Metropoulos Forum

So, I have been trying to figure out the Echoplex preamp to see if I could build one but am a bit stumped on the schematic. Some folks in another section of the forum have been trying to help out. Thought I would cross post to see if anyone had any thoughts.

The thread is about reading schematics and I threw this one in there.

http://forum.metroamp.com/viewtopic.php ... 6&start=15" onclick="window.open(this.href);return false;

here's the basic schematic made by peter of clinch fx himself, posted on another forum



for the 18 volts, a simple charge pump will do, see this page for info
http://geofex.com/circuits/+9_to_33.htm" onclick="window.open(this.href);return false;

i don't think clinch uses that particular chip, but it'll work just fine. he also recommends using some small value caps

you can use a 2n5457 instead of the very tough to find TIS58. i scored some NOS TIS58's on eBay, couldn't hurt to look i guess. the only thing i'm not certain of is how the volume control is hooked up. i haven't had the chance to experiment with the circuit, so that's up to you. other folks here have probably tried it out too, maybe they can be of assistance. hope that helped!

spaceace76 wrote:here's the basic schematic made by peter of clinch fx himself, posted on another forum



for the 18 volts, a simple charge pump will do, see this page for info
http://geofex.com/circuits/+9_to_33.htm" onclick="window.open(this.href);return false;

i don't think clinch uses that particular chip, but it'll work just fine. he also recommends using some small value caps

you can use a 2n5457 instead of the very tough to find TIS58. i scored some NOS TIS58's on eBay, couldn't hurt to look i guess. the only thing i'm not certain of is how the volume control is hooked up. i haven't had the chance to experiment with the circuit, so that's up to you. other folks here have probably tried it out too, maybe they can be of assistance. hope that helped!

Thanks, this is exactly what I am looking for. Now I need to put it into a layout. My naive question is why are the caps and resistors needed that go directly to ground - at the bottom of the schematic?

i don't know enough about electronics to say, but they do serve different functions. the 1Meg resistor at the input is setting the input impedance, and the 220k at the output is setting the output impedance.

and just realized i didn't finish one of my sentences, "he also recommends using some small value caps..." across the electrolytics in the charge pump to keep out noise from the chip.

Or 2 9 volt batteries in series giving 18 volts http://www.thepartsbook.com/SpectorEMG18VVMC.pdf" onclick="window.open(this.href);return false;

http://www.thepartsbook.com/EMG9vto18%2 ... rsion.html" onclick="window.open(this.href);return false;

xotic effects just put out a pedal that supposedly does just this, I havent heard any clips yet, anyone else know or heard anything bout these?

Hi Roger, it's quite a while since I last heard from you, but I guess this thread is getting old too

To answer some questions:

I use the LT1054 for the charge pump. It costs a little more but is a far more robust chip than the MAX1044. The pedal path is littered with broken MAX1044s because there was a small spike on the 9V supply A charge pump "Doubler" will never give double the input voltage, but. if you use Schottky diodes in the charge pump circuit, you can get close to double the input voltage. A charge pump power supply is a type of switch mode or switching supply. These generate a lot of high frequency electrical noise, requiring very good filtering.

There are very few JFETs that are an exact match for the original TIS58. If I hadn't worked in R&D as assistant technician to an analog design engineer, I would probably have never understood the significance of some of the data sheet parameters. Part of my job was to find components that could meet his specifications.

It's not easy to explain the need for capacitors and resistors that go to ground, without at least a few days worth of classroom study in electronics. The simplest explanation is that an electronic circuit must have a complete path from positive to negative, or vice versa. In this circuit, ground is negative, and these components complete the path. C3 affects the frequency and phase responses of the circuit. In later versions of the EP-3 it was not in the circuit.

Spaceace, we've talked before via PM. How are you?

The true purpose of the 1M input resistor (R2) from the gate of the JFET to ground is to provide a ground reference for the gate. It's a coincidence that it also sets the input impedance, in conjunction with R1, C1 and C2. At normal guitar frequencies, the input impedance will be very close to R1+R2 = 1.1M. At radio frequencies, the input impedance will be determined by R1 and the capacitive reactance of C1. Capacitive reactance http://www.tpub.com/neets/book2/4c.htm is the effective resistance that a capacitor will present to an AC signal. R1 and C1 create a filter to prevent radio frequencies from getting into the preamp. C1 will bleed any radio frequency signals to ground.

The output impedance is very important to the phase response of the EP-3 preamp. There's another guy selling PCBs for what he claims is the same thing as an EP-3 preamp, but he just connected a 50K pot to the end of the snippet of schematic that I posted, and that just doesn't cut it.

The Xotic EP Booster is effectively just a generic JFET Booster that has very little in common with the Echoplex EP-3 preamp. The cosmetics of the box come a lot closer to the EP-3 than the circuit inside

There are also other pedals around that use a buffer to reduce the high output impedance of the EP-3 preamp. One of the changes that came in with the EP-4 was an output buffer, and it just doesn't have the EP-3 tone.

I hope this helps.

Peter.

clinchfx wrote:Spaceace, we've talked before via PM. How are you?

Doing good Peter! Been setting up my workspace at my new house, unfortunately that means i've put off making an EP-3 preamp of my own, along with all my other crazy ideas.

I've heard you've been in and out of commission due to heath reasons, and I'm always glad to hear about someone getting one of your pedals in the mail since that means you're doing better. I hear it a lot too, the pedal is a big hit!

Thanks for the info, and stay well!

spaceace76 wrote:I've heard you've been in and out of commission due to heath reasons, and I'm always glad to hear about someone getting one of your pedals in the mail since that means you're doing better. I hear it a lot too, the pedal is a big hit!

Thanks for the info, and stay well!

Thanks spaceace,

I believe most of the health problems are well behind me now. During a recent trip to Carnarvon Gorge National Park, http://www.derm.qld.gov.au/parks/carnar ... index.html we walked about 15Km (nearly 10 miles) through some demanding terrain in one day, so I guess you'd say that we are both reasonably fit.

Mandy has Diploma qualifications in Electronic Engineering and years of practical experience both on the workbench and in management, so she is now doing a lot of pedal assembly and answering email enquiries.

Waiting time for an EP-PRE is currently no more than a month.

We're getting ready to travel throughout Australia, putting pedals together as we go. During our recent shakedown run, we sent pedals from Injune http://en.wikipedia.org/wiki/Injune,_Queensland, Emerald http://en.wikipedia.org/wiki/Emerald,_Queensland and Longreach http://en.wikipedia.org/wiki/Longreach,_Queensland.

Here's a photo of our caravan - I believe the Americans here would call it a Travel Trailer.



It's Solar powered with high efficiency LED lighting and carries enough water to remain away from civilisation for a number of days at a time. There is a good sized workbench inside.

We are at home right now finalising the design of a couple of new pedals before heading out again.

Peter.

clinchfx wrote:The output impedance is very important to the phase response of the EP-3 preamp. There's another guy selling PCBs for what he claims is the same thing as an EP-3 preamp, but he just connected a 50K pot to the end of the snippet of schematic that I posted, and that just doesn't cut it.

Hi Peter

Another fellow Aussie here visiting this fantastic forum!!

I have breadboarded your schematic with nice results. Re your comment above, would it be rude of me to ask how you hook up the output pot? Or is that your intellectual property? If it's the later, I'll understand. I tried to interpret it from the original EP-3 schematic, but not sure I've got it quite right. The taper does quite not quite replicate what you say on your website.

BTW - I am eagerly awaiting the arrival of my Metro 50w Plexi kit - should be any day now, so pumped! Can't wait to try out my EP-3 Preamp on it.

Here are some other pedals I've built.

Cheers from a chilly Melbourne

You didn't by any chance send some of your Melbourne weather up here??

It's cold, wet and miserable here in Brisbane today

Those are nice builds - good shortest path wiring instead of long wires with right angle bends running right around the box Long straight wires with right angles everywhere may look neat to some, but that kind of thing is rarely electrically neat.

I saw your Veroboard (stripboard) layout at FSB. You seem to have gone closer than anyone else to approximate the output impedance of the EP-3 preamp and you've used a 500K pot Without giving anything away, I suggest that you look at the Echo Volume pot on the original Echoplex schematic - I think you probably have a copy. Notice that the pot is not in the main signal path. The wiper of the pot is grounded, therefore the function of the pot is to control the output by bleeding signal to ground, not to pass signal to the output socket.

There has been talk of "correcting" the high output impedance of the circuit, but the high output impedance is one of the things that sets the EP-3 preamp apart from a common JFET preamp. Your SPDT switch will give you more output and lower output impedance, but at the expense of some of the special sound of the real thing. As I said in an earlier post, the EP-4 has a low impedance output buffer, and it is not regarded as having that Echoplex magic.

Also, be aware that changing the capacitor across the source resistor will have a major effect on the phase response of the circuit. Phase response really does have an effect on the way we hear things. BBE have used phase response modification for a long time. Before BBE, Bose used an equalizer to alter phase response in the 901 and 802 series speakers, to create that "in your face" larger than life sound. There is a good write-up about phase response from Analog Devices http://www.analog.com/library/analogdia ... lters.html It is written for active filters, but it does explain the concept of phase response.

Hope this helps,

Peter.

clinchfx wrote:You didn't by any chance send some of your Melbourne weather up here??

It's cold, wet and miserable here in Brisbane today

I reckon it's been the coldest bloody winter for a long time!!

clinchfx wrote:Those are nice builds - good shortest path wiring instead of long wires with right angle bends running right around the box Long straight wires with right angles everywhere may look neat to some, but that kind of thing is rarely electrically neat.

Thanks - very humbling comments. I tend to meticulously plan these builds and I've now found a great sized box that accommodates 2 x 9v batteries (I run everything I can on 18v!) and is deep enough to build allow for various switching options. I now use it as a template for all future builds, which saves a lot of time.

clinchfx wrote:I saw your Veroboard (stripboard) layout at FSB. You seem to have gone closer than anyone else to approximate the output impedance of the EP-3 preamp and you've used a 500K pot Without giving anything away, I suggest that you look at the Echo Volume pot on the original Echoplex schematic - I think you probably have a copy. Notice that the pot is not in the main signal path. The wiper of the pot is grounded, therefore the function of the pot is to control the output by bleeding signal to ground, not to pass signal to the output socket.

There has been talk of "correcting" the high output impedance of the circuit, but the high output impedance is one of the things that sets the EP-3 preamp apart from a common JFET preamp. Your SPDT switch will give you more output and lower output impedance, but at the expense of some of the special sound of the real thing. As I said in an earlier post, the EP-4 has a low impedance output buffer, and it is not regarded as having that Echoplex magic.

I have read the EP-3 schematic once again, and to be honest the penny didn't drop a while back when I first looked at it, but I think I know what's going on now. Will breadboard it over the weekend. Cheers for the tip.

I am going to ditch all the switching and going with a simple build. I will probably run it with 2 x 9v batteries, but am curious about the sonic differences compared to the charge pump (have a few 1044's lying around, although I know how you feel about these!). Will probably breadboard both just to compare.

clinchfx wrote:Also, be aware that changing the capacitor across the source resistor will have a major effect on the phase response of the circuit. Phase response really does have an effect on the way we hear things. BBE have used phase response modification for a long time. Before BBE, Bose used an equalizer to alter phase response in the 901 and 802 series speakers, to create that "in your face" larger than life sound. There is a good write-up about phase response from Analog Devices http://www.analog.com/library/analogdia ... lters.html It is written for active filters, but it does explain the concept of phase response.

That put me in my box - have a long way to go before understanding much of what's going on there!! Suffice to say, I am sticking with the .022uf source bypass cap as I wasn't too impressed with my switching options.

clinchfx wrote:Hope this helps,

Peter.

Very helpful indeed, thanks again Peter!

Thanks kd,

I've taught electronics modules and , as you can see, I prefer to provide clues to answers instead of giving outright answers.

I've always been put off by the low native switching frequency and the low current available from the 1044 and I've never used one. I think everyone is aware that I prefer the LT1054, and I believe it's worth the extra cost. During my prototyping of the EP-PRE, I ran it from one of a couple of regulated bench supplies adjusted to 18V. They are both analogue supplies and should come relatively close to the performance of a fresh pair of batteries. I couldn't hear any difference when I compared the first prototype with the next one using the LT1054.

I think we still have your Melbourne weather up here

Peter.

very interesting, for my echoplex.
i'm restoring it, and i'd like to have some spare parts, for som replacements if needed. the TIS58 seems ok, but i'd like to have some pieces in my stock. so, the 2n5457 would be the best replacement ? what hfe would be the best, when selecting the rght one ? are there some other transistors that would be good , or even better ?
it's possible to find some TIS98, but quite impossible to find some TIS58 ...

my echoplex is gonna be a monster, with cool mods ; )

cheers,
Adrien

Xplorer wrote:very interesting, for my echoplex.
i'm restoring it, and i'd like to have some spare parts, for som replacements if needed. the TIS58 seems ok, but i'd like to have some pieces in my stock. so, the 2n5457 would be the best replacement ? what hfe would be the best, when selecting the rght one ? are there some other transistors that would be good , or even better ?
it's possible to find some TIS98, but quite impossible to find some TIS58 ...

my echoplex is gonna be a monster, with cool mods ; )

cheers,
Adrien

Hi Adrien,

There were some places where the TIS58 was in stock (quarndon(dot)co(dot)uk), but now they're out of the TIS58 almost everywhere.
I've some TIS58 left, if you need some, PM me.

Dom