Common Cathode Amplifier (CCA) for LR-RIAA

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docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

Common Cathode Amplifier (CCA) for LR-RIAA

Post by docali »

Hi,

i would like to ask the forum about your opinion about my plan to implement an CCA which was described by John Broskie to use for an LR-RIAA amplifier.

The design goals are lowest possible input capacitance for use with step-up transformer, low distortion and using no capacitors where ever possible. The CCA can also be used with an LCR-RIAA. The LR-RIAA has a bit more depence on the source impedance but i think that it is quite acceptable. The CCA can be tuned for fantastic low distortion and the input capacitance is the smallest possible because the first tube does not produce much Miller capacitance as it does not amplify the signal considerably.

I think that the inductors (10H biggest) should be a realistic value that can be manufactored with good stability of inductance depending on frequency.

Best regards,
docali
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D3a_CCA_LR3k_sweep.asc
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sbench
Posts: 296
Joined: Fri Jan 28, 2005 3:45 pm

Post by sbench »

Should work reasonably well. The cathode follower input stage will indeed keep capacitance down, but this is never the lowest noise configuration. If it's quiet enough for you, it will have the other characteristics you are looking for..... Still, I'd worry about the noise.

Stephie <3
docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

Post by docali »

Hi Stephie,

your comments are very welcome! Yes, i think noise could be an issue as John Broskie already stated in his blogs the CCA has many good behaviours but also some downfalls (there are no free lunches ;-) ) which are noise and slightly higher output impedance. But there are some tricks to overcome this as you might induce a proper portion of the noise on th esecond grid or the other way make the PS very clean (iron will be your friend ;-) ).

Dou you have any idea how to boost the low frequencies how you did in your split LR-RIAA? I tried this CCA with 600 Ohm load but the low frequency fall off is to much.

Best regards,
Bernd
sbench
Posts: 296
Joined: Fri Jan 28, 2005 3:45 pm

Post by sbench »

Hi,
In your circuit, if you turn around the secondary (L2), you can add a little bass boost by a voltage divider at "U2" grid and the 3k resistor between the 2 chokes. For instance changing that 3k into a 2.9k and 51 ohm resistor and connect the tap to the grid of U2, you get a 2.5dB boost about 25Hz.

Stephie <3
docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

Noise

Post by docali »

Hi Stephie,

thanks for the hint! LF bost works well but we have global feedback in this case.

Concerning noise one can make a little twist with C1 terminating it to B+ instead of terminating to ground. This cancels noise quite well. The other way would be to eliminate R2 which was originally intended to compensate for the primary resistance of the interstage xfmr (equal currents to both tubes).

Best regards,
Bernd
Attachments
D3a_CCA_LR3k_MKII_sweep.asc
Terminating C1 to B+
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sbench
Posts: 296
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Post by sbench »

Both changes... removing R2 or bypassing to the supply rather than ground affect power supply induced noise, but neither affect the increase in tube noise caused by the addition of the input cathode follower. If you're limited by hum buzz etc, that sometimes helps a lot... if you're limited by raw tube noise, it won't.

Warmest Regards,
Stephie <3
docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

Post by docali »

Hi,
I played around a little and found that the lowest senseful load is 3 times the Tango which is 1k8 but we loose some gain.

@Dave
What's about your progress with larger RIAA inductors like 10H?

Best regards,
Bernd
Last edited by docali on Fri Jun 17, 2011 6:57 am, edited 1 time in total.
dave slagle
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Post by dave slagle »

I do 22hy for the low corner in the 9K LCR's.

dave
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Randolph
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Joined: Mon Jun 20, 2011 1:36 pm

Post by Randolph »

I must say great effort you putting on it, keep it up. And nice talking here. :)
docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

A NoC CCA LR-RIAA amplifier

Post by docali »

Hi,

following Stephies ideas i will declare this thread as NoC area for a while :wink:

Developing the idea of the CCA in a phono amplifier further we need an step-up transformer after the LR-RIAA stage. This feeds an additional CCA stage which itself has a step-down for low output imdedance. Look at the low parts count. Only a few resitors and a lot of iron but no capacitors at all!
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CCA_LR3K_2-Stages.JPG
CCA_LR3K_2-Stages.JPG (126.23 KiB) Viewed 13847 times
docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

CCA with RX-Corrector

Post by docali »

Hello,

and here we see that the CCA also works with the RX-Corrector. Nice, also without capacitors.
Attachments
D3a_CCA_NewRX-3k_sweep.asc
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docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

Post by docali »

Hello,

an even lower parts count with very good accuracy ;-)

Please look at the phase shift at 20 kHz. It's only a few degrees!

Best regards,
Bernd
Attachments
D3a_CCA_LR3k_MKIII_sweep.asc
(3.68 KiB) Downloaded 415 times
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