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Old 16th March 2012, 02:25 PM   #1
legarem is offline legarem  Canada
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Default Adding gain to this phonostage

Hi

This is the Stax Electret cartridge preamplifier ECP-1. I would like to have a little more gain from this preamplifier. i tried to bypass R104 and R113 with a 270 uF capacitor but had so much gain that the sound was distorted and unlistenable. Do someone has an idea about what to do to have more gain with this preamp ?

Thanks
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Old 16th March 2012, 02:32 PM   #2
DF96 is offline DF96  England
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That is strange - the right and left channels have quite different circuits! Is it an attempt by the maker to put two versions on the same drawing?

Bypassing the resistors as you did will stop the RIAA feedback network from operating. Your best option is to add a linestage after the preamp.
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Old 16th March 2012, 03:22 PM   #3
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Very strange design.
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Old 16th March 2012, 03:35 PM   #4
legarem is offline legarem  Canada
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Quote:
Originally Posted by DF96 View Post
That is strange - the right and left channels have quite different circuits! Is it an attempt by the maker to put two versions on the same drawing?

Bypassing the resistors as you did will stop the RIAA feedback network from operating. Your best option is to add a linestage after the preamp.

I confirm you that this is the real schematic as there is in this preamp. The sound quality is above most cartridges I tried with standard RIAA phonostage.

There's no frequency imbalance between R & L when you listen music even if R & L schematic values are different.

I use a linestage with this phonostage but would like a little more gain with both.
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Old 16th March 2012, 03:46 PM   #5
DF96 is offline DF96  England
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I am astonished. Implementing RIAA with two different circuits is a guaranteed way to get poor stereo imaging. I am unclear whether it has been caused by eccentricity or incompetence. Possibly the former, as the design is a bit unusual anyway.

An alternative explanation is a mixup between the designer and the drawing office/factory. A draughtsman got two versions mixed up and nobody spotted it. The designer, perhaps just working via a short-term contract, was never able to check that what was built is not what he designed.

I am continually surprised by the bad circuits which find their way into commercial equipment.
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Old 16th March 2012, 03:51 PM   #6
SY is offline SY  United States
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I haven't traced it out, but could it be different because of matrixing?
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Old 16th March 2012, 03:56 PM   #7
gk7 is offline gk7
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Isnīt this intended for the Stax Electret cartridge they sold in the past exclusively
(Not to be used with "normal" cartridges) ?
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Old 16th March 2012, 04:00 PM   #8
gk7 is offline gk7
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Stax CPY:
The Rare Stax CPY/ECP-1 electrostatic phono cartridge & equalizer - AudioKarma.org Home Audio Stereo Discussion Forums

I think you cannot use this phonostage with other pickups and expect it to work correctly.
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Old 16th March 2012, 04:42 PM   #9
kevinkr is offline kevinkr  United States
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I think the electret cartridge implements most if not all of the RIAA EQ internally, and only limited HF EQ is applied if at all. I have not done an analysis but the networks shown could be for HF loop stability. This design is very specific to the Stax cartridge, presumably the OP has one. There's more - the shell of the right channel RCA jack provides bias (low impedance via a source follower) to the cartridge, and the left shell provides a ground.
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Old 16th March 2012, 05:58 PM   #10
legarem is offline legarem  Canada
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Quote:
Originally Posted by kevinkr View Post
I think the electret cartridge implements most if not all of the RIAA EQ internally, and only limited HF EQ is applied if at all. I have not done an analysis but the networks shown could be for HF loop stability. This design is very specific to the Stax cartridge, presumably the OP has one. There's more - the shell of the right channel RCA jack provides bias (low impedance via a source follower) to the cartridge, and the left shell provides a ground.
There's no RIAA EQ as normal cartridge has. In the electret cartridge, there's FET followers made with 2SK53. This FET (which we can't find any data) characteristics probably change during aging and the pot in the preamp is there to correct the signal imbalance of the fets. I tested this preamp with square waves for fun. The two channels are showing exactly the same square waves at the outputs of the preamp even if both channels are not using the same parts.

As stated, the sound of this cartridge is better than many MCs I tried in the past and it is imaging really well without any problems.

As one FET in the cartridge was too weak, I had to open it to change both FETS.
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