The Phonoclone and VSPS PCB Help Desk

[protos]

Originally Posted by northernsky

Hi,

just to complete the information.

My amp is an audiosector gainclone (2 monoblocks). The first tests I did with an ipod cable to the amp (without the ipod).
When I connected a simple passive pre to the amp with shielded cables the rf noise almost disappeared.

The situation now with the phonoclone (and small caps at the IC1 input) is that I hear a very low rf noise with the input selector to CD. The noise slightly increase when I select the phonoclone pre and the phonoclone is turned on.

This is the reason why I think the the noise could vanish when I connect also in the amp the cap to the op-amp input.

One more thing: I have a Rega P3 turntable which has not the ground cable.

Renato

You should not get any rf or hum on normal inputs in a gainclone.This leads one to suspect the internal connections of the pre/amp.

[protos]

Well after further investigation this is the result:
I double checked wiring and grounding,soldering in the phonoclone but everything seems identical between channels.
What is happening is that the right channel is much more sensitive to hum and noise than the left.Something like 70 vs 30% subjectively.Also the right channel has a lot of 50hz hum which is mainly magnetic interference from the power amp transformers below.This becomes less as I increase the distance from the power amp but it needs either minimum 60-70cm or some shielding for it to reduce substantially.The left channel has almost no perceptible 50hz hum even at very high volume..The 100hz hum is more similar in volume between channels.However in the right channel sometimes every few seconds there is a cycle of extra hum coming forward and then receding without me changing or moving anything.
I can easily distinguish 50-100+hz hum because the 50hz is only evident in the right channel subwoofer.

[rjm]

Hi protos, that's very odd behavior. Since the phonoclone boards are dual mono, they should do the same thing, good or bad. One starts to suspect the shielding of the input cable and in the turntable also. For instance if you put the R channel cable in the L channel phonoclone input does the hum balance reverse?

By the way I am worried about turntables/tonearms with no ground cable to tie to the phono stage. In my case I get severe hum if I leave my TT grounding wire unattached.

[protos]

Of course I did try that.I switched inputs AND ouputs around but whatever is plugged in the right channel of the phonoclone is noisier.In fact I rewired my tonearm externally so that I could reduce any extraneous factors.So in fact I am running a two channel wire direct from the cartridge to the phonoclone sitting next to theTT.No other connections.However I am using the tonearm ground wire from before to ground the tonearm and reduce hum.
I am using Van den Hul's recommendation and slightly twisting the tone arm wires around the tube to reduce hum.It works pretty well by the way if you don't want to spend a couple of hours rewiring your tonearm internally although it does not look that neat up close.
So I narrowed it to the fact that one channel circuit board is behaving differently.

[rjm]

Ok, then what I suggest is replace the cartridge and input cable with a dummy resistor. It sounds like you've already tried running with one channel left unpowered. It would be worth double-checking the voltages to the opamps, but assuming the regulators are doing there thing this shouldn't be a problem. Try switching opamps from one channel to the other.

Is there any difference in the length or routing of the ground leads from the board to the chassis? The next thing to try is connected the board COM to chassis from a different point, the input IN- for example.

[protos]

I am getting more confused and partly it is my fault.I have to withdraw the comments about the huge difference in channels.There was a temporary faulty connection to one subwoofer which skewed the results.I will have to retest with the sub connected but right now both channels seem similarly noisy.
I have tried the dummy resistor input 15 ohms and changing com connections while checking with a headphone amplifier connected .I was still getting hum with this set up and nothing else connected but the right channel seemed a bit noisier although with the earbuds you cannot be sure of the level of 50hz hum.
I checked the voltages and they are +-10 at the opamps.

Thanks for the help always.
Perhaps I should measure with the scope now.

[rjm]

Protos was kind enough to send me a mp3 of the output. This reminded me: anyone who has concerns should first record the output of the phonoclone with a computer soundcard. It doesn't have to be ultra-high fidelity, once you get the .wav data you can make an analysis of the power spectrum and this plot gives you and everyone else a very precise idea of what's going on.

[ahaja]

>protos
Try: short inputs to ground. Then check and listen. If you hear silence that means that preamp works good.
Then if you connect TT and you hear hum that means that sth is wrong with ground connections in turntable.

K

[rjm]

@ahaja : we are troubleshooting a phonoclone, not a VSPS: you should never short the phonoclone inputs as the gain of the first stage increases immensely when you do, no phonoclone can be quiet under this condition.

@protos : sorry to air your dirty laundry in public, but here's your noise, amplified by 50 dB. (its from the first segment of the clip you sent just before the music) There is a clear level difference between channels, a 50 hz sine wave, and what looks to be a nasty diode turn off spike.

Looks to me like ground loop or straightforward noise pickup due to insufficient shielding. I'm cautiously optimistic this can be tackled.

Please give me another mp3, this time of the phonoclone output noise with the tube amp off, the turntable off, the room lights off, but keep the cartridge (or dummy resistors is that's what you have at present) plugged in.

Thanks-

/rjm

PS I include my own noise data with the same 50 dB amplification applied. It's not level matched, but as you can see your hum is not sticking that far out of the noise floor. It's pretty mild ... indicating straight noise pickup rather than circuit oscillation.

[rjm]

Alright, I have some general things to say about the phonoclone and hum.

The midband gain of the phonoclone is 60 dB. That's 1000 times, and actually at 50-60 Hz it's closer to 80 dB and 10 000 times. That's a lot of gain. 1 microvolt worth of noise, which of course is not audible at line level, will be transformed into 10 millivolts, which is.

Which means, essentially, that you have to treat the input section with respect.

I don't think enough people appreciate that. It's not like building a headphone amp, where anything goes. In an MC phonostage you have to take a lot of care to address all the potential sources of noise and interference. Mess up one thing and it will not be quiet. Good technique is paramount.

It does not seem to be necessary to shield the internal input and output leads, but I would make sure that IN+ and IN-, OUT+ and OUT- are at least loosely twisted together, separated inputs and outputs, and a good distance from the V++/COM/V-- wires.

IN and OUT are isolated from the case, COM from each board goes back to the power entry jack, and connect together there (for shared power supply). At the point where COM connects together, a wire connects over to the chassis GND lug and case, which the turntable ground connects to.

That is normally sufficient.

It should go without saying that the phonoclone must be in a metal enclosure with no gaps or holes. Somewhat more controversially, I would also suggest (insist?) that the phono cable connected the cartridge to the phonoclone be well shielded.

This is no guarantee of fixing the problem, of course, but it provides a proper baseline from which to start playing around with alternate grounding arrangements.