Telefunken V676a highpass filter removal??

Hey guys, hope all is well,

I have a pair of these in a rackmount and was using them as a line amp for a transfer the other day. Although they've been serving me well as mic pre's, i noticed the 40hz highpass is cutting off too much info for my tatses, especially for line use.....

Anyone removed the highpass filter in these?? Kubarth has a nice PDF of the orig Service manual i could email if someone would like to take a stab at it. Not being able to read German is not helping me figure this out myself. Any info from the board appreciated!
Winston (John)?? You may have racked these! You still around these parts??

Hi Bar
If you need help in translating let me know.
:sam:
Jens

Hi Jens,

I checked out the schematics and i just wouldnt even know what to remove. Just wondering if anyones done it and could steer me in the right direction. I'm hoping it's as simple as bypassing some resitor network..... :grin:

here a link the the PDF of the module:

http://www.kubarth.de/cgi-bin/diy/german/getfile?f=D%2C3%24Q%2C%230Q-C%24R.%25%5DV%2BW8V-S9A7W-E%3CG9I8V5M86YU86PN%3C%261F%0A

huh, from a quick look at the schemo (your link does not work, BTW) I've seen that C307 and C602 form a switchable high pass. Is there a three step switchable high pass on your unit?

If I'm right (take it with a grain of salt), you could replace these caps with a bigger one, say 10x bigger.

Samuel

Here´s the link:
http://www.kubarth.de/cgi-bin/diy/german/index
For me C6o3 seems to bypass the second stage of the filter (C602) but when I know it right you can switch this bypass only via remote. So what you could do is just solder a 1uF cap parallel to C602 or replace R604/606/607 with higher values as they form a frequencydepending voltage divider with C602. The first stage I do not really understand (R111/109/112), maybe someone can help there?
:guinness:
Jens

> the 40hz highpass is cutting off too much info for my tatses, especially for line use.....

It is less than 1dB down at 40Hz. What are you recording that has 40Hz, and what speakers do you play it on? I do have tracks with 32Hz drone, but even quasi-professional mastering studios don't carry that well.

Alternatively: it's broken. Can you test the frequency response? Measuring down to 20Hz is tricky: I use a meter modified to to 2Hz.

PRR

Thanks for the response. I just noticed that in a transfer from a NAGRA tape machine i have, using a racked pair of V676A's there was a real lack in bottom (using the Teles over straight transfer). Im monitoring using a Revel B15 sub, wich is nearly flat to 20hz....(1000 watt) The Mic pres work fine, but it seems the low pass filter at 40hz was cutting bottom. Theres plenty of bass info on my kickdrum sounds down to 20hz.....

I was asking about removing the filter, whether anyone had done it, if it was a simple mod, i would then like to A-B test it against one without the filter removed.

But youre saying the 40hz filter is down 1db? Doesnt it slope off? I was under the impression it was a shelf that rolled off sharply at 40hz....

Your thoughts?

(ps: thanks for the help!!)

> But youre saying the 40hz filter is down 1db?

Less than 1dB at the 40Hz measurement point and 12dB/octave for lower frequencies. The amplifier is crude enough that I can't estimate the exact filter shape, but it is probably a little sharper than a 2-pole Butterworth.

> Revel B15 sub, wich is nearly flat to 20hz

Re-label the knob 20Hz, 30Hz, 70Hz, and change these caps:

Thanks for the help, will give it a shot!! B