Designing a Phonostage with the S&B 10K LCR RIAA Module

[joe3rp]

gary,

can you please draw the schematic of this. I am getting a bit confused. Thanks in advance

Joe

[Gary P]

Hi Joe,

Here is a quick and dirty schematic of the idea. The resistor between the CCS and the input of the LCR is the terminating resistor and would need to be chosen so that the total impedanceo of the MU output and the resistor = 600 ohms.

Eli,

I don't know if you would gain anything using the diode for bias. With a CCS load the triode is already running fixed bias and gives full MU. It is something that would be easy to experiment with though.

The schematic shows how the booster CCS (or resistor) would be connected.

Gary

[joe3rp]

From you your schema, does this mean that the "10k LCR Network" is capable of taking in some dc voltage? Shouldn't there be a capacitor in the input of the LCR at its input? Or the LCR has a "built in" capacitor at its input ? Why did you put a cap in the output side of the LCR, is it needed?

thanks

Joe

[ThorstenL]

Konnichiwa,

Originally posted by Eli Duttman
The 6DS4 nuvistor is a low current type. So, a secondary current sink is the route to follow. The 6DS4 runs at a voltage somewhat less than that used for the 6922. Finding a suitable N channel FET for the secondary sink shouldn't be too difficult.

You could use other valves that can draw enough current. The lower the signal the more critical the quality of the pulldown CCS.

Originally posted by Eli Duttman
Michael Percy carries 1 KOhm Vishay 1280G trim pots. With their bulk metal foil construction, the Vishay parts seem to be the right choice for dialing the 600 Ohm impedance in. An adverse sonic impact should not be observed.

Hint. In order to make the job of adjusting the series termination resistor easy use a 600R resistor linked to a suitable capacitor. Leave the first running with the output unconnected, termination resistor.

Put some signal at 1KHz at a few mV in and measure the output AC voltage.

Connect the Resistor & Capacitor and measure the output AC voltage again. If the series termination resistor is correct the level will drop by 6db or in other words to 1/2. So if you set the output voltage without load to 1V you adjust the termination resistor untill you measure 0.5V with the RC in place..

Originally posted by Eli Duttman
FWIW, I'm thinking of using the same red LED for bias that is used in the Bottlehead Seduction.

Any time I used Diode Bias I did not like the results much.

You can make the second stage work with gridleak bias, on the first stage using an unbypassed capacitor of suitable value will not do much harm either.

Sayonara

[ThorstenL]

Konnichiwa,

Originally posted by joe3rp
From you your schema, does this mean that the "10k LCR Network" is capable of taking in some dc voltage?

Yes and no. The LCR made by S&B uses certain capacitors which I did specify and which are still used in production. These are quite high voltage in the 600R LCR (630V DC or higher). These capacitors are located between ground and the LR circuitry like shown here:



In the 10K RIAA BTW the capacitors are rated 160V DC.

Originally posted by joe3rp
Shouldn't there be a capacitor in the input of the LCR at its input?

No.

Originally posted by joe3rp
Or the LCR has a "built in" capacitor at its input ?

No.

Originally posted by joe3rp
Why did you put a cap in the output side of the LCR, is it needed?

Because you need to keep the DC off the grid of the second stage.

The difficulty of understanding this approach to coupling by niewbies let me leave it off the original article. There are several implications in using direct coupling and input termination that must be understood. HOWEVER, once they are understood and applied input termiation and DC coupling is much preferrable to the traditional coupling, sonically (yes, I have tried).

Sayonara

[jlsem]

Is there a schematic of a 10K LCR RIAA network available?

TIA,
John

[ThorstenL]

Konnichiwa,

Originally posted by jlsem
Is there a schematic of a 10K LCR RIAA network available?

Looks exactly like the 600R one, except all values scaled by the 16.66 factor the 10K impedance demands (Resistor/Inductor up, capacitor down) plus minor adjustments to fit standard RC Values.

Sayonara