2 channel Tube Spring Reverb project with switchable tanks

[rock soderstrom]

Hey folks,

what do you think of this design?
It's a 2 channel/ stereo spring reverb for line and instrument signals.

My "design" is basically based on the Lamington Reverb and this driver design with a few mods and a CF buffer to feed 10k line inputs. (for analog AUX returns of a Mixer or in front of an audio interface). The PSU is not shown, its a quite ordinary design with a dedicated mains transformer. No back to back solution as with the Lamington reverb.

Several switchable reverb springs with different decay times are planned. (plus an all pans together mode)

Is it possible to do it this way? Anything noticeable wrong?

How and where could I add a bass and treble cut?

Edit: only one channel drawn, Output tubes are ECC832/12DW7

 

[TwentyTrees]

Interesting! I'm planning something similar with some Neumann modules - particularly the switchable tanks aspect. I've no insights on your driver or recovery circuits, but subscribing as I suspect we'll be facing a lot of the same issues with switching and mechanical isolation, etc.

For a high pass filter, how about looking at coupling caps in the driver stage? If you undersized your 1uF with the highest cut you want, you can switch in other caps in parallel to lower the corner frequency as you need. You'd want a low cut before the tank at any rate, I'd imagine. Could perhaps put an inductor in series with the cap for a low pass filter, and then switch in other parallel values to raise the corner frequency. There may be issues with this approach though, given that you'll be looking at quite high output levels from the driver.

 

[rock soderstrom]

For a high pass filter, how about looking at coupling caps in the driver stage? If you undersized your 1uF with the highest cut you want, you can switch in other caps in parallel to lower the corner frequency as you need. You'd want a low cut before the tank at any rate, I'd imagine.

Definitely a good idea, I think the 1uF is too large, a 100nF will probably do.

But my wish would be a stepless solution, like I have in my test setup for example. Here a small Mackie mixer works as driver and recovery amp. Together with the tank switching this works really well!

 

[ruffrecords]

The recovery side is probably OK as long as the spring outputs 10mV or so. I am not so sure about the drive side though. An EF80 wired in pentode mode may not be able to drive the tank unless it is very high impedance.

Cheers

Ian

 

[rock soderstrom]

I was also skeptical about the transformerless driver at first, but the second link above explains the theory behind it quite well - a quasi constant current source.

Edit: the input impedance of the tanks are in the 600-800 ohm range

Here's the same driver design as a proof of concept compared to a Fender 6g15. Sounds pretty wet...



Edit: Because someone asked me, the guy in the video is someone else, not me. I think he is also menber here.

 

[rock soderstrom]

Whereby the use in front of a guitar amplifier is not in the foreground in my setup. I rather think of the integration into an analog console or an audio interface.

I'm currently building a PCB to try it out. Is not quite finished yet and will certainly still undergo a few changes.

 

[ruffrecords]

I was also skeptical about the transformerless driver at first, but the second link above explains the theory behind it quite well - a quasi constant current source.

Edit: the input impedance of the tanks are in the 600-800 ohm range

Interesting. Similar in many ways to some tube based tape head drive circuits which were also pseudo constant current for similar reasons. I checked out the EF80 datasheet and it turns out to be a meatier device than I had expected so it should do the job nicely.

Cheers

Ian

 

[Lee_M]

This is actually very similar to a project idea I've had for a while.

I was thinking of having three driver amps and recovery amps per channel (one for each tank type) and then a mixer stage for combining the outputs of the 3 recovery amps into a single output signal.

Obviously, this approach will increase the cost (you need more tubes overall, as well as three driver transformers per channel, plus a huge case to house it all) but it'd avoid the stepping/switching between different tanks, and would probably give you a lot of "in between" options for blending the tanks, which wouldn't be possible when just switching between them.

Edit - I just had a proper look at your schematic and realised it doesn't use driver transformers, so the mixed spring approach won't be anywhere near as expensive if it's done using the Lamington circuit.

 

[rock soderstrom]

I was thinking of having three driver amps and recovery amps per channel (one for each tank type) and then a mixer stage for combining the outputs of the 3 recovery amps into a single output signal.

I also thought about that first, that brings some advantages, you can mix your own super-reverb. This affects not only the different decays of the tanks, but also their sound, they all sound pretty different. Perhaps my next project.

BTW, another advantage of the OTL driver stage is that the EF80 is super cheap and still widely available. The standard tubes like ECC8x and the usual driver tubes like EL84 and 6V6 are really expensive by now. The prices are going through the roof...

 

[solkatten]

Cool design a high impedance current drive is an intresting solution Another one is a transformer couple push pull driver stage, like Kevin Oconnor does it at London Power. Clean and low noise.

 

[rock soderstrom]

Another one is a transformer couple push pull driver stage, like Kevin Oconnor does it at London Power. Clean and low noise.

Ohh, I did not know this reverb yet. So many interesting things, so little time to read it all.

 

[alphasnk]

Thanks to you, I tried the lamington circuit as well, with a 15k/8 ohm transformer placed after the 1uf cap since i didn't have the right tank for it, it works very well..

I used this transformer: Fender Style OT 3,5 W Reverb 125A20B

I also added an output transformer to be balanced out, it works very well as well!

 

[abbey road d enfer]

Thanks to you, I tried the lamington circuit as well, with a 15k/8 ohm transformer placed after the 1uf cap since i didn't have the right tank for it,

Then it's far from being constant-current drive. The output impedance of the drive circuit in the Lam is a tad below 15k. Going through the 15k:8r xfmr, the tank input is matched with the source impedance.

it works very well..

No doubt. Current drive is very much overrated. Response is equally good with a preemphasized low-Z drive.
That is because the electrical damping is very low in the Accutronics tank. Changing the source impedance results in negligible changes of the overall damping.

 

[abbey road d enfer]

I was thinking of having three driver amps

You don't need that; connect all the tank inputs in series and you're done.

 

[rock soderstrom]

I also added an output transformer to be balanced out, it works very well as well!

I guess the Edcor does the output transformer, what kind of impedance ratio does it have? 10k or 15k to 600R?

That's interesting because I added the cathode follower to be able to drive 10k inputs. The transformer is of course also a solution for that job. Maybe the Lamington Reverb is able to drive a 10k input without it, that is a impedance mismatch and the basses are missing, but you don't need them anyway.

 

[alphasnk]

I guess the Edcor does the output transformer, what kind of impedance ratio does it have? 10k or 15k to 600R?

That's interesting because I added the cathode follower to be able to drive 10k inputs. The transformer is of course also a solution for that job. Maybe the Lamington Reverb is able to drive a 10k input without it, that is a impedance mismatch and the basses are missing, but you don't need them anyway.

i used the Edcor for that purpose and it's a 15k/600 indeed!

i put an ecc81 instead of the ecc83, after comparing both i prefer how the dwell reacts with the ecc81

I'm sure we could -20db pad it and add an input transformer as well to go in balanced and all.. And we would end up with another tube pre + spring reverb 'color' or straight spring reverb..

Thank you for sharing that idea!! I was hoping you could find an inspiration in what i did for what you're trying to do, i hope it worked haha!

 

[alphasnk]

Then it's far from being constant-current drive. The output impedance of the drive circuit in the Lam is a tad below 15k. Going through the 15k:8r xfmr, the tank input is matched with the source impedance.

No doubt. Current drive is very much overrated. Response is equally good with a preemphasized low-Z drive.
That is because the electrical damping is very low in the Accutronics tank. Changing the source impedance results in negligible changes of the overall damping.

Yes i just went for having a reverberated sound lol I would have followed the schematic if i had the right tank! I ordered it, so i'll compare how both sound!

 

[abbey road d enfer]

Maybe the Lamington Reverb is able to drive a 10k input without it, that is a impedance mismatch and the basses are missing, but you don't need them anyway.

Who said that impedance mismatch necessarily results in losing bass?

 

[rock soderstrom]

Who said that impedance mismatch necessarily results in losing bass?

I said it. My humble experience is that a very high output impedance does not drive a lower impedance well. Which for me has resulted in increasing distortion and decreasing bass. I don't know if this is generally valid, but in this case it was.

 

[abbey road d enfer]

I said it. My humble experience is that a very high output impedance does not drive a lower impedance well.

That is unquestionable.

Which for me has resulted in increasing distortion and decreasing bass.

This is not necessarily the case. It depends on the global topology.

I don't know if this is generally valid, but in this case it was.

Indeed it's often the case, but it's not a universal truth.