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Old 7th July 2008, 03:36 PM   #191
h_a is offline h_a  Europe
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Again I don't have a schematic at hand, but nevertheless:

Put in simple terms, GR, BL are used to reduce power dissipation as these parts only take reliably about 200mW of heat. No audio mystery here.

Preamps are designed to run high impedance loads (power amp) of at least a couple of kiloohms. I don't know the bias current of the follower in the B1, but it's probably too small for a headphone amp. Increasing it will very probably exceed the ratings of the used jfets*. If you want to build a nifty Jfet headamp, use this one - it's again a source follower:

EUVLs DAO headamp

http://www.diyaudio.com/forums/showt...05#post1130805

The drawback is that the used powerjfets are already obsolete, however www.audioprojekte.de still stocks them (as far as I know that is).

Have fun, Hannes

*alternatively you can parallel a large number of matched jfets - happy matching!
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Old 7th July 2008, 03:55 PM   #192
MEH is offline MEH  United States
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Quote:
Originally posted by h_a
Put in simple terms, GR, BL are used to reduce power dissipation as these parts only take reliably about 200mW of heat.
Yes, I now that is a reason to use low Idss parts. Another is that you have low Idss parts in the drawer but would need to order higher Idss parts. My question is whether there is an audible advantage to using low Idss parts. Surely just saving a few hundred milliwatts of heat can't be the reason when we'll accept hundreds of watts of Class A heat in the pursuit of better sonics.
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Old 7th July 2008, 04:00 PM   #193
h_a is offline h_a  Europe
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Quote:
Surely just saving a few hundred milliwatts of heat can't be the reason when we'll accept hundreds of watts of Class A heat in the pursuit of better sonics.
Come on! Read more closely! The part does not survive reliably more than 200mW. Heat it up to 1W and it simply burns out. Even more and you get a puff of smoke.

Enjoy, Hannes
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Old 7th July 2008, 04:34 PM   #194
EUVL is offline EUVL  Europe
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You might wish to read Nelson's latest article "Leaving Class A" on the benefits of high bias.

Leaving Class A

JFETs are not too different from MOSFETs in that respect.

As long as the dissipation of the device would allow, I personally prefer higher bias.
That means using V parts, e.g. 2SK170V.

If you read my thread on Pass discrete opamp, you might also notice that I was using BF862 with 18mA bias, in exactly the same follower configuration, for the output stage of the opamp.

You need to be able to handle the heat of course.




Patrick


PS If you would believe Steen, then the DAO follower is certainly amongst the best headphone amp there is around. Ask him.




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Old 7th July 2008, 05:56 PM   #195
gebi is offline gebi  Austria
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Default B1 with lightspeed attenuator ?

hello
I'm just wondering why mr. pass for the B1 design used a traditional approach ?
the use of LDR's look innovative.
See papa's post:

http://www.diyaudio.com/forums/showt...66#post1521766

are there any serious issues with this approach ?
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Old 7th July 2008, 06:11 PM   #196
MEH is offline MEH  United States
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Thanks, Patrick. That article and similar reasoning to your own is what led to my curiosity over Papa's preference for low Idss JFETs.
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Old 7th July 2008, 06:15 PM   #197
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I don't have a preference - I simply am working with what you
guys are likely to be able to get. If you want to use V parts,
go ahead, they won'd break in this circuit.
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Old 7th July 2008, 06:58 PM   #198
MEH is offline MEH  United States
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Thank you!

As long as a circuit works reasonably well without burning up, clipping, or otherwise breaking, does this mean that in general there isn't any audible difference between JFETs of different Idss ratings?
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Old 9th July 2008, 06:44 AM   #199
Bengali is offline Bengali  United States
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Default output caps

Hi,

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?

trying to stick with lower value caps.

thx.
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Old 9th July 2008, 08:13 AM   #200
AndrewT is offline AndrewT  Scotland
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Default filoter RC time constant

Quote:
Originally posted by Bengali
10uF output cap and 1K is about 16Hz for cutoff freq(?)

What if I used 2.2uF with a 4.7K resistor? is this close to same as 10uf and 1K without affecting performance?
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.
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