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Old 3rd March 2016, 08:02 PM   #1
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Default PA03 vs Parallel 86 vs Sympatico vs ??

Hello there,

I tried to find info comparing different LM4780 amps out there, but found that info is scattered. I thought it might be a good idea to gather them together in one place.

I am a happy user of PD version of LM3886 gainclone. I would say, gainclone is the circuit that uses minimalist's approach. I started building LM 4780 kit from PD, but someone suggested me parallel 86. And then I noticed there are so many different versions.

PA03 aka Pavel Dudek's 4780 amp
Parallel 86 by Tom Christiansen
Sympatico by Twisted Pear Audio
Audio Sector Minimalist Approach
ETC

There are also commercial 3886 amps such as:
AKITIKA GT-101
AmpsLab Synergy
Audio Sector
ChipAmps
ETC

What are the main difference between them? Have you actually listened to them? What's your listening impression?

Thanks,
Doug
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Old 4th March 2016, 03:47 AM   #2
tomchr is offline tomchr  Canada
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The Parallel-86 uses two channels of the LM4780 in parallel for higher output current. The Parallel-86 can deliver minimum 14 A peak of output current. This is enough to drive a 4 Ω load all the way to the 35 V supply rails. If you lower the supply voltage to 28 V, the Parallel-86 will drive a 2 Ω load with vanishingly low THD.

The Parallel-86 is a composite amplifier. In the composite topology, a precision op-amp is used to perform error correction on a power amplifier. The end result is low noise and very low THD. The THD of the Parallel-86 is almost 10 dB below the measurement capabilities of my Audio Precision APx525 analyzer.

The Sympatico is a composite amplifier as well. It uses the two channels of an LM4780 in series for a bridged amplifier. Each channel of an LM4780 is guaranteed to be able to deliver up to 7 A of output current, but with the amplifier channels in series, that means the maximum output voltage across a 4 Ω load is 4*7 = 28 V.
This means a bridged amplifier based on the LM3886 (or LM4780) will not be able to deliver any more power than a single LM3886 (or half LM4780). To me, that defeats the purpose of the bridged design.

In addition to the incredibly low THD, the composite amplifier topology also results in incredibly high power-supply rejection ratio. I measure the same performance of the Parallel-86 regardless of if I'm using a regular non-regulated linear supply, a pair of $1200/each well-regulated lab supplies, or a switching supply (Connex SMPS300RE). With non-composite chip amps, you can count on a degradation in THD+N of about 15-20 dB (5-10x) over the performance specified in the data sheet (see attached measurement).

One note on the LM4780: TI has decided to discontinue the LM4780 along with a handful of other ICs. The LM4780 is getting hard to find in low quantity. I picked up a tube of LM4780s and offer them for sale with my Parallel-86 boards while supplies last.

Tom
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File Type: png LM3886_THD_vs_Freq_vs_Supply_28W_8R.png (132.0 KB, 787 views)
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Old 4th March 2016, 04:38 AM   #3
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Quote:
Originally Posted by Doug Kim View Post
What are the main difference between them? Have you actually listened to them? What's your listening impression?
Whilst I haven't listened to those particular incarnations of the LM3886 I would suggest (along with tomchr it seems) that PSRR is probably a fairly good measurement to use to get to grips with their differences.

If you're going to build a good PSRR LM3886 amp and not go for a composite you'll probably go the way Akitika went and have only a single, regulated supply. I'd suggest avoiding any designs which don't address the PSRR weakness of the bare naked LM3886.
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Old 4th March 2016, 04:18 PM   #4
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Quote:
Originally Posted by tomchr View Post
The Parallel-86 uses two channels of the LM4780 in parallel for higher output current. The Parallel-86 can deliver minimum 14 A peak of output current. This is enough to drive a 4 Ω load all the way to the 35 V supply rails. If you lower the supply voltage to 28 V, the Parallel-86 will drive a 2 Ω load with vanishingly low THD.

The Parallel-86 is a composite amplifier. In the composite topology, a precision op-amp is used to perform error correction on a power amplifier. The end result is low noise and very low THD. The THD of the Parallel-86 is almost 10 dB below the measurement capabilities of my Audio Precision APx525 analyzer.

The Sympatico is a composite amplifier as well. It uses the two channels of an LM4780 in series for a bridged amplifier. Each channel of an LM4780 is guaranteed to be able to deliver up to 7 A of output current, but with the amplifier channels in series, that means the maximum output voltage across a 4 Ω load is 4*7 = 28 V.
This means a bridged amplifier based on the LM3886 (or LM4780) will not be able to deliver any more power than a single LM3886 (or half LM4780). To me, that defeats the purpose of the bridged design.

In addition to the incredibly low THD, the composite amplifier topology also results in incredibly high power-supply rejection ratio. I measure the same performance of the Parallel-86 regardless of if I'm using a regular non-regulated linear supply, a pair of $1200/each well-regulated lab supplies, or a switching supply (Connex SMPS300RE). With non-composite chip amps, you can count on a degradation in THD+N of about 15-20 dB (5-10x) over the performance specified in the data sheet (see attached measurement).

One note on the LM4780: TI has decided to discontinue the LM4780 along with a handful of other ICs. The LM4780 is getting hard to find in low quantity. I picked up a tube of LM4780s and offer them for sale with my Parallel-86 boards while supplies last.

Tom
Tom,
Thanks for the detail info. It is very helpful.
With LM4780 soon going to Dodo, and because you also have LM3886 board, which do you recommend to drive Magnepan MMG (4ohm)? I don't listen to the music loud. Mostly I play them as background while working on others, so moderate power will be OK.

Doug
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Old 4th March 2016, 04:35 PM   #5
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Quote:
Originally Posted by abraxalito View Post
Whilst I haven't listened to those particular incarnations of the LM3886 I would suggest (along with tomchr it seems) that PSRR is probably a fairly good measurement to use to get to grips with their differences.

If you're going to build a good PSRR LM3886 amp and not go for a composite you'll probably go the way Akitika went and have only a single, regulated supply. I'd suggest avoiding any designs which don't address the PSRR weakness of the bare naked LM3886.
My current LM3886 (Peter Daniel version) is dead quiet with the recommended minimal power supply. I even used this amp for >100dB horn, and it was dead quiet.
But I guess it actually has poor THD?
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Old 4th March 2016, 05:59 PM   #6
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by Doug Kim View Post
With LM4780 soon going to Dodo, and because you also have LM3886 board, which do you recommend to drive Magnepan MMG (4ohm)? I don't listen to the music loud. Mostly I play them as background while working on others, so moderate power will be OK.
With the recommended power supply (Power-86 + Antek AS-2222 transformer), the Modulus-86 can deliver 40+ W into 8 Ω and 65+ W into 4 Ω both with extremely low THD.

The Parallel-86 will deliver 60 W into 8 Ω and 120 W into 4 Ω using the recommended supply (35 V). You can get about 225 W into 2 Ω, but you need to lower the supply voltage to 28 V.

The Modulus-86 does offer the lowest THD on the market, so unless you really need the extra power, I'd go with the Modulus-86.

Quote:
Originally Posted by Doug Kim View Post
My current LM3886 (Peter Daniel version) is dead quiet with the recommended minimal power supply. I even used this amp for >100dB horn, and it was dead quiet.
But I guess it actually has poor THD?
"Dead quiet" would refer to the noise floor or residual mains hum. That's what you hear with the music off, but the amplifier on.
What I measured in the graph in Post #2 was the impact of the power supply on the amplifier THD when the amplifier is delivering a signal to the load. I suspect the reason many chip amplifiers sound kinda strained and harsh is due to the interaction between the supply and the amplifier. The composite topology of the Modulus-86 and Parallel-86 remove this supply/amp interaction to the point where it disappears below the measurement noise floor. The result is an amplifier that sounds open, natural, and clear-as-a-bell transparent.

Tom
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Old 4th March 2016, 10:50 PM   #7
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Quote:
Originally Posted by Doug Kim View Post
My current LM3886 (Peter Daniel version) is dead quiet with the recommended minimal power supply. I even used this amp for >100dB horn, and it was dead quiet.
I would guess by 'dead quiet' you mean that you hear little or no hiss when there's no music playing. This isn't a good way to evaluate the PSRR because with nothing playing the power supply doesn't have to work very much at all, so there's very little ripple.

Quote:
But I guess it actually has poor THD?
THD is measured with a sinewave as stimulus which is quite unlike music. So THD in and of itself isn't particularly meaningful to perceived audio quality.

However, having said that most audio measuring devices don't measure THD, rather THD+N and that difference is crucial with an amp which has poor power supply rejection - the power supply noise at the output of the amp does contribute to the 'N' part of this measurement. Which is what you see in the plot tomchr included - the difference between an amp with a clean PSU (regulated, low noise) and a normal PSU (just a bridge and res caps).
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Old 5th March 2016, 12:06 AM   #8
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by abraxalito View Post
THD is measured with a sinewave as stimulus which is quite unlike music. So THD in and of itself isn't particularly meaningful to perceived audio quality.
The THD isn't the only parameter I measure. I do agree that measuring only THD isn't particularly meaningful. However, the complete picture presented by measurements of THD, output noise, output power, THD+N vs frequency, THD+N vs output power, IMD, etc. is meaningful if you know how to interpret the results.

Quote:
Originally Posted by abraxalito View Post
However, having said that most audio measuring devices don't measure THD, rather THD+N and that difference is crucial with an amp which has poor power supply rejection - the power supply noise at the output of the amp does contribute to the 'N' part of this measurement. Which is what you see in the plot tomchr included - the difference between an amp with a clean PSU (regulated, low noise) and a normal PSU (just a bridge and res caps).
Actually, no. The reason for the difference in THD+N of an LM3886 when using a regulated supply and unregulated supply (shown in Post #2) is because the load presented by the amplifier on the power supply creates signal-dependent variations in the supply voltage. This shows up as THD (not noise) on the output of the amp. It's the THD part of the THD+N that rises.
A good amplifier will have high PSRR, hence, reject these signal-dependent variations in the supply voltage. As a result, it will show better THD and THD+N performance at high output power.

I tend to use THD as a synonym for THD+N and count on the reader to have sufficient context to understand my point correctly. What's shown in the plot in Post #2 is THD+N (measured using an Audio Precision SYS-2700 - two measurements combined in MS Excel).

Tom
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Last edited by tomchr; 5th March 2016 at 12:12 AM.
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Old 5th March 2016, 12:14 AM   #9
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Quote:
Originally Posted by tomchr View Post
Actually, no. The reason for the difference in THD+N of an LM3886 when using a regulated supply and unregulated supply (shown in Post #2) is because the load presented by the amplifier on the power supply creates signal-dependent variations in the supply voltage. This shows up as THD (not noise) on the output of the amp. It's the THD part of the THD+N that rises.
Do you have FFTs which show that its purely harmonic distortion? I'd be interested to see them if you do.
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Old 5th March 2016, 12:54 AM   #10
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I don't have proper measurement equipment, and I don't have reference level speakers and source, most of all, my home is more like children's theme park rather than listening room, so I have no means of doing high fidelity reproduction of sound at home. So I mostly am very happy when my built amplifiers are quiet when the input is turned off.

That's why I only mentioned about dead quietness about my 3886 amp. When paired with Harbeth Compact 7 ES3, it produces somewhat bloated mid-bass, more like baritone male voice area. I don't know if this is typical for 3886 amp, or if it's just me or my system, or if it can be fixed with more advanced (complicated?) designs.
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