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dennismiller55 11th May 2020 10:45 PM

My NX Build
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Before committing this project to a proper enclosure I temporarily laid out my amp on plywood for listening test. This also gives me some practice on wiring as recommended in Andrew's "Ground Loops" pdf on his website

It sounds great. I am not good with fancy descriptions.

One question on the Power Supply and Speaker Mute Board. There is a good 3 or 4 second delay before the SSLR engages. However, turn-off time is not instantaneous. I estimate about a half a second before the indicator LED goes out and the music is muted.

It is my understanding that the mute turn-off time should be in the region of 50 micro-seconds.

Bonsai 14th May 2020 10:07 AM

Hello Dennis,

Perhaps my description of the muting function should have been clearer. There is a slight delay before the speakers are disconnected by the solid state relay during a normal power down cycle. The 50us really refers to the mosfet Relay + drive circuit switching time. Ideally you want to keep this below c. 100 us in order to remain within the energy rating of the mosfet.

When the amplifier powers down normally, the rails will collapse and 50 or 100 milli seconds later the speakers will disengage.

If the amplifier develops a fault eg short circuit across the speaker or a large DC offset, the protection circuit will switch very quickly - within 10s of microseconds.

The important thing is, you should not get any loud thumps when you power down normally.

dennismiller55 14th May 2020 10:17 AM

Thanks Andrew. I understand now. The amp is dead quiet after powering down. Looks like I have a fully functional amp.

Kaplaars 23rd May 2020 12:58 PM

3 Attachment(s)
Great that you are still supporting builders Andrew / Bonsai! Quite a while ago that I have been here. Unfortunately I have not had much time to spare. A few weeks ago I came across the boards again. Some progress, pasito a pasito as our Latam friends say ;).

The idea is to make a relatively compact build in terms of height. It was not easy to find a proper heatsink for that, so I fabricated something instead. Those radiators were actually spares for the KSA-100 clone :) Normally air is forced through them for optimal convection, but I expect this will be sufficient. A bonus is that it provides a nice shield for the cables, as I could route them underneath. The whole unit weighs more or less 3.5 kg.

thimios 17th June 2020 04:51 PM

Service to SX amplifier
4 Attachment(s)
Last two days i have problem with periodically burning fuses in the primary side of transformer in my SX amplifier.
I have used 2A slow blow and a soft start with 5x10ohm/5w.
My caps bank is 67.000uf/ rail.
I haven't any problem up to now.
I'm sure that problem is at the power supply because keep blowing fuses periodically without any load.
Not sure what happen i manage to increase the soft start resistors to 5x 22ohm(88ohm).
Measured primary side current is 400mA at the moment of power on when no load in the secondary side,just caps bank.
A. C Voltage drop on soft start resistors is 20v RMS, time before full start is about 2 sec.
Fuses are new old stock Philips.
Your opinion please.
Is it necessary to increase the soft start resistors or time further?
Given the fact that transformer is about 800 VA and the caps bank is 67000uf /rail is the 2A Fuse suitable?

thimios 19th June 2020 06:50 AM

2 Attachment(s)
Now the power supply with a load.
33R//33R each rail.
2A start current (transformer primary side) monitored.

Bonsai 19th June 2020 10:57 AM

Strange problem. Could your transformer be saturating? What’s the main voltage? What is the primary current with the secondary’s open circuit?

Might also be your fuses are not actually slo blo - maybe buy some and retry.

thimios 19th June 2020 12:43 PM

1 Attachment(s)
Hi Bonsai, thanks for answer!
Main voltage is about 225v.
Current at transformer primary side, when secondary is open circuit, is about 200mA.
Fuse body marked as T2A/250v.
Is from the same box that i used up to now in this position.
Another transformer measured for comparision, same things, current at the primary coil, when secondary is open circuit, is about 200mA.
Both transformer are salvaged from UPS.
Same dimensions, different manufacturers.
PS. Start current (caps bank+16R load) is most times 0,8-1A.See below.
Picture in post #2436, 2 A captured when playing with the power switch quickly. On-off-on-off, on.

Bonsai 19th June 2020 04:34 PM

That’s a very high primary current with the secondary’s open circuit Thimios.

Does the fuse only blow if you have the secondary’s connected to the power supply?

Note that the inrush current of the transformer itself (not related to cap charge) can be many tens of amps if you catch it at the wrong time on the mains cycle at switch on.

In an ideal transformer with open secondary’s after the switch on inrush current of the transformer, the current draw Should be 10-20 mA.

Bonsai 20th June 2020 08:01 AM

What was the fuse rating used with the transformers in their original application?

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