My amp has no trim pots for DC offset, getting less than 15mv for left channel and right channel is showing 50mv to 55mv. I know the fix will require replacing a pair of transistors, but both needs to be closely matched. Can you purchase matched sets of transistors from online stores?
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You can but you have no choice as to what the VBe will be, so no point.
'Select on test' is usual with resistors controlling the bias.
If you disclosed the make and model and a schematic, we would be able to tell you which resistor to change.
'Select on test' is usual with resistors controlling the bias.
If you disclosed the make and model and a schematic, we would be able to tell you which resistor to change.
I saw this for the R2.5 (my amp is R1.5), person who posted this mentioned the circuit is the same except that there are more transistors and rail voltages are higher.
Interesting.
IF so, you´ll need matched 2SK240.
As rare as hen´s teeth.
My take is:
1) you don´t really have a problem
2) Factory won´t be happy with you messing with the amp; if anything they may send you a suitable pair.
3) input transistor mismatching might not be the problem anyway, but something else.
4) I would cheat/improve the circuit by adding a minuscule adjustable DC voltage to one of input gates.
Generate +/- 650mV using 2 diodes, add a 10k trimmer from positive to negative and connect wiper through a 1M resistor to R11/C2 node and adjust offset to zero.
Since you are at it, do same to the other channel.
IF so, you´ll need matched 2SK240.
As rare as hen´s teeth.
My take is:
1) you don´t really have a problem
2) Factory won´t be happy with you messing with the amp; if anything they may send you a suitable pair.
3) input transistor mismatching might not be the problem anyway, but something else.
4) I would cheat/improve the circuit by adding a minuscule adjustable DC voltage to one of input gates.
Generate +/- 650mV using 2 diodes, add a 10k trimmer from positive to negative and connect wiper through a 1M resistor to R11/C2 node and adjust offset to zero.
Since you are at it, do same to the other channel.
Is the 2SK240 hard to find or finding a matched pair is hard to find ?
What do you mean by "1) you don´t really have a problem" ?
What do you mean by "1) you don´t really have a problem" ?
The 2sk240 is a rare part, though you can replace it with a matched pair of 2sk170.
60mV DC offset is less then 0.3mW into 8 ohms and generally isn't a real concern.
60mV DC offset is less then 0.3mW into 8 ohms and generally isn't a real concern.
Probably as JMF suggests in #5 (1) above, even 100mV offset is nothing to worry about. Only those who suffer from OCD or use active crossover speakers will be affected by tiny DC offsets and even those fears will be self inflicted or influenced by doomsayers insisting that their tweeters will be damaged.
My view is, don't obsess over what is little more than window dressing with numbers, like inferring that an amplifier with 0.001% THD will sound 10 times better than another with 0.01%. In such cases, where infinitesimal amounts are considered, the amplifier with slightly higher distortion levels or DC errors is likely to sound a tad better to the ear than the technically clean one, regardless. In audiophile circles that is quite often the case, too.
My view is, don't obsess over what is little more than window dressing with numbers, like inferring that an amplifier with 0.001% THD will sound 10 times better than another with 0.01%. In such cases, where infinitesimal amounts are considered, the amplifier with slightly higher distortion levels or DC errors is likely to sound a tad better to the ear than the technically clean one, regardless. In audiophile circles that is quite often the case, too.
Thanks everyone!!!
Will put back the soldering iron away for now, now, let see if I can sleep better later knowing there is a 35mv difference between my left and right channel 🙂
Will put back the soldering iron away for now, now, let see if I can sleep better later knowing there is a 35mv difference between my left and right channel 🙂
As a direct reply to your OP, you might consider our own diyAudio store, as you lie awake at night. Click on the store in the top navigation bar to find it does sell very closely matched LSK170 JFET quads, actually. They are current Linear Systems products rather than obsolete and also scarce originals and they aren't cheap but at least you know where you can get high quality matched sets which are equivalent to the OEM 2SK170.