EHX DMM Questions

Stompboxes circuits published in magazines, books or on DIY electronics websites.

EHX DMM Questions

Postby POTL » 15 Jul 2018, 00:44

Hello everyone, I started to study the principle of operation of analog delay effects
I started to study the DMM scheme and it seemed to me that it looks very strange, in comparison with the BOSS scheme.
This forum has repeatedly helped me find answers to questions and I decided to ask a few more. :D
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?
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?
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.
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?
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.
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?
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?

In addition to questions to the scheme, there are a couple of additional questions.
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?
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?
I will be grateful to your answers.
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Re: EHX DMM Questions

Postby marshmellow » 15 Jul 2018, 04:41

1. Many old effects were not TB. People weren't as obsessed as today about real copper paths™ and probably thought more like "hey a free volume control included, cool".

2. Yes it's a failure, I rambled about that a while ago in another thread. Unfortunately the Memory Man set the bad precedent with this input stage for many other delays to follow. Not only is it a bad choice with a balance between low input impedance and noise, its gain also depends on what comes before. When the volume of the guitar or another effect is changed, it also changes the gain of the stage. Same as in the Fuzz Face, which is why it cleans up so well with turning down the volume. Because while the output of the guitar is reduced which already reduces the distortion, at the same time the gain of the first stage is reduced as well, since the preceding output impedance is part of the gain network. That's why using a buffer in front of the FF doesn't work in that regard, it isolates the guitar from the FF input.
The only advantage this inverting input stage has compared to a non-inverting one, is the fact that the volume can be turned down, not only up. But that could still be achieved in a better way.

3./4. The first stage drives the output and mixer of the clean and effect signal, the blend control. The second stage is necessary, because R6 and R34 mix the clean and the feedback signal. After that comes the compander, which has to be driven by a low impedance ("zero" ideally, so an opamp is the natural choice). If you look at the internal schematic of the NE570, you see the 20k resistor that converts the signal voltage to a current. The VCA is a current in/out device (more info on that can be found in the design notes and datasheets at THAT). Any preceding output impedance (the two mixing resistors) would upset the gain of that stage.

5. The BBD has a higher output impedance and can't really drive much, so the output must be buffered. And the input must be driven from a low impedance as well, which makes these stages necessary. Certainly they could be designed differently, IC3B and IC4A could be married into one. But then one of the other non-inverting stages would have to be made inverting as well, or the polarity would be wrong for the mixing in the feedback and the blend control. In the DM2 the BBD is also driven from a emitter follower.

6. The balance trimmer is used to cancel out the clock signal. So it is left alone after the first BBD, since that signal is not of interest anyway, only after the second BBD it gets cleaned up.

7. Not sure what you mean by offset.

8. The sound of a delay is all in the filter. With that said, the clock frequency also determines how the filters have to be designed. If the DMM is your preferred sound it's due to the filters, not the IC. The MN3005 offers improved distortion and noise specs, frequency response has nothing to do with it. So you could build a DMM with MN3205s. Besides the power supply you might also have to change the signal level going into it, or distortion could be increased.

9. Its operation is described in great detail in the datasheet. I don't think there is much tuning potential here though. You could mess with the distortion trim, but I would guess that any distortion of the NE570 is masked by the BBD performance anyway. The DM2 uses different values for the rectifier capacitor and the bias resistors, build and compare. But I don't expect much to be gained from that.


If I were to build one (I have an original one, which is easily the noisiest effect I own...), I would change several things: input stage to non-inverting, second stage reduce resistors for lower noise, the two BBD buffers non-inverting and lower noise, output stage lower noise, modulation variable, maybe buffer VB (that's a whole bunch of BJT inputs drawing bias current through 100k resistors), use something other than 4558s.

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Re: EHX DMM Questions

Postby Dirk_Hendrik » 15 Jul 2018, 10:44

I was going to respond. However, why are we looking at a schematic that is "based on" a DMM? After that, why would this pedals be "identical" to a BOSS? There's more ways to build a delay.
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Re: EHX DMM Questions

Postby POTL » 15 Jul 2018, 15:29

marshmellow wrote:1. Many old effects were not TB. People weren't as obsessed as today about real copper paths™ and probably thought more like "hey a free volume control included, cool".

2. Yes it's a failure, I rambled about that a while ago in another thread. Unfortunately the Memory Man set the bad precedent with this input stage for many other delays to follow. Not only is it a bad choice with a balance between low input impedance and noise, its gain also depends on what comes before. When the volume of the guitar or another effect is changed, it also changes the gain of the stage. Same as in the Fuzz Face, which is why it cleans up so well with turning down the volume. Because while the output of the guitar is reduced which already reduces the distortion, at the same time the gain of the first stage is reduced as well, since the preceding output impedance is part of the gain network. That's why using a buffer in front of the FF doesn't work in that regard, it isolates the guitar from the FF input.
The only advantage this inverting input stage has compared to a non-inverting one, is the fact that the volume can be turned down, not only up. But that could still be achieved in a better way.

3./4. The first stage drives the output and mixer of the clean and effect signal, the blend control. The second stage is necessary, because R6 and R34 mix the clean and the feedback signal. After that comes the compander, which has to be driven by a low impedance ("zero" ideally, so an opamp is the natural choice). If you look at the internal schematic of the NE570, you see the 20k resistor that converts the signal voltage to a current. The VCA is a current in/out device (more info on that can be found in the design notes and datasheets at THAT). Any preceding output impedance (the two mixing resistors) would upset the gain of that stage.

5. The BBD has a higher output impedance and can't really drive much, so the output must be buffered. And the input must be driven from a low impedance as well, which makes these stages necessary. Certainly they could be designed differently, IC3B and IC4A could be married into one. But then one of the other non-inverting stages would have to be made inverting as well, or the polarity would be wrong for the mixing in the feedback and the blend control. In the DM2 the BBD is also driven from a emitter follower.

6. The balance trimmer is used to cancel out the clock signal. So it is left alone after the first BBD, since that signal is not of interest anyway, only after the second BBD it gets cleaned up.

7. Not sure what you mean by offset.

8. The sound of a delay is all in the filter. With that said, the clock frequency also determines how the filters have to be designed. If the DMM is your preferred sound it's due to the filters, not the IC. The MN3005 offers improved distortion and noise specs, frequency response has nothing to do with it. So you could build a DMM with MN3205s. Besides the power supply you might also have to change the signal level going into it, or distortion could be increased.

9. Its operation is described in great detail in the datasheet. I don't think there is much tuning potential here though. You could mess with the distortion trim, but I would guess that any distortion of the NE570 is masked by the BBD performance anyway. The DM2 uses different values for the rectifier capacitor and the bias resistors, build and compare. But I don't expect much to be gained from that.


If I were to build one (I have an original one, which is easily the noisiest effect I own...), I would change several things: input stage to non-inverting, second stage reduce resistors for lower noise, the two BBD buffers non-inverting and lower noise, output stage lower noise, modulation variable, maybe buffer VB (that's a whole bunch of BJT inputs drawing bias current through 100k resistors), use something other than 4558s.


1) thank you, I expected such a response :D

2) I thought there was some kind of nuance that I could not notice, it's just a non-standard solution, thanks)

3/4) I'll try to find information on the site THAT

5) I understand, it seems to me worth trying to test with the installation of buffers or amplifiers after each delay chip, as well as try to use chips without buffers and boosters, maybe this will give a difference in sound, at least I will definitely know if there is a real meaning complicate the scheme)

6) did not quite understand the answer It turns out that with two consecutive delay chips it is enough to adjust the balance only on the last one? The first one will not have much influence (but we will still have the same resistors for each output).
Did I understand you correctly?

7) I use an interpreter and sometimes he writes really strange things)
I meant that it seems to me illogical to have the last section of the circuit broken. The last cascade is a balance potentiometer and 150 ohm resistor, as I understand this resistor in theory will be the value of the output impedance.
But I am confused by one moment,
will the 150 ohm resistor mean that the output impedance is 150 ohms or is it because of the mix value adjustment that the output impedance will change?

8) Do you think that increased power (up to 15V) will not improve the sound?
I do not have any practical experience working in BBD, I'm currently studying theoretical aspects at this moment, then to get down to work.
However, I heard (or rather read about it) that the Analogman Chorus, sounds better when it is powered by increased power.

9) Thanks

10) I remembered another moment.
The active filters in IC3A / B IC5B chips are a bit unusual, they have additional resistors and capacitors. R15 / 16/23/24/28/29 and C18 / 22 in practice, in addition to filtering, this again gives us additional reinforcement, here then it is why?)


As for changing the circuit, I have a lot of ideas, in general I want to keep the brightness of repetitions, but remove the extra adjustments for me (for example Gain), as well as greatly reduce the number of components.
Most likely it will be something between DM2 and DMM, changes (deleting cascades or changing them) will be in those parts that confuse me.
I think I need to build both pedals, and then collect the model of my device and compare the result)
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Re: EHX DMM Questions

Postby POTL » 15 Jul 2018, 15:38

Dirk_Hendrik wrote:I was going to respond. However, why are we looking at a schematic that is "based on" a DMM? After that, why would this pedals be "identical" to a BOSS? There's more ways to build a delay.


Hi, it's pretty simple in DMM, I really like the sound of repeats, it's more natural, brighter, I have not heard similar repeating quality in other pedals (if you know what other pedals sound bright, I'll be glad to hear from you examples).
BOSS I took as an example of a thoughtful design of the device, it seems more intelligent and logical.
To be honest, almost all EHX effects seem strange and illogical to me, if I compare them with the BOSS or MXR schemes.
Of the old MXR pedals, probably there is not one that I liked 100%, I think they sound very cold and sometimes dirty, with maximum adjustments (strange sounds at the maximum speed of Small Clone / Small Stone), sometimes noticeable dirt in DMM, which I never noticed in the MXR / BOSS products.

The only thing that attracts me in this delay pedal is the bright sound of repetitions, if there is an opportunity, just change the filters in BOSS DM2, I just change the filters and have a thoughtful and brightly sounding delay.
Well, the mixing function of EHX is more interesting, since I can get any amount of wet and dry signal, which BOSS does not know how.
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Re: EHX DMM Questions

Postby marshmellow » 15 Jul 2018, 18:22

6. When you are working in the kitchen, you can clean up after every step of the process, or you clean everything once when you are finished with cooking. The end result is the same.

7. The 150R increases the output impedance. But the complete term is R5 + each "half" (depending on the position of the wiper) of the blend potentiometer in parallel. So in middle position you get the maximum output impedance of 150R + 5k||5k. Turn it to any other position it decreases, at each end of the travel it is only the series resistor. So overall a sufficiently low enough impedance to not cause any problems in a guitar context.

8. "Power" is not relevant in small signal audio. The higher voltage of the MN3005 reduces distortion and improves SNR. Other than that they should sound identical when applied properly. But I have never compared both ICs in the same circuit, I'm sure other people have and documented it somewhere.

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Re: EHX DMM Questions

Postby POTL » 15 Jul 2018, 22:17

marshmellow wrote:6. When you are working in the kitchen, you can clean up after every step of the process, or you clean everything once when you are finished with cooking. The end result is the same.

7. The 150R increases the output impedance. But the complete term is R5 + each "half" (depending on the position of the wiper) of the blend potentiometer in parallel. So in middle position you get the maximum output impedance of 150R + 5k||5k. Turn it to any other position it decreases, at each end of the travel it is only the series resistor. So overall a sufficiently low enough impedance to not cause any problems in a guitar context.

8. "Power" is not relevant in small signal audio. The higher voltage of the MN3005 reduces distortion and improves SNR. Other than that they should sound identical when applied properly. But I have never compared both ICs in the same circuit, I'm sure other people have and documented it somewhere.


6) Now I understand, thanks) :thumbsup

7) Yes, that's what I expected, in general, I would install a buffer or install it in the same place as the BOSS DM2

8) Got it.
Before that, I only collected distortion effects and with increasing voltage, the sound was better.
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