John Curl's Blowtorch preamplifier

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PHEONIX

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Joined 2004
Cordell power amp Jfet/BJT input

Hello Bob

It would be interesting if you could change the input to BJT and confirm which of the two inputs gave lower THD performance.

The reason I make this inquirey from what I have gathered from Scott Wurcer spice models of JFets are "usually" not as good as BJT spice models.

Regards
Arthur
 
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AndrewT

R.I.P.
Joined 2004
Re: Re: Re: Re: Re: Re: RFI

Bob Cordell said:
........... bandwidth of the amplifier limited by ..........passives than by the active closed-loop frequency response of the amplifier-proper............... In any case, I believe it is usually best that the frequency response droop at 20 kHz be kept to about 0.1 dB or less.

One thing that I like to do is right at the RCA connector where the signal comes in, run it through a series 49.9 ohm resistor and then a shunt 100 pF capacitor to the connector ground so that the interconnect is effectively terminated in 50 ohms at very high frequencies. This, of course, is a compromise, as not all interconnects have a characteristic impedance of 50 ohms,
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hear, hear.
or even a 100r feeding 100pF @ the input terminal.

I also advocate that the LF end be filtered similarly, i.e. aim for F(-0.2db)<=20Hz
 
Edmond Stuart

Edmond Stuart

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Joined 2003
data-odyssey.nl
Pavel, I'm not joking, just asking a question. According to post#7159, Bob proposes to terminate the cable only at the side of the power amp with 49.9 Ohm. So, I'm asking what to do with the other side.

BTW, "to feed it from a voltage source at the input" is impossible. Every signal source has an output impedance. Moreover, I think it's not a good idea to connect a coax cable directly (without a series resistor) to an output and loading it with a (unknown) capacitance.
 
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AndrewT

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Joined 2004
PMA said:
Bob is right, regarding HF termination (series 49.9 at the cable input). The end left open, i.e. 10k input impedance of following component is like open.
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is right at the RCA connector where the signal comes in, run it through a series 49.9 ohm resistor and then a shunt 100 pF capacitor to the connector ground so that the interconnect is effectively terminated in 50 ohms at very high frequencies.
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but Bob is proposing that we terminate the receive end with 50r.
Bob makes no mention of the source end of the interconnect.
 
Edmond Stuart

Edmond Stuart

Member
Joined 2003
data-odyssey.nl
Pavel,

This is what Bob was saying, while he was discussing input filters:

"One thing that I like to do is right at the RCA connector where the signal comes in, run it through a series 49.9 ohm resistor and then a shunt 100 pF capacitor to the connector ground so that the interconnect is effectively terminated in 50 ohms at very high frequencies. "

So, what do you think he's talking about? The pre-amp side or the power amp side? (I think the latter)
 
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ingrast

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Joined 2004
suite101.com/welcome.cfm/astronomy_astrophysics
Edmond Stuart said:


.....

So the other end needs to be improperly terminated? ....
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2 issues here,

One wants to fool the cable to think it is infinite so any signal travelling along will not find discontinuities and be partially reflected. Loading the far end with the characteristic impedance does the trick.

For the source end, 2 further issues. By the same token one wants to avoid any reflected - or backward signal fo any reason - to be further reflected which yields the familiar ringing in high speed unterminated circuits. Providing the characteristic impedance at the source does the same as before.
Morover, it can be shown - though it is not an issue for low power audio - that maximum power transfer is achieved when source and load impedance are the same, this is a further reason in RF circuits to care match source and transmission line impedances.

Rodolfo
 
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Charles Hansen

R.I.P.
Joined 2003
Once again, off topic.

But since we are there, I am surprised that there is so much misunderstanding about cables and terminations.

It is only necessary to terminate one end of a cable with the characteristic impedance. Bob Cordell's method is fine (terminating at high frequencies only), as the cable doesn't even have a constant (characteristic) impedance at low frequencies.

You can terminate the source or the load or both ends. Slightly different things happen in each case. But in all cases you will avoid the problem of multiple reflections that create a signal that looks like "ringing".

The best book to read on this topic is "High Speed Digital Design" by Howard Johnson:

http://www.amazon.com/High-Speed-Digital-Design-Semiconductor/dp/0133957241/
 
C

Charles Hansen

R.I.P.
Joined 2003
lumanauw said:
This article agrees with PMA's method :
http://www.6moons.com/audioreviews/dartzeel2/preamp_3.html
The graphs are interesting :D
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As usual, there is some misinformation here. (No surprise -- after all, this is the internet we are talking about.)

The formula given for characteristic impedance is incomplete. The full formula includes the resistance of the wire and the conductance of the insulator. At high frequencies (above the audio band), the inductance and capacitance dominate and it is acceptable to use the simplified formula. Usually.

But in the case of the OCOS cable, this is not OK. The OCOS cable deliberately introduces a controlled conductance into the insulator to create a characteristic impedance of 8 ohms. This is to match the loudspeaker, and also this technique extends the effective range of the characteristic impedance down into the audio band. Ignore the incorrect figure of 60 ohms given in the article.
 
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