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Old 12th May 2005, 03:25 PM   #1
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Default RIAA phono stage

I Hope this comes out OK, I've attached a schematic I've drawn up, first attempt at putting up a file, bear with me

Schematic is of the phono amp I'm using, before it is S&B mc Tx, after is S&B TVC.

Problem is it's pretty cr*p. Lots of noise and hum probably from the valve rectifier/ regulator. Bass is so so and the amp seems to me to be losing some clarity.

Measuring the voltages looks as if the first tube is pulling about 0.8mA and the second tube about 1.5mA.

Any ideas how to improve this or should I junk the circuit and use the very pretty case and build KYW's El Cheapo valve amp into it?

regards
Kev
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Old 12th May 2005, 05:25 PM   #2
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El Cheapo or some other passive RIAA would probably be much better than what you have, which is the usual tube active feedback RIAA, but you would still have the PS hum to deal with. I actually use tube active RIAA but with SS CCS/mu follower and a much lower impedance feedback network, but the passive RIAA is simpler and easier to build. There's also the EAR834 circuit which you can find after some searches.
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Old 12th May 2005, 05:43 PM   #3
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I don't really know the tube types but looking at the resistor values they seem quite incapable of driving a TX102. A line stage appears essential. A fresh start is probably the best suggestion.
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Old 12th May 2005, 06:02 PM   #4
Mikelo is offline Mikelo  Antarctica
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I finally gave-up on tube phono stages and built a wonderful
sounding Pearl. It is fed through a tube pre and amp, and I
think that's the best way to get a clean signal from the record
and the advantages of tube amplification. The only tube phono
stage I would try is Kimmel's hybrid stage you can find on the
welbornelabs site.
Michael
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Old 12th May 2005, 06:43 PM   #5
EC8010 is offline EC8010  United Kingdom
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I'm glad you said your circuit isn't terribly good...

Valve RIAA stages can be very good. But not done like that. As Nuvistor says, active RIAA needs a lower impedance network driven from a lower impedance. Personally, I'm not wild about active RIAA, and prefer passive RIAA. Even better, split the equalisation over three stages so that 75/3.18 is done between 1st and 2nd stage, then 3180/318 between 2nd and 3rd stages. And, as you've discovered, you need a squeaky clean HT supply and DC on the heaters to eliminate hum. You probably want to drive a bit more current through those 6N3P valves too (>5mA would be nice)...

The S&B volume control needs to be driven from a source with a low output resistance. Think cathode follower or mu-follower.
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Old 13th May 2005, 09:56 AM   #6
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Sorry, I didn't make it clear. This is a commercial product that I bought as a stopgap while I decide on what and how to build the mc, phonostage and TVC into one case.
I bought the amp 2nd hand expecting it to be at least useable now I can't for shame bring myself to pass it on to someone else before I do something with it.

So far I've done the obvious, re-routing the signal leads, screening the psu as far as possible, adding extra capacitance to smooth the 6V and 235V rails. I realised the circuit is pretty crap when I drew it out. I think the major problem is the output impedance. I think next stage is to rip out the valve power supply and install a ss bridge and a simple rc smoothing circuit. After that I would like to do something to try to keep the phono circuit but as I said in my first post I may completely renew this with a passive phono.

I don't have the ability to design circuits, I've just ordered Morgan Jones book to try to learn more. That's why I asked the question if there is anything i can do with the circuit as it stands before I go to the lengths of rewiring it into a passive stage.

BTW the case is really pretty, looks fantastic

Kevin
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Old 13th May 2005, 04:34 PM   #7
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I think you're on the right track, but why remove the valve rectifier if it's working now? An SS rectifier won't improve hum or noise, and the valve rectifier provides a turn-on delay for B+, however you will have more B+ so you can use a larger R. I think you'll find at least RCRC is needed for low hum anyway.
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Old 13th May 2005, 07:31 PM   #8
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Quote:
Originally posted by KevinTams
I've just ordered Morgan Jones book to try to learn more.
A good book. I'm trying to wear mine out before the 4th edition comes along

dave
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Old 13th May 2005, 07:35 PM   #9
SY is offline SY  United States
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If there's enough room for a buffer between the output of the second tube and the RIAA network that will improve things from dreadful to merely mediocre with some trimming of the network values. Take a look at the old Audio Research SP6B to see what I mean.
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Old 13th May 2005, 08:27 PM   #10
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Default Re: RIAA phono stage

Konnichiwa,

Quote:
Originally posted by KevinTams
I Hope this comes out OK, I've attached a schematic I've drawn up, first attempt at putting up a file, bear with me

Problem is it's pretty cr*p. Lots of noise and hum probably from the valve rectifier/ regulator. Bass is so so and the amp seems to me to be losing some clarity.
Looks like the standard stuff. I suspect the problem is layout, grounding and possibly the PSU as far as the noise is concerend. If I remember the Meixing Phono has the Valves open, shielding them may help.

For the rest, the circuit you show CAN be improved, Allan Wright did a lot of work on that, I added another little trick or two, but ultimatly the potential of this circuit is comparably limited.

With that said, gutting the case and building the "Valve El Cheapo" is a good option. ANd the El Cheapo will DEFINITLY drive the TX-102 (it was after all designed for that job!).

Quote:
Originally posted by KevinTams
Measuring the voltages looks as if the first tube is pulling about 0.8mA and the second tube about 1.5mA.
That is on the low side for the valve used, the chincese 6N3 is basically a WE 396A Clone and that in turn is a framed grid valve faintly reminiscent of a 6922.

If you want to go "noise hunting", first you need get a 'scope and learn how to use it (the cheapest Velleman Digital 'scope is surprisingly usefull, I bought one as stopgap after my old 20MHz Philips dies, but for most audio work it is just dandy. Then you can track the noise sources down and eliminate them, bump up the current, make some component and circuit changes and so on.

But it is as much work, or in some ways more, than a complete re-build.

Sayonara
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