I've been assuming all along that the volume sensitivity grows linearly with the analog input signal voltage (other things being equal), but it may well be an exponential function.  Meaning, anything beyond 2.0 V may increase loudness at ever increasing rates.  

If the original design is optimized for the [sub 1 V to 2 V max] target, we may be at  the steepest stage of that function after the 2 V threshold.  This may explain the inordinate sensitivity with sources even moderately higher than the 2 V mark.  If so, Radushka is in a greater predicament than I, just because of that 0.25 V difference.

This may also explain the apparent indifference of using driver tubes with different gains, so long as that source output does not go beyond the 2.0 V max.

If this reasoning is correct, I would expect those using 12aT7 and highly sensitive speakers and not experiencing such loudness sensitivity (an apparent paradox), most probably have sources strictly under, or at, the 2.0 V mark.

Anybody cares to comment?

Not if that same pot design is replicated in most M-T's around.  The paradox would not occur and everybody (not only the ones with highish DAC output voltage) would have experienced the same ultra sensitivity issues.

Don't get obsessed, just enjoy the challenge  ;)

that would mean that I replace the pots with high quality 100k resitors and use the mini t as a power amp? WHile adjusting the volume from the output of the DAC?

DO you think I could solve the issue by building my custom atts?
I am a little hypoglycemic, that is why my brain is a little slow on this issue.  I'll be in the kitchen helping my gf cook.
'helping'
hah

My brain is a little slow, period!  I had sushi for lunch today -- I love it!!

Sure, I actully toyed with the idea of using it as a straight ahead power amp.  You take out the volume controls and replace them with a fixed resistor.  This may or may not solve the problem, though....

With a preamp in the mix (I know you don't like the idea), you can sort pf balance things out between both components.

http://www.flickr.com/photos/71648043@N02/6908641606/

there is a 'resistor designator' bottom left, going to gnd, I suppose that one determines the resistance of the att when it is wide open? (maximum volume I presume)

the atts I am looking at are the series, not the ladder type, but they have a very good reputation, especially when paired with non-noisy resistors. I could make a non linear att, with the most used volume levels right after the zero position, so I can dial in directly :
1st click: day level volume,
2nd click : evening
3rd click: late night
linear from 4th click on

Fireblade, do you think an att would degrade the sound more than a preamp? A preamp has its own volume pot. I am worried now, what do you mean by sonic degradation?

Not at all.  In both cases it's going to depend on the quality of their design, materials, build, etc.  Although, what Erik is pointing out may suggest the attenuator won't really solve the sensitivity issue.  

I'm not qualified to suggest anything other than trying the attenuator should at least allow you that in-between volume target you're missing now.  Assuming there's no sonic degradation and the cost is less than an additional component, you'll be better off in purists terms and presumably able to dial-in the optimum volume level.

If, on the other hand, you want to take advantage of the situation and tap the benefits of riding the gain, there's always the second-component alternative, if you care to invest in it.

Radu: I'm not really saying it wouldn't be worth it.  Like caps and other passive parts, the end result is probably kind of subjective.  Fireblade stated this as I also see it -- that the type of attenuator may not be the main culprit.  With two basic types of controls, those being log taper and linear, the really nice attenuators approximate the log or audio taper with values of resistance that match the way we perceive incremental changes in volume, which is how the cheap pots in the torii, mini, also do.  Fwiw: I am honestly not in the least inclined to change them.  But this just me, only becausenI have learned after truly ridiculous amounts of comparing caps, resistors, transformers, volume controls, etc., thatmthe best one in a particular circuit is quite truly not necessarily the most expensive one.  I have had $60 capacitor in one hand, and a $0.25 one in the other, and after comparing both both decided to use the cheaper. But the opposite has also been the case.  My point is that despite the lower costnof some of the parts on this amp, they work very well, and for that I am happy!  I think a really good sounding preamp is what would do the trick in your case...but I might be completely off base.  I say this because I just tried it and it can work as well or better than not having one in the signal chain.

I am listening carefully Erik! I am still afraid that the Vcaps might have made it sound too forward compared to the original caps, so I can see in practice how expensive parts do not automatically improve things. At this point, even though a preamp might be the best solution, I don't want (can't rather, who doesn't want? )
to invest in a more than decent preamp. That is why I am focusing on the volume pots, not as much to solve the gain issue as to make sure such an important part as the volume pot gets a fair treatment.
Yeah, I should call Steve. I am just a bit intimidated...Plus I installed Vcaps in the man's amp. I am ashamed

I wasn't aware you had VCaps (Teflon or Copper-Teflon?).  This may be the 'X' variable that may explain the odd behavior and the apparent paradox.  Which means the M-T design is robust, after all, in handling moderate deviations in DAC output voltages beyond the 2.0 V mark (with the original Caps in place).  So, Erik was right in discarding a possible volume pot issue.

You may still need that stepped attenuator now to compensate for the added VCap effect, and only Steve may give you the specs for it.  It's odd, but I thought Steve mentioned once that VCaps would not fit in the Mini-Torii (there are 4 Caps total), given their size.  He stated VCaps were not an option for the Mini Torii.

Anyway, good luck!

Here is a schematic of how to add a resistor in series with the signal input before the att, which is what Erik suggested in the first place and which I now have come to consider the easiest way out, in case I still have issues with it after I install the att.



add usable steps to attenuator par raduschkamuschka, sur Flickr


resistor values par raduschkamuschka, sur Flickr

It is not the V-caps. The info on the Goldpoint page is correct. High gain amp, hgh ivoltage source and efficient speakers with no xover
equals hair trigger volume control.

However, before I did anything I would wait for Steve's reply. I think he will have a solution.

Floridaboy

What I mentioned about this are the most usual ways of dealing with excessive gain....which I have dealt with over twenty years and from designing and building some of my own amps.  But doubtless you are right that Decware knows more about there products than anyone!  We are  just sharing ideas, which in my view makes for beneficial discussion.

He could also even use a variable L pad on his driver, selected to match the impedance of the same... But yuk, it would be better to take care of it at the line level, if by no other way than by putting a preamp between DAC and amp.  
 

Erik, I understand the sharing of ideas and I don't have any audio ego.
I hope the solution is simple. Now I am curious about how the high gain is achieved in the amp.

here is what i got from Eli Duttman od DIYaudio (I will need translation :

Applying the 1:10 rule of thumb to the DAC's O/P impedance of 1.3 KOhms tells us that 15 KOhms or a bit less will be adequate.  You really need to employ a padding resistor in series with each control to tame the "hair trigger" issue.  My seat of the pants guess is that a 4.7 KOhm metal film part in series with a 10 KOhm log. taper control will do the job.  You can always increase the padding value, if control is still "touchy".

No way do I spend big money on those Goldpoint attenuators.  Hot molded carbon controls made by PEC of Canada cost lots less and certainly have a "warmer" sonic signature.  PEC controls are milspec tough and also reasonably priced.  A pair of DigiKey stock # KA1031S28-ND is what I suggest you use.

BTW, hold the interconnect cable capacitance between DAC and amp down.

Floridaboy: Noted, and I understand where you are coming from.

Raduscha: to explain the high frequency issue related to lower value potentiometers would take a long time.  I think, however, that you got sound (truly not an intended pun!) advice from Eli and elswhere.  The in-line attenuator I mentioned works very much the same.  And as I mentioned too, and of course just my opinion, that the output impedance of your DAC is just a wee bit high, but not at all unreasonably so.  Short ICs can be important in such cases, particulalrly in instances of higher output impedances in conjunction with passive volume controls, regardless of how much the cost or stepped or not.  This is because higher output Z has difficulty driving the capacitance associated with long lengths if cable.  So keeping them as ASAP (as short as possible  ::) is better than not.

The reason that your Vcaps might very mildly increased gain is because that type of cap, vs say an electrolytic (which, believe is or not, can be found in interstage coulping positions) is that they have rather low ESR (Equivalent Series Resistance).  Remeber awhile back or in an email I mentioned that even capacitors have elements of inductance and resistance? This is to what I was referring.

Here is something you can try: get a small box of some kind to use as a chassis.  Cheap plastic would be fine.  Drill four holes that are correctly sized for cheap RCA jacks.  Install the jacks.  Drill another hole to fit the shaft of a cheap potentiometer -- say 10k to start with, either linear or audio taper.  Install the volume control.  Do you know how to wire a pot as far as input, wiper (output) and ground?  One set of RCAs will be for the input.  The wiper, center lug on each section of a stereo control is the output in a standard configuration (there are different ways to wire a pot, including a shunt type of contro -- I'll talk about later, it's an idea that may help in ypur case, and have the added benefit of a potentially better sounding pot) will be connected to the output jacks.  Connect the grounds and potentiometer ground.  And run leads from the center of the input RCAs to the input lugs of the colume control.  And what you have just made is a very cheapmbut effective passive volume control.  Goldpointmsells these for a few hundred dollars.

Now plug your DAC into the input RCAs of the box, and the outputs of the box to the inputs of the mini torii.  What you end up with is what I suggested could be done using those two remaining holes in the mini torii, which is essentially a passive preamp built into the chassis. Trymit first the way I mentioned above.  Please, if I may say this, don't overly rebuild your amp right now.  Deal with the gain issues first, and then, perhaps do other modifications you might be interested in.  If too much is done too fast, you can end up feeling exasperated.  Ask me how I know! . What all of this is really sort of a variable rather than fixed voltage divider.  And apologies for spelling errors.  I have typed all of this very quickly.

Another note: the 6y6 may be my favorite tube with this amp.  It does run quite warm, but I knew that would be the case, as Florida boy also mentioned.  My dad, who is truly experienced in vacuum tube radio electronics often laughs at what I interpret as being hot as far as power supplly components go.  Moreover, I have a very good but low powered OTL amp that runs in single ended class A that will literally sizzle of I dampen my finger and touch the chassis.  Very heavy currents run through the chassis and the heatmis actually normal.  If you are not sure about anything I've written let me know and I'll explain it.

FB,

I agree.  I just want to rule out potential shortcomings and prepare myself with possible solutions.  I won't be doing anything mod-wise until I hear it, totally broken-in, but these discussions help me learn more about its intricacies.  I don't know about you (with experience in these topics), but I want to learn as much as I can.  It's all part of the fun. Have a good listening!

I really enjoy the fact that all  of a sudden the forum topic lights up and draws so much attention and investment. It has been pretty dead for a while.
Myself I specialize in psychotherapy, but I keep wanting more from my sound system and feel claustrophobic when limited. I know some of you have been playing around with audio for decades and were able to come to conclusions, but I can only make my own mistakes and find my own taste. Right now I am amazed by transparency, and how new parts can reveal more of what is hidden on those cd's. So far the setup has remained very musical. Maybe if I overdo it I will know (from experience) what it means to have an overanalytical system.
Erik, I understand very well the idea of installing volume pots on the incoming signal, before the amp's atts. I know how to solder volume pots, although I don't know much in general. I work deductively in electronics, which oftentimes doesn't take me far and sometimes backfires. Literally
After all this discussion I must say that if a new attenuator offers me ONE single click that I can use I will be satisfied, as long as it sounds better than those I have now, which I do not trust particularly. The gain issue is secondary for me and now I know how to solve it, thanks to your collective efforts, thank you very much.
So for me the question still remains whether I should get an att of the same value as the one the MT comes with (100k) or lower (50k or even 25k), given my present setup.
As for the Vcaps, my observations so far are of the hesitant kind, which dwindle from day to day and I need more time to say for sure if the soundstage is really more fwd, or perhaps I need to reposition my speakers etc etc, or perhaps I hear more detail or who knows. One thing is clear though, the sound has changed and I hear strings, let us say, which really sound like strings, metallic or not, to a degree that gives me goose bumps. That is an uncontrovertible observation, compared to the ones I mentioned before.

Radu

I hope I didn't sound like I thought you didn't understand what to do.  I  just wanted to clarify what I might have written in the way of a lousy or poorly worded description.  And I of course I don't know your electronics ability.  Steve, by the way, has to be one of nicest guys in the business.  I have talked with many designers over the years, and Steve is down to earth and a gentleman.  I of course can't speak for him or anyone, but I think he would understand the urge to tweak and experiment.  That's how this amp came into being after all.

And good grief it makes sense to have only one system that does everything well, fireblade.  Very good sense, in fact.  I think of that sometimes when I look at the two pair of huge horns staring back at me from across the room.  I love what they both do very well but differently, though!

Anyway, all have a good Sunday with your firends and family.  You'll get this gain issue solved, Raduscha!

" but I can only make my own mistakes and find my own taste. Right now I am amazed by transparency, and how new parts can reveal more of what is hidden on those cd's. So far the setup has remained very musical. Maybe if I overdo it I will know (from experience) what it means to have an overanalytical system."

It is a journey and only you can decide when and where to stop.

Here is a tip: Go slow with changes and give each one a good listening to before moving forward.

I had replaced the 240k resistor btw pins 1 and 6 with a 2watt kiwame. I had replaced 2 pairs of resistors at the same time and lost the highs but solved a nagging right channel crackle and hiss problem. I put back the original 240k and  got back my dignity. And realized at the same time that the bad resistor was one of the 47k!!

The 47 k I have installed now are 2 watt audionotes. The 2 watt is overkill and possibly a wrong decision, will have to swap with 0.5 watters. But as is it sounds great too.

Fireblade wrote on 04/08/12 at 18:37:48:


FB, I always chuckle when you say "classic bebop" because on jazz boards there are all kinds of arguments of nomenclature. "Classic jazz" is the New Orleans tradition and the 40's revival and the "Chicago Sound." "Bebop" is "modern jazz", and to call it "classic" can only make sense in this classificational world of jazz bulletin boards in referring to the very first years of modern jazz and its originators, that is "classic bebop" would just be the jam sessions at Minton's (some recordings) and so many other places that weren't recorded because of the "recording ban," and could maybe be stretched to include the first groups of Charlie Parker, Dizzy Gillespie, the Coleman Hawkins group with boppers, the short-lived Eckstine big band, etc.

Anyway, "classic bebop" is close to nonsense in that world! Gives me big chuckles to read. Thanks.

My real post is to say I commend your intentions. I hope you have on order the amp that will be THE amp for YOUR speakers and you can start right in on tuning the sound. My observation: I think you may discover as I have that there's ONE assemblage of tubes that is right for BOTH your jazz and classical recordings. One group that makes the amp sing it's unique song with the source material. I've found one that I can just pop in and spin rock, jazz, classical, Brazilian, blues, what's in my voluminous shelving. That's my prediction: you'll find one group of tubes that sets the amp free to show you what's on all the discs.

Erik, I tested 6L6 tubes in my MT and did not like the presentation one bit. Things were pretty much out of focus, although the bass was great and everything sounded somewhat fuzzy. Someone on the forum told me that it sounded pretty much like what I could expect from 6l6's. Could you please resume in a nutshell what is specific to the 6y6 in the MT, as compared to the 6v6?

Lon,

Regarding your prediction about a fixed set of tubes handling all music genres, I hope you got that one right.  It would be much easier to have a fixed set and know that the sound is relatively the best possible for the particular setting ...  Let's keep our fingers crossed, we'll see.

Regarding the tubes: I use as a major reference to the basic sound for my systems recordings that I made in my then garage apartment of bands I rehearsed with and performed with and recorded rehearsals and demo tracks for. I used the same microphones, the same tape deck, and the same space for four different groups of musicians that included acoustic and electronic percussion over several groups, electric and acoustic guitars and basses and keyboards over several groups, vocalists and reed players and one crazy trumpeter over several groups. I know these recordings pretty darned well and the room and people and I myself participate in the music-making. It gives me a good reference. In all my tube systems I've been able to find a tube set that will represent realistically what I feel is on both the electric instrument material, the acoustic instrument material, and the material with a mix of both acoustic and instrumental material. When I get this set established, I can feel confident that my purchased recordings will be represented pretty realistically.

I therefore really believe it's possible, and will be interested to see if you discover this too in time.