View Full Version : Amplifier Thump


Ivko
11-09-2005, 08:22 PM
09 Nov 2005

Power Amplifier "Cambridge A75", ser.no. A1277, manufactured in Britain by ARCAM, probably 20-25 years ago.--- During the initial application of AC power this unit causes a strong power-on thump in both speakers. With the speakers disconnected there appear approx. 45 VDC at the unloaded output terminals. Afterwards, when AC power is repeatedly turned off and on there is no audible thump or measurable voltage burst.

1) Is this the normal behaviour of this amplifier?
2) All power output transistors in this particular unit are type 2SD896 (NTE/ECG 36). What is the type number of the power transistors originally used in the A75 ampli?
3) Could anyone provide a schematic (circuit) diagram?

Regards,

ivan33@sympatico.ca

txsviking
11-09-2005, 08:40 PM
WOW Ivko, a member for over a yearand only 4 post!?
I don't think its normal but I've had an amp do that when the protection circuit
contacts were stuck and needed to be cleaned.
Someone will pitch in here with more help than that.
Maybe try a search for "turn on thump" or something similar.
Good luck and Good question.
Terry

Ivko
11-09-2005, 09:01 PM
Hello Terry:

Relay contacts? Yes, of course. Except this beast does not have anything that looks like the usual speaker relay.

Regards,
Ivko

shelby1420
11-09-2005, 09:22 PM
Welcome Ivko, not sure of the answer but i just wanted to say hello!!!

EchoWars
11-09-2005, 09:28 PM
I'm assuming that that 45V drops 'somewhat' after a second or two...

Most amps without a relay have some sort of muting that prevents the occurance of the 'thumps' you hear. Obviously, it is not working on yours.

Maybe bad caps, maybe toasted transistor or resistor. All we got at this point is guesses.

If you like the amp and want to keep it...step #1, find a service manual, or at least a schematic.

rulerboyz
11-09-2005, 10:08 PM
If you like the amp and want to keep it...step #1, find a service manual, or at least a schematic.


Here's a copy for you that you can pick up right now on ebay:

http://cgi.ebay.com/Cambridge-Audio-A75-Service-Manual_W0QQitemZ5827367998QQcategoryZ50594QQssPage NameZWD1VQQrdZ1QQcmdZViewItem

With that manual in your hand you might become the leading expert here on this amplifier by gaining some insight that none of us have.

Ivko
11-10-2005, 09:51 AM
Hello rulerboyz:

Thank you for the lead. Looking around this forum I have seen that you are an active and helpful person. Earlier I have found out this ampli was not made by Arcam but by Cambridge Audio. They have my e-mailed inquiry and I am now witing for thir response. If things don't work out I will go to your suggested ebay source. --- Thanks again.

Ivko

markthefixer
11-10-2005, 03:35 PM
Just my $0.02 here.. for informational purposes, the pioneer sx-550 does not have a protection relay in it, but it has an anti-thump circuit (and it looks like an add-on!!!!) between the power amp circuits and the pre-amp circuits.
Apparently in this unit the THUMP was coming from the powering up of the preamp circuits (including the volume, balance and tone controls) and they rigged this extra circuit to squash it without the major redesign necessary to add a protection relay and the associated circuits.

Why did I say the above??? Because if you don't see a protection relay, there are other ways to do it.