John Curl's Blowtorch preamplifier part II - Page 487 - diyAudio
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Old 3rd July 2010, 07:24 AM   #4861
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Quote:
Originally Posted by 1audio View Post
Trying to read through the article (my German is zilch) I think they say there is a difference between sine wave and MLS measurements. That is true and easily explained- the sine wave measurement uses a wide band detector so harmonics will add with the fundamental, the MLS is intrinsically narrow band, it needs to be to derive the individual frequency response. Further the MLS system is band limited by the A to D and D to A conversions.

After that I really could not make sense of the article.

If I can understand their measurement process I could duplicate it and see if I can get the same results.
My German is (slightly) better and I *think* I understand what they try to do, based on the pic below. It appears they excite the speaker with an MLS, and 'ground' the return side either through a resistor to gnd, or through the Zout of an auxiliary amp (that assumedly is not driven) via the switch. If the amp is ideal, there will be no signal to the analyzer. However, the amp is not ideal; for one it will exhibit rising Zout with freq which shows up as a rising curve on the analyzer.
In case of the resistor, the 'Zout' of the resistor is constant with freq so the curve is flat.
What I am not able to find out is the value of the resistor, and that is crucial. By manipulating that resistor value you can get almost any curve you want. Especially later when they normalize the curves to the resistor curve, not knowing the actual R value makes the results pretty meaningless.

jd
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Old 3rd July 2010, 09:49 AM   #4862
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Then why does the top line read : 8 Ohm resistor instead (statt) of the loudspeaker.
Replacement (ersatz) resistor would imply just that : 8 Ohm.
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Old 3rd July 2010, 11:26 AM   #4863
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i tryed to phone Mr.Schüller several times to get more details but sofar no luck. I will try again on monday.
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Old 3rd July 2010, 01:37 PM   #4864
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Quote:
Originally Posted by jacco vermeulen View Post
Then why does the top line read : 8 Ohm resistor instead (statt) of the loudspeaker.
Replacement (ersatz) resistor would imply just that : 8 Ohm.
I know Jacco, that I don't understand either. If we would assume that they load the amp with either the R or the speaker, driving the speaker with the MLS doesn't make sense, at least to me.
It is also not absolutely clear to me how the switch is connected; what is the pole, what are the throws? It is ambiguous for me.
This is probably one of those presentations that really need the narrative to make sense.

jd
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Old 3rd July 2010, 03:00 PM   #4865
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If you look at slide 17, it seems to imply that they drive the speaker at one terminal, and at the other terminal terminate it with either an R or the Zout of an amp. That then gives the two curves. It would make sense if the R was, in value, comparable to the amp Zout, but certainly not if R was 8 ohms.

Anybody can enlighten me here?

jd
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Old 4th July 2010, 06:18 AM   #4866
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Default Mr. Evil's JFET RIAA

I refer back to post #4812 :

http://www.diyaudio.com/forums/analo...ml#post2228265

Here is the schematics of a MC version as described with drift problems.
The schematics is correct except for that 2SK389's were replaced by 2SK170GR's matched pairs, and 2SJ109's replaced by 2SJ74 matched pairs.

This was something I did in 2005, 2 years after I started DIY. I know I would do it differently today, but I post it anyway for the sake of discussions. So don't drill on the stupidity of the circuit as posted.

I cascoded the 1st stage in an attempt to reduce input capacitance due to Miller effects of the large single stage gain.

The WORD picture is actually exported from ACAD and the conversion is not perfect. But if you open it in word and blow up the magnification, you can actually see all details including component values. It will be a LOT worse in PDF.



Patrick
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Old 4th July 2010, 09:14 AM   #4867
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Constructors wondering about assembling matched JFETs into a long-thermal time constant module may enjoy Jim William's application note AN124 at LT. He shows a pair of K369s epoxy-moulded into the cap of an apple-juice bottle, in a note aimed at measuring sub-uV noise levels in voltage references. The general construction techniques Jim uses are, as usual, works for admiration.

http://cds.linear.com/docs/Applicati...ote/an124f.pdf



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Old 4th July 2010, 09:22 AM   #4868
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Luckily we have something a little bit more elegant than an apple juice bottle :

The next best thing to 2SK389 / 2SJ109 ?




Patrick
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Old 6th July 2010, 02:41 AM   #4869
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Very nice heatsinks.
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Old 6th July 2010, 04:11 AM   #4870
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John,

You have email.

Going back to the simple phono circuit, if I stick to Mr. Evils circuit for MM carts,

http://www.diyaudio.com/forums/analo...ml#post2227434

but add a MC stage up front, how would you do the MC stage without using coupling caps ?

Something like blowtorch but double the no. of jfets ?


Patrick

Last edited by EUVL; 6th July 2010 at 04:13 AM.
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