V672 filter

[q2audio]

Hey guys,

After reading about the V475-2 15Khz filter section, it dawned on me that there is a inductor in the output stage of the V672 that I never could figure out the reason for. So, I tried the 'short it out' method with a switch so I could listen as well as see on my test gear. Interesting. With the inductor in circuit the top end rolls off just over 20Khz, with it shorted out, the -3dB point is around 240Khz (output loaded 600 ohms). My scope was in the car, so I did not look at the waveform, could be crap. Has anyone else messed with this? The inductor is followed by a 820pF polystyrene cap to ground. I didn't do enough listening to see if this removed the mid range bump that the V672 has, but maybe this would be a cool switch to add...

Tim

 

[dolo72]

Hi
This is my first post been hawking for a while(crazy knowledge here). Actually this v672 -20db 15k to 100k broadcast filter was gonna be my first question and how to get rid of it. Can I just clarify the inductor you bypassed was L201, i will try this out asap. I have noticed just listening my 672's are down in the top end compared to my sonosax and other mic amps its not drastic but I can always do with xtra highs.

Thanks for the post - Dolo :thumb:

 

[q2audio]

After posting last night I HAD to drag my scope back down to the 'lab' and see what was going on with the filter. Yes, L201 is what I bypassed. I have always measured the V672 flat to 20Khz, but with L201 bypassed I was getting a ridiculous top end response. I'm quite sure there was some ringing going on as well - if I didn't load it with 600 ohms it was really out of hand with a lot of peaks above 20K. I think the best thing to do here would be to find out the value of L201 and cut it's value by 50-75% or so to let the response rolloff without the peaks and valleys. I didn't hear any problems, but I was listening to a recording that was done on a 1/4" 8-track (not a lot of top end). I did not see any problems with no signal applied, no oscillations at about 30dB of gain.

I have always thought that the V676 had a much more open sounding top end than the V672 and this filter could be the reason. I think that the V672 does sound amazing on guitar 'as is' so I would not want to permanently remove this filter, but it would go good on a front panel switch! I also wonder if L201 and the following capacitor, C201, are the cause of the audible mid bump in these units. Some more switch flipping and listening will hopefully tell me.

Tim

 

[q2audio]

After having a disassembled V672 sitting in a pile on my bench for months, I finally got around to trying out a couple of inductor values to put in place of L201. I had a friend who has access to a nice HP LCR meter measure the original and it was right around 38 to 40 mH, depending on frequency... Chekc out the results:

Freq. L (mH)
100Hz 38.43
120Hz 38.44
200Hz 38.44
240Hz 38.39
1KHz 42.18
2KHz 39.16
4KHz 38.36
10KHz 38.12
20KHz 38.23
40K 38.81
100K 43.79

Sorry for the clumped together numbers... I can't figure out how to put space between the columns.

The 1K results are interesting. Enough change in L to make that mid bump? I need to look at the circuit again to see if it would even have that effect.

I have a couple of Fastron chokes on hand, a 22mH and a 33mH. I installed the 22mH and got about a 1.5dB bump at 20Khz that went up a bit with increasing frequency (I didn't write it down). The 33mH had about a 0.5dB bump at 20Khz and was about 4dB down at 40Khz, phase shift was a few degrees more at 20Khz than the original. I installed a switch to select between the original and the 33mH and it was quite audible... it seemed to make somewhat of a difference in more than just the top end. I'd love to hear someone else's opinion.

 

[dolo72]

Nice work Ive not touched the 672's either for a while . I'll be able to give this a try next week. By the way I was also looking at L 101 and L102 as possibly being something to do with the broadcast filter probably because they are labelled HF-drossel on my 672 data sheet. Please feel free to rubbish this theory as im a noob and this is pretty much gueswork on my part.

 

[balluda]

Has anyone tried out this filter thing? I would like to get rid of the 15khz filter aswell but I have very little knowledge about how it´s done./Balluda

 

[AndyP]

I was looking at the wiring diagram on the V672D manual and between pins 11 & 12 it says: überbrückung für tiefpass - bypass for low-pass filter. Does anyone know if there's something like this on the older V672 modules?

Yesterday I tested a couple of newly recapped V672/2's using RMAA and they did roll off above 15Khz.

Andy

 

[AndyP]

Here's the diagram I was talking about: