Spring Reverb: reducing noise

[Matt C]

I just put together a mono spring reverb unit based mostly on Rod Elliott's schematics (see below).  It's sounding good so far, but there is some hum on the output and I'm looking for advice and ideas on how to reduce it. 

Without signal, the hum/noise is registering at around -64dBFS  in my DAW.  It's an improvement over the old Furman reverb unit it's replacing, but still not great.

The only notable difference between my unit and the schematics below is that I used two reverb tanks instead of just one.  The inputs are in series, but arranged with opposite polarity.  The outputs are in series with matching polarity.  I put all the active electronics in a 1RU chassis, and put the reverb tanks in a separate 1RU chassis in hopes that it would help shield them from picking up noise.  The outer pans of the reverb tanks are also tied to chassis (and isolated from the i/o signals) to provide more shielding.  The two pieces are connected with TRS cables, with shields tied to chassis and not carrying any signal.

The noise  got worse when I installed the unit in my rack, so it seems like all the shielding still isn't effective enough, but it's not totally clear where the interference is coming from.

Anyway, I know these types of reverb units are not known for being silent, but does anyone have any other tricks up their sleeve for reducing the hum?

 

[Matt C]

the recovery amp circuit.  I did change the zobel network hanging on the tanks' output in order to adjust the midrange frequency boost, but otherwise mine is the same as this:

 

[PRR]

The tank does not hum by itself. But it is very good at picking-up hum from the environment. So where is the hum coming from?

 

[Bo Deadly]

One method for understanding where hum is coming from is to short the signal at strategic points.

Specifically try these two tests and report back:

1) Short the input of the recovery amp to ground. Is the hum reduced?

2) Short the input of the reverb tank. Meaning short R7 the 150R. Or if there's a connector, remove it and short the input to the tank. Is the hum reduced? More or less than test 1?

This will give you an idea if it's the circuit or the tank.

My guess would be it's the tank and you need shielding. Cannibalize the Furman enclosure.

 

[Audio1Man]

Hi Matt
Most of the spring reverb tanks use a phono connector input & output. This make a common ground. I would remove the drive circuit to the tank and see what the output hum level is? Short the output coil what is the hum level? Remove the connections from the coil output (no connections to the tank frame) what is the hum level? Is the tank fully enclosed in steel? A picture could help.

Using a NE5532 as drive and recovery (Common IC) may also be a problem,  Poor PS bypassing ?

I have used spring reverb tanks for many projects and changing the recovery amplifier to a full differential amplifier will make a big improvement in hum level. I hope this may help you to a good solution.
Duke

 

[Matt C]

1) Short the input of the recovery amp to ground. Is the hum reduced?

2) Short the input of the reverb tank. Meaning short R7 the 150R. Or if there's a connector, remove it and short the input to the tank. Is the hum reduced? More or less than test 1?

My guess would be it's the tank and you need shielding. Cannibalize the Furman enclosure.

I need to go back and do this again but I checked a couple things and it seemed clear that the noise was being picked up by the tank itself, not the drive or recovery amps.

Reusing the Furman enclosure won't do me any good, I already have the tanks in their own dedicated enclosure.

Most of the spring reverb tanks use a phono connector input & output. This make a common ground. I would remove the drive circuit to the tank and see what the output hum level is? Short the output coil what is the hum level? Remove the connections from the coil output (no connections to the tank frame) what is the hum level? Is the tank fully enclosed in steel? A picture could help.

Using a NE5532 as drive and recovery (Common IC) may also be a problem,  Poor PS bypassing ?

I have used spring reverb tanks for many projects and changing the recovery amplifier to a full differential amplifier will make a big improvement in hum level. I hope this may help you to a good solution.

I isolated the tanks' RCA jacks from the chassis so they are totally independent.

A differential amp seems like a good idea here, but is it realistic to get enough gain (around 40dB) while still keeping the input impedance high enough to not load down the tanks too much?

 

[Matt C]

would something like the circuit I sketched out below work as a differential recovery amp?  the super high impedances are making me nervous.  Also take note I am intentionally filtering the highs and lows out, I think as shown the corner frequencies would be roughly 175Hz and 15kHz, unless I did my math wrong.

 

[Bo Deadly]

It's not crystal clear to me why using a differential recovery circuit would reduce hum. But if you really want to do that I think it should be possible. The output impedance of the tank is usually more like 2.5k and not 20k so you could use your differential circuit with lower impedance resistors. Or if you for some reason you really want higher input impedance you could use an Instrumentation Amplifier. But again, I'm not sure it's necessary.

As someone with a degree in Science, I prefer to use a little old-fashioned scientific method and understand the problem before trying to devise a solution. Meaning characterize the noise first. Then you can make informed design decisions.

If you short the recovery amp input there should be no hum at all. If there is, there's something wrong with your recovery circuit. If you short the input of the tank and the hum is reduced even a little, then there's something wrong with your driver circuit. Otherwise it's the tank. Divide and conquer.

My guess would be that the tank is picking up the noise in which case there are two things that come to mind. One is to make sure grounds are handled properly. Make sure the tank chassis is connected to ground in one spot. If it has RCA jacks verify that only one is grounded to the chassis. My preference would be that the input is isolated and the output is connected to the relatively quiet ground of the recovery amp. If you want to be pedantic about it you could isolate both but then run a separate wire from the tank chassis to the power supply ground filter caps. Similarly, make sure the power supply caps on the the driver circuit have a separate ground wire back to the supply. Even though all of the grounds are connected together, it can matter a lot where they're connected together.

If you get grounding right and the tank transducers are sufficiently shielded, those Rod Elliot circuit should be close to about as quiet as one could hope.

One final thought: it could be that the tank is just too close to mains AC. Try moving the whole apparatus far away (2m+) from any mains socket or cables. Particularly ones that draw a lot of current. Electromagnetic interference is harder to shield from than electrostatic interference so you will need to keep it away from high current AC regardless.

 

[Tubetec]

I usually modify my tanks by installing  an insulated trs jack socket for the outputs ,so transducer outputs to tip and ring  and the ground  by another wire  to the tank  input socket .If your tank case is grounded to chassis maybe thats where the problem lies it might be  picking up all sorts of currents from the rack , maybe try isolate the tank ground from chassis with grommets,and ground it to the audio , this might be better as the tank  would act like a second layer of defense against electromagnetic energy. 
Ive had reasonabley good noise performance coupling the outputs balanced direct to a mic input  of about 3kohms,its maybe a bit low ,but it sounds ok .seeing as you have two outputs in series a bit higher might be best .
Maybe doing the series out of phase connection on the outputs of the two tanks might be worth a try, with a single ended recovery the hum from both coils might tend to cancel .
Proximity to sources of electromagnetic in the rack could be a factor also worth looking at ,extra steel sheilding on the underside of the tank might be a plan too

 

[Matt C]

It's not crystal clear to me why using a differential recovery circuit would reduce hum.

you could very possibly be right that this isn't the best solution, but my thought was that hum being picked up by the tanks' output coil would be common mode, so a differential circuit would cancel a lot of that out.

each tank has an output impedance of 2.2k, two in series making it 4.4k, I figured having a high (20k) input impedance in the following stage would be helpful.  Maybe not needed?

and yes the inputs and outputs to the reverb coils are all isolated from the tank chassis.  The tanks' chassis are tied to the enclosure chassis.

 

[JohnRoberts]

I used to manage an engineering group that designed powered mixers using spring reverbs back in the day.

The pick-up coils are magnetic, so will pick up hum from magnetic fields in the area.

Reducing the hum is generally managed by location of the pickup wrt hum sources.

In extreme cases some mu metal shielding could help.

Note: if the hum pickup is normal to the coil (not common mode) balance or differential will not help. IIRC most such circuitry is unbalanced with a robust shielded connection.

JR

PS: Spring reverbs were cheap but a PIA to design with. When they were replaced by digital efx it made design of those products easier.

 

[Bo Deadly]

Maybe doing the series out of phase connection on the outputs of the two tanks might be worth a try, with a single ended recovery the hum from both coils might tend to cancel .

I would be interested in knowing if "humbucking" reverb tanks has any effect on hum. In theory it sounds like an interesting idea. The orientation of recovery coils probably would matter a lot. They would probably need to be very similar coils (mixing / matching two different tanks might not work as well) and they would need to be oriented the same way. Meaning the two tanks should be stacked in the same orientation. Or you could remove the tray from one chassis and flip it over so that you can have the chassis openings facing each other but the orientation of recovery transducers would be still be aligned. Or, take it a step further, and connect the two trays together using standoffs so that they make one double tray. This will ensure that they are aligned perfectly. And the weight of the two together will help absorb shock. You'll need to reattach suspension springs in a suitable way of course.

As for wiring, I think you just want to wire in series and ground the center tap. Any common signal induced in both coils will be anti-phase.

Could be an interesting exercise but my guess would be that shielding / isolation is going to give better results.  Racking a spring reverb is a problem. You're going to pickup EMI and vibration. If you really want the lowest noise possible it needs to be at the end of a piece of shielded CAT6 in a closet somewhere. Then there won't be any EMI or vibration to pickup in the first place.

 

[Matt C]

I would be interested in knowing if "humbucking" reverb tanks has any effect on hum.

I remember reading a post by Jim Williams that he used to do this, but I believe it involved having the coils custom-wound, not just adjusting the wiring of off-the-shelf tanks.

I'll run some more tests on my unit and maybe try to post some pictures or drawings of how it is wired.  But from what I've done so far I'm pretty sure the noise is being picked up by the tank coils, not coming from the active circuitry

 

[abbey road d enfer]

I would be interested in knowing if "humbucking" reverb tanks has any effect on hum.

Yes. The drive coils should be connected in-phase and the recovery coils out-of phase. It has the double effect of cancelling hum due to stray field and partially cancelling the "boingy" response to transients.
Since stray magnetic fields tend to resist uniformity, orienting the tanks is still necessary.

 

[Matt C]

Note: if the hum pickup is normal to the coil (not common mode) balance or differential will not help.

I guess I'm not too clear on the difference.  What factors would cause the hum to be normal vs common mode?

 

[JohnRoberts]

I guess I'm not too clear on the difference.  What factors would cause the hum to be normal vs common mode?

For the hum to be common mode, it would need to be picked up equally everywhere in the coil as well including at both ends. If the coil converts the magnetic hum field it sees into a voltage developed across that coil winding, the hum is not common mode but a normal signal that differential processing will not cancel out.

JR

PS: I do not miss spring reverbs... There was a lot of engineering hidden behind making those cheap MI products not suck.

 

[Bo Deadly]

I guess I'm not too clear on the difference.  What factors would cause the hum to be normal vs common mode?

The hum is coming from EMI inducing currents in the recovery coil. So you can imagine at any particular instant a current flowing in one direction through the coil is going to be coming "out" of one end whilst "in" the other. That is not common mode. Common mode is when both wires have current flowing in the same direction.

 

[Audio1Man]

Hi Matt
I had suggested a differential amp for the output as I had tanks that input & output were common to the frame. No need for this if they float. If you add some form of BW filtering to the output stage is will help some, however as others have said get rid of the field by shielding or space.

I hunt the hum down using my old plastic dynamic mic and connect it to the Audio Precision in FFT mode.
Duke

 

[Tubetec]

Studio type racks are very hostile places electromagnetically and at rf , any currents flowing here are induced into the metal tin surounding your spring ,and then into the coils . You could mount the reverb pans on either grommets or even elasticated bungy cord, this would provide damping of any external vibrations and also allow you  seperately ground the reverb units from the noisey rackmount ground .

 

[Tubetec]

Ive been fooling round with the idea of spring reverbs for years ,always wondered how a spring in vaccum  might sound  ,maybe it could be used as a  time control ,the less air resistance on the springs ,surely the longer the vibrations would carry ,be interesting to mess around with ,bell jar spring reverb anyone?