Internal Hum in amp, help

[Thomas_A]

Hi all,

I'm new to this forum and this is my first post. I have got one problem with my Audio Analogue Puccini SE Mk1 ampflifier. This amplifier is modified in two ways:

1. It has added capacitors in the power stage to reduce distortion according to the italian constructor Federico Paoletti.

2. It has been equipped with a pre out and main in, where the tape loop has been sacrificed and the internal wirings been rerouted.

My system is a complete active one where two other power amps and active filters are used to drive sub, middle range and tweeter. It has been working just fine until now.

Now to the problem:

I bought a new CD-player, a Cambridge Azur 640c Mk2. I also bought a new cartridge to my record player, and I therefore soldered some new values of the loading in the RIAA stage in the amp. I also moved the system to another location, and had to reconnect all cables.

When I connected the CD-player, one fuse blew in the Puccini amp. It was the F101 fuse only (the right channel, see schematics below). I replaced it, and made measurements of all cables and checked the connections. They were all ok. When I connected the CD-player again, the fuse blew again. I was confused and checked all cables and connections again. They were all fine. Then I measured the CD-player and the right output were faulty, it did not give any signal. Maybe I got a faulty CD-player, so I got it replaced.

Now I have a problem with hum (in both channels) in my amp. The hum varies with volume setting and is much greater on the RIAA input.

I have matched and replaced also the output transistors on the right channel, but I think they all were fine. I have measured the diodes and resistors, and have not found any problem. The hum is reduced when I touch the RCA connectors or +/- loudspeaker posts. There is also a reduction when I touch parts of the chassis. The hum is still there when I connect the preamp section to an external power amp. Thus I suspect a faulty pre-amp section. I have measured the grounds too, and find nothing unusal. I have not measured the ICs.

I ask for some ideas of what I can measure to find the problem. Schematis in PDFs below:

http://user.faktiskt.se/Thomas_A/Puccini5.pdf
http://user.faktiskt.se/Thomas_A/Puc5lpwr.pdf
http://user.faktiskt.se/Thomas_A/Puc5rpre.pdf

[peranders]

Sorry, but we can't access your files, error 403.

It seems that you have tweaked the amp to death

Have you done this tweak in a big bang fashion? Normally it is wise to make small changes, one at the time.

[peranders]

Originally posted by Thomas_A
.... and the internal wirings been rerouted.

Have you full control over this improvement? Hum is often caused by ground loops or low PSRR in the circuits.

[Thomas_A]

Thanks,

I made a mistake with the files - they will be put on another site later on so you can reach them. Otherwise they are also here in post-script format (.ps):

http://www.pi.infn.it/~federico/PSE.htm


There is a ground loop yes - the amp was modified many years ago and it has worked fine for at least 6 years with no hum at all.

The problem rose after the incident with the new CD-player. BTW this player has a grounded AC pronge, if that means anything.

What also has been done is just to change the loading resistor in the RIAA to 47 kohm, and changing two caps to original values in the RIAA filter. They were previously modified to tailor the response of a Shure V15VxMR cartridge, but I have not changed to an Ortofon and restored the RIAA stage. I have measured here also, there are no shorts found.

Number two is that I have soldered new cables (RG59) but they are all fine - no shorts and low resistance i signal and shields. I have also tried other cables, the problem is the same.

I would not say it is tweaked to death, and that something happened after the CD-player was connected and the fuse blew directly (right channel only). Since the hum is reduced by touching the RCA and/or speaker leads it's a suspected ground loop problem. The chassis is also sensitive - touching it affects the hum level but not as much as the RCA/speaker leads.

Regards,

Thomas

[peranders]

Do you have a mains filter in the CDP? Do you have the CDP plugged into a grounded outlet? Is the filter connected to the signal ground?

[Thomas_A]

"but I have not changed to an Ortofon" should be
"but I have now changed to an Ortofon"

THomas

[Thomas_A]

P-A

I have not even packed up the new exchanged CDP yet. The amp is humming even without any source connected to it. Pre-out to main in, or pre-out to external amp. They both lead to hum/buzz.

I can play music (turntable) so signal goes through.

The whole setup worked just fine before using my Toshiba DVD-player, but I wanted a new separate CD-player due to the noisy and slow DVD drive.

Before the break-down I had a grounded outlet to which I connected both amp and CDP. I do not know whether the signal ground and chassis ground became connected because the CDP has a metal chassis around the RCA connections. The RCA plug grounds are also of metal. I don't know if there is a problem with the RCA-chassis-connection of the CDP that can cause a short. In that case it is a quite serious flaw of the design of the CDP. But I don't know, I have never heard about this before. It could also have been a faulty CDP since the right channel was dead (perhaps DC at output).

The main thing now is to try identify the error in the amp. If I fix it I will first try the turntable, then to look carefully at the CDP, and perhaps add som isolation rubber o-rings around the RCA-outputs so that any RCA-connectors cannot touch the metal chassis.

T

[peranders]

If you break up pre - main, will the hum stop? I'll guess the hum problem is located in the pre section.

[Thomas_A]

Maybe I've found it. I was certain that I had measured that resistor before and that it was ok, but now it is showing infinite resistance. It's the most obvious one, i.e. between signal and chassis ground, 10 ohm 2W.

[Thomas_A]

There is no hum when pre and main are separated.

I will go to ELFA tomorrow and buy the 10R 2W resistor and replace it. I hope for the best.

Thomas