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Author Topic: Mosfet Spring Reverb Driver  (Read 10427 times)

josh_777

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Mosfet Spring Reverb Driver
« on: October 13, 2008, 11:34:42 AM »

Hi,

I'm looking to make a mosfet power amp, to drive a spring reverb unit. This is to go in a modular synth, so I would ideally like it to run off the available +/- 15V supply available. I was looking at this one -http://sound.westhost.com/project83.htm   

How much power do i need to effectivley drive the Springs? R6 would need to be increased i imagine, and could i just replace ground with the -15V ??

If I'm completely barking up the wrong tree here could you suggest something more suited?
I'd like to make it mosfet based, and run off the dual power supply, but searching the forum could find little information.

Cheers for any help i get!

Josh
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teemuk

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Re: Mosfet Spring Reverb Driver
« Reply #1 on: October 13, 2008, 12:28:05 PM »

What tank are you using? They are not the same, you know. The circuit you need for the reverb driver depends on the electrical properties of the tank.
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josh_777

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Re: Mosfet Spring Reverb Driver
« Reply #2 on: October 13, 2008, 05:57:24 PM »

I'm housing it all in a marshall mosfet head, think it was a lead 100. I received the amp free from someone who couldn't get it working. I have no interest in the head, but will make use of the spring tank. This tank would have a 310 ohm impedance right?

I have basic electronics knowledge, but am clueless when it comes to designing a driver and what effect the impedance of the tank will have on it (apart from higher impedance means it won't drive as well).
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n9voc

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Re: Mosfet Spring Reverb Driver
« Reply #3 on: October 13, 2008, 08:23:49 PM »

I suggest you take a look at www.accutronicsreverb.com, they have a lot of schematics on reverb drivers and recovery amplifiers.  Even though your tank came in a marshall head, it may have been manufactured by accutronics - if not, you may find that yours matches or is very close to the specs of one of theirs.
(also a good place to learn about the differences in reverb tanks!)
I also suggest you take a look at the "Stage Center" reverb design found at www.generalguitargadgets.com, that was the design I used - If you don't want to use any of the above, it will at least give you some idea of the types of drives used!
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armstrom

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Re: Mosfet Spring Reverb Driver
« Reply #4 on: October 14, 2008, 04:22:45 PM »

Quick question... Did you use that Stage Center reverb unit in an amp or did you build it as an external pedal? If you used it in an amp, where did you put it in the signal chain? Before the preamp or between the preamp and the power amp? From studying old tube amp schematics it seems the reverb portion of the circuit is usually after the the preamp, but obviously the pedal configuration of the Stage Center would need to be placed before the preamp (unless you put it in an FX loop). Is there any advantage/disadvantage to choosing one location over the other? I am considering integrating that design into the cabinet of an amp I'm building (LM3886 power amp driven by a JFET fender blackface clone preamp).

-Matt
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josh_777

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Re: Mosfet Spring Reverb Driver
« Reply #5 on: October 14, 2008, 06:35:14 PM »

In response to armstrom... I would place it in the effects loop of the amp, (i.e after the preamp but before the power amp). That way, if you overdrive the preamp, you will add reverb to the overdriven sound, instead of overdriving the reverb which you might find too muddy.
In fact, the stage center reverb is a sort of effects loop in itself. You could build it from R4 round to R2 , and just place this in your effects loop (this part makes up the actual reverb part, the rest is just buffering and mixing, which will already be in your effects loop). I hope this makes sense.

Thanks for the replies, I am going to try and steam ahead and make a class a mosfet amp, instead of using an opamp, the discrete way might teach me a bit more.
Can anyone tell me how the power is calculated? I understand the P=VI formulas, but not how to calculate what the resistance, voltage etc is. In the schematic given, the power drawn (negating efficiency), is 88W. Is this from 40v, 2.2A and 18 ohms ? Where does the 18 ohms come from?

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n9voc

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Re: Mosfet Spring Reverb Driver
« Reply #6 on: October 15, 2008, 07:19:44 AM »

In reply to Armstom:
Built the Stage Center as a stand alone unit (with the spring reverb tank inside it was a "pedal" that is about 35 cm long by 15 cm high and wide).  I use it as both a booster and a reverb unit.  I put it right before the amplifier if using it as a reverb unit for guitar.  I found that it works great with a microphone as well - boosts a dynamic mic output hight enough to effectively drive a spare amplifier, and adds reverb to the vocals!
Good luck to both of you - Josh_777 and Armstrom.  (Armstrom, you might pose the question in the effects forum, this is a "bit" off topic for this thread - just a suggestion!)
Thanks Josh_777 for your kind patience!
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armstrom

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Re: Mosfet Spring Reverb Driver
« Reply #7 on: October 15, 2008, 03:59:53 PM »

In reply to Armstom:
Built the Stage Center as a stand alone unit (with the spring reverb tank inside it was a "pedal" that is about 35 cm long by 15 cm high and wide).  I use it as both a booster and a reverb unit.  I put it right before the amplifier if using it as a reverb unit for guitar.  I found that it works great with a microphone as well - boosts a dynamic mic output hight enough to effectively drive a spare amplifier, and adds reverb to the vocals!
Good luck to both of you - Josh_777 and Armstrom.  (Armstrom, you might pose the question in the effects forum, this is a "bit" off topic for this thread - just a suggestion!)
Thanks Josh_777 for your kind patience!
Thanks for the information. With regard to this being off topic, I'm not sure I follow. I am not asking about building an effects pedal but rather adapting one of the circuits for use as a reverb channel built in to a combo amp. Perhaps I should start a new topic regarding integrating reverb into an amp.
-Matt
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josh_777

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Re: Mosfet Spring Reverb Driver
« Reply #8 on: October 22, 2008, 06:57:11 PM »

Back on topic, if the spring driver uses a constant current source, can the power follower deliver more than the quiescent current? I'm thinking I could incorporate overload protection to prevent damaging the transducer.

I'm using the power amp detailed here: http://sound.westhost.com/project83.htm, but will run it with reduced voltage & current
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phatt

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Re: Mosfet Spring Reverb Driver
« Reply #9 on: December 02, 2008, 09:27:38 AM »

FWIW,
          I wasted many years [10 in fact] trying to build a decent Rev unit so here is my 2 cents worth.
Build a stand alone unit and be done with it.
Forget current drive you need voltage drive.
[Yes I know all the experts tell us that current drive is best but having built most of the circuits out there,, well they must have missed something.]
Don't  bother with Accutronics circuits [utterly useless]

I doubt that a big drive will kill the transducer unless you pass full bandwidth signal through it.

Rod Elliot [ESP] helped me out a great deal in the end trying to perfect my circuit so search his pages for the reverb drive ideas.

The problem with the booster circuits for opamps is that they need to run off a much higher voltage not the same 15/15 supply.
Rod E will agree with that if you care to ask him.
You really do need a big swing to get it to work as it should.

Anyway to save my babbling on about it,,
here is my "MaxiVerb Stand alone Reverb unit".

No doubt the experts will see flaws in this but still as is this is a killer tank slapping Reverb that will put a lot of other more expensive gear to shame.   [very little noise /hum].
Have fun with it,,, Phil, Nambour Aust.
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J M Fahey

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Re: Mosfet Spring Reverb Driver
« Reply #10 on: December 02, 2008, 12:27:22 PM »

Looks great. Definitely it will *drive* your tank, and then some. Congratulations.
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