Topping PA5 Review (Amplifier)

[gamerpaddy]

i found that the circuit has different values than the Ti reference document recommends.

https://www.ti.com/lit/an/slaa788a/slaa788a.pdf?ts=1665996686595&ref_url=https%253A%252F%252Fwww.google.com%252F

Ti Recommends R_fb=33k for a TPA3255, topping uses 9.9k (3x3.3k)
for R_fb_gnd they recommend 10k, but 750 ohms is being used.
the input stage is a very low noise and distortion gain stage with a gain (inverting) of about 1.77x
the output stage have a gain of roughly 1.21x
the output resistors are about 6.99k (tpa3255 has 20k input impedance, thats a gain of 0.67x)
if im not mistaken.

both in and outputs are capacitively coupled.

pretty low gain stages, very high feedback. this explains why its overall gain is so low but also why it measures so well.

i dont think heat sinking the module would help, but a softer potting compound would. im still on that thermal expansion idea with solder joints cracking.
the opa and ne5532 is rated to run at 125 and even 150°C, it wont get that hot at any time.

 

[jasonhanjk]

So which IC gets hot? The 5532 or the Opa?
Heat up due to load draws too much current or higher supply voltage.

 

[trungdtmc]

On my DAC, op amps operation at +-12vdc but NE5532s or LM4562s alway hotter than opa1612s, muse02s

 

[Toku]

i dont think heat sinking the module would help, but a softer potting compound would. im still on that thermal expansion idea with solder joints cracking.
the opa and ne5532 is rated to run at 125 and even 150°C, it wont get that hot at any time.

Their temperature is the junction temperature (Tj) of the silicon chip. This is the limit temperature below which the device will not be destroyed.
When designing equipment, we normally use the recommended operating conditions in the data sheet.
According to it, the operating temperature of OPA1612 and NE5532 is +85℃ (device surface temperature).

 

[gamerpaddy]

i was looking at the absolute max. ratings, not the recommended ones.
but if they would have overheated, they would be dead or permanently damaged by now.
it works just fine since i replaced that resistor and put it back together.

 

[norcalscott]

i was looking at the absolute max. ratings, not the recommended ones.
but if they would have overheated, they would be dead or permanently damaged by now.
it works just fine since i replaced that resistor and put it back together.

You mentioned in your earlier post that a resistor had come off the board when you disassembled the potting. I wonder if this was an issue with that part in which the solder was not quite right and at operating temperature it was losing its connection? I assume you re-soldered that resistor?

Also interesting that Topping seem to be driving this circuit to its absolute max. Would be worthwhile to re-test one of the "fixed" units to see if the "fix" is to change out some components to make it more reliable, but also not test as well.

 

[gamerpaddy]

probably, bad choice of solder, too little solder or thermal stress.

i would be interessted to figure out whats causing this leaves crackling and the other issues but 99% of the people sending their units back. gotta wait a year or two for them to show up on ebay as broken / for parts.

 

[xrk971]

Nice work Gamerpaddy. Is the main amp chip a 3255 or 3251? It makes a difference because TI recommended PFFB Rfb for 3251 is 18kohms. I wonder what Rin is set at if you happened to trace that when you were looking at the values. That would tell us the gain it is set at. I speculated earlier that opamp gain would be set at 9dB or 2.81x.

Anyhow, good to see that it is not a composite amp but just tweaked values on the PFFB circuit optimized for low distortion and reduced gain.

 

[gamerpaddy]

Rin (unless theres more resistors on the main board) is 6.99k (2k and 4.99k in series)

 

[Lräk]

i was looking at the absolute max. ratings, not the recommended ones.
but if they would have overheated, they would be dead or permanently damaged by now.
it works just fine since i replaced that resistor and put it back together.

Do you remember which resistor? That might be a clue to why the module fails.

 

[gamerpaddy]

doesnt have to be the same part in all failure cases. i dont think it was this resistor in particular since its on the left side, my right side failed.
maybe i reattached a part on the right while heating the potting compound.

 

[Lräk]

doesnt have to be the same part in all failure cases. i dont think it was this resistor in particular since its on the left side, my right side failed.
maybe i reattached a part on the right while heating the potting compound.

View attachment 238037

Hmmm...That resistor isn't the same value as the other 3 circuits.

 

[gamerpaddy]

look up smd resistor EIA 3 and 4 digit markings, its just a higher precision part. both are 680ohms

 

[Lräk]

look up smd resistor EIA 3 and 4 digit markings, its just a higher precision part. both are 680ohms

Is that 680 ohm part of the input R_ob_fb listed in Fig 2?

If so, then if it was not connected, the input would probably rail to one of the supplies and not function correctly.

 

[gamerpaddy]

it was in the input section (bottom), feedback stuff is on the top.
R_op_fb is 1.21k

 

[Toku]

A teaser photo reminiscent of the L50 ll and PA5s has been posted on Twitter.
I'm concerned about messages that say "What's the new upcoming set?"
But it is strange that there is no slit for heat dissipation!

 

[nsfgp]

Cool news/pic @Toku Yeah without the slit (and the min/max/knob-position markings) it may also be a new E50 II with a volume knob?! Just another guess.
Anyway ... typically Topping marketing ... which IMHO works pretty well for them ... SMSL should do more of these teaser things for upcoming products.

 

[Roland68]

i dont think heat sinking the module would help, but a softer potting compound would. im still on that thermal expansion idea with solder joints cracking.
the opa and ne5532 is rated to run at 125 and even 150°C, it wont get that hot at any time.

probably, bad choice of solder, too little solder or thermal stress.

i would be interessted to figure out whats causing this leaves crackling and the other issues but 99% of the people sending their units back. gotta wait a year or two for them to show up on ebay as broken / for parts.

I think it's great that you had the courage to open it and it turned out so well.

The potting compound makes absolutely no sense at this point and in such a device.
They probably wanted to protect your circuit, which in this way, in this day and age, is absolute nonsense. Even encapsulation with high-quality epoxy resin can be removed again for reengineering.
There are good ways to protect such a circuit with little effort and without effects, but apparently Topping doesn't know about them.

If the problems are actually gone without the cover and potting compound, then, as you correctly suspect, the expansion of the materials could lead to contact problems (e.g. cracking/breaking of the solder joints). One should not underestimate the forces that can occur.

Since Topping probably has no experience in this area, I guess another problem.
Poor cleaning of the circuit board before encapsulation, as well as a too cheap or incorrect encapsulation compound, can result in leakage currents over time. The same happens if there is no pre-treatment and trapped moisture with subsequent gradual oxidation.
Also, I don't see any baked-on conformal coating on the board that should be applied prior to such a potting to prevent many of these problems.

 

[Toku]

Cool news/pic @Toku Yeah without the slit (and the min/max/knob-position markings) it may also be a new E50 II with a volume knob?! Just another guess.
Anyway ... typically Topping marketing ... which IMHO works pretty well for them ... SMSL should do more of these teaser things for upcoming products.

I quickly noticed that neither photo had a slit to mark the position of the volume knob, nor did the case of what appeared to be a PA5s have a slit for heat dissipation.
I imagine this was intentionally removed to make the photo more mysterious.
Anyway, I'm looking forward to the new product news.

 

[SiamXIII]

Will it be a good amp to bi-amp it on bottom end of Kef R3, while SMSL VMV A1 drives upper part?