Larry, please help. GPXfi Incompatible with a power amp?
April 22, 2017 at 3:08 pm #20807
This is really weird but here it goes…
the GPXfi would put my power amp in protection mode if it is connected directly to the amp. There would be loud thump on one of the speakers, then silence. So far, i think my speakers survived, but it happened twice now. I am unsure whether it is just the balanced in/outs that are causing this. I haven’t the nerve to try again. The DAC’s volume was at -115db at the time.
The amp in question is an Odyssey Kismet stereo power amp. I know that the balanced in on the amp are not true balanced, but a mere “courtesy” set of inputs….and no I didn’t connect both the balanced and SE inputs at the same time.
On the other hand, if the GPXfi is connected to the preamp, then from preamp to the power amp (balanced cables throughout), everything seems to be fine.
April 22, 2017 at 11:21 pm #20808
Sounds to me like it has a significant DC offset in one or both channels. If you are brave enough, does it do the same with only one channel connected? Does it do the same with the unbalanced (RCA) outputs? Since the Pulse uses separate line buffer/drivers for balanced and unbalanced outputs this may prove to be a useful test. Just a few thoughts on diagnosing the problem.
April 23, 2017 at 12:33 am #20809
I’ve confirmed, using a cheap set of speakers, that the problem is with the balanced output (i.e DAC connected to power amp directly via balanced cables). If I connect the DAC to the amp via RCA cables, all is well. Normal listening volume is achieved w/ DAC at -40 to -30 db.
With RCA cables connecting the DAC to the power amp directly, a voltmeter test on the speaker binding post at -40db is ~1.5v, with 2v at about 30db. If I max the volume on the DAC, it reads close to 4v on the speaker binding posts. This can’t be right, as I thought 4v is closer to what the balanced out would give, at full volume.
I also did a voltage check on the speaker binding post when balanced cables are used to connect the DAC to the power amp directly. I had the DAC set to -127db (i.e. zero volume), hit play in Jriver and there was initially 15v spike recorded! I think that’s what did it to put the power amp to protection mode (and that thump on the speakers)
The weird thing is if I put the preamp in between the DAC and the preamp, either balanced cables or RCA cables, everything is fine. My theory is that the 15v initial jolt from the DAC is rectified by the tube preamp before it hits the power amp, thus the power amp does not go into protection mode in that scenario.
April 23, 2017 at 12:34 am #20810
My question to Larry is whether the DAC can be used as a preamp at all. I have the impression that it was one of the major use case of this DAC, but clearly either the DAC isn’t supposed to be used as a preamp, or there is something wrong with the DAC, at least with the balanced outputs.
I have both a GPXfi and a GPX Infinity. BOTH DACs had the same issue.
April 23, 2017 at 10:03 pm #20815
A reasonably thorough investigation there. What this tells me is that the balanced output of your Pulse has a significant DC offset, although it is curious that this anomaly only presents itself when you start playing a track. The amp appears to maintain full gain all the way down to DC, which is not too unusual for solid state amps. The preamp is blocking this DC offset, preventing it from reaching the amp. The preamp being a tube type means that it almost by definition blocks DC as most tube topologies have a substantial DC offset within their amplification chain which must be removed at the output via a blocking capacitor and/or output transformer.
I strongly suspect that your Pulse is defective, as operating directly into an amp certainly appears to be one of its design parameters. Perhaps the negative voltage rail feeding the balanced output buffers is faulty – just a wild guess without being able to poke around in the unit myself. I suppose that I could try measuring the outputs of my Pulse for comparison purposes. Hmmm… some incentive to try to find some probes and warm up my oscilloscope.
I would suggest submitting a support ticket on this matter – then be prepared to wait a week or two or three for a response. I hope that you can get this resolved in a reasonably timely manner. Also, please report what is found, as I am curious and others may find this useful.
April 23, 2017 at 11:10 pm #20816
Thank you WS. I am a noob with electronics so your input is much appreciated.
The curious thing is that it affects both a GPX Infinity AND a GPXfi that I have. i.e. OK to connect to power amp directly via RCA, but speaker-killing potential when connected with balanced cables.
You’d think that all of these things were considered during design and tested thoroughly during QC/burn-in, but damn. I am having serious doubts on whether I should even continue to use the DAC directly with the power amp with RCA cables or even through the tube preamp. Years of waiting and we are dealing with potentially a fundamental design flaw.
April 24, 2017 at 9:30 pm #20827
No. As I said above, if the output of the Pulse has some DC offset, the tube preamp will remove it.
In re-reading your post above, even the RCA unbalanced output has more DC offset than seems right. Your measurement of 4V at the speaker terminals would mean that it is pushing a constant 2 Watts DC into an 8 ohm speaker. This i a lot. I would think that a good system would have less than 1V and preferably much less than that. That 2 Watts would be a sound level of ~87-92 dB if it were an audio signal.
April 25, 2017 at 9:05 pm #20833
Ah… okay. That was not obvious and I assumed DC. Do try a DC measurement both at the output of the Pulse and amp output as you did before. I think it should be reasonably easy to probe the free end of a cable connected to the Pulse. It would be good to know this.
April 28, 2017 at 11:57 pm #20838
Oook, an update.
TL:DR – DO NOT under any circumstances connect your DAC directly to a power amp using balanced cables while using the LH ASIO drivers! Your speakers will thank you.
I went to the engineer that built my tube pre-amp to diagnose the problem, here is his theory and I think it makes sense.
We tested the following 4 test cases. Common steps are we power on the DAC, volume is set to -127db, then the tube pre (in cases 3 and 4). then the power amp, then we start up JRiver and hit play. After each test, we then shut everything down, then power every thing up again in the above sequence.
1. DAC connected to power amp via RCA cables – PASS
2. DAC connected to power amp via balanced cables – PASS with DirectSound driver, FAILS with LH ASIO driver. Protection circuit in power amp triggered.
3. DAC connected to tube preamp then to power amp via RCA cables – PASS
4. DAC connected to tube preamp then to power amp via balanced cables – PASS
We cannot 100% confirm it, but the engineer’s theory is this: when the DAC is first turned on, only RCA outs are supplied with power, no power is sent to balanced out. When we hit play in JRiver, the ASIO driver then initiates a power cycle on the balanced out, causing a DC pulse to be generated, which in turn trips the power amp protection circuit. A pop is still heard from the speakers as the DC circuit protection relay is too slow compared to the pulse.
There are no problems when connected via RCA since the power is turned on as soon as the DAC is turned on, and any DC pulse generated would not have been amplified by the power amp, which is the last thing to be turned on.
Larry, please do some URGENT investigation with the ASIO driver. We’ve tried it multiple times with the same result. The DirectSound driver does not cause a power shut down on the power amp via balanced connections but the ASIO driver will when you hit play the first time after the DAC is turned on. Subsequent start/stop of a track will NOT trigger the shut down. Our theory is that the power to the balanced out is on already on subsequent “stop/play” cycles and therefore no DC pulse is generated.
- This reply was modified 5 months, 2 weeks ago by wliucm.
April 29, 2017 at 1:17 am #20840
I haven’t tried the above test with my GPXfi, but something tells me the test results would be the same. So, effectively, this power-up logic on the balanced out affects ALL GPX(fi/Infinity). Larry, please do the right thing and ANNOUNCE this so speakers are not damaged.
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