QSXM2: Greetings from Germany / Bulgaria!

[sinigersky]

heaveno Everybody,

a month ago I have built a QSXM2 phono preamplifier and I am testing it now at my stay in Bulgaria while converting my favorite old LP to digital. My implementation of the amp is as follows:

- single Analog Devices SSM2220 / SSM2210 input pairs of transistors
- toroidal transformer, soft recovery diodes and a single Jung Super Regulator as power supply (the spec's for the best pickups say that channel separation is approx. 25 dB so I've decided not building separate regulators)
- aluminum casing from an old LAN switch
- Neutrik connectors

I am using an Ortofon OM 20 Super pickup (loaded with 440 pF) and a TOSHIBA turntable. For digital conversion, a M-AUDIO Audiphile USB is used in 24 bit / 48 kHz mode.

First listening impressions are that the preamp is absolutely superior over the original preamp integrated in the stereo. Far better mid-range, very, very clean sound! The bass is also very pleasant, I think a big improvement came from the new pickup, too. I was pleased to hear my favorite LPs with this new system!

The M-AUDIO measures 96 dB S/N ration with Right Mark audio analyzer. The idle background noise shown / recorded by CoolEdit Pro 2.1 stays about -81...-84 dBFS (dB relative to the sample's Full Scale). I noticed that when connecting the QSXM2 preamp only (without anything connected at it's input), a noise floor of -60 dBFS was detected. I then tried removing the chopper stabilized op-amps at the input (I am using TLC 2652 from Texas Instruments, which require a small PCB modification), and the noise level fell back to the sound card's -81 dBFS! The DC offset did not become worse, or at least I was not able to notice such an effect. I think this input servo ruins the noise performance, which is otherwise extreme! Here is a question about the amp. I don't quite understand why this input servo is implemented in that way. After removing the op-amps, the LEDs in the RIAA stages began to illuminate weaker! Is this ok, or should I adjust something when omitting the input servoes? I would be glad to read some comments!

Thanks to the Author for this superb project!!!

Here are some pictures of the preamp:



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[sinigersky]

Some more pictures:



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[sinigersky]

another two pictures come...


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[peranders]

When you mesaure the noise you must have the input shorted or with a load that is equivalent to a cartiridge. An open input will increase the noise. The most important thing is that the noise of the amp is substantially lower than the vinyl noise.

[sinigersky]

But could you explain the design idea of the input servos (why aren't the OP07 servos enough)?  :-o And why do the LEDs of the RIAA stages illuminate weaker when removing input servo?  :?

[peranders]

The OP07 or similar will also do. When you remove the input _BIAS_ servo you will get more offset due to input bias currents.