Friday, 2 May 2014

Boss HM-2 Heavy Metal Lite

Quite a few people have asked for a layout for this over the last couple of years, and I was always put off because with 3 x ICs and at least 3 transistors (depending on which schematic I used for the layout) there was no way it was going to fit in a 1590B, and you know what I'm like, I have a pet hate about putting a pedal that you can have in a Boss-sized box into a 1590BB.  I know some people don't mind, but I have a rack tray and space is at a premium so it does matter to me.  I'd just prefer to use an original if my own version was going to be physically bigger. 

Eventually Fredrik (Freppo) decided to do one himself, and he did and excellent job which you can find posted on the blog forum here, so I forgot about it for a few months.  But I spend a lot of time just looking at schematics to see how the manufacturers do certain things, and I was looking at the HM2 scheme, and just thought how much smaller it would be without the gyrator tone stack that's used and so decided I was going to do my own version using an active Baxandall stack instead to see how compact it could become.  This may not be capable of the "crushing lows" some people will want a HM2 for, but for others like me who just want a compact high gain distortion with a great tone control it could fit the bill perfectly.  This is a graph showing the frequency response of the new tone stack and as you can see you can still get a nice boost in highs and lows,




but this was based on an opamp supplied with +/-15V and so a larger voltage swing than the 0 to 9V we usually have in our effects, and so in practise you won't get that sort of dB boost.  It should however give you noticeably more of boost running the effect at 18V and so for anyone who wants a bit more meat out of the stack, I would suggest making sure appropriately rated caps are used and trying 18V.

This has been verified by a very nice man from Finland :o) and so is safe to build for those who fancy a Lite version of the HM2.  As noted, this won't give you the exact response you can get from the original especially at 9V, and so if you're in a Swedish death metal band you may prefer Fredrik's layout, but it's still one of the highest gain stompboxes we have on here according to Miro and I think it has a very nice tone control in its own right, and so reckon it will be of interest to some.

If anyone wants to further experiment with the response of the tone control, or maybe to use it in another effect, check out this calculator: http://www.geocities.jp/dgb_studio/bax_calc_e.htm




12 comments:

  1. Yup. I built this and compared it against the original 1988 HM-2 i have. I used LM358s for ICs and BF245A, C3198 and A733 for transistors. The output level and gain on this circuit are sufficiently close to the original - but. The original unit's Color Mix L/H controls boost the Qs of their frequencies a lot more. So in essence, this one sounds pretty close when you disregard higher halves of the Color Mix pots. I could get similar sounds out of both when i maxed all the controls on this circuit and had Color Mix knobs at the middle position on HM-2. Both (this circuit and original) had distortion maxed and volume at 3 o'clock.

    So it's a lite version due to missing halves on those controls :) Still a nice sounding high gain distortion.
    Someone should try placing Demeter Fat Control after this to see if the mids will then attack in the same manner as in original HM-2...
    +m

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    1. Actually I bet the stack could be pretty easily modded for a mid control. I'll check that out as another option.

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  2. Out of curiosity, would the possibility of removing the tone controls all together and having them "dimed" like the original Swedish buzzsaw tone is derived from exist? The way I use mine, it's a two knob with the gain being anywhere between 1 and dimed, depending on what I have in front of it (FZ2 boost/SHO clone)

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    1. No it would not. That would need the gyrators in tact, because those are responsible for boosting those frequencies.
      +m

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    2. I'm not going to go this far without doing an original for you all too :o)

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  3. Hallo. I made this pedal. I have every component on right place, but it´s not working and I don´t know why. Where can be problem? I used IC: TL072, Q1: BF245 and Q2,Q3 2SC2240. Sorry for my englsh, I am from Czech Republic. Thank you for reply :)

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    1. To begin with, two 2SC2240s won't work. Those are NPN transistors and Q3 needs to be a PNP.

      Otherwise there could be a ton of reasons why the circuit doesn't work. IC voltages are good place to start debugging.
      +m

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    2. Hallo. At first, I know I am novice in buildíng. voltages on IC1 are 1= 6,1V , 2=6,2V, 3= 6,1V, 4=0V, 5=14V, 6=11,2V, 7=11,2V, 8=11,2V and IC2: 1= 8,8V , 2=7V, 3= 7V, 4=0V, 5=14V, 6=3V, 7=3V, 8=1V

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    3. Are you sure about the pinout? It goes like this:
      1--u--8
      2------7
      3------6
      4------5

      If pinouts and the readings are correct, you are having an issue with your power supply. Assuming you are using 9V power. There should not be any voltages higher than 9V when powering with a battery or a basic 9V wall wart adaptor. The IC 1 pins 6 and 7 seem to be shorted with pin 8 as they have exactly the same voltage. And IC2 isn't getting the supply voltage at pin 8 at all.
      The voltages need to be zero on pin 4 for both opamps (which it is), and there must be the supply voltage for pin 8 on both opamps. All the rest (1-3 and 5-7) need to be around half of the supply voltage - maybe something like 4,6V.

      Now. You need to figure out why there is your supply voltage on IC1 pins 6-8. Short between the strip gaps, misplaced component or link would be usual suspect. Next up, you'd need to figure out why IC2 pin 8 isn't getting the supply voltage as it is linked with IC1 pin 8.

      When you get those sorted, you should probably try out an audio probe to trace where the signal dies.
      +m

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    4. Pinouts was onw big mistake, sorry. I am using 9V adaptor (it have about 13V if it´s short). I tried to use a 9v battery (it have 8,5V).
      And voltage is:
      3,7--u--8,5
      3,7------6,7
      3,7------6,7
      0------6,7

      5,3--u--8,5
      4,3------1,45
      4,3------1,45
      0------3,8

      What a hell is that ? :D

      So, only pins 2 and 3 on IC 2 are quite good.

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    5. All the others should be ok(ish), at least pretty close to working range.
      But the pins 5-7 on the lower IC are too low. Anyway. Seems like you're up for probing next :)
      http://diy-fever.com/misc/audio-probe/
      http://www.diystompboxes.com/pedals/debug.html

      Don't worry. You'll get it working. And as you need to more and more debugging, you'll understand more and more about what's going on.

      I bet you've had a short on your circuit at one point and that has burned the regulator on your wall wart supply (as it gives out ~13V). That supply is no good any longer as it'll pass all the ripple and that will cause audible noise and interference when used with pedals. I strongly suggest using a battery snap soldered to a standard DC plug when testing circuits. Burning a battery sets you back a few bucks, while burning good wall adapters will cost a lot more. Of course, the best solution for this is a lab power supply, but i don't think newcomer builders want to use 200 bucks to try their circuits out :)

      I usually recommend a lot easier circuits for beginners. I still think you can solve this!
      +m

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