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Old 24th October 2011, 01:37 PM   #3171
Thago is offline Thago  Belgium
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Quote:
Originally Posted by AndrewT View Post
Thago,
post a pic with the components labeled.
We can then show you which components do what.
Hi Andrew

I had the original B1-schedule from Nelson in mind:
b1_fig1.pdf
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Old 24th October 2011, 01:50 PM   #3172
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C100/200 do form a a high pass filter, but that's not why they're there... They isolate the DC at the gates of Q100/200 from the input. Also, the high-pass filter cutoff is based on C100/200, R103/203 and the input impedance of Q100/200...

Roscoe
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Old 24th October 2011, 02:17 PM   #3173
AndrewT is offline AndrewT  Scotland
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Yes, as pointed out C100 R103 form a high pass filter.
There is no low pass filter in the B1. RF is fed in and the buffer has to cope with it.

F-3dB of the HP is ~0.16Hz & F-1dB ~0.3Hz.

If the effective Zin of the actual jFET were also 1M @ very low frequencies, then the F-1dB would move up to 0.6Hz.
You could change C100 to 100nF for F-1dB ~ 3 to 6Hz.
Whether you could hear any change in the low bass can only be tested by listening. Measuring will simply confirm that >20Hz has <0.05dB of change.

100nF Teflon might sound better than 1uF MKT, in the rest of the audio spectrum. Again listening is the only way to tell.
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regards Andrew T.

Last edited by AndrewT; 24th October 2011 at 02:19 PM.
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Old 24th October 2011, 02:54 PM   #3174
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Well, no designed-in LP filter, R102/202 and Cgs of Q100/200 do form a LP filter, but it's, umm, not real close to the audio band ;-)

Roscoe
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Old 24th October 2011, 07:12 PM   #3175
Thago is offline Thago  Belgium
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Quote:
Originally Posted by AndrewT View Post
Yes, as pointed out C100 R103 form a high pass filter.
There is no low pass filter in the B1. RF is fed in and the buffer has to cope with it.

F-3dB of the HP is ~0.16Hz & F-1dB ~0.3Hz.

If the effective Zin of the actual jFET were also 1M @ very low frequencies, then the F-1dB would move up to 0.6Hz.
You could change C100 to 100nF for F-1dB ~ 3 to 6Hz.
Whether you could hear any change in the low bass can only be tested by listening. Measuring will simply confirm that >20Hz has <0.05dB of change.

100nF Teflon might sound better than 1uF MKT, in the rest of the audio spectrum. Again listening is the only way to tell.
Hi Andrew

Thank you for the clear explanation, I like to know what I'm doing.
So with lower values for C100 F-1db will be higher, with higher values F-1dB will be lower. As I have some Jantzen Silver Caps 3.3uF lying around, using them would cause a lower F-1dB, thus a larger bandwith. Is this desirable, or are there any contra indications for using such high values for C100?
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Old 24th October 2011, 07:18 PM   #3176
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Quote:
Originally Posted by Thago View Post
Hi Andrew

Thank you for the clear explanation, I like to know what I'm doing.
So with lower values for C100 F-1db will be higher, with higher values F-1dB will be lower. As I have some Jantzen Silver Caps 3.3uF lying around, using them would cause a lower F-1dB, thus a larger bandwith. Is this desirable, or are there any contra indications for using such high values for C100?
I doubt there's any real advantage to lowering the F-1 when it's already <1Hz. Generally, smaller caps of the same type tend to sound better than larger ones. Of course, there's something to be said for using what you've already got!

Roscoe
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Old 24th October 2011, 07:36 PM   #3177
Thago is offline Thago  Belgium
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Hi Roscoe

Generally, smaller caps of the same type tend to sound better than larger ones That's what I was already afraid of I guess I'll have to wait for another opportunity to use the Jantzen caps, we don't want to compromise on sound quality don't we?
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Old 24th October 2011, 07:50 PM   #3178
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Go ahead and build it. You can always change the caps later, then you'll know what sounds better TO YOU in YOUR system, which is the only thing that really matters anyway.

Roscoe
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Old 24th October 2011, 07:59 PM   #3179
Thago is offline Thago  Belgium
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You're absolutely right, after all that's the major privilege of DIY

Dirk
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Old 14th November 2011, 02:59 AM   #3180
ryanred is offline ryanred  United States
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Quote:
Originally Posted by Nelson Pass View Post
If you are concerned about the output impedance of the B1,
I suggest you try an output resistor value of your choice. The
intrinsic output impedance of that buffer is about 30 ohms, but
there would be some potential stability concerns using it
without some value of resistance, as it might oscillate into
capacitance.

If you have an oscilloscope, you can find out pretty easily.

I have access to an oscilloscope and want to try lowering the output impedance. What points should I measure to determine if there is oscillation with the lower resistor values?
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