Kofi Annan in: "Cascodin' with Steve Bench's RIAA!"

[Salas]

185/((120/47)+1)=52v OK. Why you wanna float? Having some residual hum? You must disconnect the heater voltage from earth (if it was earthed) before referencing it to a floating voltage.

 

[sonata149]

Hi Salas,

Punctual as ever - Thanks!

No, I can't hear any hum. I was just thinking that since the first (input) valve (6dj8) is configured like an SRPP (although at the cathode, not as anode load) it might benefit from a floated filament supply. I'm no circuit designer or anything of the sort.

I am so pleased with the sound of the SB phono and your HV Shunt combination that I'm redoing the whole project - i.e. mounting it in a much better looking case (although this time the regulator and PS will be mounted in same case as the phono stage -- I hope this will not degrade the sound!). My first attempt was more of an experiment and amateurish in looks. The most important changes will be a heftier transformer (toroid) and feeding the heaters with constant current LM317s as per your circuit on the first page of this thread.

You seem you don't favour floated filaments (in this case) and the fact that I don't suffer any hum without it, confirms your opinion. Your suggestions and comments are greatly appreciated.

Regards,
JA

 

[Salas]

Watch the transformer's hum field. Put it perpendicular to the phono build, and most away in a largish box. Also the mains inlet as far apart as possible from the RCAs. You can dress the toroid periphery with a grounded copper band. The secondary wire exits are the most offending for field. It can be rotated to most silent if some will be picked up. Coax for in/out is mandatory. Will be happy to see photos when you finish.

P.S. No news from Kofi, I would like he could have finished it too, since he started the thread. At least he will be encouraged by your results when he will continue it.

 

[sonata149]

Thanks again, Salas, for your build suggestions.

So you don't recommend floating the heaters.

I will post photos of my preamp-HV shunt rebuild - if it's successful!

Yes, it's a pity Kofi has "disappeared" (hopefully temporarily) from this thread. He had progressed by a large margin through this project. We'd love to see him back - quite a character!

 

[Sheldon]

I think that floating the heaters is a good idea. I did it on mine (via Morgan Jone's "thingy"). I didn't check the schema, but what you have described should work fine. Try it connected and not, and see what you think.

Sheldon

 

[sonata149]

Hello Sheldon. You too have been a great source of inspiration on this thread. What you say is true and worth trying.

You see, being un-technical and untrained in electronics (with only A/DVM for instruments) I tend to take a project in rather absolute "dos and don'ts"... But it's a positive learning experience all the time!!

Thanks for your input and regards,
Joe A

 

[Sheldon]

I tend to take a project in rather absolute "dos and don'ts"... But it's a positive learning experience all the time!!

Thanks for the nice words. Maybe the best thing about DIY is not only get the satisfaction of building yourself, but to try a few things along the way. I have a bad habit of making everything compact, which makes it more difficult to access. Give yourself a little room for mods.

Sheldon

 

[Kofi Annan]

Kofi Returns!

Hey! Sorry for the absence but there were a few things going on here that needed some attention.

UPDATE:

I finished the phono-pre today and it sounds magnificent! Its still early, but I can certainly hear a deeper soundstage and more detail than before.

Thanks so much for all the help, Salas, Sheldon, et. al.!

Pictures to follow.

Kofi

 

[Salas]

Wow! Kofi is back with a vengence! And with a fully working phono + shunt! Congrats! Hungry for details on how you solved issues in the original Kofi way please. And pics of course.

 

[Kofi Annan]

How I Solved My Problems
by Kofi Annan

Problems:

1. Outrageous hum

2. ZZZZZZZZZIP! on power down

Solutions:

1. Apparently? That wire that hangs off the back of my test turntable is like some kind of ground wire thingee or whatevs? So I grounded it? And all the hum went away? And I was like, "Wow, Kofi. You are a dumbass"? And then I said "I like totally agree"?

LOL

2. Turn off power amp before phono pre / ignore sound.

--TOTAL SUCCESS--

Kofi

 

[Salas]

How I Solved My Problems
by Kofi Annan

--TOTAL SUCCESS--

Kofi

Pics?

 

[Kofi Annan]

Working on it. It's not a masterpiece of aesthetics or anything but it matches my 300b monoblocks.

Pics soon.

Kofi

 

[manamanam]

How I Solved My Problems
by Kofi Annan

Problems:

1. Outrageous hum

2. ZZZZZZZZZIP! on power down

Solutions:

1. Apparently? That wire that hangs off the back of my test turntable is like some kind of ground wire thingee or whatevs? So I grounded it? And all the hum went away? And I was like, "Wow, Kofi. You are a dumbass"? And then I said "I like totally agree"?


LOL

2. Turn off power amp before phono pre / ignore sound.

--TOTAL SUCCESS--

Kofi

About this ZZZZZIP sound.

It seems to be some kind of oscillation when powering down. I have noticed somthing similar when playing around with heater power supply schemes.

Are your heaters somehow referenced to the ground or B+ or just floating?

 

[sonata149]

LPs sound bass-shy from Bench's RIAA compared to CD

I have enjoyed listening to Steve Bench's RIAA project with Salas regulator but I have lately become conscious that compared to CD, bass from disk (LPs) is way too shy. I was thinking maybe I have an impedance matching problem between the various components. I've been trying to find the output impedance of this preamp in this thread but no one seems to have actually stated it.

The preamp (fed by an MM cart.) goes through a C-J DF1 CD/linestage combo which has an input impedance of 50K and an o/p imp. of 200 ohms. The i/p imp. of my power amp is 50K -- so no problem there, no?. Am I correct in suspecting that the o/p impedance of the SB riaa pre-amp may be too high for the 50K input imp. of my linestage? What is the i/p impedance of the Bench riaa preamp?

Thanks to all who have contributed to this thread, especially Salas, Sheldon and of course Kofi whose questions are always relevant to newbies like me.

Joe A

 

[Salas]

See about C6, C7 (0.15uF) if they are enough for driving your line amp's impedance to LF. Try 2.2uF. SB derives output from V1 and V3 second half anodes, i.e. not buffered, which are 20K loaded. Since R5, R21 are not bypassed, I would roughly say 10K Zo for 6DJ8.

 

[sonata149]

See about C6, C7 (0.15uF) if they are enough for driving your line amp's impedance to LF. Try 2.2uF. SB derives output from V1 and V3 second half anodes, i.e. not buffered, which are 20K loaded. Since R5, R21 are not bypassed, I would roughly say 10K Zo for 6DJ8.

Thanks Salas. I'll try upping the o/p coupling caps (?) with what I have first. Will a 1uF cap (instead of 2.2) make a noticeable difference?

If you don't mind explaining to a non-tech person, in your concise but clear way: How did you arrive at the Zo at half the anode load? Is that the rule of thumb to calculate Zo for an unbypassed cathode resistor (R5, R21)? Or does it have to do with the valve's internal resistance figure? And what is the other r.o.t. (rule of thumb) for a bypassed cathode r? This is an area which isn't clearly explained in textbooks (without a lot of maths).

At the present Zo of 10K, is that too high a match for the Zin of the line pre-? What is the minimum 'happy ratio' for a quality transfer between Zout / Zin?

Thanks a lot...

 

[Salas]

1uF is kinda OK for 50K and cutting rumble on phono, still night and day VS 0.15uF to 50K, so listen to that change by all means.

Rp is in parallel to Rload. It would be 2.6k with bypassed Rk if we assume 3K Rp for 6DJ8. But the 562R local feedback ups that Zo roughly x3.5 without bypass. SB had calculated his passive HF Riaa and gain, noise, THD, as it is, so to bypass it would take partial redesign and possible worse areas of performance.

Engineers say keep a 10:1 Load:Zo for surely happy drive, they call it bridging impedance. 5:1 will lose maximum voltage transfer.

 

[Sheldon]

I have enjoyed listening to Steve Bench's RIAA project with Salas regulator but I have lately become conscious that compared to CD, bass from disk (LPs) is way too shy. I was thinking maybe I have an impedance matching problem between the various components. I've been trying to find the output impedance of this preamp in this thread but no one seems to have actually stated it.

The preamp (fed by an MM cart.) goes through a C-J DF1 CD/linestage combo which has an input impedance of 50K and an o/p imp. of 200 ohms. The i/p imp. of my power amp is 50K -- so no problem there, no?. Am I correct in suspecting that the o/p impedance of the SB riaa pre-amp may be too high for the 50K input imp. of my linestage? What is the i/p impedance of the Bench riaa preamp?

Thanks to all who have contributed to this thread, especially Salas, Sheldon and of course Kofi whose questions are always relevant to newbies like me.

Joe A

The output impedance is the plate resistor in parallel with the anode resistance. Anode resistance is the plate resistance (about 4.5k) plus the cathode resistance. Cathode resistance for an unbypassed resistor is the cathode resistor multiplied by u+1 (about 33), so 562x33 = 18.5k. So anode resistance is 4.5k+18.5k, or 23k. Therefore, output impedance is 20k//23k, or 10.7k.

With input impedance of the next stage at 50k, you will have higher distortion, and some loss of gain. You also need to scale the output cap, or you will have roll off in the bass - as you have noticed. I'd add a buffer stage (or mu coupled CCS anode load), and use a 0.47u (or higher coupling cap).

The input impedance of the Bench amp is set by the input grid leak resistor.

Sheldon

 

[Sheldon]

Keep in mind too, that the input impedance of the following component will affect the high frequency roll off of the RIAA curve. The impedance of the following component is in parallel with the output impedance of the second stage of the RIAA amp for calculating the second stage RIAA cap, as the cap is not buffered from the following stage. So, as the impedance of your preamp drops, the hf output of the phono amp will be tipped up. By my calculation, the second stage RIAA cap is a little too small already. If you increase the load by lowering the input impedance of the following preamp, it is even more undersized.

Sheldon

 

[Salas]

6800pF C1,C9 worked good into a 50K Log pot input 12B4 single stage or 6SN7 cascade pre if of any help.