Balance Control
About the two pots in the B1.. Are they for adjusting the two channels seperately or is one for main volume and the other for balance? If it's not the latter, how difficult would it be to add a balance pot?
I also can't understand why the B1 is called a 'buffer preamp' if there is no gain. Maybe because the input and output impedances are different?
Please bear with me, I'm a newbie..😱
Christo
About the two pots in the B1.. Are they for adjusting the two channels seperately or is one for main volume and the other for balance? If it's not the latter, how difficult would it be to add a balance pot?
I also can't understand why the B1 is called a 'buffer preamp' if there is no gain. Maybe because the input and output impedances are different?
Please bear with me, I'm a newbie..😱
Christo
one volpot per channel
second question - buffer in electronics is usually stage with high input and low output impedance
so , B1 is adequately called buffer
second question - buffer in electronics is usually stage with high input and low output impedance
so , B1 is adequately called buffer
Thanks, the buffer thing makes sense now.
And as for the two volume pots - one has to adjust both to raise or lower the volume, but it also serves to adjust balance. It's a bit crude, but hey, I suppose it has the least amount of circuitry and thus clearer sound.
And as for the two volume pots - one has to adjust both to raise or lower the volume, but it also serves to adjust balance. It's a bit crude, but hey, I suppose it has the least amount of circuitry and thus clearer sound.
The only way for me to have balance control. I would not do it any other way. Once you get the hang of it you will not look back, if you need balance control that is.
Ok, I have settled on the idea of getting two volume pots. 😉
But I'm wondering about matching the Fets. I bought 8x 2SK170's from a local supplier and four of them match reasonably close:
2. If I can't find better matches, should I use the closest matches in the same channel or in the same positions in the Left & Right circuits?
But I'm wondering about matching the Fets. I bought 8x 2SK170's from a local supplier and four of them match reasonably close:
8.31,
8.27,
8.40,
8.50 mA Idss.
1. Is that close enough or should I look for better matches?8.27,
8.40,
8.50 mA Idss.
2. If I can't find better matches, should I use the closest matches in the same channel or in the same positions in the Left & Right circuits?
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Use the 8.27 & 8.31 in one channel
Use the 8.4 & 8.5 in the other channel.
This will give a small offset, but the output capacitor will block the DC.
Use the lower Idss for the CCS device.
That leaves the higher Idss for the Follower.
This ensures that at quiescent the Follower operates at very slightly less than Idss and it's Vgs is very slightly negative (this is the offset referred to earlier).
Use the 8.4 & 8.5 in the other channel.
This will give a small offset, but the output capacitor will block the DC.
Use the lower Idss for the CCS device.
That leaves the higher Idss for the Follower.
This ensures that at quiescent the Follower operates at very slightly less than Idss and it's Vgs is very slightly negative (this is the offset referred to earlier).
I was inspired to take two 2SK70's at 6.33 mA and 6.63 mA Idss respectively
and measure them as current source and follower, and they settled in at
9 mV DC offset. From that you can estimate about 3 mV offset for 0.1 mA
mismatch.
and measure them as current source and follower, and they settled in at
9 mV DC offset. From that you can estimate about 3 mV offset for 0.1 mA
mismatch.
Offset voltage is just current mismatch x 1/Yfs, not true ?
And 1/Yfs for 2SK170 is ~ 30 ohm.
Patrick
And 1/Yfs for 2SK170 is ~ 30 ohm.
Patrick
@AndrewT. Thanks for your reply. I will do what you suggest. Although I had no idea initially what ‘follower’ and ‘CCS’ meant, my confusion was cleared up when I referred to the circuit description in the B1 PDF: Followers: Q100, Q200 and constant current source/s Q101, Q201.
Now to draw up a single sided PCB that I can etch myself, because having it made locally or ordering one from PassDiy + shipping is prohibitively expensive.
Now to draw up a single sided PCB that I can etch myself, because having it made locally or ordering one from PassDiy + shipping is prohibitively expensive.
There are so few components you could consider hardwiring.
Each Buffer is just two transistors and no resistors.
Then just add on the bits to complete the whole arrangement.
Do you have a plug board?
that can be used initially to do some experiments and prove the Buffer is working and allow voltages to be measured.
Then you can use that to see how you could hardwire it, or use a small piece of perfboard.
Each Buffer is just two transistors and no resistors.
Then just add on the bits to complete the whole arrangement.
Do you have a plug board?
that can be used initially to do some experiments and prove the Buffer is working and allow voltages to be measured.
Then you can use that to see how you could hardwire it, or use a small piece of perfboard.
why not get a stereo stepped pot with MFR's? Its even more minimalist (build-wise) and simple (in my view).
Ditch the alps.
That's a good idea. I do have breadboards.
I was actually thinking about using perfboard or maybe Veroboard. Great suggestions, thanks.
I never did find the need for a balance when using MFR step pots.
I dont know your experience.
😀😀😀
I don't have much eperience with this, but if two pots is good enough for Papa, it's good for me! 🙄
And since I'll probably be breadboarding initially, I'll soon find out what I need or don't need.
I don't have much eperience with this, but if two pots is good enough for Papa, it's good for me! 🙄
And since I'll probably be breadboarding initially, I'll soon find out what I need or don't need.
ok, fair enough, do let me know your experience. I designed a clone with single stereo pot,yet to test.
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Yes. Of course you can sim it or work it out from data sheets, but
it's always nice to see what actual parts do.
I have finished building a B1 and it sounds great. For the board I have used Paxolin with 0.8mm copper wire for point to point connections. It's housed in an aluminium enclosure.
My thanks go to Nelson Pass for the design. I'm grateful for the generous help from the forum members who answered my questions, and I'm also indebted to forum member Twocents who let me listen to his B1 and encouraged me and gave advice when I needed it. It really helps to have a partner in this type of crime. 😀
I still have a question, though... If the current draw is only about 20 milliamps, then why does R1 need to have a rating of 3W? Wouldn't a 1W resistor be sufficient?
Thanks in advance.
Christo
My thanks go to Nelson Pass for the design. I'm grateful for the generous help from the forum members who answered my questions, and I'm also indebted to forum member Twocents who let me listen to his B1 and encouraged me and gave advice when I needed it. It really helps to have a partner in this type of crime. 😀
I still have a question, though... If the current draw is only about 20 milliamps, then why does R1 need to have a rating of 3W? Wouldn't a 1W resistor be sufficient?
Thanks in advance.
Christo