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 1 response by mulveling

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mulveling
2,190 posts
 

The JC3 specs 48dB gain in MM mode. That’s a lot. Though it’s unclear whether this is affected by choice of outputs RCA vs. XLR. Ortofon 2M are also known for having hot oputput levels, > 5mV. This is going to result in an unusually high output level for a vinyl source. That explains your observations. You’d be better off with an MM cart in the 2.5mV range. 

Adding extra attenuators downstream means you’re generating too much extra gain you didn’t need. It’s "suboptimal", though most of our systems do it to some degree. On the question of where to place fixed attenuators (like Rothwells) - you might notice slightly different levels of attenuation based on where you insert them, due to the different input impedance of preamp versus power amp. You can sometimes get a lower noise floor by placing the attenuators between preamp and power - particularly if it’s a tube preamp w/ high gain and your speakers are sensitive. In that case, the attenuator helps de-emphasize the active preamp’s noise floor. But in most cases, it probably doesn’t matter much where you choose to put the attenuators. 

The situation will likely be different if you switch to an MC cart. JC3’s gain spec for MC mode is 64dB, which is more modest than its MM gain. This would pair perfectly to cartridges ~ 0.4mV. 

This is also a good illustration of why multiple gain options is a very good feature for phono stages. With one fixed gain (per MM/MC mode) you’re limited in your "optimal" carrtidge matchings. 

For a passive preamp insertion (Goldpoint, Khozmo etc) you’re going to have to be careful about imedance matching. A passive volume control serves simultaneously as the load impedance for the upstream and the source impedance for the downstream. First step is to list out the input and output impedances of your components. That might help choose where to put it. Then you can select the correct impedance for the passive control (usually 5K - 10K). You want to aim for at least a 10x imedpance ratio for each source-to-load interface (more than 10x is better). 

Example:

  • Source component output impedance: 500 ohms
  • Load component input impedance: 50,000 ohms
  • Passive attenutor of 5,000 ohms value: On the source size: 5,000 / 500 = 10x (Good). On the load size: 50,000 / 5,000 = 10x (Good)  
  • This is cutting it close to the 10x rule of thumb on boths sides of the passive, but it will work fine.
  • Tube preamps in particular might not like driving lower impedance loads, like a 5K - 10K passive, even if you hit the 10x ratio.
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