Official M2 schematic

[noSmoking]

hi try increasing R6 R7 is the other direction _ and +

 

[Zen Mod]

we will know everything needed , once we confirm proper Iq and what's lowest DC offset

 

[anilva]

I have used an LED and a 1k resistor for R15 on the PCB thinking that it was just a power indicator. Looks like it is in the bias circuit. Looks like I need to eliminate them out of the circuit.

I believe the LED was provisioned by Grimberg for additional bias requirements.

Regards

 

[grimberg]

I believe the LED was provisioned by Grimberg for additional bias requirements.

Absolutely not! The LED + resistor go from V+ to GND, THAT"S IT!

 

[Zen Mod]

shall we see those measurements ?

 

[anilva]

Absolutely not! The LED + resistor go from V+ to GND, THAT"S IT!

Stand corrected. I have used LED for the right purpose then. Will post the readings in a day or so as I am traveling.

Regards

 

[rolandong]

The pins/footprint for Q6/Q7 (LM385-1) regulators and JFETS are close/tight to each other. Make sure that there are no shorted solder connections. Also, there are only few resistors/components so double check values absolute correct (SOP to measure before inserting component).

 

[anilva]

Help on M2 build

please post actual schematic and pictures of build

write down what is voltage across source resistors , what is exact (minimal) DC offset you can get with trimpot on end , taking care of proper polarity (probes)

then we can help with more specific steps

Hi,

Here are the readings:

Left Channel:

Minimal offset with trimpot adjusted +2.78V

P channel Source resistor voltage .. 0.667V
N Channel Source resistor voltage .. 0.673V

Right Channel:

Minimal offset with trimpot adjusted +4.09V

P channel Source resistor voltage .. 0.688V
N Channel Source resistor voltage .. 0.690V

As mentioned earlier I have used random set of Toshiba 2Sj201/2SK1530 pair (vgs of which I have never measured) and these were removed from my working F5 amp. May be there is a huge variance on the output transistors.

I do have very well matched IRF pairs and also a Fairchild Pair with a 1 volt VGS difference between N and P parts. I can replace the outputs if that is the reason for this huge offset voltage.

Thanks in advance.

Anil

 

[anilva]

Hi,

Here are the readings:

Left Channel:

Minimal offset with trimpot adjusted +2.78V

P channel Source resistor voltage .. 0.667V
N Channel Source resistor voltage .. 0.673V

Right Channel:

Minimal offset with trimpot adjusted +4.09V

P channel Source resistor voltage .. 0.688V
N Channel Source resistor voltage .. 0.690V

As mentioned earlier I have used random set of Toshiba 2Sj201/2SK1530 pair (vgs of which I have never measured) and these were removed from my working F5 amp. May be there is a huge variance on the output transistors.

I do have very well matched IRF pairs and also a Fairchild Pair with a 1 volt VGS difference between N and P parts. I can replace the outputs if that is the reason for this huge offset voltage.

Thanks in advance.

Anil

Replaced the Toshibas with Fairchild output devices withknown vgs. Also recleaned the close spaces on the jfets and lm385s transistors. Some or all of it worked. Nicely adjusts now to under 10mv. Thanks for the advice. Now to test the amp with my Altec horns!

Cheers
Anil

 

[Zen Mod]

all you needed was either to use bigger R6 (that preferable) or smaller R7

ref. to sch you posted in #2360

I believe 56K for R6 will do the job

 

[anilva]

Thanks. I might try the Toshibas again at a later date.

Regards

 

[rolandong]

I might try the Toshibas again at a later date.

You won't want to miss that

 

[anilva]

Finally played my M2. Zero startup thump or switch off thump. Cannot make out that amp is on with ears close to speakers. When fed with music, the amp comes alive with full dynamics. Terrific dynamic range and clarity. Cannot make out which of Pa’s amps are great, because all of them are. My ears have got numbed listening to some great amps from Papa. Cannot out make out which is better.

Great addition to my Pa’s amp collection. Next is VFET 2.

Thanks to Pa and this forum for giving me years of Joy. May the tribe grow!

Merry Christmas and a great new year!

Cheers
Anil

 

[Dennis Hui]

Congratulations Anil!

 

[chede]

You won't want to miss that

+1 for Toshiba 2SK1530 / 2SJ201 !

(... with 0R33 for the source resistors and 0R0 (jumper) instead of 221 ohms for R11, as per suggestions of ZenMod somewhere in this thread; runs at 1.81 amps)

Best regards,
Claas

 

[anilva]

+1 for Toshiba 2SK1530 / 2SJ201 !

(... with 0R33 for the source resistors and 0R0 (jumper) instead of 221 ohms for R11, as per suggestions of ZenMod somewhere in this thread; runs at 1.81 amps)

Best regards,
Claas

Will do. First want to enjoy the music for some time before attempting the Toshibas.

 

[chede]

... First want to enjoy the music for some time before attempting the Toshibas.

 

[Nelson Pass]

 

[marconi118]

can you go with 0R0 source resistor with 2SK1530 / 2SJ201?

are they needed only for bias stability?

 

[Zen Mod]

this is just #2380th post in thread ;

I'm sure that somewhere earlier in the thread someone explained bias mechanismus

so - you can go with 0R source resistors and , if you're fast enough , maybe you will hear some music prior to Boom ........ Big Badaboom