Passive pre amp

[mosschops]

Hi all I am about to make a new pre amp I have looked at all the pots on hifi collective and dont know which one to get . Any advice would be good I all ready have one with a dact attenuator rated at 10k would this value be ok Many thanks Steve

[Phil Y]

Hi Steve,

Value really depends on what it is going to be connected to (from). If it is modern transistor gear, a DAC say, it should be fine but a valve Phono stage may not be so happy with 10K. The "traditional" input impedance for single ended hifi is 47K but I have noticed that with a lot of modern gear this seems to be falling for some reason.
I recently got a headphone amp and one of the reasons I chose the one I did was because it was one of very few that had 47K input impedance, most were lower.
Ideally you want what is following to have a higher impedance than the pot as well, so it is a bit of a juggling act with a passive.

After many years with a passive I recently went active and have noticed a significant improvement, especially on Phono.

Phil.

[mosschops]

Many thanks Phil it is to go with a wad kat6550 amp and cd player I will go for 50k as that is as close as i can find Regards Steve.

[Greg.]

You could save yourself a pile of money by making your own. In addition to Phil’s advice, you need to consider the design choices of Series, Shunt or Ladder. I don’t like series because the signal (depending on volume) passes through many resistors. A ladder design is favourite but it is also the most expensive. A shunt version is a fair compromise in terms of both price and performance.
Have a look at the pages on this website, especially the DIY pages. They can help you to choose resistor values.

https://www.goldpt.com/index.html

You don’t have to go with a PCB mounting. The resistors can be hardwired around the switch.

A while back, I made this shunt design stepper using Elma switches and Welwyn RC55Y precision resistors.

[Mark]

Originally Posted by Greg.

You could save yourself a pile of money by making your own. In addition to Phil’s advice, you need to consider the design choices of Series, Shunt or Ladder. I don’t like series because the signal (depending on volume) passes through many resistors. A ladder design is favourite but it is also the most expensive. A shunt version is a fair compromise in terms of both price and performance.
Have a look at the pages on this website, especially the DIY pages. They can help you to choose resistor values.

https://www.goldpt.com/index.html

You don’t have to go with a PCB mounting. The resistors can be hardwired around the switch.

A while back, I made this shunt design stepper using Elma switches and Welwyn RC55Y precision resistors.

Greg,

That looks impressive, and very compact compared with a lot of stepped attenuators I have seen which appear to be the size of a shoe box ! You mentioend that "You could save yourself a pile of money by making your own." How much did the one you made in the picture cost please ?

Thanks

Mark

[bob orbell]

Hi Steve, an Alps blue from Matthew is all you need, unless you have exceptional hearing, he does them at a good price ( look at the WD web site ) and they are made in Japan, not the junk found on ebay. Bob

[TimN]

Hi Steve

KiT6550 has a 100k log pot in the signal circuit which is literally straight through from the input sockets via an input selector switch and tape/source switch to the ECF80s and R7, 8, 9 & 10 junctions. That's probably a good place to start. You could just move that part of the circuit to an external box. Not sure what the forum rules are about sharing schematics?

But as Greg suggests, I would consider a stepped attenuator.

Tim

[Greg.]

As you mentioned your Dact attenuator, I assumed a stepper is what you’re looking for.