MFZ-1 to Hidrosis > brett or RG - explanation of technical change reasons?

ulysses · January 16, 2009, 07:29:02 AM

hey brett/RG,

i was wondering if you either of you could please give a technical explanation on the reasons behind the changes made to the MFZ-1 to Hidrosis.

there appear to be multiple changes..

such as the feedback resistor on the first opamp changed from 1M to 10K after removing the 1M input resistor (why would they design the MFZ-1 like that - seems like a recipe for a whole lotta noise) / the input now going to pin 3 of the first opamp instead of pin 2 / additional resistor and cap to ground on the feedback loop of the first opamp / voltage divider resistors changed from 2.2m to 10k / 120pf in the feedback loop of opamp 2 instead of 100pf / volume pot changed to 100k from 25k

i would be interested to know the technical reasons behind the changes - if youve got time

what i would really like to know, is why the tone changes when you increase the gain - it gets a lot trebblier when you turn the gain up - is that soemthing that can be stopped? is it possible the 33k and 2.2uf attached to the gain pot are acting as a tone control also? if we removed the 2.2uf to ground on the gain control, would that prevent the tone changes?

thanks

ulysses

yeeshkul · January 16, 2009, 01:56:09 PM

+1. I am about to build one and can't decide which one. It would be wonderful if you guys could help.

Mich P · January 16, 2009, 02:37:35 PM

Here RG comments
http://www.diystompboxes.com/smfforum/index.php?topic=50449.msg377284#msg377284
Mich P.

ulysses · January 16, 2009, 04:16:11 PM

thanks for that link

if either brett or rg has time i would be interested in the technical reasonings behind the following changes:

why is the circuit better if the first opamp is non-inverting?

why add an additional resistor and cap to ground on the feedback loop of the first opamp?

why is there a low passive low pass filter between the opamps? could you not get rid of the 2.2uf cap to ground? what is the 2.2uf's purpose if not to be part of a passive low pass filter?

cheers
ulysses

oskar · January 16, 2009, 04:53:05 PM

Ulysses, so many good questions... I can't resist to try and answer them. RG and Brett can come in and correct me later...

Ok. I link to the Hidrosis thread and schematic.
http://www.diystompboxes.com/smfforum/index.php?topic=50984.0

Quotewhy is the circuit better if the first opamp is non-inverting?

The first OP-amp of the MFZ-1 is set up as an x-1 amp. It buffers the sound but doesn't amplify the signal. In order not to give the circuit too low Z, which would shunt off high frequencies... they have big value (1M ) resistors.
They could very well have used lower values here... high value resistances is linked with thermic noise/white noise.
If you want to give your circuit a high impedance do it like Brett. ( compare it with a MXR D+ )
This is a strange way to do it especially since they've proven on the next OP-stage that they actually now what a noninverting amp is!?

Quotewhy add an additional resistor and cap to ground on the feedback loop of the first opamp?

It is given a 4.5V DC reference on the noninverting input and that is what you get on the output of OP1... why?
Because of the 2u2 cap to ground after the 2k2 resistor. The DC current will see a buffer instead and will not be amplified but the AC ( what you call sound and your neighbours call NOISE ) sees a x5 Amplifier.

Quotewhy is there a low passive low pass filter between the opamps? could you not get rid of the 2.2uf cap to ground? what is the 2.2uf's purpose if not to be part of a passive low pass filter?

Volume pot... The next cap to ground is not to loose the 4.5V DC lift we created in the first stage. DC doesn't see nothing but the next op input and the AC is shunted more or less depending on how much pain you want to inflict upon the next op-stage ( neighbours )

ulysses · January 19, 2009, 12:07:33 AM

thanks for that reply oskar

im not sure i understand fully - will have to reread your post multiple times

thanks
ulysses

brett · January 19, 2009, 01:25:29 AM

Hi
oskar is a clever fellow and has given good answers. I'll give similar answers, but in a less technical way.

Quotewhy is the circuit better if the first opamp is non-inverting?

Because the first op-amp uses a large resistor (1M) and has no gain, it is noisey. The biasing of the non-inverter in the Hidrosis is the popular low-noise technique known as "noiseless" biasing. This op-amp also provides some extra gain (almost 5x), so the Hydrosis can be a bit nastier than the MFZ-1. When cranked, you can get lots of feedback.

Quotewhy add an additional resistor and cap to ground on the feedback loop of the first opamp?

Because that's the usual way you set up a non-inverting op-amp. The ratio of the two resistors (10k/2.2k) sets the gain (and the cap blocks DC losses that would upset the bias).

Quotewhy is there a passive low pass filter between the opamps? could you not get rid of the 2.2uf cap to ground? what is the 2.2uf's purpose if not to be part of a passive low pass filter?

As oskar said, the cap allows DC connection between the op-amps. As fas as the filter is concerned, if I recall correctly, there's a similar filter on the MFZ-1, so I kept one on the Hydrosis. The idea is to roll off highs as the "drive" is decreased, making a "fatter" sound. If you want to eliminate this effect, try a smaller value cap (0.47 uF) or a 25k pot.

In my opinion, the Hidrosis is a superior circuit. That's got a lot to do with the lower noise.
I use the Hidrosis a lot, and really like it. But it could be improved.
If I were re-designing it, I would put in a mild mid hump (say, 3 to 6 dB). Sometimes it sounds a bit "flat" and lifeless compared to a mid-hump circuit like a Tubescreamer.

cheers

oskar · January 21, 2009, 05:18:50 PM

Brett!

Could you please explain the 33k on the first stage output. At first I didn't even notice it and when I did it made me rather puzzled?

-thanks

oskar

brett · January 21, 2009, 08:01:23 PM

Hi
the 33k resistor is there to ensure a volume cut out of the first op amp (by voltage division between it and the 10k pot). It also forms part of the filter.
If you want to replace the 33k resistor and 10k pot with a 50k log pot, that would work OK, too. But I'll bet it's excessively wild and crazy (and probably over-filtered) when turned up.
have fun

yeeshkul · February 07, 2009, 05:35:53 AM

Brett i was thinking about putting a 10k trim instead of the 10k feetback res to possibly lower the buzzzzz. This guy wants me to make a copy of MFZ and i suggested that your Hidrosis is really a better idea. Does Hidrosis keep the MFZ sound character? Would you recommend keeping the 33k or skipping it + 50k drive pot in case of the trim pot in the feetback?

Mich P · February 07, 2009, 08:42:11 AM

Hi Brett I have build the hydrosis and i 'm happy with.
Just one thing annoying me, it's the range feel of the
Gain pot wich is max at half of the rotation
I 've try lin, log and reverse log but without sucess
How i can smooth the range feel of it ?
Thank you,
Mich P.

yeeshkul · February 08, 2009, 01:57:45 AM

if the gain is full blast at the half of the pot then check out let's say 25-50k log pot to move the hard action towards the end ... maybe?

Mich P · February 08, 2009, 05:52:23 AM

Thanks yeeshkul that an idea
gonna try it !
Mich P.

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MFZ-1 to Hidrosis > brett or RG - explanation of technical change reasons?