B1 Buffer Preamp

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T

tschrama

Member
Joined 2001
Re: JFET grades

MEH said:
Why does the B1 (and Pass amps more generally) use GR or BL grade 2SK170s? I mean, the circuit obviously has to be designed to use some Idss grade, and Papa did design it to use GR or BL, but why? Is there something about lower Idss grade JFETs that makes them audibly superior to higher Idss parts, or is this just another example of Papa using the parts he has on hand, even though another part could have been used equally well in a slightly different design?
Click to expand...


Using V-grade will set the working point of the JFETs further into 'triode' mode, using GR-grade will set the working point of the JFETs further into 'pentode'mode.

Pentode mode in this application is more linear?..I tried Idss of 13mA and THD increased. I tried Idss of 3mA and 9mA and THD was identical but lower than Idss of 13mA. Connection a real 10K load, might change this however.. didn't measure that. I think Idss of around 9mA will be optimal..

grt, T
 
U

udailey

Member
Joined 2008
picasaweb.google.com/udailey/MyRef#
Re: filoter RC time constant

AndrewT said:
Just multiply the two R & C values to get the RC time constant.
eg.
1k0 & 10uF = 0.01Seconds = 10mS
4k7 & 2u2F = 0.01034Seconds = 10mS

Frequency @-3dB = 1/2/Pi/RC
for a single pole passive filter fed from zero source impedance and supplying an infinite load impedance.

BTW,
10mS will cut some bass from the signal. Compare to 20mS and 40mS to hear the effect.
I recommend ~90mS for power amps and >=150mS for pre-amps.
Click to expand...

That is a really helpful post. No brown nosing intended, but this is one of those important pieces that a noob puts in their little scrapbook.
Uriah
 
Babowana

Babowana

Formerly "jh6you". R.I.P.
Joined 2006
aheadamp.com
Re: output caps

Bengali said:
10uF output cap and 1K is about 16Hz for cutoff freq(?)

So we should not use anything below 10uF?

What if I used 2.2uF with a 4.7K resistor? is this close to same as 10uf and 1K without affecting performance?
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With 10uF+1K into 10k load, I estimate the low frequency roll-off at 1.5Hz.
With 10uF+1K into 47k load, I estimate the low frequency roll-off at 0.4Hz.

In the similar way as flws:

With 2.2uF+4.7K into 10k load, I estimate the low frequency roll-off at 5Hz.
With 2.2uF+4.7K into 47k load, I estimate the low frequency roll-off at 1.5Hz.

My conclusion is that you could use the combi of 2.2uF+4.7K if you do not mind the 5Hz roll-off.
 
h_a

h_a

Member
Joined 2006
Ay, I'm not sure your calculations are correct. ;)

Looking at the B1-schematic

http://www.enjoythemusic.com/diy/0708/first_watt_b1_preamplifier.htm

it's not a schoolbook high pass filter since the mentioned 1k is in series with the 10uF cap, forming a voltage divider with the real R, the 221k resistor. Assuming 10k input impedance of the power amp, one gets 221k||10k, so essentially 10k.

Using these values one obtains a corner frequency of 1.6 Hz - I think that's good enough :D

Have fun, Hannes

EDIT: Babowana was a second faster than me!
 
B

Bengali

Member
Joined 2006
Originally posted by Babowana
With 10uF+1K into 10k load, I estimate the low frequency roll-off at 1.5Hz.
With 10uF+1K into 47k load, I estimate the low frequency roll-off at 0.4Hz.

In the similar way as flws:

With 2.2uF+4.7K into 10k load, I estimate the low frequency roll-off at 5Hz.
With 2.2uF+4.7K into 47k load, I estimate the low frequency roll-off at 1.5Hz.

My conclusion is that you could use the combi of 2.2uF+4.7K if you do not mind the 5Hz roll-off.
Click to expand...

How did you calculate this?

thanks for everyone's replies. If I can use a lower value cap, 2.2uF, I could still get away with a high end cap vs. a cheaper
low grade 10uF.


I have no clue what what posted:

Originally posted by jacco vermeulen

If it's all that easy peasy, ask yourself : why the lean&mean F-Hill marketing machine picked a pricy 10uF foil cap ?
Click to expand...
 
A

AndrewT

R.I.P.
Joined 2004
it is fed from a non zero source impedance (the pot) that varies.
It is supplying a load that is not infinite.
The approximate way to guessimate the turnover frequency is to find the effective load impedance and assume that the non zero source has only a small effect.

The load impedance is the 1k0 + the 221k in parallel with Zin of the next stage.
If the next stage has Zin=10k then the load impedance is 221//10=221*10/(221+10)=9k57.
Now add the 1k0 series resistor giving 10k57.
The RC time constant is 106mS using the 10uF DC blocking cap.

If the power amp also has a DC blocking cap, you need to look at the input filter formed there as well. But this time you have two caps in series and need to account for the series pair in your calculation.
 
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