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Old 29th August 2006, 11:28 PM   #1
Sheldon is offline Sheldon  United States
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Default RIAA Phono Amp Design

After looking at a variety of phono stages here and around the net, I decided to give this one a shot - at least as an idea. The basic schematic is this one from Steve Bench's site: http://members.aol.com/sbench101/Preamps/RIAA5.gif

I picked it partly because I've done another of his amp designs and was pleased with it, and partly because it's a little more than a basic circuit and will stretch me some. I'll be using it with an MC cartridge, so will put a step-up trans on the front end. It's going into a DEQX that has an input impedence of 10K, so I added a "Broskie" follower with noise cancellation. I changed the resistors a bit on the second ECC88 to divide the PS voltage in half at the plate. I also added PS noise compensation on the cascode stage.
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Old 29th August 2006, 11:32 PM   #2
Sheldon is offline Sheldon  United States
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One problem I can see with the above, is that the 75us compensation will interfere with the PS noise cancellation at higher frequencies, even becoming additive at some point. So as an alternative I changed the circuit to an all in one pass type
RIAA network. Comments, criticisms highly encouraged.

The component values are not vetted, as I haven't included the effect of the coupling cap.

Sheldon
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Old 29th August 2006, 11:52 PM   #3
Sheldon is offline Sheldon  United States
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Another possible approach to the issue of the 75us compensation interfering with noise cancellation is to compensate the cancellation network in like fashion. This feels like shakey ground to me. No?

Sheldon
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Old 30th August 2006, 12:03 AM   #4
Sheldon is offline Sheldon  United States
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oops, error in the cancellation circuit. these are the corrected versions.
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Old 30th August 2006, 12:05 AM   #5
Sheldon is offline Sheldon  United States
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and
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Old 30th August 2006, 02:05 PM   #6
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Sheldon,

Why not just use a single 6DJ8 in the lower part of the first stage? Fully bypass the cathode resistor for lowest noise.
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Old 30th August 2006, 03:15 PM   #7
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Brian Beck
Sheldon,

Why not just use a single 6DJ8 in the lower part of the first stage? Fully bypass the cathode resistor for lowest noise.
Thanks Brian,

Seems like that would work fine. Steve showed several different phono amps, including one with a 6DJ8 for the input. He claimed lower noise for the 6072. However, I don't think he did the direct comparison of the single 6DJ8 vs. the above schematic. The unbypassed part of the cathode resistor adds back some noise, so maybe it's a wash. He liked the sound of this one. Maybe some complementarity?

Sheldon
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Old 30th August 2006, 03:41 PM   #8
Nordic is offline Nordic  South Africa
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I have a very simple single 6dj8 preamp stage for my chipamp...its basicaly the valve and couple of resitors (apart from the powersupply) it has ZERO noise...
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Old 30th August 2006, 03:50 PM   #9
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Can't add much other than this is the phono stage I built, and it is very good. I built it on a blank PCB, using the copper as ground plane and dropping the grounding points wherever the component happened to be. If you're worried about noise from the cascode(low PSRR), search Broskie's site for noise cancellation in cascodes....I've never tried his techniques.

You will need a quiet PSU for the stock Bench design.
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Old 30th August 2006, 03:50 PM   #10
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I certainly respect Steve Bench's opinion. He is one of the very few true innovators in tube audio. But it just seems to me that a 6DJ8 would perform better in the noise department if it is biased hotter than the 6072 sections. R2 would have to be reduced, and the RIAA values readjusted.

Other observations (right, wrong or half-baked):

Can you delete C4 and R10 altogether? C4 will slightly increase low frequency noise by isolating the MM resistance from the grid. I don’t see a reason for it other than very low frequency rumble reduction.

Put 10K safety resistors from both sides of pot R19 to its wiper, just in case the wiper goes open or gets dirty.

Move C3 after C1. Make it bigger than you would normally for best noise performance (“look through it” in both directions).

Can you bypass R5 (I didn’t calculate gain or overload margins here though), and then take the output right off the plate of the second stage? With an output resistance in the 2.5K neighborhood, it should drive most line stages if the cable is not very long. And then delete the whole CF with CCS.

There are probably several places to inject anti-phase PS noise. Look at the Marantz power amps for inspiration. They used capacitor dividers to re-inject anti-phase PS noise.

Just some spurious thoughts as I sit here in central Florida awaiting Tropical Depression Ernesto!
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