Modulus-86 build thread

In these days of the Internet, pictures also mean nothing - if someone wants to tell a porky, etc, it's pretty easy to fake it - and as for what it sounds like, well ...

I'm one of the few people who is not obsessed, especially these days, with taking 5 million pictures of what's going in their lives, each and every day - I just don't do pic's, unless there is a real use for doing so - and from when I started this audio stuff I have never taken a single snapshot of anything I've done - to me, this is a pointless thing to do, it never occurs to me as something that's important ...
 
I would be interested to read how the improved distortion values of mod-86 will translate in real world listening experience compared to plain 3886.

If someone posts up the distortion figures of the mod-86 when its driving a non-ohmic load (i.e. a loudspeaker), we might get a little closer to understanding the difference between it and a vanilla LM3886 in terms of real-world performance.
 
In a composite amplifier, such as the Modulus-86, the sound quality is determined mostly by the error-correction op-amp (LME49710).

When listening on my Alpair 6P full-range speakers, the only sonic signature I hear and those of the Alpair 6P (and its enclosure). The amp is transparent, both with the LM3886 and the LM3876. I really like it.

~Tom
 
If the mute resistor is chosen to draw just enough current from the mute pin (~0.2mA) to fully activate the unmute, then when the PSU charge depletes, the mute activates early for a silent shutdown.
This also works at start up by delaying the full unmute until the current comes up to near the final level.

Whereas, using a higher unmute current say around 5mA brings on the unmute very early and delays mute to much later after shutdown.
 
Member
Joined 2014
Paid Member
A couple of Caps short, which I need to order but otherwise all the passives are in place. One thing to note for those of us who dont' keep up to date with labelling trends. R13-R15 are 1% normally or 0.1% if you intend to bridge/parallel. Tom's mouser BOM orders the 0.1% parts.

I normally expect high tolerance parts to be black. And 2 of them are, but R13 is banded (but with a purple tolerance band). First banded 0.1% I had ever seen which threw me completely.

The board itself is a joy to solder tho and the mouser delivery has the locations on each bag making it quick even when doing 4 at once.
 
If the mute resistor is chosen to draw just enough current from the mute pin (~0.2mA) to fully activate the unmute, then when the PSU charge depletes, the mute activates early for a silent shutdown.

Actually, you need Imute > 500 uA to cover the worst case (according to the LM3886 spec sheet). I size the mute resistor to have Imute > 500 uA at the worst case supply voltage (low mains, high output power). I have no clicks or pops in the speakers on turn on/off.

~Tom
 
I looked up the datasheet.
I have not tried to measure whether the Imute actually required matches the datasheet numbers.

To my eyes 0.2mA (200uA) is on the flat part of the curve, i.e. full unmuting.
0.5mA (500uA) is at least double the minimum required for full unmuting.
It appears, again to me, that <0.14mA is into the muting region.

I would consider 10mA as > 500uA. That would activate and deactivate the mute/unmute function at very different start up/shut down voltages compared to using 0.2mA.
Using 1mA (>500uA) would give a different muting/umuting effect at start up/shut down.

Do you disagree?
 
Last edited:
Page 4 in the LM3886 data sheet (above the spec table):

The following specifications apply for V+ = +28V, V− = −28V, IMUTE = −0.5 mA with RL = 4Ω unless otherwise specified. Limits apply for TA = 25°C.

Emphasis mine.

Page 17:

Mute Function: The muting function of the LM3886 allows the user to mute the music going into the amplifier by drawing less than 0.5 mA out of pin 8 of the device. This is accomplished as shown in the Typical Application Circuit where the resistor RM is chosen with reference to your negative supply voltage and is used in conjuction with a switch. The switch (when opened) cuts off the current flow from pin 8 to V−, thus placing the LM3886 into mute mode. Refer to Figure 44 and Figure 45 in Typical Performance Characteristics for values of attenuation per current out of pin 8. The resistance RM is calculated by the following equation:
RM (|VEE| − 2.6V)/I8 where
• I8≥0.5mA.

The equation above ignores the voltage drop across the 1 kΩ resistors in the path, by the way... See the equivalent circuit in the data sheet.

Sure. You can look at Fig. 44, 45 and claim that less than 500 uA could be used. I have three issues with this approach:
1) The graphs are intended to show the mute attenuation, i.e. the performance at Imute = 0.
2) Due to the log scale of the graphs (attenuation in dB), it is very hard to tell the attenuation at 200 uA < Imute < 500 uA.
3) The graphs show typical performance. It does not guarantee that all LM3886es that leave the fab meet this spec. Only the parameters listed in the spec table are contractually binding (including the test conditions).

From my experience, there is no benefit of designing for Imute < 500 uA, unless you want to activate the mute function. I seem to get fine results and thump-free startup and shutdown when Imute reaches 500 uA at 3/4 of the nominal supply voltage.

If you run Imute too low (say 200 uA < Imute < 500 uA), you risk mixing some of the voltage present on the GND pin of the LM3886 into the bias current for the input stage in the LM3886. I don't think that's such a hot idea...

As you correctly point out, if you design for Imute >> 500 uA, the startup and shutdown performance will be impacted. Exactly what the impact is and at which Imute it happens, depends on many variables. I would prefer the mute current to have fallen below 100 uA when the rail voltages hit the minimum supply voltage required for reliable operation (not necessarily operation within the specs, but not full collapse of the chip bias circuits).

The bottom line is that as long as you don't have thumps/clicks/pops at startup and shutdown, and the mute function does not activate during normal operation, life is good. Designing for Imute = 500 uA at 3/4 of the nominal supply voltage works for me...

~Tom
 
Last edited:
I am close to do the final check. The manual says:

Connect the negative lead of a DC voltmeter to ground (J3, pin 2) and touch pin 4 of U4 with the positive lead of the voltmeter. The voltmeter should read -15 V (within a few percent). Touch pin 7 of U4 with the positive lead of the voltmeter. The meter should read +15 V (within a few percent

How do you identify pin 4 and 7 of U4?

Cheers,

Dominique
 
Member
Joined 2014
Paid Member
Hi Dominique

How did you decide which way round to put the ICs in? If you have the 3886 away from you as per the pic http://www.neurochrome.com/audio/wp-content/uploads/2014/09/MOD86_R1p0_Assembled.jpg you can see the dots for pin 1 on all the ICs left hand side furthest away from you.

8 pins are numbered
1 8
2 7
3 6
4 5

Note it wont hurt if you try the meter on all pins until you find the voltages you are looking for.