lm3875 and tube pre loud buzzing

[webby]

Hi all,
Can you guys help me out?
I have two chipamps and two ASL wave 8 monoblocks, running bi amped with a passive pre no problems. But when i try and use my Tube pre with the chipamps i get a very load buzzing through the speakers, the kind that makes you jump and hit the kill switch! This is the same if i just the tube pre and chipamps, no bi amping.
My aim was to run the tube pre + lm3875 for woofers + wave 8's for the tweeters.
Is this possible and what should i check?
All the tube kit is branded AV Reavox but its the same as asl i believe?

Many Thanks

Ian

 

[webby]

Bump

Bump.
No one have any ideas?
Cheers
Ian

 

[webby]

Hi all,
still no ieads?
This has got me really stumped, any help would be greatfully recieved.
Cheers
ian

 

[Nuuk]

Some more details may help!

Inverted or non-inverted chipamp?
Do you have the circuit that you used?
Do the chipampss work OK without the pre attached?
How long are the interconnects between pre and chipamps?
Is the buzzing the same on both channels?
Is the buzzing level changed by adjusting the volume control?
Is there a DC blocker on the output of the pre?
Is there a DC blocker on the input of the chipamps?
Have you tried different interconnects?

 

[webby]

Thanks for the reply Nuuk,
It a chipamp.com non inverted gainclone.
I Tried QED 1meter interconnects and .5m diy, same problem.
The tube amps run fine with either tube pre or passive. And the chip amps only buzz when the tube pre is used. Same loud buzz from both speakers and volume make no difference.
How can i check if there isa DC blocker, i dont know about either tube pre or chipamps?
Cheers
ian

 

[Nuuk]

You could try measuring the DC offset on the preamp if you have a multimeter.

Is it possible for us to see the internals of your chipamp? If so it should be easy to spot the DC blocker.

 

[webby]

Hi Again,
Here is a link to the chipamp kit i used;
http://www.chipamp.com/lm3875.shtml
Is this enough te tell if it has a dc blocker?
Save me taking it all apart!

How do i measure the dc offset on the valve pre, i have a DMM?
Cheers
Ian

 

[Nordic]

When they say DC blocker, I think they are reffering to an input capacitor... usualy a largeish polyprop cap (.22 -4.7uf, depending on lower rolloff) I don't see one.

To measure offset, set DC to lowest range and just measure outputs..

 

[webby]

Hi Again,
I measured DCV on the outputs;
Chipamp 85.4mv left output 85.6mv right output
Tube amps 0 for both
Tube pre I am not exactly where is should be measuring, I put the probe in the output RCA and the other probe on the side of the RCA for both left and right channels, but couldn’t get a stable reading. Started off at 1.5V then dropped to a few MV then 0.
Very confusing?
I guess there is no dc blocking on the chipamps then, I built the PCB’s as they came. Whet do I need to do?
Thanks
Ian

 

[Nordic]

I suggest you drop them a mail in this regard for a suggested value for the input cap.

 

[Nuuk]

I think the 10 uF Panasonic FC's are meant to be the DC blockers but I can't tell where they are on that picture!

It is weird measuring the output of a valve circuit. I have never had a stable reading on my digital meter!

 

[Nordic]

Strange Nuuk, as my little valve buffer gives a more solid DC offset (+-30mV) while it warms up

 

[Nuuk]

Strange Nuuk, as my little valve buffer gives a more solid DC offset (+-30mV) while it warms up

Is that with a digital or analogue meter?

 

[Nordic]

Cheap digital meter, got a nicle clamp meter this weekend from the cash converter shop, have to test offsets tommorrow then I'll tell you if that one works too.