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Old 12th September 2009, 03:46 PM   #1
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Default Help with RIAA stage schematics

Hello,
As I just bought a basic cheap turntable (RIAA EQ included) and have a few pieces on hand, i intend to build first a tube RIAA stage to improve the sound.
I will begin with the classic MM RCA shematics but with 2-stage 6SL7 (i have a few NOS in stock), despite the Hi-Z output :
http://www.bonavolta.ch/hobby/en/audio/rca.htm
http://members.myactv.net/~je205d/varEQph-1.jpg
http://www.lebong.de/6SL7-riaa/index.htm

I just have a few questions that could certainly help me understand some points. I understand most of the remaining scheme :

1. Can I build it in a first time using another PSU RC stages in my existing tube amplifier, after the one of the input tube (to avoid spending money on a PSU again) ? I will ground everything separately from the existing amp.

2. I am not sure i need to have the last 500k/1 Meg resistor (signal to ground) if i connect it to the next stage (i will use 250k pot). The three schematics (see links) are really different there...

3. The 180 pF cap is only seen on older schematics : no longer needed with newer cartridges ?

Thanks in advance
eric
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Old 12th September 2009, 06:07 PM   #2
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Dude,

The 180 pF. shown refers to cable capacitance, etc. That's the max. allowable capacitance. Exceed that and HF rolloff will occur.

It's easy enough to deal with the RCA circuit's major weaknesses: very poor drive capability and questionable bass extension. Look here for more info.
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Old 12th September 2009, 10:43 PM   #3
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Eric,

Can you use an existing amp power supply with another RC stage? Yep. Should you? Nope.

It's hard enough to get a decent, quiet PS with only preamp loads on the supply. Current draw from a power amp is guaranteed to cause substantial fluctuations in the power supply that will cause problems in a phono stage.

Consider how low the signal level is in the first phono stage, not the maximum signal, but a low level signal from the cartridge, one where you are trying to hear a minute detail. Then consider the power supply noise and the PSRR of the stage. You can see why the power supply must be superb for best sound here.

You'll get very good results with a current source fed shunt regulated supply to the preamp. While you can start with a passive supply, then add the SR & CS later, those two elements are crucial to great sound.

I built a variation of the RCA circuit, but using 12AX7s. Added was a cathode follower with CCS in its tail. It sounds nice, but a bit of that harmonically thin, bright 12AX7 sound is still there. Smooth plate Telefunkens sound best in it to those who've heard it.

The Beta Follower preamp I put in a Jolida chassis sounds better. It doesn't even have the advantage of the CS & SR, only a simple series regulator. The fellow I built it for was looking for simple.

Doing the Beta Follower concept, but with better gain tubes, maybe 6GK5 and 6AQ8/ECC85 or 6CW4 should work even better. I'm building one now, complete with those regs and a servo output stage.

The RCA circuit can't drive anything. You must add a follower to use it. With the Beta Follower circuit, the drive is there, but you still need a big, nasty output coupling cap. To correct either stage, use a servo output cathode follower to direct couple the preamp to the attenuator.

See Stuart Yaniger's article, I think it was "A Heretical Unity Gain Line Stage in DIY" Audio Mag, 4th quarter 2005 for the servo stage general design. This has the servo stage that sounds so good and allows you to have a coupling cap of 0.01uF for an output as opposed to 4uF if you're driving a low impedance attenuator. In that size, even expensive Teflon caps are sort of affordable.

Please note that I haven't completed the 6GK5 version yet, so I don't know how it will sound. But that RCA circuit is very good, you'll be pleased.

Stuart
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File Type: pdf RCA_RIAA_Schematic.pdf (18.0 KB, 130 views)
File Type: pdf 6GK5-6AQ8-Battery-Beta-Follower.pdf (18.1 KB, 105 views)
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Old 12th September 2009, 11:25 PM   #4
Gordy is offline Gordy  United Kingdom
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Quote:
Originally Posted by mahleriana View Post
Hello,
As I just bought a basic cheap turntable (RIAA EQ included)
If it has a RIAA Eq then you do not need the extra circuit. Or maybe you mean that you will bypass the internal circuit? OK...


Quote:
Originally Posted by mahleriana View Post
1. Can I build it in a first time using another PSU RC stages in my existing tube amplifier, after the one of the input tube (to avoid spending money on a PSU again) ? I will ground everything separately from the existing amp.
It depends... if the existing power supply has spare current capacity and the correct (or higher) voltages, then yes. (I've not calculated it but these circuits seem to take a very low amount of current). However a separate 'clean' power supply is usually best for circuits with very small signals, as noted by sepolansky.


Quote:
Originally Posted by mahleriana View Post
2. I am not sure i need to have the last 500k/1 Meg resistor (signal to ground) if i connect it to the next stage (i will use 250k pot). The three schematics (see links) are really different there...
In the circuit http://members.myactv.net/~je205d/varEQph-1.jpg the final stage is a x1 cathode follower and before that is a 250k volume control pot.

If your 'next stage' has a high input impedance you may be able to delete the cathode follower. (And if it has a way to get a tiny current to the grid of the next tube then you may also be able to delete that last 1M resistor).


Quote:
Originally Posted by mahleriana View Post

3. The 180 pF cap is only seen on older schematics : no longer needed with newer cartridges ?
In circuit http://www.bonavolta.ch/hobby/en/audio/rca.htm the 180pF is marked as 2% so may be the last part of the RIAA Eq (forming an RC filter with the output impedance of the stage). However, as Eli mentioned, the output cable may already have that.
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