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Thinking of going passive, and need advice

50.0.191.24
Posted on March 25, 2015 at 04:45:30
cdrayson
Audiophile

Posts: 33
Location: San Francisco
Joined: December 26, 2012
 I've got a homemade pre-amp based around a board designed by Stan Warren, and after upgrading the chassis and volume pot, I've got some serious buzz in the left channel. I suspect it's a problem with the now 18 year old board, so I'm thinking of just taking the board out of the circuit, and going passive.

Here's my concern: I'm big on my vinyl, and I'm concerned about whether my phono stage can drive the amp. My phono stage is a Sonic Frontiers SFP-1 Signature, which has an output impedance of 1K Ohms. My cartridge in a 2.5 millivolt Blackbird, and calculating the 37 dB gain, that puts the output at ~1.8V. My amp is an Adcom 5400, with an input impedance of 49.9K, and the volume pot is a 25K Goldpoint.

Think it'll work?

 

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RE: Thinking of going passive, and need advice, posted on March 25, 2015 at 10:27:31
Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008
 The gain for the MC input is 57 dB for an output voltage of 1.76V According to the GFA 5400 manual on the Adcom site, the input sensitivity for 125W output is 1.125V so there should be no problem. Keep the IC cables short for best results.

 

RE: Thinking of going passive, and need advice, posted on March 25, 2015 at 13:49:40
fantja
Audiophile

Posts: 15518
Location: Alabama
Joined: September 11, 2010
 Welcome! cdrayson-

I hope you find another inmate to assist you.

 

RE: Thinking of going passive, and need advice, posted on March 25, 2015 at 13:55:53
Dman
Audiophile

Posts: 7211
Location: Kansas
Joined: January 28, 2001
 Palustris is right.

Besides, IIRC (and having worked at SFI many years ago), if things dont' get up and boogie, can't you internally select the gain in the SFP-1 Sig?

Cheers, and good listening...
Dman
Analog Junkie

 

RE: Thinking of going passive, and need advice, posted on March 25, 2015 at 14:02:04
BCR
Audiophile

Posts: 2444
Location: connecticut
Joined: April 7, 2009
 I gave that a try a few days ago. I used my Quicksilver phono preamp that has a gain of 44 db with an output impedance of 3 K ohms, using a Grado cartridge with 1.5 mv output. My amps are Quicksilver with an input impedance of 100 K ohms and an input sensitivity of 1.9 volts. The outcome was I could not get much volume, so I went back to using a line stage in the loop. Hope you have better luck!

 

Keep the interconnect cable short, posted on March 25, 2015 at 14:04:24
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 -between the control and the amplifier. Otherwise the cable will introduce artifacts that were likely not there with the active preamp.

Its not likely BTW that the circuit board in the preamp has failed in such a way as to cause buzz. It is far more likely that something associated with your recent 'upgrade' is the culprit, based on your post.

 

RE: Keep the interconnect cable short, posted on March 25, 2015 at 16:09:55
cdrayson
Audiophile

Posts: 33
Location: San Francisco
Joined: December 26, 2012
 This is cdrayson - I changed my moniker to bigmoose to keep consistent with other forums.

Thanks for the replies, folks. The board has always had some hum to, which is why I suspect it. I may try isolating the grounds for the outputs, and see if that helps, too.

 

RE: Thinking of going passive, and need advice, posted on March 26, 2015 at 10:30:14
dvb
Audiophile

Posts: 2444
Location: Vancouver
Joined: August 18, 2000
 I have 2 TT set-ups, both using Passive Pres.

Short ICs and low capacitance ICs are good advice.

Personally, I'd be concerned if the gain is less than 40 dB.

One of my Phono Pres (an inexpensive ART USB) has adjustable gain, and more gain sounds better. The other is a Slee Reflex which sounds very good with its 40dB output.

 

RE: Thinking of going passive, and need advice, posted on March 26, 2015 at 10:46:07
Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008
 Yes, mathematically this won't work since posivite 44dB related to 1.5mV is 237mV and you need 1900mV to drive the amp to clipping.

 

RE: Thinking of going passive, and need advice, posted on March 26, 2015 at 11:05:31
BCR
Audiophile

Posts: 2444
Location: connecticut
Joined: April 7, 2009
 Would it work with a cartridge with a 5 mv output? Thanks!

 

RE: Thinking of going passive, and need advice, posted on March 26, 2015 at 12:08:57
Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008
 5mV input with 44dB gain will give you 790mV output, so less than half what you need. The 1900mV input sensitivity of the Quicksilver amp is about double what a typical amp requires for input voltage to drive it to clipping. I am disposed to believe that the Quicksilver amp is intended to have a gain structure that reduces the noise and distortion of a typical preamp/amp combination. Most preamp/amplifier combinations have too much gain which degrades the signal. Normally, the excess gain is reduced in the preamp's volume control so as not to overdrive the amp; but it makes no sense to have a high gain preamp and then an amp that is overly sensitive. Each of the superfluous gain stages add noise and distortion. It appears as if the Quicksilver amplifier is engineered to require an active preamp and the benefit is lower noise overall.

 

RE: Thinking of going passive, and need advice, posted on March 26, 2015 at 12:33:27
BCR
Audiophile

Posts: 2444
Location: connecticut
Joined: April 7, 2009
 Thanks for the information. How do you arrive at those figures of the 790mv? Inother words what is the calcuation you use?

 

RE: Thinking of going passive, and need advice, posted on March 26, 2015 at 12:37:00
Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008
 I use a handy online dB calculator. See below.

 

RE: Thinking of going passive, and need advice, posted on March 26, 2015 at 12:46:30
BCR
Audiophile

Posts: 2444
Location: connecticut
Joined: April 7, 2009
 Thanks again!

 

RE: Thinking of going passive, and need advice, posted on March 27, 2015 at 01:18:19
Ozzy
Audiophile

Posts: 7588
Joined: September 21, 1999
 The input sens. of the Quicksilver amps is your problem. 1.9 is pretty low. My mid monos were 1.25 and that wasn't enough for me either. I went with a CJ amp that works great and sounds better than the quickies too.

Oz



Don't worry about avoiding temptation. As you grow older, it will avoid you.
- Winston Churchill

 

RE: Thinking of going passive, and need advice, posted on March 27, 2015 at 03:44:46
BCR
Audiophile

Posts: 2444
Location: connecticut
Joined: April 7, 2009
 It's only a problem if I want to use a passive preamp .It has plenty of gain when I use an active line stage. Inmate Palurtris explained the reason for the high input sensitivity of the Quicksilvers in this post. I was just curious why a passive would not drive the amps.Thanks for the info!

 

RE: Thinking of going passive, and need advice, posted on March 27, 2015 at 13:10:36
soundsalot
Audiophile

Posts: 5
Joined: March 6, 2015
 A good passive is the Axiom assembled by luminousaudio on ebay. They can customize the impedance to your needs, making it match your system. I use an older version, no complaints, never looked back. Perhaps a smidgen less open than a Penny and Giles conductive plastic pot, but good enough for most of us mortals. Ive used it with low output grados 1.5-2.5mv, Nad PP2 set on MM, and KAB rumble filter feeding into different amps without issue. Usually any modest limitation of going passive is preferred to running the signal through additional caps n resistors of an active gain stage. Decades into the audio "hobby", I am glad we finally reached the truth in audio... transparency via quiet, simple circuits, with the passive attenuator eliminating the pesky preamp once and for all.

 

RE: Thinking of going passive, and need advice, posted on March 27, 2015 at 13:58:33
SgreenP@MSN.com
Audiophile

Posts: 3537
Joined: April 23, 2007
 Passives never worked for me. They sound anemic...no energy.

 

Try thinking of passive in a different configuration..., posted on April 1, 2015 at 00:55:46
stehno
Manufacturer

Posts: 739
Location: Oregon
Joined: November 8, 2001
 Yeah, I thought the same as you as I swore I'd never go passive because of my love for dynamics.

However, last year upon installing my 575wpc mono block amps, it wasn't long before the dynamics, particularly a note's initial attack were so in my face, that it took me further from the absolute sound rather than closer to it.

It was as though my ears were 18 inches from a note's initial attack and then seated in the front row for the remainder of the note.

I swapped out the active pre and went for a passive pre and now with the 575 wpc amps all the music is on the soundstage exactly where it belongs and my ears are maybe in the 5th row for most performances with a nice realistic distance between my ears and the music.

And believe it or not, the system is still one of the most dynamic systems I've encountered only now the dynamics are far more natural and slightly rounded with a realistic distance from my ears.

What I now realize is that with the active gain stage and the 575 wpc amps, I was experiencing an electronics induced "jump factor" that though impressive to some simply is not realistic.

Think of it this way. A .45 caliber weapon fires 5ft from your ear. At that distance, every time it fires, it will induce a jump factor. Now imagine that same .45 caliber weapon firing 50 feet away from your ear. The dynamics are still there but there is no jump factor.

Try a passive pre with some very high-powered amps and you just might hit the right combination.

Besides that benefit, there's also the added sonic benefits of 1 less power supply dealing with noisy AC and 1 less power supply inducing its own internally-generated vibrations that distort the sound.

Then there's the distortions induced by an active pre-amp's amplified gain stage that you should not encounter with a passive,

In other words, the sonic benefits of going passive with high-powered amps can be numerous and significantly more musical.

Additionally, if you go with a passive volume attenuator built into the source component, there's 1 less set of IC's and there's 1 less chassis to capture and trap air-borne and internally-generated vibrations that induces serious distortions into the signal.

And there's one less shelf required, less real estate space required, and a cleaner simpler system.

All of which translates into potential improvements in musicality.

 

RE: Thinking of going passive, and need advice, posted on April 6, 2015 at 14:03:28
cdrayson
Audiophile

Posts: 33
Location: San Francisco
Joined: December 26, 2012
 To play it safe, I bought a Rogue 99 on Audiogon. I appreciate all the advice.

 

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