Deluxe Memory Man mod project part 1

[Primus]

I am finally getting around to modding my DMM so I opened her up today to see what exactly was going on inside. I noted:

Already true bypass by blue 3PDT
MC4558CN ICs in all positions

What I intend to do is replace U1 with a TL072 for a cleaner input signal. I still want the regens to be dirty, though. What I am going for is warm and tape sounding but not harsh. I was wondering if it would make a difference if I chose different op amps for the regen signal, for example the JRC4558D sounds great in my tube screamer (much better than the one I replaced, anyway).  Would it be possible to adjust the gain on the JRC4558s by replacing a resistor w/ a pot? I also know of other pedals that have  Bass and Treble cutting pots for the regen signals so you could get dub effects. Another thread had mentioned doing this by adjusting caps, but could it be done a different way with the DMM using a pot? I don't have the schematic because the link is broken in the schematic page.

One last thing, my DMM (which I mentioned is true bypass) makes my amp hum a little bit. I don't understand why it could be humming since it runs on AC power and is therefore ground loop isolated by the power transformer. Any thoughts?

Thanks in advance.

[LyleCaldwell]

Primus,

I redid a DMM a few months back.  First, the tone of the DMM is due to three main things - the memory chips, the input impedance, and the gain structure.  The 4558s were used because they are cheap, but they really just raise the noise floor.  You can replace all of them with TL072s without negatively affecting the tone, and the noise floor drops quite a bit and the dry sound (when active as your pedal is TB) has more clarity while the delays still have all the character you like.

So try TL072s in place of every 4558 - it's a cheap experiment and I think you'll agree with me.

Now, the stock input impedence is really low and there's a ton of potential gain on tap - much more than is needed, even using the DMM as a boost.  So replace the 100K resistor in the input with a 1M pot.  Here's the schematic:  http://www.freeinfosociety.com/electronics/schemview.php?id=381.

The stock gain structure has a 100K resistor before the opamp and a 1M pot in the feedback loop (this is the volume pot).  By changing the 100k resistor to a 1M pot, you can set it to the stock impedance and when the volume pot is on full, have 10x gain.  But you don't need 10x gain, and the stock impedance is really low and loads down the pickups in a passive guitar (particularly with single coils).  Put in the 1M pot and you can raise the input impedance and lower the overall gain available, finding a balance between a higher input impedance and still having enough gain on tap.  I liked the input pot set to about 500K, which still had enough gain (2:1 at max though I didn't use it on max) for a nice boost.  Really improved the sound and gave it more depth.

As to noise, the stock design is really terrible in terms of grounding.  All the jacks are connected to the chassis and the ground is just series from jack to jack to PCB to transformer, along with using the chassis as a ground path.  Replace all the jacks with nylon Switchcraft/Neutriks, or use shoulder washers on all jacks.  Run the grounds from the jacks and the PCB to the grounding point of the transformer.  Now there's only the one path to ground, and the chassis (which will be grounded only at the transformer as well) can shield but not serve as a ground path.  This should greatly reduce hum.

If you have any hum/noise at that point, the stock wire is terrible and runs all over the place.  Replace all the wires to/from the jacks and PCB with shielded wire.

[Toney]

Ha heh, had to do a double take there......(Max voice) " he's modding his Digital Multimeter, second time this month" then ohhh , duh, Deluxe Memoryman.......ah so.

[Primus]

Thanks, there is a lot of great advice in there. I'll try the other TL072s as well as the new grounding path.

[LyleCaldwell]

Try the 1M pot as well - makes a huge difference and will cost you $1.50.  If you don't like it, put the 100K resistor back in.

[LyleCaldwell]

BTW, the "dry out" jack is useless as it's a passive split.  You can't use it musically unless you have a buffer before the DMM, and if you have a buffer odds are you can split before the DMM.  So that chassis hole is a good place to put the 1M pot if you want it externally adjustable.

[twabelljr]

Ha heh, had to do a double take there......(Max voice) " he's modding his Digital Multimeter, second time this month" then ohhh , duh, Deluxe Memoryman.......ah so.

That's funny, me too! The sad thing is, if there was not a link to the schematic, I'm not sure I would have figured it out. lol

[rockgardenlove]

I also thought he was modding his multimeter lol.  I got especially confused once he mentioned a stomp switch.

[Primus]

Ha ha, whoops! Changed the title of the post!

I should be clear about what I mean by gain... I am looking to make the regen cycles break up a little. That is why I was considering staying with the 4558s and not a TL072, because I am interested in some of the breakup that overloading the circuit yields. Is that what the mod you are suggesting does?

Any ideas about how to control bass and treble rolloff? This is really the most important potential modifcation for me. I'm okay with the idea of using a daughterboard and building a low-pass filter. I should reference this Mark Hammer post:

"No.  The trimpots are for other things.  The easiest, cheapest, least invasive way to insert a treble cut is to locate the input and ground lugs on the Feedback/Regeneration pot, and solder a cap between them.  A 0.1uf cap is probably a reasonable place to start, but there is no special magic value to aim for, other then something which produces enough of a treble cut that you can detect the occurrence of even less treble on each repeat.  Bigger values for more treble cut, smaller for less.  Indeed, you may be able to leave the pedal plugged in, take out an assortment of caps with long enough leads, touch them to the pot lugs and select a suitable value by ear.

If you want to cut the bass increasingly with each repeat (great for dub music and all Augustus Pablo fans), identify the cap in series with the regen path, just before where it gets mixed back in to be shipped to the BBD again, and reduce its value.  It will probably be set for maximum bandwidth (we'll say that's 20hz rolloff just off the top of my head) so you will want to cut it by about 80% for starters to raise the low-end rolloff by 2 octaves or so.  If, hypothetically, it were 0.1uf, resulting in a 20hz rolloff, you would probably want to replace it with .018uf to raise that rolloff up to around 110hz, or .012uf to raise it to 167hz."

I guess I could use a 5 position toggle with caps, but I'd rather have a knob.

Also, what's a good source for the nylon washers? Mouser?

[LyleCaldwell]

The mods I suggested retain plenty of gain and breakup, just without loading down the pickups and with less noise.  The DMM already loses treble on each regen, but follow Mark's suggestions (check the schem) to further change the tone on regen.

Mouser has the washers, as does Small Bear (I think Small Bear lists the Mouser part number).

[Processaurus]

Hi, if you have the DMM with the three prong AC cord, the hum you are getting is a ground loop, from the third prong, not anything to do with the grounding scheme EH uses on their 1/4" jacks.  A big piece of metal like the chassis has such a low resistance there probably is little or no benefit to star grounding the jacks.  On many hum-free Boss pedals they don't even bother to run a wire to one of the jacks, they just use the chassis to ground one of them, and run a ground wire from the board out to the other.  Try plugging the AC cord into the same outlet or plug strip as your amp and see if that gets rid of the hum.  That worked for a friend of mine with a hum problem using the DMM (it hummed even when the effect wasn't on).

Good idea, Lyle, about using the direct out for another knob, that way you don't hurt the resale value/look.

Cosmetically you can mod the DMM  by putting some metal washers between the pots and chassis, so the knobs don't stick up so high.  That way you can turn them with your feet and not worry about bending the shafts.

Primus, for distortion on the repeats, how about putting a Red Llama/ CA tube sound fuzz in between where the dry signal splits off and the delay circuitry starts.  Its a very simple 1 chip circuit that is neutral frequency-wise and clips the signal softly when it has low gain; you can see it gently rounding off the tops of the signal its clipping on a scope.  That would be a sure fire way to get your delay sounding as grungy as you want.

[LyleCaldwell]

If a DMM is grounded as I suggested, then the only way it will hum is if the gear is hooked up to different outlets, particularly if the different outlets are on different circuits.  Gear should always have a common supply.

It's never a good idea to use the chassis for an audio ground.

Hi, if you have the DMM with the three prong AC cord, the hum you are getting is a ground loop, from the third prong, not anything to do with the grounding scheme EH uses on their 1/4" jacks.  A big piece of metal like the chassis has such a low resistance there probably is little or no benefit to star grounding the jacks.  On many hum-free Boss pedals they don't even bother to run a wire to one of the jacks, they just use the chassis to ground one of them, and run a ground wire from the board out to the other.  Try plugging the AC cord into the same outlet or plug strip as your amp and see if that gets rid of the hum.  That worked for a friend of mine with a hum problem using the DMM (it hummed even when the effect wasn't on).

Good idea, Lyle, about using the direct out for another knob, that way you don't hurt the resale value/look.

Cosmetically you can mod the DMM  by putting some metal washers between the pots and chassis, so the knobs don't stick up so high.  That way you can turn them with your feet and not worry about bending the shafts.

Primus, for distortion on the repeats, how about putting a Red Llama/ CA tube sound fuzz in between where the dry signal splits off and the delay circuitry starts.  Its a very simple 1 chip circuit that is neutral frequency-wise and clips the signal softly when it has low gain; you can see it gently rounding off the tops of the signal its clipping on a scope.  That would be a sure fire way to get your delay sounding as grungy as you want.

[Primus]

I'm not convinced that the grounding issues has to do with the third prong. The wiring in my 100-year-old apartment has no ground wire, so I use a cheater to plug in the memory man. I have also plugged it into a modern wiring system both on and off the same ground as the amp with no difference. I am going to try isolating the jacks first, then maybe I'll try doing crazy ground wiring.

I haven't heard of the chip you are talking about. I'll go poking around. I am mainly psyched about the high pass filter on the regens right now.

[Primus]

Well, I'm still waiting for some parts to come in but I couldn't resist playing around with the op amps. The configuration I tried was with a TL072 in U1 and TI JRC4558Ds in U2-U5. I plugged it in and boy did it sound gorgeous. I should also comment that it changed some of the characteristics of the knobs. For example, it's harder to make the pedal feedback into overload.

I have a question about the overload sounds. When you overload the pedal you typically here a wshhhwshhwshhwshhwhsh sound like wind blowing over a microphone. Then as this gets louder, it starts to clip and sound bad and you get wshhKKwshhKKwshhKK. I was wondering what part of the circuit was overloading there, the op amps and is it something that I can reduce? Maybe by making U2-U5 also TL072?

Also, Mark Hammer suggested a way to do a treble cut:

"If you want to cut the bass increasingly with each repeat (great for dub music and all Augustus Pablo fans), identify the cap in series with the regen path, just before where it gets mixed back in to be shipped to the BBD again, and reduce its value.  It will probably be set for maximum bandwidth (we'll say that's 20hz rolloff just off the top of my head) so you will want to cut it by about 80% for starters to raise the low-end rolloff by 2 octaves or so.  If, hypothetically, it were 0.1uf, resulting in a 20hz rolloff, you would probably want to replace it with .018uf to raise that rolloff up to around 110hz, or .012uf to raise it to 167hz."

Can someone help identify what cap he is talking about in the schem?:

http://www.freeinfosociety.com/electronics/schemview.php?id=381.


Love,

Me

[LyleCaldwell]

It's the memory chip that gets overloaded.  One of the trim pots (sorry, forget which one) controls that.  Be very wary of adjusting them as they are non-intuitive and very interdependant.  If you want to play around with them, mark their current positions before doing anything.

Sorry, don't know which cap that is.

Well, I'm still waiting for some parts to come in but I couldn't resist playing around with the op amps. The configuration I tried was with a TL072 in U1 and TI JRC4558Ds in U2-U5. I plugged it in and boy did it sound gorgeous. I should also comment that it changed some of the characteristics of the knobs. For example, it's harder to make the pedal feedback into overload.

I have a question about the overload sounds. When you overload the pedal you typically here a wshhhwshhwshhwshhwhsh sound like wind blowing over a microphone. Then as this gets louder, it starts to clip and sound bad and you get wshhKKwshhKKwshhKK. I was wondering what part of the circuit was overloading there, the op amps and is it something that I can reduce? Maybe by making U2-U5 also TL072?

Also, Mark Hammer suggested a way to do a treble cut:

"If you want to cut the bass increasingly with each repeat (great for dub music and all Augustus Pablo fans), identify the cap in series with the regen path, just before where it gets mixed back in to be shipped to the BBD again, and reduce its value.  It will probably be set for maximum bandwidth (we'll say that's 20hz rolloff just off the top of my head) so you will want to cut it by about 80% for starters to raise the low-end rolloff by 2 octaves or so.  If, hypothetically, it were 0.1uf, resulting in a 20hz rolloff, you would probably want to replace it with .018uf to raise that rolloff up to around 110hz, or .012uf to raise it to 167hz."

Can someone help identify what cap he is talking about in the schem?:

http://www.freeinfosociety.com/electronics/schemview.php?id=381.


Love,

Me

[jxoco]

It looks to me like pin 10/11 of the NE570 goes to a 7.5k and a 1k and then a .47uf, then to the feedback control. From the wiper of the feedback control through a 22nf (.022uf) and a 100k back to the input of the first half of the NE570, the input.
I guess you could mess with the .47 or the .022uf.