Solid state preamp grain compared to valves

[anthonyTD]

No, the input of the power amp, by your own explanation is now nearer 1k, not 100k, at low volume, due to the low resistance imposed on it by the pots wiper position,with respect to ground!

I presume by "amplifier" you're referring to the power amplifier. If so, let's look at some figures. Using the same example as above with a 10k pot and a power amp with a 100k input impedance, with the pot down near the bottom at say, 10%, the pot has a resistance of 9k above the wiper and 1k below it. In that instance the pot has an output impedance of 1k in parallel with 9k, which works out at 900 ohms. The input impedance of the power amp is still 100k. The load the CD player sees is (near as dammit) 10k.

[mikmas]

I think that because they use output transformers a valve amps frequency output will vary with the load it sees from the speaker.

I see (kind of) .... so how does that manifest itself in terms of what we hear?
The author of the article refers to a "substantial effect on the frequency response of the connected speaker" - would that be:

- a frequency imbalance?
- distortion?
- noise?

... none of the above?

[anthonyTD]

We'l just have to agree to disagree!

No, it isn't - it remains at 100k.
At this point perhaps this particular discussion is over, lest it descend into "yes it is, no it isn't".

[anthonyTD]

Hi Mike,
See post 241, it may answer in part.
A...

I see (kind of) .... so how does that manifest itself in terms of what we hear?
The author of the article refers to a "substantial effect on the frequency response of the connected speaker" - would that be:

- a frequency imbalance?
- distortion?
- noise?

... none of the above?

[Marco]

At this point perhaps this particular discussion is over, lest it descend into "yes it is, no it isn't".

Thing is, one of you must be right, and the other wrong [providing you're both talking about the same thing], as what you're discussing isn't subjective

In that respect, it would be good to find out which.

Marco.

[Barry]

The effective input impedance of the amplifier, as seen by a source up-stream of the potentiometer will be: (resistance between the wiper and ground) in parallel with 100K [the quoted input impedance of the amplifier]) + the remaining resistance of the potentiometer (between the wiper and the source terminal).

For the example given, where the wiper divides the total resistance range into 1K (between wiper and ground) + 9K, the effective input impedance as seen by a source will be (1K // 100K) + 9K = 0.99K + 9K = 9.99K. So virtually unchanged from 10K.


In the other case where the wiper divides the 10K resistance range into 9K + 1K (so 90% full voltage, or about -1dB), the effective input impedance as seen by the source will be (9K // 100K) + 1K = 8.26K + 1K = 9.26K. Again, not too far removed from 10K.


I think the confusion might have been due to the ambiguity of exactly where the 'input' impedance is being calculated.

[anthonyTD]

Indeed!
Thanks Barry,
Which was why I think both of us decided to agree to disagree, the main thing that our discusion has hopefuly acheived is helping people understand the main diffrences between active, and passive preamps, and how attenuation is applied, and their posible affect on diffrent combinations of equipment, even if some of it ended up a bit confusing!
A...

.




I think the confusion might have been due to the ambiguity of exactly where the 'input' impedance is being calculated.

[Marco]

I'd just like to touch again on something Alan posted earlier, regarding cables, and specifically his comment on: "driving the capacitance of interconnect leads" See key bits in bold:

Now keeping things simple, imagine the output of the passive/active preamp driving the capacitance of the interconnect leads:

With a passive with its associated larger output resistance the hf frequency response is always curtailed, very much in the ultrasonic range (to our ears) in virtually all cases, BUT even though we cannot hear the frequency where the response falls there is always an alteration to the phase response.

With an active preamp with a much lower output resistance there is much more drive to charge/discharge the capacitance of the interconnect leads, so the hf response is affected much less, with the correspondingly smaller effect on the phase response.

So my hypothesis is that the missing bit with the passive approach is phase accuracy.

Alan mentions phase accuracy as possibly being the 'missing bit' with passives, which may well be the case, but what about the respective abilities of both passive and active designs at, as he says, driving the capacitance of interconnect leads, and passives curtailing the HF frequency response, as a result of them having a higher output resistance?

Could that be a contributory factor for the 'soft' sound, lacking in drive and punch, some of us perceive with passives, compared with their active counterparts?

Certainly experience tells me that passives are much fussier than actives about cables, both in terms of length and capacitance.

In that respect, using ones that are too long, and/or too high in capacitance, softens the sound. That's what I heard repeatedly during my experiments with the Goldpoint (and other passives I tried), which is why I ended up using 0.5m interconnects of the lowest capacitance I could find.

But....Perhaps no matter how short or how low in capacitance the accompanying cables are, ultimately they act as the defining limiting factor in passives, by curtailing the HF frequency response to a degree, still audible enough to soften the sound?

Actives, despite having their own issues, negate that problem by having a much lower output resistance, and so simply don't suffer from that issue to anything like the same degree, by 'driving' cables better, and in turn negating the propensity to soften the sound.

Just throwing that one out to folks for consideration!

Marco.

[anthonyTD]

From my point of view, as I said, Alan has a point where he mentions impedances having an affect, which was the point I was trying to make, if you put a conventional Attenuator in the front end of an amplifier, it will be the dominant factor as far as the input sensitivity of the amplier is concerned, when you vary the pot, you will be varying the input sensitivity of the power amp, due to the way the Attenuator is connected to the amplifier! And yes, I do believe that while this is going on, any cables that are connected between the amplifier, Attenuator, and Source to a degree" will also have an affect.

I'd just like to touch again on something Alan posted earlier, regarding cables, and specifically his comment on: "driving the capacitance of interconnect leads" See key bits in bold:



Alan mentions phase accuracy as possibly being the 'missing bit' with passives, which may well be the case, but what about the respective abilities of both passive and active designs at, as he says, driving the capacitance of interconnect leads, and passives curtailing the HF frequency response, as a result of them having a higher output resistance?

Could that be a contributory factor for the 'soft' sound, lacking in drive and punch, some of us perceive with passives, compared with their active counterparts?

Certainly experience tells me that passives are much fussier than actives about cables, both in terms of length and capacitance.

In that respect, using ones that are too long, and/or too high in capacitance, softens the sound. That's what I heard repeatedly during my experiments with the Goldpoint (and other passives I tried), which is why I ended up using 0.5m interconnects of the lowest capacitance I could find.

But....Perhaps no matter how short or how low in capacitance the accompanying cables are, ultimately they act as the defining limiting factor in passives, by curtailing the HF frequency response to a degree, still audible enough to soften the sound?

Actives, despite having their own issues, negate that problem by having a much lower output resistance, and so simply don't suffer from that issue to anything like the same degree, by 'driving' cables better, and in turn negating the propensity to soften the sound.

Just throwing that one out to folks for consideration!

Marco.

[Marco]

Understood, Anthony. I'm just wondering though, ultimately which effect will be more audible (although of course it may vary from system to system): the former, as you describe, or the limiting factor of the interconnects, in terms of a passive's inability to 'drive' them as efficiently?

After all, when all said and done, the signal his to pass through those cables to the next stage of the amplification chain (the partnering power amp), and so any bottleneck present there is bound to have an influential effect.

If HF frequency response, as Alan says, is being curtailed to any notable degree, it'll always be heard on a good system

Marco.