Lateral Mosfet amp with DC offset >100mv

[Javajiven]

I built a pair of Lateral Mosfet Amps (Renesas LMosfet Randy Slone designs from his High Power Amp book) and they both ended up with >100mv of DC offset at the speaker outputs. Recently I beta matched a bunch of small signal transistors for the differential input pairs and soldered them into the amps. This fixed one amp, I was able to rebalance the differential input with the balance trimpot and rebias to 45 mv idle and had < 15 mv DC offset. The other amp remained at >100 mv DC offset, and has some audible hum from the speaker. Is this a cap problem? Or do I need to try different differential input pair transistors? I have a sig gen and an oscope. Can these be used to track down the source of the DC offset? I can post a schematic for the amp tomorrow if it is helpful.

Thnks!

[AndrewT]

It's just possible the Slone circuit is badly designed.
We get quite a bit of feedback on Slone's designs and much of it is related to builder's problems when they follow Slone's advice.

[Mooly]

Hum will probably be down to wiring problems, ground loops etc. Input and feedback ground need to be referred to a common "star point". If you are using a connection taken between the main reservoir caps then that's not good enough.
DC offset is down to the circuit design... 100mv is a lot. Can't really comment without knowing the circuit.

[richie00boy]

Check for bad joints on the board that is humming. Is it's ground point exactly the same as the quiet amp?

[Javajiven]

Yes, both amps are grounded identically. One is the right channel, one is for the left. I guess I'll have to pull the board and go over it with a magnifying glass and check for cracks or poor solder joints.

It looks like I should add a schematic of the design I built. It might make it easier to figure out some possible problems areas for DC offset.

On this particular design, the differential input pairs apparently need to be closely matched (and there are a pair of pairs to add extra complication, a pair of npn, and a pair of pnp transistors). Any mismatch presumably can be tuned to a minimum via a trimpot... it balances the two pairs ). I wish I could remember the name and exact functionality of the scheme. Anyway it worked out well on one amp with < 15 mv of DC offset, but not so well on the other. However, I'm not sure if this is the only area of possible mismatch in the system. I need to break out the oscope and compare the two channels to see what differences I can detect. I need to get another set of scope leads I suppose. It will be easier to compare the two amps if I do. Will the signal with DC offset appear to be riding on DC? Is there anyway to see the DC offset with an Oscope? I look at trigger pulses and waveforms on oscopes all the time for my work. This, however, falls outside my experience.

There seems to be a certain ammount of poor regard for Mr. Slone's amp designs. I can say that some simpler designs I've built have worked out better... especially for the novice DIYer that I am.

[Bluetron]

Originally Posted by AndrewT

It's just possible the Slone circuit is badly designed.
We get quite a bit of feedback on Slone's designs and much of it is related to builder's problems when they follow Slone's advice.

Randy Slone passed away on April 2nd.

[homemodder]

One of his designs is not workable, something tells me this is what you tried to build. This design suffers from high and unstable dc offset. Could you show us a schematic.

Slones designs arent bad, theres problems with one of them, the others work perfectly, then there is his amps he use to sell called optimos. Well now, build yourself a pair and youll have a very big surprise how good these amps sound and measurements are impressive too. Most underrated amp around this forum I would think, nothing special to its design except one particular stage, Ill say no more, theres one thread relating to the sound quality of these amps that says it all.

Javajiven if you have built the design im thinking you did, you wasting your time youll always have problems with it, youll have to start over from scratch, I can supply you the optimos schematics privately if you interested in building it. Parts are about the same and interchangeable.

Rest in Peace Mr Slone.

[mlloyd1]

that's too bad ... i've talked to him; he was a really nice guy, knew quite a bit and liked talking audio.

mlloyd1

Originally Posted by Bluetron

Randy Slone passed away on April 2nd.

[Carl_Huff]

Randy passed from pneumonia April 2nd. He was a good guy. I've built and still have both his OptiMos and Totem Pole amplifiers. Both design are outstanding and have always been trouble free.

[multisync]

virtually every modern ( last 40 years or so) oscilloscope's input amplifier is dc coupled. Somewhere near the input connector is a switch labeled AC DC. The problem you may have if you are using a 10x probe is that if the scopes sensitivity is to low you may not see 10 mv of offset very well. Use a 1x probe or see if there is a switch on your probe that says 1x 10x. 10 x means 10 times higher input impedance at the cost of 1/10th vertical sensitivity. If you scope has 10mv/div sensitivity and your probe is a 10x probe and you are measuring a 10mv signal the display will move about the thickness of the horizontal trace. If your probe is 1x or switchable the trace will move up or down 1 division up or down depending on +/- input. The display will probably be a bit fuzzy as well at those millivolt levels. With a 1x probe the input resistance of your scope will more than likely be 1 meg ohm.

When taking any measurements with a 1x probe you are putting a 1 meg resistor in parallel with whatever circuit you are testing to ground. For SS stuff this will have minimal effect in most cases, with tubes and their high value grid resistors, plate resistors etc a 1x probe will change the circuit parameters! That's 1 reason why 10x probes and higher exist.