Solid state preamp grain compared to valves

[Primalsea]

A passive attenuator be it a volume pot or otherwise will attenuate the noise floor of everything before it, as well as the music signal. So if the volume is too loud and needs to be reduced anyway it’s a bit of a win-win situation, especially if the impedances have been thought out in order to not have any objectional HF roll off.

A preamp (with gain) generally will amplify the signal and the noise, and add more noise. However, exactly how this is done depends on the circuit design. Valve pre’s tend to have a passive volume pot as the very first thing and being a passive attenuator reduce the noise floor and signal before amplifying whats left, not always a good thing in every case, but can work well. So what you have is a passive attenuator followed by gain. SS pre’s often have the volume pot buffered. I think it can become a bit of a head-scratcher as the wanted signal and the noise may not have the same output impedance. If you buffer the signal you do not have any gain as such but you also lower the output impedance of the noise, making it potentially harder to deal with.

[anthonyTD]

Something that has occured to me, while discussing this thread with Barry, is; before we can generalise on the effect of a passive attenuator network, we really need to know the type of input circuit configuration it will be used with, ie; some amplifier inputs have series resistors, or capacitors, which tend to isolate [to a point] the effect the attenuator might otherwise have in comparison to an amplifier without.
This is where I fear some of the confusion has risen from!
A...

[Marco]

A passive attenuator be it a volume pot or otherwise will attenuate the noise floor of everything before it, as well as the music signal. So if the volume is too loud and needs to be reduced anyway it’s a bit of a win-win situation, especially if the impedances have been thought out in order to not have any objectional HF roll off.

A preamp (with gain) generally will amplify the signal and the noise, and add more noise. However, exactly how this is done depends on the circuit design. Valve pre’s tend to have a passive volume pot as the very first thing and being a passive attenuator reduce the noise floor and signal before amplifying whats left, not always a good thing in every case, but can work well. So what you have is a passive attenuator followed by gain. SS pre’s often have the volume pot buffered. I think it cam become a bit of a head-stracther as the wanted signal and the noise may not have the same output impedance. If you buffer the signal you do not have any gain as such but you also lower the output impedance of the noise, making it potentially harder to deal with.

Hi Paul,

Could you correct the typos above, so that it doesn't spoil an otherwise good post, and thus becomes clear what is was you meant? Ta!:)

Marco.

[Marco]

Something that has occured to me, while discussing this thread with Barry, is; before we can generalise on the effect of a passive attenuator network, we really need to know the type of input circuit configuration it will be used with, ie; some amplifier inputs have no series resistors, or capacitors, which tend to isolate [to a point] the effect the attenuator might otherwise have in comparison to an amplifier without.


Did you mean with, rather than without? Without doesn't really make sense. Otherwise, that's a VERY good point, and probably explains why passive preamps, although a technically more elegant solution than their active counterparts, aren't guaranteed to perform optimally in all set ups.

Marco.

[anthonyTD]

Sorted!
A...

[Marco]

That's more like it:)

Marco.

[Yomanze]

I thought the opposite was true and that we are not very good at picking up on phase errors?

Also that any phase error would only exist over a very narrow frequency band and therefore be unlikely to be audible?

We are very good at picking up phase errors. Speakers have by far more of an influence on this though e.g. port / crossover phase shift, group delay etc.

I would rather have excellent phase and time domain performance than a flat frequency response, any day.

[anthonyTD]

I think the only reliable way of knowing what affect [if any] a passive attenuator may have on influencing the following circuitry is; to test the system as a whole, otherwise we end up generalising, or trivialising what some perceive as a real, and obvious trait of using such Attenuators in a known system.
The phase shift of an RC filter and its frequency response are intimately linked and are at the ends of its passband.

As I commented way back in the thread, the roll-off - and therefore phase shift - of a passive preamp can be at frequencies well beyond audio frequencies.

[Ali Tait]

Should have my new pre shortly, uses an AVC. Look in products-signal- inductive volume control-

http://www.tribute-audio.nl

[anthonyTD]

I shall wait with interest for your evaluation Ali.:)
Should have my new pre shortly, uses an AVC. Look in products-signal- inductive volume control-

http://www.tribute-audio.nl

[Ali Tait]

Yes looking forward to hearing it. Tribute are amongst the best audio transformer makers on the planet.

[anthonyTD]

:)
Yes looking forward to hearing it. Tribute are amongst the best audio transformer makers on the planet.

[Barry]

Hope they sound awful, as Tribute are no longer supplying the DIY market, so none of us can procure them now.

Nah only joking! - will read your findings with interest. Are they a two winding design or an autoformer design?

[walpurgis]

We are very good at picking up phase errors. Speakers have by far more of an influence on this though e.g. port / crossover phase shift, group delay etc.

Very true in my opinion. I'm sure I'm sensitive to that. I don't tend to like multi driver speakers and much prefer dual concentrics or full range drivers, probably for this reason. Mind you, even single driver speakers have phase anomalies.

[struth]

It is without question some form of distortion. In the case of CD with an active pre-amp you are taking a signal, amplifying it and then attenuating it again before passing it to the power amp. That's sub-optimal compared to simply attenuating the signal from the cd player, it seems to me?

Sounds like a gain issue with the higher input of cd

[anthonyTD]

Overload margins of equipment is certainly worth considering' as to the cause of a certain type of nastiness experienced with some systems.
A...

[Edward]

We are very good at picking up phase errors. Speakers have by far more of an influence on this though e.g. port / crossover phase shift, group delay etc.

I would rather have excellent phase and time domain performance than a flat frequency response, any day.

In my recent DSP experiments/playground this is precisely what I found. I was shocked how much difference a tiny bit of time correction can make versus say changing crossover points or even having overlapping (due to operator error) freq ranges.

[Ali Tait]

Hope they sound awful, as Tribute are no longer suppling the DIY market, so none of us can procure them now.

Nah only joking! - will read your findings with interest. Are they a two winding design or an autoformer design?

Lol cheers Barry. They are an autoformer design, from what I’ve read built to a higher spec than the Slagles.

Ant, of BTE designs is putting it in a chassis for me, he’s tried it out in his system and he now wants a pair!

Should be able to buy these from Tribute, I believe it’s only bespoke diy transformer designs he’s no longer producing. Don’t know what the cost is new, but I’d imagine not cheap.

[Barry]

Lol cheers Barry. They are an autoformer design, from what I’ve read built to a higher spec than the Slagles.

Ant, of BTE designs is putting it in a chassis for me, he’s tried it out in his system and he now wants a pair!

Should be able to buy these from Tribute, I believe it’s only bespoke diy transformer designs he’s no longer producing. Don’t know what the cost is new, but I’d imagine not cheap.

That's good to know. No, they won't be cheap - transformer design is becoming a lost art and winding them with care is a time-consuming business.

Despite the acknowledged simplicity and elegance of a resistive attenuator, I've always admired a TVC as the change in output impedance is more benign. And an autoformer can confer some advantages over a two coil design.

Again, I'll be be reading your findings with interest.

[Ali Tait]

I will post up my impressions when I’ve given it a good listen.

[DSJR]

Oh heck, I read a few pages and got a headache ;)

I suppose the best thing always is to try it and see (I should take my own advice here ;) ).

I'd also add please that it seem to me that many 'active' preamps in the 70's and 80's (some valuable today) were musically awful, removing so much of what's there and active as sonic processors rather than 'acoustically transparent' switchers and adjusters of volume and gain. The best active models are pretty transparent to the signal now I feel.

I use passive and active preamps, but not always can they be substituted in my stuff here...