Harrison 32 series modification : Q factor

Hi there,
I have cloned the EQ section of the 32-series.
It sounds very nicely but I would like to add a switch for changing the Q factor of both Lo-Mid and Hi-Mid ranges .
I have already tried to modify R313 and R323 but nothing changed...
So I am looking for the involved resistors and/or capacitors.
Any ideas ?
Thanks

here they are

Thank you for helpin Martin
Actually I added a 37.4k and a 100k (by-passable with a switch) in series with the 9.1k and I noticed no difference...
What value should I choose ?

Can't help you. The way the circuit is laid out did my head in. One of the more gifted guys here will probably help. I think you picked the wrong Rs to play with, they seem to affect the + - dB range of the pot rather than the Q. Thats my guess

you need to change pairs of caps for each...  for the low mid C304 and C305... you will need to switch them independently so  2 pole switch required.

Scaling them further apart but still centered on the tuning frequency gives a wider bandwidth, closer together narrower.

JR

Hmm.. this is a bit offtopic altough vaguely related but I remember reading somewhere that back in the day filters were done using an RC design and doubling the filter circuit gave a 2nd order design (6db/oct -> 12db/oct etc) which also affects the Q value (early designs would self-oscillate at certain points where the feedback result would be a pure sine wave)

Then there's resonance which to my understanding is the result of the same feedback phenomenon but I think it isn't what one would be looking for in a console.

I've achieved the same effect with low quality mp3's played through a Formula Sound PM-90 mixer.

JohnRoberts said:

you need to change pairs of caps for each...  for the low mid C304 and C305... you will need to switch them independently so  2 pole switch required.

Scaling them further apart but still centered on the tuning frequency gives a wider bandwidth, closer together narrower.

JR

Click to expand...

Thanks, Jim, I'm gonna try this. I guess if the gain is modified too I'd have to play simultaneously with a resistor to compensate...

efinque said:

Hmm.. this is a bit offtopic altough vaguely related but I remember reading somewhere that back in the day filters were done using an RC design and doubling the filter circuit gave a 2nd order design (6db/oct -> 12db/oct etc) which also affects the Q value

Click to expand...

Not so much the Q than the phase-shift.

(early designs would self-oscillate at certain points where the feedback result would be a pure sine wave)

Click to expand...

2nd-order phase-shift is bound to create positive feedback and instability (oscillation).

I just replaced film caps by values closer than the central freq to get narrower bandwith. It didn't modify the Q so much. The same result if I put same values (8.2nF) for C307-C308...
When I put a variable resistor between pin 9 and GND it actually increased the gain to 20db (instead of ca.12dB). It is logical because it is a proportional Q vs gain topology...
So the target is to keep the same gain with narrower BW... Could it be possible... or not... ?

YannLu said:

So the target is to keep the same gain with narrower BW... Could it be possible... or not... ?

Click to expand...

You would need an additional opamp in order to use positive feedback; major mod...

Mmm I guess... too bad...
I really thought it would be easier. I've already cloned the MR4 EQ but it's very different circuitry...

abbey road d enfer said:

You would need an additional opamp in order to use positive feedback; major mod...

Click to expand...

Where to place that opamp ?

YannLu said:

Where to place that opamp ?

Click to expand...

After all you may not have to add an opamp, but you need to seriously alter the topology. See attached.

Thanks but I don't want to re-design the circuitry 

Are you the vendor of the Harrison Clone units?
So you want the service note that tells you how to do this.!!!..........
it is not one switch, the cap ratios have to change (this really does the Q) as do the resistors others have mentioned. The service note you don't have also tells you how to do peak/shelving on hi range, but you can derive that from MR series circuits anyway.

porkyc said:

Are you the vendor of the Harrison Clone units?
So you want the service note that tells you how to do this.!!!..........
it is not one switch, the cap ratios have to change (this really does the Q) as do the resistors others have mentioned. The service note you don't have also tells you how to do peak/shelving on hi range, but you can derive that from MR series circuits anyway.

Click to expand...

Thanks for the infos Porkyc !

Actually I gave up the Q factor mods on the 32-series clone. Indeed the Q is defined by the capacitors values...
But I succeed in the hi shelve/bell switching mode (given above in this thread) !

Anyway I just finished a stereo version (both channel perfectly paired and fully linked ) of that fabulous EQ in a 1U rack version Picture just above

As I worked for Harrison for 20 odd years, I am touchy about copyright and intellectual property!! (The Great River lunchbox version is a licenced product).
The original 32series had sharper Qs and was deemed "squeaky" so the cap ratio was changed to lower them..
I never understood why the top did not have shelf/peak when the bottom did. The later MR/TV series and PP1 film boards had SVFs anyway, as did everything else afterwards.
The unit looks good with its Rickenbacker knobs.

Which op amp did you use as a substitute for the HA4605?

living sounds said:

Which op amp did you use as a substitute for the HA4605?

Click to expand...

MC33079 (quad opamp)