Topping A70 Pro HP & Preamp Review

[mike7877]

Have you ever measured whether the preamplifier outputs are powered by the op amps, the Tang-ku-la modules, or the output stage?

I think the simplest test would be measuring the voltage drop on the output stae first: 10,000,000 ohms vs. 8 ohms.
A70 Pro's single-ended output impedance is stated to be 0.1 ohms, and I think I saw it measured somewhere to be 0.04 or 0.07, so my Klein CL380 will have enough resolution (3 decimal points of resolution up to 3.999V) to detect the drop with 0.04 ohm output impedance (voltage drop from 3.999V to 3.9965, so 3.997 (rounded) if my calc is correct)

One leg of the XLR output should match the single ended output voltage with the A70 Pro in low gain mode (0dB gain). I'll measure the XLR output before and after connecting the 8 ohm load again to the tang-ku-la output - if there's a voltage droop at all, it's through the Tang-ku-la. If it remains the same? Op amp.


Do you think I should do anything different?

(the reason I didn't use RCA out is I think the path is a bit less direct)

(bal in, se out is 3dB worse than se in/se out)



edit: because of Klein, I was going to use 1kHz as the frequency for these measurements, but dang! Klein you let me down...
Spec is 400Hz high, 45Hz low... Not that 400Hz isn't enough, it's just a slight change in what I thought I'd be doing
(I thought that the "true RMS" feature (for accurate power measurements) started from all the way down 20/30Hz allll the way up to 2,000Hz)
Anyway, for fun, I'll be finding the frequency below 45Hz at which 3.999V measures 3.995, and the frequency above 400Hz at which 3.999 drops to 3.995 as well
(if anyone's interested - I think it's likely one could expect pretty similar results for at least their clamp meters made around similar times (late 2010s through the '20s)

 

[Roland68]

I think the simplest test would be measuring the voltage drop on the output stae first: 10,000,000 ohms vs. 8 ohms.
A70 Pro's single-ended output impedance is stated to be 0.1 ohms, and I think I saw it measured somewhere to be 0.04 or 0.07, so my Klein CL380 will have enough resolution (3 decimal points of resolution up to 3.999V) to detect the drop with 0.04 ohm output impedance (voltage drop from 3.999V to 3.9965, so 3.997 (rounded) if my calc is correct)

One leg of the XLR output should match the single ended output voltage with the A70 Pro in low gain mode (0dB gain). I'll measure the XLR output before and after connecting the 8 ohm load again to the tang-ku-la output - if there's a voltage droop at all, it's through the Tang-ku-la. If it remains the same? Op amp.


Do you think I should do anything different?

(the reason I didn't use RCA out is I think the path is a bit less direct)
View attachment 454883
(bal in, se out is 3dB worse than se in/se out)

I just wanted to point out that there are three possibilities where the signal for the preamp output can come from.
I'm old-fashioned and pedantic when it comes to this sort of thing, and I simply measure which wires are switched in each mode and where they come from or lead to. However, this can be very dangerous for the device (short circuit during measurement, static electricity, etc.).

 

[Bow_Wazoo]

Luckily, I discovered this thread. I had exactly the same problem with the a90 back then.
What's about the L70?

 

[mike7877]

I just wanted to point out that there are three possibilities where the signal for the preamp output can come from.
I'm old-fashioned and pedantic when it comes to this sort of thing, and I simply measure which wires are switched in each mode and where they come from or lead to. However, this can be very dangerous for the device (short circuit during measurement, static electricity, etc.).

Wait a second... if the vertical pair of op amps (1656+1612) are the XLR inputs, and the pair to their right is the condensed (single ended) form (for the signal to go through a half sized switched resistor volume network), and the bottom op amps (lone pair) are after all the relayed resistors ,yet before tang-ku-la... then the XLR outputs HAVE to be coming from either the 6120A2 (final stage) or the t'ang-ku-la (driving 6120A2).

Reason being, the single OPA1612 which receives audio signal from the end of the series of relays... it couldn't possibly be outputting the stereo XLR outs on the rear, because its supply voltage doesn't allow for 56V peak to peak - there's no way... The 1612 has to be going to the tang-ku-la which, as a part of its duties, takes the single ended signal and re-doubles it again (to balanced) so that the 4x TPA6120A2s (which are each stereo chips running in parallel) can be driven in pairs, half of each running in opposite phase for "balanced" output. Also, it makes sense that a good part of the gain is done in the tang-ku-la modules, with their goal being low noise. The 6120A2s are also isolated (ground plane on the bottom, heat sink on top, double row of capacitors wide side, and tang-ku-la modules other wide side, capacitors small side, with only like 20x 0.7mm exposed to potential RF.

Sorry this is disorganized, it's late and this is my second late night lol

 

[Roland68]

Wait a second... if the vertical pair of op amps (1656+1612) are the XLR inputs, and the pair to their right is the condensed (single ended) form (for the signal to go through a half sized switched resistor volume network), and the bottom op amps (lone pair) are after all the relayed resistors ,yet before tang-ku-la... then the XLR outputs HAVE to be coming from either the 6120A2 (final stage) or the t'ang-ku-la (driving 6120A2).

Reason being, the single OPA1612 which receives audio signal from the end of the series of relays... it couldn't possibly be outputting the stereo XLR outs on the rear, because its supply voltage doesn't allow for 56V peak to peak - there's no way... The 1612 has to be going to the tang-ku-la which, as a part of its duties, takes the single ended signal and re-doubles it again (to balanced) so that the 4x TPA6120A2s (which are each stereo chips running in parallel) can be driven in pairs, half of each running in opposite phase for "balanced" output. Also, it makes sense that a good part of the gain is done in the tang-ku-la modules, with their goal being low noise. The 6120A2s are also isolated (ground plane on the bottom, heat sink on top, double row of capacitors wide side, and tang-ku-la modules other wide side, capacitors small side, with only like 20x 0.7mm exposed to potential RF.

Sorry this is disorganized, it's late and this is my second late night lol

As far as I know, no supply voltage in the A70 Pro exceeds +/- 15 volts.
Since neither a DC/DC converter nor a step-up converter is installed, anything else would be a miracle. The operating voltage of the TPA6120A2 is also limited to +/- 15 volts.

 

[DrCWO]

Using the A70 Pro as an Amp for speakers.
Some of you in this thread have tried unsuccessfully to use the A70 to drive speakers.
I've now done it with success and I'm really impressed with the results.

So let's start from the beginning...
I run a 2x2 active system in my home. More details you can find here and in the posts after this:

Multi-Channel, Multi-Amplifier Audio System Using Software Crossover and Multichannel-DAC

(Grin) No pics of the wiring behind it all?? :rolleyes:;) Thank you again for your kind encouragements! You can find my "wiring behind" photo as of May 30 2022 in my post #540 (before the revival of LP player); please allow me showing it again here.:)

audiosciencereview.com

One special thing regarding my horn speakers are the custom BD drivers by AER that have an impedance of 16 Ohm.
I also have to mention that the crossover for the horn cuts it off with a very steep slope below 65Hz (green curve, red is XO for Woofer).
The crossover is realized as a FIR filter within Roon.

Here are my thoughts before buying the A70:

• As @amirm found out the A70 delivers 10V in 12Ohm. So the A70 will probably not be overloaded by the BD drivers.
• By cutting off low frequencies, where most of the energy is located and which the horn cannot transmit anyway, the load on the A70 is further reduced.

The A70 now happily sits beneath my RME ADI-2/4 Pro DAC, which is driven by a rooExtend-Box acting as a Roon RAAT Streamer. It replaces the Hypex NCx500 amplifier I was previously using. I connected the speakers to the balanced output on the front panel of the A70 using a custom-configured cable.

To A/B the output of both amplifiers (A70, Hypex NCx500) I installed my updated LC30 Meter-Box (Photos a little further down in this post). I calibrated the output of both amplifiers with a 1kHz tone and a multimeter connected to the speakers to 0.01dB.

Even with critical listening I cannot distinguish between the two amplifiers

The A70 also works for very high volumes as the horn speakers are so sensitive (approx. 100dB / 1W / 1m).
Happily listening now with an even smaller setup as before.

Best DrCWO

EDIT:
Volume at the A70 is fixed to -8.5dB. The volume of the 2x2 system is controlled by the ADI-2/4 both analog outputs (Horn, Woofer) coupled. To use the device volume control of the ADI-2/4 by Roon the Roon Extension rooADI is activated in the rooExtend-Box.

 

[Am1n985]

Recently purchased an A70 Pro in the hopes it would power my SJY Horizon Closed Carbons sufficiently (11-ohm).

Initially, I ran them off of my SMSL DO400's internal amp, sounded okay but definitely felt it lacked volume (especially with some EQ'ing and negative pre-amp, I like bass shelfs and I cannot lie) to the point where it would just distort at the volume level I'd like to listen to.

In comparison, my Hifiman HE1000 Stealths ran really well on the DO400, had plenty of headroom - which is as expected, they're much easier to run in comparison.

Now, using the DO400 as a Dac, connected via XLR to the A70 Pro, and outputting to the SJY's via XLR (tried 4.4mm and 1/4" too) I can barely reach 'loud' volumes on them without the A70 briefly cutting the sound when the bass drops, example tracks are 'Hecate' by Amanti and 'do re mi' by Blackbear, consistently cuts out when the bass kicks in - no messages appearing on the amp that people have mentioned earlier in this thread.

I've tried various levels on the DO400, less on that, more on the A70 etc, even using an Apple dongle to a 3.5mm/RCA directly into the A70 Pro (bypassing the DO400 entirely), same issues occur at similar output volume levels. The HE1000 Stealths do get loud on the A70 Pro, but ultimately cut out too (at a much higher volume compared to the SJY's, but still).

Any ideas? I'm fairly new to this world but have researched a fair bit, my understanding is it is the 'dc protection' kicking in on the A70 Pro.

I never had these issues on my previous all-in-one setups like the K11 R2R, Topping DX5 II (before one channel shit the bed and I had to send it back for a replacement, still waiting for this) and now the SMSL DO400.

 

[Roland68]

Recently purchased an A70 Pro in the hopes it would power my SJY Horizon Closed Carbons sufficiently (11-ohm).

Initially, I ran them off of my SMSL DO400's internal amp, sounded okay but definitely felt it lacked volume (especially with some EQ'ing and negative pre-amp, I like bass shelfs and I cannot lie) to the point where it would just distort at the volume level I'd like to listen to.

In comparison, my Hifiman HE1000 Stealths ran really well on the DO400, had plenty of headroom - which is as expected, they're much easier to run in comparison.

Now, using the DO400 as a Dac, connected via XLR to the A70 Pro, and outputting to the SJY's via XLR (tried 4.4mm and 1/4" too) I can barely reach 'loud' volumes on them without the A70 briefly cutting the sound when the bass drops, example tracks are 'Hecate' by Amanti and 'do re mi' by Blackbear, consistently cuts out when the bass kicks in - no messages appearing on the amp that people have mentioned earlier in this thread.

I've tried various levels on the DO400, less on that, more on the A70 etc, even using an Apple dongle to a 3.5mm/RCA directly into the A70 Pro (bypassing the DO400 entirely), same issues occur at similar output volume levels. The HE1000 Stealths do get loud on the A70 Pro, but ultimately cut out too (at a much higher volume compared to the SJY's, but still).

Any ideas? I'm fairly new to this world but have researched a fair bit, my understanding is it is the 'dc protection' kicking in on the A70 Pro.

I never had these issues on my previous all-in-one setups like the K11 R2R, Topping DX5 II (before one channel shit the bed and I had to send it back for a replacement, still waiting for this) and now the SMSL DO400.

This shouldn't actually happen with the A70 Pro.
Are you sure the XLR cable on your SJY Horizon Closed Carbon is truly symmetrical? Or isn't defective?
Unfortunately, there are pseudo-symmetrical cables on the market where the ground pins are connected. That would be the first thing I would check and rule out. To do this, you need to use a multimeter to check whether the two ground pins in the XLR connector are connected.

This should be ruled out first, before spending a lot of time chasing a nonexistent problem.

I deleted my previous post because I apparently forgot my text.

 

[Am1n985]

This shouldn't actually happen with the A70 Pro.
Are you sure the XLR cable on your SJY Horizon Closed Carbon is truly symmetrical? Or isn't defective?
Unfortunately, there are pseudo-symmetrical cables on the market where the ground pins are connected. That would be the first thing I would check and rule out. To do this, you need to use a multimeter to check whether the two ground pins in the XLR connector are connected.

This should be ruled out first, before spending a lot of time chasing a nonexistent problem.

I deleted my previous post because I apparently forgot my text.

Checked and no continuity.

For both headphones I've used 3 different XLR, 2 different 4.4mm and 1 1/4" cable. For connecting the DAC/Amps, 2 new sets of balanced cables, 3 sets of RCAs - so I'd like to think I've ruled out cable issues.

Also, tried different sources, USB, Bluetooth, direct iPhone -> Dongle -> 3.5mm/RCA -> A70 Pro.

It came with the latest firmware.

 

[Roland68]

Checked and no continuity.

For both headphones I've used 3 different XLR, 2 different 4.4mm and 1 1/4" cable. For connecting the DAC/Amps, 2 new sets of balanced cables, 3 sets of RCAs - so I'd like to think I've ruled out cable issues.

Also, tried different sources, USB, Bluetooth, direct iPhone -> Dongle -> 3.5mm/RCA -> A70 Pro.

It came with the latest firmware.

Secondly, I would check these songs; perhaps the A70 Pro is behaving completely correctly.

Please set up a low cut at 20 Hz and 48 dB/octave and test again with these songs. Please also test the same thing again at 30 and 40 Hz.

 

[DrCWO]

Hecate' by Amanti and 'do re mi' by Blackbear,

Both tracks are very bass heavy
So if you put it a bass shelve this may probably overstress the A70.

Maybe you need to get a NCx500 for your Headphone

 

[Am1n985]

Secondly, I would check these songs; perhaps the A70 Pro is behaving completely correctly.

Please set up a low cut at 20 Hz and 48 dB/octave and test again with these songs. Please also test the same thing again at 30 and 40 Hz.

Tried those to no avail and multiple other songs with higher frequency bass (60-80hz ish) with the same results.

 

[Am1n985]

Both tracks are very bass heavy
So if you put it a bass shelve this may probably overstress the A70.

Maybe you need to get a NCx500 for your Headphone

At this rate I'll pull out the JL Audio 300/4 I have sitting in my drawer, or one of my heftier amps, 500rms per channel should be enough, right?

 

[Roland68]

Tried those to no avail and multiple other songs with higher frequency bass (60-80hz ish) with the same results.

Then I'd forward the issue to Topping, since the HE1000 Stealth also shuts down your A70 Pro.

It would be very interesting to know if your A70 Pro has a problem, or if it's a general issue with the A70 Pro.

 

[Roland68]

Tried those to no avail and multiple other songs with higher frequency bass (60-80hz ish) with the same results.

Topping claims that the A70 Pro works down to 8 ohms, but apparently, in Amir's test, it also shut down at 12 ohms and high power.
Perhaps this is something Topping could change with a future firmware update.

 

[oleg87]

Can you try EQing in a subsonic filter? Perhaps an overly sensitive DC protection circuit is kicking in?

 

[Roland68]

Can you try EQing in a subsonic filter? Perhaps an overly sensitive DC protection circuit is kicking in?

He already has, see posts #1.250 and #1.252

 

[oleg87]

He already has, see posts #1.250 and #1.252

Whoops, sorry, didn't see that.

 

[Am1n985]

I gave up, trialled a SMSL H400 (SMSL's answer to the A70 Pro, pretty much), that could go a little louder but ultimately cut out too, returned both. I think both are not designed for the high current demand the SJY's ask for, thinking I should lean towards an Aune S17 Pro Evo, or S9C Pro (all in one, simpler).

 

[DrCWO]

Aune S17 Pro Evo, or S9C Pro

I don't think you'll have any luck with either headphone amplifier.

According the Specs the Aune S17 Pro Evo delivers 7,5W at 32 Ohm.

According to @amirm's measurements, the A70 delivers 8.22 W into 32 ohms.
Therefore, I expect the same inadequate results from the significantly more expensive Aune S17 pro.

The S9C Pro only has a power output of less than 5.5 W at 32 ohms. Therefore, it will reach its limits even faster.

I therefore recommend buying a power amplifier for your insensitive headphones, as most headphone amplifiers deliver less power at 32 ohms than the A70.

An excellent choice may bet the Topping B100 amplifier. He delivers 100W at 4Ohm which means 12.5W at 32Ohm. This will probably be sufficient. You need an adapter from Banana to XLR or 4,4mm TRS and you're done. Cheap solution and power without end...

If this still isn't enough get a NCx500. It will deliver 500W at 4Ohm which means 62,5W at 32Ohm.

Best DrCWO