Heartbeat-like thumping in speakers? ST-70 > PAS-3

DC92 wrote:I've run my system for years; however, this phenomenon is new. I run an ST-70 into a PAS 3. If I run the 70 straight out, the thumping disappears. So it's either in the RCA connectors to the PAS 3 or it's in the PAS 3 somewhere.

I don't know a better way to describe it than like a heartbeat pulse but it's a repeated pattern . . . like . . .Baa ba ba ba . . . Ba ba ba ba . . . but the tone of a heartbeat . . .

Any experiences with such a thing? Even if  I disconnect everything but the ST-70 the thump happens. And while it generally happens to the right (stage left) speaker, it does occasionally happen to left (stage right) speaker.

It's a maddening sound. Like the tell-tale heart or something . . .

Thanks!

peterh wrote:

DC92 wrote:I've run my system for years; however, this phenomenon is new. I run an ST-70 into a PAS 3. If I run the 70 straight out, the thumping disappears. So it's either in the RCA connectors to the PAS 3 or it's in the PAS 3 somewhere.

I don't know a better way to describe it than like a heartbeat pulse but it's a repeated pattern . . . like . . .Baa ba ba ba . . . Ba ba ba ba . . . but the tone of a heartbeat . . .

Any experiences with such a thing? Even if  I disconnect everything but the ST-70 the thump happens. And while it generally happens to the right (stage left) speaker, it does occasionally happen to left (stage right) speaker.

It's a maddening sound. Like the tell-tale heart or something . . .

Thanks!

If your PAS is a non-X it has a small DC at it's output. That could affect the input of the ST70 as it
has dc to the input tube grid.

Try a small cap in series ( 0.1 would do just fine) between the PAS and the ST70. The most
convenient point is probably in the PAS right at the output posts.

Bob Latino wrote:

DC92 wrote:I've run my system for years; however, this phenomenon is new. I run an ST-70 into a PAS 3. If I run the 70 straight out, the thumping disappears. So it's either in the RCA connectors to the PAS 3 or it's in the PAS 3 somewhere.

I don't know a better way to describe it than like a heartbeat pulse but it's a repeated pattern . . . like . . .Baa ba ba ba . . . Ba ba ba ba . . . but the tone of a heartbeat . . .

Any experiences with such a thing? Even if  I disconnect everything but the ST-70 the thump happens. And while it generally happens to the right (stage left) speaker, it does occasionally happen to left (stage right) speaker.

It's a maddening sound. Like the tell-tale heart or something . . .

Thanks!

Usually a repetitive thumping noise is caused by one or more power supply capacitors that are not doing their job. Try replacing the power supply capacitors and any associated resistors in the B+ power supply. In general this noise is called "motorboating" because sometimes it does sound like a motorboat. It may also be a sound that repeats at regular intervals (as in your case).

Bob

audioregenesis wrote:What instructions did you follow when you bypassed the tone controls? Proper instructions should include the addition of a 1.0uF coupling capacitor to prevent excessive DC voltage from appearing at the output of the PAS. Power amplifiers, like the ST-70, which are not AC coupled at the input, may become unstable if too much DC is present. A stock PAS (non X) will exhibit about 100mV of DC at the output, which the ST-70 will tolerate, however, bypassing the tone controls without including the capacitor will elevate that to about 800mV, which can be a problem for the ST-70.

audioregenesis wrote:The curcioaudio instructions, which have been out there quite a while, oddly, did not include the capacitor. It is quite important, and why it was included in the PAS 3X, which has the tone bypass feature incorporated into the special tone control pots. The audioregenesis tone bypass instructions mimic the way it happens in the 3X and, of course, include the capacitor in exactly the same way.

http://www.audioregenesis.com/documents/PAS%20Tone%20Control%20Bypass.pdf

At this point, it may be easier to just add a capacitor at the output jack of your PAS as peterh described, although I'd perhaps go little larger than .1uF to take better advantage of the now lower output impedance of the PAS, should you ever require it.

Jampot wrote:I'm a bit late to this so perhaps the problem is resolved. Do you use power line network adaptors by any chance? I do and find that they induce mains borne noise similar to that you describe in my phono stage. No apparent interference on other inputs so it may become audible due to the increase in gain.

Hope you get it sorted.

Jim

audioregenesis wrote:Basically all it means is adding a capacitor in series with the wires that connect to the output jacks. It can be as simple as unsoldering the wires at the jacks, solder one lead of the capacitor to the jack, and then solder the wire to the other end of the capacitor. Keep the leads of the capacitors short so everything is neat, compact and secure. The capacitors can be low voltage to keep the size down, and any good quality film capacitor will work well. Perhaps 0.22 - 0.47 should be fine.