John Curl's Blowtorch preamplifier part II - Page 3280 - diyAudio
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Old 17th January 2013, 03:35 PM   #32791
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Did I say that the AD797 or the 4562 were mid-fi quality?
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Old 17th January 2013, 03:48 PM   #32792
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Quote:
Originally Posted by Esperado View Post
It would be interesting to see in witch context (power margin, brand, place in the circuit, you find them so 'audible' ? May-be we could find some reason ?
I use cheap metal film resistors from mostly Maplin Electronics UK rated 0.6W & Parts Express US rated 0.5W + a few others types.

I didn't distinguish between them.

I also avoid trimmers ( 22 turn cermet types ) as much as is possible or design the circuits so that their o/p is filtered or diluted - I think they sound even worse than fixed resistors but I didn't do a study on this - studying these resistors does not really interest me - the solution is simple - buy better resistors - I hope I can find some good ones that are cheaper than the Caddocks - that would be great ! I will research more on this later this year.

As I explained already I heard a difference in all amplifier circuit positions - even as VAS CCS ( dual BJT ) feeder resistor ! The overall effect when all are changed is not small.

If anyone reading this finds it hard to believe - why not just try it just once - one amp with all cheap metal film and another with Caddocks TF020 or MK132 - you may be very pleasantly surprised . . . or you may not hear any difference - but personally I would be too curious not to try it.
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Old 17th January 2013, 03:52 PM   #32793
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AD797: sample
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Old 17th January 2013, 04:15 PM   #32794
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Quote:
Originally Posted by john curl View Post
Did I say that the AD797 or the 4562 were mid-fi quality?
Yes, you did.

Quote:
Originally Posted by john curl View Post
I have nothing personally against IC's any more than I have against American Cars.
They both serve a significant purpose in what they are designed to do. AND they really help to make low distortion audio measurement equipment.
Then why don't they work 'perfectly' for hi end audio? Well it is not because they measure so low in lower order harmonic distortion, we have been beyond that barrier, for the last 30 years or more. It is the HIGHER ORDER distortion, or more fundamentally, the 'dropout' in the transfer function that generates higher order distortion that is what the ear seems to not be tolerant of. ANY hint of higher order distortion, 7th and 9th especially, and you sink to mid-fi.
They ARE ICs, right?
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Old 17th January 2013, 04:15 PM   #32795
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Quote:
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I have a design in front of me that uses a 4562 with a 350 ohm load. Do you see a problem with this? '-)
Yes, the current capability of this OPA is 42ma. With 35Ohm load, it allows... 14.7 volts -)
We are far from the 100X margin we like :-)
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Old 17th January 2013, 04:17 PM   #32796
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Example of loading: Am I wrong?
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Old 17th January 2013, 04:19 PM   #32797
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Partial schematic with 'loading'
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Old 17th January 2013, 04:20 PM   #32798
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John,

As a matter of interest - how low would H6 - H19 have to be that you think it is no longer audible and spoiling the sound ?

In spice when I was designing a servo circuit I could see that a poor design raised the harmonic noise floor from -150db to -125db with H1 10V peak o/p and in reality I could hear the sound had lost it's transparency.

So I try to aim for -150db H6 - H19 noise floor in spice but I have no idea what is happening in real life.

Lookin for a reality check !
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Old 17th January 2013, 04:20 PM   #32799
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Quote:
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AD797: sample
OK, so you're saying you don't have anything with a 10k load (home audio) to support your contention? Or comparative data with a "high end" preamp like Parasound or Blowtorch?

Do you test your Porsche with a tow bar and a trailer?
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Old 17th January 2013, 04:54 PM   #32800
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Quote:
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Partial schematic with 'loading'
I don't understand.
The output load is 2K (less the charge of the following circuit input i don't know).
The feedback resistor is 530 Ohm: Why ?
As it look like an inverting configuration, we need to know the source impedance to have an idea of the gain + the charge induced by this strange low value for feedback resistance, while they suggest 6 x more in TI application notes.
Notice TI measure the best figures for 2K load, i would use this value as a minimum, and certainly not drive a 600 Ohm load without an additional buffer..
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Last edited by Esperado; 17th January 2013 at 04:58 PM.
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