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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Transformer based 6db preamp
Transformer based 6db preamp
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Old 4th July 2020, 03:44 PM   #1
happyrabbit is offline happyrabbit  United States
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Default Transformer based 6db preamp

My latest diy preamp adventure. -/+ 24v Linear Supply. Sparkoslabs regulators. LSK489 for the jfetís. 6 layer PCB (s/g/p/p/g/s). Startup / shutdown muting.

Balanced -> LL1676(1:2) -> jfet follower -> 10k ladder -> jfet follower -> dc blocking cap -> RCA

I will post measurements soon.. the in/out are flipped on the case.
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Old 17th July 2020, 03:05 PM   #2
happyrabbit is offline happyrabbit  United States
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Input @ +4dbu -> 1k @ 1V

16bit picoscope & NTI balanced signal generator.
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Old 17th July 2020, 05:14 PM   #3
Mark Tillotson is offline Mark Tillotson
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10dB/division is much easier to evaluate than 14dB/division as a vertical scale...
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Old 17th July 2020, 10:05 PM   #4
Ken Newton is offline Ken Newton  United States
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Good clean looking construction.
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Old 18th July 2020, 02:42 AM   #5
happyrabbit is offline happyrabbit  United States
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Mark,

I have only had the picoscope for a month.. I am no expert on the software. I could not find a way to scale @ 10db/division. Sorry !

KEN,

Thanks ! not super exotic but I find the design enjoyable with my solid state amp.


Dwight
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Old 2nd September 2020, 03:06 PM   #6
happyrabbit is offline happyrabbit  United States
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Round #2

Transformer configured 1:1. Isolated the PS better plus additional filtering. Removed Jfet v-regís and replaced with low pass filters. Waiting on new case. Also integrating Tortuga Audioís new ldr volume attenuator.

Electronic Stepped Attenuator | ePot.V3 Mini | Tortuga Audio

Dwight
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Old 2nd September 2020, 04:59 PM   #7
analog_sa is offline analog_sa  Europe
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Transformer based 6db preamp
How does the 1684 compare to the 1676 soundwise?
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Old 3rd September 2020, 01:03 AM   #8
happyrabbit is offline happyrabbit  United States
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Almost there..
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Old 5th September 2020, 01:09 AM   #9
happyrabbit is offline happyrabbit  United States
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Frank sounds absolutely lovely. Still have a little hum with my 97db speakers.. but otherwise itís a killer preamp.
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Old 7th September 2020, 06:44 PM   #10
sgrossklass is offline sgrossklass  Germany
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Quote:
Originally Posted by happyrabbit View Post
Still have a little hum with my 97db speakers.. but otherwise itís a killer preamp.
Ever heard of an oxymoron?

Your power amp isn't also IEC Class I by any chance? (If not, does its power xfmr have a shield winding?) With all the work you've put in, it would suck to have a ground loop in the setup.

Also, XLR pin 1 hookup is not AES48 compliant by the looks of it, should go straight to chassis instead. And twisted pair arguably isn't first choice for hooking up RCAs, consider something closer to coaxial wiring.

Not sure I would trust a Picoscope for audio work, I mean, it's a scope and not an audio analyzer. Would suggest rechecking distortion via a decent USB soundcard / audio interface input on a battery-powered laptop. The levels displayed earlier would not be very much to write home about.
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