Roland Space Echo Pre Amp

[jcober]

Hello all!

I joined your lovely forum as I am seeking some information from folks who are much more well versed in the world of electronics than I am myself! I have been pouring over the service manuals for the Roland RE-150 Space Echo in the hopes of faithfully recreating the wonderful pre amp circuit found inside. I own an RE-150 and it has become a staple in my sound that I use 100% of the time. Unfortunately, I tour a lot and though I have often brought the Space Echo with me, I dont see this as a viable option in the future (my nerves cant handle it anymore!!). I have been searching for a suitable, road ready replacement, not just for the echo, but for the pre amp as well. After trying some stompboxes that loosely recreate these sounds (and being a bit disappointed), I figured I would try my hand and recreating the thing myself.

My question(s): I have isolated, in the block diagram where the circuit begins and ends (from Instrument in, to Direct out) and I can see the pre amp circuit from J03 through to around Q5 and on to jumper (J) 14 on the schematic as well as the output amplifier surrounding IC3 to the direct output (with the H, M, L output resistors). I am wondering, with my somewhat limited understanding of the "rules" where can I make a clear distinction and isolation of the circuit to begin creating a revised schematic and subsequently begin building this for use as a stompbox? Are there components that arent dircectly related to that signal path that will be necessary for operation? For instance, I see that only 1/2 of the 4558 Op amp IC is being used specifically for the output amp and the other half is dedicated to the echo (100% wet) output, which I would not care to include. Could I use only half of that specific opamp IC? Are there any other components that arent directly related to that isolated circuit, but may be a necessity to actually making this thing produce any sound at all? If anyone has any helpful information, I (and my road weary Space Echo) would forever be in debt to you!

Here are the service notes for the RE-150, block diagram being found on page 1 and the pre amp schematic on page 3 :https://ia600208.us.archive.org/20/items/roland_RE-150_SERVICE_NOTES/RE-150_SERVICE_NOTES.pdf

Im going to throw together a revised schematic today, based on what my understanding of the circuit is to give you a better idea of what I am thinking and perhaps one of you brilliant humans could verify that I am on the right path to making this work?


Thanks so much in advance and I apologize if this is a ridiculous and naive pursuit!
J

[Ice-9]

Just a quick look at the schematic but what jumps out is the components around Q3 and Q4 form the instrument pre amp, ie all components between C7-C9 can be considered as the preamp section. The main dry signal now goes to one half of IC1 via VR3, R20, R21 and C10, You can then follow the signal out through C13 and R56 into the output op amp half of IC3. You can use both parts of one dual op amp instead of splitting them between two IC's.

That is the basic signal flow as I see it.

Q5 is used as a signal echo switch/bypass and would not be needed in a dry only circuit.

[jcober]

Marvelous! I cant believe I didnt see it this way from the block diagram. You cleared things up wonderfully for me, thank you. Just to make sure, you do mean that the dry signal goes to the 56k resistor at R58 from C13 and not to R56, correct?

[wavley]

If it were me, I'd only bother with this part of the circuit.  The 150 fixes the miserably stupid input impedance of the other Space Echoes, that's why so many folks do the Billy Zoom mod to there's.  I did that to my 101, my 501 has a good chunk of what you need to have the important parts of an FA-1 without the eq part.



Maybe put a regular old buffer on the output of the pot and use a 4558 or since you only need half, just use a 741.

[Ice-9]

Marvelous! I cant believe I didnt see it this way from the block diagram. You cleared things up wonderfully for me, thank you. Just to make sure, you do mean that the dry signal goes to the 56k resistor at R58 from C13 and not to R56, correct?

Yes that's correct, it is R58 which is a 56k resistor not R56 


The snippet Wavley has shown is the important pre amp part

[jcober]

so really, the direct output section has no effect on the sound? How necessary is the section surrounding IC1? Keeping in mind that I want this to be as close to an RE-150 being used without a tape as possible, you figure the pre amp is the sole contributor to that wonderful sound I hear? I figure it couldnt hurt to just breadboard that pre amp from input to the 20k-b volume pot and test it out! Another thing I am wondering, on the schematic it looks like it is receiving +15v...is that correct? any way I can run this thing at +9v and if so, how would I go about doing this? I really wish I could offer more than my gratitude for this information...your knowledge is invaluable!!

[jcober]

so I threw this together really quickly and aside from the IC symbol being incorrect (didnt have time to create a new symbol in the CAD library), does this look about right? I also didnt include the output amp, but if you're saying it is unessesary, I may just throw a simple linear power boost at the end and see how it turns out! Still wondering about the +VDC, though...

[t_mbaker]

A couple of quick observations on your schematic:
-R2 shouldn't be connected to C1 and R4, in the RE150 service manual only the line crossovers with a black dot on them are connected. R2 simply goes from the junction of R1 and R3 to ground.
-Similarly, there shouldn't be a connection from the collector (pin 2) of Q2 to the wiper (pin 2) of RV1.
-Everything from R8 on (imho) isn't needed, look up 'AMZ basic buffers' for some more useful circuits.

[jcober]

ah yes! my mistake! Thanks for picking up on that! Im going to breadboard this today with a buffer and simple JFET boost at the end of this and see how it sounds! Will report back. Thanks again for your help folks 

If anyone is remotely interested, Ill post an a/b of this and the actual RE-150 and see how they compare

[thermionix]

If anyone is remotely interested

Fire it up, man!

[jcober]

Finally got around to putting this thing together and much to my shagrin, I only get a loud buzzing sound when it is engaged I half expected this upon my first attempt, but now I am wondering what I could have done wrong! I couldnt find the listed 2SC732TM transistor, so I went with a similar 2SC1815 as a replacement. I dont imagine this would be the issue, but perhaps it is? I double checked the orientation of the transistors and have gone through the full diagnostic process and cant seem to figure it out! Perhaps you fine folks have an idea...I have a funny feeling that this circuit needs something other than +9VDC, but I really dont have the know how to rectify this. In the meantime, I will keep hacking away at this thing and report back if this melon scratcher gets sorted

[jcober]

Here is a bit of an update for those who may care:

I got this thing finished and sounding great!! I added a boost circuit (switchable by a DPDT toggle) to the end of it to use as an overdrive if I care to as well as being able to just use it for the preamp. I threw it in a box and quickly did up some (questionable) graphics for it. Im so pleased with the results. Ill make a demo of it if anyone would care to hear it beside the Original Space Echo unit!

Thanks so much for your help everybody.

[bartimaeus]

I love it, looks great! I've always wanted to build a standalone space echo pre...

Mind explaining what was wrong with your first attempt that you fixed by the time you made a box for it? Did the shielding of the enclosure solve the buzz?

[Squarewavesurfer]

Was this circuit able to run off 9V afterall? 

Also, what type of capacitor should be used for the 1uF cap (C3 in your schematic, C9 in the re-150 schematic) ?  The re-150 schematic shows it has polarity so I am guessing it's an electrolytic.


I did up a schematic based on the info shared here.  I included a DPDT switch to bypass the preamp as I will be incorporating the circuit within a delay pedal and I like options

[Squarewavesurfer]

For anyone interested, I can confirm this works great as per this schematic using 9V power:

[antonis]

I'd split R15 into 270k + 220k and put an electro cap (10 - 22 μF) between their joint and Source..
(it's a pity for JFET CS amp to exhibit only 478k2 input impedance..)

[Squarewavesurfer]

I'd split R15 into 270k + 220k and put an electro cap (10 - 22 μF) between their joint and Source..
(it's a pity for JFET CS amp to exhibit only 478k2 input impedance..)

I didn't see this reply sorry.  Do you mean like this?

[antonis]

Do you mean like this?

Yeappp..


It should be more easy to just connect cap between Source and R13/R14 joint (classic bootstrap configuration) but then R14 should be set in parallel with R18 upsetting AC load line..
Now, R14 is "isolated" from Source via R15A high value resistor while R15B apparent value is 220k times 1/(1-A), where A = (g_m*3k3) / (1 + g_m*3k3)

[Wavelength]

Gang,

Was just checking this out today. Really why is there a voltage divider biasing the JFET? It's only set at 0.323V from my calculations, why not ditch the whole thing and throw the 470K to ground.

A better JFET in my experience is the LSK189 better input voltage capable of some pretty high biasing to ground. At 1ma typically 1.21V biasing which is nicer than 0.3+0.323 @ like what 150-180uA in my book plus much lower noise.

If your going to run the JFET really low current then maybe use a LDO because it's going to pick up some noise.

I just did a folded cascode Clean Boost (18dB, transformer output 4Vac capable with 9V supply) with a LSK189 feeding a 2N404 with a new LDO regulator using a J113 loading an LM431 shunt regulator. Worked really well -68dB of noise drop and only 323nVrms self noise at 10Hz.

~~~
So what voltage was the 150 running the preamp section at?

Thanks,
Gordon

[jafo]

What's Q4 doing in the circuit? It's an emitter follower buffer, hence no amplification; my guess is that the JFET's output impedance is still too high for whatever follows Q4?