PWD and amplifier input impedance

I am trying to better understand the effects of amplifier input impedance relative to its source, in my case a PWD mk2 (I run this amp-direct).

I have begun looking for a new amplifier. PS Audio’s upcoming unit will be on my short list. In the interim, I have an amp from another small manufacturer coming this week to demo. The input impedance of this unit is low by amplifier standards (8000 ohms).

Does this pose any relevant concerns technically, and what would I be listening for as far as audible effects (if any).

As always, any insight is much appreciated.


The PWD’s output impedance is lower than most at 100 ohms unbalanced and 200 ohms on the balanced output. This should work fine with that 8K input impedance of your amp.

I am curious as to what amp you have that has such a low input impedance. This would be okay for most solid state sources but would be a bit low for most tube products.


It’s an H2O S250, made by Henry Ho. A small production affair, he makes a dual mono ICE based amp with linear power supplies. They seem to have quite an underground following, and Henry has been great to deal with. Outside of shipping costs, it’s a free in home demo, so what the heck, eh?

Again, it’s a demo only at this point, and won’t even receive it until the end of this week.

Do tell us what you hear.


Will be happy to… presuming a small technical glitch doesn’t foil the whole thing!

Apparently there is common mode DC on the speaker outputs. Honestly, we are reaching the limits of my knowledge of what exactly this entails, but the manufacturer has instructed me not to connect this to a subwoofer. I use a Thiel external crossover which taps the speaker outputs and creates a line level output to a subwoofer, and am currently contacting Thiel and the subwoofer manufacturer to make sure there are no issues.

Stuff like this makes me wonder whether it’s all that bad to just go with a company you trust and be done with it. I am loosing enthusiasm for “sampling the amplifier landscape” at this point.

More to come…

I have been using an H20 Audio Signature Stereo 250 amp in conjunction with an Audio Research preamp for a few years which replaced my tubed Audio Research VT-100 MKII amp. I do have a PWD MKII DAC, but have not used it directly into my amp as I need more than one input and also want tubes somewhere in my system. In my 2 channel system, and to my tastes, my H20 amp is superb sounding and the first SS amp that could compete, and in fact sound better to my ears, than my former tubed-amplifier, the VT-100 MKII.

As for build quality and sound, the H20 is top notch, but nothing fancy to look at. Also Henry Ho’s customer service is second to none. He is a one man operation and does not have marketing to sell his products, but rather, only through word of mouth from delighted customers. For anyone interested, here is a link to a review of the H20 S250 Signature Stereo amp:

Henry told me in an e-mail that “anything with an output impedance of 600 ohms or less is just about perfect for the H20” when I asked him what preamps would be a good match for his amp. From the information provided above by wingsounds13 regarding the output impedance of the PWD MKII, at appears that the PWD might work directly into the H20.

Henry does recommend that the H20 be used with a high quality Active Preamplifier, due to its low input impedance according to the owner’s Manual. However, like everything else, home audition is a must to tell if something is compatible in your system and sounds good to you. It is certainly worth a shot to give his amp a try. From my experience the H20 also sounds best with a good shielded power cord like the PS Audio AC12 or equivalent. BTW I found that my H20 thrives on the PS Audio P10 power plant. Paul MCGowen himself thought that it would—Thanks Paul!!!

Good luck and let us know if it works out for you.



Bob, this is a fascinating review. Thanks for posting the link.


Your most welcome.

Here is the original done by Seajan Ebaen on the original 250 mono block amps:

It will be most interesting to hear the new amp coming out by PS Audio too. :smiley:


I received the amplifier yesterday and did about 2 hours of listening last night.

While I must be cautious as I really haven’t listened to many competitors, the amplifier is wonderful and completely outclasses my current amplifier in about every conceivable way. I have never heard percussion produced like that, that I can remember.

Henry tried to send me his Fire pre-amp, which looks like it may be quite a product itself. This is in keeping with nordicbob’s comments on his recommendation to use a pre-amplifier. I would love to have tried it, but monetarily it’s not in the cards and I really don’t want to have to insert a pre-amp unless all else fails. I will say that it is pretty nice being run directly from the PWD.

The only thing that kills me is this DC potential on the speaker ouputs… Thiel tells me that their crossover should be fine, but Dr. HSU of HSU research has some reservations. I prefer connecting a sub via the high level outs through this crossover, and it would be a bummer if I had to give this up.

I have a pair of ncore monoblocks coming from an acquaintance sometime in the next couple of weeks. Down the line, I hope to hear Paul’s creation. If people are interested, I’d be happy to comment.

Thanks for the additional information here, guys.


Ryan, I may have missed it, but what is your current amplifier?

Sorry, it may not be clear in the sig…

PS Audio’s own GCA-100.

It’s clear. :slight_smile:

I ignore sigs, although in circumstances like this I should remember to check.


You piqued my curiosity, so I finally connected my PWD MKII directly to my H20 amp. It did not take long to find that much of the magic that I had in my system, at least to my ears, was lost without my Audio Research preamp in the chain. I am not trying to convince anyone one way or another that a “Top Notch” preamp in the chain is better than none at all, but I personally have never had any luck in getting good sound without a good preamplifier in my system no matter what gear I had at the time.


Despite usually ignoring sigs myself, here I end up putting everything but the kitchen sink in mine :stuck_out_tongue:

nordicbob said: You piqued my curiosity, so I finally connected my PWD MKII directly to my H20 amp. It did not take long to find that much of the magic that I had in my system, at least to my ears, was lost without my Audio Research preamp in the chain.

Careful! Your comments are on the threshold of pushing me out of my delusional state of grandeur!

I only have PS Audio PCA-2 pre amplifier at present, and the PWD is superior going amp direct. As I don't think I can keep all the knives in the house out of my wife's hands, I simply MUST believe that no pre-amp is better than ANY pre-amp! If I get on a pre-amp fix, I fear for what she might do.

I like what I am hearing, so I will try to block your comments from my conscience X_X



Oh dear! I had better keep my mouth shut as I don’t want to jeopardize harmonic bliss between you and your wife :blush: My wife was not too happy when I purchased my first power conditioner many years ago which cost $600 :open_mouth: “You did what? Paid $600 for a set of plugs!”

However, if you are crazy enough to decide to audition another preamp, do try to keep all sharp objects away from your wife unless you can keep it undercover :blush:

Good luck.



I am exaggerating a bit, of course, but it would indeed be nice to have some sort of private account for these kinds of things… of course, hiding an appropriate amount of cash in it would probably not go unnoticed. So for now, it’s hiding the sharp objects!

I’ll never forget the first time when I was traveling on behalf of Genesis Technologies that I was asked to come into the customer’s kitchen during a break in the listening session. The fellow opened the fridge, beckoned me to come over and stand with him hidden from view by the open fridge door, and handed me a large wad of cash.

"Here. I need to replace the power amps in the system and I’ll contact you later about the details. I’ve told my wife they cost just under $1000 each."

The amps he was replacing were $10,000 each monoblocks.

You know, the fridge might be a better investment vehicle than some others I am engaged with… maybe that’s where I’ve been going wrong :-))

I once sold a pair of large floor standing speakers locally to a gentlemen who paid $3000 in cash for them, all in new $20 bills. He told me that he had a private stash for such audio deals as his wife was ready to divorce him if he kept spending money on audio. He said that his next problem was to try to get the new speakers set up in the house in hopes that his wife would not notice them despite his old speakers being only about half as tall and a different finish. I never heard the outcome, but to this day have never forgotten about that undercover deal B-)

I understand and appreciate the humor of the stories, but what terribly sad relationships.