FirstWatt J2

[Peppe]

For the FE CCS (Q1 in EUVL schematics in #1393) I have 2 pairs of 2SK170-BL to choose from with idss of 8.51 and 8.52 mA, and 8.97 and 9.25 mA.

Which pair is the best for the job and is the answer related somehow to the what is said below?

For a jfet ccs lots of degeneration is a good thing.

 

[Peppe]

Is it the use of a variac still valid for first turn on an optocoupler biased amp?

 

[Zen Mod]

this one, yes

 

[Peppe]

I have an offset problem (I think).

By turning all the way counterclockwise Rdg (50R trimpot) in the schematics below I can get the DC offset at the output to 80 mV with inputs floating, and 800 mV with inputs shorted.

Both channels behave the same. PSU is +/- 22.5V under load and bias current for Q5 is 1.4A.

I did use IRFP240 for Q4 and 2SJ109-BL for Q2-Q3. I do not have the BOM on the laptop to check other values at the moment.

What should I look into to reduce the offset?

.

 

[Ylab]

Making R33 a little bit higher can get you there. 330 would get you close to 0mv offset I think

 

[EUVL]

If the offset is negative, then the current source has too much current, and you need to increase Rdg.
If the offset is positive, then the current source has not enough current.
You can try to increase the 300R resistor to 330R or 360R.
I always short the input while adjusting DC.


Patrick

 

[generg]

For the FE CCS (Q1 in EUVL schematics in #1393) I have 2 pairs of 2SK170-BL to choose from with idss of 8.51 and 8.52 mA, and 8.97 and 9.25 mA.

Which pair is the best for the job and is the answer related somehow to the what is said below?

Nelson recommended around 8-10mA for the pair doing the CCS for the J-Fets. so he took two SK170 GR adding to the 8-10mA.

 

[Peppe]

Thanks. Currently I am using the pair with higher idss (one per channel since EUVL PCB has the FE halved, see #2,184).

Furthermore, with R33 = 330R the offset is reduced but not enough. So I have to play a bit more there.

 

[Marcus Halberstram]

I'm finally finishing my J2 clone. I have` been sitting on these SemiSouth JFETs for a long time. Thanks to @Nelson Pass, @Zen Mod, @EUVL, @rhthatcher, @xrk971. Especially to Nelson.

 

[KevinHeem]

Do you think this sounds like a crazy idea? Building two J2's and wiring them for the option of balanced (bridged) operation. I have enough Fet's to build two. My speakers need a little more drive than a single J2 can provide I'm afraid. I know I'm probably giving up dominant second for third. Anything else I should consider?

 

[Zen Mod]

bigger speakers?

 

[Marcus Halberstram]

In bridged operation, each half of the bridge will see half of the load impedance. Seems like not a good fit for the J2. I think some have paralleled output devices, which seems like a better plan.

 

[marconi118]

half damping factor

 

[Marcus Halberstram]

and same power output (no increase)

 

[Itsmee]

Do you think this sounds like a crazy idea? Building two J2's and wiring them for the option of balanced (bridged) operation. I have enough Fet's to build two. My speakers need a little more drive than a single J2 can provide I'm afraid. I know I'm probably giving up dominant second for third. Anything else I should consider?

If the J2 has enough output voltage to get the spl you want, build a crippled F4 and put it between the J2 and your speakers.

 

[6L6]

Kevin… are you running out of gain, or clipping your amp?

 

[KevinHeem]

Kevin… are you running out of gain, or clipping your amp?

Well, neither to my knowledge...it's just my experience that an amp with more wattage and a high damping factor produces a better sounding mid-bass/bass with my speakers. Some smart a** may say "get different speakers" but I've listened to many speakers of differing type's and keep coming back to what I have.

 

[6L6]

100% on board with build around your speakers. They are the hardest part to get right.


There’s nothing wrong with the idea of trying bridge J2 monos. How will you drive them? Have you got an XLR out preamp? Transformer? Bridging adaptor of some other kind? Lots of ways to do it.

 

[KevinHeem]

Yes, I have a couple of options having XLR output.

 

[Rush]

Well, neither to my knowledge...it's just my experience that an amp with more wattage and a high damping factor produces a better sounding mid-bass/bass with my speakers. Some smart a** may say "get different speakers" but I've listened to many speakers of differing type's and keep coming back to what I have.

Kevin,

This is my experience with why a more powerful amp has better control of the mid-bass/bass. It's mostly that the power supply of a higher wattage amp has more current available, lower impedance , not so much the higher voltage swing.
The other factor, lower output impedance (higher damping factor) could be achieved with a little feedback.

Rush