I agree that there's probably nothing wrong with the amp. I've got an S200/II which also has a slight thump. It's been that way since new, so I suspect that's just a normal part of the power on stabilization.
---Gary
so , even if RMS , that means around 1W@8R
safe even for Lowthers
tnx ; I'm having fun , trying to make it like that
I suppose I could offer to do the relay mod that I did for the other guy with the PS Audio amp that was an early s/n and didn't have the improvements that later s/n of that model had. That unit does get quite warm after a while. Had it running about 10 hours continuous with 10V of 1KHz sine wave into the dummy loads. Heat sinks are 122°F after all those hours, but do to the mass, I can feel the radiant heat from 4' away. Can heat a room with this amplifier. 
Offset at output after five minute warm up is about 4mV in both channels.
Avoiding a turn on thump would require precise matching of neg and pos power supply components to ensure identical risetimes on startup. Most every transistor power amp uses a relay to aboid presenting this spike to the speakers.
The .02 ohm resistance of good relay contacts is relatively small compared to the wire it's connected to, and not to mention the several ohms of resistance in the speaker voice coils, so it's nothing to be concerned about, as long as the relay contacts are of good quality and don't degrade significantly over time.
From what I can tell, there seem to be many flavors of the PCBs in this amp model. As the customer didn't want to spend a lot to bring the amp back up to par, I am reluctant to disassemble the piggyback baord from the power output board unless there is a good reason to do so. Since there seems to be nothing obviously wrong with the amp, it would be adding unnecessary labor charges to his bill.
From what I can tell, there seem to be many flavors of the PCBs in this amp model. As the customer didn't want to spend a lot to bring the amp back up to par, I am reluctant to disassemble the piggyback baord from the power output board unless there is a good reason to do so. Since there seems to be nothing obviously wrong with the amp, it would be adding unnecessary labor charges to his bill.
No, thats wrong. If the preceding stages are already operational when the power transistors come up and
the DC gain is 1 (cap in the feedback loop) there will be no thump to speak of.
My amp has no output relay and is quiet at turn on and turn off, the SymAsym and the The diyAB "Honey Badger" Class AB Power Amp, both discussed here on diyaudio.com don´t have a "thump".
With a 3V turn on thump there is obviously something wrong with the amp. But if your
customer accepts a workaround instead of fixing the problem ...
No Threshold amp has an output relay BTW.
My QSC Powerlight 6.0 PFC doesn't have a relay and doesn't thump real bad when powered on either, but it's power supplies come up slowly using soft-start switching power supplies that track eachother. This Threshold amp appears to have a standard linear power supply which charges up at maximum possible rate.
A push-pull power output stage will eventually balance itself through servo feedback loop however, depending on the time constants in that loop, stabilization may take a few fractions of a second. If one rail doesn't come up at precisely the same time as the opposite rail, the output of the amplifier is going to swing the difference until either the servo loop catches up, or the supply rails balance. That's why on amplifiers that have blown a rail fuse, the speaker output sits at the opposite rail's potential, minus the forward drop of the transistor junctions in the still-connected rail.
The customer with the PS Audio amp specifically requested the relay, because later s/n of his 200c/x were updated by the manufacturer with relays. I would hate to be the owner of an expensive pair of loudspeakers that were fried because the output stage failed and put DC across the voice coils. A relay , with DC protection circuitry, is simply good engineering practice.
The three amps I have mentioned all use a simple supply and do not exhibit a "thump". The few caps are due for exchange anyway and the feedback cap and / or those in the current sources might well be the reason for the thump. I'm pretty sure Threshold did not ship the amp with such a defect, at least I never experienced it on the Thresholds I heard so far.
There are four 4.7uF and two 10uF electrolytics on each driver PCB. I'm going to replace them for good measure, however, I will be really surprised if that has any impact on the startup thump. There is always some fluctuation in output voltage until the rails stabilize. Virtually every transistor amp I have ever seen produces a pulse at the output during charging of the rails. Lower powered amps don't have relays, but almost all high power amps do, lest they damage speakers, or at the very least elicit complaints from their owners!
I did a full recap job on the driver boards of the S/300 II this week. I hand-selected the capacitors for measured capacitance by measuring and grading all of the capacitors. Matched sets went into both channels.
The turn on pop is 30% better, but not completely gone. Right channel produces a 2 volt positive going spike at turn on, left channel produces about the same, but negative going. Also, about 4 seconds after shut down, there's a slight 'ringing' sound heard from both speakers, similar to the sound of a spring letting go, but electrically produced. The amp otherwise performs well and runs pretty cool unless driving big signals into heavy loads for a long duration. I'm measuring about .035% in the left and .06% THD in the right channel at medium power levels. Noise is down under 1/2 a millivolt and offsets are under 2mV after warmup.
At this point, I can't declare anything wrong with this amplifier and I'm going to return it to the customer.
The turn on pop is 30% better, but not completely gone. Right channel produces a 2 volt positive going spike at turn on, left channel produces about the same, but negative going. Also, about 4 seconds after shut down, there's a slight 'ringing' sound heard from both speakers, similar to the sound of a spring letting go, but electrically produced. The amp otherwise performs well and runs pretty cool unless driving big signals into heavy loads for a long duration. I'm measuring about .035% in the left and .06% THD in the right channel at medium power levels. Noise is down under 1/2 a millivolt and offsets are under 2mV after warmup.
At this point, I can't declare anything wrong with this amplifier and I'm going to return it to the customer.
I noticed there are two small relays on the piggyback cards on the S/300 II that are not shown in the schematics I've been able to find on this forum. Does anyone have a schematic of the circuit that drives the relay coil? I'm trying to determine if the relay is closing too soon at startup, as it seems they close instantly upon power up and maybe that's causing the pop at power on.
Today, the large replacement electrolytic capacitors arrived and I installed them, but am waiting on new 2-1/2" clamps, as the old 3" clamps do not fit. The new capacitors are physically smaller, despite higher 90 volt rating and 42,000uF capacitance.
Alas, the amplifier STILL makes an audible pop when powered on, though not the first time from a cold start. The first time is pretty quiet. Shut it off and wait a minute, then turn back on, makes a louder pop (about 3 volts positive going spike on the 'scope.
What I've observed is that the small relays on the PCBs close immediately on power up--there is no delay. The schematics I have don't show the circuit that drives the relay coil. How are these driven? Are they supposed to close immediately or after a delay, at power on? I'm starting to suspect that the lack of delay might be the cause of the pop. Customer confirms that the amplifier did not pop at startup in the past. Both channels do this.
It would really help if I could find a schematic of the relay driver circuitry. It's not shown on the two different S series schematics that I downloaded here.
Alas, the amplifier STILL makes an audible pop when powered on, though not the first time from a cold start. The first time is pretty quiet. Shut it off and wait a minute, then turn back on, makes a louder pop (about 3 volts positive going spike on the 'scope.
What I've observed is that the small relays on the PCBs close immediately on power up--there is no delay. The schematics I have don't show the circuit that drives the relay coil. How are these driven? Are they supposed to close immediately or after a delay, at power on? I'm starting to suspect that the lack of delay might be the cause of the pop. Customer confirms that the amplifier did not pop at startup in the past. Both channels do this.
It would really help if I could find a schematic of the relay driver circuitry. It's not shown on the two different S series schematics that I downloaded here.