Do all CJ legacy preamps do this?

[Tuggs]

Can't say I noticed this before, since I've only had my PV6 for a couple months with occasional play. Over the past couple of days, I've been swapping preamps, amps, CD players around in my second system doing comparisons, so I've been paying particular attention to everything.

When I reconnected the PV6, I've now noticed that if a tuner is connected (first tried it with a Sherwood, then a McIntosh) along with a turntable and CD player in Tape 1 or Aux, there's a thump through the speakers when switching from Tuner into phono and a bit of a pop switching back into Tuner. This is with nothing playing, and the volume knob around 8:00; the higher the volume, the worse the effect. Thinking it was the Sherwood, I tried different interconnects, reversed the power cord, used either the variable or fixed outputs. Disconnected the 300 ohm twin lead. All to no avail.

However, with nothing plugged into the Tuner inputs, or with the CD player instead plugged into Tuner, there is no thump or pop. Of course, if I have the tuner or the turntable playing, I don’t notice anything unusual going from one to the other due to the volume knob being at a normal listening level (around 7:30).

I’m probably making a mountain from a mole hill, but I’ve been used to McIntosh silence in operation. But for the life of me, I don’t understand why it’s fine with the CD player plugged into Tuner, but with a real tuner plugged in, it balks. So is this normal for older CJ?

[admin]

I don't think it's anything to worry about. It may be just the manual switching causing a quick voltage change in the lines. In all honesty, when I switch sources, I always have the volume all the way down, but that is just me and not necessary.

[roberto]

Hola Tuggs,

Admin is right. Usually that is due to a spark voltage between the switches. The problem is at both Speakers? Between the switches there is a cap .1 uF on each channel to prevent this. Just change the caps and problem solved. You can turn the volume down while changing inputs to prevent this unwanted transient or send the unit to an electronic workshop for a recap the unit.
This transient is annoying, we know, buy it will not harm your speakers or your amp.

[Tuggs]

Thanks for the input Roberto.
Looking at a schematic, I see the only two .1 uF caps are C13 and C19, the coupling caps between V1 & V2, and V4 & V5, respectively. Would those be the ones? I'm no EE, so how would those two, in the middle of their gain stages (phono V1, V2, V3; line V4, V5, V6), have an effect on the switching. I figured it would be C15, C16, or C17, since those are on the lead-in to the phono input at the selector switch.

[roberto]

Hola Tuggs,

That value might be not exactly (the one that I sent)...are those at the rotary switch location? Odd numbers are for L channel and even numbers are for right channel usually. Can you forward your schematic to have a look? My email address is conradjohnson.costarica@gmail.com
Do not hesitate to do it!
Happy listening!

[Tuggs]

Per Roberto's recommendation, a couple weeks ago I re-flowed the solder connections of the two 2470 pF "dissipation" caps on the PV6's phono section output to see if that fixed the thump and pop during switching. No change. It was suggested I replace those two caps. However, as before, when I plugged yet a different CD player into the Tuner jacks, the thump and pop is non-existent.

If those two caps are actually bad (I haven't pulled them and checked their values with a multi-meter), could it be the CD player's output section is absorbing that switching transient instead? Conversely, when I tried the two different tuners, the thump/pop was there, so they did nothing to help the problem.

The simple answer is "just keep your CD player plugged into the Tuner jacks." Notwithstanding that, I'm always trying to further my knowledge by troubleshooting electrical malfunctions, as it not only helps me, but maybe I can pass something else on to my aviation maintenance students.

[roberto]

Tuggs,

Oh God, the behaviour of electronics...it is not always as we might think it is. Those caps are spark killers. That's what they are doing there. Try to put in parallel connection a .1uF @ 200V right where the switch is, and no signal will pass through the caps, only will kill the spark or transient.

Happy listening!

[plurn]

I wonder if this is due to the presence of DC offset on the outputs of your tuners, and the PV6 not filtering out this DC offset, whereas your previous McIntosh pre probably filtered this out.

Excessive DC offset is a common cause of thumps and pops.

If you have a multimeter you can easily check for DC offset. Set the multimeter for DC millivolts (or DC volts) and just measure at the RCA outputs of your tuners. Compare it to your CD players.

There should be very little DC reading if any - the closer to zero the better. Have not looked up appropriate numbers recently, but as a guess, I would expect anything above 50mV DC (0.05V DC) is bad. I'd prefer to see a much lower value then that though.

If that is the issue, there are various ways to correct it which we can discuss if that turns out to be the cause. Or you could just put it down to incompatibility between those tuners and your PV6.

In the meantime I think you should turn the volume down when changing inputs. Thumps and pops can cause damage to speakers, especially if they drive the power amp into clipping.

[roberto]

Hola Plurn,

What a fantastic idea and advice. Coupling caps are sometimes needed for absorb that unwanted DC working voltage that are in some stages or come from other components of the system. So, you use the plus (+) of the capacitor to where the plus (+) voltage is...and if you have minus (-) voltage, then you use the minus (-) there. This cap should be in series connection. The use of quality caps is a must, because the DC will not pass through to the other stage, but it will the AC (in this case, the music signal). I am trying to explain this for the members that might have this problem, and this is the way to fix it. Value of the cap? 2.2uF to 3.3uF is typically used for this purpose. Voltage? Use 50% more of the DC value where you want to do the coupling.

If I am mistaken, or saying wrong things, please correct me. I am a little bit rusted...some years that I don't work on servicing electronics. Just minor problems...my eyes are not too good, and working with electronics, definitively you must see what are you doing!!!

Happy listening!