Feedback problem?

[ameise]

hi freaks,

I cut some dubs today, some of them had quite a lot of highfreqz
so it blew my fuse on one channel a couple of times..

I did a nice cut after that...but then with the next track
i got a funny noise (well not sooo funny afterall):

one channel somehow "catched" a highfrequency noise after playing back some seconds over the amp and drivecoils...

i turned off the (music) signal immediately, but the noise stayed and did not went away until i switched of the amp after a few seconds...

sounded to me as if that's a feedback problem, as if the feedback channel went into some endless loop of playing back its own noise...

well well i teared down the whole studio now,
and i will check every little cable...
i will remeasure the feedback (btw: flozki i cannot find
this super step by step feedback adjusting how-to...is this online somewhere here?)

i started with measuring the drive and feedback coils resistance:
at least that seems to be normal...no short circuit and no broken coil (hopefully)

maybe someone likes to guess what happened here...
i will investigate tomorrow morning...

btw: it's an SC99 with minicut-preamp and 2 x 250 W solidstate power amp

greetings
ameise

p.s. how many feedback in db do you use for your vinylium systems? -5db or more?
sx74/sal is -12 or? would be interesting...

[duomo]

it sounds like a phase problem in one channel , i have the same problem with lowfreqz some years ago, and it was the cutting amp which had no stable phase in that range. So it could be there or a phase problem in your minicut...
I have a sx74 and a modified sal and cut with - 8dB here . I can go up to -12dB but then the freqz response is not flat any more , (too much hifreqz and lowfreqz) and the sound gets "liveless" for me -8dB is perfect...

[ameise]

thanks for the reply!

yes somehow a phase problem, but where from...
maybe the amp itself got damaged or the opamps
on the preamp?

i now made up a workflow:

first disconnect head
and put in dummy head

as it is just one channel: switch channels
and see if the problem also switches sides

change cables

change amps

re-adjust everything from scratch

i like the idea that it is just a cable with loose connector
(because i can easily fix it)

well i will see...

greetz
ameise

[flozki]

on a kingston dubcutter with vinylcut vc200 5-6 db at 5khz.
on a minicut and if you know what you are doing go to 8-9db.
depending on how good your head is...
i think 8db is great.



the adjust prozedüüüüür.hehe. hope i dont forget anything with my jetlagged brain...


1. put in a 500mA fast acting fuse.
2. riaa off.
3. fb to zero
4. drive level to zero.
5. fb mon level to max.
6. 5khz tone and watch on db meter for anything around -10 to 0 db (fb mon full cranked up... i then have around 100-150 mA cutter current.
7. fb level adjust: increase FB level. achtung: on a sc99 the fb goes up first to +1 then it drops. drop to -6 or anything you want...
8. drive level adjust: cut 1khz tone and adjust ref. level to riaa specs... maybe not with the ameise test tone record! hehe.
9 adjust fb mon so you have same levels for input and feedback...
10. remove the 500 fast fuses , put back the 630mA slow blow fuses or aluminum foil , whatever you prefer....
11. dont forget to switch riaa on....

[ameise]

jo thanks flo!

btw: any idea what might cause the highfreq noise
on one channel?

om shalom
mart

[flozki]

hmm. maybe you should turn off your mobilephone or pirate radio station during cutting?

or even worse a loose feedback coil? but that you can check if you only get 5-6 db of feedback...then that needs reglueing...

or shitty feedbackcables?

[ameise]

i never turn off my mobile when cutting, just in case some groupies
call and need a real deep cut...ähem sorry...

hmm.. ok loose feedback coil

i guess that is something that needs proper fixing in switzerland...