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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Some more pictures:
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another two pictures come...
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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.
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? :?
The OP07 or similar will also do. When you remove the input _BIAS_ servo you will get more offset due to input bias currents.