Question about impedance matching

[ohshitgorillas]

I am very very close to pulling the trigger on the Eros 2 at the discounted price, however, I have one concern before I do. My preamp is a Schiit Saga, so I am a bit concerned about the fact that it has an input impedance of 10k ohm and the Eros has an output impedance of 4k ohm, with a recommended preamp input impedance of 15k ohm.

Is the difference between the 10k and 15k going to really be that significant? Has anyone tried this combination? As much as I would love a BeePre, it is well out of my budget, and I really do like the Saga enough to want to keep it.

Any advice will be greatly appreciated, thank you!

 

[Paul Birkeland]

My preamp is a Schiit Saga, so I am a bit concerned about the fact that it has an input impedance of 10k ohm

Wow, that's insanely low for a preamp, but I see that it's switchable between passive and active.  For an active preamp, a 10K input impedance is absolutely unacceptable, but for a passive it is a disadvantage you have to settle for.  While the Eros would be far happier to play into a 10K load than the 12AX7 based phono preamps that are out there, you might consider modifying the Saga to have higher input impedance on one of the inputs. 

Is the difference between the 10k and 15k going to really be that significant? Has anyone tried this combination? As much as I would love a BeePre, it is well out of my budget, and I really do like the Saga enough to want to keep it.

You could ask Schiit about replacing the volume pot with a 50K pot instead of 10K if never use it in passive mode.  I was hoping they had a different preamp that didn't have such low input impedance, but they are all 10K...even the tube ones.

 

[ohshitgorillas]

Unfortunately the Schiit Saga is a resistor ladder attenuator, not a volume pot, and can't be replaced as easily. I have considered building the AMB resistor ladder preamp as a passive unit, but I really like the tube gain stage on the Saga and it would probably be more expensive than the Saga anyway, after casework is factored in.

Doc B said over email that it should be fine, maybe a little bass looseness/warmth, and my subs are one of the weak points of my system anyway. So I pulled the trigger.

 

[Paul Birkeland]

https://www.dealsan.com/buy/motorized-potentiometer-alps
It looks like there are some other substitutes.

 

[ohshitgorillas]

I actually do run it in passive mode anyway, I am running 10wpc into 98db speakers. Mine is one of the old versions before S/+, it has a tube that I can turn on/off but doesn't use a motorized pot. The pot switches on/off depending on whether the remote has been engaged, and to reengage the pot requires turning it down to the lowest step that the remote went to. Kind of a weird one, but I don't mind.

Would modifying one of the inputs just involve putting a couple resistors across the inputs? Like say 5k or 10kohm each? Because I can do that easily.

 

[Paul Birkeland]

You would want to put a 10K resistor between the center pin of each RCA jack and where it connects on the PC board. You'd also want to not run it in passive mode configured like this.

 

[ohshitgorillas]

Thanks. I'll consider it if the effect is notable enough, however, I suspect that the Eros is going to be such a huge upgrade over my current Hagerman Bugle2 (DIY solid state phono pre, remarkably transparent for something powered by a switcher) that I won't really notice, at least at first.

Just curious, since impedance matching is something I'm still struggling to understand... why wouldn't I want to run it in passive mode with those 10k resistors? Would it threaten to damage anything or just sound bad? What's the issue there?

 

[Paul Birkeland]

The idea behind an active preamp is to provide a very high impedance load to your sources.  High impedance loads are very easy to drive, so if you have a 100K volume pot at the input of your preamp, any source is going to be completely happy driving that.  After the volume pot, an active preamp will have at minimum an active buffer stage with a low output impedance, which allows it to drive long cables and difficult loads.  For example, maybe you have a solid state amp with a 10K input impedance and a powered subwoofer with a 10K input impedance, thus a 5K load.  An active preamp with a 600 ohm output impedance won't have any issues with this.

With just a 10K pot, you will have higher output impedance.  This will cause issues with long cables and will compound issues with low impedance loads after the level control.  Putting a 10K resistor in series with the 10K pot will make these issues worse. 

There isn't a "damage" issue here, but more of a bad sound issue.  For those who sell passive preamps to completely gloss over these issues, as they aren't all that easily understood.

 

[ohshitgorillas]

I think I see. An active preamp's gain stage will have a low output impedance by design, whereas a passive preamp is just passing along the total impedance is between the source and the preamp's pot, and having those extra resistors there would just be further increasing the output impedance of the signal. Did I get that right?

I just checked and my amp's input impedance is 50 kohm.

Rather than opening up the Saga and smashing the warranty and resale value (not that I have any desire to sell...), would I be able to add 10kohm resistors in the RCA cables I'm using to connect the Eros to the passive preamp? I usually make my own cables anyway, so that would be very easy.

 

[Paul Birkeland]

Yes, but be sure they are in the end plugging into the Saga and not the end plugging into the Eros.