Classe DR9 biasing info

[marknoir]

Hello. I tried to use a recently acquired DR9 amp in bridged mode, but it promptly shut down, and went into protection ON/OFF cycle. It works fine in regular stereo. I was told that the bias may have drifted. How do I check it?
Which pot is the bias trim, and where do I measure it? Thx

[gni]

Measure the bias across the output resistor. . . . it is most likely
a 2W to 5W resistor raised off the main board. . .0.22ohms to 0.5ohms.

If you measure each one, they should all be within a decibel or so
of each other. . .

Let's say you measure 15mV across a 0.22ohm output resistor,
then the bias current will be 68.2mA. A class A amplifier will probably
have considerably more voltage across the output resistor at idle.
Maybe in the neighborhood of 80mV.

Measure the other channel. . . it should be close to the same. . . .

If not. . .look for trim pots. . .if there is one per channel. . .it is probably
the bias adjust. . .if there are two. . .the other might be the DC-offset.

Measure DC-offset at the output. . . it should be less than 60mV. . .
if it is higher. . .something might be wrong with that channel. . . .

I would suspect that from the behavior of the amp when attempting
to run it in bridge mode that either the load impedance was too
small or the channels are not biased similarly.

Time to upload a picture and let us see.

Make sure to attach clips that are insulated and won't move
when turning amp back on.

[marknoir]

Thanks for the info. I got a service manual from Classe! I checked the bias and DC offset. Somebody played with bias and it was completely out of wack. But even after I returned all to spec, it still shuts down in mono, even into a regular 8 ohm load. I beleive there is a problem with either bridging board or protection board. Me and the seller will come to some sort of decision on what to do with it.

[gni]

That is some amp layout. Maybe the supply caps are dry causing
some bad current causing the protection to kick in.

[marknoir]

Doubtful. That would happen in either mode. Amp worked for nearly two days fine in stereo mode EVEN WITH BIAS OUT OF WACK! As soon as you switch it into bridged mono - caboom! 5 minutes into playing you hear CLICK! Relays open. Besides, according to Classe, not only these caps would last nearly forever, but when they go, they go slowly, and the first thing you notice is hum and buzz. Which is normal for any supply caps. The amp is dead quiet.

But it does look great! A real work of art, with all components numbered, lettered, color-coded etc. My later DR15 is not nearly as pretty inside - just like a regular amp. This DR9 is an early one, with ENGRAVED face plate. My other one is a later version with silk-screened face.

[Classe DR-9]

Hello Mark,

Were you able to get a full repair manual?

Do you mind if I ask who you spoke with to get the manual?

All I could get was a pdf schematic of one channel and a hand written note that says Bias = 18.

No other info.

I have two DR-9s and I'd like to set them up for mono but I want them to be as close to each other as possible.

Thanks.

[marknoir]

Hi

No, I couldn't get the full manual. Same story as you. But bias instructions were enough.

I brought the amp for repairs to a facility in Long Island, and they butchered it! I eneded up digging in to it myself, completely rebuilding the protection board and that solved my problem. I don't remember bias figure off-hand, it was a while back, but 18 sounds like it may be correct.

[Classe DR-9]

Thanks for the reply Mark,

I only got a one page schematic, no bias instructions at all, just a hand written note that said Bias = 18.

I found something for the DR-15/DR-25 this morning that has a lot more detail and schematics fo it with bias instructions that in some places reference the DR-9 as well so I'm going to study them for a bit.

[Classe DR-9]

Mark, I noticed there are a couple of coils going to the speaker terminals on the inside of your amp, I have two DR-9s, both manufactured in 1990 from what I can tell, one has them the other doesn't, I'm trying to determine when they were added.

I tried to email you but I don't have enough posts yet, I wanted to compare serial numbers if you were willing.

I'm still working on getting a complete manual from Classe, I sent another email last night.

[marknoir]

Hello

My email is marknoir@optonline.net

Both my amps have those coils. One of my amps is early, with etched letters on front, one is later, with silk-screened letters. I don't really know what these coils do. Amount of inductance and resistance there is minimal, so what are they?

Early ser # is 8811512, late - 98920778

Hope this helps