passive preamp:how do they work what are the requirements

hum4god · « **on:** 24 Feb 2005, 08:14 pm »

hi
i am looking for a pre amp for my aloia power amp and was wondering if a passive preamp might be an option.the amp has an input impedance of 60 k ohms.
i have heard of requirements like 100 k ohms and high gain sources etc.
i would use the pre to connect two cd players one with a tubed output stage and a tv.
any info how the passive pre works and compares with active units is much appreciated.
specific model recommndations are also highly welcomed.
thanks

rbrb · « **Reply #1 on:** 4 Mar 2005, 05:38 pm »

Here are reviews of a Creek and an Antique Sound Lab in the Canadian mag UHF.

http://www.uhfmag.com/Issue62/Issue62.html

tschanrm · « **Reply #2 on:** 4 Apr 2005, 01:47 am »

Read this too, it gives a good overview as to why preamps were created. Sorry, can't give any recommendations to any passive preamps, the only ones I've heard I made myself.

http://www.stereophile.com/reference/917/index.html

muralman1 · « **Reply #3 on:** 4 Apr 2005, 01:55 am »

Active preamps are necessary for full appreciation of ICE amps, if not all related amps.

nirmal · « **Reply #4 on:** 29 Aug 2005, 05:04 pm »

Hi,
Try this website http://www.mfaudio.co.uk/index.htm Seems to have a few favourable reviews. A bit pricey though.

Cheers
Nirmal

flintstone · « **Reply #5 on:** 29 Aug 2005, 07:58 pm »

Your amps input impedance is fine...the rest depends on your cd players output (most are 2V but some higher...some lower). Your amps sensitivity and your speakers sensitivity also come into play.

If you cdp is 2v output and your amp will be driven to full power by that 2v or less...you should be good to go.

I have used the Bent Audio passive before with great results, Placette Audio passive is another good one I've tried.

For less money:

http://www.ftaudio.com/

Dave

avahifi · « **Reply #6 on:** 29 Aug 2005, 10:41 pm »

A passive preamplifier actually is not a preamplifier at all, inasmuch as it does not amplify. Well - there are some that use some kind of step up transformers, but that is another can of worms.

Essentially a passive preamp is a volume control in a box with some switching functions.

The advantage musically is that it eliminates all active preamp circuit functions, and if the active preamp circuits are not of very high quailty they deserve to be eliminated.

The disadvantage muscially is that is eliminates all active preamp circuit functions. Those functions should be designed to provide a easy pure resistive load for the sources to drive, eliminate out of band garbage, drive the sh--- out of all downstream loads no matter how goofy they are, and not screw up in and of themselves. Lots of designers drop the ball here.

A passive preamp essentially connects your sources directly to the power amp, thru the likely significant amount of distributed capacitance of the cables between the source and the passive preamp, its internal load, the distributed capacitance of the cables between the passive preamp and the power amp, and the internal distributed capacitance of the amplifier's input circuits. Whew!!! If you wonder what a capacitive load does to the music signal, see all the test results done over the years showing this, they cause a leading edge spike and ringing on the signal, the bigger the load the worse the results. In addition now that load makes demands on the drive current capability of the source that it likely does not have.

So with a passive preamp you have the distortion of the additional capacitive load on the source, and from taxing its drive current capability. With an active preamp, you have the distortion of the preamp's active circuits themselves.

Which is worse? It depends on the quality of the preamp active circuits, the load driving capacity of the source, and the amount of distributed capacitance of the cables. Your results may vary.

In general we would suggest short, low capacitance well shielded interconnect cables, sources with excellent current drive capability (a spec not usually talked about) an amplifier with a pure resistive input impedance, and a very very low distortion active preamp line stage providing a resistive input load and high output drive current.

Hope this has not confused you too much.

Frank Van Alstine

JLM · « **Reply #7 on:** 29 Aug 2005, 11:09 pm »

Always good to see your input here Frank.

chadh · « **Reply #8 on:** 30 Aug 2005, 02:20 am »

Integrated amplifiers like the 47 Labs Gaincard, or the Audio Zone Amp 1, or the Portal Panache (and I'm sure many others) are essentially passive volume controls followed by a power amp. Given that these designs (a) necessarily get a set of interconnects out of the way; and (b) allow the amplifier designers to ensure that the input impedence of the power amplifier is sufficiently high for their particular passive volume control, does this sort of design necessarily look attractive then?

It always seemed weird to me that anyone would make an integrated amplifier in which you have one gain stage (the pre-amp) then attenuate that gain, and then amplify again. If these capacitance and impedence issues remain in an integrated unit then it obviously starts to make sense - but by the sounds of it, a well designed unit involving a passive volume control followed by an amplifier with appropriately high impedence would avoid at least some of concerns of using a passive attenuator.

Of course, things can't be that simple or else everyone would be doing it...

Chad

avahifi · « **Reply #9 on:** 30 Aug 2005, 09:51 pm »

Unfortunately, all things are not equal. In order to have adequate overall gain, usually it is necessary to build the integrated amplifier with an audio amplifier gain higher than normal to make up for lack of gain in the missing preamp section. This is usually done by making changes in the feedback loop (less feedback equals higher closed loop gain). Unfortunately this changes the operating parameters of the amplifier (distortion, bandwidth, stability, etc.) so that just increasing the gain means all things are not equal.

Again, which provides the most desireable results, a higher gain (and typically higher distortion) amplifier section, or adding a line preamp section ahead of a tried and true amplifier section? Again, the results can vary.

This is such fun.

Frank Van Alstine

WEEZ · « **Reply #10 on:** 30 Aug 2005, 11:24 pm »

Hi Frank,

..so are you saying that the minimum amount of gain is best?- what would be optimum?

Most amps have around 26db of gain. Some pre-amps have as much as 20db of gain- such as yours. A typical integrated has maybe 32db of gain.

What's the best balance?

(this is a sincere question- not trying to be a wise guy

)

WEEZ

Steve · « **Reply #11 on:** 19 Oct 2005, 09:20 pm »

Hi Hum,

Saw your post and thought I would provide a different view.

Any system requires a minimum of gain, after the source to reach full output power. Some use separates, as in a preamp with active gain and basic amp, and others use a passive preamp, buffer, or transformer with all the voltage gain in the amp (integrated amp).

So using a passive doesn't actually rid a stage with gain (unless one has both an integrated amp and gainstage preamp) because the gainstage is already incorporated in the integrated amp.

The integrated was developed in order to save money and rid an interconnect. Unfortunately, another problem arose, that of passive volume control and interconnect capacitance interactions.

Using a buffer stage or transformer should help with the IC capacitance problem, but then one still has a gainless stage that will lower the sonic quality. One has to decide which might be the least problematic.

One other plus is that the external gainstage preamp has the advantage of eliminating the need for the (preamp) gainstage in the amp Plus taking the place of a buffer stage, all at the same time, thus saving a stage.

In an integrated, one doesn't have much choice but to have an additional stage, whether transformer or buffer stage. I guess one could mount a selector switch and volume control in the amp, and then one could bypass the buffer stage or tranny, and interconnect cable. But then there are problems in that mode as well.

I suggest shooting for a minimum number of high quality stages. (I count a stage as including an active device(s) or even a transformer.)

Hope this helps Hum.