John Curl's Blowtorch preamplifier part II - Page 450 - diyAudio
Go Back   Home > Forums > Member Areas > The Lounge

The Lounge A place to talk about almost anything but politics and religion.

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 31st May 2010, 08:22 AM   #4491
PHEONIX is offline PHEONIX  Australia
diyAudio Member
 
Join Date: Jul 2004
Location: Australia
Quote:
Originally Posted by SY View Post
Problem is, they were also driving the RIAA off that second stage plate, which was a lot lower impedance at high frequencies, not to mention high capacitance. This screwed up the open loop gain enough to require positive feedback (cathode-to-cathode resistor). Marantz interposed a cathode follower, but chose one that compromised the low distortion driving ability for the RIAA network's capacitance.
Hello Sy

Are you saying that the 12AX7 cant be used for an RIAA stage because it cant drive the the RIAA filter network without distorting.
  Reply With Quote
Old 31st May 2010, 08:31 AM   #4492
PMA is offline PMA  Europe
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
Yes. Not only that triode, but also opamps give better results when buffered, especially in MC preamps.
__________________
Pavel Macura
http://web.telecom.cz/macura/audiopage.html
  Reply With Quote
Old 31st May 2010, 10:59 AM   #4493
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by PHEONIX View Post
Hello Sy

Are you saying that the 12AX7 cant be used for an RIAA stage because it cant drive the the RIAA filter network without distorting.
It can't be used for a phono stage of the Dynaco/Marantz type without distortion and slew limiting. Current and transconductance is too low, plate resistance is too high. To make things worse, the cathodes in that circuit are degenrated, raising the plate resistance even more (with a 2k2 cathode resistor, the effective plate resistance is nearly 300k!)

There are other ways to use that tube successfully in phono amps and exploit its excellent linearity, just not that way.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 31st May 2010, 10:19 PM   #4494
PHEONIX is offline PHEONIX  Australia
diyAudio Member
 
Join Date: Jul 2004
Location: Australia
Quote:
Originally Posted by SY View Post
It can't be used for a phono stage of the Dynaco/Marantz type without distortion and slew limiting. Current and transconductance is too low, plate resistance is too high. To make things worse, the cathodes in that circuit are degenrated, raising the plate resistance even more (with a 2k2 cathode resistor, the effective plate resistance is nearly 300k!)

There are other ways to use that tube successfully in phono amps and exploit its excellent linearity, just not that way.
Sy

Is the Paragon preamp by Bruce Moore an example of a better use of the 12AX7 in a Phono stage.
  Reply With Quote
Old 31st May 2010, 10:32 PM   #4495
R.I.P.
 
Join Date: Dec 2001
Location: Schaffhausen Switzerland
Quote:
Originally Posted by PHEONIX View Post
Sy

Is the Paragon preamp by Bruce Moore an example of a better use of the 12AX7 in a Phono stage.
Not really, as he uses it in Cascode, and it's not the ideal tube for a cascode. Too low transconductance, leading to far too high distortion.

The gain sections of a 12AX7 phono such as the Marantz 7, ARC SP3 & 6 etc are how a 12AX7 should be used, just that it needs a heafty high current, hi transconductance CF after it to drive the capacitance of the RIAA network, and any following cable capacitance etc.

Suitable tube for this job would be a minimum of a 6DJ8 at 10mA, better still a 6H30 at 20mA. A 12AX7 at 1mA, or a 12AU7 at 5mA don't cut it.

Regards, Allen
  Reply With Quote
Old 1st June 2010, 12:24 AM   #4496
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
Default limitations of 12AX7

Well, I don't have $1000 for a preamp. I enjoyed my PAS2 for 30 years, then when volume pot supply problems developed, went onto a Sony TC250 tape recorder as a preamp, then when 100 electrolytic capacitors and the source switch needed replacing in the TC250, bought a Herald "disco" mixer with 4558 opamps driving the RIAA capacitors. I had to replace the 4558's in the disco mixer with 33078's to get the midrange to sound nearly as good as the PAS2, (remove the hiss) and the disco mixer hum still exceeds the PAS2, unless I hold my hand over the mixer faceplate or turntable headshell. (yeah, the turntable ground is hooked up, also the ground to the wall plug pin) While the PAS2 design might not be ideal, it was close enough to sound really good with good speakers (bought only recently for $600) in A-B comparison with a Steinway console ($1000) sitting between the speakers. No previous speaker has been good enough to even approach doing decent piano sound. I'm not using the PAS2 not because of the sound, but because the volume pot I bought from Stereo Cost Cutters was a reject that sounds good but requires vice-grip pliers to adjust the volume.
__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 1st June 2010 at 12:33 AM. Reason: speaker cost, volume pot
  Reply With Quote
Old 1st June 2010, 06:51 AM   #4497
diyAudio Member
 
john curl's Avatar
 
Join Date: Jul 2003
Location: berkeley ca
I understand your position indianajo, I started my hi fi with a used PAS 3 in 1963. I like used hi fi equipment. My speakers are used, my tuners are used, my video projector is used. In fact, almost everything that I have is either used or I had a hand in the design, itself. However, what we are talking about here, is the highest quality hi fi that we can think of, and that can be made. In your price range, I would consider a passive preamp and some good relay switching.
  Reply With Quote
Old 1st June 2010, 08:06 AM   #4498
PHEONIX is offline PHEONIX  Australia
diyAudio Member
 
Join Date: Jul 2004
Location: Australia
Quote:
Originally Posted by Allen Wright View Post
Not really, as he uses it in Cascode, and it's not the ideal tube for a cascode. Too low transconductance, leading to far too high distortion.

The gain sections of a 12AX7 phono such as the Marantz 7, ARC SP3 & 6 etc are how a 12AX7 should be used, just that it needs a heafty high current, hi transconductance CF after it to drive the capacitance of the RIAA network, and any following cable capacitance etc.

Suitable tube for this job would be a minimum of a 6DJ8 at 10mA, better still a 6H30 at 20mA. A 12AX7 at 1mA, or a 12AU7 at 5mA don't cut it.

Regards, Allen
Hello Allen

Do you have a link to the Marantz 7 circuit you are talking about , I dont no the circuit details.

Regards
Arthur
  Reply With Quote
Old 1st June 2010, 08:33 AM   #4499
gk7 is offline gk7
diyAudio Member
 
Join Date: Oct 2007
Location: Vienna, Austria
Marantz 7c Schematics:
File Sharing Marantz 7
  Reply With Quote
Old 1st June 2010, 05:02 PM   #4500
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
Hi Allen, 6DJ8 might have suitable technical specs but they sound hard and gritty to me no matter when they have been in a circuit... so no go.

Now what about those RESISTORS??

_-_-
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  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



New To Site? Need Help?

All times are GMT. The time now is 01:26 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