Adjusting the bias - advice please

[bigburner]

I run two amps (an RB-1080 and a Harman Kardon PA2400) off a single preamp (an RC-1070) in a 4.1 music configuration. The volume coming from the speakers attached to the HK amp is slightly louder than the volume coming from the Rotel side of the system. I therefore have two options - either adjust the Rotel up or the HK down.

Is this possible? If so, how easy is it to do? Is it something that an amateur like me should do or should I get it done by an authorised service centre?

[soundhound]

It won't be easy for you to do. The easiest thing to do would be buy a mate to you're favorite amp, or, being its only truely a 2 channel configuration, take 1 set of outputs from the RC-1070, into 1 amp, and parallel the speakers off from that 1 amp. You can then sell the other amp. That way you keep the 4 speaks, volume will be matched, and the sound will be identicle. Just make sure the amp you use can handle 1/2 the impedance that the speakers state, or just run the speakers in series which will double their rated impedance.

[Taito]

Hi Bigburner,

Not something that can really be done internally -would require redesigning at least one stage of the amp. Also, the problem may not be so much due to the amplifiers' gains being mis-matched, as speaker efficiency -are they the same speakers or not?

Are you running the 4.1 configuration for critical listening or just for parties etc.?

Probably the easiest (and cheapest) solution would be to buy a reasonably inexpensive (but good) used line-stage preamp.

Run one of the outputs from the 1070 directly into the 1080 (via your sub, from memory ), and the other into the second preamp (for level adjustment) and into the HK. Cheap, easy, adjustable, without degrading sound of HK channels too much.

Hope this helps, Ben.

[bigburner]

Thanks guys. I use my second amp/speakers to fill my room with sound at party time or when I want a blast. They do a damn good job too. I have an old (1975) Pioneer integrated amp that would do nicely as a second preamp. I was just hoping that I could avoid using another piece of equipment. My second pair of speakers are my 12 year old B&W DM610i's, and they still sound great.

So Ben, have you built that sub yet?

[Taito]

Unfortunately no! It's those damn finances again. Looks as though it might actually have to wait until I finish my elec. eng. degree and start making some real money (just over a year now). I don't see the point in going halfway here. When I do get it done, I want it to be a keeper!

By the sounds of things, that is a nice add on to your system you have going there.

[mjb]

I run two amps (an RB-1080 and a Harman Kardon PA2400) off a single preamp (an RC-1070) in a 4.1 music configuration. The volume coming from the speakers attached to the HK amp is slightly louder than the volume coming from the Rotel side of the system. I therefore have two options - either adjust the Rotel up or the HK down.

Is this possible? If so, how easy is it to do? Is it something that an amateur like me should do or should I get it done by an authorised service centre?

You could try using a passive pre-amp (or a home made attenuator/pot) after the pre-out to reduce the level to the HK amp, but if the amps have different input sensitivities (which they do) you might find yourself fiddling with it at different volume levels.

[Andrew Pratt]

I don't know about that HK model but I know back when I had the PA5800's that there was a discussion on adjusting its bias via some pots. If you google it you might find what you're looking for and then see if it can apply to your amp....still its risky and a passive pre might be all you need. Creek makes a decent one that's not too expensive.