Baracuda Build (silverdog kit Rev2)

[Qiaozhi]

Haven't read every post so my have missed something but my offset cermet is 25K

The PCB shows the value as 25k, but the original schematic shows 100k. Impulses's board has a 20k installed.

Possibly connected to pin 7 ? and +-5V and a 270K resistor on pin one

That's the way the PCB is wired. It works, but it's not the recommended method to use.
I intend to use the offset null method shown in the datasheet.

@Q, It looks perfect... 40106 and 5534 look perfect and the power supply makes sense at last...

@Koala, the schematic connects the 5534 as per the datasheet... all other schematics deviated for some unknown reason...

I'll breadboard it tomorrow and see if I can spot any issues... hope I can find enough empty breadboard ... looking forward to it...
Mike

I'll wait until you've confirmed the changes before starting the PCB layout. My intention is to keep component placement similar to the original. Otherwise, if we make too many changes, it will cease to a Baracuda.

If you look at the data sheet r21 should go to pin 7 and the voltage on pin 7 (which should be/is +5v) used for off set, not batt gnd

http://circuits.datasheetdir.com/25/...4-circuits.jpg

Well spotted. That was my mistake, and the 22k resistor should be connected to +5V.

Q won't the 7660 still be in trouble with 12 volts applied? Maybe just specify the 7660A or a 7662 and/or add a diode in series with the supply.

The schematic is just showing a generic 7660, and the parts list will specify a 7660A. I'll change the component name to 7660A on the schematic to avoid confusion, unless we add FatBob's modification below.

On My Bara I knew the 12V was going to be a problem so I added a LDO set to 9V.

3 Lipo's input will range from 12V - 9V roughly and the LDO works with 300 mv differance.
I have a lot of MAX667's so I used that for mine.

At the same time I increased the supplies to +-9V for a little more headroom. I haven't tried this
yet as I'm finishing up my Surf just now.

[ATTACH]35886[/ATTACH]

This might be a good modification to add to the Baracuda, as it would allow any version of the 7660 to be used. It looks like the original design is intended to use a battery pack with a nominal 9V (i.e. 6x 1.5V AA cells), but many constructors see the maximum voltage listed as 12V, and then connect a 12V battery pack, which eventually damages the 7660A,

A corrected schematic will be uploaded soon.

[Koala]

Wasn't the TL1054 a simple option? With a max of 16V.

[Qiaozhi]

Wasn't the TL1054 a simple option? With a max of 16V.

The LT1054 is also a possibility, but I'm not sure whether it will synchronize correctly with the current arrangement.

OK - I'll borrow one from my MPP and see what happens ...

[Qiaozhi]

Here's an updated schematic. -> Geotech Baracuda - Schematic.pdf
As before, please see if you can spot any mistakes.

The offset null adjustment on the NE5534 is the same as specified in the datasheet, and the 22k resistor is now correctly connected to +5V.
I have replaced the 7660 with an LT1054, which is a drop-in replacement with a maximum voltage input of 15V. After inserting the LT1054, I increased the supply voltage to 15V without any issues, and (more importantly) no release of the magic smoke. The 5V rails started to drop out with 7V at the input.

So far, the updated design appears to be quite stable.

[raygdunn]

I have replaced the 7660 with an LT1054, which is a drop-in replacement with a maximum voltage input of 15V.

Hi Q and Coala,

The LT1054 is great news! I've tried five ICL7660S and all reach finger burning temperature after 1-3 minutes on 12V supply! (Stable at 11ma input to 10V then up to ~90ma by 12V, makes them feel like they were really a 10V version)

Mike also pointed out that my envisaged 11.8V max. from a 3 cell LiPo is low. The fully charged voltage is actually 12.6V.

This sounds like an excellent solution for the higher voltages.

Ray

[Old cart]

Here's an updated schematic. -> [ATTACH]35888[/ATTACH]
As before, please see if you can spot any mistakes.

The offset null adjustment on the NE5534 is the same as specified in the datasheet, and the 22k resistor is now correctly connected to +5V.
I have replaced the 7660 with an LT1054, which is a drop-in replacement with a maximum voltage input of 15V. After inserting the LT1054, I increased the supply voltage to 15V without any issues, and (more importantly) no release of the magic smoke. The 5V rails started to drop out with 7V at the input.

So far, the updated design appears to be quite stable.

This is all very good news. The questions being now, is this really the "patched" Baracuda or the new Geotech design? I think it is important that we clearly differentiate which is which lest we introduce more confusion.

[Qiaozhi]

This is all very good news. The questions being now, is this really the "patched" Baracuda or the new Geotech design? I think it is important that we clearly differentiate which is which lest we introduce more confusion.

It's both. The Geotech Baracuda REV-A (when it's finally released as a PCB) will be a corrected and slightly modified version of the original Baracuda. If anyone wants to patch their original Baracuda, I can post a list of changes required. These changes essentially consist of connecting the pull-up resistors to 0v instead of +5V, and some modifications to the power supply smoothing capacitor connections, plus one additional capacitor and a value change. The offset null on the NE5534 can stay as it is. A final [optional] update is to unplug the 7660S, and plug in an LT1054.

I'll create a change document later.

[eclipse]

C25, C26 are not needed. Will there be a non-motion switch, the original schematic has it.

[Old cart]

Comments on new supply design

On My Bara I knew the 12V was going to be a problem so I added a LDO set to 9V.

3 Lipo's input will range from 12V - 9V roughly and the LDO works with 300 mv differance.
I have a lot of MAX667's so I used that for mine.

At the same time I increased the supplies to +-9V for a little more headroom. I haven't tried this
yet as I'm finishing up my Surf just now.

[ATTACH]35886[/ATTACH]

I have long thought this is the best way to go. Still it has the problem that the negative supply is not really regulated. It can vary a lot with changing loads due to the high 50-100 ohm output impedance of the charge pump.

I would think the best way to go on the new design is:

1. 12 volt battery- powers only the transmitter. Assuming 8 cells and a low voltage limit of 1.1v / cell this could vary from 12 to 8.8 volts.
2. 8 volt LDO pre-regulator powers the charge pump and the plus 5 regulator.
3. The charge pump also drives a 5 volt minus regulator. This reduces the effective output impedance of the charge pump.
4. Optionally a separately plus 5 regulator is used for the timing circuitry to isolate the digital noise. Alternately, additional filtering could be applied to the timing supply circuit.

I know this is bunch of regulators! They don't cost much other than board space and a slight additional quiescent load though.

I also think higher supply voltages for the first stage would be good idea to extend dynamic range and would be interested to hear what Fat Bob's experience is if he tries this.

The goal of all this is to take the simple PI as far as it can go in terms of raw performance.

[Qiaozhi]

C25, C26 are not needed. Will there be a non-motion switch, the original schematic has it.

Another eagle-eyed Geotech member!
The reason that C25 and C26 are not required is because the LF412A is a dual opamp.
There's some good teamwork going on here.

[raygdunn]

Re: New schema

Hi Q,

The existing board had a 4001 diode across the battery connections. I guess this is intended to help save the circuit on a reverse battery connection. I've seen it on another schema. How useful is it?

The threshold control is now 2M2 + 50K variable + 27K. It was 39K + 20K + 27K. So different, I'm wondering how it works out.

Ray

[Michaelo]

I know the question was addressed to Q but I had to comment...

Protection by means of a parallel diode should only be used if accompanied by a in-line fuse... (if the power is connect in reverse, the diode conducts thereby blowing the fuse). If no fuse is present, at best the diode will handle the current for a short time but the supply may be damaged, at worst the diode will blow and damage to the circuit will follow...

A series diode is a far better solution but of course you looses .6 of a volt and that's not always ideal, a better solution might be a polyfuse (it can take several hits before requiring replacement). On the other hand, if the power is connected via a one-way connector, you shouldn't need any protection but I would still add some

Speaking of connectors... on the redesign of the bara, can we use a connector for the power, audio and the external pot or at least make the holes larger... Also, how about two medium sized 4700uF caps in place of the huge 6800uF one...

Just noticed the cap is now 4700uF, I still would like two of them

Mike

[raygdunn]

I know the question was addressed to Q but I had to comment...
Protection by means of a parallel diode should only be used if accompanied by a in-line fuse... (if the power is connect in reverse, the diode conducts thereby blowing the fuse). If no fuse is present, at best the diode will handle the current for a short time but the supply may be damaged, at worst the diode will blow and damage to the circuit will follow...

Hi Mike

Good point. I still haven't sorted out a fuse on mine. What I've found in my previous build is that any low value fuse blows, with the initial surge, presumably to fill the capacitors. My PI-1 has a 1A. I blew a load of fuses in my meter before relying more on a cheap bench supply with current reading and limiter.

A series diode is a far better solution but of course you looses .6 of a volt and that's not always ideal, a better solution might be a polyfuse (it can take several hits before requiring replacement). On the other hand, if the power is connected via a one-way connector, you shouldn't need any protection but I would still add some

OK so I'm way out of date on fuses for most purposes. I'm still thinking of a old wire fuse in the power wiring. Just searched on "polyfuses" and found some nice PCB components.☺ I think this problem goes beyond one way connectors, if one is targeting the project to include beginners. I blew a couple of 555s when I first started, have been working with cars and positive earth in recent years. The other aspect is limiting damage, when a component is inserted the wrong way round, including the problem of inconsistent pinouts (e.g. the 2N2222).

It feels like something that could usefully be designed in, for my amateur viewpoint.

Speaking of connectors... on the redesign of the bara, can we use a connector for the power, audio and the external pot or at least make the holes larger... Also, how about two medium sized 4700uF caps in place of the huge 6800uF one.

Just noticed the cap is now 4700uF, I still would like two of them

Mike

The current PCB has only 3300uF. Reduces the size problem, but is that enough uFs?

Ray

[Michaelo]

Hi Mike

Good point. I still haven't sorted out a fuse on mine. What I've found in my previous build is that any low value fuse blows, with the initial surge, presumably to fill the capacitors. My PI-1 has a 1A. I blew a load of fuses in my meter before relying more on a cheap bench supply with current reading and limiter.

OK so I'm way out of date on fuses for most purposes. I'm still thinking of a old wire fuse in the power wiring. Just searched on "polyfuses" and found some nice PCB components.☺ I think this problem goes beyond one way connectors, if one is targeting the project to include beginners. I blew a couple of 555s when I first started, have been working with cars and positive earth in recent years. The other aspect is limiting damage, when a component is inserted the wrong way round, including the problem of inconsistent pinouts (e.g. the 2N2222).

It feels like something that could usefully be designed in, for my amateur viewpoint.

The current PCB has only 3300uF. Reduces the size problem, but is that enough uFs?

Ray

I think my first transistor was an AC127 (looked like a tiny valve but made from plastic) ... it only took me 35 seconds to kill it

Some years back I came across a similar problem, cap charging blowing the fuse every time you turned it on...

The answer in those days was to include a soft-start... which meant adding a resistor to slow charge the cap and then clicking in a relay to short out the resistor after a few seconds... I think the solutions to everything in those days was to add a relay

Mike

[Michaelo]

Rev2 to Rev3

As the availability of a Rev 3 board is probably some time off, I thought it might be worth while doing a Rev 2 to Rev 3 upgrade...
With the exception of the changes to the main amp circuitry (which has no issues in Rev 2 but might improve in the new configuration), the rest of the fixes should be very easy to implement...

As I have already made so many changes to my current baracuda (not related to this post), I will need to purchase another bara and perform the upgrade on that.
This should allow testing of the new design and also allow me to provide the upgrade procedure for anyone who wishes to upgrade their existing Rev 2 boards...

Also, being able to upgrade from Rev 2 to Rev 3 should allow for current stock to be sold (basically negate the negative effect afforded by a new design) otherwise, outstanding tooling costs for Rev 1, would have top passed to the new board making it more expensive...

Mike