[petrL]

Hi,

I'm happily using Promitheus audio standard TVC for almost 2 years. Now I connected new amp, Transcendent sound SE-OTL, and found the system sometimes lacking gain. The problem is my DIY dac, which has sub-standart output level. Before going to active preamp, I'd like to ask if there is any chance to re-connect TVC primary to have little step up. I know some TVCs have 2 primaries, which enable this, but not sure about Promitheus (6dB up would do it for me).

[guest48077]

Contact Nick at Promitheus.

I know a 6db step up can be added when you order for an exrta 80 bucks. You could most likley get his advice on one for a DIY project as well. It would be a shame to send it to him just for that.

[Brown]

Did you loose gain with your previous amp ? Unless the input sensitivity is too high for the TVC with the new amp. Hopefully under 1 mv.  Now Does your DAC have an output of a minimum of 2V ?
   Before you delve into an active linestage borrow a DAC with more output. Only way to tell.
   

[petrL]

Brown,
I'm not sure if I follow you, but the new amp has low gain only 9dB. I tried standard CD player (the transport) and it is of course louder, but 2-3 more clicks on TVC would be nice to have. The DAC is even quieter. Still good for critical listening, but sometimes one likes to crank it up...

undertowogt,
I think the gain option should be taken into account during manufacture, most likely a tap on the primary is added for $80. Probably too late now. Anyway I'll check with Promitheus, if part of the primary winding can be disconnected.

[Quiet Earth]

I don't think 9dB of power amplifier gain is enough for a TVC to work properly unless your source is very robust (>3V) and your speakers are very efficient.

A TVC will work its best when you think the music is plenty loud and the TVC attenuation is no more than 14 dB down. So, from zero attenuation (volume up all the way), dial back to minus 14 dB and tell us if it is plenty loud. If it is not, then you need an active preamp with that amp/speaker and source combination.

The reason why I say 14dB maximum attenuation is for a good reason. Most TVCs are in the "impedance friendly" zone and are transferring the energy their best when they are attenuating the most (volume way down). As you go above (louder than) the 14dB attenuation point, things start to fall apart and you get the symptoms of "no drive" and "wimpy dynamics", just as you describe.

When you have a lot of system gain, a TVC is your best friend. Hope that helps some.

****Addendum : Adding the +6dB feature will not solve the problem. I have that option on my TVC, and while it does add gain, it makes the drive a little worse. It's an impedance trade off for gain, and it's better to not use it at all.****

[petrL]

Quiet Earth (uh), this is good point. Yesterday I opened the TVC (while listening dac direct to amp). The modification would be messy and surely there will be some trade off because of necessary higher drive current. I think I'll go for ative preamp and be using TVC as attenuator. So, according to you, the TVC will be best connected between preamp and amp?

[Quiet Earth]

Sorry for any confusion. TVC or active pre amp. One or the other.

TVC situation : it is way too loud when the volume control of your active pre amp is barely on. You have no range in the volume control. It is only loud and way to loud. You can't dial in a comfortable volume setting.

Active pre amp situation : You tried a TVC or passive volume control and while yes it is very clear, there is no drive and the volume control must be up nearly all the way for normal listening levels.

There is one other option for you to keep your TVC. Improve the analog output signal level of your DAC (you said it was substandard), or upgrade to a DAC that has a stronger analog output signal level.

How efficient are your speakers? (Sensitivity@1W, 1M and nominal impedance?)

[petrL]

My speakers are Klipsch RF-7, should be 102 dB/2.83V/m, but in the most important range they are below 8 Ohm (minimum 3.5 Ohm), so that real sensitivity would be more like 99 dB/W/m. I have enough volume with my phono setup, around 5 clicks on TVC remain to maximum. The problem is DAC. It is basicaly TNT1541 from www.tnt-audio.com. As for its output stage gain, I'm not able to see if it can be increased or not (given the voltages). As a preamp I plan to scratch built the grounded grid circuit also from Transcendent. It has gain of 12 dB, which I suppose would bring 6 clicks on TVC and more or less balance the level with phono set up.

[Quiet Earth]

You totally nailed it. 

As a preamp I plan to scratch built the grounded grid circuit also from Transcendent. It has gain of 12 dB, which I suppose would bring 6 clicks on TVC

Yes! That would be the ticket.
I would also try to leave any volume control out of the design since you will need the full 12dB of gain. Think of it as a DAC head amp  .

Hey, I wonder if there's a market for this? The mini DAC booster. Hmmmm.......
 

[Quiet Earth]

Just a side note for those that might be interested.

I have enough volume with my phono setup, around 5 clicks on TVC remain to maximum.

 I would say that you are probably at the very edge of the impedance friendly zone. My TVC isn't a Promitheus unit, but I'll bet it is very similar in performance. Mine would be 12dB down from zero attenuation at 5 clicks from max volume. In other words, it would be -3, -6, -8, -10, -12dB. That's just the way it is made.

The impedance chart (for mine) shows that this is where the output and input impedances of source and amp really start to matter. From -12dB, things gets drastically worse with each click towards zero. On the other hand things seem very trivial at only 14db attenuation, and a non-issue further down.

PetrL,

It might be interesting for you to build that gain stage of 12dB and put it after the TVC. That way your phono stage would also benefit. It's worth a try anyway. I think the main thing to remember is that you probably bought (or built) an amplifier that was designed to work with an active preamp. Low gain amplifiers are getting more popular now that the audio community admits that most people have too much gain with CD players and active preamps.

Oh, yeah..... I hope nobody minds that I am posting in this circle. I just like to talk about TVCs and system gain.

TVC users unite! 

[trebejo]

A TVC will work its best when you think the music is plenty loud and the TVC attenuation is no more than 14 dB down. So, from zero attenuation (volume up all the way), dial back to minus 14 dB and tell us if it is plenty loud. If it is not, then you need an active preamp with that amp/speaker and source combination.

The reason why I say 14dB maximum attenuation is for a good reason. Most TVCs are in the "impedance friendly" zone and are transferring the energy their best when they are attenuating the most (volume way down). As you go above (louder than) the 14dB attenuation point, things start to fall apart and you get the symptoms of "no drive" and "wimpy dynamics", just as you describe.

I don't see why 14db is the magic number...

According to the tvc faq at Nick's site, the TVC is always reducing the output impedance of the input source (e.g. a DAC). So although you get a lowering of the impedance with greater attenuation, does that necessarily mean that the sound quality is *bad* when the attenuation is up to -10db or more?

I'm right on the fringe of that range--sometimes I attenuate at -18db, sometimes up to -12db--and, well, maybe I'm just not golden-eared enough to distinguish between these settings, but more to the point, it's literally impossible to make an a/b comparison since, well, changing the recording changes what you are comparing!

I've heard things that sound fine even if brought up to -10db, and I've listened to things that sounded mediocre even though they were pressed for a -18db setting. The quality of the recording seems to matter more than the setting of the TVC.

I wish Nick would clarify some of these issues, though. There are two that could probably be answered pretty quickly.

1. Is the attenuation in 3db steps for the first 12 steps, and 2db steps for the next 12 steps? Silly me, I had assumed that it was 2.25db per step.

2. When the tvc faq uses the example of 10% volume, 50% volume, etc., how does this correspond with the steps on the volume knob?! If I had to bet money on it, I would calculate the following "percentage" for, say, the famous -14db setting:

Voltage ratio = 10^(db/20)
                    = 10^(-.07)
                    = .1995 ~ 20%

That means that your boundary of -14db corresponds to a step-down of output impedance by a factor of 1/(.1995)^2 ~ 25, which seems pretty impressive. If I make it go an extra click to -12db (as I often do), this becomes

Voltage ratio = 10^(-0.6)
                    = .2511 ~ 25%

and the step-down of output impedance is now at 1/(.251)^2 ~ 15.8, which is not as impressive as 25 but still seems pretty darned good!

I suppose things may get downright magical if the attenuation is set at -30db and then we have

Voltage ratio = 10^(-1.5) = 0.0316 ~ 3.1%

with an impedance step-down of (1/0.0311)^2 ~ 1000. But just because it isn't magical, that doesn't mean that it sounds bad!

[petrL]

I was supposing that Promitheus TVC had steps by 2dB. I think the size of steps changed during production, though.

Anyway, I went back to the schematics of TNT DAC, studied little differential amplifiers - and found the possibility of increasing the gain of the DAC. I decreased the already quite large emitter resistors from 270 Ohm to 100 Ohm, which brought theoretical gain of 9 dB. This is enough for most CDs. The TVC has now 5-6 steps left for original volume, but I listen mostly 3-4 steps before maximum. Also at the maximum. Moreover I like the sound. It is more dynamic, vocals are clearer, although the distortion increased and maybe the sound is more tiring. The SE-OTL is so smooth... (and at the same time detailed!) I'll build the GG preamp and then probably fiddle with the emitter resistors again, somewhere between.

[Quiet Earth]

I don't see why 14db is the magic number...

You are right trebejo, it's not the magic number. I do think it's a good starting place for those that are having trouble using a TVC. If someone says that they "find the system lacking gain" or they just don't seem to have enough drive with their TVC, then asking if they are in the benign 14dB range is a good place to start. After all, there's no way to know everyone's  source/amplifier combination.

The reason I like the -14dB number is based on some very limited data I have for the Stevens and Billington TVC. Basically, it shows that going between -14 and -10dB there is a gradual decrease in the input impedance of the TVC. Little as it may be, it starts to act as a load to the DAC. At -8dB there is a noticable decrease (not gradual) in input impedance, and the output impedance of the transformer starts to go up. At -6dB and -3dB the changes are getting grossly worse.

So, it's the rate of performance change as you near full volume, rather than a magic number.