Passive pre-amp

[OO1]

Anyone use one.  I can't see much of a downside as a line stage unless your sources don't put out enough voltage.  Just looking for thoughts.

 

Thanks, Carl

a bit of reading info - one page -http://www.rothwellaudioproducts.co.uk/html/the_truth_about_passive_pre-am.html

[CECAA850]

 

Anyone use one.  I can't see much of a downside as a line stage unless your sources don't put out enough voltage.  Just looking for thoughts.

 

Thanks, Carl

a bit of reading info - one page -http://www.rothwellaudioproducts.co.uk/html/the_truth_about_passive_pre-am.html

 

 

Informative read.  Thanks.

[CECAA850]

I have had extremly good results with entry level DVD players which can be purchased off the floor for $15 -20 bucks or new for $25.00 - $40.00. Every couple of years they throw a little new technology at the DVD to keep it fresh and I simply buy a new one. I do modify mine and no I am not sharing what I do. Stock DVD players from Sony and Toshiba are now 24 bit 196KHz sample rate and make very good sounding CD players for next to nothing. Buy one at Walmart if you do not like it take it back. Cost you nothing to try but your time. I am running a Toshiba right now because it has track ID on the player but I actually prefer the new Sony and it has a better/nicer remote and is the same price. Best regards Moray James.

 

Thankd Moray.   I don't think I'm going to have a disc player on this particular system.

[Shakeydeal]

If I need to get an $1100 DAC to work with a passive pre, I might as well get a conventional pre and a moderately priced DAC.  I do appreciate the links and info. 

 

Carl

You could do that, but it won't sound nearly as good. You would have to spend in excess of 5K to get a an active pre that will equal a good passive, all things being equal. So where is your savings?

Shakey

[paul79]

I can't stress the importance of the USB input enough for what you are trying to do though man. The DAC I linked to with a good passive, will get you some world class results for not allot of dough all things considered.

[erik2A3]

What Mark said.

The source ouput impedance (DAC, CDP, etc), the value of the potentiometer used in the passive, and the input impedance/sensitivity of the power amp are important considerations. As a rough guideline, the usual standard is a 1/10 ratio between source output impedance and the load it sees. For example, if the output impedance of a linestage preamp is 1k ohms (which is on the high side), the input impedance of the amplifier should ideally not be lower than 10k ohms. In practice, these two figures tend to be quite a bit different, particulalrly in the case of vacuum tube amplifiers, which generally have both quite high input impedances and high sensitivity -- which for passive preamps are good and desirable characteristics.

I have made many 'passive' in-line attenuators over the past couple of decades. Unless we are talking about TVCs, as mentioned above, a passive preamp is nothing more than a vołume potentiometer between the source and amplification. Even though such a passive device does NOT amplify (and in fact does the complete opposite), I use the term 'preamp' simply to identify its position in the signal chain -- as in the fact that it (the passive device), as far as the audio signal is concerned, comes before and is in front of the amplifier. Thus, PRE - amp.

Audio always involves choices between compromises, and this can also be the case with passive in-line attenuators. If we have an amplifier with an input impedance of say 100k ohms and an input sensitivity of 1 volt (or in many cases even less), a passive preamp becomes (subjectively) a very possible option. But what happens if one simply plunks in a vołume control between the source and this amplifier? The inter-stage impedance relationship becomes greatly changed. The load seen by the signal is now NOT that of the amplifier (which at 100k -- or even 1meg ohm in some cases) but rather the value of the potentiometer. Of course one could choose a VC with a resistance value of 100k (which is the same as that of the amp), and that would be nice for the source signal, but it's what happens on the output side of the vołume control that things become less desirable. As someone also mentioned above, a conventional pot will alter the impedance relationship considerably depending on the position of the wiper on the volume control. So, one must necessarily find the best compromise between the value of the volume control and the impedances of the source and amp. One vintage power amp I recently completely took apart and rebuilt has an input impedance of 1meg ohm, and the potentiometer I installed on it works fine because the ratio between it (the VC) and the 1 meg ohm grid leak resistor (which is what establishes the input impedance of the amplifier) is suitable.

So, to put all this in light of Justin W's amp, which I'm just guessing may have an input impedance of approx 100k or so, a more suitable value for a passive in-line potentiometer would be in the range of 10k or so. But this bring about another possible compromise in that the impedance seen by the source is now lower, too, which is less desirable. BUT, it's again what happens on the output going to the amplifier that is important: the 10k pot will have a more appropriate impedance relationship with the amp than did the higher value 100k potentiometer. But because a passive does not possess gain (again, a TVC is an exception to this in that they, depending on how they are configured, CAN in fact step-up the strength of the signal between the primary and secondary winding), one is then confronted with the effects of cable length and the associated increases of capacitance and resistance. Higher output impedances (as in the case of high value conventional potentiometers) have difficulty driving capacitive loads (ie long interconnects), and these issues present themselves most commonly in the form of high frequency losses. However, there is something of a remedy for this: a very small value capacitor (as in picofarads) can be used as a high pass filter on the VC, and this will provide a small amount of EQ at lower vołume rotations -- allowing for less attenuation of high frequency information. On music instrument amps this is sometimes referred to as a 'bright switch' and it can be switched in and out of the circuit as desired.

One solution for Justin's amp, which I have done more times than I can remember, would be to replace the resistor on the grid to ground of the first stage with a vołume control of the same value. In other words, the volume control would not only provide the necessary working impedance for the amp, but would also provide adjustable gain. Channel to channel tracking is not always ideal on those less expensive carbon strip pots, though, and one way get around that problem is to either use dual mono pots for individual channel adjustment and balance, or buy a better contro such as a blue Alps, which are not only very quiet, but generally have much more even channel to channel tracking between the two mono sections of the potentiometer. Goldpoint also sells very nice stepped attenuators using surface mount resistive networks. Because these consist of precision switches, the cost is far greater than the usual kind of control. Goldpoint also sells nicely built passive preamps with choices of stereo, dual mono, source selection switches, etc.

In-line attenuators are incredibly easy to make if you know how to solder. Good luck!

[John Chi-town]

I use the Adcom GFP 750 preamp.  Has both passive and active stage.  Developed by the great Nelson Pass.  Very highly aclaimed pre amp.  Used prices are between $850-$1,000 depending on condition and whether or not owner still has a functioning remote.  See links below for more details. 

 

 

http://www.stereophile.com/solidpreamps/133/index.html

 

http://www.hometheaterhifi.com/volume_7_4/adcom-gfp-750-preamplifier-11-2000.html

 

Best regards,

John

[A1UC]

If I need to get an $1100 DAC to work with a passive pre, I might as well get a conventional pre and a moderately priced DAC.  I do appreciate the links and info. 

 

Carl

http://app.audiogon.com/listings/da-converters-eastern-electric-minimax-2015-02-14-digital-33411-royal-palm-beach-fl

 

I had the plus version and really liked it , this would be a awesome Dac for the money 

[erik2A3]

A sort ironic thing about preamps: more often than not, they are used not to boost a signal but to attenuate it; to reduce its strength. Sounds odd, I know, but here's what I mean: take an average CD player with a fixed output and connect it directly to an amplifier with both very high input impedance and voltage sensitivity (many tube amps, particulalry vintage examples, fit this description) For example, I recently rebuilt a LEAK ST20, which has a notoriously high input sensitivty. What would happen If I were to connect a CDP to this amplifier without first going through a device (either active or passive) with an attenuator of some kind? I would instantly blow out the windows of my house and shatter my ear drums......

So, we use a preamp which, even though it may be capable, as the case may be, of 10,15,20,30 dBs of gain, would be used to attenuate and moderate the CDP's full-strength signal rather than boost it. Of course that's not always the case, in that there are many amplifiers that not only would benefit from an active linestage, but in fact where one is mandatory for optimal performance. A primary benefit of a preamp has really less to do with an increase in gain, but rather impedance buffering between the preamp ouput and the amplifier -- in essence making the two more electrically compatible with one another. One can even make or buy unity gain buffering stages for the same purpose -- to provide suitable inter-component impedance matching.

[erik2A3]

Here is an example of an impedance buffering I mentioned above (I am providing the link for illustration purposes, only and as an example)

http://www.decware.com/newsite/zbox.htm

[bobdog]

I use the Adcom GFP 750 preamp.  Has both passive and active stage.  Developed by the great Nelson Pass.  Very highly aclaimed pre amp.  Used prices are between $850-$1,000 depending on condition and whether or not owner still has a functioning remote.  See links below for more details. 

 

 

http://www.stereophile.com/solidpreamps/133/index.html

 

http://www.hometheaterhifi.com/volume_7_4/adcom-gfp-750-preamplifier-11-2000.html

 

Best regards,

John

This is what I use as well.  It sounds great in either mode and has a theater bypass mode as well.  The passive section is more than just an attenuator it corrects the output impedance issue.  A truly great pre that competes with units many times it's price.  No phono section is the only minus.

[John Chi-town]

 

I use the Adcom GFP 750 preamp.  Has both passive and active stage.  Developed by the great Nelson Pass.  Very highly aclaimed pre amp.  Used prices are between $850-$1,000 depending on condition and whether or not owner still has a functioning remote.  See links below for more details. 

 

 

http://www.stereophile.com/solidpreamps/133/index.html

 

http://www.hometheaterhifi.com/volume_7_4/adcom-gfp-750-preamplifier-11-2000.html

 

Best regards,

John

This is what I use as well.  It sounds great in either mode and has a theater bypass mode as well.  The passive section is more than just an attenuator it corrects the output impedance issue.  A truly great pre that competes with units many times it's price.  No phono section is the only minus.

 

 

Bobdog;

 

I agree both passive and active are very sweet sounding.  I find myself favoring the passive mode in my setup about 80% of the time, which is determined by source material, etc.  I find the bass response ever so slightly less "muddy" in the passive mode.  Very slight, I am sure that cable choice could make a bigger difference. 

 

I remember a quote by Nelson stating that the passive mode was nothing more than a "wired pair".  Nothing more, nothing less.  Essentially the same pre-amp as the Pass Labs Alpha 1.

 

Best regards,

John

 

Best regards,

John

[CECAA850]

A lot of very good info here, thanks.

[Bubo]

There are two different devices being discussed here.

 

Preamps - This is an active device with gain, impedance matching, and attenuation.

 

Attenuators - This is a passive device (no gain), which just attenuates an existing signal before passing it on. LDR, and the infamous misnomer "passive preamp" which is usually just a pot or stepped attenuator.

 

What matters?

1. The output voltage and impedance of the source device

2. The input sensitivity and impedance of the power amp

 

You must have enough signal to drive the the power amp to full output

You must have a way to attenuate the signal from full to zero output

You must have a proper impedance match between the source and the amp so as not to cause increased distortion, loss of signal

You must be sure the output impedance is low enough to drive the amount of capacitance in the cable you use to connect the two devices without loss of high frequency

 

Example:

Assume your power amp needs 2V for full output and has 100k ohms input impedance.

 

If your source can output 2V and 5k ohms or less, and has a volume control, you may be done. If no volume control, add a passive of some sort and you may be done.

 

If your source is less than 2V you will need to amplify it. You will need some gain through an active device - a preamp. A typical linestage preamp has 10dB to 20dB gain. The output impedance varies by brand and model and could be as low as 90 ohms (e.g. JMA Peach), or as high as 100k ohms for antique preamps with no buffers.

 

Get the specs from the source and the power amp, and the numbers can tell you what you will need.

 

 

Totally agree, you need enough signal to have something to attenuate.

 

You can do a search for "head amps", some 12vdc battery driven for a turntable, CD players and perhaps DACs are hotter signals.

 

When you have all of your numbers, post them and someone smarter than me can tell you if the math works.....

 

http://www.audiokarma.org/forums/showthread.php?t=639796&highlight=head+amp

[allegro]

On 2/20/2015 at 3:09 PM, Shakeydeal said:

I got one from Abacus Music. It uses the Tortuga Audio LDR board. Here is one for sale that is different than mine, but uses the same board from Tortuga.

 

 

 

http://www.audioasylumtrader.com/ca/listing/Preamplifier-SS/Tortuga/Preamp/127333

 

The distortion of the Tortuga LDR3 has been measured with THD about 1% at normal listening levels and IMD around 4%. Very high for a modern preamplier. I had a Tortuga LDR3 and Schiit Saga in my home system at the same time. The output impedance of the Saga using the active tube buffer is 180 ohms which works quite well with the 10K ohm input impedance of my tube amp. The Saga was more transparent than the Tortuga in my system. I found the sound of the Tortuga somewhat colored and fatiguing to listen to. LDRs are known to have higher than normal distortion but have achieved somewhat of a following among some audiophiles who like novel gear.

[TubeHiFiNut]

+1 for the Saga using the 6SN7 buffer. 

 

Very transparent. Easy to integrate into your system due to the low output impedance of the tube buffer. Plus you get a remote. 

 

At $350, the Saga has to be one of the best audio buys out there.

 

Just my opinion.

[Cornman]

On 2/21/2015 at 5:05 PM, moray james said:

the reason for getting a quality (more expensive) DAC would be to extract more info from your disks. No preamp can make more information come out of your DAC. A better volume control system, one which does less to the information given to it by the source will let you hear more clearly what was extracted from your disk. The idea of a passive is that it does less so it has less opportunity to mess up the sound. Better DAC's mean better digital cables and you may well find that you have exceeded your available budget. I think that most folks simply want to do the most with what they have (available budget). I have had extremly good results with entry level DVD players which can be purchased off the floor for $15 -20 bucks or new for $25.00 - $40.00. Every couple of years they throw a little new technology at the DVD to keep it fresh and I simply buy a new one. I do modify mine and no I am not sharing what I do. Stock DVD players from Sony and Toshiba are now 24 bit 196KHz sample rate and make very good sounding CD players for next to nothing. Buy one at Walmart if you do not like it take it back. Cost you nothing to try but your time. I am running a Toshiba right now because it has track ID on the player but I actually prefer the new Sony and it has a better/nicer remote and is the same price. Best regards Moray James.

Moray do those dvd players have volume control on the remote , and enough output voltage to go straight to your amp ? If so it would save me a lot of money on the purchase of a new Oppo .My 980H is getting long in tooth . Is there some other advantage to the Oppo's ? Have loved my Oppo but big buck compared to the Sony .

[glens]

It appears as though if you want a new Oppo player you'd better hurry.  Their website no longer shows them and says "Farewell."

[PrestonTom]

With all due respect, most of you folks are spending (or advocating) far too much money on this. 

If you do not need a phono section and you are not using a balanced input on the amp,  then 1) tell me the output voltage of the source (keep it simple and make it Volts RMS) and 2) tell me the input impedance (input Z) of the load (amplifier). Then we can figure out an attenuator for you that will work fine. Otherwise, construct a DIY version of the B1 buffer (see details over at DIYaudio / Pass sub-forum). You guys are making it way too complicated and expensive. All this assumes you are using high-efficiency speakers.

-Tom

[joessportster]

On 2/20/2015 at 9:57 AM, CECAA850 said:

Anyone use one.  I can't see much of a downside as a line stage unless your sources don't put out enough voltage.  Just looking for thoughts.

 

Thanks, Carl

Heres another highly received Passive that uses a light diode to adjust volume

 

http://www.thehornshoppe.com/the_truth_pre_amp.html

 

I have been using it for 4.5 months and I have to say it is what they claim, There is a thread dedicated to the pre on Eds site..........If you want a Demo Carl that could be arranged.