Stage Center Rever - Microphonic sound

[Dror3332]

Hi all,

I have built the Stage Center Reverb from GGG site with the TL074 IC
I did exactly what is written in the schema.
I tried it on two Reverb Tanks : 8DB2C1D and 4EB2C1B
Both had a very microphonic sound like I put it in a big church even for the small amount of dwell.
I'm looking for more quiet and bright reverb sound.
Any help will be appreciated

Thanks

Dror

[jimbeaux]

Dror;

I have built a couple of these & was very impressed with the sound. There's a bright switch - which definitely works.

You may want to try using an accutronics reverb tank sold by Paia (www.paia.com) - since this was the one that the circuit was designed for.

Just my $0.02.

Jimbeaux

[jimbeaux]

Dror;

Did a quick check - the construction article only mentions that the accutronics reverb tank is available from Paia - but doesn't give a model number.

I have the construction article on the "Hot Springs Reverb" (also by C. Anderton & available from Paia), and it lists the accutronics model # as 1FB2B1D.

    -Jimbeaux

[Dror3332]

Thanks Jim,

I thought more about twicking the circuit I have already those two accutronics that I mentioned before.

Did you tried it with these kind of reverb tanks?

[jimbeaux]

No - I actually bought my two units as Kits from PAIA back in the 80's (stage center reverb kit is no longer offered by PAIA - to my knowledge)

And the Reverb Tanks came with the kits - so I haven't tried any other tanks with the SCR.

J.D.

[Dror3332]

Thanks

[Mark Hammer]

Reverb pans vary from very low (8 ohm) to semi-medium (1k) input impedances.  Not all op-amps can drive the lower loads provided by some reverb pans.  Rather than designing the SCR to accommodate *any* reverb pan, Craig Anderton used the op-amps available at the time and recommended a particular reverb pan (provided by PAiA) which would complement the limited current-drive capabilities of that op-amp.  

Of course, since that time, the choice in op-amps has widened up considerably.  Since you have built the SCR using a TL074 (which does not hold up well when delivering current to ultra-low loads), you will want a replacement that is more oriented towards looking at loads in the <600ohm range.  A perfect choice would be the LM837.  This is a quad version of the more familiar LM833 (a dual op-amp).  The 833 and 837 are intended to be able to deliver current to low loads, and are similar to the NE5532 (dual) and 5534 (single) in that respect.  Both the 833 and 5532 are often used as headphone drivers, an application very similar to driving the input of a reverb pan.  My guess is that you may enjoy the performance of the unit more with such a chip change.

On other fronts it sounds to me like you may need to cut the bass in the driver section a bit too.  The SCR provides a low-end rolloff in the driver section (IC1b) of about 360hz, and a high-end rolloff of around 1540hz.  Change C4 to maybe .018uf, and C1 to maybe 180pf.  Dropping the low end will decrease the overall amplitude of the driving signal sent to the reverb pan, so a little more gain may be needed.  Since we don't know exactly how much gain, the sensible thing to do is to replace R7 (470k) with a 220k fixed resistor and 500k pot in series, to provide a variable resistance of 220k to 720k, and a wide range of possible driver gains.

Note that the "Dwell" control in the SCR is nothing of the sort.  It is a reverb level control.  The traditional "Dwell" control is, in fact, precisely what I have described above: the control that determines how hard the springs are pushed and and how long they shiver in response - dwell.

You should probably stick a 10-22pf cap in parallel with R9 to keep a lid on hiss, although with an LM837 you may have little of it.

[Dror3332]

Thanks a lot Mark
I need to process what you wrote here  
I will try it and post the results.

[Dror3332]

Reverb pans vary from very low (8 ohm) to semi-medium (1k) input impedances.  Not all op-amps can drive the lower loads provided by some reverb pans.  Rather than designing the SCR to accommodate *any* reverb pan, Craig Anderton used the op-amps available at the time and recommended a particular reverb pan (provided by PAiA) which would complement the limited current-drive capabilities of that op-amp.  

Of course, since that time, the choice in op-amps has widened up considerably.  Since you have built the SCR using a TL074 (which does not hold up well when delivering current to ultra-low loads), you will want a replacement that is more oriented towards looking at loads in the <600ohm range.  A perfect choice would be the LM837.  This is a quad version of the more familiar LM833 (a dual op-amp).  The 833 and 837 are intended to be able to deliver current to low loads, and are similar to the NE5532 (dual) and 5534 (single) in that respect.  Both the 833 and 5532 are often used as headphone drivers, an application very similar to driving the input of a reverb pan.  My guess is that you may enjoy the performance of the unit more with such a chip change.

On other fronts it sounds to me like you may need to cut the bass in the driver section a bit too.  The SCR provides a low-end rolloff in the driver section (IC1b) of about 360hz, and a high-end rolloff of around 1540hz.  Change C4 to maybe .018uf, and C1 to maybe 180pf.  Dropping the low end will decrease the overall amplitude of the driving signal sent to the reverb pan, so a little more gain may be needed.  Since we don't know exactly how much gain, the sensible thing to do is to replace R7 (470k) with a 220k fixed resistor and 500k pot in series, to provide a variable resistance of 220k to 720k, and a wide range of possible driver gains.

Note that the "Dwell" control in the SCR is nothing of the sort.  It is a reverb level control.  The traditional "Dwell" control is, in fact, precisely what I have described above: the control that determines how hard the springs are pushed and and how long they shiver in response - dwell.

You should probably stick a 10-22pf cap in parallel with R9 to keep a lid on hiss, although with an LM837 you may have little of it.

Hi Mark,

I can't find the LM837. 
Any ideas for suitable replacement that can deliver current to ultra-low loads?

Thanks

[Eric H]

I can't find the LM837. 
Any ideas for suitable replacement that can deliver current to ultra-low loads?

Thanks

It's always helpful to post a schematic when asking for help on a circuit

You may be able to put a discrete buffer before the pan fairly easily --but I'm not familiar with the design.

-Eric

[Dror3332]

I can't find the LM837. 
Any ideas for suitable replacement that can deliver current to ultra-low loads?

Thanks

It's always helpful to post a schematic when asking for help on a circuit

You may be able to put a discrete buffer before the pan fairly easily --but I'm not familiar with the design.

-Eric

Ohhh   No problem! it's the stage center reverb in GGG site
http://www.generalguitargadgets.com/diagrams/stage_center_reverb_sc.gif
But you must read what Mark wrote here.
He recommended to work with LM837 instead of TL074
So now I have to try it.

Dror

[Peter Snowberg]

Use the Force.

http://www.accutronicsreverb.com/schemat.htm

[Peter Snowberg]

Think about circuit #2, using an NE5532 for the opamp. The input impedance is a bit low for guitar use, but if you have this stage after a buffer which also supplies the dry signal to the output. (a perfect use for the other opamp in the 5532)

[Mark Hammer]

Gah!  I'm not THAT authoritative!

The LM837 is a quad version of the LM833, one of several available dual op-amps that are comfortable dumping lots of current into the low impedance loads typical of reverb pans.  I suggested it because quad packages CAN be easy to do layouts for.  However, a pair of 5532's or LM833's is fine and may even work better in your case.  Hell, the only place where op-amp choice is going to make a huge difference anyways will be in the one that drives the reverb pan input.  So, if you can score one of the aforementioned dual op-amps and stick it in the socket for the input/output of the reverb pan and use whatever the heck you want for the other two op-amps, that'll be fine.

Absolute worst-case scenario, you buy one 5534 and use that to drive the reverb pan input, and whatever the heck you have on hand for the other 3 op-amps.

Sorry for the wild goose chase.

[Dror3332]

Gah!  I'm not THAT authoritative!

The LM837 is a quad version of the LM833, one of several available dual op-amps that are comfortable dumping lots of current into the low impedance loads typical of reverb pans.  I suggested it because quad packages CAN be easy to do layouts for.  However, a pair of 5532's or LM833's is fine and may even work better in your case.  Hell, the only place where op-amp choice is going to make a huge difference anyways will be in the one that drives the reverb pan input.  So, if you can score one of the aforementioned dual op-amps and stick it in the socket for the input/output of the reverb pan and use whatever the heck you want for the other two op-amps, that'll be fine.

Absolute worst-case scenario, you buy one 5534 and use that to drive the reverb pan input, and whatever the heck you have on hand for the other 3 op-amps.

Sorry for the wild goose chase.

I think I'll use two LM833. That will be the simple way for me
Thanks Mark

[Dror3332]

Gah!  I'm not THAT authoritative!

The LM837 is a quad version of the LM833, one of several available dual op-amps that are comfortable dumping lots of current into the low impedance loads typical of reverb pans.  I suggested it because quad packages CAN be easy to do layouts for.  However, a pair of 5532's or LM833's is fine and may even work better in your case.  Hell, the only place where op-amp choice is going to make a huge difference anyways will be in the one that drives the reverb pan input.  So, if you can score one of the aforementioned dual op-amps and stick it in the socket for the input/output of the reverb pan and use whatever the heck you want for the other two op-amps, that'll be fine.

Absolute worst-case scenario, you buy one 5534 and use that to drive the reverb pan input, and whatever the heck you have on hand for the other 3 op-amps.

Sorry for the wild goose chase.

Hi Mark,

I just noticed that the LM837 is not quite a quad version of the LM833. cause the LM833 can't realy drive a 600Ohm load.

Thanks
Dror

[Mark Hammer]

Hmmm....I was always under the impression that the LM833 could do those sorts of tricks since it was always lumped in with the NE5532.  Seems I was wrong.  As you note, though, the LM837 quad version seems to be able to drive low impedance loads, and we know the NE5532 can, so at least you have a few choices.

[puretube]

I don`t believe, that simply swapping IC`s in that schem
will "de-microphonicate" the sound...

[petemoore]

  I'm still having the SCR circuit on the bench for now.
  I'm trying to sort out what was typed here...
  Mention of a buffer ? That would be on the driver output ? Would I want that if I'm ordering the right tank ?
  Also, I haven't settled on a tank for the circuit, there are many to choose from and I haven't sorted out the impedances, [recommended 310 ohm...? I'm guessing higher input ohmage rating for the tank would be the 'side to lean toward' when choosing a tank?] if someone could point a tank out for me that would be nice  , but I'll reread Mark's post and probably be sorting that out.
  Also about the dwell, or how hard the springs are pushed by the driver, would not there be a fairly easy method to implement a 'real' dwell control?
  I have board space and time to think about what I might want to add.

[Dror3332]

I didn't knew about the NE5532 .

I want to say that I made the change that should reduce the low frequency,
  I must say that it's match better and it's reducing the microphonic freq .
Thanks a lot Mark

Now I want to add high freq without losing the current low freq. Can it be done?

Thanks
Dror