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Old 5th April 2005, 10:54 PM   #11
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Default Simple 12AX7 / ECC83 Phono Preamp

I will likely give the RCA schemtic a whirl with 12AX7.

Here is another simple design I came across:

http://www.turneraudio.com.au/htmlwe...implephono.htm
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File Type: gif simpletriodephono450w279h.gif (21.4 KB, 2151 views)
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Old 29th June 2005, 04:55 AM   #12
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Default The "RCA circuit with RIAA Eq"

I have that('66 RCA tube manual), that schematic, and I would like to consctruct that phono preamp to connect my phonograph directly to my headphone preamp.

I can build it; it is a very simple circuit.

BUT...

QUESTIONS...

The +250V; where does it come from (this question goes for all 12AX7/7025-based preamps)? Can I incorporate a solid-state voltage control? I need to build a self-contained preamp; a unit that goes on it's own shelf.

If I was put in an empty room and told that I needed to construct this thing I would use a 250 V transformer and a couple of diodes wired for full-wave rectification. I wouldn't know how to filter the voltage, so as to make it a "smooth", or "constant" 250 V, though.

OTHER QUESTIONS...

Is -V applied to anything?

Is any voltage applied to the heater of the tube?

If anyone has an answer could they please post a detailed message or simple shematic on the subject of the +V control?

Should I know the answer to all these questions by now?

Thanks,

Trevor
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Old 29th June 2005, 07:49 AM   #13
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Default Re: The "RCA circuit with RIAA Eq"

Konnichiwa,

Quote:
Originally posted by Sir Trefor
I can build it; it is a very simple circuit.

BUT...

QUESTIONS...

The +250V; where does it come from

<snip>

Should I know the answer to all these questions by now?
May I STRONGLY urge you to read the Phonopreamp Thread I referred to earlier?

http://www.diyaudio.com/forums/showt...022#post526022

You may find most of your answers there.

Sayonara
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Old 30th June 2005, 02:50 AM   #14
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Arrow Yes... I am getting closer

Thank you, Keui

The thread was very long, and I have difficulty maintaining focus while reading something as aimless as a forum thread, but I did gather some things from it.

I also did what I should have done before posting; I checked my personal resources.

I found out that the heater of a 12AX7 is powered by AC/DC 12.6V(series), 6.3V(parallel). That answers one question.

I will post more later.
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Old 30th June 2005, 03:47 AM   #15
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I have attached a shematic I made that shows the extent of my knowledge of voltage control; rectification- that's all I know. But Keui, did you mention that a 100% regulated is not necassarily crucial to the tube's function? According to "A Taste of Tubes"(pub. by Sonic frontiers in 1998) A Voltage regulator consists of a (1)trasformer, (2)rectifier, (3)Filter, (4)regulator, and a (5)bleeder/divider. That explantion also includes a helpful schematic; but the schematic uses several tubes and appears needlessly complex for this application.

What is an RC filter? NOT what does and RC filter do... what is it physically composed of? Is it similar to a capacitor?
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Old 30th June 2005, 07:54 PM   #16
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Default Correction

I was thinking about the schematic I posted yesterday while I was driving today, and I realized it isn't a full-wave rectifier; I just wanted to post this so that I don't lose too much credibility.

-Trevor

PS- I have found most of the information that I need in my tube manuals.
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Old 30th June 2005, 08:41 PM   #17
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Default Re: Correction

Konnichiwa,

Quote:
Originally posted by Sir Trefor
What is an RC filter? NOT what does and RC filter do... what is it physically composed of? Is it similar to a capacitor?
An RC filter is composed out of R(esistor) and C(apacitor) elements. I gave an example (including circuit diagram and performance graphs) in the cited thread that allows a simple passive supply to be build for the Phonocircuit I recommend and which has a noise performance by far superior to ANY active regulation circuit and which offers much better transient load tolerance etc, at the price of a few capacitors and resistors of moderate quality.

You also want to supply the Valve heaters of the phonocircuit with DC to avoid noise from the heaters coupling into the signal circuit. A simple regulated 6V supply is sufficient.

BTW, the simplicity of the circuit I suggest, also of it's powersupply is rather deceptive, the performance is very high.

Sayonora
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Old 30th June 2005, 09:28 PM   #18
cerrem is offline cerrem  United States
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A very simple low parts count RIAA pre-amp I know of consist of just a single EF86 per channel....
It actuallt sounds much better than the crudy RCA ones...
The RCA ones are slow sounding and distorted....total lack of detail and air....not to mention the time constantsd were off the mark..
If interested let me know I will post it...
Chris
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Old 1st July 2005, 06:17 AM   #19
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Quote:
Originally posted by cerrem

If interested let me know I will post it...
Very interested....

dave
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Old 1st July 2005, 07:36 AM   #20
cerrem is offline cerrem  United States
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Here is the EF86 schematic...
Use just the first stage......
I remember that I recalculated the R and C values to getter a tighter RIAA curve....can't find the notes right now...

My free picture Host will not support the size of this PDF....
SO I will Email it to you...

Chris
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