John Curl's Blowtorch preamplifier part II

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There isn't one but the one you used would be a bit more accurate if you calculàted one layer if insulation.
So, more like this then ?.
twisted pair4.png
Awaiting what you find.

Dan.
 
It is almost impossible to "find" anything. :)

Besides the "audiofile cable cook book", it is good to read articles like
http://eeeic.eu/proc/papers/55.pdf

An to improove understanding of behavior in coax cables.

Online calculators are based on very simplified formulas, but some cable characteristics are real and measurable with ubiquitous instruments..
Some characteristic no, but with network analyser, all cable "problems" could be enlighten.

It is nice and ok to make some expriments, but it is also necessary to follow some technical elements, not only audiofile subjective(preferable wrong, or distorded) claims.
I mean "distorded" by huge subjective expectation.
Some audiophile claims are road to "technical nowhere"...
 
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Use a slow blow fuse for low distortion vs current. But, then protection may not be fast enough to protect sensitive input/hardware. However, fuses are really to prevent fires.

There are faster, low distortion ways.... maybe protection like A-P uses would help you best.

I thought I had blown one of the A-P inputs a month ago when I accidentally put +15vdc on the input... in dc coupled mode. Wouldnt measure distortion. Input must be blown.
After ordering new pair of matched input modules and new protection devices...... going thru thier check list... all seemed OK. I tried it again and it is still working fine. It had just latched up. Now I have a couple rare matched input modules lying around. A-P approach doesnt add distortion and is fast acting.

THx-RNMarsh

That was just what I am doing at the moment. See if I can design-out the fuses.
At -120dB even looking at the circuit intently increases the distortion ��

Actually there's another reason to try to get rid of them. They are physically large thus prone to pick up hum and RF, especially on a 1Meg input.

Jan
 
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Use a slow blow fuse for low distortion vs current. But, then protection may not be fast enough to protect sensitive input/hardware. However, fuses are really to prevent fires.

There are faster, low distortion ways.... maybe protection like A-P uses would help you best..........I thought I had blown one of the A-P inputs........A-P approach doesn't add distortion and is fast acting.
THx-RNMarsh
What do A-P do ?.

Dan.
 
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Can you fry a Jfet in such a way that it becomes noisy but it's other characteristics don't change?

Still trying to figure out the excess noise in my amp... Replaced 6 BJTs and it's still noisy. Actually slightly noisier.


Is this with the input shorted?

Note also, you can swap D and S is some JFETS - is that not your problem?

If you are using a current mirror LTP load, be sure to degenerate it - un-degenerated can result in excess noise (see discussions on this point by BC, JC et al on the forum)
 
Note also, you can swap D and S is some JFETS - is that not your problem?

We have a winner! Thanks, I have been having a lot of pin order issues lately.

This may have disastrous consequences when I begin to use my set of obscure 2N5551/5401 which have guaranteed Hfe>200 and a different pin order. I found them on Mouser one day and had to double check.
 
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So, more like this then ?.
View attachment 551780
Awaiting what you find.

Dan.

A bit. But if you read the cite by Coldy then you should understand twinaxial cable is my solution. Less RF pickup and lower impedance. But have to get samples to verify design predictions.

Then use a zobel to correct loudspeaker impedance to match the cable into the low rf frequencies.
 
A bit. But if you read the cite by Coldy then you should understand twinaxial cable is my solution. Less RF pickup and lower impedance. But have to get samples to verify design predictions.
Then use a zobel to correct loudspeaker impedance to match the cable into the low rf frequencies.
Ok, I was thinking this is what you are doing.
How is the shield extra capacitance going to effect in audio band, esp for mid to long cable runs.
Is it worthwhile to bond the driver baskets to the shield ?.

Dan.
 
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That was just what I am doing at the moment. See if I can design-out the fuses.
At -120dB even looking at the circuit intently increases the distortion ��

Actually there's another reason to try to get rid of them. They are physically large thus prone to pick up hum and RF, especially on a 1Meg input.

Jan

A good solution is the use of what Littlefuse call 'nanofuses'. Very small, SMD yet field-replaceable.
I think I go with that.

http://www.mouser.com/ds/2/240/Littelfuse_Fuse_157-39684.pdf

The AP solution only protects against overvoltage but in my case that is not a pressing issue - it's more the overcurrent which has to do wih making current limit resistors as small as possible to prevent excessive noise.

I realize this might not make sense now, but all will be revealed in the fullness of time:cool:

Jan

Jan
 
On fuses, from Bob Cordell's book:

At low frequencies the audio signal can heat up and cool down the fuse element within a single cycle, causing the resistance of the fuse to vary as a function of the signal amplitude. This leads to distortion because the attenuation of the fuse resistance against the load impedance changes as a function of signal swing [5]. Fuses are often undersized with respect to the peak audio current they may be called on to pass, recognizing that a smaller fuse will provide relatively more protection and that with normal audio signals, such high currents are brief events much shorter than the time constant of the fuse element. The cold resistance of a 2-A 3AG fuse was measured to be 78 mΩ, while its resistance when passing 2-A DC was 113 mΩ. This represents a 45% increase in fuse resistance.

Amplifier THD (due to the fuse) is calculated by normalizing the fuse distortion voltage to the amplifier output voltage...................At 20 Hz, amplifier distortion due to the fuse is calculated to be 0.0033%
 
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On fuses, from Bob Cordell's book:

At low frequencies the audio signal can heat up and cool down the fuse element within a single cycle, causing the resistance of the fuse to vary as a function of the signal amplitude. This leads to distortion because the attenuation of the fuse resistance against the load impedance changes as a function of signal swing [5]. Fuses are often undersized with respect to the peak audio current they may be called on to pass, recognizing that a smaller fuse will provide relatively more protection and that with normal audio signals, such high currents are brief events much shorter than the time constant of the fuse element. The cold resistance of a 2-A 3AG fuse was measured to be 78 mΩ, while its resistance when passing 2-A DC was 113 mΩ. This represents a 45% increase in fuse resistance.

Amplifier THD (due to the fuse) is calculated by normalizing the fuse distortion voltage to the amplifier output voltage...................At 20 Hz, amplifier distortion due to the fuse is calculated to be 0.0033%

In general, that calculation is not correct. It is as Bob said, the delta-fuseimpedance against the load impedance on the fuse is what determines the distortion, although in some cases it may come out the same as your calculation.

Jan
 
The distortion of a fuse comes into play when it is used in series with the loudspeaker. Not a common practice anymore.

A 5x20 mm standard fuse the type you would use to protect a tweeter at 2 amps cold is .043 ohms and at rated current .075 ohms. It is also rated for 4.2 amps squared / millisecond. So in that use I would expect a distortion increase of .094% third harmonic distortion. At full power continuous tone. Which is below the normally expected threshold of perception. Of course you might just have a typical 20 dB minimum dynamic range, which would drop that at least another order of magnitude.


BTY his measurements of a 3AG 2A fuse are close to the data sheet values.
 
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In general, that calculation is not correct. It is as Bob said, the delta-fuseimpedance against the load impedance on the fuse is what determines the distortion, although in some cases it may come out the same as your calculation.

Jan


You also did not say how much distortion your circuit could tolerate. If going below .01% with a fuse, might be a problem..... but you can easily test with and without fuse. The THD will increase by about 6db/oct below 100Hz for typical fuses we use. The faster blow speed types -- the faster the fuse element heats up and varies/modulates its' R.


THx-RNMarsh
 
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