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:
http://www.lulu.com/shop/thomas-henry/making-music-with-the-ne570-compander/ebook/product-17459222.htmlYou'd find most of what's in the book on the web these days, with a bit of hunting.
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