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does DC loop feedback in a cascade feedback pair mitigate turn-on/off thumps ?

freddi

Member
Paid Member
2005-08-16 4:21 pm
does DC loop feedback in a cascade feedback pair have any effect (good/bad/neither) upon turn on/off transients in a preamp with no timed relay protection?

what are the bad points of applying DC loop feedback in the circuit below?

around 45 years ago I accidentally turned off a Citation IV preamp connected to a Citation 12 power amp - the turn off transient blew 4 Dynaco A35 woofers.

Van Alstine had AC feedback in his "Super Pas" mod hi-level 12ax7 stage, photoflash caps in its power supply and it was well behaved w, my DH200 - not the best sound but could be switched on and off without destroying speakers and perhaps the amp itself.

for a phono preamp these days, I'm more interested in a bare bones version of Stu Hegeman's Citation I/IV and with an anode follower hi-level stage.

TbbnIp8.jpg
 

6A3sUMMER

Member
2016-06-07 6:50 am
If the 300V B+ comes up before the 12AU7 is warm, then 300V charges the 3.3uF cap at the output . . . Bump.

But if the 12AU7 is warm before the 300V comes on, then the 12AU7 will have a plate voltage (perhaps at 150V). Then 150V charges the 3.3uF cap at the output . . . smaller Bump.

Likewise, when B+ goes away, the 3.3uf cap's charge dumps into the output (goes negative) . . . Bump.

Possible solution is muting during turn on and during turn off.
(Muting relay contacts shorting across the output during charge and discharge times of 3.3uf).

Negative feedback does not solve this problem, when the tubes are not warmed up. It also can not solve the problem of B+ coming on, or going off.

Just my opinions.
 
Last edited:

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
does DC loop feedback in a cascade feedback pair have any effect (good/bad/neither) upon turn on/off transients...

No. The key problem is that output DC level must transition from zero to 150V and back at turn-on, and again at turn off.

For much of the turn-on/off phase the tubes are dead or hardly alive and not in control.
 

freddi

Member
Paid Member
2005-08-16 4:21 pm
thanks to all - - FWIW, Van Alstine's earlier SuperPas used PAS's tube rectifier, large drop R's and photoflash caps to obtain a slow turn on -off of the B+ - that seemed to work pretty well with regards to turn on/off surges for the circuit involved with 200V to the phono board and 300v to the high level board.

Are there any advantages whatsoever with DC feedback in a cascade pair? (maybe in an RIAA stage ?)
 
Are there any advantages whatsoever with DC feedback in a cascade pair? (maybe in an RIAA stage ?)

Yes, the coupling cap voltage can be a little lower!

For turn-on thump, a cathode follower using a tube with a really hefty cathode will turn on very, very slowly (BX7, BL7, EL84/34, etc).

Turn on/off thump blowing speakers is also a symptom of too much gain with sensitive speakers. Padding the amp inputs or using trim pots would also help.