Echoplex EP-3 22VDC Preamp Schematic

[Marlowe]

Since I just joined the forum, I thought I might offer this up. On various sites, I see many questions about the Echoplex EP-3 used by Van Halen, Jimmy Page, my hero Eddie Hazel and others. I wondered about it as well.

Sometime back, I was noodling about in my parts bin and came across a 9VAC 300ma adapter used to charge my old Nokia phone. I had some MPF102 JFETs and had never tried an EP-3, so I built this. The 9VAC adapter goes into a simple voltage doubler. Because of the low current draw of the MPF, the filter cap charges to 24VDC, which after the drop across the 6.8K resistor came to exactly 22VDC on my unit. A larger current-rated adapter may cause higher voltages on top of the stacked caps. The 300mA 9VAC adapter I used (a Nokia phone charger, but a 9VAC answering machine adapter would also probably work well) worked perfectly.

You see many criticisms on forums about 9V EP-3's not having the correct vintage voltages. Well, this one does. I have built it, it works and works WELL. I was skeptical about the mojo associated with this preamp, but it DID make a difference in how my JCM800 preamp felt and sounded. I like it very much! The MPF seems to work well in this otherwise note-for-note copy of the EP-3 circuit. I've got about 500 hours on mine so far, so the circuit seems stable.

One warning, it's NOT a boost as much as a tone-coloring buffer. It has a very mild gain. What makes it special is that it creates an accentuated high mid which seems to impart a clarity on the guitar signal and really comes through in distorted amps. These are my impressions. You may build it and hate it. So it goes!  

Of course, it is tied to an adapter. Sorry about that, but I didn't have a charge pump in my junk box! I mounted mine in my rack. Forgive the crudeness of the drawing. I drew it up quickly for a fellow on an amp forum. If you build it, I hope you like it! Bill.

My Greatest Respect to Mike Battle, the Echoplex circuit's designer!

[Kipper4]

What's the point of the parrallel resistors on the output?
I'd be tempted to miss them out and put the output pot to ground ?
Unless its an impedance thing or I'm missing something else?

[Marlowe]

Ahh. That's a part of the preamp's "mojo". Originally, the preamp had the output of the tape circuitry tied to that junction of the 100K and the Level pot (the Level was originally the Blend control). Since you don't need that part of the circuit, I left it out. On the first prototype, I wired in some compensators to mimic the tape circuitry. I tried it with and without the compensators and honestly couldn't tell a difference. If you're turning it toward the "tape" side as it's wired, you're cutting down the signal anyway...

If you want the preamp to actually perform its "mojo", you truly need the level control wired like that. It isn't so much a drive as a buffer with a slightly varying medium output impedance.  You get maximum output about 1/2 to 3/4 full on the Level. It actually affects the preamp's output impedance, and thus affects how the preamp drives subsequent preamps, effects  and amplifiers. The preamp is subtle, but after trying it, I know why those guys had to have it in their rigs.

Bill.

[Kipper4]

Thanks for clearing that up Bill. Appreciated.
Rich

[Marlowe]

Anytime man! If you build it good luck!

[jubal81]

Looks like the bias Voltage "Y" probably needs to go to the 1M resistor on the gate instead of taking it to ground.

[Marlowe]

Actually, "Y" is the second 9VAC lead from the adapter. It attaches to the junction of the two stacked caps which form part of the voltage doubler circuit ( at quick glance it DOES look like the standard op amp artificial ground, but it's not).

I drew the schematic for a Van Halen fan on an amp forum and he had little experience with building circuits. So I labeled lead 1 from the adapter "X" and lead 2 from the adapter "Y".  Since the leads are on a 9V AC adapter, they are interchangeable. The first lead of the adapter connects in the usual way to the junction of the forward and reversed diodes (please notice one of the diodes is  reversed). The second lead "Y" connects to the junction of the two stacked capacitors. The system charges on alternate cycles to double the voltage at the top of the stacked caps. The end result (on my circuit, at least) is 24VDC at the top of the two stacked 47uF/16VDC caps. After the voltage drop across the 6.8K resistor, you get 22VDC - exactly the voltage of the original EP-3.

The circuit functions perfectly exactly as drawn, even if the schematic is rough as a back country road.  But thanks for spot checking me! I'm old as dirt and my eyesight is going, so heaven only knows what I might have put in there!  

Regards, Bill.

[tubegeek]

Looks like the bias Voltage "Y" probably needs to go to the 1M resistor on the gate instead of taking it to ground.

that's not a DC bias node, it's an AC input node for the voltage doubler. The only voltage feed for the circuit is the +22V node.
EDIT: Marlowe in ahead of me!

[Marlowe]

Forgive me! 

[Marlowe]

Just a quick note before anyone calls me on it:

I'm sure someone familiar with the EP-3 will notice that I left off a grounded resistor between the junction of the JFET's .1uF output cap and the 220K resistor going on to the level control. In the original circuit there was a 470K resistor connected from the .1uF and 220K junction and which went to ground. You can put this resistor in if you'd like, but I thought it made little difference and to be quite honest, I felt the preamp needed all the help it could get drive-wise. A purist might want to install it.

Regards, Bill.

[mth5044]

Nice - I like your voltage doubler use. Have you tried the effect at 9V DC to see if there is a difference?

[tubegeek]

Forgive me! 

How can I hold anything against a guy who talks like this: "Such a lot of guns around town and so few brains. You're the second guy I've met within hours who seems to think a gat in the hand means a world by the tail."

[Marlowe]

Tubegeek, you're a class act!  

Mth, I have tried the EP at 9V. Actually, I built a 9V EP3 for a handicapped friend a while back. He won't use ANY effect which uses an adapter or AC power (yet he'll use an amp, go figure). Despite my advice, he insisted on a battery-operated EP-3. I have to say, mojo aside, I think the EP3 lost something in the voltage translation. It's not a great deal, maybe even inperceptible to many players. I think the EP-3 at 9V is less dynamic than at 22V. Mine sits in my rack,  after my pedalboard overdrives (where it made a HUGE sonic improvement in my opinion) and before my tube preamps. While the EP-3 is certainly hyped, there IS something it does to the sound of a rig.

If you want to build the schematic I put up at 9V, it will work, but you'll have to adjust the Drain resistor down to about 18K, I think. The circuit shown is 90% original Echoplex with only the MFP standing in for the original JFET.

One thing I will say: don't use a J201 in this circuit! It is DARK and LIFELESS (and doesn't bias properly in the original circuit)! I  use a great circuit (not the Till circuit) for a buffer that uses the J201 which is very uncolored and clean. The J201 does not do the EP-3 thing at all. The MPF sounds very good here.

Also, a larger-current rated 9VAC adapter (mine was 300mA) might cause a higher voltage at the top of the stacked caps. The circuit draw miniscule current, so there's no reason for a monster adapter.

"I'd straighten your tie a bit, Harry. Still, I'm proud to have you following me!"  

[tubegeek]

A reasonable compromise for your battery-loving buddy would be two 9V in series, I would think. Low current draw means reasonable battery life, and 18V isn't THAT far off from 22.

A version like that would be as cute as lace pants, if you ask me.

[Marlowe]

Quite true about the batteries. To be honest, I just wanted to get him the simplest thing I could! 

Bless him, he's one of the few people I know who's scared of being electrocuted by a 9V battery...

You guys have a great weekend!

[Marlowe]

You know, thinking about it, Tubegeek makes an excellent point. The circuit shown (minus the power section, of course) ought to work fine with two 9V batteries in series.

So, if you don't want to do the voltage doubler, or have an 18V wall wart, you could always try that before building the voltage doubler. The 18VDC adapter would probably still need a filter cap at the head of the Drain resistor.

I believe the 22K Drain resistor would still allow the MFP to work at 18V.

Anyway, whatever works for you works for me! Regards to all!

[pinkjimiphoton]

i really hate to break it to you guys, but the "preamp" in an echoplex is nothing but a buffer. period. 

i own 2 ep3's. if you put them in front of any amp, or in the effects loop, tonal change is really negligable other than the way buffers usually color tone.

it definitely doesn't make you any "louder". jmo. ymmv.

the guys selling the ep preamp as a secret weapon are trying to make money. god bless 'em.

but we didn't use these for the preamp in them. we used 'em cuz they were pretty much the only game in town for echoes.

if ya don't believe me, i'll put my plex in an a/b box and you can tell me if ya hear a difference. i have done this, and don't.

no dis intended to anyone, especially the op, looks like a nice circuit!

[Plexi]

Great great thread!

Anyone have the schematic of the first post?

Thanks

[pinkjimiphoton]

plexi, i've seen schematics for this circuit all over the place, i'd hit google, bro

[Plexi]

Thanks..!
I did, and found nothing so specific about what are commented in this thread.

BTW, I've open a thread about this preamp:
http://www.diystompboxes.com/smfforum/index.php?topic=116939.msg1093671#msg1093671