HiFi Subjectivist ¦ Audio Forum ¦ NVA User Group

I'm looking to invest in a Decware valve power amp and Steve (from Decware) recommends the use a 100k passive pot. I own an NVA P50sa (mk 1) and not sure of the specification.

Thanks.

The P50SA is 10k.

We can make a bespoke one with 100k - with the same Seiden switch.

Same price but non-returnable, as it is a one-off.

savvypaul wrote: ↑Mon Jan 27, 2020 3:56 pm The P50SA is 10k.

We can make a bespoke one with 100k - with the same Seiden switch.

Same price but non-returnable, as it is a one-off.

OK useful thanks. Why would a manufacturer recommend a 100k passive pot? What's the maths involved in matching a power amp usually?

wackyraces wrote: ↑Mon Jan 27, 2020 4:06 pm

savvypaul wrote: ↑Mon Jan 27, 2020 3:56 pm The P50SA is 10k.

We can make a bespoke one with 100k - with the same Seiden switch.

Same price but non-returnable, as it is a one-off.

OK useful thanks. Why would a manufacturer recommend a 100k passive pot? What's the maths involved in matching a power amp usually?

viewtopic.php?f=29&t=46107&hilit=10k+impedance

viewtopic.php?f=3&t=45558&p=80521&hilit ... nce#p80521

wackyraces wrote: ↑Mon Jan 27, 2020 4:06 pm Why would a manufacturer recommend a 100k passive pot? What's the maths involved in matching a power amp usually?

100K is the usual pot value for a valve amp.

SteveTheShadow wrote: ↑Mon Jan 27, 2020 4:21 pm

wackyraces wrote: ↑Mon Jan 27, 2020 4:06 pm Why would a manufacturer recommend a 100k passive pot? What's the maths involved in matching a power amp usually?

100K is the usual pot value for a valve amp.

Why, why does a valve amp differ from a SS one in this respect?

Why, why does a valve amp differ from a SS one in this respect?

Valves are high impedance voltage based things. So while the input pot for a amplifier won't matter to the amp if its 100k or 10k, it will matter to whatever is driving the pot. In the time of valves, consumer phono stages, preamps, tuners and tape decks assumed they were going to be driving a valve input device so the output impedance would likely be 1-5k which would be fine for a 100k pot. Pro valve kit may assume they were driving a balanced 600 ohm load, so the output stages were small power amps with coupling transformers. The extra work to drive a low impedance cost money and valves, so using a 100k pot avoided all that and was the cheap solution for consumer use as its was likely only driving a short cable.

Transistors are low impedance current devices, and adding a few extra in the output stage costs almost nothing, so low impedances are more likely.

....thanks! And I thought that a passive pre-amp was solely there to provide incremental resistance to the - for example 2V output - of a CD player / DAC. So, 10K, 50K, 100K "so what" - it's stil a 2V output with incremental trimming of the output based on in-line resistors is it not?

I'm not an electrical engineer so, I wouldn't know.

it's stil a 2V output with incremental trimming of the output based on in-line resistors is it not?

Not really, A pure voltage (or current) source only exists in theory. In practice the output will also include a source resistance (well really an impedance, but lets try and keep it a little simple). Likewise the load the passive is driving will not be a infinite resistance, it will have a load resistance (again actually a impedance). The whole thing source, potentiometer, and load consists of a set of resistors in series and parallel, and around this network will be another network of inductances and capacitances, both from the source, load, and pot, but also from the cables between the source and pot and pot and load. The whole complex network will have a frequency response that depends on all the individual value and also the position of the pot.

So, no, its never as simple as you may think. You may not need to know all that, but you did ask.

As to why Decware suggest a 100k pot before their power amps, the simple way to find their reason for that is to ask them,

Thanks again - useful information