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Passive pre-amps...

 

Bronze Member
Username: Unbridled_id

ChicagoUsa

Post Number: 55
Registered: Mar-04
Hello all, do any of you have much experience with passive pre-amps? I just purchased the ft audio little wonder and pair it with a nad c272...
I find a dramatic improvement over my denon dra-685 stereo receiver... What say you all?..
 

J. Vigne
Unregistered guest

Passive pre amps can, in the right circumstances, give excellent sound for minimal money. They do not represent the best solution for all systems, however. Since the output impedance changes with the volume control setting, the sound quality can also change depending on the source and the amplifier. The quality of the volume control can change the sound of a passive pre amp by enormous amounts since it is essentially the only "component" in the signal path. The very best VC for a passive unit is typically the step type made up of individual resistor pairs. They are expensive, but, the sound is a huge leap from a potentiometer. A true passive pre amp has no buffer stages to maintain the output impedance or capacitance so care must be used when placing a passive unit into a system. With no buffer stages the passive units usually have a small range on the VC where the sound works best. Outside that small range the sound will tend to be flat. Not all sources can drive a power amp to full rated power. Turntable preamps will often have too low an output voltage to adequately drive most power amps. Cables can have a dramatic affect with a passive pre amp that you wouldn't find with an active pre amp with the buffer stages. Typically cable runs between pre and power amp need to be as short as physically possible when using a passive pre amp. Anything over a few feet will introduce added resistance and capacitance that can drastically alter the sound.

If you have a passive pre amp that fits into your system you should be hearing much better sound than anything in an active unit that you could buy for several times the price.


 

Bronze Member
Username: Unbridled_id

ChicagoUsa

Post Number: 57
Registered: Mar-04
Thanks mr Vigne, your knowledge on audio matters is impressive... I try to basically listen to those with more knowledge and learn what I can.
I was initally going to get the nad c162 pre to pair with the nad amp. For a similar amount of money I wanted a pre-amp that produced better sound but didn't cost two or three times the price... My searching on audio-asylum led me to the idea of passive preamps. This is what separates the ft audio little wonder from other passive pre's (I do agree that a stepped attenuator would be an improvement over the alps pot which the unit has, this has been confirmed by those who have implemeted the change) is the x-coupler circuit. > The LW1 contains an impedance-matching circuit called the X-coupler, which gives it the ability to look at the source, volume control, and the amplifier as a complete package. Reportedly, the actual load impedance of the source is the X-coupler and the amplifier input impedance. The actual source impedance of the amp is the source and the X-coupler. The X-coupler is designed for a wide range of combinations and can drive amplifier loads as low as 10k ohms. This, in theory, should help reduce the shift in tonal balance that results from the changes in impedance caused by the varying positions of the volume control.<
I have only about 50 hrs on the amp and pre and speakers so I think some more break-in may be in order... Thus far I have noticed no lack in dynamics or bass so I suppose the passive works in my system... It does sound nice I must admit even with my overall lack of audio knowledge (I am learning).. Thanks for your feedback mr vigne...
 

J. Vigne
Unregistered guest

If you have a link to the web page for the unit, I'd appreciate the information. I've seen a few passive units that claim more or less what you describe and I'm not at all clear how this can work without an active circuit somewhere. So far all I've found are claims this happens and no explanation how.

Glad you've found an inexpensive path to good sound. It will leave more money for music or to send a few dollars to a charity.


 

Bronze Member
Username: Unbridled_id

ChicagoUsa

Post Number: 58
Registered: Mar-04
www.ftaudio.com. paullam@sasktel.net (the dealers email)..
 

J. Vigne
Unregistered guest

Thanks.

 

Silver Member
Username: Diablo

Fylde Coast, England

Post Number: 193
Registered: Dec-04
I've been looking on the web for some technical info on this X-coupler thing. Not found much, though.

One comment from the seller of these units was "The sucess of our FT Audio is because of my buddy Fred's proprietary designed X-coupler which look at the source impedance and poweramp loading impedance and do appropriate passive buffering. That sounds a bit 'active' to me - if it actually 'decides' something.

The seller also requires a long 'break in' period "Per Paul's recommendation, I played music through the unit for about 200 hours. The majority of this time, I left the power amp off. This has a nasty snake-oil feel.<shudder>

Has anyone found any circuit diagrams of this magic technology?

Some links -
http://www.harmonicdiscord.com/forums/viewtopic.php?topic=3520&forum=7&8
http://archive.audioreview.com/01/0EF86971.php
http://www.audioreview.com/cat/amplification/preamplifiers/ftaudio/PRD_118274_15 91crx.aspx

I'm thinking of building a pre-amp which also includes passive paths as an option, so would like any sugestions as to what this technique might be. I'm much too mean to spend $495 in order to take one apart to find out what makes it tick! :-)

Regards,
diablo
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 4830
Registered: May-04


It is probably a transformer coupled device. The transformers will allow a constant output impedance. The volume control sounds like a stepped resistor type. If that's what this unit amounts to you could save yourself $100 -150 by putting the pieces together; but you'll also have the labor to figure into the equation. The best values on stepped resistor type volume controls are found in the units you assemble yourself. How good are you at soldering in tight spaces and do you have some really fine solder and a heatsink on hand?


 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 4831
Registered: May-04


What sort of pre amp and power amp are you currently using, diablo?
 

Silver Member
Username: Diablo

Fylde Coast, England

Post Number: 194
Registered: Dec-04
Jan,
After re-reading the FAQ, I had started to wonder if a transformer might be involved.
I've just had a google and almost instantly found this interesting device. I doubt if this the sort used in the FT Audio though. Will have to look further and ponder on the possibilities.

I bought a NAD T753 for my main listening area a few months ago. The plan was always to feed the pre-outs for the front speakers through a 'better' stereo power amp, using the 'stereo direct' mode where appropriate.

I've been much more pleased with the sound of the NAD than I imagined I would be. I decided to leave things as they were, for the time being anyway.

Unfortunately, I seem to have developed an almost irresistible lust for a Prima Luna valve amp, probably the ProLogue Five model.

Given the fairly high output voltages and low impedance of many source components (maybe up to almost 4v rms and 100-500 ohms), it occurred to me that it might be a good idea to have the stereo leads connected to the amp as directly as possible. Hence the thought of building a modular pre-amp with passive sections. The pre-outs from the NAD would be connected as well, of course.

The design is at the 'what if' stage at the moment. It may have some simple class A amplified inputs, a headphone amplifier and eventually, an electronic crossover. Possibly even some bypassable tone controls. <shock> The design will keep the signal paths for the passive parts as short and as isolated as possible.

Electronics used to be a hobby of mine, a few years ago. So I'm not worried about soldering. I may still be capable of calculating some circuit values, as long as they don't involve much calculus. :-)

I'm starting to have more free time these days, so any projects like this may keep me from more evil pastimes.

I really shouldn't look at sites like this - if I didn't then I wouldn't know about the Prima Luna stuff and would be quite happy with what I already have!

Regards
diablo
 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3581
Registered: Dec-03
diablo,

I do not know if I have specifically enthused to you about my new Prima Luna Prologue Two.

The PL Five looks a bit like the power amp stage of the PL Two. It ought to do good things with NAD pre-amp outputs. If you would like a separate pre-amplifier, too, you could use the tape monitor out from the NAD receiver, which is also passive, and turns the whole thing essentially into a neutral source selector. Plus a high quality FM tuner, of course. I have an NAD T760 and also an older stereo NAD pre-amp, mostly used (before I moved) for its phono input. I will try these combinations myself one day One of the nice nice things about NAD receivers is the connectivity
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 4856
Registered: May-04


I'm not certain the tape outs are passive on a NAD receiver but I haven't checked. I would guess they would run through some buffer circuits if nothing else. However, the tape monitor inputs are most often the cleanest input on a receiver or pre amp. Not much is usually put in line with the tape in other than a few switches for routing purposes. If you haven't tried going into the tape inputs with your source, I'd suggest you give it a try. Usually it cleans up the sound noticeably over the "CD" or "DVD" inputs which tend to have active circuitry involved even when run "direct". On my Audible Illusions pre amp, the tape in is essentially a passive circuit to the VC's with nothing more than a buffer to maintain output impedance. The difference between going into "line in" and "tape in" on the AI is readily apparent.


Next I would suggest trying a decent VC wired in line with your source to get a feel for "passive pre ampery". After hearing the difference between passive (even with some restrictions) and active, you might be in a better position to decide what you want to design.


I would think the link you provided is referring to a device not too unlike what the FT Audio employs. Passive pre amps have come through stages that began with just plugging your CD player's front panel VC through your amp, to better pots and switching, to better VC's (stepped ladder type) and no pots and there seems to be a trend toward X-former coupled pre amps showing up in the magazines. There's a magazine called "AudioXpress" from the folks who did "Glass Audio" and "Speakerbuilder". They ran a construction article on a passive X-former coupled pre amp in their Jan. '05 issue. It's not in their back issue archives yet, but you could probably find it by giving them a call:

http://www.audioxpress.com/magsdirx/ax/index.htm




 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3582
Registered: Dec-03
Excellent, Jan.

Small comment on using tape monitor. You suggested this to me for connecting my stereo pre-amp ouput to another pre-amp input on the 5.1 receiver. I don't think I thanked you at the time. It is by far the best solution. Thank you, now.

What might be relevant to diablo is that, using "Tape monitor" out, I noticed that the stereo pre-amp worked as a source selector for all the various inputs even with its own power off. That means it is passive. It was only with the phono input that it needed power on.

Whether an NAD T753 will work in this excellent way, supplying a clean signal passively to another pre-amp input, I do not know. One would have to try it. Anyway, diablo is thinking of using the T753 as the pre-amp for a tube/valve power amp. I think that could work well and would be very interested to know.

My only niggle with the Prologue Two is lack of connectivity. No pre-amp outputs or power amp inputs, or tape monitor. In UK, Icon Audio in Leicester sell a similarly-priced tube/valve integrated that has tape outs, at least, but only by direct sale. As I said, I have a preference for dealers, and service, which I certainly got in the PL2 purchase. I have no idea what a mess it would have turned into with direct sale, or what the total shipping cost would have been for sending units back and forth.
 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3585
Registered: Dec-03
diablo,

Thank you for the "interesting device" link. I think that would be a good solution. You could miss out the T753 altogether. I was considering purchase of a valve power amp, but it had no volume control. If I had know about this, I might have gone ahead.

If you have not yet seem it, you might be interested in the thread Tube Talk.
 

Silver Member
Username: Diablo

Fylde Coast, England

Post Number: 195
Registered: Dec-04
Thanks for the replies, chaps.:-)

I've done a check on the the NAD tape loop circuit, It will not output anything unless the power is on, so using the T753 as a passive pre-amp is a non-starter. The tape loop may still come in handy though.

I will try out connecting a source to a power amp using resistors at some point. Certainly before buying any of the S&B TX-102 transformers. I've found some details of pre-amps using these, varying from 550 euros for a kit, to 8,500 Canadian dollars, fully assembled. There is also some DIY info around, see the pics on this link for example!


I started looking at various valve amps about 2 years ago. I was looking at the cost, in parts and time, of rebuilding my ancient Quad II/22 system and came to the conclusion that it probably wasn't worth it, so maybe one of the Chinese built amps would be a good alternative. I was a little tempted by the Icon Audio products which John mentions. I can''t remember when the Prima Luna came to my attention, maybe it was from John''s posts in tube talk.

So, yes, I have read some of that thread. When reading it, I sometimes get the impression that a pair of 2 watt monoblock SET amps, with a bag of NOS 300Bs from various dates and manufacturers is the ONLY way to go. But I soon shake that out of my head -- leave that stuff to those who are really dedicated. :-)

Regards,
diablo
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 4916
Registered: May-04


'Tis a bit hairshirt.
 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3596
Registered: Dec-03
diablo,

Myself, I would investigate fixing the Quad II/22.

http://www.iaguk.com/quad/service.htm

HiFi News recommends Icon Audio. I read some things about various UK amp makers. I also thought about
http://www.affordablevalvecompany.com/

If you have the skills and interest, and time, there are also kits.

http://www.worldaudiodesign.co.uk/
http://www.hificollective.co.uk/kits/glasshousehome.html

I do not have time and space for "hobby" electronics, these days, unfortunately. The PL2 is excellent and it was also good to buy through a quality dealer. I am sure there are many in your general area.
 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3597
Registered: Dec-03
Re kits there is also http://www.diyhifisupply.com/ which My Rantz drew our attention to on "Tube Talk".
 

Silver Member
Username: Diablo

Fylde Coast, England

Post Number: 196
Registered: Dec-04
The PL2 is excellent and it was also good to buy through a quality dealer. I am sure there are many in your general area.
Quality dealer? Nah, we don't have any of those in the sticks. Nearest Prima Luna dealer that I can find is a 3 hour drive away in the Midlands. What's the chance that he can demo the amp with Castle Conway 3 speakers??

Will try to negotiate a trial period with one of the London dealers.

I will probably be retiring in 2-3 years, so I may decide to rebuild the Quad stuff myself. Might get some SET kits instead though! :-)

I've noticed some pre-owned Quad II-forty amps on UK Ebay with starting bids of £1500.
I assume that the PrimaLunas are of fairly similar design, probably sound 90+% as good for a quarter of the new price of the Quads. I could buy two of those for the probable actual price of a second-hand Quad and properly bi-amp my speakers using an active crossover (eventually). That might be better, perhaps? But who knows?
I is only a simple lad and is all confoosed.

Thanks for the suggestions though. :-)
Regards,
diablo
 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3598
Registered: Dec-03
diablo,

"probably sound 90+% as good for a quarter of the new price of the Quads. "

That is exactly what my dealer said. And they sell the Quads, too. It is one shop I've been to that basically has everything they like and is not tied to any particular franchise. It is frightenening up-market but the staff are generous with their time and there is absolutely no hard sell.

I would have thought Manchester and Liverpool have some good dealers. A lot of class hifi comes from Yorkshire and Lancashire, expecially speakers.

As I said, I have only recently moved to the capital, here, after many years in a strange land. I am totally agnostic about where the sticks really is.

Frank A kindly named some dealers in and around London. I always recommend www.bada.co.uk as one way to find a reputable dealer anywhere in UK.
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 4939
Registered: May-04


"I've noticed some pre-owned Quad II-forty amps on UK Ebay with starting bids of £1500.
I assume that the PrimaLunas are of fairly similar design, probably sound 90+% as good for a quarter of the new price of the Quads.

Well, probably not, in my opinion. But I've not heard the PL's, just read about them. The transformer quality is the heart of a tube amp and few did it as well as Quad. The manner in which the grid/plate/cathode voltages are derived and tied into the X-formers are tricks that every company years back did to make their amp a bit different and a bit more efficient. Plug things like "UltraLinear", "Current dumping" and "Unity Coupled" into a search engine with valve amplifier or vacuum tube amplifier attached to come up with some comparisons. See if any of it makes enough sense to lean in one direction based on engineering principles.

Here's something I found today for j.c.:

http://www.webace.com.au/~electron/tubes/screens.htm

http://search.yahoo.com/bin/search?p=fixed%20bias%20McIntosh%20vacuum%20tube%20a mplifiers



Naturally the sound is what you are after, but the engineering will determine the overall quality of the design and will affect the longevity of the equipment. Personally, I find the fact so many Quad, Marantz and McIntosh products are still being used and fetch high prices to be a selling point for the designs they implemented.




 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3599
Registered: Dec-03
I agree with Jan, diablo. I have no idea what "90% as good" means.

There was a review in HiFi news of the Quad II-forty, in Nov. 2000, before my renewed interest in such things. As, I said, if I owned one, I would look seriously at repairing the II-22.
 

Silver Member
Username: Diablo

Fylde Coast, England

Post Number: 197
Registered: Dec-04
The Quad II/22 setup had its faults when I was last using it, about 25 years ago. Hiss, hum, lack of treble. Most of that would be due to the pre-amp, I suppose, which I wouldn't be using. It stopped working on one channel and I recall diagnosing a faulty output transformer at the time. So, essentially I have only one channel of Quad II left. I would probably need to buy another with a similar serial number to get the thing working properly. Anyone got a spare one with a serial number around 55394? :-)
I could send them off to Quad for service, but I doubt if that would be a cheap option.

I really wanted to believe the advertising, especially bits like " Custom designed output transformers Encapsulated low hum, wide bandwidth for great bass and extended highs. Designed by Marcel Croese and one of the most prestigious transformer designers in the USA", because I know that the transformers are a major part of the deal.
I like the pictures of good construction, as in this 6moons review.
I liked the idea of having two PL5s bi-amping my speakers. On a cold winters day, I could arrive at my house and turn on the amps to warm-up my hands.

I'm neither rich nor poor, I could buy four channels worth of the Quad II-forty, to do my bi-amping thing, without much problem on financial grounds. But is it worth it? (don't bother to point out that this question can only be answered by me - I know that :-)).

I could easily afford a PL5 and could use it in my second system if it gets replaced by something else eventually.

So, will I be overcome by the temptation and get one (or two)? Or listen to wise counsel and think again?
Don't know...

Best regards,
diablo


 

Silver Member
Username: Diablo

Fylde Coast, England

Post Number: 198
Registered: Dec-04
Jan,
I've not looked at a lot of the links which your last post directed me to, as yet.
I'm not looking to buy or about a month or so, if then. So I have a fair time to contemplate these ideas before making any decisions.
Thanks,
diablo
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 4956
Registered: May-04


I was not trying to persuade; just add logs to the fire. A bad X-former is almost certain death for an amplifier. At that point it becomes scrap for the next repair. I didn't mean to imply old always sounds better than new. There is very little "new" in tube amplifiers.

The PL products and other more recent lines have received excellent reviews and comments from owner such as John. Read and listen; there is a touch of nationalism in the comments both John and I have made. Whatever destination you choose, it sounds as if you're headed in the right direction.




 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3600
Registered: Dec-03
diablo,

"Nationalism"; No, I plead innocent! I just happen to be where I am.

Why this allegation....?! (Smiley face).

It seems to me from your previous post that you will really benefit from some contact with a professional dealer, who will understand your questions and requirements. Also, a good one will undoubtedly demonstate a PL5 (choose your pre-amp) and a Quad II-forty, to compare. Obviously you would make an appointment with the dealer, it is hardly likey they will have those already wired up and with other components similar to your own. It is good that you have some time and do not want everything by yesterday, a common theme on this forum.

As regards repairs, I would still investigate. Quad will service everything they ever made, as I understand it, and give a warranty on the result. There are also independent repairers around the world, especially the UK. They advertise in magazines. There are also many Quad enthusiast web sites.
 

Silver Member
Username: Diablo

Fylde Coast, England

Post Number: 201
Registered: Dec-04
John,
There are lots of Quad dealers around, even some within half an hour's drive of my house, not all of whom stock the valve stuff, though I suppose they might get some in if I rang far enough in advance.

I still can't find any PrimaLuna dealers nearer than the Midlands though. I checked out the dealer list which you linked to earlier. None seem to have them.

Maybe if I get a trial of the PL5 and pre-arranged taking it along on a visit to my nearest Quad dealer, together with my speakers? Wonder how they would handle that? :-).

I'm a little puzzled by Jan's reference to nationalism. The Quad stuff was/is English designed and now made totally in China, as far as I know. The PrimaLunatic design is based on existing Chinese models, which are probably copies of English or American originals, but incorporating American-made transformers, and circuits added or modified by a Dutchman.

No. Still can't work that one out. :-) *

Regards,
diablo

* I originally mistyped that as 'still can't wok that one out', must be that the Chinese are taking over the world, including our minds. lol
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 4971
Registered: May-04


SEE, SEE!!! SEE what I mean!!!

As they say on the Underdog cartoons, "They're everywhere, they're everywhere!!!"




 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3601
Registered: Dec-03
diablo,

My first link on Sunday, August 07, 2005 - 05:19 pm: was to Quad HQ in Huntingdon, and gives a phone number. I am sure they would be pleased to advise.

I only know one dealer with PL and Quad valve amps in stock; it is where I got my PLPL2. Unfortunately it is in London W1. I do not know if they carry the PL5 but I would guess so, or would get one. They also have the complete range of McIntosh and many other makes that may seem exotic from a UK point of view. There are probably similar dealer, though, a bit nearer Fylde Coast. Glasgow seem well endowed with HiFi dealers. That would also a bit of a treck of course.

Jan,

I do not understand.... (puzzled smiley).
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 4974
Registered: May-04



I do not understand.... what? (puzzled smiley).
 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3617
Registered: Dec-03
No I do not see what you mean, Jan; what are everywhere.....?!

diablo,

There are some recent posts that seem relevant to your dilemma on Leaving Quad for Krell/Levinson?.
 

Silver Member
Username: Diablo

Fylde Coast, England

Post Number: 204
Registered: Dec-04
John,

Dilemma? After some more looking around it started to look like trilemma, then a multilemma. :-)

I'm now thinking about taking your advice re the Quads - well partly, anyway.
I've rescued the working Quad II from the attic. With a couple of minor mods, I should be able set it up with a direct connecton to a CD player to assess the sound.
I've noticed that the EF86 valves are different makes, which may not help. I think I have some spare ones, along with a couple of GEC KT66s which may sound different to the Marconis now in place. Tube rolling for beginners.
With the help of Google, I've found this useful site, where the chap is using passive input to his bi-amped Quads. It seems that whatever you want to do, someone else has already done it and posted lots of info on the internet.

If everything works out wonderfully, then I will get three more Quad IIs off Ebay at maybe £150 each. With a few valves, connectors, caps and resistors, the total should be less than £1000.:-)

Then I'll start looking at the passive pre-amp stuff again!

Regards,
diablo
 

Gold Member
Username: John_a

LondonU.K.

Post Number: 3628
Registered: Dec-03
diablo,

Thanks. What an amazing web page!

There is such a lot of common sense there, as well as hard-won knowledge and expertise. Isn't is wonderful that such people simply want to share what they have learned?

I wish you luck. I do not have such courage.

One day....!
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 5061
Registered: May-04


Some useful information on the Quads might be found at the bottom of this page:

http://www.quadesl.com/schematics.shtml


 

Bronze Member
Username: Harrypalmer

Bellevue, WA

Post Number: 12
Registered: Feb-06
Quads like in ATV's?
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