Help wanted on SRPP preamp

[jtcc1015]

I have recently built a SRPP line-stage preamp using 6SN7. The circuit, as seen in the attachment, was modified from a circuit I got from the Web. The original circuit used ECC82.

I am quite satisfy with outcome of this project. This is a very high definition and dynamic circuit.

However, I find the gain is about 13dB which is little too high to my liking. Since it is zero feedback design, the gain is not readily adjustable.

So my question is:
Can anyone kindly advise me on how to lower the gain of this circuit to about 10 to 11dB?

BTW, the load impedance is about 20k Ohm.

 

[Frank Berry]

You can reduce the gain of the stage by removing the 100uF capacitor on the cathode of the bottom section.

 

[fscarpa58]

Hi

I call it a mu-follower, more gain
and less distortion in respect with an SRPP.

I think you are probably speaking
about 26 db not 13. ( formula is 20*log(Vo/Vi) )

So I argue you want 20-22 db

1) remove the 100u cap, but you'll have only
a 0.5 db reduction
2) reduce the 22k res to a 2k one, the gain is
now about 22 db but the sound will be different.
3) if you like that sound and accept a further gain
reduction, remove the 22k, 1Meg, and .47u cap
net and go with a true SRPP with a gain of
about 18 db.

A question

what's the purpose of the 30k input res ?


Ciao
Federico

 

[fdegrove]

Hi,

It's a mu-follower alright...

One thing to keep in mind: Zout will go up when you remove the Ck bypass, going form mu-follower to srpp will make it worse still.

My advice would be to go with SRPP and bypass the cathode only if you need the lower Zout.

what's the purpose of the 30k input res ?

LOL....Reducing the gain....a little?

Regarding the mu-follower: 0.470µF is unnecessarily big (and slow).
For working into a 1M impedance you'd be better of using 0.100µF or 0.220µF at most.

The 1µF output coupling cap can be made larger if Zin of the amp is lower than say 47K.

Cheers,

 

[fscarpa58]

Hi Frank, all

Zout will go up when you remove the Ck bypass, going form mu-follower to srpp will make it worse still.

Correct.

for the original circuit we have a Zout of about
600 Ohm and 1k with and without Ck respectively

changing 22k res to 2k res we'll have
1400 and 2800 w and w/o respect.

finally the SRPP

2500 and 4700 with and without Ck

Ciao
Federico

 

[fdegrove]

Hi,

All this time I was wondering why that circuit look so familiar....

He, he...

It's "Fabricato in Italia", by the good folk at Megaherz no less:

TRIOPRE........

Ciao a tutti,

 

[jtcc1015]

fdegrove,

ha ha! you got it. Yes, the original circuit was from Megahertz. I tried a 0.33uF in place of 0.47uF, but didn't like it. It sounded too dry.

Thanks for your advice.

fscarpa58,

The 30k is the integral part of the electronic volume control circuit which is a DIY kit made by a very popular China based DIY shop, "Lite Audio". Refer to the attached photo, you can see there are 2 relays boards stack. This kit didn't use any IC to do volume control, instead they use whole bunch of 5V miniature relays and Halco resistors to perform volume control just like those manual rotary tappers. I believe they inserted a 30k series resistor into the output of the circuit to prevent or damp any resonance or transient that may be generated during relays operation. BTW, this kit can do 100 steps volume control, 4 sources selection, and come with a remote control.

A question, will the bias current increase drastically afte I've changed the 22k to 2k??

Thanks for your advice.


Regards,

 

[fscarpa58]

thank you

will the bias current increase drastically afte I've changed the 22k to 2k??

in my sim it goes from 3.35 mA up to 4.5 ma.
so the bias from about 2.75 V to 3.7 V.

Attention: in case 2) 22 db refers to situation without cap.
note that the presence or absence of the cap has more effect
in case of 2k than 22k.


Ciao

Federico

P.S. fine stuff. what about its price?

 

[jtcc1015]

Hi Federico,

The volume control kit sell for about USD 95.00 in Hong Kong. It is little bit cheaper than a manual 23-step tapper made from Japanese gold-plated selector, and definitely much cheaper than those made from Elmar rotary selectors.

Thanks again for your advice.

Regards,
Johnny