Adding gain to this phonostage

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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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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.
 
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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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.
 
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.
 
ECP-1 schematics

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.

Is it so that one has to be inverting and one non-inverting, like the amplifiers for e.g. the Technics ECP-450 pick-ups? Maybe I have to magnify the schematics...
 
Leads of coil/magnet type cartridges are internally "flipped".

Correct. Not possible here.
If I remember correctly, with the strain gauges like Technics 450 you could have positive polarity bias on one channel and negative on the other, and thus keep the phono amplifiers identical. (Or do memory not serve me well.) Not possible with the Stax as the bias in this case are for JFETs in the cartridge, not simple strain gauges.
 
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