Some findings with the BTDR-2 (small reverb brick)

[culturejam]

I have messed around with the Accutronics / Belton BTDR-2 brick quite a bit now, and I thought I'd post a few simple things I've noticed. This a really cool little device, which allows for a relatively low-cost reverb that can be delivered in a 125B (or maybe smaller) enclosure.

• Modulation
There is a pronounced modulation effect on the output. It's a pitch-bend that is reminiscent of the sound of Rick's Little Angel, but it's more of a pitch dip than a true chorus type sound. Sometimes it sounds like there is a relatively fast LFO speed. The whole modulation sound is rather unpredictable, which is pretty cool and also perplexing.

I sort of wonder if there really is an internal LFO modulating the pre-delay time, or if the internal delay circuits (presumably multiple PT2399-like circuits) are perhaps drawing enough current from the regulator to cause a dip in voltage on the supply pin? I should probably hook up the old volt meter and check that out.

I dig the modulated sound, but it would also be cool if we could figure out how to turn it off.


• Stereo Outputs
The datasheet (which is laughably inadequate) shows both output pins tied together in parallel for mono application. I have confirmed that it will work with just one output connected. However, this knocks down the overall reverb level quite a bit, as you might imagine. But it could be a useful feature in some applications to add a switch to toggle between stereo and bridged mono (or maybe just a more/less reverb switch for mono).

Another use for the second output might be for a secondary feedback loop, although I'm not sure this would be any better/different from just taking the feedback from the bridged outputs.


For reference, here is the first simple circuit I came up with using the brick:
http://www.madbeanpedals.com/forum/index.php?topic=4594.0

This one does not have feedback of any kind, but I'm working on a more complex version that does. And I'm still exploring the benefit of a secondary feedback path.


GTR2 and other who have worked with this brick, I would love to hear your thoughts/input on this nifty little quirky thing. 

[gtr2]

I haven't tried it or really looked into it yet but I'm thinking that the internal pt2399's clocks are modulating the power supply rail as listed in this wonderful doc

http://www.freewebs.com/valvewizard2/PT2399_Data_Notes.pdf

I'm wondering if using the docs Fig. 2 5V regulation would stop the modulation.  I don't have a suitable transistor at the moment (some on the way) to breadboard it.  But my reasons for breadboarding were for a different pt2399 project 

I found the stereo outputs useless for anything I wanted to do anyways as it's just two mono outs.  But surely one of them could be tied into a feedback loop with an opamp stage of some sort.  There isn't enough flow to push it for a dwell loop on its own.  Not sure what the technical term would be, loading effect 

Just my measly two cents...

Josh

[culturejam]

I haven't tried it or really looked into it yet but I'm thinking that the internal pt2399's clocks are modulating the power supply rail as listed in this wonderful doc

Okay, but why doesn't it happen (or why isn't it audible) with a single PT2399? Do you think the effect is cumulative?  (this is more rhetorical than me actually demanding that you answer   )

I'm wondering if using the docs Fig. 2 5V regulation would stop the modulation.

If it really is the internal clock causing the modulation, then I don't see how improving the power supply source would stop it. But it's certainly worth trying out.

Also, Merlin notes that the clock causes noise. He doesn't say it causes pitch modulation. Not sure how much to read into that.

I think a 2N5089 or MPSA18 should work fine in place of the BC tranny.

I found the stereo outputs useless for anything I wanted to do anyways as it's just two mono outs.  

Right, but it does give more overall output (reverb) volume. I hooked it up with a switch so that I could toggle between one output and two outputs bridged mono, and it's quite noticeable.

But surely one of them could be tied into a feedback loop with an opamp stage of some sort.  There isn't enough flow to push it for a dwell loop on its own.  Not sure what the technical term would be, loading effect  

Oh yes, it can be done. I have a working prototype right now with feedback. You can set it for nearly infinite reverb that degrades into noise like a standard 2399 delay with the feedback cranked. It will also do runaway oscillation. Setting it up that way is great for the noise-rock and ambient guys.

I'll post what I have working as soon as I get all the values finalized.

[gtr2]

I've always felt that plain jane Pt2399 delays had a bit of natural modulation inherent in the chip.

I don't have a brick to breadboards anyways to try the regulation circuit 

[culturejam]

I've always felt that plain jane Pt2399 delays had a bit of natural modulation inherent in the chip.

Interesting. I've never noticed this. And I don't recall hearing it on the old Belton brick that I experimented with. Different ears, I suppose. 

[night-B]

Mumble mumble... One output straight, one output followed by a pt2399 delay, blend... Mumble mumble

[jubal81]

I'm really impressed with the way the modulation becomes more pronounced at the tail of the decay.
I'd be willing to contribute some moolah toward buying one if someone wants to do some exploratory surgery.

[stecykmi]

I've never played around with the belton, but the pitch modulation effect may come from clock skew between the individual pt2399's on board, meaning the internal oscillators on board drift slightly, and drift relative to each other. this would cause a "chorus" type effect since the delay times will be slightly out of sync.

if the brick were an on-chip solution (ie squeezed into an IC rather than a module), all the clocks would be probably in a phase-locked loop to prevent this from happening.

[Ettore_M]

Mumble mumble... One output straight, one output followed by a pt2399 delay, blend... Mumble mumble

I can see where you going with it...

[culturejam]

I've never played around with the belton, but the pitch modulation effect may come from clock skew between the individual pt2399's on board, meaning the internal oscillators on board drift slightly, and drift relative to each other. this would cause a "chorus" type effect since the delay times will be slightly out of sync.

if the brick were an on-chip solution (ie squeezed into an IC rather than a module), all the clocks would be probably in a phase-locked loop to prevent this from happening.

Aha! Now that is smart thinkin', sir! 

[Jack Deville]

I personally think a little bit of modulation in a reverb simulator is a good thing.
Too much and it can get retarded and begin to resemble a chorus or flanger effect.
Without a little drift, it might sound stiff, mechanical and "fake."

Consider what the circuit is attempting to simulate.
You may say: well, the boundaries (walls) of the space aren't moving around!
Right!  But!  The reflections are subject to all types of distortion as they do not bounce off a surface in a straight line, but rather simultaneously omnidirectional.  So maybe there is a little doppler type stuff happening, compounded by collisions with other reflections, air pressure/temp, frequencies absorbed by reflecting/pickup material etc etc etc.

All I'm saying is that if the part is a few PT2399s, well, a little drift in clock frequency is a good thing.

On another note, try probing around the Current Control pins ( 7, 8 ).  I think you'll find the answers to your  initial question.

[jkokura]

I agree Jack. Afterall, perhaps the most commonly used reverb these days is the Boss RV-5, and it's widely held that it's best mode is the modulated setting. I use it that way.

My experiments with the BTDR-2 has also seen a very musical effect come out. What I appreciated is the 'chimey trails,' which is the worst description compared to the actual effect. I've utilized that stereo effect to create a bit of a variable dwell circuit, allowing for the breadth of both of the stereo outputs to just a single output. I find that with both fully engaged, there's an amount of 'shimmer' for lack of a better term. It's like an upper octave has been added, but so quietly that you almost don't believe it's there.

That's just what I hear messing around on my breadboard. The Prototype pedal is gonna be ready soon.

Jacob

[Jack Deville]

Like this?

http://www.youtube.com/watch?v=qrjt5aDkAsQ

But with the center control at a lower setting?

[jkokura]

That's what I'm talkin about, but to a much subtler degree. Its not anything Ive done to create it, because my circuit is simply a modified extension of the data sheet, but that's the effect I perceive that I believe might be inherent in the block.

Jacob

[culturejam]

I didn't mean to suggest that the modulation should be removed. I just think it would be cool if it could be controlled or turned off/on. If not, no biggie.

On another note, try probing around the Current Control pins ( 7, 8 ).  I think you'll find the answers to your  initial question.

You mean pins 7/8 on a PT2399?