monacor: SA-800; Solid State Stereo Amplifier

Hi

I want to resolve an issue with this amplifer.

 

The units design doesnt disconnect the speaker terminals from the internals when you switch it off so basically when you power it down it just gets quieter before crackling into silence. then the load drops and it comes back for 1 second of quiet sound before completely going off.

 

im wondering without changing the unit to switch the speaker outputs when power is off. is there any simple cheats to getting it quiet. other than putting a load on the capacitors to draw them down fast.

 

great to be here on RM with my first post

 

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Hi Joel.

I have two suggestions, the second being the safer and far more elegant solution, which is what I would recommend. 

1. If you’re looking for a ‘simple cheat’ without modifications to the amplifier, as you say, you could try using a small mains transformer with a low voltage output winding to hold a couple of relay contacts closed, those contacts being in series with each speaker line.  The primary of the transformer would be connected to the switched output socket at the back of the amplifier.  When the amplifier is switched ON, the contacts would close immediately enabling the speakers to be connected to the outputs of each amplifier channel.  The difficulty comes when switching OFF since you need a fast release of the contacts to disconnect the speakers in order to silence all the spurious thumps and clicks which follow.  For this reason, it would be prudent to energize the relay coils using a.c. from the low voltage transformer secondary since there would be no appreciable stored charge as there would be when using a d.c. circuit with smoothing capacitor.  The low voltage transformer would also provide the creepage and clearance distances required for electrical safety when using this approach.
2. Recommended approach.  Your fellow compatriot, Rod Elliott, is a veritable authority on audio/hi-fi design in all its aspects and I hold him in high esteem.  I’ve used his design for Loudspeaker Protection and Muting and found it to be simple, effective and reliable.  This is the better course to take since the schematic for the Monacor amplifier suggests that no protection is provided for the loudspeakers should the ‘amplified diode’ bias transistor or one of the power transistors fail.  More modern amplifier designs usually take this into consideration and provide such protection since speakers often cost more than the amplifiers that drive them.  Looking at photographs of the amplifier, Rod’s circuitry would likely fit inside quite easily with minimal modifications to the amplifier, but you could construct it as a separate item.

For those members interested in audio design, Rod’s pages at Elliott Sound Products are well worth a look.  

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