MN3205 Amplitude and Vgg

[boga]

Hi!

I am trying to build Arion SAD-1 clone which has basically the same schematic as Boss DM-3. After some struggling I put only the BBD part togehter on a breadboard.

I have done some measurements and I can see that from about Vin = 250mVrms the output signal of MN3205 does not increase any more and gets distorted (becomes a squarewave). Is this normal behavior?

I can see measurements that go up to -4dBm in the datasheet which corresponds to 489mVrms.

Another thing is that I connected Vdd to Vgg input of MN3205. After that I got much better results. Why is vgg = 14/15*Vdd?
Is this wrong to connect Vgg to Vdd or not?

I have added my measurement schematic and oscilloscope pictures.

Measurement schematic

Undistorted signal, Vgg=Vgg, Vin=300mVrms, Vout=224Vrms

Distorted signal, Vgg=Vgg, Vin=700mVrms, Vout=318mVrms

Undistorted signal, Vgg=Vdd=7.5V, Vin=729mVrms, Vout=841mVrms

Distorted signal,  Vgg=Vdd=7.5V, Vin=1.44Vrms, Vout=1.05Vrms

Hopefully someone can shed some light on this! 

[Mark Hammer]

The datasheet indicates an "input signal swing" of 0.36VRMS with 2.5% THD, so what you report sounds well within the realm of expectations.

[boga]

The datasheet indicates an "input signal swing" of 0.36VRMS with 2.5% THD, so what you report sounds well within the realm of expectations.

OK. Thank you for the quick answer. Good to hear! 

So how do you actually read this parameter from the datasheet?
What does it mean that the input swing of 0.36Vrms has THD of 2.5%?
You can somehow calculate the output signal swing from that?
Or how do you conclude that the output is within the given range?

And do you have any idea about the Vgg question?
Is it OK to connect Vdd to Vgg or not?

[Mark Hammer]

Really more a matter of interpreting what the English is intended to convey than a matter of technical knowledge.  In this case, the manufacturer states, about halfway down page 2 of the datasheet, that, if you feed in 360mv RMS, with a 5V supply and Vgg (the bias voltage) set at 14/15 Vdd, you can expect 2.5% distortion.  It also states that "typical" distortion is 0.8%, which one may presume is only attainable with a smaller input amplitude than 360mv.

I'll note that this is precisely why some folks' obsession with increasing the supply voltage of analog delays from 9 to 12 or 15v is misplaced enthusiasm.  The change in headroom may improve performance of the op-amps in the signal path, but they are generally not applying much gain, anyway.  Unity gain of a half-volt signal will be as clean with a 9v supply as with an 18v supply.

More critical, and the real fidelity bottleneck, is the limitations of the BBD, which can not be improved very much at all with changes to the supply voltage.  It is what it is.

[ElectricDruid]

The MN3205 datasheet gives 360mV as a *minimum* input voltage ("Input signal swing"), Mark. It's a pity that it doesn't commit itself to any maximum or typical levels. It's very little help, in short.

I'm surprised that the level is so low. Other chips in the series cope reasonably well with levels up to a volt or a volt and a half or so. It looks to me like you experiment with Vgg=Vgg wasn't working correctly for some reason. The second set of results are more what I'd expect to see, but I don't think that's *because* you connected Vgg to Vdd, rather that it's *despite* the fact you connected Vgg to Vdd.

If they chose an odd number like 14/15Vdd for Vgg, they had a reason, although I don't know what it is (BBD transistor design not being my strongest suit!!). My advice would be that unless you know why Vgg=Vdd is better, go with 14/15th.

HTH,
Tom

[Mark Hammer]

Yeah, it does say minimum.  Weird.  I checked a bunch of other Matsushita datasheets, because I thought it might be a typo, but they all say "minimum", albeit with a different (larger) maximum voltage swing.  The exception is the Radio Shack datasheet for the MN3002 which has the voltage swing listed as maximum (still at 2.5% THD).  So I'm not sure what to believe.  The one pattern that seems to hold is that the 32xx series has more headroom, and BBDs with fewer stages tend to have more headroom.

[Rob Strand]

Do you have pin 1 grounded?

Edit 1: 
Another thing I noticed is the Boss DM-3 has a diode from pin 3 of the MN3102 to ground.

Edit 2:
And the Arion has two diodes from pin 1 of the MN3102 to pin 5 of the MN3205.

Edit 3:
Ibanez  AD99 and Ibanez CD10 both have a diode from pin 3 of the MN3102 to ground; as per Boss DM-3

[Phoenix]

Where's your low-pass filtering on the output (and input for that matter)? Without that, the output from a BBD is essentially meaningless...
Looks like clock noise to me!

[ElectricDruid]

Phoenix is totally right. I'm sorry, I didn't look at the schematic. Without filtering, the output "level" won't really make any sense, since it'll be covered in clock spikes.

T.

[Fender3D]

Do you have pin 1 grounded?

Edit 1: 
Another thing I noticed is the Boss DM-3 has a diode from pin 3 of the MN3102 to ground.

Edit 2:
And the Arion has two diodes from pin 1 of the MN3102 to pin 5 of the MN3205.

Edit 3:
Ibanez  AD99 and Ibanez CD10 both have a diode from pin 3 of the MN3102 to ground; as per Boss DM-3

4:
TC Electronic SCF has Vgg set by clock frequency (rectified)

[boga]

Thank you very much guys for so many responses, ideas and comments! 

My last setup was crap as it was put together on a breadboard and after some time it was not working at all (must have been connection issue, but try to find the broken jumper wire or bad connection on a breadboard - like a needle in a haystack!). So I soldered the same schematic together on a prototyping board. Now it is at least working constantly and reliably.

From my quick experiments today the results seem to be very similar. But I will answer in more detail soon and also add new measurement results.

Do any of you have a working original or clone of Arion SAD-1 or DM-3 so you could take some measurements from an original pedal?
Or any schematic which uses MN3205?

In a meanwhile have a happy new year!   

[boga]

Hey, Guys!

Here's my update. Basically still struggling! The issue is still the output amplitude. I will explain below.

I put the same setup together on a prototyping board which I had on the breadboard. Now my setup looks like this instead of that mess! 

Did some measurements again and this time fed the signal coming out of the BBD through reconstruction filter also. The anti-aliasing filter doesn't make difference right now if I am using monotone signal which satisfies Shannon-Nyqvist criteria.

I have used 440Hz signal and oscilloscope. I also measured the voltages on Vgg (or pin 8 ) and voltage on Vdd pin (number 5). I have used 2 different chips - V3205 and MN3205 (actually I have more of each and they all act the same way).

V3205 Pin 8 or Vgg connected to pin 8 or Vgg of MN3102:
Vin = 273mVrms, Vout = 253mVrms, V3205, Pin 5 = Vdd = 7.6V, Pin 8 = Vgg = 2.96
Vin = 1.19Vrms, Vout = 269mVrms, V3205, Pin 5 = Vdd = 7.6V, Pin 8 = Vgg = 2.96

V3205 Pin 8 or Vgg connected to Vdd:
Vin = 312mVrms, Vout = 396mVrms, V3205, Pin 5 = Vdd = 7.6V, Pin 8 = Vgg = 7.6
Vin = 608mVrms, Vout = 777mVrms, V3205, Pin 5 = Vdd = 7.6V, Pin 8 = Vgg = 7.6
Vin = 1.19Vms, Vout = 1.0Vrms, V3205, Pin 5 = Vdd = 7.6V, Pin 8 = Vgg = 7.6

MN3205 Pin 8 or Vgg connected to pin 8 or Vgg of MN3102:
Vdd = 7.6, Vgg = Vgg, MN3205, Pin 5 = Vdd = 7.6V, Pin 8 = Vgg = 3.25

MN3205 Pin 8 or Vgg connected to Vdd:
Vin = 599mVrms, Vout = 777mVrms, MN3205, Pin 5 = Vdd = 7.6V, Pin 8 = Vgg = 3.25

So I have tried with 2 different chip types - V3205 and MN3205, they behave very similar. I have tried different MN3102 drivers which I have (all from the same patch). The issue and the key seems to be the Vgg voltage. If I mesure the Vgg coming out of the MN3102 then I get the correct voltage which is 7.06V or 14/15Vdd (Vdd = 7.60V). As soon as I connect the Vgg coming out of the MN3102 to V3205 or MN3205 pin 8 which is Vgg (input) I get much lower voltage. You can see from above that it is around ~3V. This must be the reason why the amplitude is limited and distorted!?

Why does the voltage drop so much? Could these MN3102 chips be faulty?

The problem if I connect the Vdd to Vgg pin is that the BBD does not start-up corractly after turning off. So I still have to connect it to MN3102 Vgg pin first and then I can use a jumper to connect the 3205 BBD vgg pin to vdd and then I get a nice amplitude.

I also tried to connect diode to MN3102 Vdd pin, basically connecting the supply through forward biased diode. This also didn't make any difference.

This is becoming a rocket science and I am going crazy!!!  This shouldn't be so hard. What the hell could be the problem?

[ElectricDruid]

What happens if you "manually" set Vgg to 14/15Vdd with a potential divider?

I've never come across the Vgg from the MN3102 getting dragged down so much, but at the same time, I never had cause to put a voltmeter on it while it was running.

Tom

[boga]

What happens if you "manually" set Vgg to 14/15Vdd with a potential divider?

Tom

Yep! That was going to be my next step anyway... so we are thinking in the same direction.

I also received my Behringer VD400 pedal (which should have quite the same schematic) and I am going to measure what are the voltages over there.

[Yazoo]

I have just built a version of the Arion SAD-1. It works but needs tweaking. The Repeats pot is only effective for about 30%. If I turn it up any higher the effect gets completely out of control. I also had trouble setting the bias trimpot. I am going to replace this with a multi-turn trimpot for better control.

Why are there 2 diodes connected to pin 1 of the MN3102? I realise they drop the voltage down from a standard 9 volt supply by around 1.4 volts. The datasheet for the MN3102 states that the most suitable VGG supplied to the MN3205 will be given when both the MN3205 and the MN3102 share the same power voltage. On my build, VGG is 7.35 volts, way lower than the recommended 14/15 of the datasheet.