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Thump sound at turn off of a power amp

I use two Audio Research D-250 Mk II Servo amps. These are from the same era as the D-115 mentioned in this thread, and share some circuit similarities. In addition to the two opamps that are feeding the screen supply and front-end voltage regulators, the D-250 Servo model has a DC-servo circuit employing opamps to keep the DC-coupled gain stages at the proper operating points. I had noticed a significant thump at turn off when I first got them (mentioned in the reviews, too). I attributed it to these opamps shutting down before the slow decay of the tubes. After I replaced the main B+ power supply caps with double-the-capacity caps, they made Zeus-worthy thunderbolts. Since I'm using them in a bi-amplified arrangement, the directly-driven midrange and tweeters were getting the full brunt of this thunder. So I used a Schottky diode and 30,000uF of capacitance to hold up the opamps for a while longer, similar to what Tube Mania suggested. I had to get up to this high level of capacitance, which is not prohibitively large at a 25 volt rating, to quell most of the disturbing wallup. There still is a pop left. It believe it comes from the front-end inverter used to convert the unbalanced input signal to a balanced signal. The D-115 uses a single triode in this stage, while the D-250 uses a pair of triodes here. This stage is the only section of the circuit that can cause an imbalance in the amp at shut-down, thereby causing a differential voltage transient. The pop is maybe 10 or 15 dB lower than the original big thump, so I'm living with it.
 
Old thread but I've got D115 which upon the turn on makes thumping noises and flashes in the output tube screens . The sequence is that while the tubes warm up it is quiet than it starts humming and the thumps and flashes begin for a 10 -15 seconds than it stops and the amp plays fine. Kind of annoying since it's intermittent. All the tubes check out fine. When I pull out the V1 input tube left channel starts oscillating -thump , thump, thump :)
 
I may be that on turn off some of the HT supply is dropping out too quickly. It could be the supplies to the input stages - phase splitter. Maybe some of the caps have dried up a bit. I had a problem like that. Basically the negative bias on the output should collapse in order to dischage the HT to both output valves in a symmetrical way. If the input stage collapses first that can produce a thump on the grids which is differential.


The thump thump thing may also be dried up coupling caps.
 
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It actually happens on turn ON . Cleaning regulator ( 6550 12AT7) sockets tubes pins helped a little. It thumps only when turned on from overnight rest. Subsequent turns ON not do not produce thumps and flashes. I'm replacing the aging PSU caps since they are marginal . The V1 power supply (304v) is stable so no shorts or obvious faults.
 

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The V1 power supply (304v) is stable so no shorts or obvious faults.


Are you quite sure ?

My first instinct is in turn to put a scope probe across each of the DC supply rails (use x10-x100 probes) and see if there is any instability on power up, also power down. Do this sequentially. If the instability is suspect and cannot be seen then it is highly probable that RF could be produced beyond the Scope bandwidth. This where I always use an old AM radio which will most certainly 'squegg' or chirp.



Just my view, generally I don't like hybrid amps which use both semi's and tubes in critical operating parts. This happened to an amp when I was living quite close to the powerful UK Droitwich transmitter (1961) which sent 200Khz in every wire. Some of us oldie's remember too well at the time that the 405/625 line TV sets needed no antenna.





rJ
 
Update regarding the turn-off thump of Audio Research amps such as the D-250 and D-115 and others: I happened to notice that the arcdb.ws site has recently posted a turn-off muting modification for these amps. This pdf file includes some handwritten notes from ARC. Basically the mod is simple; it adds two relays that short the outputs immediately upon turning off the amp.

https://arcdb.ws/Database/D250/ARC_D250_turn_off_mute_modification.pdf

I have not implemented this mod in my two D-250s yet, but the relays are on order to do so.
 
Dunno what others think; but the idea of using relays to short the output during turn off, to me seems a horrible solution for quite an expensive amp! A deeper investigation is needed. Point is, tube amps of all varieties have been round now for 80 yrs or so and one would have thought such problems were by now ironed out.
 
I agree with the above. Is the complexity of these amplfiers justified by the sound? Like exotic old cars, I have seen a couple of these amps on 2nd hand sites for low prices because they are not working, and due to the lack of affordable specialist resources to fix them.

Defeats the point of building with tubes, IMO.
 
The circuit topology in this family of ARC amps from the mid-eighties bears almost no resemblance to most other tube amps, so 80 years of tube amp history, most using variations of RCA tube manual circuits, is not applicable to these amps.

As for “a deeper explanation is needed”, I have explained what is going on in a prior post in this thread. No mystery really.

The relays shorting the output do seem brutish, but the only energy being dumped through the output tubes after shorting is a short-lived thump of maybe a couple of volts. Should not cause a problem.

These amps can be converted to fully balanced, which is the natural state of this circuit. The added inverters up front convert what is essentially a balanced amp into a single ended (RCA jack) amp. In the eighties balanced had not yet caught on in a big way. By eliminating the inverter, the thump is eliminated.

Are these workarounds worth it? I think so. As always, YMMV. These amps will go toe to two with the latest from ARC and certainly from other tube amp manufacturers. The mighty D-250 is one amp that can drive Apogee ribbons with authority. Several of my Apogee buddies either have acquired or will acquire these amps for their Apogees.
 

PRR

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Amplifier at full voltage has self-control. Amplifier at zero voltage has no self-control. The transition can be sudden, "thump".

for 80 yrs or so and one would have thought such problems were by now ironed out.

Most of my life, amplifiers thumped. Like hum, it was expected. We knew to power-off the preamp last, but the power amp usually thumped. The idea of NO!-thump is a change of fashion.

And yes, we lit our homes with incandescent wires heated with coal-fired electricity and paid our bill with a check.
 
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its normal.
its called counter emf of the output transformer.


get a 5A dpdt relay and switch the speaker in and out.
I used 115V aircraft ones ( cheap and lower dropout) and connected it to the violet wire off the power transformer primary (100V tap).
I took a 100 ohm 12W resistor and went across the nc circuit to speaker common so when the relay is de-energized, the transformer had a load. when the relay closes, it switches to the speakers.

I got a D70 and I have heard the D125 quite a few times.

That "LOUD thump" is NOT normal, making a little thump, no problem, by a BIG THUMP is a problem.

These are rather old amps by now....

bias the tubes
how worn out are the tubes?

The amp might need some rebuilding by now?
 
I had a D70 with similar problem. I knew it had something to do with front end circuit shutting off before the power stage shut off that created a sudden thump. I just never had the time, skill, nor stamina to remedy the problem so I sold the damn thing! Early WZJ designed ARC products are notorious for overcomplicated circuits. And the resulted sound never justified that kind of complexities. Just look at their phase-splitter circuits for god sake!