Parallel opamp: an extreme audiophile amplifier made of parallel multibrand op-amps

[PRR]

...I came across this.
https://diyaudioheaven.files.wordpress.com/2013/03/opampamp.jpg Op-amp | DIY-Audio-Heaven
... three decades before mine...
It looks like made from 10 x 4 quad opamps by Motorola (LM324?)...

I did a thing with mass array of CMOS logic inverters, probably 35 years back. HOT!! And wimpy.

That guy's opamps are (as he says and links) "{SGS-Thompson} L272M IC’s salvaged from old Technics SLP202A CDM4/19 driver boards"... i.e. servo motor drivers. And duals not quads. Will swing over 1 Amp peak (but not for long). Gain-bandwidth a mere 350KHz (a '741 opamp is 3X faster). OK, slew is 1V/us which is '741 turf.

 

[GianlucaG]

Thanks. When I magnified the pic I had recognized the M for Motorola on the chips, and this misled me......TCA0372??

 

[GianlucaG]

It should be pretty straight-forward to figure out. You can always start with Doug Self's circuit.

Personally, I'm not a fan of the master-slave setup as the slaves will add their own THD to that of the master. I would rather create one giant paralleled opamp and use one feedback loop around it. That's harder to do, but not impossible.

The output resistors are probably on the order of 0.1-1 Ω.

Tom

If you read the site (50thanniversary) carefully, in the section where the thermal dissipation calculations are made, you can find out that the balance output resistor for each operational amplifier is 5 ohms.

The master-slave (M-S) configuration in this schema has three advantages.

1) it increases the damping factor (DF) by about +14dB (calculated via LTSpice)

2) it reduces distortion by about -20dB

(Power Boosters)
(http://www.tubecad.com/2013/04/19/Better%20Headphone%20Amplifier.png)

3) it allows you to use higher values of balancing resistors, reducing opamps stress


The higher the DF, the more control the amplifier can operate on the speaker's inertia, hence, the more precise the speaker movement is controlled, the better defined the bass is and the cleaner the sound is. Solid state amplifiers always have higher numbers, tube amps low.

In itself, the amplifier without M-S configuration still has a very low output impedance: 5ohm /60opamp= 0.083ohm, hence, the damping factor onto 8 ohm is about 100.

Thanks to the M-S feedback wired around all the slaves opamp, the output impedance is highly reduced and the damping factor of this amplifier rise to 500 (five times more, with output impedance of only 0.017ohm).

Since I'm connecting the speaker with two AWG 11 cables soldered in parallel and running 7 meters, the cables' resistance to be added to the output impedance of the amplifier is 0.029ohm.

Thus, by adding the cables' resistance, in the first case (without M-S), the actual DF is 71, and in the second case, (with M-S) the actual DF results to be 174.

Both are anyway good values, but as a general rule between an amp with a low damping factor and one with a high damping factor I would want the one with the higher DF to better control the speakers. FYI I never tested the "50th anniversary" by excluding the M-S feedback, even if the mod is very simple (but I do not want now to disassemble it anymore, also due to the high cost of thermal pad replacement...).

 

[U130421]

0.002% THD+N is the limit of 16 ADC used for measurement . The lm3886 has 2uv noise in its input.

 

[deonaudio]

Found this thread, great. Something I always wanted to try. A few questions. As I imagine, “slave” op-amps are in unity gain configuration, right. How about using something like 20 or 30 LME49713 TO-99 metal can op-amps per channel as slaves.. Those are high-quality and 100mA output current, Current Feedback. TO-99 would be very nice for cooling. Could use some of them for something... exotic..

http://80.93.56.75/pdf/0/5/5/2/1/05521640.pdf

Regards

 

[tiefbassuebertr]

Different opamp sound different

"The 50 Anniversary" also combines mixed types of opamps. If you listen carefully, you may perceive some tonal changes and psychoacoustic variations when connect identical or different opamps (either in series or in parallel).

My preferred opamp is the AD826 (ADI), which is smooth, open, clean and sounds true to life in mids and voices: this opamp reproduces the best female voice I've ever heard among any op amp tested.

To me also the LTC family of fast bipolar opamps LT1355, LT1358, LT1361 and LT1364 is audio superb!

The LT1358 (LTC) is warm, bright, tonal rich and sounds forward.

The LT1364 (LTC) is dynamic, fullness, punching bass and sounds accurate.

Consequently, for the "master" amplifier it was chosen the AD826 (ADI) which alone pilots, at a gain of two, an equal mix of 59 (per channel) parallel AD826 (ADI) and LT1364 (LTC) in unity gain configuration. The pleasing parallel combination of the AD826 (ADI) with LT1364 (LTC) compensates for each other's sonic difference and exhibits the lowest listening fatiguing.

Both AD826 (50Mhz; 350 V/us) and LT1364 (70Mhz; 1000V/us) opamps exhibit outstanding technical specifications, quite difficult to find all in one sole amplifier: bipolar, single stage, handling all capacitive loads, suited to drive cables, unity gain stable, hi-current output (50ma min), wide bandwidth (>50Mhz), extremely high slew rate, low-impedance driving capability (150 ohm), class AB output stage, easy to assemble in DIP package.

Also LT1208 (LTC) has stunning sonic and technical performances, comparable to AD826 (ADI), but it has half of its current capability, hence, it was discarded.

The pre-amplifier section is made by one half of AD826 (ADI) as buffer input in series with one half of LT1358 (LTC) that provides most of the gain after the volume control: both are biased in class A by using Warren Young (tangentsoft.net/audio/) constant current source JFET cascade configuration. They have their own dedicated power supply using best-in-class complementary LT1963A/LT3015 (LTC) low noise low dropout regulators.

GianlucaG copyright, Italy 2018

The AD826 (and the single version AD825) are also from my view one of the best choice.
Check out my thread (started today) under
Choosing of best sounding OP AMPs for the lowest possible THD+N -really the best Way?

 

[U130421]

AD825 is the jfet version of AD826, both have open loop bw above 10khz where others are less than a hundred. The AD8008 is my other favorite.