Low gain preamp options

Another one to consider is the Herron. Keith makes them with switchable gain and will work with you to get the values you want. I auditioned the 360 last year. It's a nice piece, very well thought out and engineered. He's a terrific guy.

The Backert Labs Rhumba 1.3 has 8db of gain but they can adjust it at the factory to other levels. It is also tubed with balanced input and outputs. 

https://backertlabs.com/rhumba-1-3-preamp/

Check out Bottle-Head tube preamps. Available as kits or finished units. 

@wrm57 I missed that difference between the LTA MZ3 and the pre. Before the level 2 upgrade option I thought they were pretty much the same except for the tape loop and single chassis. The MZ3 (with 12AU7s to drop the gain a smidge) is one that I’ve been giving serious consideration. I need to take another look at the pre with the input specific gain control. I’m probably overthinking the gain issue, but I was short on things to obsess over this week.

@jaytor thanks for the link. I hadn’t heard of Hattor before, but it definitely warrants some serious investigation. A passive now with a tube buffer later if needed might keep my options open. I need to do additional critical listening comparing the passive to the buffer on the Saga+ to get a more specific idea what I like better about the tube.

@georgehifi it’s rare that I’m out pessimisted. Thank you. 

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

Check out the Hattor preamps. This is from the same folks that make the Khozmo attenuators which are highly regarded. I'm using a Khozmo attenuator in my DIY preamp and an thrilled with the sound quality and reliability.

They have a model with a tube output buffer. The standard gain is 9db, but they may be able to configure a lower gain if desired. They are very easy to work with. 

http://www.hattor.com/tubeactivestage.html

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

The LTA MicroZOTL preamp appears to check all your boxes. Looks like you can drop the gain by 10db, from 10-12db down to 0-2db.  No balanced output, though.

https://www.lineartubeaudio.com/products/microzotl-preamplifier

@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,
not sure where you're pulling 2k from. I have one amp now with 22k input impedance, and there are a some others that I'm interested in at 15k and 10k. I just don't believe in limiting my future options without reason. I also have the bad habit of leaving more than one amp connected to my pre even though I only have one amp powered at a time. Makes for an easier change, but it can present more of a challenge to a preamp. When I finally get around to experimenting with subs (that typically have class D amps) being able to handle multiple outputs simultaneously with a combined impedance under 10k might become more important. My arbitrary maximum of 500ohms would allow 10:1 ratio with two 10k loads in parallel. 

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

@cat_doorman,
This is the preamp that includes all the desired features that you list in your post. btw, I have speakers that have efficiency >2.6%. So i understand the need for controlling gain i.e. Voltage in audio components. It is very important for sound quality. With this preamp, output voltage is adjustable from 0~35 volts. Hope this helps.


https://www.decware.com/newsite/ZTPRE.html

look at the rogue audio RH 5 linestage headphone amp.  
front panel adjustable gain toggles of 3, 12 and 16db.  
sounds transparent and very dynamic.  love the sound.
https://www.stereophile.com/content/rogue-audio-rh-5-preamplifier-headphone-amplifier

Thanks for all the suggestions. I actually have looked into a lot of them. (I did make an effort to search before posting.) I’ll probably need to resign myself to some sort of compromise. Maybe 10-12dB isn’t too much. It seems like some of my desired features are mutually exclusive.

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. I also like whatever tone a tube may impart and am interested in maybe getting a little more of that than the hybrid buffer of the Saga+ provides. I think a tube stage must provide more tone with gain though. This leaves adjustable gain preamps that seem to actually adjust the input attenuation rather than adjust the gain. (Still not sure on this.)

Maybe I’ll need to do more research on custom, DIY, and brush up on my electronics. If the actual gain of the tube circuit can be adjusted in conjunction with and opposite to input attenuation then there can be a neutral effect on total gain while listening for what combination gives the best tone or presence (or whatever magic it is that I like about tubes). Once that is adjusted a separate volume control and source selector could be added. I can’t be the first person to want something like this. Maybe it just isn’t worth the trouble.

Anyone recommend a good book on audio tube circuit design? The few EE classes I took years ago only covered the basics in solid state.

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

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.. 

Not tubed, but it would be a mistake to not consider the Benchmark LA4, with user variable input sensitivity. Meets all your other requirements, and the price is right. 

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

I just went through this with my system.  I was running direct from DAC to amp but the digital volume was unpredictable and appears to "bit strip" in some cases.  It does not sound right unless you turn it up.  I also can get quite loud volume without the preamp.  So this leads me to believe a passive will work.  My amp has 50 kohms input resistance.

I talked to Roy Mottram at tubes4hifi.  We are going to try a buffered passive preamp.  It will be one of his stock preamps without the gain tubes.  I can run volume only or tube buffer and if that doesn't work right we can add a small amount of gain.  He also sells a Chinese preamp that has 6 dB of gain for $500.  The one I am getting is a custom SP12.  I will be receiving it next week. 

Regardless  there are several vendors mentioned that can do this.  PS  Decware is 6 months out, last I knew.   I am getting mine in about a month. 

Van Alstine's tube preamp might be close to what you're seeking with a "low gain" toggle�. One review gives the standard gain as 20dB, but they don't say what it is with the low gain in use. Output impedance is either 560 or 600, so a bit above your desired specs.

https://avahifi.com/collections/preamplifiers/products/fet-valve-cf-slr-preamplifier

How about something like this:

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

Steve might be able to do it with two outputs.

@verdantaudio it’s premature to start quoting custom equipment. I’m still trying to see what standard units are available.

@tablejockey the recent thread on the EVO400 volume control did give me some concern and prompted this thread. Previously I figured I’d be okay with anything under 12dB of gain, but I’m starting to have doubts, especially if I go with even more efficient speakers (>100dB).

This  polarizing brand in threads, appeared to have checked of all of your requirements, but I see it does have 10db gain. So if 4dbs extra  doesn't cut it-nevermind(as Emily Latella would say)
https://www.primaluna-usa.com/primaluna-evo-400r-tube-preamplifier

all tube(including rectifier-dual mono)
even lower output impedance
balanced in/out(unknown if it's wired as "real balanced or faux, as some) 
2 sets of outputs

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

So...I certain that I can have the gain customized on my Art Audio preamps.  The Conductor and Conductor Simply have adjustable gain.  The Conductor I had switched between 0dB and 25dB, the latter driven by gain for the headphone amp.  

Basically, depending on what you need explicitly, Art Audio can probably accommodate with one exception, the chassis is not large enough to support XLRs.

If you tell me what you want explicitly, I can get a quote and timing from Tom.  

A pre-amp is very important for many reasons. As example for driving multiple amps such as bi-amping or mono blocks. Most of the problems come from high power amps with very efficient loudspeakers but you can always pad down the pre-amp's output but it's very hard to add gain you don't have.  

A passive might work with the amps I have now, but I've been looking at some others that have input impedances in the 10k range. When running at full volume that may be fine but at lower volume I think it might cause issues. If I remember right poor impedance matching can get you some roll off on the low end which is where you would theoretically want to boost it to compensate for perceived roll off at lower volumes. (Fletcher-Munson) 
The other thing is I really like whatever the tube in the Saga+ is doing. I think I'd miss that. When I first got it I did a lot of comparing to my MF A300 integrated preout even going so far as to use it as a pre into the A300. I'll need to start paying attention to how much I actually use the volume control. Maybe I'd like even more tube sound even if it means having to put up with only a very small portion of usable adjustment. I really hope the eventual solution isn't to spend $10k+ on a vinyl rig and record collection.

I added a passive pre with a remote that was almost perfect for a guy. He would NOT give up his tricked out Fisher 500 pre. He couldn't get up to change the volume either.  The guy had it made for 4-500 dollars.. Silver traces and good pots.. I thought it was Dodd, wonderful work.. 20-25 years ago.. It lost nothing in SQ that I could hear.. La Scala will show noisy or bad equipment.. 100+ sensitive.. OH YEA..

Regards

Aren’t variable or adjustable gain preamps just adding in another attenuator on the input? So yes, you do get more useful range on your volume control, but you’ve also knocked down your signal so you can then amplify it again. Maybe that’s the only way to get the circuit to do anything useful so your output impedance is regulated. There might be a minimum required gain to keep the circuit linear. That might be all a buffer does for all I know - attenuate the signal and then amplify it by the same amount for no net gain. Wasn’t planning on doing enough research on circuits for a DIY project. I was really hoping I just hadn’t found what I think should exist or someone could tell me a good reason why it doesn’t exist.

I should state that I agree with the gain issue given digital component output voltages. Two volts seems to be the minimum. Too many amps with far too low a sensitivity rating. I have a Line Magnetic integrated which is 0.2V sensitive. Even in triode mode at 15wpc driving Tannoy Sterlings (91dB, 8ohms) can't get past 9 o'clock in my 12'x15' room. 

Talk to Aric of Aric Audio. He makes 3 tube  preamps at different price levels having variable output gain, low output impedance, and 2 sets of outputs. Aric has a nice website.