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toxic ingestion 29th April 2013 05:18 AM

Troubleshooting F5 - Dennon AVR 4603 hookup
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Troubleshooting F5 - Dennon AVR 4603 receiver hookup

I built the standard F5 a couple years ago and it sounds incredible and works well with one exception involving a Dennon AVR-4603 receiver and a Pioneer flat screen HDTV. Schematic attached. (Of note, I can’t find a schematic of the AVR-4603. If anyone has a link, I’d appreciate it.)


1. A CD player or ipod directly into the F5 sounds fine, powering 8 ohm speakers. The F5 has 24 volt rails, is biased at 1.3 amps with offsets < 5 mV, and this has been confirmed again today.

2. Picking up the preamp output from the Dennon receiver into the F5 also works just fine when using various inputs into the Dennon, such as CD player or auxiliary input (ipod).

3. BUT, if a television (Pioneer Elite PRO-111FD flat panel HDTV) is connected to the Dennon receiver (HDMI) AND the television is powered (even on standby), then when the preamp outputs of the Dennon are connected to the F5, the speakers powered by the F5 will click/pop loudly, with cones alternating at various frequencies between fully out or fully in, and the F5 begins to quickly overheat. The frequency of the loud clicking/popping with alternating and sustained maximal movement of speaker cones begins immediately with F5 power-up, first alternating every second or so and then slowly increasing in frequency. I quickly turn off the F5 to prevent burning up the speakers or the F5, and haven’t let it go more than several seconds. It does not matter what input is selected on the receiver. The F5 does get hot.

4. In fact, even if the Dennon receiver is without power (unplugged from the wall), if the TV is connected to the receiver and the F5’s input is connected to the output from the Dennon’s preamp, and the TV is on standby, the popping/clicking will occur as soon as the F5 is powered up. (I’ve never attempted to turn the TV “on” while the F5 is connected.)

5. If the TV is actually unplugged from the wall (not on standby), then everything is fine. Or if the HDMI cable between the TV and receiver is unplugged at either end, then everything is fine.

So, to investigate this, I spent some time with a scope and DVM today.

Preamp Output Across Dummy Load

First, I took the outputs of the preamp of the Dennon receiver and connected them to 100K ohm resistors to somewhat mimic the input impedance of the F5, and them monitored waveforms and AC and DC voltages across the preamp output in different situations:

1. With no input to the receiver and the receiver powered up, I got a flat waveform and 0.000 VDC by the DVM. This seemed very good.

2. And this remained so even when the TV was connected to the receiver, whether the TV was on standby or not, and whether the receiver was off or on. This threw me a bit. I thought that I would see some DC offset or wild waveforms on the preamp output as soon as the TV was connected and on standby, but the preamp output was a perfectly flat waveform, 0.000 VDC. (Photo attached)

3. When a CD was played and used as input into the receiver, there was no DC offset and the AC signal stayed perfectly centered about 0 V, with appropriate AC voltage out (for example, 0.05 to 2 VAC), changing with volume, of course. And things looked fine on the scope, even with the TV connected and on standby. Again, this was with a 100K ohm load across the preamp outputs and no connection to the F5.

Preamp Connected to F5

So, then I disconnected the TV from the receiver, hooked up the preamp outputs of the receiver into the inputs of the F5 and put 10 ohm power resistors across the F5 outputs (not risking speakers). I monitored waveforms with the scope and voltages by DVM across the power resistors in different situations.

1. With the TV disconnected, the F5 behaved fine with typical waveforms and voltages at speaker terminals across the power resistors. Again, speaker offset with no signal into the receiver was < 5 mV.

2. Then, I powered down the F5, put the receiver on standby, and connected the TV to the receiver by HDMI. On powering up the F5, and without an input signal into the receiver, complex and changing waveforms appeared across the power resistors, with large voltage swings of at least several volts by DVM. I kept adjusting sweep frequencies and gain on the scope, but could never get a constant waveform to photograph.

I also have an F5t with double MOSFET outputs (4 per channel) and double gain (decreased feedback) but no diodes. Exactly same issue.


1. No abnormal signal is seen on the output of the receiver’s preamp, even with the TV connected, when a dummy 100K ohm load is used across the preamp output.

2. No abnormal signal is seen on the output of the F5 when taking input from the receiver’s preamp output as long as the TV is not connected to the receiver AND/OR as long as the TV is unplugged from the wall, without any power.

3. As soon as both the F5 and the TV on standby are connected to the receiver, fluctuating waveforms of various patterns appears on the output of the F5, even with no input into the receiver, and even if the receiver is without any power at all.

Anyone have some insight as to what is occurring here? I wondered if the TV doesn’t sense some sort of capacitance through the receiver when the F5 is powered up, triggering chaos, but I really have no idea. Attempts by the TV to talk to the receiver, I wouldn’t think, would produce large voltage outputs out of the preamp.


bk856er 29th April 2013 05:57 AM

I'm no expert, but have you ruled out ground loops between the TV (and/or cable, satellite, etc.) and the receiver? Test with the TV coax input disconnected. Test with everything powered through a common powerstrip. Verify proper house wiring and cable/sat installation.


toxic ingestion 29th April 2013 02:31 PM


Interesting. I'll perform some experiments later today and post my findings tonight. I hope it is something this simple. Things are on different outlets, and the receiver is through a power conditioner. We'll see what is found, as I am open to most any suggestion. And I continue to look for any other insights.

Barniboy 29th April 2013 05:30 PM

Hello Toxic
I also suspect some floating ground issue, particularly when reading your point 3 above: Problem remains with the Denon is switched off.
Is your F5 power supply centerpoint connected to chassis and your mains earthlug at one, and just one point ?
The Denon probably is.
The TV too.
This leaves the mains conditioner. The two mains lugs are probably 60 VAC each (on your side of the pond), like a safety transformer.
But is the earth lug fed thru ?

rif 29th April 2013 06:05 PM

I had/have a ground problem with a denon receiver caused by the incoming coax cable wire, that then propagated through the hdmi connections, and onto to the sub amp. Hope that helps a little

toxic ingestion 29th April 2013 09:13 PM

Everyone - thanks for your suggestions. There appears to be a common theme, and we will know in several hours whether this is the case. I am really wondering whether the coax ground (shield) carries through from the junction box outside the house (which should be grounded to a water pipe or copper rod) all the way through the house, through an inline amplifier (which may or may not be grounded) to the back of the receiver. Again, I hope it is something this simple.

rif 29th April 2013 09:30 PM

That's pretty much caused my ground loop, but I was advised not to change anything due to safety concerns. I have Verizon fios which uses an ont in my garage to convert fiber to coax. It grounds to both a 3 prong outlet AND a ground wire from the ont going outside directly to the electric meter. If I remove the ground on the meter, no hum. But I was told that was dangerous.

toxic ingestion 30th April 2013 02:03 AM

Schedule changed and can't experiment tonight, but will do so tomorrow. Hope to post something in about 30 hours. Of note, on checking, the coax cable was never hooked up to the receiver when all the testing was being carried out, so the coax will not be part of a ground loop problem if such turns out to be the case. Tomorrow will inspect the electrical panel for tight connections and appropriate cable runs, appropriate grounding within the panel, etc. I will then confirm correct grounding of power outlets in the room. Then we will connect every piece of equipment up to the same power strip being fed from a single outlet and see what we have.

bk856er 30th April 2013 02:18 AM

Sounds like you're on the right path. It can be a nuisance, but it might be helpful to disconnect every single interconnect and wall plug and just have the absolute minimum required to test the configuration. Then introduce one thing at a time and test again, etc. For my AV equipment the magic ticket was a quality power strip accommodating all the gear - star ground topology I suppose. I understand Cab/Sat coax feeds are also a common problem.


AndrewT 30th April 2013 02:49 PM

I had to add an aerial cable isolator to my friends installation to attenuate a hum problem.

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