BBDs biasing doubts

[Dimitree]

of course I would, I just don't know if I have the permissions to share it here, since it is property of E.H.

btw here is a link to it:

Code Select Expand


http://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CDMQFjAC&url=http%3A%2F%2Fguitar-gear.ru%2Fforum%2Findex.php%3Fapp%3Dcore%26module%3Dattach%26section%3Dattach%26attach_id%3D15663&ei=0kwgVO7DMtPaaJiLgcAI&usg=AFQjCNGVIoAqpVjn1b62SG59NB2ukyoejA&sig2=WBPLg_FQwdAJnNX5Zjf2uQ&bvm=bv.75775273,d.d2s&cad=rja


I'm using BL3208 in place of MN3005, and of course inverting voltage (-15V -> 15V)

I don't mind about running the LFO at 15V or 9V
My doubt is: is it worth running the opamps at 15V, and the BBDs at 9V? this way I could get some more headroom for the dry signal, and maybe the compander would work better?

[armdnrdy]

Can you post a regular link to the schematic? I can't do anything with the format that you posted.

[Dimitree]

Try this
http://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CDMQFjAC&url=http%3A%2F%2Fguitar-gear.ru%2Fforum%2Findex.php%3Fapp%3Dcore%26module%3Dattach%26section%3Dattach%26attach_id%3D15663&ei=0kwgVO7DMtPaaJiLgcAI&usg=AFQjCNGVIoAqpVjn1b62SG59NB2ukyoejA&sig2=WBPLg_FQwdAJnNX5Zjf2uQ&bvm=bv.75775273,d.d2s&cad=rja

[Dimitree]

up, please

[Fender3D]

... this way I could get some more headroom for the dry signal, and maybe the compander would work better?

I'd say you aswered yourself already...

How much will it differ from stock layout? 2-3 tracks, 4 maybe and a regulator IC?

[PRR]

> Try this

You are posting the link to the Google Search. This will not always work for someone else.

The true link appears to be:

http://guitar-gear.ru/forum/index.php?app=core&module=attach&section=attach&attach_id=15663

This connects to a small PDF file of the schematic for a 1995 DMM.

[Govmnt_Lacky]

Does anyone happen to have a link to or a copy of the (4) MN3008 version schematic of the DMM? I'm looking for the New Sensor/EHX schematic. I already have a redraw but, I would like to verify. 

Sorry for the hijack 

[Dimitree]

> Try this

You are posting the link to the Google Search. This will not always work for someone else.

The true link appears to be:
http://guitar-gear.ru/forum/index.php?app=core&module=attach&section=attach&attach_id=15663

This connects to a small PDF file of the schematic for a 1995 DMM.

yes it is indeed this one.
My doubts is, if I power the audio signal path using +/-15V (and the rest using 9V, given that the clock requirements are meet), would U2B opamp give the wrong bias voltages to the BBDs?

[Fender3D]

Yes and nope...

I mean: you must set bias voltage (with VR8), then you'll always trim it for a proper BBD working bias voltage...
Issues may arise if U2b doesn't work properly with a bias slightly "off"...

In that case, U2 should be supplied with (decoupled) BBD supply.

If you're in hurry and don't mind breadboarding, you may safely run IC2 with (decoupled) BBD supply, without worrying about headroom (it is after a compressor, afterall...)

*
decoupled:
add a 10R resistor and a 100uF cap (as the block between A and B on schematic)

[Dimitree]

Yes and nope...
I mean: you must set bias voltage (with VR8), then you'll always trim it for a proper BBD working bias voltage...
Issues may arise if U2b doesn't work properly with a bias slightly "off"...

what do you mean? if I set the proper BBD bias voltage using VR8, then U2b may not like that particular voltage?

[Fender3D]

Yes,
TR8 sets bias for both IC2b and BBD.
If I recall, BBD's bias should be slightly higher than 1/2 Vcc, then ~5V with 9 V supply.
Best op-amp swing will be @ ~7,5V with 15V supply...

It will depend on how much signal you have @ IC2b's output...

[Dimitree]

and what if I tie U2a (positive input) direcly to 7.5V voltage divider (supply C, just like U3a), and connect the center lug of VR8 to BBD input (maybe using a resistor like on the other trimpots)? If so, do I need a DC blocking cap?

[Dimitree]

this is what I mean:



(on the left the original schem., on the right I highlighted the mods in yellow)

[anotherjim]

That looks good to me. You've isolated the BBD bias from the opamp's DC.

[armdnrdy]

Pin 8 (VGG) of the 3205 should be 14/15 of the BBDs voltage....in this case 9 volts. I'm not sure what that 6.8K is going to do for you.

The VGG voltage is usually achieved with a voltage divider...or the MN3201 and 3101 clocks also provide the proper VGG voltage.

The VGG for the 32XX series at 9 volts should be about 8.4 volts.

A voltage divider consisting of 1.6K on top, and 22K on the bottom will get you there.

It's also a standard practice to connect a 1µf cap at the junction of the two resistors for better stability.

[Fender3D]

It will work ok, with the added cap..

Larry:
100:106,8=14:14.952   

[armdnrdy]

I don't understand your equation Federico  

Ah...got it. I now see how the bias trimmer is used as the bottom resistor of the voltage divider.

So..6.8 on the top and 100K on the bottom equals 8.4 volts.

I don't believe I've ever seen it done like that. Very clever!

[Fender3D]

PS

MXR flanger redone....

[armdnrdy]

PS

MXR flanger redone....

When I was getting your help on that thing all those years ago....I didn't understand a thing!

and I haven't revisited that circuit since!

I'll take a look. 

[Dimitree]

thank you, but is 1uF value appropiate for the cap I added?

another question, why R25 is inside the opamp loop and not before R39? what's the difference?