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General Discussions => Drawing Board => Topic started by: Samuel Groner on October 22, 2005, 07:17:55 AM

Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on October 22, 2005, 07:17:55 AM
Hi

Just successfully finished a slightly simplified version on breadboard: [removed]

My design criterions were:
* good audio performance
* low noise, OSI around 1k ohm (for 1:2 mic input transformers)
* class A output into 600 ohm up to clipping
* stable at noise gains of 2 or lower
* offset preferably below 20 mV
* reasonably high CMRR and PSRR
* running on +/- 18 V
* input- and output-protected
* easy to squeeze into a 2520 footprint
* easy to build (no matching and selecting)
* cheap and easy to source parts

Q1/Q2 could be replaced with 2N4403s if you don't mind higher input bias current and a tad more noise.

As shown, this opamp could rival the noise performance of the 990! Output swing is close to 34 Vpp.

Detailed report and Gerber files to come (you might need to give me a few month though, as I have to write my master thesis first...).

[Edit latest documentation: SGA-SOA-1_documentation.pdf (http://www.sg-acoustics.ch/analogue_audio/discrete_opamps/pdf/SGA-SOA-1_documentation.pdf). And pictures: SGA-SOA-1.html (http://www.sg-acoustics.ch/analogue_audio/gallery/SGA-SOA-1.html).]

Samuel
Title: Preview On New Discrete OpAmp
Post by: Bauman on October 22, 2005, 08:17:49 AM
Hey thanks for sharing!!! I love new opamps  :twisted:
Title: Preview On New Discrete OpAmp
Post by: radiance on October 22, 2005, 08:36:06 AM
Yeah, Thanks man! :thumb:
Title: Preview On New Discrete OpAmp
Post by: Dan Kennedy on October 22, 2005, 11:59:42 PM
Try the FPN560/FPN660 available from Mouser for the outputs.

A bit faster than the 170/180 s, but still enough for a lot of loads.

Or, try multiple paralled 4403, 4401's, with some ballasting resistors in the bases.

Not huge gains, in any of the above, but some.
Title: Preview On New Discrete OpAmp
Post by: Family Hoof on October 23, 2005, 12:55:25 AM
Congratulations, Samuel! I have greatly enjoyed reading all of your amplification threads and now I see where all that information has gone. Looks like a very tidy design to me. So you're saying that none of the components need to be matched, not even the diff pair? Care to explain why? Thanks so much for sharing! I will definitely order or build one when you get the 2520 footprint worked out.

-Jens
Title: Preview On New Discrete OpAmp
Post by: bcarso on October 23, 2005, 01:25:00 AM
Quote from: "Dan Kennedy"
Try the FPN560/FPN660 available from Mouser for the outputs.

A bit faster than the 170/180 s, but still enough for a lot of loads.

Or, try multiple paralled 4403, 4401's, with some ballasting resistors in the bases.

Not huge gains, in any of the above, but some.


Those look like some interesting parts Dan---thanks for bringing them to our attention.  Pretty high min. beta (250 @ 500mA) and it's pretty flat with Ic.  The tall TO92 style package is nice for dissipation too.
Title: Preview On New Discrete OpAmp
Post by: bcarso on October 23, 2005, 01:26:03 AM
Quote from: "Family Hoof"
Congratulations, Samuel! I have greatly enjoyed reading all of your amplification threads and now I see where all that information has gone. Looks like a very tidy design to me. So you're saying that none of the components need to be matched, not even the diff pair? Care to explain why? Thanks so much for sharing! I will definitely order or build one when you get the 2520 footprint worked out.

-Jens


I wouldn't count on low offset Jens, but for most applications who cares.
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on October 23, 2005, 06:22:56 AM
Thanks for your interest!

Quote
Try the FPN560/FPN660 available from Mouser for the outputs.

Yes, these are interesting parts. I wanted to order a few 2SA1680/2SC4408 (basically the same, probably slightly faster, but lower beta), but never did. For this design, I opted for the TO-126 parts because I wanted the output to be short-circuit proof. The drivers may dissipate several watts in this condition, and that's too much for a "modified TO-92". If you don't need that, your suggestion may allow us to make C2 slightly smaller and thus improofing slew-rate and linearity.

Quote
So you're saying that none of the components need to be matched, not even the diff pair? Care to explain why?

I think we should put it the other way round: I was looking for a design that works well even if we don't match parts.

For a differential pair to be well balanced, we need (at least) two conditions: the two transistors should be matched (i.e. for beta, Vbe, stray capacity...) and their operating conditions (i.e. collector current and voltage etc.) need to be equal.

For audio use, the most important condition is equal collector currents, as this sets a lower limit to the linearity at high frequencies (unequal currents reduce the ability to cancel even-order distortions). In this design, the value of R3 is crucial for this (thus the silly SPICE-predicted 1% 1k62). The first prototype shows about 9% current imbalance (with a 1k6 instead of 1k62 and running on +/- 15 V though - so this might improve), which is enough to dominate distortion performance at high frequencies. The resulting distortion is mainly 2nd harmonic, so sonically probably OK.

A current mirror would reduce this imbalance dramatically, but it needs another three parts, which I didn't wanted here.

This morning I had an idea how to slightly modify this design for better current balance; set R3=1k8 and parallel it with a selected resistor. This resistor is selected by measuring the voltage drop across R3 and R5 and throwing these two voltages into your calculator (I have to think about the formula, but it's rather easy).

Matching Q1/Q2 with regard to Vbe would improve offset a bit, but it remains too bad for direct DC coupling (especially when considering the significant drift), so this doesn't help much.

Matching beta does reduce offset current, but this is helpful with equal DC resistance only - for audio, we mostly have different conditions.

I hope you understand my reasoning - it's not easy to put all these considerations into words!

Samuel
Title: Preview On New Discrete OpAmp
Post by: cuelist on October 23, 2005, 07:44:41 AM
Quote from: "Samuel Groner"
I hope you understand my reasoning - it's not easy to put all these considerations into words! Samuel


Samuel,

I think you have come up with a very nice circuit. No too simple, not too complicated. As you know, engineering is all about compromises - making the right ones. :wink:

I look forward to building it myself, maybe when I get some quality DIY time (around christmas they way things look now... phew!).
Title: Preview On New Discrete OpAmp
Post by: bcarso on October 23, 2005, 12:33:31 PM
Quote from: "Samuel Groner"
For a differential pair to be well balanced, we need (at least) two conditions: the two transistors should be matched (i.e. for beta, Vbe, stray capacity...) and their operating conditions (i.e. collector current and voltage etc.) need to be equal.

For audio use, the most important condition is equal collector currents, as this sets a lower limit to the linearity at high frequencies (unequal currents reduce the ability to cancel even-order distortions). In this design, the value of R3 is crucial for this (thus the silly SPICE-predicted 1% 1k62). The first prototype shows about 9% current imbalance (with a 1k6 instead of 1k62 and running on +/- 15 V though - so this might improve), which is enough to dominate distortion performance at high frequencies. The resulting distortion is mainly 2nd harmonic, so sonically probably OK.

A current mirror would reduce this imbalance dramatically, but it needs another three parts, which I didn't wanted here.

This morning I had an idea how to slightly modify this design for better current balance; set R3=1k8 and parallel it with a selected resistor. This resistor is selected by measuring the voltage drop across R3 and R5 and throwing these two voltages into your calculator (I have to think about the formula, but it's rather easy).

Matching Q1/Q2 with regard to Vbe would improve offset a bit, but it remains too bad for direct DC coupling (especially when considering the significant drift), so this doesn't help much.

Samuel


Be aware though that collector current matching, via selection of R3, is a slippery slope strategy due to the temp coefficient of Q4 Vbe.  The negative tempco of the Q5 collector current working with the effect of R6 does alleviate this a bit.

Again, for most audio apps, who cares about d.c. offset.  The exceptions occur when one is hell-bent on eliminating coupling and d.c. blocking caps.
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on October 23, 2005, 12:44:54 PM
Yes, things drift around a good deal. By biasing D3/D4 with very little current it is possible to almost cancel the drift over a pretty wide temperature range (in simulation at least), but I felt better with more electrons through R6.

The idea behind selecting R3 is basically to reduce HF distortion variation between different units, it doesn't change anything about drift performance, of course.

Samuel
Title: Preview On New Discrete OpAmp
Post by: cuelist on October 23, 2005, 03:49:47 PM
Samuel,

Why doesn't R11 have a companion on the +V side?
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on October 24, 2005, 01:54:59 AM
The (simulated) positive PSRR of this design is about 20 dB at very high frequencies (> 1 MHz), whereas the negative goes towards 0 dB (without R11/C5). Such low PSRR would open a door for negative-supply related stability issues. R11/C5 corrects this to 20 dB as well.

A similar RC filter on the positive supply wouldn't hurt, of course, but it doesn't help too much either - PSRR (at very high frequencies) starts to get dominated by the output stage. So we would need to move the RC filter in front of Q6/Q7, which is somewhat unhappy as it might inject some rectified distortion currents into the small-signal stages when driving heavy loads. And it (significantly) lowers max. output swing.

Actual PSRR will depend on layout and the resonance frequency of the cap.

Samuel
Title: Preview On New Discrete OpAmp
Post by: [email protected] on October 24, 2005, 04:29:34 PM
Nice!!!   :grin:

Fully biased opinion: I like to use a simple current mirror on LTPs. It helps reduce high frequency distortion at all gain settings (but it really shows its advantages at higher gains) and minimizes offset at the output. Just two cheap transistors...
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on November 12, 2005, 08:14:49 AM
In the meantime I was able to do some more testing; it is unity gain stable even on breadboard and without R11/C5. Some informal listening test did not reveal any problems.

2520 layout in progress...

Samuel
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on January 27, 2006, 03:47:55 AM
A short update on this project: I've been very busy with my diploma thesis and I had trouble sourcing the pins, so this got delayed a bit.

In the meantime I applied some changes to the output stage, here's the current schematic: [removed]

After several iterations I ended up with a PCB layout that I like: [removed]
It got a double ground plane and should be pretty easy to assemble.

Before I post Gerbers I'd like to build a prototype--may take another few weeks, sorry!

Samuel
Title: Preview On New Discrete OpAmp
Post by: [email protected] on January 27, 2006, 10:50:18 AM
Samuel,

I got thousands of pins. I would be glad to share.

Cheers,
Tamas
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on January 27, 2006, 11:33:33 AM
Thanks for the generous offer! However I ordered them already and they should arrive in a few days. It was a 2k5 minimum order, but fortunately they weren't that expensive and I'll hopefully build more than two or three 2520-footprinted discrete opamps in the next few years. :wink:

Samuel
Title: Preview On New Discrete OpAmp
Post by: kruz on January 27, 2006, 01:18:33 PM
Samuel, i would need some if you want to get rid of some extras.
I'm in Italy so we are not much far, shipping would be cheap.
Please contact me at kruz --at-- beatbazar --dot-- come
Thank you!!
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on January 31, 2006, 02:39:33 AM
Done a few days back, but no response from you?

Samuel
Title: Preview On New Discrete OpAmp
Post by: kruz on January 31, 2006, 04:35:20 AM
i've just replyed to yoru email
sorry for the delay :oops:
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on March 25, 2006, 02:42:38 PM
Finally I found time to stuff and test the prototype. Works like a charm! I think Fabio will do a board run, if anyone needs Gerbers let me know and I'll see what I can do.

Within the next weeks I'll write a detailed documentation with parts list, stuffing instructions and application notes. For the time being here a temporary parts list: [removed]

Samuel
Title: Preview On New Discrete OpAmp
Post by: mnats on March 25, 2006, 04:44:50 PM
Great news! Looks like there will be more DOA building going on in the future.

One question though - what are you using for inductors? I didn't see them on your parts list.
Title: Preview On New Discrete OpAmp
Post by: strangeandbouncy on March 25, 2006, 05:22:33 PM
Wow!

    another DOA! life just gets better and better . . .


        Wonderful news. ican;t wait yo try.


         ANdyP :thumb:
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on March 26, 2006, 02:55:39 AM
Quote
What are you using for inductors? I didn't see them on your parts list.

Thanks for pointing this out--completely forgot them. I updated the parts list. Any standard HF inductor with low resistance and reasonable resonance frequency should work in place of the part I indicated.

Samuel
Title: Preview On New Discrete OpAmp
Post by: bovox on March 26, 2006, 10:08:51 AM
Hey Samuel, I´m not trying to be smart, but you have to take a look
at the M-100 from former NTP in Denmark ( you can find it at Gyraf´s
resources) and tell me what your improvements are?! I can answer one for my self; M-100 runs in class B which I think is a pity because  its a wonderful sounding amp if you not strain it; thats-no heavy load under 5-10kohms.
Cheers, Bo
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on March 27, 2006, 01:27:38 AM
Quote
You have to take a look at the M100 from former NTP in Denmark and tell me what your improvements are!

Perhaps let's say it like this:
* substantially lower voltage noise
* class A drive up to 600 ohm
* input protection
* output short-circuit protection
* improved high-frequency PSRR
* higher open-loop gain (and thus lower distortion)
* lower unity-gain transit frequency (and thus improved stability)

I do not pretend to have any significant invention in my design; it's all about compromises and that's a compromise I like. It is very easy to improve the performance in many respects if you give up the simplicity of these designs and add a few more parts, but that's not what I wanted here.

BTW, I measured the input offset voltage of the first prototype yesterday and found--much to my surprise--that it is below 1 mV. Either I've been lucky or these Toshiba transistors are extremely consistent and SPICE predicts the quiescent conditions very well.

Samuel
Title: Preview On New Discrete OpAmp
Post by: Bauman on March 31, 2006, 02:05:11 PM
We are about to make some boards of Samuel DOA, check here:

http://www.groupdiy.com/index.php?topic=14712&postdays=0&postorder=asc&start=0

 :guinness:
Fabio
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on April 13, 2006, 04:13:10 PM
I uploaded the first revision of the documentation: [removed]

More to come ASAP! Please report errors and typos...

Samuel
Title: Preview On New Discrete OpAmp
Post by: [silent:arts] on April 13, 2006, 05:08:09 PM
Samuel, great work and documentation, thanks alot.
looking forward to the pcbs :thumb:
Title: Preview On New Discrete OpAmp
Post by: tommypiper on April 13, 2006, 05:46:20 PM
Very exciting!  Excellent documentation.  Can't wait to see the remaining subjects completed.  Samuel, how can a Swiss write such good English? :grin:
Title: Preview On New Discrete OpAmp
Post by: bovox on April 25, 2006, 11:34:29 AM
Hey Samuel, i made a breadboard of this your nice OP,but I have some trouble.
The slew and therefore freq resp. is poor.
I get neg. peak when the square curve goes down, upwards its very slow.
I´ve messured the idle current and they are fine, just as you predicted.
The outp.offset is about 0,8V. The MPSA 18 has about 950 as hfe.
I took away the C2+R8 and it became a figure of 5 better in responce.
I´ve followed your fine desciption but I cant get it work properly.
What to do?
Cheers Bo
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on April 25, 2006, 01:15:43 PM
At which gain/configuration did you test the opamp? Can you measure the voltage drop across R3, R4, R6 and R7 and post them? It looks to me as if one of these resistors could have a wrong value, at least it would explain the large offset and the asymetric slewing.

Removing C2/R8 will of course make the amp unstable at low gains.

Samuel
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on April 25, 2006, 03:07:12 PM
I uploaded the latest revision of the documentation: [removed]

Samuel
Title: Preview On New Discrete OpAmp
Post by: bovox on April 25, 2006, 03:18:54 PM
:oops: -my mistake, I thought I checked evry...
the fault was that the 1k6 never was connected,you pointed me into the right direction. Thanks! So now it reeds 8,2 mV !! offset with gain 1 and 10kohm in the feedback loop. ´This is a fast amp and it reeds square wave above
100kHz but it rings and it could be wise to compensate.
It still has a ringing and an overshot of about 10% mostly on the negative side.  (measured at 10kHz.)
Soon I´m gonna listen to it as well.
Cheers Bo
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on April 26, 2006, 01:01:35 AM
Glad it worked out! Check the simulated impulse responses in the new documentation. With my board and test gear they are pretty close to what I measure.

What happens when you short the 10k feedback resistor? 10k is a bit high in value as it will start to decrease stability together with the rather large capacity of the input transistors.

DC offset should go down as well, I guess.

Samuel
Title: Preview On New Discrete OpAmp
Post by: bovox on April 26, 2006, 07:19:20 AM
Yeh, fine information! I tested shortening the 10k in feedback loop.It
didn´t like that,but when I was around 3kohm it measured really well.
When I came less then 300 ohms it did tend to get nervous. Maybe
cause I´m still in breadboard-land.
Cheers Bo
Title: Preview On New Discrete OpAmp
Post by: clintrubber on April 27, 2006, 12:41:02 PM
Quote from: "Samuel Groner"
I uploaded the latest revision of the documentation:
SGA-SOA-1_documentation.pdf (http://home.datacomm.ch/gronerfamily/DIY/SGA-SOA-1/SGA-SOA-1_documentation.pdf)

Samuel

Thanks, nice to have all the info compiled and as thorough as this.
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on May 27, 2006, 03:36:04 AM
I was able to finish a new revision of the documentation which includes a mic pre circuit in the app notes: [removed]

Samuel
Title: Preview On New Discrete OpAmp
Post by: Kit on May 27, 2006, 07:42:55 AM
Well done!! :thumb:
Title: Preview On New Discrete OpAmp
Post by: Carl_Huff on May 28, 2006, 11:13:59 PM
R9 & R10 are 2.7 ohm 1% metal film 1/2 watt resistors.  I am having a problem finding a source for those buggers.  Anybody have a suggestion where I might find them?  I am near Los Angeles, USA

The usual suspects (Newark, Digikey and Mouser) don't carry them.
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on May 29, 2006, 02:15:10 AM
You might check if you find a 0.6 W. I use Farnell 946-7068.

If you are sure that you'll never short the output, a 0.25 W well be fine as well.

Samuel
Title: Preview On New Discrete OpAmp
Post by: AnalogPackrat on May 29, 2006, 12:09:10 PM
Carl,

On page 485 of the current Mouser catalog you'll find Vishay BC 5% 1W metal film resistors which are quite small for 1W and come in 2.7ohm.  Buy more than you need and match them by hand--you should be able to find, say, 2 or 3 good pairs in a set of 10.  You'll need a decent meter to read better than ~0.2ohms or precision at the low end, though.

A P
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on July 30, 2006, 01:16:15 PM
Latest revision of the documentation: [removed]

Includes the summing amplifier schematic and description as well as a few minor changes in the mic pre.

Samuel
Title: Preview On New Discrete OpAmp
Post by: Bo Hansen on July 31, 2006, 07:17:15 AM
Samuel,

First time I see your op-amp design/documention, very nice work and description.

--Bo
Title: Preview On New Discrete OpAmp
Post by: gyraf on July 31, 2006, 07:47:08 AM
That is a really, really nice paper..!!

Thanks, Samuel..!

Jakob E.
Title: Preview On New Discrete OpAmp
Post by: bcarso on July 31, 2006, 06:19:16 PM
Quote from: "Samuel Groner"
Latest revision of the documentation: SGA-SOA-1_documentation.pdf (http://home.datacomm.ch/gronerfamily/DIY/SGA-SOA-1/SGA-SOA-1_documentation.pdf)

Includes the summing amplifier schematic and description as well as a few minor changes in the mic pre.

Samuel


Nice job.  Looks like the numbers on input bias current are wrong though---I should think you meant 1 uA, not 1 nA.  Those would be some high-beta devices indeed otherwise!
Title: Preview On New Discrete OpAmp
Post by: kiira on July 31, 2006, 07:16:34 PM
That is really terrific documentation... makes me wish I had bought a few SGA-SAO1 boards!  :razz: It would be great to see the headphone amp schemo sometime... I'd like to build a discrete RIAA/phones/line amp sometime.

Kiira
Title: Preview On New Discrete OpAmp
Post by: Dan Kennedy on July 31, 2006, 08:46:42 PM
Hey Kiira,

Good to see you are still out there and still building!

Nice work Samuel.
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on August 01, 2006, 12:31:34 AM
Quote
Looks like the numbers on input bias current are wrong though--I should think you meant 1 uA, not 1 nA.

Oh yes--thanks for pointing that out!

Samuel
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on August 06, 2006, 02:19:11 PM
Finally added the headphone amplifier schematic and description as well as the input bias value correction from -1 nA to -1 uA: [removed]

Samuel
Title: Preview On New Discrete OpAmp
Post by: ruairioflaherty on August 06, 2006, 07:37:51 PM
Hi Samuel,

I just wanted to thank you for the superb documentation you've written for this project.  It's detailed yet totally readable for an electronics beginner like me.  Speaking of which I just got a copy of the Radio Designer's Handbook 4th ed which is some seriously intense sh*t - it takes all my meagre brain power and a strong cup of coffee to digest a few pages of that mo-fo.

I hope I can build some of your SOA when I'm schooled,

cheers,
Ruairi
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on August 17, 2006, 01:27:06 PM
Thanks for your comments--I hope you'll like the amp as much as its documentation!

Fabio asked me if it is possible to fit BC550/BC560/BD139/BD140 instead of the suggested transistors and I thought I'd post my answer here so that all interested can read it:

It is if you accept that:
* voltage noise is higher (didn't calculate how much, but it's more than just half a dB)
* distortion gets a tad higher because beta of Q4 and hence open-loop gain is lower
* it is likely a pain to fit the transistors because Q1-Q5 have a different pinout

In order to maintain correct bias of the input stage I suggest the following two changes:
* R3 = 1k69
* R4 = 720

For Q1, Q2 and Q4 it is important to fit the highest beta rating with the suffix "C".

Samuel
Title: Preview On New Discrete OpAmp
Post by: Bauman on August 17, 2006, 01:39:37 PM
Thanks Samuel!

Ya know, it was just a WHAT IF in my head... :)
Now waiting for the right transistors to hear this nice opamp :)

Probably I will try the BCs/BDs on one board just for the fun of it :D


many cheers!
Fabio
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on August 18, 2006, 12:40:30 PM
Quote
I wonder if there will be any issues about that, being symetrical and all.

I suspect that the inherent imbalance from NPN to PNP is much larger than the manufacturer difference--but only a careful measurement would tell us the true story.

Samuel
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on November 11, 2006, 04:01:04 AM
I wonder if somebody got his board from Fabios batch running? I just heard from mnats that his boards tend to short to the ground plane, so we'd appreciate your feedback. I did not have this problem on my boards, but I got them from another board house and without silk screen.

Samuel
Title: Preview On New Discrete OpAmp
Post by: mnats on November 23, 2006, 03:04:24 AM
(http://www.mnats.net/images/sga-soa-trio-small.jpg) (http://www.mnats.net/images/sga-soa-trio.jpg)
As Samuel mentioned earlier, I've been having some problems building his design on boards supplied by Fabio. At work, I commonly hand-solder parts with pads spaced three to a millimeter, so I'm fairly confident my soldering skills weren't to blame.

So far, I've built three SGA-SOAs working very meticulously - measuring or testing every component and double checking the location before soldering it in place. Samuel's documentation is very useful and his assembly sequence was extremely helpful. Still, the first one didn't work.

The first module was drawing excessive current. The problem was traced to a short to ground at the base of Q3. No fault was visible even under intense magnification, but measurement showed the pad was definitely shorted to ground. My theory is that the solder is able to make a bridge just under the silk screen layer out of sight just under the white ink. The affected pad had white ink between the pad and the ground plane which formed a neat radius around part of the pad. Here's an image of the pad after I had fixed the short (click for enlargement):
(http://www.mnats.net/images/sga-soa-q3-1-small.jpg) (http://www.mnats.net/images/sga-soa-q3-1.jpg)

I removed the transistor and used solder wick on the top pad and the short was gone. To ensure that problems didn't recur, I carefully scratched off the ink around the pad.

Number two gave me similar problems, though on a different transistor. When I pulled the transistor and used the same trick with the solder wick, I still had a short. Finally I discovered that while removing the transistor with a heat pen one of the ceramic capacitor pads had gone short as well when the solder reflowed. The same sort of bridge was found with white ink surrounding part of the radius of both pads.

On my third board, I tried deliberately to make a short, but found I was unable to do so. Closer inspection revealed that the ink did not make a complete bridge between the pad and the ground plane. Even so, I removed any ink from between the pads and ground plane before stuffing the whole board. This one worked without any problem first shot.

Hope this helps anyone who is experiencing problems with the batch of boards from Fabio. I believe Samuel is reworking his files to prevent this problem from occurring in the future.

Can anyone else share their experience?
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on November 23, 2006, 06:40:40 AM
Thanks for posting--I've been much too busy to do that before.

I marked the pads which seem to be the most imperilled ones here:
SGA-SOA-1_PCB_corrections.pdf (http://home.datacomm.ch/gronerfamily/DIY/SGA-SOA-1/SGA-SOA-1_PCB_corrections.pdf)

I'm very sorry that this happened. My boardhouse edits the overlay print in case there is an overlapping region, that's why I never noticed that I should change my footprint library. I hope you get the boards running--don't hesitate to contact me with further questions.

Samuel
Title: Preview On New Discrete OpAmp
Post by: hitchhiker on April 11, 2008, 02:06:34 PM
Hi Samuel,
 
  I bought a pair of SGA-SOA-1 kits fron ptwownkid and built up your mic pre design as drawn in the documentation paper. I like it very much! Thank you!  I only had a 6 pos switch so I used gain setings 3 thru 8 and it covers my needs well. I'll install the pad someday to complete it.  I can't help thinking this would be a nice pcb to offer with the 990 footprint.
Sounds fast, punchy and mid present yet smooth and pleasing. I went with transformerless output and a Lundahl input.
 

Cheers! :guinness:
Lance
Title: Preview On New Discrete OpAmp
Post by: Samuel Groner on April 11, 2008, 03:32:31 PM
Thanks, Lance, for your feedback; I'm glad you like the result.

Quote
I can't help thinking this would be a nice PCB to offer with the 990 footprint.

In fact I allredy have a slightly more complex implementation (with additional transformer balanced output and some other features) with fully tested PCB ready. I just haven't finishing my homepage on analogue audio electronics (which will host the Gerber files) yet...

Samuel
Title: Preview On New Discrete OpAmp
Post by: skipwave on April 13, 2008, 01:45:19 AM
Yes, many, many thanks Samuel. I learned much from your documentation and posts here on the forum. I really appreciate you sharing your knowledge and keen, incisive observations.