MN3008 BBD circuit - modifications help

[ilcaccillo]

Hello,
I have a Pioneer SR-60 unit and although it says "Reverberation Amplifier" it's actually a Delay, it uses a MN3008 BBD delay chip.



I would like to have a MIX knob, so I could have 100% WET when needed, as in stock mode there's no provision to change the MIX between Wet and Dry.

It would be great also to be able to change the Delay Time and the Feedback level.

here is the schematic for the unit:




https://i.imgur.com/JVhDN9V.jpg

The schematic is a bit confusing to me because of all the switching going on,
I need help from you guys.
How do you think I could implement a MIX Knob?

Thank you so much

[ilcaccillo]

This is the datasheet for the MN3008:

[bluebunny]

[IMG HEIGHT=nnn] . . . [/IMG]

Just sayin'... 

[ilcaccillo]

[IMG HEIGHT=nnn] . . . [/IMG]

Just sayin'... 

Done,
just left the schematic bigger so it's easier to see the details.

Thank you so much

[bluebunny]

You didn't need to post different pictures.  The same ones, but with HEIGHT=nnn, would have appeared at a reasonable size, but would be clickable to show the whole thing.

Like this:

  <-- click!

[11-90-an]

Changing the values of R39 and R41 should help... (R39 for wet, R41 for dry..)

I recommend just pulling out R41 and relacing i with say, a 25K potentiometer..

[ilcaccillo]

Changing he values of R39 and R41 should help... (R39 for wet, R41 for dry..)

I recommend just pulling out R41 and relacing i with say, a 25K potentiometer..

Thank you, I will try that out

How about the Feedback? I'm not getting how the delay feedback is setup in this circuit

[11-90-an]

If Im not mistaken, VR1 is a dual-gang pot, and it controls both how much wet reverb is mixed in, and the feedback... 

Changing the values of R39 and R41 should help... (R39 for wet, R41 for dry..)

I recommend just pulling out R41 and relacing i with say, a 25K potentiometer..

A correction from what I said earlier, when you change R41, that would be a DRY control.
The REVERB knob is a WET mix knob already...

[ilcaccillo]

A correction from what I said earlier, when you change R41, that would be a DRY control.
The REVERB knob is a WET mix knob already...

The reverb knob mixes more or less effected signal with the dry but the dry signal is always there, you can't achieve 100% wet.

I would like to be able to have a Mix knob to balance between completely dry and 100% fully wet

thanks

[anotherjim]

That schematic is one of those where you cannot size it on a screen so you can see all that you need to see without zooming out and then can't read the detail that is then too small!

R41 for dry indeed, but just making it variable doesn't let you totally kill the dry. If you split R41 into x2 10k in series and then either add a kill switch that shorts the middle of the resistors to 0v or, instead of a switch, a 100k pot to 0v to make it adjustable.

[11-90-an]

Or maybe follow what the theremin fuzz did... 

Place a 100k pot's outer lugs on the wet and dry resistors are. (Take note of the order!)
Connect the middle lug to ground.

P.S. you should pull out R41 and R39 before this...

[ilcaccillo]

Or maybe follow what the theremin fuzz did... 

What is the Theremin Fuzz?

Thank you

[moid]

Pretty sure they are referring to this:
https://www.parasitstudio.se/thereminfuzz.html

[ilcaccillo]

Thanks.

It would also be simple to install a Kill switch, it solves the problem

[ilcaccillo]

Just for testing purposes I was to cut Kill the Dry signal by lifting one leg of R41 (Left channel) and R42 (Right Channel).
It works perfectly and I can install a Kill Switch there, thinking in something like this

[ilcaccillo]

Anyone knows how could I change the Delay time? even if very limited?

Thank you so much

[11-90-an]

The delay time in this ckt is based on the clock freq

I tried to simulate the circuit and tried to adjust the speed, but i don't really get it...

Here's a more specific schem of the clock...



P.S. don't mind the top and bottom schematics, those are something else...

[anotherjim]

It's a standard 2 inverter oscillator. The 39k+10k is the path usually made variable. You could get a lot of range but the BBD can't be clocked too fast or it gets errors (noise/distortion) and too slow the clocking noise breaks through the filters. If you fit a pot in place of the 10k, you can go faster (shorter delay) at zero resistance and slower (longer delay) at high resistance. A 22k to 50k pot value should be worth trying.

[ilcaccillo]

It's a standard 2 inverter oscillator. The 39k+10k is the path usually made variable. You could get a lot of range but the BBD can't be clocked too fast or it gets errors (noise/distortion) and too slow the clocking noise breaks through the filters. If you fit a pot in place of the 10k, you can go faster (shorter delay) at zero resistance and slower (longer delay) at high resistance. A 22k to 50k pot value should be worth trying.

Thank you much for your explanation and suggestions anotherjim.


I spent some time today trying out the suggestions and spend a little bit of time tweaking the values so that the control range provided by the potentiometers was nice,
These were the values I liked:



For the FEEDBACK the 51K resistor with a 200K Log pot in series gives a good range between few repeats and it goes into Oscillation in the full rotation.

On the DELAY TIME, this one is tricky. A 100K or 200K Pot instead of R132 works nicely for slowing the delay time, but there's problems... Probably what anotherjim explained.

With longer delay times there's a constant High Frequency whine pitch note that goes down in pitch as you slow down the delay and it's quite present.
I suspect that that high pitch note or noise is also there on the stock delay time but at an higher pitch above the hearing range.
Slowing down the delay brings that note into the audible range and it's quite irritating. (after the pot reaches around 70K)

Is there any solution for this? Or at least a way to attenuate?
I don't have any noise besides that constant high Pitch sound

Thank you so much for your help, I had great fun today trying your suggestions​

[anotherjim]

and too slow the clocking noise breaks through the filters

I said, and that's the whine you get.
Clock noise appears as either...
A clear tone at the clock frequency if that is in the audio band or...
A metallic ring modulation (called aliasing) comprising of a difference frequency between audio and clock frequencies - this can occur even when the clock is above audio range. or...
Both!

You can't easily fix this. A more sophisticated design might have voltage controlled filters so that as the BBD clock frequency falls so do the filter cut-off frequencies. This would soon get very dull sounding though but conversely, it could then be brighter at short delays than it currently is.
You can't simply fix the whine with an extra filter on the output either - you have to filter more before the BBD too.
The important filters are called anti-alias (before BBD) and reconstruction (after).
A good write up and description here...
https://www.electrosmash.com/boss-ce-2-analysis
...and the fixed filter design used is too complex to easily make adjustable with a simple control.