Diamond Cascode, a Diamond-like buffer/current booster - diyAudio
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Old 2nd August 2013, 11:29 AM   #1
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Default Diamond Cascode, a Diamond-like buffer/current booster

I've mentioned (and posted schematics) of what appears to be a simple and novel diamond-like buffer in other threads (Discrete Opamp Open Design, etc.). I've now built an opamp current-booster using this output stage, in the DIP-8 form factor. This thread is for the purpose of collecting the info about this topology and implementation(s) in one location, for easy reference.

1. LTSpice simulation schematic with LME49860, with and without the diamond cascode current booster.
2. THD20 FFT at 4v output amplitude into 600R load, with and without the current booster.
3. Eagle schematic.
4. Eagle layout.
5. Component-side picture of prototype.
6. Solder-side picture of prototype (with SOIC-8 LM318M installed).

Implementation notes:

1. The most difficult part of the assembly is the SOT363 BC847BPN dual NPN/PNP. However, it's manageable with a toothpick, standard soldering iron (could also be used as a diagnostic test for Parkinson's), and some no-clean liquid flux.
2. All other actives on component side are SOT23.
3. All passives are 0603 with large pads, which will also accommodate 0805 with some care.
4. From the schematic, all the SOIC-8 opamp pins are terminated at the DIP8 pins, except pin 6 (output), which drives the diamond cascode current booster, whose output goes to the DIP8 pin 6.
5. As a consequence, just about any SOIC-8 single opamp could be used - an LME49990 will probably be the king-of-the heap.
Attached Images
File Type: jpg lf07_dlb.jpg (85.7 KB, 831 views)
File Type: jpg lf07_dlb_4v600r_thd20_fft.jpg (71.2 KB, 803 views)
File Type: png lf07_sch.png (5.4 KB, 778 views)
File Type: png lf07v10_brd.png (41.2 KB, 759 views)
File Type: jpg lf07_lm318_cs_12x9.jpg (254.8 KB, 744 views)
File Type: jpg lf07_lm318_ss_12x9.jpg (175.5 KB, 266 views)

Last edited by linuxguru; 2nd August 2013 at 11:41 AM. Reason: addendum
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Old 5th August 2013, 10:32 PM   #2
rsavas is offline rsavas  Canada
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Wonderful job. Are you able to measure this device? DC offset?
BOM?
Q1= SST4393?
Q2=BC847PN
Q3=MMBT5550?
Q4=MMBT5401?
U1= LME49860,LME9990

Last edited by rsavas; 5th August 2013 at 10:35 PM.
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Old 6th August 2013, 09:10 AM   #3
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Quote:
Originally Posted by rsavas View Post
Wonderful job. Are you able to measure this device? DC offset?
BOM?
Q1= SST4393?
Q2=BC847PN
Q3=MMBT5550?
Q4=MMBT5401?
U1= LME49860,LME9990
Thanks - I haven't specifically measured the DC offset, except at the output of a MyRef Rev C with an LM318M installed as U1. In that case, it was negligible - 0.6 and 1.4 mV respectively in the two channels at the main speaker output.

Preliminary BoM:

R1: 1k to 1k5/1%/0603 (for Idq of ~1 to 0.6 mA in the MMBF4393)
R2, R3: 3k3/1%/0603
R4, R5: 220R/1%/0603
R6, R7: 22R/1%/0603
Q1: MMBF4393/SST4393 (there may be better SOT23 NJFETs for a CCS)
Q2: NXP/Infineon BC847BPN SOT363
Q3: MMBT5551/BC817/MMBT2222 (any medium power NPN)
Q4: MMBT5401/BC807/MMBT2907 (any medium power PNP)
U1: Almost any SOIC8 single opamp - LM318, LT1028, LF356, OPA627/827, LME49710MA, LME49990, etc.

The existing prototypes use LM318M (~6 qty) and LME49990 (1st unit assembled this morning, tested working - will do another one and audition shortly).
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Old 16th August 2013, 01:27 AM   #4
Struth is offline Struth  Canada
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Hi Guys

I do not see this as a diamond at all. Q3,4 are merely a bias regulator. Drive from the opamp is applied to the base of the PNP EF. You might find an improvement in performance if you added a cap across the EF bases.

To have this really be a diamond buffer in any form, the opamp output would be tied to the bases of Q3,4.

Also, the no-clean solder (Kester 245) is fine if you simply leave the flux residue in place - chip it off mechanically if you want to after it dries, but totally stay away from the water-clean solder. This garbage leaves residue that eats the board.

Have fun
Kevin O'Connor
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Old 16th August 2013, 04:33 AM   #5
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What strikes me as interesting is the change in the relative levels of the 2nd,3rd (etc) harmonics in the simulation. The circuit seems to cancel some 3rd and make the 2nd higher, which may mean that (assuming such things are audible) the circuit will sound "better" than the opamp alone!

Also wondering what the performance would be if Q3/4 were not bootstrapped to the outputs and instead went to the rails?

If the function of Q3/4 are only for bias regulation, then the above connection ought not alter the distortion products.

Also why not use a standard VBE multiplier or other more or less passive WRT the signal bias method and see how that looks?

I keep looking at J1 and thinking "mirror"... not sure why, or if that would be of any benefit...

The other thing is, don't drink too much coffee before soldering!!
Linuxguru, so far I think this is an interesting Diamond Like circuit.
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Last edited by bear; 16th August 2013 at 04:36 AM.
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Old 16th August 2013, 06:37 AM   #6
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Quote:
Originally Posted by bear View Post
What strikes me as interesting is the change in the relative levels of the 2nd,3rd (etc) harmonics in the simulation. The circuit seems to cancel some 3rd and make the 2nd higher, which may mean that (assuming such things are audible) the circuit will sound "better" than the opamp alone!
Yup, the discrete output stage is biased with about 5-10 mA quiescent current, so it's in Class-A for typical high-Z loads and line-level or similar swings. So it tends to have Class-A sonics. The second part is that the CCS also biases the opamp output stage to Class-A, thus improving the audible sonics of those opamps that benefit from sinking constant current into the output pin (classic ones like NE5534, LF356, LM318, as well as some recent ones).

I've physically built and auditioned it with an LM318 - the sonics become Class-A, i.e. airy, smooth, less fatiguing.

Quote:
Also wondering what the performance would be if Q3/4 were not bootstrapped to the outputs and instead went to the rails?

If the function of Q3/4 are only for bias regulation, then the above connection ought not alter the distortion products.
Q3, Q4 also have a (weak) lower cascode effect through the emitters of the output stage BJTs. I didn't actually sim it with the collectors tied to the rails, but it should also work with bias adjustments, without the cascode effect.

Quote:

Also why not use a standard VBE multiplier or other more or less passive WRT the signal bias method and see how that looks?
It's a bit trickier to trim a standard Vbe multiplier in SMD, ie. to set the quiescent operating point. There are some alternatives that may work OK.

Quote:
The other thing is, don't drink too much coffee before soldering!!
Linuxguru, so far I think this is an interesting Diamond Like circuit.
Thanks - a shot of Jack Daniels may work better :-)

Last edited by linuxguru; 16th August 2013 at 06:46 AM. Reason: typo
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Old 16th August 2013, 06:57 AM   #7
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Quote:
Originally Posted by Struth View Post

I do not see this as a diamond at all. Q3,4 are merely a bias regulator. Drive from the opamp is applied to the base of the PNP EF. You might find an improvement in performance if you added a cap across the EF bases.
Good idea - it's reasonably high-Z at the CCS, so a small X7R may work - say 100 nF.

Quote:
Also, the no-clean solder (Kester 245) is fine if you simply leave the flux residue in place - chip it off mechanically if you want to after it dries, but totally stay away from the water-clean solder. This garbage leaves residue that eats the board.
I use an IPA-soaked tissue for manual cleaning, and it seems to work - I don't use water-clean stuff.
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Old 16th August 2013, 10:00 AM   #8
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Quote:
Originally Posted by linuxguru View Post
Good idea - it's reasonably high-Z at the CCS, so a small X7R may work - say 100 nF.
I checked it in the sim with 10 - 100 nF between the bases of the EFs, and H2 (and the evens) reduce quite dramatically with 100 nF. Unfortunately, H3 and the odds increase slightly, so there may be no net gain in sonics, even if THD20 decreases overall.

Nevertheless, it's a useful option, especially if the output stage is biased heavily into Class-A and there's sufficient room to decrease H2 while still allowing it to stay dominant.
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Old 16th August 2013, 03:06 PM   #9
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I was wondering about running a simulation of those other circuit variants to show what the differences might be...
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Old 16th August 2013, 08:19 PM   #10
Struth is offline Struth  Canada
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Hi Guys

Good suggestions, Bear.

I would think that the main benefits you are seeing in any sim of this circuit compared to the raw opamp results is mostly the result of the bias shift of the opamp output due to the current source. This is an old trick, beginning simply as a resistor to one rail from the output.

Have fun
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