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:scratch: Some wires crossed:lol:

[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:scratch: Some wires crossed:lol:

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...

[Reffc]

Progress:

Chassis kit arrived and assembled. Need to form a few more holes for selector and input RCAs. Components strewn in a neat and orderly pile (not):

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1181_zpsoml7xu3z.jpg

Additional holes drilled and deburred, and starting to fit RCAs:

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1182_zpsa29qsva4.jpg

Front selector switch hole formed:

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1183_zpsggc44rmz.jpg

Bits bucket!

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1184_zps4oo15tek.jpg

RCA's all fitted. One of my irrational pet hates is seeing input RCA sockets with randomly aligned nuts. OCD made me align these (scratch was already present on case which was unfortunate):

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1185_zpsc5shnd48.jpg

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1186_zpswttsm6qt.jpg

Switches fitted to front:

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1188_zpsfck23qgo.jpg

Starting to wire up inputs:

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1189_zpszwtmot3z.jpg

View of internals so far. Wiring is a mix of Klotz OFC screened mic cables and Furutech low capacitance shielded coax:

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1190_zpsub2qgcx7.jpg



http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1191_zpsvdrumgs9.jpg



http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1195_zpsxodjok18.jpg


That's me done for the evening. Will finish it off tomorrow. Just one more input to wire up.

[Barry]

Looks like the case and knobs are worth more than the components! :D

I tried a "10K pot in a box" years ago - despite the elegance and the Mies van der Rohre's maxim of "the less is more", I prefer to use an active preamp regardless of source.

[Reffc]

The input sensitivity of the power amp means that no gain is needed Barry, and many integrated valve amps today use just a passive control section. I'll suck it and see. The case was sized on the bases of converting it to active if I don't like the end result Barry, hence the IEC socket which I've fitted "just in case". My Lumley ST40 used a passive control unit and that was one of the best sounding valve amps I've heard, so have high hopes for this. Cost wise, it isn't as much as you might imagine. The case was selected as it's strong enough to support the STA25 which will sit atop. In fact the width is almost identical so it should look quite neat when set up. Still waiting for the arrival of the '25 but I know that Will from Radford's on the case ;)

[337alant]

Paul
They are a very nice case and I am using one in my own pre amp at the moment :)
I used a passive pre for a few years and had the best results with 2 x 10k sfernice pots and an elma switch. I now use a goldpoint stepped attenuator in place of a Pot
I also found it beneficial to keep all cableing as short as possible between source and power amp so you may wish to consider using pot and switch extentions to keep the internal cabling down to a minimum

Alan

[Reffc]

Paul
They are a very nice case and I am using one in my own pre amp at the moment :)
I used a passive pre for a few years and had the best results with 2 x 10k sfernice pots and an elma switch. I now use a goldpoint stepped attenuator in place of a Pot
I also found it beneficial to keep all cableing as short as possible between source and power amp so you may wish to consider using pot and switch extentions to keep the internal cabling down to a minimum

Alan

Interesting Alan.

Yes, the cases are pretty nice and everything goes together well.

10K pots won't work for me. I have spoken with the designer of my MC preamp and he's advised 20 to 50K pots. I trust his judgement implicitly. I have 50K Cosmos pots (very good pots indeed) which possibly better Blue Alps, the standard reference for good vfm pots, so will be using those to start with. Stepped attenuators offer more cost for no real benefit at present. I want to experiment with the design using good vfm components first to confirm values. Once I have it right, I will invest in a ladder-type 47 step attenuators and a better quality input switch. The additional length of cables (all 15 to 20cm) will make next to no difference over an extender kit with my sources. The Radford's high-ish input impedance makes for a relatively easy load. I have kept output cables from volume pots short. Pot to power amp is about 0.75m (ie case to amp is about half a metre and I'll trim back further to make them as short as needed once the power amp arrives). It'll be fine.

[Sovereign]

Looks great Paul.
My dual mono LDR is quite fantastic, it was heavily modified by Chris Daly where he used my pre as a prototype, it came back sounding a lot more full bodied and three dimensional. I built an over-specked PSU for it.
I have tried against a few actives and I keep going back to the LDR.

[Reffc]

Thanks James. Yes, trying to keep things simple. Heard some good things about the LDR kits but haven't seen any measurement for things like S/N or comparisons with a good stepped attenuator. I tend to favour a quality ladder type stepped attenuator solution for a passive control unit (funds permitting!). It's just a bit of fun at the minute and I have an active to compare it against.

[spendorman]

I like the STA25 III, can't be far short of 40 years that I've had mine, been utterly reliable. It will be interesting to hear what you think of it.

I have the matching SC22 and FMT2 tuner.

Here is the STA25, temporarily driven by the pre out of this NAD.

https://c7.staticflickr.com/6/5230/5687938702_b4e5b621d5_z.jpg (https://flic.kr/p/9ECb1E)NADford STA25 III (https://flic.kr/p/9ECb1E) by A60man (https://www.flickr.com/photos/41679262@N02/), on Flickr

[Reffc]

I like the STA25 III, can't be far short of 40 years that I've had mine, been utterly reliable. It will be interesting to hear what you think of it.

I have the matching SC22 and FMT2 tuner.

Here is the STA25, temporarily driven by the pre out of this NAD.

https://c7.staticflickr.com/6/5230/5687938702_b4e5b621d5_z.jpg (https://flic.kr/p/9ECb1E)NADford STA25 III (https://flic.kr/p/9ECb1E) by A60man (https://www.flickr.com/photos/41679262@N02/), on Flickr

I like the MkIII Alex and reviewed one of Radford Revival's restored MkIIIs a few years ago, so had one here in the system for a week or two driven from a Lumley PP40 passive pre. They worked very well together. Personally, I'd have the switches and speaker terminals facing the rear as it looks tidier but many seem to prefer having them facing front as in your photo.

[spendorman]

Is your review available? Link? I would be interested in seeing it.

My stuff is usually in a state of flux! Moving stuff around all the time, so having connection facing me is convenient!

Yesterday, I changed amps, feeding the Rogers BBC LS3/6 now is the cheap TPA3116 amp, not at all bad.

[Arkless Electronics]

Looks like the case and knobs are worth more than the components! :D

I tried a "10K pot in a box" years ago - despite the elegance and the Mies van der Rohre's maxim of "the less is more", I prefer to use an active preamp regardless of source.

Exact opposite here!

[Reffc]

Completed, all bar the lettering which I'll sort in due course.

I've also added a chassis earth and ganged it up with the pot switch earth to be taken back to the main amp's earth.

Photos of (almost) completed control unit. Won't get a chance to try it out for a while so it'll sit around for a while no doubt.

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1198_zps8utebyfk.jpg

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1199_zpsxxzicvyg.jpg


http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1200_zps0myzpd6n.jpg

Front, minus decals/lettering (to follow):

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1203_zpsjjxxyshr.jpg


http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1209_zpsuhrka4ez.jpg


Rear view: IEC is there but left unconnected whilst I use this as a passive control unit.

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1213_zpsgcse6dgy.jpg

Top:

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1214_zpsnbc6aig9.jpg

http://i1140.photobucket.com/albums/n577/24pacman/Hifi/IMG_1216_zpsu54p4rr5.jpg

[JohnJo]

Very nice, tidy job Paul.

That's a high end looking case!

[spendorman]

I been going on about the Radford, should have said what a nice job you have done of the passive unit.

What's the IEC power socket for?

[struth]

to fool the uneducted hoi paloi ;)

[Reffc]

I been going on about the Radford, should have said what a nice job you have done of the passive unit.

What's the IEC power socket for?

The Radford is the real star in this story ;)

Thanks Paul for your kind comments anyway. IEC came with the case kit, so it was fitted 1) to blank the hole in the chassis and ii) as in time, it may become an active project but for now, it's intended as control unit for an STA25.

[Reffc]

Very nice, tidy job Paul.

That's a high end looking case!

Thanks John. The case is a nicely made thing although it needed a fair few more openings and personally, I'd have a slightly different arrangement at the rear. I selected that case pertly as it has to support the power amp which is 17 or 18Kg (still only about half the weight of the Emille!)

[Barry]

to fool the uneducted hoi paloi ;)

It's hoi poloi (to the educated ;))!

[tubehunter]

Yes a nice case

But I wouldn't be running the left and right channels together in the same cable.
I've experienced channel corruption when I've done so.

Duncan

[Radford Revival]

Great job Paul, also, exquisite choice in amplification I must say ;)

[struth]

It's hoi poloi (to the educated ;))!

This my point

Ps nice job Paul

[JohnJo]

Just realised this would be a nice visual match for the JE Audio monoblocks I had here recently.
http://i1229.photobucket.com/albums/ee462/castle48/85CF33ED-DB77-4165-BEE5-5037A50B5D52.jpg (http://s1229.photobucket.com/user/castle48/media/85CF33ED-DB77-4165-BEE5-5037A50B5D52.jpg.html)

[PaulStewart]

to fool the uneducted hoi paloi ;)


It's hoi poloi (to the educated ;))!

Actually to the educated, it's Hoi Polloi and it's not the Hoi Polloi as that would translate as the the many. Hoi is the definitive article in Greek. :) Try and keep up at the back :lol:

[Barry]

οἱ πολλοί - τοῦς πόλις

(Et in arcadia ego.)

[Macca]

Caeser adsum jam forte

[Barry]

Haha

Caesar adsum jam forte
Brutus aderat
Caesar sic in omnibus
Brutus sic in at

[struth]

Caesar had some jam for tea, Brutus ate a rat. Caesar was sick on the bus, Brutus vomited into his hat.

[Barry]

10/10 for pronounciation, now provide a correct translation.

[struth]

And write it out 100 times,... Or i will cut your balls off! John Cleese as a Centurian;)

[Macca]

'People called the Romans, they go to their homes?''

That scene is exactly how they taught Latin at my school.

[Reffc]

I appear to have stumbled into a thread for senes linguae studentium intendit ad malum :ner:

My mistake was that I thought this one was about DIY control units :doh:

That'll teach me to offer a contribution ;).

[Macca]

Mea culpa.

[Arkless Electronics]

οἱ πολλοί - τοῦς πόλις

(Et in arcadia ego.)

"I am in paradise" ?:scratch:

[walpurgis]

Mea culpa.

Me 'avin a cuppa :)

[Barry]

"I am in paradise" ?:scratch:

Sorry, I should have written: "Et in Academia ego".

[montesquieu]

Mea culpa.


Me a cowboy
Me a cowboy
Me a Mexican cowboy.

[walpurgis]

Hmm. Seems a drop of the old Glenlivet has got the better of Tom. :lolsign:

[montesquieu]

Hmm. Seems a drop of the old Glenlivet has got the better of Tom. :lolsign:

There's only one thing I can add to that and it's:

Strawberry Jampots.

(I expect Paul to get the first one .. he'll probably get this one as well but who else?)

And back on topic - nice job on the Passive, Paul :)

[Reffc]

There's only one thing I can add to that and it's:

Strawberry Jampots.

(I expect Paul to get the first one .. he'll probably get this one as well but who else?)

And back on topic - nice job on the Passive, Paul :)


I do get the first one Tom...very astute of you to have picked up what was recently said by someone so influential (!)

[montesquieu]

Strawberry Jampots - mnemonic for iambic pentameter - which often goes as a couplet with a line of dactylic hexameter .... from your Virgil and Homer, Ovid etc ... many hours of 'fun' in the classroom. (As beautifully parodied in the Billy Bunter books)

[spendorman]

My school was so good that it was approved, no chance of learning Latin, or much else for that matter.

[struth]

Head of school was a mr Fagan :D

[spendorman]

History teacher was Mrs Crump, hit students with a steel rule on the back of the head for no reason at all. I failed CSE History, and Geography too.

[PaulStewart]

"I am in paradise" ?:scratch:

Et in arcadia ego, is literally Even in Arcadia, there am I. As to meaning it was inscribed on a tomb in a Poussinpainting called the Arcadian Shepards. The meaning is thought to be Even in paradise, there is death.

[Reffc]

Some improvements to the prototype now made. Extension kit fitted, signal paths shortened, lettering added:

http://i1140.photobucket.com/albums/n577/24pacman/RFC%20Cables/IMG_1277_zpsdl9fh5hr.jpg


http://i1140.photobucket.com/albums/n577/24pacman/RFC%20Cables/IMG_1280_zpspnklztlt.jpg


http://i1140.photobucket.com/albums/n577/24pacman/RFC%20Cables/IMG_1281_zpsehovsqam.jpg


http://i1140.photobucket.com/albums/n577/24pacman/RFC%20Cables/IMG_1285_zpsnrq5maeo.jpg

[Macca]

Looks a million dollars Paul. Now how does it sound?

[brian2957]

Very nice Paul :D