2. amps-preamps
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Low gain preamp options

I’ve read lots of preamp suggestion threads and discussions on amp matching. I don’t know what percentage of people have digital only systems, but it seems like there is a missing niche for preamps.

Most DACs have at least a 2V output which exceeds the input sensitivity of most amps. So unlike a phono input there isn’t a real need for gain in most cases. The problem with a passive preamp is some amps don’t have high enough input impedance. Also a preamp allows longer cable runs (if needed). Then there’s the whole tube warmth or tone that some people are so fond of in taming perceived digital harshness.

I’ve read about lots of tube preamps, but even the low gain ones are still in the 10dB range. It would be nice to have a useful amount of volume adjustment instead of silent to too loud with just a wiggle.

The only unit that comes to mind that’s even close is a Schiit Saga+, but it seems like even mid-fi systems should be able to do better. Don’t get me wrong, I’m using one now. It’s great for $400, but at some point it’s going to become the weak link in my system (if it isn’t already). 

List of desired features:
<6dB gain 
<500 ohm output impedance
tubed output
at least 2 sets of outputs
balanced in/out optional

Anything out there that fits the bill?

cat_doorman

Showing 6 responses by georgehifi

georgehifi's avatar
georgehifi
10,312 posts
 
I’m using a Khozmo attenuator in my DIY

Good but I found Dact also switched smd resistors, is the best value for money without being charged stupid Hiend hifi money, who needs "roller bearings" on the shaft lets get serious really.
http://www.dact.com/html/attenuators.html
georgehifi's avatar
georgehifi
10,312 posts
 
@georgehifi,
not sure where you're pulling 2k from.



Just citing the worst of which Class-D's input impedance can be.
Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 
Because I want the option of using some low input impedance amplifiers, I don’t think a straight passive will be a good fit for me.
Only valid reason in this case for not using one, but why anyone would want an amp with 2k input impedance is wrong also, as you can’t even use tube preamps into them either! And even solid state preamps that have output coupling caps too small for that 2k impedance, as the bass rolls off if it is.

Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 

You usually want to keep the volume toward the high end of its scale. you want to amplify as much as the input signal as possible, not a small portion of it.. then you’re just amplifying mostly noise..



Correct, you want to use as much as possible of the sources output, not dispose most of it to ground with the volume control, only after which to amplify it yet again (noise and all) by the active preamps output gain stage, which btw also adds even more colouration/distortions. That's why passives are the answer, in most cases.

Nelson Pass,
"We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.
Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.
Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.
What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.
And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp."

Cheers George

georgehifi's avatar
georgehifi
10,312 posts
 
I also can get quite loud volume without the preamp.
If it's loud enough you don't need more gain.

I talked to Roy Mottram at tubes4hifi. We are going to try a buffered passive preamp.
No need, you have 50kohm input impedance on your amp, a $49 Schiit Sys will do the job perfectly.
And without any distortions/colorations the buffered one will introduce (and it’s probably just an opamamp buffer)

Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 
Most DACs have at least a 2V output which exceeds the input sensitivity of most amps.
So unlike a phono input there isn’t a real need for gain in most cases.
There’s many phono stages now that have 60db of gain, so no extra gain needed with them either.

The problem with a passive preamp is some amps don’t have high enough input impedance.
This is true, especially with Class-D, the industry standard was 47k or 50k input impedance for amps, but some Class-D’s are a silly 2kohm!! even 1kohm!!! input impedance, not good with passives and most tube preamps!!!.
You should have 1:10 ratio or more, output to input impedance, to be matched.
EG: If you have a 10k passive it will be at worst position 2.7k output impedance, the lowest amp input impedance "should" be 27kohm or higher, no damage will occur if it’s not, say 1:5 ratio or even 1:1 just cuts the volume down lower and becomes even more of a voltage divider.

Cheers George
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