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Old 16th October 2005, 08:41 PM   #1
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Default Phono Pre-Design Input

Found this on the web (thanks Chris), any comments or drawbacks you tube heavy hitters can see? Input much appreciated, I chose this because I have the completed (& my first, see earlier post) tube PSU.

Thanks, PB
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Old 16th October 2005, 09:29 PM   #2
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Looks like a place to start to me. One thing though: the 27k resistor
on the input tube cathode is obviously wrong. I'm guessing it should
be 2k7, or 2.7k ohms. Otherwise, fine. Keep cables short, including
interconnects. (There are no doubt more advanced preamps, but it
is not such a bad idea to start with a simple design.)

Morgan
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Old 16th October 2005, 10:29 PM   #3
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Thanks - can you confirm some voltage ratings on the unmarked caps? I think I got it figured but better safe than smoked!

PB
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Old 16th October 2005, 11:18 PM   #4
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Morgan is correct - that resistor should be 2.7 K.ohm, or even 2.2 K.ohm (manuals give an optimum of 1.5 K.ohm, but it is not critical - valves have a spread anyway). I have quickly checked the response on a program and it appears to be in order for RIAA. You will have a nett gain of about 230.

The coupling capacitors (0.1 uF and the last one which I have not noted down) should be 400V. The 3.3 nF and 10 nF can be low voltage, typically the small 100V types. But the two cathode capacitors could also be low; 16 - 25V.

Just a small point; I do not know how experienced you are with valve amp construction. Remember to earth the metal (screening) tube in the middle of the sockets. Some folks miss this and it can lead to inadvertent capacitive coupling between electrodes and high frequency oscillation if not done.

Have fun!
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Old 17th October 2005, 12:48 AM   #5
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That's QUITE close to the circuit found in RCA receiving tube manuals. It has the same strengths and weaknesses. The biggest weakness is absolutely wretched drive capability. As shown, that phono section has to be built on the same chassis as the line section and a 500 KOhm volume control is in order.

Here's a link to a thread Planet10 (DIY moderator) and I were involved in fairly recently that discussed the RCA circuit at some length.
RCA Phono Section
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Old 17th October 2005, 01:51 AM   #6
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Eli - Yaai!

I did want to add about the high output impedance and forgot dismally - thanks for pointing that out. (Perhaps I should not be up this time of night.) Also, the cathode bypasses are too low. Somehow I registered 470uF and analysed with that. The RCA circuit's 25uF is definitely too low, and so will be 47uF. Capacitors are small and inexpensive these days, and I would suggest 470uF. Some folks make heavy whether of using electrolytics here; in my opinion that is a bit overdone - this is an entry-level circuit. If PB wants he could use the grid-leak method of bias which works for an ECC83 for low level signals. That entails another 2 R-Cs, but they replace the cathode bias R-Cs.

Now I will go to bed before I overlook more things.
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Old 17th October 2005, 01:34 PM   #7
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Konnichiwa,

Quote:
Originally posted by Johan Potgieter
the cathode bypasses are too low. Somehow I registered 470uF and analysed with that. The RCA circuit's 25uF is definitely too low, and so will be 47uF.
Actually, with 47uF as in the circuit discussed you get in the second stage a dominant pole at 5Hz and in the first stage at 2.5Hz (close estimate) which with the permissable 500K load on the output (as given as for the RCA circuit it resembles) and the RIAA Network & coupling network having each around 1.5Hz turnover means the 10hz pole from the second stage becomes dominant.

In other words, as is, the -3db point of the phonostage will be well below 10Hz. So, the 47uF used by the circuit are EMPHATICALLY NOT TOO LOW in value.

Also, my ECC83/ECC88 Phonocircuit was designed mainly to overcome ALL the problems in the RCA circuit.

Sayonara
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Old 17th October 2005, 01:45 PM   #8
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Kuei is this your circuit? If so thanks, credit where credit's due.

PB
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Old 17th October 2005, 02:04 PM   #9
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Konnichiwa,

Quote:
Originally posted by philipbarrett
Kuei is this your circuit?
Nope, mine is a bit less compromised.

It was originally intended for a commercial product that ended up never build and was hence released into the public domain here:

http://www.diyaudio.com/forums/showt...624#post335624

This is my circuit:

Click the image to open in full size.

As you can see, unlike some others I am morbidly concerend to keep the signal circuit completely free of electrolytic capacitors (no Black Gates allowed either) simply because I actually tested them blind against a wire bypass and feel them to be completely unacceptable.

Key changes compared to the RCA Circuit are:

1) Converted second stage to ECC88/6DJ8/6922 to reduce output impedance, the Circuit is fine to drive a 10K load (tested)

2) Used gridleak bias on the second stage and removed the cathode bypass capacitor in the first stage, re-arranged first stage for low anode voltage operation which usually gives much lower noise

3) Changed the the RIAA circuit to whta I call "Kondo" or "semi-split" circuit, this has the advantage to allow for completely seperate adjustment of the capacitors in the circuit

4) Moved the coupling capacitor behind the RIAA so the RIAA capacitors dielectric is "soaked" (avoiding any issues from "soakage" [which is also known as "dielectric absorbtion"] on the signal) and the interaction between coupling cap & first RIAA cap (and thus on the RIAA EQ)

Sayonara
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Old 17th October 2005, 02:30 PM   #10
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Your corcuit was actually my 1st choice, I really liked the elegence & simplicity. I have already built a 350V PSU so was looking to leverage that into a design, hence the other choice. Maybe I should re-work the PSU?

PB
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