Lexicon 501 monoblock amps- different output levels??

autoconsortium · January 2012

My Lexicon monoblock amps are acting strange. One gets very hot, the other barely warm. Also, the output level (volume) is different between the two so I compensated by using the balance setting. The amp that has the lower output level, gets very hot putting out the same volume as the amp that has the higher output...this one barely heats up, even at very high volumes. I have, within the past 4 months replaced my adcom preamp with a lexicon DC1, and the problem seemed to exacerbate with the new pre. Any ideas on what could be causing this? The amps still sound wonderful and function perfectly otherwise. As an fyi, they are non-common and are ground strapped and running polk sda srs. They have been in use on my system for about 6 years.

F1nut · January 2012

Could be the DC offset. Did you ever check with Lexicon before strapping them?

FTGV · January 2012

Did you check the position of the series /parallel switches on the back to confirm they are both set the same?That amp is the functional equivelant of the Bryston 7B ST.It will be non common ground with the switch in the series position but common ground when in the parallel position but power output will differ for each mode.Other wise the overheating likely relates to a biasing issue.

autoconsortium · January 2012

They are set to the series position. I can remove the ground strap wire in this position? When I got them 6 years ago and hooked them up I got the nasty loud noise through the speakers that you get with non-common ground amps...but I'm not sure if the switch was on series or parallel. Just scared to repeat that experience.

autoconsortium · January 2012

And what about the heat issue???

FTGV · January 2012

It should never be strapped or have the SDA cable connected when in series mode.It would be fine to do so in parallel mode but you'll take a hit in the power deparment unless you are running a low impedance(ie2 ohm) load.

FTGV · January 2012

autoconsortium wrote: »

And what about the heat issue???

As mentioned previously above it's likely a biasing issue,but could be protesting the strapped outputs..Shut them off and switch them to parallel mode and run them awhile that way and see if they don't work better.

FTGV · January 2012

Well did get a chance you try them in parrallel mode?

autoconsortium · January 2012

I did, and THANK YOU. They still need the grounds strapped or I get the nasty noise, but now the volume output is equal between left and right channel AND they warm up to the same temperature...they also sound much better.

FTGV · January 2012

Your welcome and yes they should be strapped when in the parallel mode.

dorokusai · January 2012

Great job FTGV!

Your channel bias could be off when this happens also. In that case I'd suggest taking it to a shop and get hold of the schematic or service manual. If you have an amplifier that you plan on keeping, always try and get the service docs and manuals for it.

The heat is a product of the bias having drifted out of tolerance and it will eventually blow a fuse when it's completely shot. You don't want it to get this far.

FTGV · January 2012

dorokusai wrote: »

Your channel bias is off, take it to a shop and get hold of the schematic or service manual. The heat is a product of the bias having drifted out of tolerance and it will eventually blow a fuse when it's completely shot. You don't want it to get this far.

Read post #9,they are now operating at same temp.

dorokusai · January 2012

I read it. I was just commenting for other readers and perhaps should have used different verbage in the past tense I guess.

Fixed... :arrowu:

FTGV · January 2012

Thanks champ.

autoconsortium · January 2012

Yes, great job and very good advice. So, if you may...explain how this series and parallel thing affects monoblock amps. I understand the meaning and difference of the two terms, but can't seem to wrap it around my noggin how exactly it pertains to these amps. When I think 'series' with respect to electronics I think of...say, my kid's remote control car takes 4 double A batteries...when you stack them in the battery compartment they are hooked up in a 'series', where if I had 4 double A batteries and all the negatives were attached to negative and all the positives were attached to positive...'parallel'.

FTGV · January 2012

Though mono blocs each of those chassis's contain two complete channel assemblies much like would be found in a stereo unit.This gives it the flexibility to be configured in either the parrallel or series mode.In the series mode they are configured in what is commonly refered to as bridged mode.In this mode one channel amplifies the positive or non inverted half of the signal while the other channel (within the same unit)amplifies the negative or inverted half of the signal.Amplifying the signal differentially like this gives greater voltage swing thus giving greater power output potential into impedances in the 4-8 ohm range.
Typically bridged amplifiers don't like driving low impedances so for 1-4 ohm loads the parallel mode would be used.In parallel the two channels are operating as such and because the output transistors of both sections are working in tandem greater current (,but lower voltage) is available to drive the demanding lower impedance load.

Does that help?

Lexicon 501 monoblock amps- different output levels??

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