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Old 19th April 2009, 04:35 PM   #1
hunsoop is offline hunsoop  United States
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Question tube pre amp for phone stage

Hi,
I am trying to build a tube pre amp for phone stage. People said that marantz 7c is a pretty good for phone stage. But i can't get the schematic. Can anyone help me? What do you guys think, as a beginner, of chanlleging marantz 7c pre?
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Old 19th April 2009, 05:03 PM   #2
SY is offline SY  United States
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It's a collectors' item. It is a classic circuit. It's actually not very good.

There are many newer designs which will do better. You'll want to look for something with excellent headroom, very rapid overload recovery, and relative insensitivity of RIAA conformance to changes in tube parameters with aging.

What sort of cartridge are you using and what's your level of constructional competence? That info can guide recommendations.
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Old 19th April 2009, 05:22 PM   #3
kevinkr is offline kevinkr  United States
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I agree with SY wrt to his comments on the Marantz 7 phono pre-amplifier topology and would extend this to cover almost all vintage designs. (They just aren't that good.)

There are some free designs on my site which might be of interest.

Again your skill level and ability to source parts plays a role in what you want to tackle, but even more important is the cartridge you want to use this phono pre with, best results would be achieved with standard moving magnet types or high output MC types - low output types will require exotic circuitry and tubes for good noise performance and are beyond the ability of most constructors - alternately a good mc step up transformer could be used.
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Old 19th April 2009, 09:32 PM   #4
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Default I would not use a 12ax7a.

I would look to newer designs with other tubes; 6GK5, 6ER5 with solid state constant current sources and passive EQ between the stages rather than global feedback. Kurt Strain had a good design with 6GK5-6ER5. Steve Bench has a nice cascode with a 12ay7/6072a and a 6DJ8/7308. Both of these can be found on the web. Volt Second has some nice solid state constant current sources. The EQ requirements of a RIAA preamp are a collosal mountain to climb which generally calls for a cascode in modern terms.
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Old 19th April 2009, 10:45 PM   #5
kevinkr is offline kevinkr  United States
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Default Re: I would not use a 12ax7a.

Quote:
Originally posted by grhughes
I would look to newer designs with other tubes; 6GK5, 6ER5 with solid state constant current sources and passive EQ between the stages rather than global feedback. Kurt Strain had a good design with 6GK5-6ER5. Steve Bench has a nice cascode with a 12ay7/6072a and a 6DJ8/7308. Both of these can be found on the web. Volt Second has some nice solid state constant current sources. The EQ requirements of a RIAA preamp are a collosal mountain to climb which generally calls for a cascode in modern terms.

I generally agree, however for a newbie good 12AX7A are relatively easily sourced, are well understood and can provide decent if not wonderful performance when properly employed.

I have experimented with cascode connection, mu followers, and srpp and I think the single triode with a ccs or resistive load still generally sounds best, despite the appeal of a more complex topology on purely technical grounds.

I like the 6072/12AY7 in cascode as well, but good quiet ones are getting a bit hard to find. (And above still holds true)

My current fave is the D3A connected as a triode. (My current pre amplifier with tube gyrator loads, passive eq, and a triode connected D3A for input and 5842 for output manages very close to 50dB of gain with RIAA correction losses included.)
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Old 21st April 2009, 06:42 PM   #6
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Default Why not the RCA tube manual RIAA preamp

It uses a 12ax7a/7025. I could be an ideal Newbie build. You can later add a White cathode follower to the output which would buffer it with maybe a 12bh7a and you could later add those solid state constant current sources instead of the plate resistors. But don't forget a good power supply with a CHOKE or two. If the B+ noise floor is not clean then neither will the audio! Use an LT1086 for the filament supply. As you skills inprove, you could even change later to a 12ay7/6072a and a cascode up above it. Ray
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Old 21st April 2009, 07:43 PM   #7
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Similar to RCA phono:

12AX7-passive RIAA-6DJ8 with grid leak bias
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Old 22nd April 2009, 12:32 PM   #8
Alex M is offline Alex M  United Kingdom
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Default Re: tube pre amp for phone stage

Quote:
Originally posted by hunsoop
Hi,
I am trying to build a tube pre amp for phone stage. People said that marantz 7c is a pretty good for phone stage. But i can't get the schematic. Can anyone help me? What do you guys think, as a beginner, of chanlleging marantz 7c pre?
How about the Vacuum State FVP/SVP preamp? This is a relatively simple design, with three 6922 pre channel, but uses a JFET at the phono input to get very low noise. The FVP/SVP beat some very sophisticated pramps in this group review. You can start with a fairly simple 250V HT supply, but you can get serious gains in performance by upgrading to the SuperReg shunt regulator.

I have built the differential version of the FVP/SVP, the RTP3, and although I wouldn't recommend it as a project for a beginner, it sounds fabulous.

Alex
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Old 22nd April 2009, 12:40 PM   #9
SY is offline SY  United States
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It's an excellent design, but definitely not for a beginner. Cascodes in general require immaculate power supplies and grounding, with the complications multiplying in hybrid circuits.
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Old 22nd April 2009, 02:19 PM   #10
tinitus is offline tinitus  Europe
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Default Re: Re: tube pre amp for phone stage


No mention of Broskies Aikido Phono yet

I suppose it works ok, and an easy affordable project

I suppose a simpler lower gain riia would work with step up trafo too
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