Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
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« 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. And pictures: SGA-SOA-1.html.]

Samuel


Bauman

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« Reply #1 on: October 22, 2005, 08:17:49 AM »
Hey thanks for sharing!!! I love new opamps  :twisted:
cheers!
Fabio
_____________________
www.thediypill.com/store

radiance

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« Reply #2 on: October 22, 2005, 08:36:06 AM »
Yeah, Thanks man! :thumb:
"Knowing that you are dreaming, however, does not automatically guarantee full rationality.
Then again, being awake doesn't ensure good thinking, either." -  Lynne Levitan

Dan Kennedy

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« Reply #3 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.

Family Hoof

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« Reply #4 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

bcarso

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« Reply #5 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.

bcarso

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« Reply #6 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.

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
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« Reply #7 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

cuelist

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« Reply #8 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!).
Mr K

bcarso

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« Reply #9 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.


Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
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« Reply #10 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

cuelist

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« Reply #11 on: October 23, 2005, 03:49:47 PM »
Samuel,

Why doesn't R11 have a companion on the +V side?
Mr K

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
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« Reply #12 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

[email protected]

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« Reply #13 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...

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
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« Reply #14 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

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
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« Reply #15 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

[email protected]

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« Reply #16 on: January 27, 2006, 10:50:18 AM »
Samuel,

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

Cheers,
Tamas

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
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« Reply #17 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

kruz

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« Reply #18 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!!

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
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« Reply #19 on: January 31, 2006, 02:39:33 AM »
Done a few days back, but no response from you?

Samuel


 

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