Threshold s300 Amp Power on Thud

[nellson]

Has anyone heard of this problem occuring?

On power-on a loud thud is now heard in my loudspeakers which concerns me greatly as it may cause damage.

The amp is working and sounding fine apart from that?


any known parts that fail or should be replaced for this problem?

Thanks in Advance

Richard (nellson)

 

[Zen Mod]

electrolityc caps ,no doubt;
time to replace all small ones on main PCBs

 

[nellson]

thanks on the reply.

I had a quick visual before posting to see if any leaky caps were present, they look ok but are definitely old.

I have had a look and notice only a 47uf electrolytic, tantalum and mylar cap on the board. whilst having the amp apart is there anything else that should be done?

It looks easiest to drop the bottom panel out(with toroidial and filter caps on it) so the 2 side boards are accessible to work on at this stage.( comments?)

I guess i'll try replacing the 47uf cap and see if it helps the problem.


thanks

Richard

 

[georgehifi]

While your at it replace that tantalum bias circuit C5 I believe with a good quality plastic film , it's been known to clap out and give trouble. Here's the circuit if you need.

Cheers George

 

[georgehifi]

Oops, that's the S300 mkII opto bias this is the S300 mkI just in case.

 

[Nelson Pass]

The only phenomenon I recall is that the feedback cap could
store a charge on turn-off and if you did not wait a significant
amount of time before turn on, that charge would create a thump
as the amplifier removed it. That is why you will see a resistor
across feedback caps in these designs - to bleed off a turn-off
charge.

You could try placing a large electrolytic cap across the bias
circuit - the Vbe multiplier, or in the case of the optical circuit,
across the opto-transistor.

 

[nellson]

Have replaced the 0.15 tant and 470uf electrolytic on both channels and thud is still present.

Does anyone else have a turn on thud on there power amp and what is considered acceptable?

I have attached s300 power supply schematic.

Has anyone had problems with the switch/breaker and what is the criteria for selection of the 2 capacitors 0.01uf off the switch?

 

[wayle45]

Threshold S300ll Bias Adjustment

Nellson: Hi, It seems that you have schematics for the Threshold S300 ll. I'd like to set the bias and the DC offset but I'm new to this procedure. Are the blue pots the bias adjusters? Are there DC offset pots anywhere too? Please help- I'm a "Nervous Nelly" about this. Thanks, Wayne

 

[georgehifi]

Re: Threshold S300ll Bias Adjustment

Nellson: Hi, It seems that you have schematics for the Threshold S300 ll. I'd like to set the bias and the DC offset but I'm new to this procedure. Are the blue pots the bias adjusters? Are there DC offset pots anywhere too? Please help- I'm a "Nervous Nelly" about this. Thanks, Wayne

Here ya go.

Cheers George

 

[wayle45]

Threshold S300 ll

High five!, George - Thanks, Wayne

 

[wayle45]

Threshold S300 ll

This amp seems to lack smoothness or ease of delivery. While checking the bias, I noticed a number of power transistor replacements. They read MJ15011. 10 amp, npn, bipolar. This bank shows lower output than the other. Is this the correct substitution for the S300 "stasis" ? Could this cause nonlinearities in the amplifier output? I think the amp is worth a fix. Thanks, Wayne

 

[analog_sa]

S300 Series II

I wonder if anyone still remembers ( 🙂 ) what type the 1ohm emitter resistors are. Carbon composition? Brand? Some of them are cracked on mine.

Initial audition was not too promising but there is plenty of good metal to be reused 🙂 Very tempted to replace the PS Mallory with something civilised...

 

[Nelson Pass]

They were IRC wire-wounds. You can sub the Panasonic
3W metal films without problems.

😎

 

[Nelson Pass]

Re: Threshold S300 ll

They read MJ15011. 10 amp, npn, bipolar. This bank shows lower output than the other. Is this the correct substitution for the S300 "stasis" ? Could this cause nonlinearities in the amplifier output? I think the amp is worth a fix. Thanks, Wayne

While these will function, you want the MJ15024 - 27 series
which work better.

 

[analog_sa]

They were IRC wire-wounds. You can sub the Panasonic
3W metal films without problems.

😎

Many thanks Mr Pass. If they are indeed wirewound i may just live with the cracks.

 

[ANADIGIT]

SA/12e and S/550e

Hi,
Does anyone know parts different between SA/12e and S/550e ?

I try to reduce S/550e bias but I still can't do it ! (now 125mA/transistor and I need only 25mA/transistor)

P.S. I don't have S/550e schamatic but I have only SA/12e schematic on hand 🙂

Thanks
Anadigit

 

[nhojaji]

Hello, can someone send me the s300-II power supply and the main board schematic?
Is there a part list with detail information available for this amp?

 

[DeltaStar]

Hi, Is it a common thing for the S300 to thump on turn on? My mid ribbons don't like it at all. The S200 doesn't do it at all.
thanks

 

[kosscher]

Hello. If someone can advice replacement to the tantalum capacitor 0.15uF 35v, what the type and nominal would be good choice?
The same question about orange drop 0.1uF and mkt 10uF 100v.

Thank you.

I also thinkg about dual bridge rectifier, but this will take some time to stady...

Would appreciate your advice.

 

[Zen Mod]

0u15 - just put decent non-polarized solid cap instead

other two - leave them as is , they're not electrolytic