Passive Pre

[Reffc]

Coming this way soon.

Intended to be the mate of a tasty STA25. Intention is for purity of sound through minimal signal path = as few components as necessary. Lets just call it the RFC-PP25 for now

Starting points:

1. looking at the input sensitivity of the '25 = 750mV for full output.
2. Assessing output impedance and coupling capacitance of source drivers.

In my case, worst case = 600 Ohms

Next stage: Type of passive.

I'm not a fan of TVCs as they can introduce as many issues as they solve and I just think they're unnecessary (with no further need to justify that statement which is my personally held opinion).

I want a means of balancing output to account for room effects, so some form of balance is desirable. I also require minimisation of any risk of ground loops and the case must be strong enough to support the STA25

Answer:

Screened case, large enough in area and strong enough = milled alloy frame.

Sufficient inputs and a decent selector switch with each input individually earthed (no common earths to be used) as this ticks the minimising ground loop issues.

Quality volume pots, one per channel. This remove the need for an extra components (balance control) in the signal path and also means that one can precisely tailor balance.

Captive outputs...no RCAs needed as the pre will be designed to sit under the power amp so I will hard wire the output wiring.

Volume pots selected are the excellent Tocos Cosmos pots. I have selected 50K pots which may be marginal with the STA's 100K input at 1/4 value (ie -6dB) but should, in reality, be ok. This will provide sufficient load for the source...I hope! The coupling capacitance of the source is large enough to pull down any high pass issues to well below audible threshold so bass shouldn't be compromised.

Selector is a cheap and cheerful one for now, just to test the control unit. If it proves a hit, it will be upgraded to something better quality. I will be using make before break switches to avoid any unwanted pops and crackles.

In the event that a different pot value is needed, I'll try it, and in the event that doesn't work, I'll revert to a buffer design for that particular source. Suck it and see time.

Photos to follow.

[JohnJo]

Very interesting Paul, now where's that popcorn thingee and the one with all the sign boards with "useless without photos" on them?



How do you think those pots compare with Alps Blue?

[Macca]

Didn't realise you had an STA 25 Paul. Is that a recent acquisition?

[Arkless Electronics]

Coming this way soon.

Intended to be the mate of a tasty STA25. Intention is for purity of sound through minimal signal path = as few components as necessary. Lets just call it the RFC-PP25 for now

Starting points:

1. looking at the input sensitivity of the '25 = 750mV for full output.
2. Assessing output impedance and coupling capacitance of source drivers.

In my case, worst case = 600 Ohms

Next stage: Type of passive.

I'm not a fan of TVCs as they can introduce as many issues as they solve and I just think they're unnecessary (with no further need to justify that statement which is my personally held opinion).

I want a means of balancing output to account for room effects, so some form of balance is desirable. I also require minimisation of any risk of ground loops and the case must be strong enough to support the STA25

Answer:

Screened case, large enough in area and strong enough = milled alloy frame.

Sufficient inputs and a decent selector switch with each input individually earthed (no common earths to be used) as this ticks the minimising ground loop issues.

Quality volume pots, one per channel. This remove the need for an extra components (balance control) in the signal path and also means that one can precisely tailor balance.

Captive outputs...no RCAs needed as the pre will be designed to sit under the power amp so I will hard wire the output wiring.

Volume pots selected are the excellent Tocos Cosmos pots. I have selected 50K pots which may be marginal with the STA's 100K input at 1/4 value (ie -6dB) but should, in reality, be ok. This will provide sufficient load for the source...I hope! The coupling capacitance of the source is large enough to pull down any high pass issues to well below audible threshold so bass shouldn't be compromised.

Selector is a cheap and cheerful one for now, just to test the control unit. If it proves a hit, it will be upgraded to something better quality. I will be using make before break switches to avoid any unwanted pops and crackles.

In the event that a different pot value is needed, I'll try it, and in the event that doesn't work, I'll revert to a buffer design for that particular source. Suck it and see time.

Photos to follow.

I completely agree with you on TVC's.

Personally I would make pots 10K if your minimum driving impedance is 600R

[Reffc]

I completely agree with you on TVC's.

Personally I would make pots 10K if your minimum driving impedance is 600R

Ideally, I would like to lower the pot value for 100K input on the STA25, but as it sees 600R in parallel with about 50K/4 (-6dB), then it should work ok, and 50K gives the source a fighting chance of driving into the higher impedance pot. 10K I would have thought a little on the low side?

[danilo]

One man's solution :
http://www.zenmod.in.rs/amplifying/iron-pre/
Use a single pot per channel as balance control ?

[Reffc]

One man's solution :
http://www.zenmod.in.rs/amplifying/iron-pre/
Use a single pot per channel as balance control ?

I've already designed and built an op-amp based buffer. It's similar to the one in the link and currently out on loan.

I prefer twin volume pots as a balance control. A stereo volume pot has no less in the signal path and a balance control more. My current amp (Emille) uses twin volume pots and I quite like that set up.

[Arkless Electronics]

Ideally, I would like to lower the pot value for 100K input on the STA25, but as it sees 600R in parallel with about 50K/4 (-6dB), then it should work ok, and 50K gives the source a fighting chance of driving into the higher impedance pot. 10K I would have thought a little on the low side?

You mean raise the value? You don't need to match anything to the STA25 BTW. If as you say your highest source impedance is 600R then 10K is no problem... provided the source can erm.. source the required current to do it without incurring distortion. Virtually all SS sources will have no problem at all as will most valve stages which will usually have a cathode follower at the output... providing it's not a weedy cathode follower and a high signal level which requires a lot of attenuation. ie volume right down!

The only real problem would be with valve sources which don't use any cathode follower and therefore have a high output impedance. This will usually be vintage gear but there are a few modern examples around... these ideally need a 220K/470K region pot for best results but obviously in this rare case a 10K pot would be even worse than a 50K pot.

The STA will see the 12.5K worst case output impedance of the 50K pot as effectively in series and not parallel with the 600R source impedance BTW. The 600R is the source impedance of a generator remember and not just a passive resistor doing nothing. Think of it as the internal resistance of a battery The output impedance of the source forms a voltage divider with the 50K of the pot so, if the source were 50K then the max output signal is halved and the worst case output impedance from the pot is now 25K, and at full volume rather than at half volume

[Arkless Electronics]

I've already designed and built an op-amp based buffer. It's similar to the one in the link and currently out on loan.

I prefer twin volume pots as a balance control. A stereo volume pot has no less in the signal path and a balance control more. My current amp (Emille) uses twin volume pots and I quite like that set up.

You've posted this whilst I was posting mine.... Op amp buffer? I thought you were making a passive for use with an STA25 Some wires crossed

[Reffc]

You've posted this whilst I was posting mine.... Op amp buffer? I thought you were making a passive for use with an STA25 Some wires crossed

The buffer was designed and built a year or two ago Jez for another project and I don't want/need to use it...it's just an option for worst case.

Yes, I agree with what you've written above. Soz, yes, it will be in series and not parallel with as you say so should be fine. The worst case source is a valve source with cathode follower/capacitor coupled but the cap value is high enough not to present any problems whatsoever.

I'm worrying over nothing and just sharing thoughts out loud which is dangerous. I'll learn one of these days...