BBD delay as simple as possible

[allesz]

The schem above was the resul of a discussion started by user stfala here: http://www.diystompboxes.com/smfforum/index.php?topic=112152.0

It give a decent single repeat in the range of 100-150 msec, but I want more: I would like to have some control on the number of the repeats but:

- Connecting the two asterisc on the schem with a sort of level controls does not work.
- I subbed the bjt with another TL072; now I have two inverting buffers, one for the delayed signal and one to (try to) feed that signal back to the bbd input. This idea does not work too  : I don't get more repeats, just distortion.

I can only connect the echo signal back to the input, but this messes quite bad with the dry signal (thought the IC buffer works a little better).

Is there a right way to connect the delayed signal to the bbd without messing with the input?

Do biasing the bbd input (like in the dm2 for example) will help?

[anotherjim]

To get repeats you really need a mixer at the input to the BBD, similar to the output mixer of the second op-amp. You might get away with a passive resistor mixer there, but it's never the  best solution IMHO.
To add a level control for the repeats, you could add a 500k pot in series with the 47k out of the transistor. That would give variable attenuation of the delayed signal, but not completely turn it off.
If you use the same circuit to mix  repeats into the BBD, another pot in series with signal coming from the transistor emitter would work the same.

You have the BBD input bias at 1/2 Vcc reference (Vb). That could be around 4.5v (if you're using 9v power) and it could be that the particular BBD you have is happy with that, but they do vary on the working bias level from chip to chip. If you put a 10k trimpot in between the 2 10k divider resistors and take Vb from the wiper of the pot, you can set the bias anywhere between 3 & 6 volts to suit the particular chip.

Joining the asterisks was never going to work. The op-amp output is a very low impedance so it will just fight the transistor (which is also low impedance), but like I said earlier, resistors (4.7k? 10k?) added after each output and joined together to BBD pin7 will mix the clean and delayed signal without any fighting. But using another op-amp instead gives another opportunity for a filter cap on its feedback loop should it prove to be needed.

Your circuit is all directly DC coupled. You may just about be getting away with it, but the output op-amp has a gain of about 2. If the output of the transistor has a DC level any different from Vb, then that difference is multiplied by 2, and as it's inverted, the output is pushed down by that amount. It would be advisable to use a capacitor from the emitter to AC couple the output of the transistor which would remove this bias offset. The choice of capacitor value gives you the option (small value cap) to lower the bass response a bit, which can improve clarity when the echoes are dense. Something like 100nF or lower. The same reasoning goes for input repeat mixer if you add one.

[allesz]

Thank you anotherjim.
After some paint work I can show you actually where I am, sorry for the schem "quality" 



The point is, of course where to connect the question mark point...
Not before the input cap, like we said...
So, before attempting to decouple the 3205 input with caps and then find a way to bias that damned pin, you suggest to put a couple of 4,7K/10K in front of 3205's pin 7 and of the question mark, and join them togheter (of course I will put a volume pot (10K?) between the question mark and the 4,7K/10K resistor; did I understand right?

Btw, where can I buy question marks (only quality items please) in Europe?

[Fender3D]

What's easier than the application circuit in datasheet?
Check it out, you may simplify the input and output filters, and use your BJT for high impedance input buffer

[allesz]

Good point bro.... Maybe cutting some cap and resistor. .. At the moment I have the same amount of op amps afterall.
Btw, did someone tried it?

[armdnrdy]

Question for you allesz.

You never really gave your reason for exploring a minimal parts count BBD delay.

To make it as small as possible?

As an exercise?

Do you want it to sound the best that it can? or...are you willing to sacrifice low parts count/size for quality of sound.

[PRR]

The mixer goes AT the input of the BBD.

"Repeats" really means a potentiometer-- if everything were exactly unity, repeat would be infinite; but things are not unity and you don't want infinite repeat.

I don't believe the transistor needs that 220K.

I do agree there should be some DC blocking.

Changes marked with red slashes.

http://oi57.tinypic.com/2zzj0pu.jpg

[allesz]

Thanks for the help people 

Of course this is more of an exercise: with the same parts count you can build a way better delay with the usual pt2399 chip; but it's just something I had to try, just for the challenge. I mean, today you can buy really good delay (digital or analog) for little money.
Then, if it useable, I will be happy: a single repeat of 100 msec is quite short, but 100 msec with two repeats is not too bad for my simple, unprofessional and nerdy diyer slapback needs.

Thanks for the input PRR, I will give it a try.... but I would like to avoid that kind of output buffer, and the bjt buffer for the delayed signal: inverting buffer (I am a total noob about them) appear to offer more filtering opportunities.... and we need a lot of filtering here. 
Tonite, instead of compulsive breadboarding, I studied the 3205 datasheet and the app schematic, thinking about Fender3d's words....

I was thinking about this signal flow: bjt input buffer (non inverting),then the signal goes to the output and to an IC (inverting) buffer (maybe with some filtering in the feedback loop), then to the 3205 (straight or with a small resistor?). The echoed signal then goes in another IC inverting buffer (heavy filtering do apply here) and from this buffer goes to the output (I could even put a mix control) and to the input of the first inverting buffer (with some kind of pot, of course) to give repeats.

[Scruffie]

Needs refinement, not sure it'll work as shown but a quick idea I just sketched out to play with;



Has pre & de-emphasis and a sallen-key all set to 1591hZ, the mixing is done at the low pass filter.

[allesz]

OMG! Thank you a lot for the contribution Scruffie!

I started thinking about a BBD project since I saw your EH Full Double Traking conversion for 3205.... others are too complicated to do by myself, unless I buy a kit, of course.

I think I will have to go home, get the family out, and lock me in 

[Scruffie]

No problem

Something like this version is probably better, the dry wont be filtered so much and it cuts a few parts out, once again it's only a quick 5 minute drawing so no promises it'll work as is.

It's a bit like an electric mistress really with a heavy sallen key on the BBD output.

[allesz]

An humble question, since today I will have some breadboard time: does the vr network substitute tha classic two 10k resistors used to bias the IC?

[Scruffie]

Yes it does, you can have it with 2 x 10k if you want but the trimmer is better, opamps are a bit more forgiving of bias than BBDs.

I can see a few issues with that drawing now, try it first though, it should still work.

[allesz]

Thank you.
Today I tried the first one. No dice.
Started debuggin, tore everithing apart, then realized the battery amp I was using was amlmost dead.
Rebuild everithing. .. right now I have a working input buffer
Will report my progress.

[allesz]

Finally a little bit of success tonite 
I will post the schematic asap.

[allesz]

I upped the "time" control capacitor to 470pF, before it was shorter but not noisy, now at the longest setting it wines a little..... probably I should add some filtering; it's quite dark sounding right now...

I had to put some gain on the op amps, no more plain buffers; somehow the volume was less than unity; now, maybe, I have a little boost, but it's not too big and, btw, I don't like true bypass delays.

The schem in the end is part mine, part PRR, marteen an Scruffie: it borrows and mixes from everyone 

I called it the echopathit because... well, you have to try it.

Of course I would be happy to improve it.... of course I'll need some help.

[allesz]

You can ear that the echo is quite dark sounding and, as long as the repeats get longer, a sort of wistling note emerges.... it can be perfected... but it's not too bad (apart for the guitar playing ).

https://soundcloud.com/allesz/echopathic

[allesz]

Eliminated the 470k resistor and 22nf cap from the bjt buffer;  then modified the echo filters: a 10nf in place of the 3,3 nf that now goes to the emitter of the bjt, not to ground.
Then upped the 10k resistor in the time control network to 22k
Did a comparison with my digital delay: it can get aroun 300 msec, and wining only starts at around 200 msec.
I wonder if using 2 3208 would improve noise.

[Scruffie]

3208 if they're like the 3008 (CBA to check right now) have slightly better specs than the 3205... but having 2 in series will likely negate that.

200mS is what the chip is specified to give, we drag more out with lots of filtering and noise reduction, getting more than that with a single 2 pole low pass filter and a little bit of input filtering and no compander from one chip is a very tall order.

For 300ms you need a clock frequency of ~6.75kHz just for reference, quite audible...

You could try dropping the value of the 4n7 on the transistor to 1n, it'll raise the filter point to 1kHz but give it a steeper roll off... or you could make the caps 22nF & 2n2 and it'll be about the same frequency and steeper.

[allesz]

Thank you. I will try those values asap.
I am thinking about a layout.... no 1950a this time, but even in a 1950b it will be quite tight.

I have the small 3208s... I was thinking about swap the bjt for another tl072: in order to have one more buffer between the two bbds.