passive v. active preamps, could use some input

ooheadsoo · « **Reply #20 on:** 22 Mar 2004, 11:32 pm »

Yeah, I think it really depends on system matching. It's hit and miss unless you're specifically aiming for it.

WEEZ · « **Reply #21 on:** 23 Mar 2004, 02:04 am »

My 2 cents worth-

Stay away from passive preamps- frequency response can change with volume attenuation, and impedences could be a problem if you ever change your source- which with the changes going on with digital these days- is a likely possibility.

There are many excellent choices today in active pre-amps- some even at reasonable cost (AVA;IRD;Musical Design;hell, even Conrad Johnson PV-10B's can be had on sale or used for a reasonable sum)

And who knows- someday you might want to use "other" sources such as analog from a good turntable/arm/cartridge setup (which beats digital hands down, if you ask me).

For what it's worth...

zybar · « **Reply #22 on:** 23 Mar 2004, 02:21 am »

Weez,

I have to say that I disagree. I have directly compared in the last month or so the Placette passive vs. AVA T7, MiniMax, and North Star active preamps. Although none were "bad", the Placette produced the best sound by far "in my system".

Like any setup, you need to match gear carefully (as you do with an active preamp or other gear). I don't find that a properly built passive fluctuates in performance based on the volume level.

Lastly, what does a passive have to do with listening to analog? The majority of preamps out there don't have built in phono stages, so you need an external one anyway. I have used an external phono stage with a passive to very nice effect in past systems.

It is posts like yours that scare people away from passive preamps. A passive preamp can produce excellent music (just like an active preamp). It would be a shame for somebody to shy away because of all the stereotypes and false information out there.

Just another two cents...

George

Oxia · « **Reply #23 on:** 23 Mar 2004, 02:24 am »

Quote from: rkapadia@ROOP

Active preamplifiers are generally required when the input has a high "input impedance".

Actually, the opposite is true. The input impedance of an amplifier is a measure of how much current it will draw and the higher it is, the more difficult a load it is. Conversely, the output impedance of a preamp is a measure of how freely current flows from its output and the lower it is, the more capable it is to drive difficult loads. Consequently, when driving difficult loads (i.e. amps with low input impedance), a preamp with low output impedance is best suited. Generally speaking, solid-state active linestages have the lowest output impendance.

Passive preamps are best suited to amplifiers with relatively high input impedance. I've heard the "rule of thumb" that your amp's input impedance divided by your preamp's output impedance must be 10 at minimum. VTL, I believe, says a factor of 200 is minimum. Who's right? I would defer to the manufacturer of your amp. Ask them, and they should give you the best recommendation.

Some more things to consider regarding passive preamps:

1. Without proper impedance matching, the combination of source impedance (preamp) and interconnect cable can act like a low-pass filter, as the high frequencies will be rolled off at a rate equal to the source's output impedance times the capacitance of the cable being driven. For this reason, passives should be mated with shortest possible interconnects of the lowest possible capacitance.

2. Since passive linestages are not buffered, its output impedance will vary as its volume attenuator is turned. This is because the resistance of the volume pot (or stepped attenuator, or whatever it's using) gradually decreases as your turn the volume up. When this function is combined with the capacitance of the IC used, this might result in non-linear frequency response (treble rolloff) that varies with the volume setting.

In short, the are a large number of circumstances (e.g. the voltage output of your source, the gain of your amp, the sensitivity of your speakers, the capacitance of the cabling you use, your preferred listening level, etc.) that may determine whether a passive will work best in your system or not.

zybar · « **Reply #24 on:** 23 Mar 2004, 02:29 am »

Oxia,

If things can fluctuate with volume level, how come I don't hear that with the Placette? Do you think it is something special with the Placette or maybe the fact that the other factors are positive in my system?

Thanks for your post. I knew that a passive could sound different depending on the associated gear, but this puts some sceience behind what I have heard.

George

Oxia · « **Reply #25 on:** 23 Mar 2004, 02:42 am »

Hi zybar,

While it is possible to experience frequency rolloff with passives, this is a worst case scenario that simply stresses the importance of impedance matching. There are certainly strengths and weaknesses in any gear. Fortunately, the weaknesses of passives can be ameliorated by matching it with an amplifier of sufficiently high input impedance and interconnects of low capacitance. If you suffered no ill effects when using the Placette, I would suspect that you took care to ensure that your downstream components were well matched to it. Out of curiousity, what amp were you using?

Cheers,
O

zybar · « **Reply #26 on:** 23 Mar 2004, 02:46 am »

I am using Rowland 201 mono blocks.

They have an input impedance of 40k ohms.

George

Horsehead · « **Reply #27 on:** 23 Mar 2004, 02:52 am »

The Placette worked extremely well with my Pass X350 which had an input impedance of 22k ohms.

Oxia · « **Reply #28 on:** 23 Mar 2004, 03:08 am »

Oh, I should have seen it in your system description -- extremely nice setup. It's a shame though that Placette doesn't provide the output impedance specifications for their passive, as that could have been illuminating. As an aside, the adjustability of your speakers could also be a positive factor. Scientific explanations notwithstanding, I do believe that your ears are the final arbiter of what sounds "right". So if your system sounds right to you, who am I to second guess. Enjoy.

Horsehead · « **Reply #29 on:** 23 Mar 2004, 03:34 am »

I brought up the Pass because of it's relatively low input impedance and the fact that it was able to mate so well with a passive- something I was concerned about. You actually raised several important issues and I checked with both Pass and Placette to see if they thought there would be an impedance issue. Both said no and after listening I guess they were right. The only "spec" I know about on the Placette is a constant 9K input impedance for a given source at any volume setting. Not sure what this actually means for the output impedance and amplifier matching issue though.

Mad DOg · « **Reply #30 on:** 23 Mar 2004, 10:29 am »

Quote from: zybar

Weez,

I have to say that I disagree. I have directly compared in the last month or so the Placette passive vs. AVA T7, MiniMax, and North Star active preamps. Although none were "bad", the Placette produced the best sound by far "in my system"...

i have to agree w/ zybar here...i've directly compared Jon L's Bent passive to the IRD, MiniMax, and Aragon Aurum and the only preamp that I didn't clearly prefer the Bent over was the Aurum. The Bent in my system clearly produced better sound in my system over the others...

Marbles · « **Reply #31 on:** 23 Mar 2004, 01:10 pm »

Passive pre is a Bent TX102, amps are Symphonic Line Kraft 400's...input impedance of the amps is 10 KOhms.

I'm very happy

Transformer based passive pre-amps act differently than resistive based passives.

Please don't lump all passive's under the same umbrella.

flintstone · « **Reply #32 on:** 23 Mar 2004, 05:45 pm »

I had the Bent passive in my system for a time. The most important factor with this unit IMO is not with the Bent at all, it seems to mate well with many amps. Your CD player must have enough output to drive your amp to full power...mine did not. On the other end of cd player output..a cd player with to much output will cause the Bent to (over-load ??) if thats the correct word.

My system with the Bent was fantastic at the volume levels I could listen but with my Krell Ksa-250 amp and Apogee Duetta Signature speakers I could never get enough volume in my very large room.

What I did hear was great and If I ever change speakers I would love to own another one of these units.

Dave

P.S. As I stated above, the Technics has a hidden volume control. If you push display twice on the remote and then toggle left on the toggle botton it will come up on the players face menu. Toggle up or down to control volume to your amp...defalt setting is full volume so make sure you turn the volume down before you play music.

Paul L · « **Reply #33 on:** 24 Mar 2004, 05:25 am »

The crucial criteria of using a passive controller is the source output must be capable to drive the power amp. In Flinstone's case, not enough volume has nothing to do with the passive. It is the source cannot drive the Krell to full power.

I know that at most of our customers who change back to active preamp is because they change source without trying out. When the source is not capable to drive the poweramp, they have no choice but going back to active unless they change their newly acquired expensive (mostly) source equipments but which will be very unlikely.

maxwalrath · « **Reply #34 on:** 25 Mar 2004, 02:11 pm »

Thanks flintstone, looking at the remote I never would have guessed there was a volume control. Now I know...always read the manual. When it's working there's plenty of power and the sound is great, but when it's not.....

I've had some pretty major problems with the direct Odyssey hook-up though. To start, when the CD player is hooked directly into the amp, there is an audible hum or fuzz at all times. I think it's in the midrange area, it seems to be coming from the ribbons of the RM1's. The noise is always there, including when the cd player is turned off. It is much worse when the s-video is hooked up to the tv, then music becomes unlistenable. During these listening tests, the only cords the Technics would have hooked up are the power cord and the audio outs. The noise disappears when the connection goes through the modwright beta.

That's not the worst of it though. Sometimes when I start the Technics up there are bad pops or thumps coming from the speakers each time there is an action by the player. When the player first reads a disc or when I hit play, there's a pop/thump. It's when there's just about any action. After that there won't be any music coming from the system exept the fuzz sound, except after the pops the fuzz is a little worse than before. The weird thing is it seems to be completely random as to when the Technics decides to play discs without problems, and when it makes me think I'm blowing out my speakers.

maxwalrath · « **Reply #35 on:** 25 Mar 2004, 06:30 pm »

anyone have any idea what's going wrong here?

rosconey · « **Reply #36 on:** 25 Mar 2004, 06:32 pm »

after 4 pages what did you expect

Rocket · « **Reply #37 on:** 25 Mar 2004, 10:57 pm »

Hi Max,

How easy is it to use the Technics variable volume control? In my experience and what i have done is to always buy a cdp and use the remote volume control to control the sound levels. Very easy method to use.

Btw everytime i remove the preamp from my system and connect the cdp directly to the amp i experience a loss in music pleasure. By this i mean the top end is less refined, the soundstage is reduced and basically it Sux.

Recently i had a shootout with my aksa 100 (nirvana) and a bel canto evo amp and we used a cdp direct to the amps. Both amps in my opinion sucked as the sound was a bit rough. When i put my aksa back into my system (perpetual tech p1a/p3a modwright, n.e.w. preamp etc) it sounded much better.

I have Never experienced what you are describing, but it doesn't sound good as you may damage your ribbons.

If you Have to sell your Modwright (btw mine is breaking in well and i think does sound better than my n.e.w. p3a tube preamp) you might as well try the luminious preamp. I would recommend using a preamp in your system.

Best wishes

Rod

ooheadsoo · « **Reply #38 on:** 25 Mar 2004, 11:00 pm »

Rocket, sounds like you have the same problem I have, which is cables that are too long. I solved mine by bumping up the sensitivity on my power amp. Is it possible for you to do that with yours?

Rocket · « **Reply #39 on:** 25 Mar 2004, 11:21 pm »

Hi,

ic's are all less than 1 metre in length.

the sound is fine in my system as long as i use a preamp.

i was over a friend's house when we used his cdp direct into the amps. imo the system didn't sound as good without a preamp.

i've just bought the modwright beta preamp and will take it over to his house next week for a demonstration.

regards

rod