New Pre amp thingy

[colin.hepburn]

Hi All
Ok so here is the new Buffer Pre I had it up and running last night just with a single input for testing running it from the WDPSU3s 200-volt line and it sounds very good a little darker perhaps and has more depth and wider sound staging than the WAD CF Pre.

I am going to keep running it for a time just to see if I will stick with it then I may put it in the CF Pre Case with a new relay board from Mathew

The good thing is as I had most of the parts to build it to hand it has only cost me less than a tenner to date for the PCB which uses three valves two 6922/12au7 I am using two Russian 6H1-EB and a JJ12A7
Take a look at the Schematic and give you opinion on it images here

https://ibb.co/F0RFqNv
https://ibb.co/Bff7D4r

[bikerhifinut]

I'm assuming that the tails of the cathode resistors are connected to earth/HT -ve? Otherwise, that circuit hasn't a hope in hell of working as its not a completed circuit as far as the valves are concerned.
Otherwise, it's very strange.
You've got a negative feedback loop from the output to the grid of the ECC82 input valve, and also the ECC82 has got degenerative local feedback due to having no bypass capacitor on the cathode resistors(s), which strikes me as a bit OTT and I haven't worked out the feedback ratio but it's likely the first stage could offer no gain whatsoever. So what's the point of it?
I see the first stage is a bootstrapped cathode stage which you see often on cathode follower stages, which gives a huge increase in input impedance, in which case if I got it right, that 0.47uF capacitor on the input is more than a bit too large in value.
Then it feeds a sort of White cathode follower with an active load, where the bottom valve is being modulated by the output from the top valve, much like the aikido design of J broskie. The claimed advantage is an extremely low output resistance.
I am sure it works Colin assuming that the actual build has a return to Ht-, but frankly it looks weird to me.
It looks like the designers decided to use 3 active devices where one would do.
Is it a buffer? This in my book means a unity gain stage with high input Z and a low output Z, preferably using a high current capable device so it will drive a long high capacitance cable.
Or is it meant to have a bit of gain? If the ECC82 is biased by the 5.6k unbypassed resistor it won't have a right lot of gain anyway and the negative feedback will likely wipe most of that out.
I will stick with my Aikido CF I think.
For the record, the new WD CF with an optional active load will no doubt do everything that circuit does.
In my book, a cathode follower should neither add, nor take away anything from the original signal.

At the end of the day if it does the trick for you Colin then that's what matters.

Andy.

[bob orbell]

To add my bit it does look very strange, as Andy has said, the HT appears to go nowhere and the Aikido CF has excellent PSRR, that makes an inky black silence, I think the CF Pre from Matthew would also be a good choice. Keep us posted on this strange circuit Colin. Bob

[A Stuart]

Looks like they omitted to draw an earth return. Hopefully that omission was not translated by computer into how the board was built.

Colin - Where did the design come from?

Alastair

[colin.hepburn]

Hi All
Well, I did ask for your views on this thing I know nothing about it other than it came from china off eBay. I was just looking for some sort of cheap project to play about with over a year ago now. I build it up and it just sat on the bench until the weekend there.So just gave it a tryout


Andy went in to it a bit but over my head thou when it comes to explain circuit’s maybe you could come back on it with a more laymen’s terms on it doing
I did note that it pulls down the 206volts from the PSU3 pre ht line to 174 volts that’s all I have checked on it so far it is intended to run on an HT. between 180/240volts

Here’s a cleaned copy of the circuit with some voltages on it with the 180v Ht version
Colin
https://ibb.co/yp8kjkg

[pre65]

Colin, can you give me a link to the Ebay supplier please ?

[colin.hepburn]

Originally Posted by pre65

Colin, can you give me a link to the Ebay supplier please ?

Hi Philip
Heres the link
https://www.ebay.co.uk/itm/1pc-Gold-...72.m2749.l2649

[A Stuart]

That one (post #5) shows the missing earth connection.

[bob orbell]

Colin, the WD PSU III, is designed to provide about 5mA to phono and about 15mA to the pre, your new pre thingy, is obviously consuming more, you will need to reduce the value of R4, if you have taken the supply from pre output, the PSU will supply about 50mA. Instead of loosing power to resistors, regulated power supply's are supreme, that is why I have gone that route. Bob

[bob orbell]

Colin, another problem may be the two Russian 6H1 tubes, I think these consume a lot more current both HT and heater, (double the heater current of a 6DJ8) 6DJ8 / 6922 would been much less power consuming. Bob