Bellini "No-Cap" preamplifier

The "normo-audio-designer" has always avoided the capacitor problem.

The rule was:


or even

Well, capacitor after capacitor, the signal into a preamplifier usually has to go across at least three of these "strange" components.
You don't know how strange are they ? Well, give a look at these pages:
You'll agree with me that there is only a component more perverse and non-ideal than a capacitor: an inductor !
But this is another story.
Coming back to the Bellini preamplifier, we wanted to have a "No-Cap" preamplifier; this is not a new history, a lot of other company have done the same job.
Anyway we did it in our way.
It's now time to download the zipped poscript (635KB) file of the Bellini preamplifier.
A first look at the Bellini block diagram reveals nothing of particular, except for the "DC correct/Protection/Stand-by" system; we'll explain this later.
The Phono stage is the same of the Puccini S.E. integrated amplifier, except for R15 and JP3 (R115/JP103 for the right channel) that if installed with all the phono components not fitted let the input becoming a normal "line" input.
The signal present at the output of this stage, toghether with all the other signals coming from other inputs, goes to the "Listen" switch as to the "Record" switch.
This latter has the sixth position grounded, avoiding larsen problems with the tape loop (you can't record on tape if you're listening a tape !). The "Tape out" is buffered via IC10/IC110.
After the "listen" switch there is a circuit quite similar to the Puccini, with the difference that even the first of the two operational amplifiers (i.e. IC6/IC106) have gain; then the ALPS volume pot (VR1); at the end of the chain some other gain with IC7/IC107.
Well, nothing of special 'till now; but what if there is a DC coming from some input ? Here come the DC servo-loop.
This is a double integrator built over the OP-77 "Ultra-low offset voltage operational amplifier" (IC8/IC108); when this configuration is used in a loop (the out of this IC control IC7/IC107 via a summing point with R25/R125), the DC at the output is controlled and become as low as the input Vos of the OP-77 (10uV typical in this case).
The time constants (R26, R31, C55, C56; add 100 for right channel) and the relative gain of the loop are choosen for have a pole in the transfer function around 1.8 Hz.
IC9/IC109 are window comparators, sensing the integrator's output voltage. If this voltage exceed +10v or -10v, then the output (FAULT) drive IC201 (NE555) via R201, switching off the Bellini's outputs and avoiding potential problems if the system is "out of control" (In this case both the red and the green LEDs are ON, thus obtaining an orange light that indicate something is going wrong).
The same IC (the NE555 timer) is used at the power up as a delay (few seconds), and is driven by the SW201 front panel piezo switch: every time the button is pushed, the IC202A "toggles" it's output, driving Q200 and Q201 ON or OFF.
But what happen if there are some devices with a high DC level connected to the input ? With the gain of four in the signal path, there is the possibility that the protection circuit (i.e. the comparator) switch off the output, making impossible to use the devices.
For this, we decided to have one of the inputs with a jumper, choosing if the coupling must be DC or AC; this is AUX2, that is AC coupled by factory default. The user can always move jumpers to obtain the input DC coupled as others inputs.
The power supply is divided in three sections: CHL (T1, IC1, IC2), CHR (T101, IC101, IC102) and SERVICE (T200, IC200).
All the transformers are Talema toroidal 15+15 Vac. The phono stages are powered via low value resistors (R32. R33, R132, R133), in order to avoid interactions with line stages.
Output RCA jacks are double, for helping in bi-amplification.

Errors !

A simple test can reveal if you have this kind of problem: listen to a CD as usual, at the normal volume level you use; switch to Phono, with no vinyl disc; if you hear the CD music on background, you have a 2.2 revision or earlier. Correction is made cutting the track and connecting again via a short wire that do not run parallel and/or near the other track.


Nothing to say but the fact that we supposed to use an infrared remote control at least for the volume; for this reason you'll find two holes back of the ALPS pot, in order to install the motorized version.

If you have time ...

Click here for an internal view of the Bellini preamplifier
Another interesting "mod" could be the use of the second pair of RCA outputs for a phase inversion, in order to drive in bridged mode the Donizetti power amplifier. You can use some of the popular ICs like the SSM2142. But remember to make some mods into the amplifier too, or you'll loose the protection circuit !

Or, you can leave the Bellini as is, and put the same chip into the Donizetti (it's cheaper, you'll use less cabling !)

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