Volume problem with Audiophonics Purifi amp.

[d2k5000]

I recently acquired an Audiophonics LPA-S400ET amp for my RAAL Audio SR1a headphones.

(For those of you who do not know, they are ribbon headphones that require a fairly hefty speaker amp to a converter box or specialized headphone amp to drive them.)

I had some issues with the Purifi unit when it first arrived, but Audiophonics was very responsive with their customer service, and most of the issues were resolved. The issue that seems to persist is the low volume output of the amp. I had previously been using a Cambridge audio AXR100 amp that has a 100 wpc output. That amp can drive the RAAL headphones to ear blasting levels with room to spare.

In contrast to the Cambridge audio amp, I can actually set my topping D90 to nearly full output and still listen comfortably with the Purifi amp. How could a 100 watt amp be way more powerful than an amp rated at 227 wats at 8 ohms? Am I missing something here? I don’t notice anything wrong with it other than the volume. It sounds very clear distortion free.

I know that there are different gain settings, I suppose that it could have been accidentally set to its lowest setting.

Any thoughts on the matter would be appreciated.

 

[Blumlein 88]

227 watts is only 3.5 db louder. Gain differences could account for it as PaulD said. Though with the 4 volt output of the D90 it looks like even at low gain you'd have enough power. Maybe try the +12.5 gain setting.

From the maker of the amp you have:

Audiophonics interface board
The 2 modules are driven in phase opposition, to cancel the "Power-Supply Pumping" effect. This allows a greater stability of the power supply, distributing alternately on each rail the calls of current and thus increasing its capacities.
We have opted for a 2-level voltage regulation, to optimise the power supply to the OPAs and the auxiliary voltage supplied to the modules.
One Texas Instrument LM4562 OPA per channel is used. These are very low distortion OPAs offering the highest transparency with a very low THD. They are mounted on a DIP8 socket, allowing their replacement if needed.
To suit all uses, we have opted for 3 gain settings, selectable by jumper on the interface card :

• Gain of +7.2dB, i.e. a total of 20dB (default) - Maximum power reached with a signal of -4.2V RMS.
• Gain of +12.5dB, i.e. a total of 25.3dB - Maximum power reached with a signal of -2.25V RMS.
• Bypass: In this case, the signal passes directly from the inputs to the modules, allowing the lowest possible gain of 12.8dB. This mode can be interesting when using a strong preamplifier, capable of driving a 2 to 4K Ohm load and reaching a voltage of -10V RMS.

 

[ririt]

To check how gain is set-up you will have to remove to cover of the amp and look how the jumper are set.
it is explained here (but in french). May be an english version is available.
https://www.audiophonics.fr/fr/blog-diy-audio/36-configurer-gain-bypass-sur-module-ampli-purifi.html

 

[Audiophonics]

You can try to play a 100hz sine, and measure the amplifer output with AC voltmeter (with or without load).
It will give you the amplifier output RMS voltage.

Same for D90 output which should reach ~4V RMS between hot and cold pin.

https://www.szynalski.com/tone-generator/

 

[morpheusX]

it is explained here (but in french). May be an english version is available.
https://www.audiophonics.fr/fr/blog-diy-audio/36-configurer-gain-bypass-sur-module-ampli-purifi.html

There is already an English version
- https://www.audiophonics.fr/en/blog...-bypass-on-purifi-amplifier-module-ampli.html

 

[d2k5000]

Thanks for the help everyone. I am impressed by the knowledge of the forum members here. I think that I will grab a volt meter and an engineer friend of mine too measure the RMS voltage.

 

[d2k5000]

Update: Checked the jumpers, and they are in the default gain position on the amp. I borrowed my Dad's Tectonics oscilloscope, and it looks like the Topping D90 is measuring 4.2 Volts as expected. Next step would be to measure the amp's output and make sure that it is right. I was hoping that somebody could check these figures and see if they seem right.

We would expect a 20db gain in default mode: Dac voltage times 100. So about 400 volts. Would the expected VRMS output in default mode with a 4.2v input be 420V?

Thanks

 

[NTK]

...
We would expect a 20db gain in default mode: Dac voltage times 100. So about 400 volts. Would the expected VRMS output in default mode with a 4.2v input be 420V?
...

20 dB in voltage is 10X, not 100X.

 

[d2k5000]

20 dB in voltage is 10X, not 100X.

Thanks for pointer NTK! That would have been a big mistake.


Just measured the amp with the oscilloscope. I am running it by playing a 100hz sine wave from my Topping D90 which is giving off 4.2 volts.

I am seeing 33.2 volts RMS and about 94.4 volts peak to peak. Does this seem right for the amp? RMS seems a little lower than I was expecting, but not by a huge amount.

Feedback would be apreciated!

 

[NTK]

Thanks for pointer NTK! That would have been a big mistake.


Just measured the amp with the oscilloscope. I am running it by playing a 100hz sine wave from my Topping D90 which is giving off 4.2 volts.

I am seeing 33.2 volts RMS and about 94.4 volts peak to peak. Does this seem right for the amp? RMS seems a little lower than I was expecting, but not by a huge amount.

Feedback would be apreciated!

It seems the gain is 2 dB below what is expected. That means with your DAC outputting 4 V rms, you will only be able to drive your amp to 63% of full power.
[Voltage gain in dB = 20 * log10(V_out / V_in) = 20 * log10(33.2 / 4.2) = 17.96]

That shouldn't matter a lot since 2 dB in loudness is not much. Alternatively, you can set the jumpers to the high gain mode. Or ask Audiophonics.

Was the amplifier driving to a load when you were measuring? Have you tried measuring with a lower input voltage level? Post an oscilloscope screen shot if you can.

 

[d2k5000]

You will have to forgive my technical illiteracy. I am afraid I made mistake again. I had the volume set to 75% on the tone generator. I am now getting 43.4 RMS voltage.

 

[NTK]

You will have to forgive my technical illiteracy. I am afraid I made mistake again. I had the volume set to 75% on the tone generator. I am now getting 43.4 RMS voltage.

Not a problem at all. We've all made and learned (hopefully ) from these kinds of mistakes. Glad that your new amplifier is in fine working order. Now relax and go enjoy some music.