[JoshK]

FWIW, Tact gear, when using both the preamp and amp, doesn't lose bits as you adjust volume. That's one of it's great strengths. The preamp passes volume attenuation off to the amp/DAC.

The level of playback is controlled by adjusting the voltage of the power supply. To reduce the volume the voltage of the power supply is reduced. This means that the volume control is no longer part of an active circuit.

As a Tact user, I'm really excited about the SB2 mods. I would, natuarally be most interested in u ...

With SB2's measured jitter you are more likely to increase its jitter with jitter reducers than decrease.  I'd skip em.

[zybar]

FWIW, Tact gear, when using both the preamp and amp, doesn't lose bits as you adjust volume. That's one of it's great strengths. The preamp passes volume attenuation off to the amp/DAC.

The level of playback is controlled by adjusting the voltage of the power supply. To reduce the volume the voltage of the power supply is reduced. This means that the volume control is no longer part of an active circuit.

As a Tact user, I'm really excited about the SB2 mods. I would, natuarally be most interested in u ...

I am using a modified SB2 into my TacT 2.2x preamp and it is a great match!  I set the SB2 to max output and adjust volume via the TacT.

George

[Wayne1]

Richard,

Would you be kind enough to point me to a link where the voltage control scheme is discussed?

I am aware that Tact is using the TI Equibit design in their amps. This is the same basic design the Panasonic uses in the SA-XR series of  receivers. The voltage control of the output stage is only for a narrow range at higher levels. At low volumes a "standard" digital volume control is being used.

This would only be the case if one were using a Tact preamp with a Tact amp. If one were to use a Tact preamp/digital EQ with another brand of amp, the same bit dropping type of volume control would be used.

Josh is correct. You would want to keep the digital signal path as very short as you can. The measured jitter of the SB2 is extremely low. I do not think these would cause any improvements

[csero]

FWIW, Tact gear, when using both the preamp and amp, doesn't lose bits as you adjust volume. That's one of it's great strengths. The preamp passes volume attenuation off to the amp/DAC.

The level of playback is controlled by adjusting the voltage of the power supply. To reduce the volume the voltage of the power supply is reduced. This means that the volume control is no longer part of an active circuit.

As a Tact user, I'm really excited about the SB2 mods. I would, natuarally be most interested in u ...

Even the tact is dropping bits. The PS voltage adjustment works only in narrow  (~20 dB) band, below that it is like any other conventional digital volume control. You simply can't reduce the PS voltage of the output H bridge ad infinitum.

[zybar]

I thought because TacT does something with the bits, that you didn't lose any resolution until you went to a certain volume level or lower?

Since I have no expertise in this area (like that wasn't obvious already), let me try and dig something up and post it.

Wayne, if you want me to move the discussion to the digital circle that's fine by me.

George

[Wayne1]

George,

If you want to copy and paste the posts from Richard S on, that's fine with me.

I would like to keep the posts here as well. I don't want to shut the thread down here. I am fine with talk about other products.

I really want people to understand that the SB2 is not unique in "dropping bits". ALL digital volume controls do it to some extent. The SB2 DOES take the 16 bit digital information and changes it to 24 bit on both the input to its internal DAC and the S/PDIF output. That is somewhat unique

If you want to move it for greater visibility, cool.

[dwk]

Using the term 'dropping bits' is at best very misleading, and in the context of the units we're talking about, probably wrong.

'Dropping bits' did happen in very early digital volume implementations where everybody was limited to 16 bit output samples. It's probable that some cheezy computer interfaces and/or software also do this, although the higher-end stuff certainly doesn't.

*IF* we assume that all these units are using a 24 bit word size, then the artifacts of the computational aspects of digital volume control are down at -144dB below full scale, and hence of no concern to anybody.

The practical limitation of virtually any well-implemented digital volume control is the low-level linearity of the DAC chip used.  This is slightly different than the S/N or THD+N number, but it is related.  In essence, when the LSB of the input signal gets shifted down to the point that it starts hitting the low-level limts of the DAC, then distortion will occur.  For good DAC chips, this limit is maybe -110dB or so.  This is 14dB below  full-scale for CD signals, so you "should be able to" attenuate the signal by that much with pretty much 'perfect' results.

It's possible that dithering the signal at either 17 or 18 bits rather  than letting it 'fall into' the DAC noise floor would result in better subjective performance, but I have never tried it.

In practice, the crux of the issue with digital volume control is to ensure that you never need too much. If you can adjust the overall gain of the system so that you're never having to use more than ~10dB of attenuation, you should be in good shape.   This is probably a challenge  to do in many cases, though, since everyone seems to like really hot voltage levels, and amps always seem to have way too much gain.

[tdangelo]

If I remember correctly someone from Tact told me the 2.x has full resolution until the volume is below an indicated 76.

[RichardS]

Wayne
According to the Tact website:

5. Level control is supplied by power supply voltage keeping resolution, signal to noise ratio and distortion constant at all normal listening levels.

see: http://www.tactlabs.com/Products/MS2150/MS2150_Frames.htm

I'll see if I can find a more thorough online discussion of the particulars.

Re: jitter boxes, do you have the output jitter figures at the digital out of the SB2?

The Genesis Lens places the (stripped) digital signal into a RAM buffer and reclocks it to practically eliminate correlated jitter at its output. Similar benefits to using a HD as source. It can also add dither to increase the apparent bits to 20, which I've found to increase tonal richness and liquidity. For some reason. I like it better than just using an undithered 16-bit signal and letting the Tact upsample (the amp does upsample to 24/192).

I also use the Camelot between the Tact pramp and amp (supposedly reduces jitter to below 20ps) since I'm sure more jitter is generated along the way. As much as I agree with Josh's premise of keeping the digital signal path short, I've found these two units to appreciably add to the sense of realism. But, I'd still like to get rid of them if I can do so without losing much (heck, I'm using 5 digital cables).

I have a Mac G4 in a nearby closet (with firewire, USB and USB 1 ports) with external firewire drives. What would you recommend as the best way to take the signal out into the SB2. Wired connection is no problem (though the computer is 30 feet from the audio).

Thanks

[Wayne1]

RichardS,

A lot of that link is in marketing speak. I believe that the TacT uses the exact same method as the Panasonic receiver. That is just as csero explained it. Yes the power supply voltage is reduced to control volume FOR A PORTION OF THE VOLUME RANGE. I think the marketing guys just didn't use the whole statement.

The jitter figures that have been published are:

> playback paused: *55 ps*
> playing AIFF: *57 ps*
> playing FLAC: *57 ps*
> playing mp3: *58 ps*

These were measured at the internal DAC input. I have not seen anything taken at the S/PDIF output. These were also taken using a linear power supply instead of the switching supply that comes with the SB2. The 12.28 crystal was also lifted. That is another mod that I am now including in the "basic" package.

As the SB2 does ouput a 24 bit signal, I don't see much advantage to using the Genesis Lens.

If you have a network, that is what the SB2 has been designed to be used in. Either a direct wired ethernet connection to your router or wireless 802.11G. It can be hooked direct to a single computer using a crossover cable.