bought 6 and decided to play around. I took out the ones came with the board and there was a bit of mismatch when I checked with DMM. I measured the new ones and they are all very close (closer than what came out of the board). I soldered the matching new pair and DC offset is now 15mV (previously 24). Is the difference audible? I don't think so but it was a fun little project and I'm happy with the progress.
Thanks for the tips.
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Old 4th April 2013, 03:48 AM   #28
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Great , it's good fun tweaking but as you find, some notional ideals like zero DC offset are often a waste of time as they don't show any any obvious benefit apart from knowing you have....uh...zero DC offset.

The awkward truth is that the actual Vbe and Hfe matches are more important for the lowest amplifier THD condition than the Vbe alone is for the DC offset. That in turn results in max. cancellation of even order, mainly desirable 2nd harmonics. And so it goes on, lurching about from bad to worse when you start tweaking some parameters. FWIW, you then have to tweak the current mirrors, Q11,13 I think, to null the offset. The mirrors will fight any change the LTP, to maintain equal current in Q6,8 as they are designed to

Yes, I did quote the wrong parts for the input pair as you guessed. I can't recall why other than I guessed because I couldn't read it at the time of typing and forgot to amend it later. That seems to happen a lot to me when I try to avoid losing the post completely trying to view earlier attachments if I opt to make an advanced reply. That is a right PITA and a good time to quit and go to bed.
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Old 4th April 2013, 05:09 AM   #29
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Originally Posted by Ian Finch View Post
FWIW, you then have to tweak the current mirrors, Q11,13 I think, to null the offset. The mirrors will fight any change the LTP, to maintain equal current in Q6,8 as they are designed to
Yes, that was my next on the list. I bought extra of those as well since they are cheap. I'll report back.
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Old 4th April 2013, 06:24 AM   #30
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Are 5551 and 1815 interchangeable?
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