John Curl's Blowtorch preamplifier part II - Page 3632 - diyAudio
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Old 5th March 2013, 08:18 PM   #36311
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Quote:
Originally Posted by PMA View Post
That is the real issue, further issue is RFI coupling through the transformers - they WORSEN the situation.
I was just back at my bench working on exactly that problem! For fun we have been building an assortment of microphone preamplifiers. The first one is a a single op amp gain stage and a dual opamp servo. Transformer input at 1/2 ratio. Local AM station came in at -50 (re 1 V).

Ah the reference designs from some manufacturers!

The power supply is in a steel box and connected by a simple 4 conductor cable (+15, -15, +48). Power supply noise is 5 mV (Or -70 db re 15V @ 10 Mhz bandwidth). The circuit has just enough PSSR to get this down to 10's of uV in the output at full gain.

So with the 3 mV of RFI and power supply noise, it is not a very good preamp!

Adding an input filter and an RC to the PS will probably bring it up to acceptable.
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Old 5th March 2013, 08:31 PM   #36312
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Originally Posted by jan.didden View Post
I don't think that is the case; I believe it is only used for tuning purposes, NOT to generate DH.

jan
No varactors were/are used to generate microwave frequencies at odd harmonics (usually) of a pump frequency. There was a 1mW at 120GHz one at ISSCC. The operation is not simple varying the capacitance but IIRC they snap on and off rather fast.
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Old 5th March 2013, 10:25 PM   #36313
DF96 is offline DF96  England
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Varactors were also used as low noise parametric amplifiers.
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Old 6th March 2013, 03:43 AM   #36314
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Unfortunately, a 'really high quality' all jfet input stage is very expensive today, due to the complexity of the circuit and very high cost of the ultra low noise jfets.
A transformer could be used, to get a balanced input with less complexity. I personally have not found a transformer at any price point that I consider as good no transformer at all, so I have elected to not use any in these very expensive phono stages. One having a retail price of about $19,000, the more expensive one probably priced at $60,000.
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Old 6th March 2013, 06:09 AM   #36315
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Originally Posted by 1audio View Post
What Bill told me, when we were discussing his circuit, was that transformers do not work well at higher frequencies, where imbalance in windings becomes difficult to control.
Yes, but typically the noise in question is at the lower frequencies. Either that or it's well above the audio band where it can be taken care of with simple filtering.

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I wish the common mode was that good. I worked with Jensen on this exact issue for an isolation transformer for subwoofers. The common mode hum transfer was good but not always good enough, even with shields etc.
What, was this some sort of large home theater system where you're running 100 feet of cable to an active subwoofer or something?

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With as little as 60V common mode the hum was no longer acceptable.
How on earth are you getting the cable to pick up 60 volts of common-mode noise?

se
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Old 6th March 2013, 02:17 PM   #36316
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A sense of perspective has to brought into the balanced-single ended debate.
OF COURSE, when you are trying to move audio signals hundreds of feet, with parallel connections to other audio systems, in an environment that you don't know or can't control, THEN transformers can be a wonderful solution to ground loops, and common mode hum pickup. That is what it was originally designed for.
However if, like me, you have a phono player connected with maybe 4 feet or 1 meter of cable, it is just a 'waste of time' to have a balanced input. This is why I don't use balanced inputs in the CTC Blowtorch, and I don't have any problem.
However, IF I put the phono player in another room, let's say with 5 meters of cable, maybe a good balanced input or a transformer would be an ideal solution. This is WHY we put balanced in for the Constellation phono preamps. Rich guys sometimes actually do put their phono player in another room. But look at the COST! 4 times as many expensive jfets per channel. In my design, 45 per channel, count them! And you guys squawk about the cost of purchasing 2 low noise jfets per channel. '-) Could we reduce the number of jfets used? Yes, BUT at the cost of higher input noise and/or less dynamic range. For a 'best' phono preamplifier that we can design, we chose the all out, not compromised approach. If we didn't, someone else perhaps would, and we would not be the 'best' effort possible then. It is a design choice.
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Old 6th March 2013, 03:01 PM   #36317
1audio is offline 1audio  United States
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Quote:
Originally Posted by Steve Eddy View Post
What, was this some sort of large home theater system where you're running 100 feet of cable to an active subwoofer or something?
The target was a commercial product intended to be ina blister pack in about 2000 retail stores. It must always work or its not worth doing.



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How on earth are you getting the cable to pick up 60 volts of common-mode noise?se
Example: One ungrounded AV receiver with a grounded subwoofer. Chassis leakage at the AV receiver sets its chassis at 1/2 line voltage. Very small current through the transformer to ground on the other side. Shields in transformer. Unbalanced cables ( consumer, RCA subwoofer output) Source impedance maybe several hundred Ohms, load impedance 20K. The leakage gets mixed with the audio on a high impedance transformer. A low Z transformer requires an active circuit to drive it which A) is too expensive a solution and B) requires power supplies and approvals. Once the circuit is active we did not need a transformer BUT at 5X markup its too expensive a product to have a customer base.
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Old 6th March 2013, 03:09 PM   #36318
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Quote:
Originally Posted by john curl View Post
actually
I heard some also place the balanced pre-pre, CD transport, balanced DAC, and balanced preamplifier in the adjoining chamber.
(could be my imagination playing havoc again)
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Old 6th March 2013, 03:16 PM   #36319
Waly is offline Waly  United Kingdom
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Originally Posted by john curl View Post
But look at the COST! 4 times as many expensive jfets per channel. In my design, 45 per channel, count them!
Designing with expensive unobtanium parts is a JC trademark. Not looking for a modern solution and "sticking with what works" since the flower power times is another catch for the price-no-objection audio fashion market.

Since the target customers are those with deep pockets and a good chunck brown matter dangling in the deep space between the ears, complaining about "costs" is nothing but hypocrisy.
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Old 6th March 2013, 03:19 PM   #36320
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I think balanced for home use is overkill, and noisier (transformer approach of course not so). For long runs, then it serves it purpose well.
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