Impedence Matching Help Needed


I've been hoping to build a passive preamp system, but am confused about impedances and voltages. The idea is this:

Sources:
(1) Audioengine DAC (for multimedia sources) (output: 2V, 2 ohms)
(2) NAD CDP (output: 2.2V, 200 ohms)
(3) Mani phono preamp (output: 5V, 75 ohms)... also have Yaqin tube phono preamp I could use (output: 0.7V, 500 ohms)

Here's where I sort of 'create' a passive preamp:
(1) Passive 5-way input selector
(2) Schiit sys passive ALPs attenuator (input: 10K ohms, output: 5K ohms max)
(3) Schiit loki passive EQ (input: 47K ohms, output: 75 ohms)

Then all this goes onto a Parasound Classic 125 WPC power amp (input: 33K ohms)

I will adjust the amplifiers gain/sensitivity down until the passive attenuator is at around 2 o'clock to minimize the increased S/N that comes from too low of a volume setting. I plan to use the shortest cables possible to reduce resistance?capacitance?.

I would love to hear from those who are more experienced with the math behind this if this should work or not. I've heard of passive systems sounding thin and would prefer to avoid that. I'm also considering purchasing an Onkyo A-9150 (~$400 new) and going active, but I'm trying to avoid the 'character' that active preamps tend to add and I don't have a budget for a high end active brand.

Keep in mind I also have an Onkyo Integra home theater setup with preamp outs, but I'm hoping to use y-adaptors on the sources and primarily use the above system for 2-channel listening (only turning on the Onkyo HTR for movies). So, in a way the 2-channel system is a HT-bypass setup.

Thanks for any help from the impedance experts!


Fb927ac4 ffc7 44ee 9f23 2d93ff1223cdrobertjason75
I really really think this is all very complicated and not worth anything for it.

Get a decent preamp and be done with it.
I would advise against any schiit preamps. I have not had success with them in my systems.
To add to the previous responses, from a technical standpoint the main concern I would have with the proposed configuration is not related to impedance compatibility, but rather that it may not provide you with enough gain for the phono source.

Your reference to the Mani phono stage providing an output of 5 volts appears to be incorrect. Assuming you would be using it with a moving magnet cartridge, from a sonic standpoint its 42 db gain setting is likely to be the optimal choice among the four gain settings it provides. That setting is correctly stated at the Schiit website to boost a 2.3 mv cartridge output to 300 mv. Moving magnet cartridges usually provide somewhat more output than that, under the standard test conditions their output voltage ratings are based on. Let’s optimistically assume the cartridge can provide 5 mv under the standard test conditions. 42 db of gain would boost that to 630 mv.

Assuming the specs on your Classic 125 amp are similar to the currently produced NewClassic 2125 v.2, the amp requires an input of a bit more than 1.1 volts to be driven to full power, or roughly twice as much as the combination of a 5 mv cartridge and the 42 db gain setting of the Mani would provide under the standard cartridge test conditions. While some recordings can have dynamic peaks which significantly exceed the standard cartridge test conditions, and while you might never need to drive the amp to its maximum power capability if your speakers have relatively high efficiency, at best you would probably find yourself using the volume control close to its max position on many recordings. And that would be with the rear panel level control on the amp at max.

Also, btw, I would definitely not count on cranking up the settings of all of the gain controls on the equalizer to be a good way of addressing that issue, as I suspect doing so would degrade sonics significantly.

But to answer your question about impedance, I suspect there would not be any significant issues with the proposed configuration. Although I can’t say that with certainty, in part because the specified output impedances are undoubtedly based on a mid-range frequency such as 1 kHz, and do not reflect the highest output impedances at any audible frequency. And in part because I suspect that the 2 ohm output impedance specified for the Audioengine DAC is for its headphone output, and the output impedance of its line-level output is probably a good deal higher. The Audioengine website does not make that clear.

Regards,
-- Al


Went the passive way once with a pre-amp. Thought it sounded fair until we added a buffer circuit. Not passive anymore, and sounded exceptional compared to the weak sound that the passive produced. Maybe if the math all worked out, the story could be better.
Thank you for the great answers, especially Al for the phono cartridge info. My phono carts (pickering xv-15, shure m91ed, and [email protected] 5.5, 5.0, and 3.5mV respectively). So, they probably wouldn't drive the amp well enough after passing through the mani.

I'm leaning away from passive after reading your comments.

I have a yaqin ms-12b I can use as a tube buffer that should simplify this whole idea for now. With this I could bypass the schiit mani altogether since it has a phono section. I'll get an active preamp down the road, I'm just paranoid that they try to 'do too much' and arent neutral. I'm on a budget so a used NAD or new Onkyo might suffice. My goal is clean refined sound that doesn't have the harsh highs or bass bloat associated with cheap gear. Speakers are dynaudio and are great. It feels like a good preamp is the last piece of the puzzle.

So, I’ve been searching far-and-wide for a $300-$400 active preamp that will do the trick. So far I’ve been looking at integrated’s with preamp outputs because they are affordable (ie.. onkyo preamp only $1200, onkyo intergrated with preamp outputs $400). SUCH a huge difference in price and I can only think it’s because separates tend to have a richer customer base. I’m sure there are ’some’ audible improvements between these two, but I doubt it’s 4X’s.

I couldn’t find much in the used marantz, denon, or yamaha with preamp outs. All of their intergrateds have ’line out’ or ’recorder out’. However, Onkyo and Pioneer (same company) seem to have some mid-level integrateds with preamp outs.

Now it comes down to something like a NAD C 162 used or an Onkyo A-9150 new (both come in under $400).

The onkyo’s preamp outputs are 1V the NAD’s are 17V. That seems insane to me. I wonder if one would be a better match for my Parasound 2125 amp than the other. The amps input impedance is 33K ohms and input sensitivity is 1V. I’m guessing the Onkyo will perform as well as the NAD since BOTH meet the 1V input requirement? I’d rather have the newer product since I’d probably get a bit paranoid with an NAD that has 13 year old parts in it. The NAD is currently on eBay for around $300. The Onkyo can be had for a little more and is getting good reviews so far. The nice thing about an integrated with pre-outs is I can move it to another room as a complete system down the road.

More info: My speakers are Dynaudio and I am sensitive to overly harsh highs that come with cheap electronics. Sources are NAD cd player, laptop streaming Netflix, and 30 year old Technics turntable.
Preamp output voltage specs can be misleading. The NAD spec you referred to, which is actually ">15V" according to this document) is its maximum output voltage **capability.** Its actual output voltage at any instant of time will be the voltage that is sent into it, increased by its gain, and reduced by whatever amount of attenuation its volume control is set to introduce.

You’ll note that the sensitivity of its line-level inputs is specified as 150 mv "ref 0.5V." Which means that with its volume control at max an input of 150 mv will produce an output of 0.5 volts. That is a gain of 20 x log(0.5/0.15) = 10.5 db, which is a very reasonable number.

Specs for the Onkyo indicate a line-level sensitivity of 200 mv, and a "rated output" voltage of its pre-outs of 1 volt. That corresponds to a gain of 20 x log(1/0.2) = 14 db, which again is a reasonable number.

Regarding impedance, the 75 ohm output impedance of the NAD and the 500 ohm output impedance of the Onkyo’s pre-outs should both be suitable for driving a 33K load.

Also, speaking of misleading specs, Parasound usually specifies input sensitivity based on the input voltage required to drive their amps to an output of 28.28 volts, rather than the more usual practice of basing the sensitivity spec on the input voltage required to drive the amp to full power. In this case, though, that doesn’t make a great deal of difference, as 28.28 volts into 8 ohms is 100 watts, and 28.28 volts into 4 ohms is 200 watts. Those figures being not a great deal less than the maximum power capability of your amp, especially when considered in terms of db. That, btw, is why in my previous post I referred to "a bit more than 1.1 volts," rather than to 1 volt, as being required to drive the amp to full power.

Regarding how sonics may compare between those two alternatives, I’m not in a position to comment.

Regards,
-- Al