What is Zinky Thinking?

[bob p]

Sorry for the lame pun, but I'm looking at a Zinky/Fender schematic and I'm not following what he was thinking.

On the attached Vibro King schematic I've highlighted the area that's got me scratching my head.




V4A is used as a recovery stage from the effects loop. The output of V4A goes through two high pass filters in series, then to the volume pot at the input to V4B which drives the tone stack.

I'm not sure why they decided to put these two high-pass filters in series at the output of V4A:

The first filter, comprised of C5 and R21, looks like it amounts to a subsonic filter.

The second filter, comprised of C6 and R15, looks like it has a musically relevant corner frequency in order to throw away some LF content.

Why have these two filters been placed in series? I'm thinking that C5 and R21 doesn't even have to be there. Am I missing something obvious?

[d95err]

Can't see any reason for C5 and R21 either. Could this be a leftover from a previous version of the design that accidentally got left in?

[bob p]

This is the service manual for the amp. It includes the schematics for Rev A, E and F. That extra filter is there in all of the revisions.

[octal]

Well, the 2nd filter will roll off at a 6dB/ octave slope in (your terms) the musical frequency band. The first stage "subsonic" filter will roll off at a much lower frequency, but its roll off will combine with the 2nd filter to form a 12dB/octave filter below the corner frequency of the subsonic filter.

[octal]

Looks like the 2nd portion of the filter has a corner freq of about 90Hz.

[The Dude]

I know that some of the Zinky amps used some rather "unique" speakers. Possibly this is some special tuning for the oddball speakers?

[bob p]

I honestly don't have a clue about the oddball speaker thing. This amp used 10" Alnico Jensens.

Regarding the filter slope -- you could repeat the first filter over and over again to increase the slope, but I'm thinking the corner frequency is going to be so low as to render it irrelevant.

[octal]

Without seeing the full schematic- I know some of the Zinky designs used no NFB in the output stage. Is this one of them? So, expanding on what the Dude said-- if used with a speaker with a high Q bass resonance...the rolloff filter might be to try to keep things a bit tighter in the low end given the lack of damping from the power amp. (Assuming a no or low NFB design.)

[bob p]

> without seeing the full schematic...

the full schematic was posted in the 1st post, and the complete service manual was posted in the 3rd post. what's missing?

[The Dude]

I see the schematic. I don't see NFB.

[bob p]

regarding speaker Q:

I don't have the specs for the Jensen 10" alnico speakers but I remember a little bit about speaker Q in general.

Qms is a measurement of the control coming from the speaker’s mechanical suspension system (the surround and spider). I don't recall the Jensen 10" alnicos to have much in the way of a stiff mechanical suspension. I've always thought them to be loose and floppy with flimsy cones.

Qes is a measurement of the control coming from the speaker’s electrical suspension system (the voice coil and magnet). I don't recall the J 10" alnicos having a whole lot of voice coil/magent strength. Certainly nothing in comparison to what modern ceramic magnet speakers have.

Qts is the ‘Total Q’ of the driver.

Qts = (Qes * Qms) / (Qes + Qms)

That said, I can't imagine how a floppy alnico Jensen in an open backed cab would make this stuff relevant. This stuff would really matter in a sealed or ported cab, but for open cabs nobody even bothers with Thiele-Small math.

Of course, I'm no expert. Anyone?

thanks everyone for posting.

[bob p]

I see the schematic. I don't see NFB.

you're right -- the NFB is what's missing! lol.

that brings up another question, sort of my own hijack:

What's the real difference in performance between an open loop system like this amp with no NFB, and an NFB circuit that has a really high value resistor or pot inserted into the NFB loop?

my experience has been that adding a high value pot (or switchable fixed resistor) in series with the amp's regular NFB resistor cuts the NFB down to the point that the amp sounds like it's behaving in open loop mode, even though you've only reduced the NFB without going truly open loop. In those cases the amp sounds like it doesn't have NFB even though it has a small residual amount. How does that compare as far as speaker damping is concerned? Is the near-zero NFB condition with the high value resistor going to cause speaker damping to be improved significantly over the open loop condition? just trying to determine if it's better to go one way vs. the other.

[The Dude]

Agreed. IMO, at some high resistance value, the NFB is effectively similar or the same as with no NFB. In order to have variable NFB, there needs to be a pot there. Choosing a high value pot allows basically for "NFB defeat" without adding a switch. I think the difference between no NFB and NFB through a high resistance would be roughly equivalent.

[octal]

> without seeing the full schematic...

the full schematic was posted in the 1st post, and the complete service manual was posted in the 3rd post. what's missing?

It's what I'M missing. I just saw the yellow, highlighted portion. Sorry!

[The Dude]

......That said, I can't imagine how a floppy alnico Jensen in an open backed cab would make this stuff relevant. This stuff would really matter in a sealed or ported cab, but for open cabs nobody even bothers with Thiele-Small math......

The speaker is going to "flop" and try to reproduce the LF content regardless of cabinet- of course, less efficiently in an open back cabinet. In fact, I would think filtering would be more relevant in an open back box where there is no cabinet effect on maximum cone excursion. In a closed cabinet, cone movement can be limited by cabinet volume and porting. I'm thinking that the filtering in this amp is to tighten low end and make it less "mushy" by using a HPF to limit low frequency. It would also increase amp efficiency by cutting those unwanted lows.