BBD delay as simple as possible

[thomasha]

Hi, nice to see you guys so fast back to the discussion!

I guess that you have to set the TL072 bias so that it works at the BBD desired bias.

A complication in my build, an not shown in the schematic, is that I bought a v3205SD. I never pay attention to the leters, but in this case they were important... It's the 5v version of the chip...For it to work I had to reduce the voltage below 8v.
http://coolaudio.com/docs/COOLAUDIO_V3205SD_DATASHEET.pdf

Since a voltage drop was required I used a 7805 to get 5v and power the clock and BBD at this level. I'm biasing the BBD separately with a trimmer, cap and 100k resistor.

If the clock is at higher voltages, like 9v, VGG will be off and it won't work, right? I have to test that, but I think it would at least require a special bias for VGG.

The twin-t works, but it's a heavy drop in signal right in the middle of the bandwidth, and needs to be changed for each setting of the clock.

[Mark Hammer]

As for the "high-pitched noise", as discussed in another thread here, consider inserting a 5k trimmer between pins 3 and 4 of the BBD, and connecting all the stuff presently connected to those pins to the wiper of that trimmer.  If the circuit has enough lowpass filtering, and/or uses a clock frequency well above the human hearing range, some pedals can afford to skip balancing the two outputs.  But you don't have much filtering, and are aiming for a longish delay time (i.e., low clock frequency) so balancing the two complementary outputs with a trimmer to cancel out the clock noise a little more is probably a good idea.

Ideally, the bias voltage for the BBD is something other than what is used for Vref in the rest of the audio path (Vb in your diagram).  If you look at the circuit for the Zombie chorus, you'll see that it uses a single Vref for the entire circuit, effectively feeding it "through" the op-amp, but it is not simply half V+ (i.e., 4.5V).  I take it that John Hollis used that as a sort of compromise that would come closer to the ideal bias for the BBD, but still provide enough headroom for the dual op-amp.

[Scruffie]

The 320X series are advertised to work at 5V which made them appealing for battery but they can actually take 11V (absolute) max as that datasheet states. There are various pluses and minuses at different voltages depending on the BBD type which that datasheet doesn't provide, find the original Panasonic one to see all the specs.

From a quick glance, with higher voltages you get S/N improvements, less gain loss, better frequency response and headroom but at the expense of worse THD.

The MN3102 just has a simple voltage divider internally so the Vgg will adjust with supply.

Edit: But the clock & BBD must be at the same supply voltage or else you will run in to problems.

[Scruffie]

Ideally, the bias voltage for the BBD is something other than what is used for Vref in the rest of the audio path (Vb in your diagram).  If you look at the circuit for the Zombie chorus, you'll see that it uses a single Vref for the entire circuit, effectively feeding it "through" the op-amp, but it is not simply half V+ (i.e., 4.5V).  I take it that John Hollis used that as a sort of compromise that would come closer to the ideal bias for the BBD, but still provide enough headroom for the dual op-amp.

The bias varies for the BBD type and for the supply voltage, the datasheet actually confirms it's not too far off half supply for this chip (a little lower for 5V, a little higher for 9V) obviously a trimmer is better as that's just an average but if it's not creating nasty distortion!

[Mark Hammer]

The 320X series are advertised to work at 5V which made them appealing for battery but they can actually take 11V (absolute) max as that datasheet states. There are various pluses and minuses at different voltages depending on the BBD type which that datasheet doesn't provide, find the original Panasonic one to see all the specs.

From a quick glance, with higher voltages you get S/N improvements, less gain loss, better frequency response and headroom but at the expense of worse THD.

The MN3102 just has a simple voltage divider internally so the Vgg will adjust with supply.

Edit: But the clock & BBD must be at the same supply voltage or else you will run in to problems.

Useful to know.
Again, one needs to appreciate that the MN32xx series was developed to cope with what would have likely been a more common 9V battery supply at that time.  So the recommended 5V voltage was for the purposes of maintaining a stable reliable bias voltage, even as the battery-voltage waned from 9.6V (fresh) down to around 7V.  With few players depending on 9V batteries these days, preferring to use a wall-powered external supply, using higher supply voltages is a realistic option.

[thomasha]

As for the "high-pitched noise", as discussed in another thread here, consider inserting a 5k trimmer between pins 3 and 4 of the BBD, and connecting all the stuff presently connected to those pins to the wiper of that trimmer.

I tried it with a 10k trimmer and it really helps. The setting is a little off-center and the whinning is considerably reduced in comparison to center position. There is a narrow band where I could hear a drop in the noise.

The 320X series are advertised to work at 5V which made them appealing for battery but they can actually take 11V (absolute) max as that datasheet states. There are various pluses and minuses at different voltages depending on the BBD type which that datasheet doesn't provide, find the original Panasonic one to see all the specs.

good to know, will check that.

I completely changed the circuit today. First I introduced another tl072, both sides working as 3rd order Sallen-key filters, at a frequency around 3.5kHz.
Just realized that boss also did that, but with 10k resistors. In mine I put 10k, 4.7nF, 100k, 2.2nF and 30k 330pF, kind of lossy.

The second opamp of the first IC is now the de-emphasis stage, with similar frequency response as the first stage (2.2n and 4.7k). It's also the new output (need to finish this schematic). I kept the 33k 15n low pass filter between the anti-aliasing and the de-emphasis stage, at the place of the 47k mix resistor.

What is interesting is the gain of 20 for frequencies above 10kHz, was that really the intention? Boss has a much lower gain (5 times) and a lower frequency corner of approx. 2kHz.



It looks much more complicated than previously, but it helped. I'm one compander away from the dm2 apparently, but it's at my limit of ICs for a 1590a (equivalent surface area of 5 Tl072s).

An experiment that I did was test 5v supply vs 9v supply for the v3205sd and the clock. I measured the output signal after the anti-aliasing filter, while the input was fed with a 600Hz sine wave. I had to adjust the bias for both cases, but I could get 2 times more signal out of the BBD with the 5v supply than with 9v. I used a cheap oscilloscope, and completely forgot to check the RMS amplitude at the input of the BBD...

When I tested it with my guitar the 9v configuration sounded distorted and quieter, it was also more difficult do adjust the bias of the bbd with the 20k trimpot. The "usable" range of the bias was narrower than with 5v.

Good news is that I got longer times without the clock noise, but it is sounding much darker. I'm still planning to work on the emphasis stage and check the boss values.

[ElectricDruid]

I tried it with a 10k trimmer and it really helps. The setting is a little off-center and the whinning is considerably reduced in comparison to center position. There is a narrow band where I could hear a drop in the noise.

Yeah, it's a pain to have another trim, but it does provide an advantage if you're using low clock frequencies. If the clock is much higher, the filters generally can deal with it.

I completely changed the circuit today. First I introduced another tl072, both sides working as 3rd order Sallen-key filters, at a frequency around 3.5kHz.
Just realized that boss also did that, but with 10k resistors. In mine I put 10k, 4.7nF, 100k, 2.2nF and 30k 330pF, kind of lossy.

I checked your values in the tool you mentioned earlier, and I'm not seeing anything lossy. Gain=1, like it should be.
http://sim.okawa-denshi.jp/en/Sallen3tool.php
If you're losing a lot of signal through the filter, something's wrong.

An experiment that I did was test 5v supply vs 9v supply for the v3205sd and the clock. I measured the output signal after the anti-aliasing filter, while the input was fed with a 600Hz sine wave. I had to adjust the bias for both cases, but I could get 2 times more signal out of the BBD with the 5v supply than with 9v. I used a cheap oscilloscope, and completely forgot to check the RMS amplitude at the input of the BBD...

..then go and check! It doesn't make sense that you get more signal out at a lower voltage.
My own experiments with these BBDs at 5V suggested that the maximum input was around 1Vpp at that voltage, so you might be able to get as much as 1.8 or 2Vpp at 9V supply (ooh! Woo! wow! etc ).

When I tested it with my guitar the 9v configuration sounded distorted and quieter, it was also more difficult do adjust the bias of the bbd with the 20k trimpot. The "usable" range of the bias was narrower than with 5v.

Again, this doesn't sound right. Something else is different between the two set-ups.

Good news is that I got longer times without the clock noise, but it is sounding much darker. I'm still planning to work on the emphasis stage and check the boss values.

More filtering = less clock noise = darker. You can't reduce the high end to get rid of the signal you don't want without also reducing the high end of the signal you do want. As you say, pre-emphasis might help here, to offset the darkening effect of the filters somewhat.

HTH,
Tom

[Scruffie]

Save yourself some time and hassle, find the schematic for the 'Economy Memory Man', that pulled nearly 300mS out of 3 x SAD1024 and was about as simple as your schematic and gives you a spare op amp as you don't need the gain recovery stage between BBD's.

[allesz]

I agree with Scruffie: if you have to over complicate the project, maybe it's better to build something else, like a DM2 or another tested and confirmed project.

The way to keep it simple would be to avoid clock noise and then avoid a lot of filtering.

I, for example, in the end built the effect without pre and de enphasis and just a 200 pf (exactly two 100 pf caps in parallel) and a 100K time pot; you don't get wining but you also are stuck with short time repeats (around 200 ms probably), wich doesn't get more than slapback.

I also discovered that if the max avaliable repeat time is short you can just use two repeats instead of just one (it is not the same of course, but it gets the job done). Also I found I really nice effect that is not easy to obtain with a standard analog echo: I like to put a lot of repeats (just on the verge of feedback) with a really short time (difficult to obtain with commercial units), the effect is a sort of unmodulated metallic flanger that I really like.

In fact, of the two I boxed, one is in a 1950A (but it use a 3208 instead of the bigger 3005), that I use only for the described effect; the actual delay can be called a doubling effect.

Another simple work around would be to use two 3205 for longer delays (longer... around 400 ms :-)), but the clock IC can drive only one 3205....

[allesz]

Sorry, I forgot to add that my chips were also the low voltage type. In fact I put in the power supply a protection diode and a 470 Ohm resistor in series, they both keep the B+ a hair under the maximum 9V supply for the BBD.

So I agree with Electric Druid, is should work good also at 9V.

Btw, could a voltage regulator  be a way to separate the audio power line from the clock line?

[ElectricDruid]

Btw, could a voltage regulator  be a way to separate the audio power line from the clock line?

Yes, it would probably help separate the two power supplies. But that assumes that the shared power supply is the reason you're hearing clock whine. It probably isn't, especially if you're using low frequency clocks. At low frequencies, the losses through the delay line are worse and noise (including clock noise) goes up. And since you have the clock frequency within the audio bandwidth, you'll hear it. There's really no getting away from that. You can have "simple", or "long", or "quiet", but you can't have all three. There's a reason so many classic analog delays used pretty much every trick in the book - it was the only way to make them acceptably quiet.

If I were you, I'd pick two out of the three I mentioned and focus on getting those optimised.

[allesz]

Well, if the noise is coming also from the audio path, I think you are totally right. And I choose simple and quiet.

When you hook up a BBD, even an humble 3208, and you get incredibly long delays... yes terribly noisy, but incredibly long, you get mad: I couldn't admit that it was impossible to get rid (in an easy way also!  )  of the whining. I did my (modest) best, and gave up.

But I don't give up to hope that someone will find a compact and clever solution, possibly misusing and abusing some component, to the problem.

[thomasha]

..then go and check! It doesn't make sense that you get more signal out at a lower voltage.
My own experiments with these BBDs at 5V suggested that the maximum input was around 1Vpp at that voltage, so you might be able to get as much as 1.8 or 2Vpp at 9V supply (ooh! Woo! wow! etc ).

I'm testing it again, it is probably a problem with my breadboard. I have some caps that like to jump out of place, wouldn't be the first time...

Save yourself some time and hassle, find the schematic for the 'Economy Memory Man', that pulled nearly 300mS out of 3 x SAD1024 and was about as simple as your schematic and gives you a spare op amp as you don't need the gain recovery stage between BBD's.

Never had seen those! Thanks, that is exactly what I'm looking for at the moment.

I did my (modest) best, and gave up.

It is way better than you think. After making it more complicated and achieving similar results I think I'm going one step back and try a smaller version too. I really want to try the twin-T filter again. I'm having lots of fun by experimenting and annoying my wife with high pitched noises 

[allesz]

Thanks, I don't want to dismiss the thing, by any means.
Being totally uneducated in electronics, I can build effect, tube amps, even an analog delay! Thanks to all the great diyers and DIY forum(s).


My maximum values with no whining is 200pf and 100k for the time network. How far can you go with the 1 Meg pot?

[thomasha]

How far can you go with the 1 Meg pot?

Haven't measured the pot, but the clock was at 5.7kHz when there was no audible whine.

At the moment it is kind of too dark, like a blanquet over the amp. But I want to remove the extra stages and start from your initial idea, just cutting a little more signal at the second stage. Your schematic sounds much clearer.

I also tested the supply of the chip again. Using a sine wave, 500hz, 0.38v at the cap in front of the BBD (I'm setting the bias with the trimmer).
I adjusted it for max. output and measured after the balance trimmer. With the max. signal coming out I changed the clock speed to have the same 0.38v at the output, which resulted in a clock rate of 4.47kHz for 9V (8.82).

When using 5V  at same clock (had to adjust speed), I had to adjust the bias trimmer again and got for max. output 0.44v at the output.

When I increase the clock rate the signal at the output also increases, so I guess I should have done the measurements at usable clock rates. But the pedal is so dark now that even at 3kHz the whine is not bothering me. The problem is that at this rate the wet signal has aliasing noises (crispy sound).

It looks to me that the chip likes 5v better, at least at this clock rate and input signal.

Now the balance trimmer at the ouput made a huge difference! You should give it a try. It will extend your delay for a couple of ms.

PS: I checked different clock frequencies and calculated the gain of the BBD for 9v and 5v supply (0.37v at the input of the BBD, 500Hz sine)

[ElectricDruid]

The most interesting idea for me is the use of a narrow notch filter to remove the clock whine. Now, even that's going to have limitations no matter how deep the notch is, because the clock whine isn't a sine wave (and therefore is not just a single frequency) but it might well make more difference than anything else.

Bonus points if you can do it using some switched capacitor filter chip so that it tracks the clock frequency automatically.

Oh, hang on, bang goes "simple" again...

[j_flanders]

Isn't clock whine simply the (one) clock frequency? The sum of of the two out of phase clock signals?
If you set the balance trimmer exactly 'right', there should be no clock whine at the output of the bbd. Especially if you have only one bbd.
That one trimmer, when set correctly once , should solve it for any chosen clock frequency/delay time.

When I watch the bbd output without input signal I see one clock (whine) frequency on the scope and when turning the balance trim, I can pretty much remove it completely. It might still show up on the scope a little bit but at levels that are absolutely inaudible.

Aliasing distortion / heterodyning / sum and difference harmonics or whatever you want to call it: the 'distortion' that results from input signal + clock frequency can never be dialed out.
On a scope you can clearly see the 'harmonics' generated from input signal + clock frequency.
For example a clock frequency of 10kHz and a sine wave input of 1500Hz generates spikes at:

11.5KHz, 13kHz, 14.5kHz etc but those don't really matter much because a typical guitar speaker rolls off after 5kHz or so.
The ones that matter are: 8.5kHz, 7kHz, 5.5kHz, 4kHz etc. because those can be heard more clearly.
The higher order harmonics are lower in amplitude though.

The typical solution to minimise those are:

1) anti-aliasing filter: remove high frequencies from input.

2) use a clock frequency that is high enough (>  2 x max input frequency)
If your lowest clock freq is 10kHz, don't let anything over 4,9kHz in.
The first difference harmonic will be 10kHz - 4,9kHz= 5,1kHz.
You can in theory filter out anything above your highest input frequency of 4,9kHz.
This way you can remove all clock and still preserve all input. In theory, because no filter will be steep enough.
Or vice versa: if your max input signal frequency is 4,9kHz, don't let the clock come down below 10kHz.
This is just taking into account the first (and strongest) harmonic, the second and third etc will also be present and to avoid those you'll have to reduce the input range even further or increase the clock frequency.

3) LP filter at the output of the bbd, filtering out anything above the highest input frequency.

4)use more bbd's (although you'll run into problems with the maximum capacitive load for the clock)

As said earlier, the compander only solves the white noise:
1) compressor makes the input signal twice as loud
2) signal goes through bbd, constant level white noise (regardless of the level of the input signal) gets added
3) expander makes everything half as loud. The input signal and echo are back at the original level, the white noise that got added along the way is cut in half.

[thomasha]

Well, I ended up abandoning my schematic...it was too dark.
The echopathetic is still brighter with some extra filtering. The version with the sallen-key filter after the BBD sounded better, with the additional Twin-T I could increase the pot to 150k. But I still had to try the economic memory man.

It was not as long as I expected, quite like the echopathetic. After hearing some videos of the other versions that I could find I ended up using the schematic of the memory man with the chorus switch:


I just used the normal clock because I'm not interested in the chorus function. Instead of the front buffer/booster I used this opamp to mix both signals and recover some of the signal lost in the filters, it was slightly quieter than when the pedal was bypassed. Even with unity gain filters it sounds quieter, could be the gain of the BBD at longer times or just the impression caused by less high end.

Well, for it to work I also changed the input to the inverting side, so that it's not grounded anymore and the non-inverting imput is connected to the  4.5v divider. The only adjustment required to run it with a single supply.

But now I'm not sure if I still can add the feedback at the same node (now the imput of the circuit).
What do you think? Can the feedback be connected to the input? The delayed signal will be filtered multiple times anyway, but what if someone uses a fuzz face in front of it?

[ElectricDruid]

Instead of the front buffer/booster I used this opamp to mix both signals and recover some of the signal lost in the filters, it was slightly quieter than when the pedal was bypassed. Even with unity gain filters it sounds quieter, could be the gain of the BBD at longer times or just the impression caused by less high end.

Well, for it to work I also changed the input to the inverting side, so that it's not grounded anymore and the non-inverting input is connected to the  4.5v divider. The only adjustment required to run it with a single supply.

Sorry, I don't understand this part. Why use the op-amp A1 to mix signals when op-amp A2 already does that? And why mess about with A1 to get gain when it already does that too? (Boost input)

[thomasha]

Good point, but I thought boosting after the BBD would be better?

I though boosting at the input would only result in more distortion from the BBD, and the V3205 has a lower headroom than the MN3005, so If I hit it too hard it distorts with some clock noise over the notes (only when strumming, not a constant noise). I thought boosting afterwards would reduce that, because I boost the first repeat, which gets more audible, while the second repeat comes through the feedback loop way quieter, not presenting this distortion. Maybe it worked better with the MN3005 and the +-15V supply.

Instead of using the dry signal after A1, which I removed, I use the signal after A2, still dry, but already filtered, like in the schematic below:


So that the blend occurs at the end of the circuit. I even put a high pass filter, but I'm not sure if it really makes a big difference.