Buffer of mxr Zackwylde

[preciousmolina666]

1.) im not familiar with this style of buffer. it use many components instead of resistor to 4.5+ since 4+5 is available. how does it do compare to the common tubescreamer buffer?
2.) r14 is 22k this is usually 10k,why 22k,i tried to change it 500k and i got more Gain/SNR? whats the drawback

My chain: Humbucker-> buffer-> audio interface

[marcelomd]

Hi,
1- The transistor is biased wit the 1M8+100k to VCC and the 2M2 to ground, which is a bit over 4.5V

[Fancy Lime]

Hi,

this is indeed a strange way to bias a buffer if there is already a 4.5V point in the system. But even apart from that, I cannot see what purpose the buffer serves anyway in this schematic. If the fork between effect and bypass was behind the buffer, sure. But it's before the buffer, so the buffer has no effect in bypass mode. And with the effect switched on, the only thing the buffer does is lowering (!) the input impedance compared to a properly designed non-inverting opamp stage. Very mysterious design choices...

Andy

[POTL]

Hi,

this is indeed a strange way to bias a buffer if there is already a 4.5V point in the system. But even apart from that, I cannot see what purpose the buffer serves anyway in this schematic. If the fork between effect and bypass was behind the buffer, sure. But it's before the buffer, so the buffer has no effect in bypass mode. And with the effect switched on, the only thing the buffer does is lowering (!) the input impedance compared to a properly designed non-inverting opamp stage. Very mysterious design choices...

Andy

Usually the idea is that the buffer organizes a high input impedance of the circuit; this allows reducing the bias resistor of the input of a non-inerting amplifier and in theory reducing noise.

[MrStab]

my assumption was that using two large resistors to bias like this usually doesn't allow for a stabilising cap, as it would be in the high-impedance signal path, but placing it after the 100k resistor allows for such a cap by isolating it somewhat from the input.

[Fancy Lime]

Hi,

this is indeed a strange way to bias a buffer if there is already a 4.5V point in the system. But even apart from that, I cannot see what purpose the buffer serves anyway in this schematic. If the fork between effect and bypass was behind the buffer, sure. But it's before the buffer, so the buffer has no effect in bypass mode. And with the effect switched on, the only thing the buffer does is lowering (!) the input impedance compared to a properly designed non-inverting opamp stage. Very mysterious design choices...

Andy

Usually the idea is that the buffer organizes a high input impedance of the circuit; this allows reducing the bias resistor of the input of a non-inerting amplifier and in theory reducing noise.

Usually, yes. But in this case, that is not what the buffer does. In fact, if you ditch the buffer and change R7 to 1M or higher, you get a higher input impedance. And that resistor hardly contributes to noise at all and even a 10M here would be completely eclipsed by the (horrible in terms of noise) 1M gain pot. The buffer itself contributes more noise than the reduction of R7 saves. So, I really don't get it.

my assumption was that using two large resistors to bias like this usually doesn't allow for a stabilising cap, as it would be in the high-impedance signal path, but placing it after the 100k resistor allows for such a cap by isolating it somewhat from the input.

But the 4.5V biasing network R8-R9 has a stabilization cap as well: C7. Now if any of the parts in this circuit were to draw a lot of bias current, then it would make sense to have two, but that is not the case here. So...?

Andy

[antonis]

Totally agree with Andy..

The whole circuitry around Q1 could be omited and raise R7 value to 1M..
(C6 could stay to 18nF in case of many stock caps..)

[MrStab]

took me a while to spot where C7 was, i didn't even realise there was another divider! so there appears to be two bias sources, then - wtf?!

would the common-collector benefit from its own biasing network like this in some way, seeing how the R8/R9 bias only seems to feed the op-amp stages? maybe the designer was really anal about having 1M9/2M2 for a precise voltage to compensate for the NPN's diode drop, used 1M8 and 100K to achieve this, and thought they'd just stick a cap in there cos why not?

you wouldn't have found Randy or Jake doing any of this nonsense.

[Elektrojänis]

Adiing to all other strange things, there is not much point using a transistor to control tha status LED... Or can someone explain why someone would want to do it like that.

I wonder if that schematic is correct at all...

[MrStab]

other diagrams on Google Images seem to be the same, except this one:

https://codesafe.tistory.com/m/325?category=166076

...which seems even more wrong, lol. but maybe not. maybe it's not meant to just be a buffer stage.

just noticed it's using MPSA14. dunno how a Darlington would necessitate this strangeness, though.

[patrick398]

Adiing to all other strange things, there is not much point using a transistor to control tha status LED... Or can someone explain why someone would want to do it like that.

I wonder if that schematic is correct at all...

So you can use a DPDT switch instead of 3PDT no?

[Fancy Lime]

Adiing to all other strange things, there is not much point using a transistor to control tha status LED... Or can someone explain why someone would want to do it like that.

I wonder if that schematic is correct at all...

So you can use a DPDT switch instead of 3PDT no?

You could do that just as well without the transistor.




Another possible reason for all this wierdness is just simple bad design. Never assume that big companies that make these things for profit never screw up. In fact most companies that have made a specific kind of product forever get things wrong on a regular basis that should have been sorted out decades ago. Anyone had their car recalled because the manufacturer suddenly forgot how to design brakes properly for that one model despite having designed brakes forever and without problems? It happens with all sorts of thing, no reason to think MXR would be immune.

Andy

[PRR]

...Never assume that big companies that make these things for profit never screw up. ... Anyone had their car recalled because the manufacturer suddenly forgot how to design brakes properly for that one model despite having designed brakes forever and without problems?...

My Honda is now on its *THIRD* recall for air-bags that kill people. "For profit" they bought bags from Tanaka, who was apparently careless for decades. Honda will give me yet another airbag, but I'm tired of driving an hour to the dealer every few years for the same issue.

Compared to that: this plan may use a few more parts than essential, but they are *penny* parts (and nobody dies). If MXR goes broke, a few penny-parts extra won't be the big reason.

[MrStab]

Another possible reason for all this wierdness is just simple bad design.

yknow what i keep thinking? that the input stage is actually from a different circuit altogether, and it's been tacked onto the front of an op-amp clipping stage as though someone really insisted it remain intact. it looks like 2 completely different designs stuck to each other with zero streamlining. that would explain the two bias dividers, but not R13/C8.

i can't help but wonder about the accuracy of the diagram in the PDF i posted - it does seem more complete than the OP, with an output buffer etc., but 100k/2M2 biasing the signal then a 1M8 in series before the Base makes no sense to me either. they've gone to the effort of making a PCB layout, though.

this is interesting, i like this. i hope it isn't a complete screwup from the designer, but it likely is at this point!

[bool]

I don't think theres is any screwup there - they most likely went to "marry the subckt x with the subckt y and see whichever interacts in the best way".

If you looked f.e. at P.Cornish schems theres a lot of similar "glueing" of subckts going on...

[Elektrojänis]

it looks like 2 completely different designs stuck to each other with zero streamlining. that would explain the two bias dividers, but not R13/C8.

That could well be it... And it could even be the reason for the odd status led arrangement.

Maybe it got partially copied from something with true bypass with led on a DPDT... Somehow they didn't quite get it right so and they ended up simplifying it. It's not true bypass anymore and not even buffered bypass but it works... Send it to the factory and ask them to make 10000.

They could have simplified it more by throwing out the transistor and a few resistors, but those really cost only pennies.