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Old 14th December 2008, 02:04 PM   #1
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Default Kofi Annan in: "Cascodin' with Steve Bench's RIAA!"

So...

Kofi is putting together an analogue package for a friend and is considering building a tube phono preamp. I use Thorsten's El Cheapo right now and I think it sounds great, but I'm looking to make a different one for comparison's sake and I've read some good things about Steve Bench's RIAA.

Anyone else have an opinion on this RIAA stage they'd care to share? Any power supplies (regulated or just well filtered) that I should consider? Any other phono stages worth considering?

Kofi?
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Old 14th December 2008, 02:11 PM   #2
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It's very good...just be sure to supply it with a very well filtered B+. The input cascode has a very low PSRR.
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Old 14th December 2008, 02:27 PM   #3
SigFire is offline SigFire  Germany
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Hi Kofi Anan, ....
.....if I had the time I'd built that one by Allen Wright:

http://www.vacuumstate.com/index.tpl...755.5849974914

IIRC Allen Wright refers to the Loesch Pre somewhere in his PreAmp Cookbook.

(Uppermost schem, left part ;-) )

Hope this helped

Greez

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Old 14th December 2008, 03:21 PM   #4
MRupp is offline MRupp  Germany
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Two different Loeschs and hence two different amps
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Old 14th December 2008, 04:10 PM   #5
SigFire is offline SigFire  Germany
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.......ooops, sorry indeed for not takin' this into account while postin' "quick'n dirty". I knew that there where 2 Loeschs.
Referin' to A.W.'s book I seemed to have overlooked the expression "legendary" before the name Loesch. ;-)
(It's been quite a long time since I had time to look through it.)

Greez

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Old 14th December 2008, 04:29 PM   #6
Sheldon is offline Sheldon  United States
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Here's my take on Steve's phono amp. In the final version, I removed the bypass resistors for the LED. http://www.diyaudio.com/forums/showt...19#post1083419

The supply, as shown in the next post works well, but more complicated than necessary. The Miada regulator part is mostly there to protect the current regulator from over voltage if there is no current draw in event of a fault down the line. Also, I wanted to make one for the experience. If you design the supply front end so that the voltage with no load other than the bleeder resistor, doesn't exceed the rating of the current regulator, you can omit it. You can also probably omit the Morgan Jones filament lift with DC filaments, or sub in a simpler one. But the MJ version is pretty straightforward and uses cheap parts.

John Broskie also has some nice shunt regulators, based on Mosfets; http://www.tubecad.com/2007/07/blog0115.htm, and even some kits.

If you look carefully, you will see that Allen Wright's phono amp is actually pretty similar to Steve's amp. He puts his super linear cathode follower on the back end, I used the follower from Broskie's Aikido, and used his noise canceling scheme. If your next stage has an input impedance of 100k or more, you can omit the follower (may have to tweak the final RIAA cap some). In the Wright amp, use a 50K load, and you can omit the follower. If no follower is used, you either have to use short interconnects (their capacitance is in parallel with the final RIAA cap), or tweak the final cap with your interconnects in place.

Sheldon

edit: I see that Salas just posted a shunt reg., and for Steve's amp, of all things: http://www.diyaudio.com/forums/showt...67#post1685067
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Old 14th December 2008, 04:36 PM   #7
Salas is offline Salas  Greece
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Default Re: Kofi Annan in: "Cascodin' with Steve Bench's RIAA!"

Quote:
Originally posted by Kofi Annan
So...

Kofi is putting together an analogue package for a friend and is considering building a tube phono preamp. I use Thorsten's El Cheapo right now and I think it sounds great, but I'm looking to make a different one for comparison's sake and I've read some good things about Steve Bench's RIAA.

Anyone else have an opinion on this RIAA stage they'd care to share? Any power supplies (regulated or just well filtered) that I should consider? Any other phono stages worth considering?

Kofi?
Steve Bench's is the best you can make. Have made Loesch's grid leak cascade one you linked as well. Comes second. You may need to change C1 from 5n7 to 6n8 for the Bench if you make it. It measured and sounded tiped up in my case with the original value.
It will only sound good with an active regulator. Mosfet Maida does a good job. But my recent developement for the Steve Bench PSU may interest you. Upgrades it strongly.

See link.
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Old 16th December 2008, 01:44 AM   #8
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Alright! Thanks for all the responses!

I think this is a winner. I'm thinking I'll CLC before the regulated schema and I'm guessing I won't have to twiddle much.

Salas-- it looks like you've got one supply per channel if I read your wiring correctly. The Bench RIAA only draws aobut 10mA for both channels. Did you experience any issues with this?

I understand that some regulators require a minimum current to function properly. I don't think this one is subject to that, but I just wanted to check.

Thanks.
Kofi
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Old 16th December 2008, 02:06 AM   #9
Salas is offline Salas  Greece
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No issues Kofi. Common high voltage shunt supply actually. That supply has constant current, no dependence on load. There is a separate heater transformer (toroid) on that photo, and gives juice to 2 heater regulators. One 12.6 one 6.3. The cascode sounded better when the heater supplies of lower and top tubes got totally separate and different for voltage. Of course that is something not easily guessed. You were right to assume that maybe I used more HV circuitry.
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Old 16th December 2008, 04:28 PM   #10
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Thanks! I should have noticed that it was single channel...

One more-- in your post, you mentioned:

Quote:
In this case a 230V tension is applied at the input, 60mA constant current, and 20mA idle load at 190Vreg by the Steve Bench cascode RIAA for 2 channels.
I'm guessing this means that the PSU will have to supply about 80mA of current for both channels including the rectifier. I just wanted to confirm as I was going to run a CLC in the front end and needed to know approximate current draw.

Since we're dealing with CLC and no resistors in the supply before the regulator, I'm thinking the current draw estimation need not be exact. I'm just looking to supply about 230V in, per your post, and I should be fine.

I think this means that I could get away with about a 180V mains transformer with solid state rectification (180 x 1.414 = 254V less approx. 10 - 20V for L resistance) and get the recommended 230V.

Sound right?
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