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View Full Discussion Hi @pospos . Sure, bridged load with 16 Ohms nominal and 12 Ohms min is fine. The amp has output current limit around 0.9 Amperes peak.
I joined the drop, but I still have a few (!) questions, if you don't mind :)
1- How is THD vs frequency? Would you have a such a graph, for both high and low loads (300 ohms/16 ohms for example) at a given power?
2- Any IMD figures to share for low loads?
3- Does the amp emit a pop/thumb/noise when turned on and off, or is it completely silent?
4- Is it DC coupled, and is there any form of DC protection?
5- What is the DC offset (max and stabilized) for SE and balanced outputs with shorted inputs?
6- How long does it take for the amp to stabilize to its full potential when turned on (DC offset, distortion, etc.)
7- Would you have a curve of the output impedance vs frequency? (I see that it is approximately doubled at 1.5kHz compared to DC on the Benchmark AHB2, looking at the damping factor graph, but I don't know if this amp will share a similar behavior)
8- The specs show distortion figures for 1mW and 100mW, and then the 1% distortion point (aka max power). How does the amp behave between these two points? Any THD vs power curve to share for low and high loads? Or maybe figures at mid power (-3dB relative to the 1% THD point) at different loads? That would be most useful.
Sorry for the load (pun intended) of questions!
Thanks in advance.
1. Added to backlog. Problematic since 555 doesn't compute such low thd properly during sweeps.
2. Added to backlog.
3. No pop / thump whatsoever at turn on/off. Dead silent.
4. It's ac coupled at the input to eliminate any dc from the source / DAC, but is otherwise dc coupled throughout. It probably voids the warranty and I officially don't recommend it, but the 4 caps are right there waiting to be shorted to make it dc coupled.
5. Small dc offset. No servo. I've added this data request to the backlog.
6. Thd and dc offset stabilize within a few seconds or less. The thermal time constants are very short and the die temperatures vary very little.
7. Now you're just making work for me :)
8. AAA THD is very flat between 1mW, 100mW, and -3 dB. But! The APx555 has an issue wherein it doesn't compute such low thd properly during sweeps. We rely on fft to get accurate results. But ffts are in freq domain and the AP GUI doesn't permit moving the data to a sweep graph without using export / import functions and some manual Excel post-processing. A real pain in the butt.
Thank you so much for taking the time to answer all those questions.
It is really priceless to have a member of the design team shim in and answer customer's questions in such a open manner!
My purpose is certainly not to cause you any unnecessary pain is the place you mention, so to simplify questions 1 and 8 a set of discrete measurements would certainly be adequate.
For example for the THD vs frequency simple measurement at 100Hz, 1kHz (already done) and 6kHz would be perfect, at different loads (300 ohms, 32 ohms, 16 ohms?)
For THD vs power a simple 1kHz measurement at say -1dB, -3dB and -10dB at the different loads already mentioned would be perfect.
Regarding the IMD measurement already published, at what power are they measured?
Thank you !
Maybe just a suggestion, de 555 doesn't compute very well during sweeps but has a very nice feature of sending a serie of discrete tones, don't have one available but it's fairly easy to find and I really like this option for stuff like THD by frequency
Any updates on these?
hi @pospos . It's a tricky thing. The amp isn't designed or tested for such low impedances, even if you're driving very little power there. I mentioned the current rating too. Converting higher your effective load impedance (as seen by the amp) with a transformer would certainly take the load off the amp. Sorry, as an engineer I can't make a more definitive recommendation without testing stuff - hope you understand!
Hi Andrew, thank you for this honest and informed response!
My amp will ship in mid-May and I have a question about the 12 Ohm impedance limit. I have (or will rather) a pair of Verum Audio 1's (12 Ohm)...
So the question(s) is:
Thanks in advance.
- How does one go about not overheating the amplifier? Or void the warranty! With such headphones?
- Will a single-ended impedance adapter do the trick?
- What about a balanced XLR cable? Does one simply add 16 Ohms of additional resistance on the signal leads inside the XLR connector?
The manual says:
Permissible Headphone Impedance
The 789 balanced and unbalanced outputs are designed for headphone loads with a minimum 12-ohm impedance,and can tolerate loads between 12 ohms and 8 ohms if the amplifier is driven to no higher than 4 V peak across the load. Usage outside of these parameters may void the warranty. Do not use the 789 to drive loudspeakers.
These are conservative guidelines.
I don't recommend your #2 or #3.
Thanks for responding so quickly, Andrew.
Could you elaborate on why it's not recommended to use such adapters? If not, no worries.