2. amps-preamps
  3. 194088-170624

gain staging and attenuation

Hi everyone - Thanks for your help. This is a 'set up question' but I hope is of some general interest. I am adding a phono set up to what was a streaming only system. The streaming system fed an RME2 DAC into a Mcintosh MC462 power amp. Previously I used an ARC LS25 II pre in the chain, but I thought the direct feed and DAC volume control sounded better. When adding the phono I used an Ortofon 2M blue into a parasound JC3+ phono pre and then went into the ARC preamp. This then went directly into the power amp via a good quality switch (Goldpoint), letting me select DAC or phono. All connections are balanced. The plan was to learn alignment etc with the Ortofon blue, then later switch to a moving coil.

Here is the issue - there was a tremendous amount of gain in the phono side of the system. The volume was so loud that I had to use the ARC in its lowest gain setting and even then barely turn up the volume. I solved the problem by removing the switch and instead using a Goldpoint passive pre with volume control. Even for that I had to plug Rothwell 15 dB attenuators into the back of the passive pre.  

My quesion is where to use attenuation and where to use gain. The ARC has a bypass setting that puts everything at unity gain and removes all the volume controls and switches etc from the signal path. This sounds notably better so I use the ARC simply in that setting, as a buffer. Should I put the passive preened attenuator between the phono pre and ARC, or is it better to put the attenuator and phono pre right before the power amp (with a short cable to the power amp).  I have read all sorts of discussions about this. Is there a good reason for a certain arrangement? I guess I could see the passive pre with attenuator, followed by the pre, as a sort of thrown together preamp, with switch and attenuation followed by buffer. Thanks much for your help. 

arhgef

Showing 2 responses by arhgef

arhgef OP
61 posts
 

Based on mulvelings detailed instructions (thanks!) there are not many ways to do this. Passive pre has to go before the ARC preamp because the input impedance of the power amp is about 22K, and the passive pre is 10K (or maybe even 25K). From reading, my understanding is that the passive pre will have the same output and input impedance, and that the impedance is barely affected by whatever volume setting you use on that pre. Should the attenuator also be treated as a separate device with input and output impedance the same?

Kr4 - Thanks for the helpful pointer. I guess it is best to prioritize impedance matching over putting gain first, though effects on noise make sense. 

Audio_guy - at acceptable volumes the ARC is at about 7 or 8 o’clock while  on its lowest of 3 gain settings. Frankly, it is pretty old and seems to sound better when used in bypass mode  New pre would come in handy.   

 

arhgef OP
61 posts
 

Just a final note to write up the solution here. Thanks for everyone’s help. I was using the variable attenuation of a passive pre to adjust volume because I wanted to leave the ARC at unity gain. I thought it sounded markedly worse with volume controls etc engaged. I did want to have that ARC in there to buffer the passive pre.
 

After much experimentation I ended up getting rid of the passive pre and simply using the ARC volume control, but putting a Rothwell 15 dB attenuator in the input of the ARC.  This enabled me to use the ARC volume at around 12 o’clock with its gain set at low. Certainly sounded better than using the volume at 7-8 o’clock. Noticeable improvement in the richness of the sound and the soundstage. Still not sure if the “low medium high” knob on the ARC alters gain and the volume knob is attenuation of that gain, but this works. It clearly sounded better to use the attenuation at 12-3 oclock (rather than 7-9 o’clock) even if that meant using a lower “gain” setting. Shout out to the Rothwell attenuators. No noticeable effect on sound quality at this position in the chain and enabled me to use optimal settings on the pre, just as Rothwell describes.  

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