Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Analogue Source Turntables, Tonearms, Cartridges, Phono Stages, Tuners, Tape Recorders, etc.

Valve Itch phono
Valve Itch phono
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 29th March 2014, 03:00 PM   #1701
dirkwright is offline dirkwright  United States
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by stajo View Post
The little I know of MM carts say they need to be tuned in with a small amount of pF. A tranny would be devestating in that logic. Thanks for working the link Dirkwright!
There's usually more than enough capacitance in the cable.
  Reply With Quote
Old 29th March 2014, 03:44 PM   #1702
SY is offline SY  United States
On Hiatus
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Valve Itch phono
It would take a very special transformer to work with an MM. You have a source impedance that varies with frequency and gets quite high in the treble, so you can't optimize the load for the secondary. The leakage inductance and parasitic capacitances will also cause a severe HF rolloff because of the high source Z.

Fortunately, the levels are high enough that you can achieve a high CMR balanced in without transformers.
__________________
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
  Reply With Quote
Old 29th March 2014, 04:39 PM   #1703
dirkwright is offline dirkwright  United States
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by SY View Post
It would take a very special transformer to work with an MM. You have a source impedance that varies with frequency and gets quite high in the treble, so you can't optimize the load for the secondary. The leakage inductance and parasitic capacitances will also cause a severe HF rolloff because of the high source Z.

Fortunately, the levels are high enough that you can achieve a high CMR balanced in without transformers.
Yeah, that's basically my conclusion also.
  Reply With Quote
Old 1st April 2014, 01:08 AM   #1704
fdegrove is offline fdegrove  Europe
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
Originally Posted by Salas View Post
I looked into it a bit and I think it could be running just at 10mS in total as it is, with such a Jfet CCS about 6mA is to be expected. So something like 1.5mA per triode. Half the transconductance of a 5mA 2SK170GR for the whole composite pre pre in other words. 5mA per triode can boost to 20mS for 3dB better SNR. DN2540 can provide and its actually a good CCS down to a 20-25mA minimum level. Two in self biased cascode is even better high Z vs. Freq.
The Itch itself could push its input transconductance a bit also if there is raw DC spare for 320V B+ and R3 became 330 Ohm in combination. Little thing, but it could be better driving its EQ network in the highs. To maybe avoid double up experiments and more Miller for Stajo.
Thanks for looking into it.
Is this doable with the low 24V PS that feeds the 4 triodes of the ECC88 stage as it is now or do I need a raw supply higher voltage for that?

I'm lost about the second part of your observation though. Does that refer to the first stage of the itch being fed from a dedicated 320V supply and ditto CCS?
I'd agree that the phono preamp would benefit from a dedicated supply on a pro stage basis, just not sure if that is what you had in mind.

Cheers,

P.S. Went to the bedroom on my knees laughing a few nights ago after reading you last PM....
__________________
Frank
  Reply With Quote
Old 1st April 2014, 01:32 AM   #1705
Salas is online now Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Valve Itch phono
Those are depletion mode NMOSFETs work as -2VGS(OFF) JFETs. They should work stably when you got VGS*2 to spare across them.

The second part is just a suggestion to Stajo for using a little more B+ and a smaller current setting resistor in the μ Follower to push the 6N2PEV a bit in the circuit as is.
Just bcs he was thinking of doubling up on input triodes. He could achieve almost the same effect with a single. He could also use 1N4148 instead of an IR LED to achieve even more current and gm although few mV of grid current effect over the 47K may start to appear.
  Reply With Quote
Old 1st April 2014, 01:57 AM   #1706
fdegrove is offline fdegrove  Europe
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

O.K. I think it is just about doable with only just a few volts left to spare.
The important thing is that the composite block of triodes is kept into the starved voltage zone of roughly 12VDC plate voltage.

As for the second part, personally I would not // the triodes of the input section as it will also require the RIAA values to be recalculated.
All in all I feel that there's more to lose than to win by doing so but that's just my opinion.
May as well opt for single triodes in that case.

I also recall you mentioning that the channel separation of the Itch is maybe not so great as it is due to the two channels sharing one envelope.
However, as the 6N2P has an internal shield (just like a ECC88) this should not be affected provided this shield is grounded. Which it is as I understand it.
IOW, a channel separation of 30 to 35dB which is in the range of what most MCs have on offer anyhow should easily be feasible. Unless compromised by careless layout.

Cheers,
__________________
Frank

Last edited by fdegrove; 1st April 2014 at 02:13 AM.
  Reply With Quote
Old 1st April 2014, 02:07 AM   #1707
Salas is online now Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Valve Itch phono
Looks like working, I set up a quick sim while we were posting. 1mV pk-pk input. Looks alright for output? R1 got to be a rheostat mode trimmer.
Attached Images
File Type: png 2014-04-01_040516.png (9.6 KB, 172 views)
File Type: png 2014-04-01_040332.png (24.1 KB, 171 views)
  Reply With Quote
Old 1st April 2014, 02:12 AM   #1708
Salas is online now Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Valve Itch phono
Quote:
Originally Posted by fdegrove View Post
Hi,

O.K. I think it is just about doable with only just a few volts left to spare.
The important thing is that the composite block of triodes is kept into the starved voltage zone of roughly 12VDC plate voltage.

Cheers,
Should go up around 15-16V plate with 5mA per ECC88 if the sim model is any good. Is that a problem? Looks normal it should bias up a bit with more current used.
  Reply With Quote
Old 1st April 2014, 02:25 AM   #1709
fdegrove is offline fdegrove  Europe
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

That's magnificent.

It could even be integrated with the Itch if we steel the raw voltage somewhere from the B+.
I'm actually very happy as it provides a long term solution since the J-FETs in TO92 package are being slowly phased out, etc. Plus it quite likely outperforms the J-Fet current source anyway.
Great, thanks a bunch for simming this.

Ciao,
__________________
Frank
  Reply With Quote
Old 1st April 2014, 02:33 AM   #1710
fdegrove is offline fdegrove  Europe
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
Should go up around 15-16V plate with 5mA per ECC88 if the sim model is any good. Is that a problem? Looks normal it should bias up a bit with more current used.
Not if the 7824 can be kept in place as I don't think the MPSA14A (cap mult.) can have much more than on its input(need to check this)
The load line of the tubes should still be O.K. though. All in all I don't think the few volts more will affect the input impedance in any appreciable way.
At first glance: should work fine.
(If not, I don't understand anything anymore. Won't be a first. )

Ciao,

P.S. Owe you a fine Belgian beer or two.
__________________
Frank
  Reply With Quote

Reply


Valve Itch phonoHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Got the build itch again. 80w kt88 itsmejto Tubes / Valves 44 27th September 2008 12:49 AM
valve phono stage wanted for mc brobarrie Swap Meet 1 13th August 2007 09:17 PM
Basic DIY Valve Phono Stage matthew2456 Tubes / Valves 5 29th March 2007 05:11 AM
Phono pre, valve or jfet lndm Analogue Source 7 4th April 2006 03:11 PM
phono preamp design for c3m postie valve fergs1 Tubes / Valves 1 29th May 2005 03:12 PM


New To Site? Need Help?

All times are GMT. The time now is 05:08 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki