Boss HM2 clone: increase gain?

[Cotoletta]

Hello, I built an HM2 and works great, but I have two minor issues: the first looks like there isn't enough gain. I drew this schematic: http://i.imgur.com/kx61cQu.jpg Any value tweakings to have a little more gain? Oh the schematic shows TL072, but I'm using some RC4558.
The second thing is that I have seen some schematic with the resistor (R9, 68K) on the non inverting input of the first opamp connected to VCC or VB (half the power supply). Currently I have it to VCC as the schematic, sounds great and all but the gain control is a little difficult to use, almost ON/OFF. (Yes, the pot taper is correct) Should I connect that resistor to VB?

Bonus guts shot: http://i.imgur.com/UZYpJQx.jpg

[antonis]

You should connect the R9 to Vb (Vcc/2) to have "equal" distortion (swinging between positive and negative signal half cycles)..

I'm not an expert but propably this is the cause of not properly working gain pot - the op amp is hitting the positive rail minus a couple of volts (it depends on the specific op amp) without any amplification so you have distortion only on the negative cycles (without necessary being bad..)

If you think that you need more gain after connecting the R9 to Vb, you may increase a little the R6 resistor (Q3 collector) and/or decrease R23 (Q3 emitter) or R26 (but this will result in altering the cut-off frequency..)

[Cotoletta]

You should connect the R9 to Vb (Vcc/2) to have "equal" distortion (swinging between positive and negative signal half cycles)..

I'm not an expert but propably this is the cause of not propelry working gain pot - the op amp is hitting the positive rail minus a couple of volts (it depends on the specific op amp) without any amplification so you have distortion only on the negative cycles (without necessary being bad..)

If you think that you need more gain after connecting the R9 to Vb, you may increase a little the R6 resistor (Q3 collector) and/or decrease R23 (Q3 emitter) or R26 (but this will result in altering the cut-off frequency..)

Thanks, Ill try these tweaks and let you know!

[antonis]

Sorry for not proposing you some indicative resistor values but I'm not good on calculating gain stages with resistor feedback, Miller compensation, Amp topology and Norton equivalents..

You may read the 2. Input Booster Stage here: http://www.electrosmash.com/big-muff-pi-analysis for a more detailed calculation way..

[duck_arse]

I'd say take the 68k to V/2 as well, but then I'd also say that it looks like a cap is missing between the transistor collector and the 68k. what happens to the transistor volts if you leave the 68k disconnected?

[antonis]

it looks like a cap is missing between the transistor collector and the 68k.

The Eye of the Duck...!!!! 

[Cotoletta]

I'd say take the 68k to V/2 as well, but then I'd also say that it looks like a cap is missing between the transistor collector and the 68k. what happens to the transistor volts if you leave the 68k disconnected?

Umh where is this missing cap exactly? I'll try to disconnect the resistor when I have the pedal back.

[antonis]

Between Q1's C (or R11, C10, R22) and R9's joint to U1A's 3 pin..

[duck_arse]

I'm not saying it is definately missing, but it looks to me to be not there. I couldn't find any original boss diagrams of this circuit, only redraws/reverses, but it seems an odd sort of configuration there. someone like prr or rg would be able to pick the eyes (and a possible vlaue) outta this.

[ashcat_lt]

With a cap before R9 the thing will definitely pull up to the positive rail going to VCC like that.  You would have to send R9 to VBias if the two stages were AC coupled. 

We always have to ask what you think you mean by gain.  Are you looking for more distortion, or more output volume?  First step for output volume would be to make R21 smaller, or the level pot bigger, or both.  As is, it drops 6db even at max.

[Cotoletta]

Hey thanks for the replies guys. I found something interesting about R9: in the original schematic (http://i.imgur.com/HgeTiH4.png) that resistor (called R25) is connected to VCC. At first I thought this was a typo, but further confirmation from some photos of the pcb I found (and edited): http://i.imgur.com/cZwrrJ3.jpg http://i.imgur.com/eZCzWwW.jpg Looks like the components references matches, so R25 definitely goes to VCC.

[DiscoVlad]

The Distortion (gain) pot on these is well known for not doing much.

From the original Boss drawing it looks like Q6 inverts the signal, which is then split off to the "gain" pot and Q7 which inverts the signal again.
The inverted and [now] non-inverted signal is then fed differentially into the op-amp, with the Gain/Dist. pot setting the amount of inverted vs non-inverted signal. I guess it was a good idea, but in practice the only difference in the sound is at the ends of the pot's rotation.

Like everyone else I'm not sure that 68k should be pulled up to VCC, I can check from both original Jap. and Taiwan HM-2 pedals to see if that is the case or not.
Oh it is pulled up to VCC? Crazy.

[duck_arse]

geeze, boss really went to town on this pedal. two "G" taper pots and a "D" (whatnow?) taper that's know to not work good, along with the bizarre, how-does-that-work, we'd-better-change-it biasing scheme.

I'd still be interested in the dc voltages around that stage, as is and when it made more sense. abd when the dist pot worked more gooderer.

[DiscoVlad]

On closer inspection: the non-inverting input of the op-amp is directly connected to the previous transistor, so that stage helps set the bias of the opamp.

It's probably worth noting that the when the HM-2 was replaced by the HM-3 they did away with that 2 transistors + op-amp gain setup in favour of a single opamp whose gain was set by altering the feedback resistor which is a much more conventional setup.

It's IC1a in the diagram: http://i.imgur.com/dlIsUiW.jpg

They also removed the series clipping diodes, used transistor gyrators in the filter section, and maybe tweaked some values but otherwise the circuit topology is essentially the same.

[DiscoVlad]

Just measured the voltages at IC1 of my Taiwan and Japan HM2s:

Power source: 12V exactly into the DC Jack (board hasn't been modified for the 9V PSA)
All knobs at the "Swedish chainsaw" setting i.e. All turned to max. Pedal is switched on with an empty 1/4" jack plugged into the input.

these are the voltages from the Taiwan, the Japanese is within 0.1V at the same pins.

IC1
Pins 1 and 2 are tied on the board: 8.34V
3: 4.67
4: 0
5: 7.62
6: 7.63
7: 7.67
8: 12 (this should be 9V according to the schematic.)

Pins 5, 6, and 7 are the ones connected to the transistor stage.
Pin 1 and 2 don't look right to me, unless they're getting biased by the gyrator stages or IC3 they should be around 4.5V like pin 3?

Something went wrong here, these readings are bad.

[DiscoVlad]

A little more work in spice: The Distortion pot doesn't set gain but it appears that gain is set by the fixed transistor stages and op-amp IC1b.
As before Q6 inverts the signal and amplifies it a little (but not enough to clip), which is then split off to the distortion pot and Q7 which inverts the signal again. In simulation, the signal at the Q7 collector/op-amp + input is quite substantially clipped. I'm not sure if this is due to the transistor, the Op-amp gain, or the clipping diodes in the feedback loop.

[duck_arse]

more voltages, please! how about the Q7 volts, and the "9V" volts? that pin 3 voltage is right for a 9V supply, but the pin 1 and 2 are wrong, they should be following pin3, as it is a voltage follower. and the R25//R27 form an ordinary looking voltage divider which would produce a dc bias for the opamp of less than V+/2, so either the transistor is doing some influencing, or there really is something fishy.

[DiscoVlad]

Right, trying again...

New...ish 9V battery.
Pedal switched on, input grounded. all knobs maxed.
Measured using a Fluke 117, not the cheap Jaycar meter.

Battery voltage measured between pin 2 (9V in), and 3 (0V) on the board: 8.27V

IC1 Voltages:
1: 4.14
2: 4.14
3: 4.11
4: 0
5: 3.63
6: 3.63
7: 3.62
8: 8.27

Q7 Voltages:
1 E: 7.58
2 C: 3.63
3 B: 6.98

[duck_arse]

so the fluke fixed a couple of odd points in the circuit. I might gat around to breadboarding this soon, the 68k is still troubling me.

a jaycar meter? are you another cobber?

[DiscoVlad]

I think the biasing for Q7 is also affecting this. a 68k + 10k voltage divider should only have 1.15V at the output on 9V if my maths is right.

so the fluke fixed a couple of odd points in the circuit. I might gat around to breadboarding this soon, the 68k is still troubling me.

a jay car meter? are you another cobber?

Worse, NZer.

The meter that gave the bad readings was one of those cheap small yellow DMMs kinda like this: http://www.multimeterwarehouse.com/dt830bf.htm