Promitheus Audio TVC - the best preamp!

[jai1611]

Besides input sensitivity, you also need to look at the amount of gain in the power amp. Ideally you want 30db or so of gain on the power amp along with low input sensitivity.

I've had good luck with a Jungson JA-99c and with a pair of tube monos.

 

[pratimbayal]

Yes gain is also an important issue .

My power amp has 31 db of gain - I informed Clause of Odyssey about using passive pre before bringing the power amp.

 

[tirthankar]

I am not sure how gain will be determining the effectiveness of a passive pre like a tvc.
It will be great if someone can explain.

As I understand, gain will basically determine the output level of an amplifier for a particular level of input. It is more dependent on the speakers involved.

For example for a typical full ranger which can be driven by say a 12w set, the amp does not need a high gain, just high enough to achieve its rated power for the given input voltage.
But for a speaker with lower sensitivity which needs a higher amplifier power, the gain will have to much higher for the same input sensitivity of the amp.

To summarize, for two amps with same input sensitivity, the one with higher gain will have a higher output power. But the amount of gain required, and hence the output power, will depend more on the speaker.

One can of course argue that if an amplifier has a high enough gain then even a low voltage can allow it to give sufficient output to drive the speaker load, hence reducing the requirement of a high input sensitivity. But that is basically circling the actual problem.
An amp with .5 v sensitivity rated at 50w can achieve its output with 500mv of input. An amplifier with higher of sensitivity will need a much higher gain in comparison, increasing logarithmically.

A lower input voltage to achieve the required output levels is preferable to a higher gain to keep down distortion. Of course, we are talking about sanely priced amplifiers.. not the insanely priced ones with state of the art design and components, in which case these things cease to matter.

 

[jai1611]

I think you are mixing up gain and power. A 2w tube amp can still have the same gain as a 100w solid state.

Most active preamps have 10-12db of gain.

 

[tirthankar]

I think you are mixing up gain and power. A 2w tube amp can still have the same gain as a 100w solid state.

Most active preamps have 10-12db of gain.

Power is very much related to gain.

The voltage gain ratio will determine the voltage output for a particular input. The db gain is taken as a log of that gain ratio. Power ratio becomes more complicated as current and hence impedance becomes involved, but it's still related.

A 2 w tube amp with the same gain as a 100 w solid state will have an insanely low input sensitivity.

Active preamps have a gain stage, which is why they are active.

 

[jai1611]

Among other things, I tried the Promitheus with a Lyrita 2A3 (2wpc, used as a power amp). With a pair of 94db single driver speakers, that sounded louder at the same volume setting compared to a 70wpc Acoustic Portrait amp. That particular AP amp is something delibrately designed with low system gain and needs an active pre to shine. The 80wpc Jungson had noticeably higher gain compared to the AP - much more than the modest difference in power would suggest.

I'm not sure about the science behind this, just sharing my experience of various amps with a TVC, which has led me to conclude that circuit gain is important to keep an eye on.

Edit: some links for my audition experience with some other power amps and a pair of insensitive Dynaudio bookshelves here:

http://www.hifivision.com/amplifiers/8129-krell-400xi-7.html#post118094

 

[2chFreak]

Gain of a pwr amp is approx from 20~32 dB. Generally most manufacturers like to take a midway path & set Gain at 26 dB.

Ideally for a passive to work well

1. Gain should be as high as possible, preferably above 30 dB
2. Input sensitivity should be as high as possible, @ the min closer to 0.775 V and if lower all the better. 0.5 V and lower will work very well.
3. Input impedance of the pwr amp should be at least 10k ohms or more. The higher, its "kinder to the passive" to work less harder. 47~50k ohms are great figures.
4. The output drive to the passive, from the source should be as high as possible, ideally above 2.5 V and nicer if 3 V or a bit more.
5. The output impedance of the source should be as low as possible, 100 ohms is a good value, better if lower.
6. Finally, IC lengths greatly matters. Keep them short to half a metre and use as low a capacitance cable as possible, the lower the lesser roll-off of HF.

these are good practise guidelines of a Passive in a system. If correctly followed, its tough for an active pre to come & knock its socks off unless its a mega expensive one.

 

[iaudio]

Among other things, I tried the Promitheus with a Lyrita 2A3 (2wpc, used as a power amp). With a pair of 94db single driver speakers, that sounded louder at the same volume setting compared to a 70wpc Acoustic Portrait amp. That particular AP amp is something delibrately designed with low system gain and needs an active pre to shine. The 80wpc Jungson had noticeably higher gain compared to the AP - much more than the modest difference in power would suggest.

I'm not sure about the science behind this, just sharing my experience of various amps with a TVC, which has led me to conclude that circuit gain is important to keep an eye on.

Edit: some links for my audition experience with some other power amps and a pair of insensitive Dynaudio bookshelves here:

http://www.hifivision.com/amplifiers/8129-krell-400xi-7.html#post118094

Jai, Same here, I felt the same when I paired the Prometheus passive pre with the AP 70wpc PM1 power amp!

 

[tirthankar]

Ideally for a passive to work well
....

Thanks for the concise and to the point post.

However I have a few doubts.. hope you don't mind.

1. Gain should be as high as possible, preferably above 30 dB
Can you please explain why you think gain by itself is so important? I mentioned earlier, as I understand, gain can be expressed as a function of input sensitivity and output power.. and as long as an amp has a low enough enough sensitivity along with a high enough output voltage on the source, and enough power to drive the speakers.. why do we need to be concerned about gain separately?

2. Input sensitivity should be as high as possible, @ the min closer to 0.775 V and if lower all the better. 0.5 V and lower will work very well.
Agreed

3. Input impedance of the pwr amp should be at least 10k ohms or more. The higher, its "kinder to the passive" to work less harder. 47~50k ohms are great figures.
I know it is important for a resistor based passive as in that case the passive and IC along with the amplifier input becomes a part of the load to the source output. But is it also that important for a transformer based passive like the prometheus TVC? Isn't the transformer in the circuit going to take care of bulk of the impedance mismatch problem?
Of course it is always important to have the load input impedance/source output impedance ratio as high as possible as any circuit.. preferably in the order of 10 or more, but do we need to take care of it as meticulously as we need to for a resistor based passive?

4. The output drive to the passive, from the source should be as high as possible, ideally above 2.5 V and nicer if 3 V or a bit more.
Agreed

5. The output impedance of the source should be as low as possible, 100 ohms is a good value, better if lower.
Agreed.. along with point 3

6. Finally, IC lengths greatly matters. Keep them short to half a metre and use as low a capacitance cable as possible, the lower the lesser roll-off of HF.
Agreed

Also I am not too surprised about the AP being a poor match for passives. As I see their power amps have input sensitivity much higher than 1v. Unless the source has a great drive it will be tough to get it working good without an active.

 

[2chFreak]

1. Gain should be as high as possible, preferably above 30 dB
Can you please explain why you think gain by itself is so important? I mentioned earlier, as I understand, gain can be expressed as a function of input sensitivity and output power.. and as long as an amp has a low enough enough sensitivity along with a high enough output voltage on the source, and enough power to drive the speakers.. why do we need to be concerned about gain separately?
.

Gain is the ability of the amplifier to boost an incoming low level signal at the input to a loud enough output at the other end of the amplifier. Consider a 200mV signal of 1khz coming into the amp. The higher the Gain, the louder the output will be. If the Gain was 20 dB, the output will be X but now this same 200mV signal with 30 dB Gain amp will be hugely louder. Its 10 dB more than output X.

3. Input impedance of the pwr amp should be at least 10k ohms or more. The higher, its "kinder to the passive" to work less harder. 47~50k ohms are great figures.
I know it is important for a resistor based passive as in that case the passive and IC along with the amplifier input becomes a part of the load to the source output. But is it also that important for a transformer based passive like the prometheus TVC? Isn't the transformer in the circuit going to take care of bulk of the impedance mismatch problem?
Of course it is always important to have the load input impedance/source output impedance ratio as high as possible as any circuit.. preferably in the order of 10 or more, but do we need to take care of it as meticulously as we need to for a resistor based passive?

It is always good or nicer to have as high as input impedance as possible. This Ratio that you mention off, 1:10 then just gets better & better.

 

[jls001]

It is always good or nicer to have as high as input impedance as possible. The Ratio that you off then just gets better & better.

Asking in general terms: if I remember correctly from electrical engineering classes, according to the Maximum Power Transfer Theorem, the maximum power will be transferred from a source to a load if the impedance of the load matches that of the source, as viewed from the output terminals of the source. And a corollary to the theorem is that even after meeting the above condition, only half of the power output of the source can be transferred to the load.

So my question is: why do equipments have low output impedance and very high input impedance?

 

[2chFreak]

So my question is: why do equipments have low output impedance and very high input impedance?

to minimize distortion in the transmission line of audio signal transfer & to maintain highest integrity of that signal

 

[milpai]

My recent experience in "gain" is something I have not understood. I have been using the Marantz SA8260 with RCA outs to my Promitheus TVC. Then the TVC is connected to my Parasound A21 using RCAs. My standard volume position on the TVC was 13.
Last week I installed XLRs between the TCV and the A21 and I got a gain in my system. This was a welcome boon, as it put meat on bones - literally - for the music. I am so thrilled, like a kindergartner. Here is what I experienced:

AudiogoN Forums: Cable conundrum: Parasound A21 with RCA AND XLR

Why should the same volts from CDP to TVC provide me with much more gain when I change the RCAs to XLRs between the TVC and amp? Anyone care to explain please? Thanks in advance

 

[2chFreak]

My recent experience in "gain" is something I have not understood. I have been using the Marantz SA8260 with RCA outs to my Promitheus TVC. Then the TVC is connected to my Parasound A21 using RCAs. My standard volume position on the TVC was 13.
Last week I installed XLRs between the TCV and the A21 and I got a gain in my system. This was a welcome boon, as it put meat on bones - literally - for the music. I am so thrilled, like a kindergartner. Here is what I experienced:

AudiogoN Forums: Cable conundrum: Parasound A21 with RCA AND XLR

Why should the same volts from CDP to TVC provide me with much more gain when I change the RCAs to XLRs between the TVC and amp? Anyone care to explain please? Thanks in advance

6 dB Gain from unbalanced single ended to now symmetrically balanced via XLR at the power amp inputs. This is absolutely normal & a crime if it doesn't happen.

Enjoy it

 

[jai1611]

The Promitheus is not designed to do a proper unbalanced to balanced conversion. Nicholas had told me that there is a loss in sound quality. In my system, it also led to higher noise levels. There was some extra gain though.

 

[milpai]

2chFreak,
Thanks for letting me know.

jai1611,
I have the Reference C-Core that is dual mono. Nicholas mentioned to me that it was completely balanced. If you go through the link from audiogon where I posted my experience, I actually find that the system sounds better at lower volumes now and bass is "full". Noise levels have also dropped as strings have fuller body and seems to form "with confidence" in mid-air. I am thoroughly enjoying the improvements in my system.

 

[sonosphere]

Hi,
Recently I got possession of Promitheus TVC. Thanks Sidvee, it was your opinion in this thread that gave me confidence to buy it blind, without audition. I am in disbelief as I hear this pre, for reasons that Sidvee has already explained in detail. Transparency, dynamics, imaging, bass, details, freq-extension... It does all this and the bugger doesn't even consume an iota of electricity :lol:

There's one bit of "unexpected experience" I would like to share...
This unit was unused probably for months (soulforge correct me if I am wrong). When i hooked it up and played the first song, I heard > disproportionately loud bass, ok mids and rolled off highs, no imaging, no depth

I kept my nerves, and kept it playing... after 1 hour highs started showing up.... next day bass reduced to correct proportions, mids and highs sounded better, imaging started appearing... after 3 days and approx 8 hrs I am in disbelief.

So much written about this unit on this thread but no photo? So here it is below (not my unit though)

Thanks to our FM soulforged who took great pains to pack the unit (literally Bomb-proof packing) and had it delivered to me in Pune. (Jai, the DAC-19 is really shining thru this pre I am sitting transfixed looking at the empty space between the speakers since two hours, and folks at home are thinking I've lost it totally :lol

Regds,

 

[soulforged]

^^^ You are right...they really did not see much playtime at my place. I was in two minds about letting it go but I'm glad it went to someone who is liking it...the best part about the TVC is it transparency...and the Adcoms may just be the right match for them.

 

[mpw]

The TVC will not be suitable for quite a few power amps i suspect. One must study well before buying this product and i am happy that we have another member with Promitheus TVC.

One more on the boat for company )

Even with amps that seem to have the tech spec to accept a passive TVC.. the sound signature is no guarantee.. you may like it or you may not since the TVC brings warmth into the system.

Yesterday i heard the Parasound A23 + Promitheus TVC... and i was not too impressed with the combo ( not with this individual components. ) ( As an individual amp.. the A23 shines thru ) ( The Parasound ticks the tech spec to accept the promitheus TVC )

The system matching may be a surprising factor - it went very well with my NAD C356 + Epos and if sonos is happy.. the Adcom would be accepting the TVC well too..

mpw

 

[sonosphere]

...the best part about the TVC is it transparency...

I forgot to inform reg another unexpected surprise...

Actually my soundcard was sounding quite good while being connected directly to the Adcom power-amp. Sometime ago, I did a comparo between direct connection, and thru a preamp (it was pre-section of Cyrus integrated amp) to see what difference a good pre can deliver. Result: The pre gave better body to each instrument and overall presentation. Most significant improvement > bass became much tighter than before.

With Promitheus, I got one more benefit, apart from tighter bass.... surprisingly it brought out low-level details that I had never heard in direct connection. (I actually believed that even a good active-pre can add a subtle veil over low-level details, especially at low volume)

Now, I am going thru a ritual many audiophiles go thru after a significant upgrade... I am re-playing all old songs that had good detailed recording (A.R Rahman, western classical, Kraftwerk etc.) and with all new details showing up, they sound like new songs to me .

Regds,