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Building your own stompbox / Re: EHX DMM Questions
« Last post by POTL on Today at 10:17:13 PM »
> I just in case checked the connection of two resistors from two sources in the simulator and this does not affect the gain factor.

If you drive *one* of the two inputs to the mixer, the mix loss is half, so the result is unity.

When driving both inputs with *different* signals, it is not simple.

Yes, unity-gain 2-pole filters are a thing. They can be fussy, or need extreme values, for sharp corners. Also before the internet we used whatever equations we could find, and the with-gain form is simpler and was once more common. (I don't think Lancaster's book even touches unity-gain?)

IC4_B (thanks) is a trim for the several-dB spread in BBD gain.

How compact do you need to be? With SMT parts you can probably make it smaller than the knobs.

I did the simulation again capturing a little bit of most of the circuit, you're right both resistors affect the gain, I made a mistake, thanks for your answer)
I am interested in diley in the case of 125B, I use mostly SMD, but in addition to the compactness I'm interested in the same circuit reduction, vactically I think about how to collect a full clone of DMM and in parallel to collect the second diley making adjustments to it and in practice to understand how important these or other stages of the scheme and how you can change it without losing sound.
Building your own stompbox / Re: Foxx Select-a-fuzz.
« Last post by bmsiddall on Today at 09:54:56 PM »
Another belated bump on this. The YouTube video has me salivating!
My DIY / Maker hobbies extend beyond stompboxes. Right now I'm working on a RetroPi arcade system using a raspberry pi and making my own oldschool dual player arcade joystick.

This site, while specific to making arcade controllers, has a lot of good info that may be useful to stompbox builders. Info on materials, including wood, acrylic plastic and metal, tools for cutting, drilling and routing these materials, construction layouts for boxes and panels, etc. Maybe helpful, maybe not, maybe interesting for anyone interested in old school arcades!


Building your own stompbox / Holoflash Finish
« Last post by DougH on Today at 09:32:22 PM »
Am I just late to the party on this? This is the first Iíve seen of it. Looks beautiful!

Did you make the trace cuts under the IC?  (to separate pins 1-4 from pins 8-5)
Building your own stompbox / Re: EHX DMM Questions
« Last post by PRR on Today at 08:48:31 PM »
> Mostly, there are no "errors", only "decisions".

Errors have happened.

It was an error to build the Comet jet-liner with squarish windows and a rivet hole in the corner. However this was not clear from the existing knowledge of aluminum fatigue, or revealed in extensive testing, and it took massive post-crashes testing to narrow down the sequence leading to blow-out and death.

A happier sequence on the Mosquito bomber. This was plywood; plywood they understood. Prototypes are expected to stand 120% of design load. The first proto's wing fell off at 60%, and it is suggested they expected this. They beefed it up in stages to 116% of design load. The Mosquito was an excellent warplane in part because it was very light for the work it did. (OVER-building is a different kind of error-- some German WWI planes were built and flew like trucks, lighter Gnomes flew rings around them.)
Building your own stompbox / Re: BOSS DD-2 only output dry Signal
« Last post by tone5 on Today at 08:41:03 PM »
Thanks so much,i will keep trace this week.

+1 agree with Slowpoke.

Pin 46 is not audio. you'd be better checking for audio on pin 14 of R73 or on pin 6 pf the following 5534 op-amp.

No delayed audio could be caused by all sorts of things. Those 4066 switches might not be working, for instance. Or it could be something in the Compressor/Expander stages, as Slowpoke suggested. Or it could be a dead transistor in one of the output filters. Or a dead FET (Q8?) further along.

So long as the micro controller is working, none of them are super-difficult to fix. It's just a question of working through the circuit logically until you get to the point where the signal disappears.

Building your own stompbox / Re: BOSS DD-2 only output dry Signal
« Last post by tone5 on Today at 08:31:28 PM »
This is a very helpful information.
i will follow this step trace signal.Thank you

Welcome to the forum and hopefully we can help with your problem.

Digital pedals can be a bit difficult to repair but I think that your DD-2 won't be too hard.
You found an audio signal going to pin 31 of IC7. Good. But the signal you found on pin 46 is not audio. It is data and will not sound anything like your initial audio input.
The delayed audio output from IC7 is found on pin 14 of the R73 resistor network. R73 is a R2R resistor network which converts the digital data from IC7 to an analog (audio) signal. If you use an audio probe you should be able to hear the delayed audio on pin 6 of IC6 assuming that IC7 is working. It is most likely fine and the problem will be elsewhere.

Old pedals can suffer from dried out electrolytic capacitors. The BOSS digital pedals are prone to this problem due to internal heat buildup. The pedal works for years and then suddenly stops working.
On the schematic, look at IC2 (NE570 compander) and the electrolytic capacitors connected to it (the symbol for these capacitors have a + symbol next to the capacitor). Using an audio probe check for audio on pin 7 of IC2 (this is audio going to IC7) - You probably have audio here anyway as you have some signal on pin 31 of IC7. Then check for delayed audio on pins 14 & 15 of IC2. Most likely you will not have delayed audio available. C39 and C34 are notorious for causing the problem that you have described.
Good luck and let us know what you find.
Building your own stompbox / Re: Filters for BBD devices
« Last post by PRR on Today at 08:21:58 PM »
> bbd circuits with switched cap filters?

Switched-cap filters have similar problems to BBDs.

I recall some filters clocked considerably faster than 2X Nyquist limit, so you could set your analog filter higher and less sharp. I dunno if you can even buy those chips now.

I dunno how hard I would work to "improve" a lousy Kb delay. I just got 4GB for $40. While these mobo sticks are not simple to interface, surely small serial RAM can be found and driven delta-sig, at high bitrate and long delay.
Building your own stompbox / Re: EHX DMM Questions
« Last post by ElectricDruid on Today at 08:16:44 PM »
1) I am confused by the bypass operation mode, in this circuit we have not a true bypass. The gain controller will work even when the pedal is in bypass mode.
Why is it so?
Non-true-bypass circuits were the normal thing until not so long ago. The holy grail at that point was "click-free switching", and things that provided a buffered bypass were seen as a benefit not a liability.

2) The first stage is an inverting amplifier, so the input impedance is R2 or 100K, which is below the recommended 500K-1M, is it a design error or is there any advantage in this?
No, there's no advantage in this. A higher input impedance would be better, but again, this is something that's only really become an important selling point in more recent designs. In the late-70s/early-80s no-one knew or cared, frankly. Except the engineers, and they weren't the ones buying pedals.

3) What is this cascade made for? If you look at the BOSS DM-2 scheme, there are no additional amplification stages at all, only 3 stages of filtering on BJT, 2 filters at the input and output, and also the input buffer, which looks much more logical.
3 stages or 2 filters on input and output sounds like a cascade to me. That doesn't sound so different. You've got to have a certain amount of gain and a certain amount of filtering - you choose how you want to implement that.

4) After the first stage of amplification, we have a non-inverting amplifier (IC1B) with a flat frequency response, but giving an additional gain of 6dB, why so many amplification steps?
The second stage is really a mixer for the input signal and the feedback signal. If the first stage had been used as a mixer as well, turning the Level up would have also boosted the Feedback - not what you want. It's not a conventional inverting mixer. Instead they've done a passive mixer into a non-inverting gain stage. You don't see this arrangement so much these days, and you can find articles online discussing the value of one approach over the other, but it's a valid choice.

5) after each delay chip, we have inverting amplifiers (IC4A & IC4B). Why are these amplifiers added to the circuit? They create additional noise and take up additional space on the board along with the components. Again, in the BOSS DM2 there are no additional amplifiers after the delay section and the pedal works fine without losing any loudness.
One of the English language's more horrible expressions is "There are lots of ways to skin a cat". E.g. you can achieve the same result in lots of different ways.
I doubt adding an extra op-amp in-between 4095-stage BBDs adds any noise that is significant. The BBDs will produce so much more noise than the op-amp that the op-amp's contribution will be entirely negligible.

6) After the first delay chip (IC6), the outputs are R41 & R42 resistors, although in all other delay effects there is a trimpot for fine-tuning the balance. Why did EHX put resistors, because they do not guarantee a tonal balance setting?
Good question. Maybe they decided that since the two chips were using the same clock it didn't matter and they could trim out the clock noise as far as possible on the output. Doesn't that work? Have you tried it? Why should the second chip care if the first chip provides a signal with clock noise, after all - the two are sampling at the same rate. It *should* be possible to simply balance the two signals at the eventual output. But I know that not everyone did it this way.
Ultimately, it's a compromise, like all designs. "Improve" it and add more parts, or keep it simple? Which do you choose? You can't do both.

7) The offset scheme looks standard, but after it there is no buffer, and this adjustment will affect the input impedance, in contrast to the BOSS pedals.
Is this also a design error?
Mostly, there are no "errors", only "decisions". Some of those decisions might look bad with time, but they probably made sense at the time. Occasionally, you can find a genuine error - I think there was one in one of the early ARP synth filters, which they finished up having to fix in a later revision.

8 ) Repetitions in the DMM sound brighter than in other delay effects, Memory Boy, DM2, Carbon Copy give blurry repeats.
What is the reason? This is due to the power supply of 15V input to the circuit or with filters in the audio part? This is interesting, since I want to collect a copy, but the MN3005 chips seem too expensive and I want to buy the MN3205. The problem is that the MN3205 only works from 9V, which means that if the brightness of the repeats depends on the increased power (15V), then my idea does not make sense.
What is the reason for the brightness and readability of repetitions? Filtration or increased power?
Almost certainly *not* the extra power. Why would that increase treble? The combination of filtering and companding is much more likely to be the cause. I haven't analysed the circuit closely, but do they do any pre-emphasis/de-emphasis too? They could boost the treble going in, so that when it is cut later by the filters, the cut is not so noticeable. This helps reduce noise/hiss where it is most objectionable and also makes the repeats seem more "present".

9) The NE570 chip remains a mystery to me. I understand that it works in compressor / expander mode, compresses the signal to the delay section and returns it to its original state after the delay section. But I do not understand what components affect, in Datasheet I did not find the information, where can I read it?

Thomas Henry's little book is the best source I know about the NE570 compander, but it's expensive for what you get:

You'd find most of what's in the book on the web these days, with a bit of hunting.
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