Single Ended Stereo Preamp - Shared power supply

[StarTrucker]

On Power amps, I've seen sharing the B+ directly between channels, but what about on Single ended amps?

This preamp has a mono schematic and I housed 2 of them in the same chassis to run in stereo off the same power supply. They're single ended and I get oscillation/interference between the channels. The problem is in channel 2 but I get junk in channel 1. Disconnect ch 2's B+ and the problem goes away in both channels.

The B+ ends with a cap to ground (last PSU Cap) between the channels, so I figured any junk would go to ground.

This is troubleshooting, but the real question is how to share a power supply between 2 SET amps, so I posted it here in the drawing board forum.

 

[Rusan]

Which tube type are you using? Parallelling two triodes effectively creates a new, single triode with twice the transconductance and half the anode resistance. Which, of course, makes it much more susceptible to funny business like oscillations, on top of what was already a worst-case scenario for power supply rejection. It'll only be aggravated further by low Rp, high Gm types like 6922/6DJ8, 5687, ECC99, etc. If using such types, grid stopper resistors of adequate value are of double importance, with one per grid connection and very short leads.

How well is your B+ filtered and decoupled? How long are the cathode resistor connections to the ground side of the filter cap? Do the grounds run through the chassis? Do the grounds or the signal leads run close by the output transformers, which could be inducing a feedback loop?

In such a simple circuit (if you don't have any hum going on), it's quite possibly a lead dress/wiring issue of some type in Channel 2.

 

[StarTrucker]

Which tube type are you using?

12AY7 with both halves parallel'd. Ua100d I the circuit, and the PSU is in a separate enclosure with a 2foot run to the audio circuit which carries B+, signal and chassis grounds, and heaters.

How well is your B+ filtered?

4 stages RC


Is the schemo I posted correct where you can just link the two output transformers together from the B+ supply? Nothing needed to knock down cross talk?

 

[Rusan]

Is the schemo I posted correct where you can just link the two output transformers together from the B+ supply? Nothing needed to knock down cross talk?

No issue there. However....

....the PSU is in a separate enclosure with a 2foot run to the audio circuit which carries B+, signal and chassis grounds, and heaters.

Is the decoupling cap that the output transformer primaries are connected to housed in the PSU chassis, or the audio chassis? A two-foot long wire from the grid leak and cathode resistors of those two amplifier stages to the ground side of that cap could be an issue.

 

[StarTrucker]

Nope, the B+ enters into the audio chassis and is immediately connected to a 22uf cap. This then goes to the OT primary. The negative side of this cap is the ground for this parallel triode.

 

[dai h.]

hi,

my (novice) understanding was that a separate decoupling cap helps (with crosstalk) since it's a part of the signal path, like this:



source: http://www.nutshellhifi.com/library/Pix-A.gif

 

[ruffrecords]

Is there any decoupling local to the preamp where the HT enters? If not try adding 220uF electrolytic and a 220nF film across the HT at that point..You problem is possibly due to the inductance of the HT cable from the PSU to the preamp.

Cheers

Ian

 

[StarTrucker]

Here's a more complete schemo. Add the highlighted blue cap and increase the size of the electrolytics in the preamp?

How does inductance cause noise? Like an antenna?

 

[ccaudle]

How does inductance cause noise?

Inductance creates an impedance which rises with frequency. The noise referred to is the voltage variation of the power supply with current draw of the load circuit. The voltage across an impedance follows the AC version of Ohms law, so v=iz, where i is the frequency dependent current of the load, z is the impedance at a particular frequency, and v is the AC voltage generated across the impedance. That leaves the original power supply voltage minus the noise voltage at the load (or plus the noise voltage when the current load is decreasing).
An ideal class A stage does not have power supply current variation with signal, but any variation caused by non-ideal behavior is also not rejected by a class-A stage, the noise can show up directly in the output.

 

[Rusan]

The fact that Channel 2 is injecting audible artifacts into Channel 1 means unwanted inductive feedback loops are not only likely happening, but it could well also mean that (as Ian indicated) your B+ connection isn't adequately decoupled at the lowest and highest frequencies. In other words, the circuit's AC ground at those frequency extremes has excessively high impedance. If you still have the issue after following Ian's wise suggestion to more adequately decouple the B+ connection at the low/high frequency extremes, the culprit is possibly an inductive feedback loop of some sort.

I noticed from the schematic you have AC on the heaters, which means 600mA of current (assuming 6.3V) flowing parallel for two feet with the signal, circuit ground, and B+ wires. Even if the heater wires are twisted tightly to break up the AC fields, that's a loooong run. Mr. Faraday and Mr. Maxwell tell us this is a problem, even under the best of circumstances.

Now, consider that the cathodes and plates in your particular circuit are in-phase relative to one another, but in anti-phase to the control grid. If you're inductive-crosstalking between the various connections in that two-foot run, you'll be creating induced signals that are in-phase and 180° out-of-phase with respect to the various elements. Any positive feedback created here does what positive feedback does, which is turn your amplifier into an oscillator. But, because we also have reactive elements (caps, transformers) involved, there is also frequency-dependent phase shifting involved, so "the plot thickens," so to speak. Add it all together with the fact (as ccaudle noted above) that a transformer-loaded single ended stage has virtually zero noise rejection of any kind to begin with, then parallel two triodes to halve the plate resistance (virtually its only guard against artifacts that are riding on the B+) and make it that much worse, and well, you get the idea.

Out of curiosity, what is your raw B+ immediately after after the rectifier diodes, and what are the values of the series CRC filter resistors?