It took me a while to identify the issue it occurred in, but I finally located it. A 1983 issue of the Canadian edition of Electronics Today had a spring reverb project that included several useful features, including an optoisolator-based limiter to prevent spring-splat. Interestingly, it includes a variable feedback path from the recovery stage back to the driver stage to extend the decay of the reverberation.
I imagine a different issue of one of the other international editions of ETI (Australia, UK, India, et al) may have had the same project, but I wasn't going to sift through all of them. Here is the issue, courtesy of the American Radio History site, and whoever graciously scanned their copy. Haven't built it myself, but it looks promising.
https://www.americanradiohistory.com/Archive-Electronics-Today/Canada/80s/ETI-1983-07-Canada.pdf
Thanks Mark! It just so happens I've been looking at spring reverb circuits the last couple of days, so that's very timely.
Thus far, this is the best page I've found on the topic:
http://roymal.tripod.com/reverb.htm (http://roymal.tripod.com/reverb.htm)
(linked from here: http://roymal.tripod.com (http://roymal.tripod.com) - there's some other useful stuff too).
Well thanks, Tom! Much appreciated.
QuoteI imagine a different issue of one of the other international editions of ETI (Australia, UK, India, et al) may have had the same project, but I wasn't going to sift through all of them.
I don't remember seeing that one in the au version of ETI. Interesting how the local variants of ETI have different projects. That's happened number of times now.
QuoteInterestingly, it includes a variable feedback path from the recovery stage back to the driver stage to extend the decay of the reverberation.
Probably something that hasn't been explored enough for spring reverbs. (Where's mac?)
A few of the earlier smaller tube amps used the amp output to drive the reverb, then feed the reverb output back to the input. The motive here was to save a reverb driver more than anything but by default it achieves a natural regenerative feedback loop. The problem with those is you can't control the amount of feedback separately from the "once through" signal so you were forced to keep the reverb at a low level, making it sound weak.
There's a spring reverb project in a fairly new issue of Everyday Practical Electronics. If anyone's interested I can find which issue. The mag is available as a digital download if it can't be found online.
Quote from: ElectricDruid on March 15, 2019, 03:43:03 PM
Thanks Mark! It just so happens I've been looking at spring reverb circuits the last couple of days, so that's very timely.
Thus far, this is the best page I've found on the topic:
http://roymal.tripod.com/reverb.htm (http://roymal.tripod.com/reverb.htm)
(linked from here: http://roymal.tripod.com (http://roymal.tripod.com) - there's some other useful stuff too).
You may have come across these articles, but in case not:
http://sound.whsites.net/articles/reverb.htm
http://sound.whsites.net/project34.htm
I recently pulled an old spring tank out of a 70s transistor organ, so I'll be playing with these soon :D
QuoteInterestingly, it includes a variable feedback path from the recovery stage back to the driver stage to extend the decay of the reverberation.
Probably something that hasn't been explored enough for spring reverbs.
(Where's mac?)
:icon_lol:
I tried feedback, but as the spring was not isolated from speaker, street traffic and flies wings, small amounts of feedback made it resonate badly.
I did not go further.
This is the first driver for the speaker-piezo,
(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53520&g2_serialNumber=3)
Thanks to Rob, I changed to coil-to-piezo, and coil-to-coil with a similar all-opamp driver circuit.
Feedback is easy to add.
https://www.diystompboxes.com/smfforum/index.php?topic=120207.0
mac
QuoteI tried feedback, but as the spring was not isolated from speaker, street traffic and flies wings, small amounts of feedback made it resonate badly.
I did not go further.
Maybe some fly-spray will fix it ;D. I guess you can only have feedback in small doses. Maybe some sort of filter on the feedback can help tame the beast.
Those old amps had the speaker shaking the whole works so the reverb would have to be backed off somewhat.
QuoteThis is the first driver for the speaker-piezo,
One cool thing about that circuit is the loading on the piezo levels the response from the voltage drive on the drive side.
QuoteThanks to Rob, I changed to coil-to-piezo, and coil-to-coil with a similar all-opamp driver circuit.
Feedback is easy to add.
It was all your hard work.
Quote from: Mark Hammer on March 15, 2019, 03:35:31 PMInterestingly, it includes a variable feedback path from the recovery stage back to the driver stage to extend the decay of the reverberation.
From my DIY SS spring reverb bookmarks collection:
Another example of a circuit using feedback:
https://kassu2000.blogspot.com/2015/10/spring-reverb.html
(https://1.bp.blogspot.com/-MRA8djIwm5k/VhrKb3iuUQI/AAAAAAAACEY/onJy8ckAa_U/s1600/Spring-Reverb%2B%25281%2529.png)
The absence of any trimming of the low end on that feedback path is, um, questionable. The "anti-shimmer"?
Quote from: Mark Hammer on March 16, 2019, 09:16:04 PM
The absence of any trimming of the low end on that feedback path is, um, questionable. The "anti-shimmer"?
U1B takes care of that ;D
Very interesting thread and links, I think the ETI circuit has been discussed here before some years ago?? (EDIT:here https://www.diystompboxes.com/smfforum/index.php?topic=111671.msg1028607#msg1028607 (https://www.diystompboxes.com/smfforum/index.php?topic=111671.msg1028607#msg1028607)
Another thought is to follow a (long) spring reverb with a vca and trigger an envelope generator with the audio and optionally superimpose that envelope on the reverb. Has anybody experience with something like that?
Quote from: MaxPower on March 15, 2019, 06:49:47 PM
There's a spring reverb project in a fairly new issue of Everyday Practical Electronics. If anyone's interested I can find which issue. The mag is available as a digital download if it can't be found online.
Mornin' all, here you go -
https://www.dropbox.com/s/uo0mjeoh3lpqmbg/Everyday%20Practical%20Electronics%20-%20April%202018.pdf?dl=0
Your status as "prince of a man" is re-affirmed, Stephen. Many thanks.
> absence of any trimming of the low end on that feedback path is, um, questionable.
As Greg says, there's heavy low-cut in the forward path. Bass won't build up.
C6 R10 is 350Hz. R9 C4+5 is also 350Hz. The recovery stage has 35Hz. Feedback gain will peak above 1KHz (where you hear it) and deep rumble is seriously suppressed.
You're right. I should have looked at more than simply the feedback path.
Late last year I tried building the Gaussmarkov version of the G. Forrest Cook spring reverb and late in the process encountered a number of inconsistencies between the build docs and the schematics and the original Cook page. The Gaussmarkov page had been revamped and old comments were gone. I had built the Stage Center Reverb as one of my earliest DIY efforts, ten years ago; it was successful but didn't sound good.
I just built a Surfybear spring reverb -- sounds amazing but really only works with guitar, not decent for my many keyboards and line level sources.
Still would love to build something for broader applications.
This ETI one looks do-able!
Strategy
Those are interesting comments, Strategy. What do you think makes the difference between a spring reverb "for guitar" and one "for keyboards"? Levels is the obvious first answer, but is there anything beyond that? Perhaps keyboards need a more open reverb path since they might have more treble end than a guitar...or not, I dunno.
The reason I ask is I was studying reverb circuits looking for something suitable for an organ. I had an old Yamaha transistor organ from the Seventies and it did a fantastic Reggae stab sound if you turned the reverb up and played stacatto chords. Bliss. I want to get back there, so I guess I'm looking for something that can get pretty "twangy" when it's turned up too much or driven fairly hard. And that's a clue...I suspect that how hard you drive a reverb spring might affect the sound a bit. Does anyone have any experience to offer having tried different drive levels?
Thanks,
Tom
QuoteI want to get back there, so I guess I'm looking for something that can get pretty "twangy" when it's turned up too much or driven fairly hard. And that's a clue...I suspect that how hard you drive a reverb spring might affect the sound a bit. Does anyone have any experience to offer having tried different drive levels?
Increased drive level helps with electronic noise and mechanical noise but I don't remember the tone changing much.
You could try a fairly high cut-off high-pass filter with a bit more post boost to push the level up. Follow that with a small amount of low-pass filtering.
Quote from: ElectricDruid on March 19, 2019, 07:07:33 PM
Those are interesting comments, Strategy. What do you think makes the difference between a spring reverb "for guitar" and one "for keyboards"? Levels is the obvious first answer, but is there anything beyond that? Perhaps keyboards need a more open reverb path since they might have more treble end than a guitar...or not, I dunno.
I think EQ is part of it. That's all baked in to Surfybear with some of the bass rolled off I think - what you'd want for guitar and a surf tone. But in this particular case it's mainly a levels thing. It just does not like line level; distorts. Could it cause damage? Not really wanting to experiment there :)
QuoteIt just does not like line level; distorts. Could it cause damage? Not really wanting to experiment there
It depends if the drive side or the recovery side is distorting. In the first case you can reduce the amount of drive then add more gain in the recover. On the drive side the drive amp can distort or or the drive amp can overdrive the input coil of the spring. What's happening becomes fairly clear when you look at the waveforms with an Oscilloscope.
Good spring tank is important I believe
I did read something somewhere how some emulations suck at simulating real "surf" style reverb, while they May Be good for something else
Quote from: j_flanders on March 16, 2019, 08:46:54 PM
Quote from: Mark Hammer on March 15, 2019, 03:35:31 PMInterestingly, it includes a variable feedback path from the recovery stage back to the driver stage to extend the decay of the reverberation.
From my DIY SS spring reverb bookmarks collection:
Another example of a circuit using feedback:
https://kassu2000.blogspot.com/2015/10/spring-reverb.html
(https://1.bp.blogspot.com/-MRA8djIwm5k/VhrKb3iuUQI/AAAAAAAACEY/onJy8ckAa_U/s1600/Spring-Reverb%2B%25281%2529.png)
Quick question - what's with the C2 and C4 duplicates laid cathode to cathode? I've never seen that before.
The back to back caps are to make a non-polar cap. In that position and with bi-polar power, the signal will swing positive and negative about 0v. It is quite common where a large (uF range) non-polar cap is required.
+1
If you wanted to find/buy a 10uf NP cap, that might be both tough and expensive, not to mention bulky.
Quote from: anotherjim on June 05, 2019, 04:19:53 PM
The back to back caps are to make a non-polar cap. In that position and with bi-polar power, the signal will swing positive and negative about 0v. It is quite common where a large (uF range) non-polar cap is required.
Interesting! I still don't fully understand how this works, but thanks for your explanation.