Help wanted on SRPP preamp - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 26th November 2004, 08:28 AM   #1
diyAudio Member
 
Join Date: Jul 2004
Location: the 3rd rock from the Sun
Question Help wanted on SRPP preamp

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.
Attached Files
File Type: pdf 6sn7 srpp.pdf (21.5 KB, 286 views)
  Reply With Quote
Old 26th November 2004, 09:00 AM   #2
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
You can reduce the gain of the stage by removing the 100uF capacitor on the cathode of the bottom section.
__________________
Frank
  Reply With Quote
Old 26th November 2004, 10:44 AM   #3
diyAudio Member
 
Join Date: Nov 2003
Location: Genova, Italy
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
  Reply With Quote
Old 26th November 2004, 01:37 PM   #4
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
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.

Quote:
what's the purpose of the 30k input res ?
LOL....Reducing the gain....a little?

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

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

Cheers,
__________________
Frank
  Reply With Quote
Old 26th November 2004, 01:57 PM   #5
diyAudio Member
 
Join Date: Nov 2003
Location: Genova, Italy
Hi Frank, all

Quote:
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
  Reply With Quote
Old 27th November 2004, 02:19 AM   #6
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
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,
__________________
Frank
  Reply With Quote
Old 27th November 2004, 06:37 AM   #7
diyAudio Member
 
Join Date: Jul 2004
Location: the 3rd rock from the Sun
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,
Attached Images
File Type: jpg img_1688a.jpg (85.1 KB, 375 views)
  Reply With Quote
Old 29th November 2004, 10:10 AM   #8
diyAudio Member
 
Join Date: Nov 2003
Location: Genova, Italy
thank you

Quote:
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?
  Reply With Quote
Old 29th November 2004, 04:44 PM   #9
diyAudio Member
 
Join Date: Jul 2004
Location: the 3rd rock from the Sun
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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
JFET SRPP RIAA Preamp wrenchone Analogue Source 249 27th March 2013 11:20 PM
jfet srpp preamp yves Solid State 12 23rd June 2012 05:50 PM
Preamp Opinions Wanted DC Dave Solid State 3 6th April 2004 10:41 AM
Lowering gain in 12AX7 SRPP preamp net-david Tubes / Valves 26 23rd December 2003 05:55 PM
Preamp with 6sn7 & 6sl7 SRPP ??? Gregouille Tubes / Valves 4 18th June 2002 10:41 PM


New To Site? Need Help?

All times are GMT. The time now is 09:48 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2